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Electrocatalytic reduction of lignin related phenols in a stirred slurry reactor for green synthesis… Wijaya, Yanuar Philip 2021

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  Electrocatalytic Reduction of Lignin Related Phenols in a Stirred Slurry Reactor for Green Synthesis of Renewable Chemicals and Fuels by  Yanuar Philip Wijaya  M.Eng., Korea University of Science and Technology, 2014  B.Eng., Parahyangan Catholic University, 2009  A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF  THE REQUIREMENTS FOR THE DEGREE OF   DOCTOR OF PHILOSOPHY in The Faculty of Graduate and Postdoctoral Studies (Chemical and Biological Engineering)  THE UNIVERSITY OF BRITISH COLUMBIA (Vancouver)  February 2021 © Yanuar Philip Wijaya, 2021  ii  The following individuals certify that they have read, and recommend to the Faculty of Graduate and Postdoctoral Studies for acceptance, the dissertation entitled: Electrocatalytic Reduction of Lignin Related Phenols in a Stirred Slurry Reactor for Green Synthesis of Renewable Chemicals and Fuels         submitted by Yanuar Philip Wijaya   in partial fulfillment of the requirements for  the degree of Doctor of Philosophy                        in  Chemical and Biological Engineering        Examining Committee:  Kevin J. Smith, Professor, Chemical and Biological Engineering, UBC     Supervisor Chang Soo Kim, Principal Research Scientist, KIST-UBC Biorefinery on-site Laboratory   Co-Supervisor Elöd L. Gyenge, Professor, Chemical and Biological Engineering, UBC     Supervisory Committee Member Heather Trajano, Associate Professor, Chemical and Biological Engineering, UBC   University Examiner Dan Bizzotto, Professor, Chemistry, UBC                                           University Examiner Elizabeth Biddinger, Associate Professor, Chemical Engineering, City College of New York                            External Examiner   iii  Abstract Electrocatalytic hydrogenation-hydrogenolysis (ECH) is a prospective route for valorization of lignin derivatives, mainly for synthesis of organic chemicals. This environmentally benign process enables the integration of biorefinery and renewable electrical energy for clean fuels and chemicals production. Electrochemical water and/or proton reduction facilitates in situ, continuous generation of chemisorbed hydrogen on an electrocatalyst surface. However, most conventional ECH studies were operated at low current densities with low Faradaic efficiencies, owing to diffusion limitations of the organic molecules to the electrode surface. Polar organic electrolyte, which could facilitate the solubility of non-polar organic substrates in aqueous electrolyte, has not been extensively studied for the ECH purposes.   This work presents the ECH of lignin model compounds (e.g., guaiacol and phenol) using dispersed metal catalysts (e.g., Pt/C, Ru/C, Pd/C) in diverse aqueous electrolytes under mild conditions (25–60 oC, 1 atm). The stirred slurry electrochemical reactor (SSER) configuration enables ECH operation at high current densities (> |100 mA cm-2|) and efficiencies (>50%) due to the improved mass, heat, and electron transfers between the reacting molecules and catalyst particles. Different catholyte-anolyte pair effects were investigated under potentiostatic and galvanostatic conditions whereby the electrocatalyst activity was found to be dependent on electrolyte pH and composition. In the process development, electrocatalytic reduction of bio-oil substrates was conducted using organic solvent-mixed acidic electrolytes for mild depolymerization.  Acid-acid and neutral-acid catholyte-anolyte pairs were efficient for ECH of guaiacol and phenol, capable of resulting in high conversions (>90%) and efficiencies (>70%). This electrolyte pair combination enables the synergy of electrocatalyst and electrolyte, which could improve iv  Faradaic efficiency and extend the pH-dependent catalyst options. Polar organic solvents (e.g., isopropanol) improve the reactant solubilization and affect proton stabilization for enhancing dehydration reaction, however they could also hinder substrate reactivity owing to competitive adsorption on the catalyst and suppressed ionic activities in the electrolyte. Electrocatalytic hydrodeoxygenation (HDO) of alkyl guaiacols in the mixed electrolytes could produce cycloalkanes at the low temperatures, suggesting the potential of ECH routes for the synthesis of alkane fuels, besides the value-added chemicals. Finally, challenges and opportunities for future development of electrocatalytic pathways for lignin valorization are discussed.  v  Lay Summary Biomass is naturally abundant carbon source for energy and chemical production. Water is also renewable resource which can be used to produce hydrogen via water splitting reactions. Together, biomass and water can be utilized as renewable feedstock for chemical energy storage via electrochemical process. Lignin, the major part of woody biomass, is natural polymer composed of aromatic compounds which have potential uses in the production of renewable chemicals and polymeric materials. Electrochemical reduction pathways offer a promising strategy for the upgrading of lignin derivatives. This study aims to investigate and develop the mild electrosynthesis of industrially valuable chemicals from lignin-derived phenolic compounds using stirred slurry reactor in various electrolytes. Reaction parameter effects are studied with the objective to enhance the electrochemical process efficiency and its practical applications under environmentally benign conditions. In a broader context, this research contributes to the development of electrochemical process for clean energy production and renewable chemical and fuel synthesis from biomass.     vi  Preface This Ph.D. dissertation consists of seven chapters. Chapters 1–5 have been published in peer-reviewed journals. A version of Chapter 6 is in preparation to be submitted for publication. The Ph.D. study and research, including literature review, experimental design, reactor setup, data analysis, and dissertation preparation, were conducted by Yanuar Philip Wijaya under the supervision of Professor Kevin J. Smith, Professor Chang Soo Kim, and Professor Elӧd L. Gyenge. All the experimental works were performed at Chemical and Biological Engineering Department and KIST-UBC Biorefinery on-site Laboratory in the University of British Columbia. The list of publications included in this dissertation is given below: 1. Y. P. Wijaya, T. Grossmann, R. D. D. Putra, K. J. Smith, C. S. Kim, E. L. Gyenge. “Electrocatalytic Hydrogenation of Guaiacol in Diverse Electrolytes Using a Stirred Slurry Reactor.” ChemSusChem 13, 629–639 (2019). A version of this manuscript is included in Chapters 3 and 4. This manuscript was prepared and written by the author with direct guidance and intellectual contributions from Prof. E. L. Gyenge, Prof. K. J. Smith and Prof. C. S. Kim. The experimental works were planned and conducted by the author with some parts of the experiments executed by T. Grossmann. Technical support for GC-MS instrumental analysis was provided by R. D. D. Putra following some insightful discussions. TEM measurement was performed by Bradford Ross at Bioimaging Facility (BIF), UBC. 2. Y. P. Wijaya, K. J. Smith, C. S. Kim, E. L. Gyenge. “Synergistic Effects Between Electrocatalyst and Electrolyte in the Electrocatalytic Reduction of Lignin Model Compounds vii  in a Stirred Slurry Reactor.” Journal of Applied Electrochemistry (2020). A version of this manuscript is included in Chapters 3 and 5.  This manuscript was prepared and written by the author with direct guidance and intellectual contributions from Prof. E. L. Gyenge, Prof. K. J. Smith, and Prof. C. S. Kim. The experimental works were planned and conducted by the author with some assistances and technical supports from Priyanthika Adinamozhi, Daichi Hirata, and Robertus D. D. Putra. 3. Y. P. Wijaya, K. J. Smith, C. S. Kim, E. L. Gyenge. “Electrocatalytic Hydrogenation and Depolymerization Pathways for Lignin Valorization: Toward Mild Synthesis of Renewable Chemicals and Fuels from Biomass.” Green Chemistry 22, 7233–7264 (2020). A version of this manuscript is included in Chapters 1, 2, and 7. This manuscript was prepared and written by the author with direct guidance from Prof. E. L. Gyenge, Prof. K. J. Smith, and Prof. C. S. Kim. 4. Y. P. Wijaya, R. D. D. Putra, K. J. Smith, C. S. Kim, E. L. Gyenge. “Mass Transport and Kinetic Study of Electrocatalytic Reduction of Guaiacol in a Stirred Slurry Reactor with Methanesulfonic Acid Electrolytes.” (2020), in preparation. A version of this manuscript is included in Chapter 6. This manuscript was prepared and written by the author with direct guidance and intellectual contributions from Prof. E. L. Gyenge, Prof. K. J. Smith, and Prof. C. S. Kim. The experimental works were planned and conducted by the author. Numerical analysis was supported by R. D. D. Putra following some insightful discussions. viii  In addition to the journal papers, this work has also been presented as conference abstracts in the proceedings of international conferences, as follows:  • Y. P. Wijaya, K. J. Smith, C. S. Kim, E. L. Gyenge. “Electrocatalytic Hydrogenation of Guaiacol Using a Stirred Slurry Reactor with Carbon-Supported Platinum Catalyst in Different Aqueous Electrolytes.” The 236th Electrochemical Society Conference, Atlanta, United States (2019). • Y. P. Wijaya, K. J. Smith, C. S. Kim, E. L. Gyenge. “Electrocatalytic Reduction of Lignin Model Compounds in a Stirred Slurry Reactor: Mild Approach for Synthesis of Renewable Chemicals.” The 237th Electrochemical Society Conference, Montreal, Canada (2020). [Abstract accepted, conference meeting cancelled due to COVID-19]  • Y. P. Wijaya, K. J. Smith, C. S. Kim, E. L. Gyenge. “Mild Synthesis of Cyclohexanol and Cyclohexanone via Electrocatalytic Reduction of Phenols Using Dispersed Metal Catalysts.” The 238th Electrochemical Society Conference, Honolulu, Hawai (2020). [Abstract accepted, conference meeting held virtually due to COVID-19] • Y. P. Wijaya, K. J. Smith, C. S. Kim, E. L. Gyenge. “A Comparative Study on the Catalytic Performance of Pt/C, Ru/C, and Pd/C in the Electrocatalytic Reduction of Phenolic Compounds.” The 26th Canadian Symposium on Catalysis Conference, Vancouver, Canada (2020). [Abstract accepted, conference meeting postponed due to COVID-19]      ix  Table of Contents  Abstract ......................................................................................................................................... iii Lay Summary .................................................................................................................................v Preface ........................................................................................................................................... vi Table of Contents ......................................................................................................................... ix List of Tables .............................................................................................................................. xiii List of Figures ...............................................................................................................................xv Nomenclature ............................................................................................................................ xxii List of Abbreviations .................................................................................................................xxv Acknowledgements ................................................................................................................ xxviii Dedication ...................................................................................................................................xxx Chapter 1: Introduction ................................................................................................................1 1.1. Current trend of worldwide energy supply and demand ...............................................1 1.2. Lignocellulosic biomass as a renewable carbon-based energy source ..........................2 1.3. Lignin and bio-oil as natural sources for renewable chemicals and fuels .....................5 1.4. Renewable hydrogen from water electrolysis ...............................................................7 1.5. Research scope, objective, and outline ..........................................................................9 Chapter 2: State of the Art and Literature Review ..................................................................12 2.1. Prospects of electrocatalytic hydrogenation (ECH) technology ..................................12 2.2. Reaction mechanisms in ECH of organic compounds ................................................13 2.3. Electron transfer mechanisms in electrochemical hydrogenation ...............................15 2.4. Electrocatalysis mechanism in organic electrosynthesis .............................................17 x  2.5. The roles of electrocatalyst and electrolyte in ECH of organic compounds ...............18 2.6. Recent progress in ECH of lignin derivatives and the knowledge gaps ......................19 Chapter 3: Experimental Methodology .....................................................................................30 3.1. Electrochemical cell: Stirred slurry configuration .......................................................30 3.2. Cyclic voltammetry and electrolysis experimental protocols ......................................31 3.3. Product analysis and catalyst characterization ............................................................33 3.4. Figure of merit calculations .........................................................................................34 3.5. Kinetic parameter estimation methods ........................................................................36 3.6. Design of experiment and factorial analysis methods .................................................36 3.7. Materials and reagents .................................................................................................37 Chapter 4: Electrocatalytic Reduction of Phenolics in a Stirred Slurry Reactor ..................38 4.1. Electrochemical characterizations: cyclic voltammetry analysis ................................39 4.2. Thermodynamic analysis of ECH of guaiacol .............................................................41 4.3. Control experiments in ECH of guaiacol ....................................................................42 4.4. Potentiostatic ECH of guaiacol and phenol .................................................................45 4.4.1. Effect of different catholyte-anolyte pairs at controlled cathode potential ............. 45 4.4.2. Impacts of electrolyte conductivity, cathode potentials, and cathode materials ..... 54 4.5. Galvanostatic ECH of guaiacol and phenol .................................................................61 4.5.1. Effect of different catholyte-anolyte pairs at constant current and temperature ..... 61 4.5.2. Impact of acid concentration, current density, and temperature ............................. 68 4.6. Catalyst reusability tests in ECH of guaiacol ..............................................................75 4.7. Summary of the chapter ...............................................................................................79 xi  Chapter 5: Synergy between Electrocatalyst and Electrolyte in Electrocatalytic Reduction of Phenolics .......................................................................................................................................81 5.1. The synergistic effects between electrocatalyst and electrolyte ..................................82 5.2. Platinum group metals for hydrogenation and catalyst characterization results .........83 5.3. Catalyst performance analysis in different electrolyte pairs .......................................86 5.3.1. Comparative investigation in galvanostatic ECH of guaiacol and phenol ............. 86 5.3.2. Catalyst loading effect and carbon balance analysis............................................... 90 5.4. Electrolyte concentration effect in ECH of guaiacol .................................................101 5.5. Influence of catalyst slurry in water reduction ..........................................................104 5.6. Summary of the chapter .............................................................................................105 Chapter 6: Electrocatalytic Reduction of Phenolics and Bio-oil Using Mixed Organic and Aqueous Electrolytes .................................................................................................................112 6.1. The importance of organic solvent in ECH of lignin derivatives ..............................112 6.2. Mass transport and kinetic study in ECH of guaiacol using MSA electrolyte ..........113 6.2.1. Investigating the effect of mass-transport related parameters .............................. 113 6.2.2. Reaction network and mechanism study in the ECH of guaiacol ......................... 121 6.2.3. Impact of catalyst concentration and stirring profiles in a slurry reactor ............. 123 6.2.4. Reaction order and rate constant determination in the guaiacol ECH pathways .. 132 6.3. Factorial analysis on the galvanostatic ECH of guaiacol ..........................................140 6.3.1. Design of experiments and the guaiacol ECH factorial experimental results ...... 140 6.3.2. Individual and synergistic effects of the factor variables on the responses .......... 142 6.3.3. Optimization of process conditions ...................................................................... 147 6.4. Impact of different organic solvents in ECH of guaiacol ..........................................151 xii  6.5. Electrocatalytic upgrading of bio-oil substrates ........................................................155 6.6. Summary of the chapter .............................................................................................178 Chapter 7: Conclusions and Recommendations .....................................................................181 7.1. Conclusive summary and key findings ......................................................................181 7.2. Recommendations for future work ............................................................................183 References ...................................................................................................................................189 Appendices ..................................................................................................................................202 A.1. Stoichiometric analysis of the ECH of guaiacol ........................................................202 A.2. Mass-transfer limitation assessment in the ECH of guaiacol ....................................204 A.3. Catalytic reaction steps in ECH of guaiacol ..............................................................207 A.4. Formulating rate law and rate-determining step ........................................................208 A.5. MATLAB codes for the ECH of guaiacol kinetics ...................................................216 A.6. Statistical analysis results from the ECH of guaiacol factorial experiments .............219 A.7. GC-MS chromatogram results from the ECH of phenolics ......................................223 A.8. Physical properties data for lignin model compounds and organic solvents .............226    xiii  List of Tables Table 1.1. Typical structure and operational characteristics of the four major classes of electrolyzers for hydrogen production ............................................................................................ 9 Table 2.1. General comparison between ECH and TCH for reductive upgrading pathways. ..... 13 Table 2.2. Summary of the electrocatalytic reduction of lignin derivatives from various publications. .................................................................................................................................. 23 Table 3.1. List of materials and reagents used in this work. ........................................................ 37 Table 4.1. Standard Gibbs free energy, enthalpy, standard potential and temperature coefficient of the standard potential for reactions in ECH of guaiacol. ............................................................... 43 Table 4.2. Summary of the control experiments in the ECH of guaiacol .................................... 44 Table 4.3. Summary of the results from ECH of guaiacol in different pairs of aqueous electrolytes under potentiostatic control........................................................................................................... 50 Table 4.4. Summary of the results from ECH of phenol in different pairs of aqueous electrolytes under potentiostatic control........................................................................................................... 51 Table 4.5. Conductivity and pKa of the electrolytes at room temperature (± 25 oC). ................. 52 Table 4.6. Summary of the results from ECH of guaiacol (or 2-methoxycyclohexanone) in different pairs of aqueous electrolytes under potentiostatic control. ............................................ 59 Table 4.7. Summary of the results from ECH of guaiacol using different cathode materials at various applied potentials. ............................................................................................................ 60 Table 4.8. ECH of guaiacol in different catholyte-anolyte pairs under galvanostatic control. .... 66 Table 4.9. ECH of phenol in different catholyte-anolyte pairs under galvanostatic control. ....... 67 Table 4.10. Characteristics of the carbon-supported Pt catalysts used in this study. ................... 77 xiv  Table 5.1. Plausible proton/water reduction and hydrogen evolution reactions in acid, neutral, or base solutions ................................................................................................................................ 83 Table 5.2. Characteristics of the carbon-supported metal catalysts ............................................. 85 Table 5.3. ECH of guaiacol in different combinations of electrolyte pair concentration using Pt/C in acid-acid and neutral-acid catholyte-anolyte pairs. .................................................................. 96 Table 5.4. Summary of carbon balance calculation results in the ECH of guaiacol .................. 100 Table 5.5. Effect of catalyst loading on the organic carbon recovery in different solutions. .... 100 Table 5.6. ECH of guaiacol under galvanostatic conditions with different concentrations of neutral-acid catholyte-anolyte pairs using Pt/C, Ru/C, and Pd/C. .............................................. 111 Table 6.1. Characteristics of the Pt/C catalysts with different metal contents. .......................... 114 Table 6.2. Guaiacol ECH rate constant approximation results from the galvanostatic experiments at different temperatures (40–60 oC)........................................................................................... 139 Table 6.3. Summary of the results from galvanostatic ECH of guaiacol in MSA electrolytes for factorial analysis. ........................................................................................................................ 141 Table 6.4. Summary of the factor-response interaction and correlation in the factorial ECH of guaiacol experiments. ................................................................................................................. 143 Table 6.5. Optimization results of the guaiacol ECH. ............................................................... 147 Table 6.6. Summary of physical properties of the organic solvents used in this work. ............. 153 Table 6.7. General composition of bio-oils categorized by the chemical functional groups ..... 156 Table 6.8. Summary of the galvanostatic ECH of bio-oil experimental results. ........................ 166 Table 6.9. List of specific compounds identified in the bio-oil samples by GC-MS analysis. .. 173 Table 6.10. Thermodynamic data for ECH of bio-oil-relevant phenolic compounds ................. 174 Table 6.11. Summary of the results from ECH of bio-oil representative monomers.................. 175 xv  List of Figures Figure 1.1. World total primary energy supply and demand (consumption) by source. ............... 2 Figure 1.2. Lignocellulosic biomass structure consists of cellulose (crystalline), hemicellulose, and lignin (amorphous). .................................................................................................................. 3 Figure 1.3. General schematic illustration of lignocellulosic biomass transformation into bio-products (fuels, chemicals, and/or polymeric materials). ............................................................... 4 Figure 1.4. Representative lignin structures: (a) the primary building blocks (monolignols), (b) the network with the interunit linkages................................................................................................. 6 Figure 2.1. Electron transfer mechanism in electrochemical hydrogenation reactions, involving two distinct pathways: direct electroreduction followed by electronation-protonation (EP) and electrocatalytic hydrogenation (ECH). ......................................................................................... 16 Figure 2.2. Different operation modes of electrodes in electrosynthesis applications. ............... 17 Figure 2.3. Three common techniques for the electrocatalyst preparation and application in the ECH of lignin derivatives, including electrodeposition, impregnation or spray deposition, and stirred suspension. ......................................................................................................................... 21 Figure 2.4. Plausible elementary steps in ECH of guaiacol to cyclohexanol: Illustrative reactions between guaiacol, chemisorbed hydrogen, and the intermediate products. .................................. 22 Figure 3.1. A sketch of electrochemical cell setup for ECH of guaiacol and phenol .................. 31 Figure 4.1. Effect of guaiacol and phenol on the Pt gauze cyclic voltammograms in different electrolytes .................................................................................................................................... 40 Figure 4.2. Comparison on the ECH of guaiacol over activated charcoal (AC) vs. over carbon-supported platinum (Pt/C). ............................................................................................................ 44 xvi  Figure 4.3. Effect of catholyte-anolyte pairs on ECH of guaiacol and phenol under controlled cathode potential at -2.0 or -2.5 V vs. Ag/AgCl. .......................................................................... 46 Figure 4.4. Color changes of the catholyte product sample after 4 h guaiacol ECH ................... 52 Figure 4.5. Acid concentration effect on guaiacol conversion and hydrogenation product yields after 4 h reaction in the acid-acid and neutral-acid electrolyte pairs. ........................................... 53 Figure 4.6. Guaiacol conversion, product yield, and Faradaic efficiency as a function of applied potential (a); Current density and temperature as a function of applied potential (b).. ................ 57 Figure 4.7. Polarization curves for three different cathode current feeders (Pt, Ti, and Ni gauze) in the acid-acid and neutral-acid electrolyte pairs. ....................................................................... 58 Figure 4.8. Galvanostatic ECH of guaiacol in the two most effective catholyte-anolyte pairs: acid-acid and neutral-acid using jacketed H-cell. ................................................................................. 64 Figure 4.9. Galvanostatic ECH of phenol in the two most effective catholyte-anolyte pairs: acid-acid and neutral-acid using jacketed H-cell. ................................................................................. 65 Figure 4.10. The effect of (a) acid concentration, (b) superficial cathode current density, and (c) temperature on the ECH of guaiacol using the catholyte-anolyte pair of H2SO4 electrolytes after 5 h of electrolysis................................................................................................................................. 70 Figure 4.11. The dependence of guaiacol conversion rates on the initial substrate concentration (CGUA) under galvanostatic and potentiostatic conditions. ........................................................... 71 Figure 4.12. Graphical kinetic analysis for determination of reaction order for ECH of guaiacol based on Arrhenius equation. ........................................................................................................ 72 Figure 4.13. Graphical kinetic analysis for determination of reaction order for ECH of phenol based on Arrhenius equation. ........................................................................................................ 73 xvii  Figure 4.14. (a) Cathode current feeder effect on guaiacol conversion, product selectivity, and Faradaic efficiency; (b) Comparison on the electrocatalytic performance of the Pt/C catalyst: fresh vs. spent (A – in acid environment, B – in basic environment).. .................................................. 76 Figure 4.15. Adsorption-desorption isotherms for the carbon-supported Pt (5 wt.%) catalysts using N2 physisorption for fresh and spent catalysts. ............................................................................. 77 Figure 4.16. TEM micrographs for the 5 wt.%-Pt/C samples before and after reactions ............ 78 Figure 5.1. Comparative study on the electrocatalytic performances of Pt/C, Ru/C, and Pd/C in ECH of phenolics. ......................................................................................................................... 81 Figure 5.2. Adsorption-desorption isotherms for Pt/C, Ru/C, and Pd/C catalysts using N2 physisorption. ................................................................................................................................ 85 Figure 5.3. Catalytic performance of carbon-supported metals in ECH of phenol with different catholyte-anolyte pairs: (a) acid-acid, (b) neutral-acid. ................................................................ 88 Figure 5.4. Catalytic performance of carbon-supported metals in ECH of guaiacol with different catholyte-anolyte pairs: (a) acid-acid, (b) neutral-acid. ................................................................ 89 Figure 5.5. Effect of Pt/C catalyst loading in the ECH of guaiacol with different catholyte-anolyte pairs: (a) acid-acid, (b) neutral-acid. ............................................................................................. 91 Figure 5.6. Effect of Pt/C catalyst loading in the ECH of phenol with different catholyte-anolyte pairs: (a) acid-acid, (b) neutral-acid. ............................................................................................. 92 Figure 5.7. ECH of guaiacol in acid (H2SO4)-acid (H2SO4) catholyte-anolyte pair with different concentrations. .............................................................................................................................. 94 Figure 5.8. ECH of guaiacol in neutral (NaCl)-acid (H2SO4) catholyte-anolyte pair with different concentrations. .............................................................................................................................. 95 xviii  Figure 5.9. Effect of catalyst loading in the ECH of guaiacol in neutral-acid catholyte-anolyte pairs with: (a) Ru/C, (b) Pd/C. ...................................................................................................... 98 Figure 5.10. Schematic of the reaction pathways for the guaiacol and phenol ECH and the activity order of the carbon-supported metal catalysts. ............................................................................. 99 Figure 5.11. ECH of guaiacol in neutral (NaCl)-acid (H2SO4) catholyte-anolyte pair with different concentration combinations using 5 wt.%-Pt/C .......................................................................... 107 Figure 5.12. ECH of guaiacol in neutral (NaCl)-acid (H2SO4) catholyte-anolyte pair with different concentration combinations using 5 wt.%-Ru/C ........................................................................ 108 Figure 5.13. ECH of guaiacol in neutral (NaCl)-acid (H2SO4) catholyte-anolyte pair with different concentration combinations using 5 wt.%-Pd/C ......................................................................... 109 Figure 5.14. Catholyte pH profiles from the ECH of guaiacol experiments showing the catalyst effect on the water reduction reactions. ...................................................................................... 110 Figure 5.15. Catholyte pH profiles from three different experiments: (1) ECH of guaiacol, (2) Blank electrolysis, (3) Electrolysis over slurry catalyst (dashed lines). ..................................... 110 Figure 6.1. ECH of guaiacol in MSA electrolyte (0.2 M) under potentiostatic control showing the effects of (a) metal content, (b) stirring rates, and (c) catalyst concentration. ........................... 116 Figure 6.2. ECH of guaiacol in H2SO4 electrolyte: (a) 0.2 M, (b) 0.5 M comparing the performance of Pt/C and Pt/Al2O3 ................................................................................................................... 119 Figure 6.3. ECH of guaiacol in different acid electrolytes, including sulfuric acid (HSA), perchloric acid (HPA), and methanesulfonic acid (MSA) with different concentrations: (a) 0.2 M, (b) 0.5 M under potentiostatic conditions ................................................................................... 120 Figure 6.4. ECH of guaiacol under potentiostatic and galvanostatic conditions in H2SO4 (0.2 M) and MSA (0.2 M) electrolytes, respectively investigating the initial reaction rates ................... 123 xix  Figure 6.5. ECH of guaiacol in MSA electrolyte (0.2 M)  at different temperatures: 40 oC, 50 oC, 60 oC using 5 wt.%-Pt/C (at 10 wt.% concentration) for the 1st reaction order. ......................... 125 Figure 6.6. ECH of guaiacol in MSA electrolyte (0.2 M) at different temperatures: 40 oC, 50 oC, 60 oC using 5 wt.%-Pt/C (at 7 wt.% concentration) for the 2nd reaction order ........................... 126 Figure 6.7. ECH of guaiacol in MSA electrolyte (0.2 M) at different temperatures: 40 oC, 50 oC, 60 oC using 5 wt.%-Pt/C (at 10 wt.% concentration) for the 2nd reaction order ......................... 127 Figure 6.8. Guaiacol conversion, product selectivity, and Faradaic efficiency at different temperatures and fixed current (j = -182 mA cm-2) after 4 h investigating the effects of catalyst concentration and stirring profiles .............................................................................................. 128 Figure 6.9. Graphical analysis for the apparent reaction order determination in ECH of guaiacol using MSA (0.2 M) electrolyte pairs with different catalyst concentrations, stirring rates, and stirrer sizes. ............................................................................................................................................ 129 Figure 6.10. Arrhenius plots for the determination of activation energy and pre-exponential factor in ECH of guaiacol using MSA electrolyte pairs at different temperatures (40–60 oC). ............ 130 Figure 6.11. Concentration and Faradaic efficiency profiles from ECH of intermediate reactant: (a) phenol, (b) cyclohexanone, (c) 2-methoxycyclohexanone. ................................................... 133 Figure 6.12. Schematic procedure for kinetic analysis in the ECH of guaiacol to determine the reaction rate constant. ................................................................................................................. 134 Figure 6.13. Concentration profiles for the actual and model data fitted by non-linear regression method as the rate constant approximation results from the galvanostatic ECH of guaiacol at different temperatures (40–60 oC). ............................................................................................. 138 Figure 6.14. Model 3D surface graph visualizing the synergistic effects of the significant factors on guaiacol conversion. .............................................................................................................. 144 xx  Figure 6.15. Model 3D surface graph visualizing the synergistic effects of the significant factors on Faradaic efficiency. ................................................................................................................ 145 Figure 6.16. Model 3D surface graph visualizing the synergistic effects of the significant factors on cyclohexanol selectivity. ........................................................................................................ 146 Figure 6.17. Model 3D surface graph visualizing the synergistic effects of the significant factors on cyclohexanone selectivity. ..................................................................................................... 146 Figure 6.18. Contour plots for optimization of guaiacol ECH process conditions that give maximum values for all responses based on 1st Design of Experiment. ..................................... 149 Figure 6.19. Contour plots for optimization of guaiacol ECH process conditions that give maximum values for all responses based on 2nd Design of Experiment. .................................... 150 Figure 6.20. Impact of different organic solvents in the ECH of guaiacol under potentiostatic conditions.. .................................................................................................................................. 154 Figure 6.21. Average molecular weight analysis results by GPC for bio-oil samples from ECH experiments at constant current and mixed aqueous and organic electrolyte. ............................ 167 Figure 6.22. Compound distributions in the samples from bio-oil ECH experiments for the MSA catholyte solution mixed with: (A) ethanol, (B) isopropanol, (C) acetone before and after reactions (~20 h) based on a solvent- and oligomer-free basis. ................................................................. 168 Figure 6.23. Compound distributions in stack columns showing the evolution of lignin derivatives (including phenols, aromatics, and guaiacols) during the ECH of bio-oil.................................. 169 Figure 6.24. Compound distributions in stack columns showing the evolution of lignin derivatives (including phenols, aromatics, and guaiacols) during the ECH of bio-oil.................................. 170 Figure 6.25. GC-MS chromatogram of the dried sample from bio-oil ECH experiment in the MSA electrolyte mixed with acetone before and after 25 h reactions.................................................. 171 xxi  Figure 6.26. GC-MS chromatogram of the extracted sample from bio-oil ECH experiment in MSA electrolyte mixed with acetone before and after 25 h reactions.................................................. 172 Figure 6.27. Visual appearances of catholyte mixture in the ECH of cerulignol using acidic electrolyte (H2SO4, 0.2 M) and possible reaction pathways for ECH of cerulignol and ECH of creosol under the operating conditions in this study. .................................................................. 176 Figure 6.28. Product distributions from the ECH of mixed phenolic reactants, including cerulignol, creosol, and guaiacol (or phenol) ................................................................................................ 177 Figure 7.1. Concept design of a distributed biomass processing station. .................................. 188 Figure A.1. Product distribution of a complete guaiacol conversion via: (a) 100% demethoxylation–ring saturation and (b) 100% ring saturation–demethoxylation pathways as a function of the conversion of intermediates based on stoichiometric analysis. .......................... 203 Figure A.2. Plausible reaction network in the ECH of guaiacol under the experimental conditions in this work. ................................................................................................................................ 207 Figure A.3. (a) Correlation between experimental and calculated rate data for ECH of guaiacol. (b) Profiles of the surface coverage ratio between adsorbed guaiacol and chemisorbed hydrogen compared to guaiacol concentration over time ........................................................................... 215 Figure A.4. GC-MS chromatograms from the ECH of guaiacol in the pair of NaCl (0.5 M) and H2SO4 (0.2 M) electrolytes over 5 wt.% Pt/C catalyst before and after reaction. ...................... 223 Figure A.5. GC-MS chromatograms from the ECH of cerulignol in the isopropanol-mixed H2SO4 (0.2 M) catholyte over 5 wt.% Pt/C catalyst before and after reaction. ...................................... 224 Figure A.6. GC-MS chromatograms from the ECH of mixed phenolic reactants (cerulignol, creosol, and guaiacol) in the isopropanol-mixed MSA (0.2 M) catholyte over 5 wt.% Pt/C catalyst before and after reaction ............................................................................................................. 225 xxii  Nomenclature ∆𝐺𝑅0   Standard Gibbs free energy of the reaction (kJ mol-1) ∆𝐻𝑅0   Standard enthalpy of the reaction (kJ mol-1) ∆𝑆0   Standard entropy of the reaction (J mol-1 K-1) 𝐸𝑟𝑒𝑑 0    Standard reduction potential of the reaction (V vs. SHE) 𝜕𝐸𝑇0𝜕𝑇   Temperature coefficient of the standard equilibrium electrode potential (mV K-1) 𝜂 cathode  Cathode overpotential (V)   Specific conductivity of electrolyte (S m-1) φ   Metal dispersion (%)   Density of liquid, solvent, or electrolyte (kg m-3) p  Density of catalyst (kg m-3)   Dielectric constant (F m-1)   Viscosity of liquid or solution (Pa∙s) A  Pre-exponential factor (s-1 mol-1 or M-1 s-1 mol-1) BP  Boiling point (oC) C.B.   Carbon balance (%) CG  Guaiacol concentration (M) CH,a  Anolyte proton concentration (M)  CH,c  Catholyte proton concentration (M) Deff  Effective diffusivity (m2 s-1) dp  Active particle diameter (nm) Dpore  Average pore diameter (nm) xxiii  E  Cathode potential (V vs. Ag/AgCl) Ea  Activation energy (kJ mol-1) F  Faraday’s constant (= 96500 C/mol e-) F.E.  Faradaic efficiency (%) Hads  Adsorbed (chemisorbed) hydrogen I  Operating current (A or C s-1) j  Superficial current density (mA/cm2) k  Rate constant (s-1 or M-1 s-1)  kS-L  Mass-transfer coefficient at solid-liquid interface (m s-1) Mn  Number average molecular weight (g mol-1) MW  Weight average molecular weight (g mol-1) NW-P  Weisz-Prater criterion pH  Potential of hydrogen pKa  Potential of acid dissociation constant rA  Reaction rate per mass of catalyst (mol s-1 g-1) ra  Rate of adsorption (mol s-1) rd  Rate of desorption (mol s-1) Rd  Rotation (dial) speed (rpm) Re  Reynolds number R/M  Reactant to metal molar ratio (mol/mol) Rp  Radius of catalyst particle (m) rs  Rate of surface reaction (mol s-1) Rx  Conversion rate per mole of catalyst (mmol mmol-1 h-1) xxiv  rx,0  Initial reaction rate (mol s-1 g-1) S   Selectivity of product (%, based on carbon mole) SBET  Surface area of catalyst, based on BET method (m2 g-1) Sc  Schmidt number Sh  Sherwood number Smetal  Surface area of metal, based on CO chemisorption (m2 g-1) SR  Stirring rate (rpm) T   Temperature of the reaction (oC) Vpore  Total pore volume of catalyst (cm3 g-1) X  Conversion of reactant (%, on molar basis)  xxv  List of Abbreviations AC  Activated charcoal  ACC  Activated carbon cloth AEM  Anion exchange membrane Ag/AgCl Silver-silver chloride ANOVA Analysis of variance BET  Brunauer-Emmett-Teller BJH  Barrett-Joyner-Halenda CAN  Carboxylic acid number CE  Counter electrode CEM  Cation exchange membrane CI  Confidence interval CV  Cyclic voltammetry ECH Electrocatalytic hydrogenation-hydrogenolysis ECHDO Electrocatalytic hydrodeoxygenation ECO Electrocatalytic oxidation EP  Electronation-protonation ET  Electron transfer FPO  Fast pyrolysis oil GC-MS Gas chromatography–mass spectrometry  GPC  Gel permeation chromatography HAT  Hydrogen atom transfer HDO  Hydrodeoxygenation xxvi  HER  Hydrogen evolution reaction HOR   Hydrogen oxidation reaction HCl  Hydrochloric acid HClO4  Perchloric acid H2SO4  Sulfuric acid IEA  International Energy Agency KA  Ketone-alcohol KCl  Potassium chloride KH2PO4 Monopotassium phosphate KOH  Potassium hydroxide MSA  Methanesulfonic acid NaCl  Sodium chloride NaOH  Sodium hydroxide Na2SO4 Sodium sulfate ODE  Ordinary differential equation OER  Oxygen evolution reaction ORR  Oxygen reduction reaction PCET  Proton-coupled electron transfer Pd/C  Palladium on carbon PDI  Polydispersity index PEM  Proton exchange membrane PGM  Platinum group metals PhAN  Phenolic acid number xxvii  PT  Proton transfer Pt/Al2O3 Platinum on alumina  Pt/C  Platinum on carbon  RDS  Rate-determining step RE   Reference electrode RT  Retention time Ru/ACC Ruthenium supported on activated carbon cloth Ru/ACF Ruthenium deposited on activated carbon fiber Ru/C  Ruthenium on carbon  RVC  Reticulated vitreous carbon SSCE  Standard saturated calomel electrode  SEC  Size exclusion chromatography SHE  Standard hydrogen electrode SOEC  Solid oxide electrolysis cells SSER  Stirred slurry electrochemical reactor TAN  Total acid number TCD  Thermal conductivity detector TCH  Thermocatalytic hydrogenation-hydrogenolysis TEM   Transmission electron microscopy  THF  Tetrahydrofuran TOF  Turnover frequency WE  Working electrode WSBO  Water-soluble bio-oil   xxviii  Acknowledgements First of all, I would express my sincere gratitude to my supervisors, Professor Kevin J. Smith, Professor Chang Soo Kim, and Professor Elӧd L. Gyenge for their support, advice, and encouragement throughout my research and study. Four years ago, I was given the opportunity to embark on a new chapter of life in Canada, and it was Professor Kim who paved the way for this exciting challenge. Since Professor Smith approved my application, I could come and study at Chemical and Biological Engineering (CHBE) Department, the University of British Columbia (UBC) with a Four Year Doctoral Fellowship (4YF). In the first year, when my research direction was not yet clear, Professor Gyenge inspired me through his lecture and, thankfully, he was willing to be in my supervisory committee. I consider myself fortunate to be able to learn from their diverse expertise, such as in heterogeneous catalysis, biomass processing, and electrochemical engineering fields. The financial support from UBC 4YF with a collaborative project from Korea Institute of Science and Technology (KIST) is gratefully acknowledged.  I would like to thank Joel Lau, Tobias Grossmann, Priyanthika Adinamozhi, and Daichi Hirata for their assists in my research, especially in the experimental work, with great attitude. My gratitude also goes to Dr. Kwang Ho Kim and Dr. Robertus D. D. Putra (Robe), the members of KIST-UBC Biorefinery on-site Laboratory, for the technical support and insightful discussion during the lab work and activities.   I would also thank the fellow students and the members of CHBE Applied Electrochemistry and Fuel Cell Laboratory (past and present): Dr. Amir Dehkhoda, Dr. Colin Moore, and Dr. Pooya Hosseini Benhangi for their support during my initial and early stage of work in the lab; Alexander Jameson, Sheida Arfania, and Yu Pei for their cooperation and cheerfulness during the time of working together in the lab.  xxix   I also extend my gratitude to the members of CHBE Catalysis Research Laboratory (past and present): Dr. Haiyan Wang, Dr. Shida Liu, and Lucie Solnickova for their help in introducing the lab instrument operations; Majed Alamoudi, Abdullah Althobaity, and Samira Shirvani for their cooperation in the lab management and for the nice conversations about academic life.   My sincere appreciation to all the CHBE staff (past and present): Marlene Chow, Gina Abernethy, Richard Ryoo, Miles Garcia, Brittany Ji, Kristi Chow, Doug Yuen, Ken Wong, Amber Lee, William Wijaya, and Richard Zhang for their assists during my research and study.  There are a number of people outside my academic circles who need special mention. My heartfelt thanks to Pastor Russell Mackay from Metro Baptist Church for his kindness and spiritual advice; Pastor Daryl Lim from House For All Nations (HFAN) Church for the friendship and brotherhood; Marshall Law for being a great hiking partner; David Hartono for his kind hospitality; Jovian Varian and Robe for taking the time to have some fun activities and enjoy soccer games together in the midst of hectic school days; Kwan Lam and the Takagawas for helping my initial settlement during my first year in Vancouver; and to all my brothers and sisters in HFAN community and House On The Rock Care Group for their support and care.  Special thanks are owed to my parents, Cornelius D. Wijaya and Hoo Lie Ing, and my siblings, Mikhael Peter Wijaya and Natania Pamela Wijaya, who all live in Indonesia, our home country, for their love and prayers despite the years of being away physically. I am indebted to them for the time we could have spent and the memory we could have made during the past years of my study.   Last but not least, to my wife Sandra, love and thanks. I am grateful for her patience and encouragement during the ups and downs of life in this Ph.D. journey. I owe her time and favor that could have been invested on each other together. Without her enormous support, I would not be where I am today. Looking forward to the next chapter of life together with our firstborn.   xxx  Dedication I thank God, whom I believe in Jesus Christ, for keeping me alive and well during this doctoral study. I am grateful that this thesis can be finalized amidst the global outbreak that has affected people all over the world in many different and unprecedented ways.  Every good and perfect gift is from above, coming down from the heavenly Father  (James 1:17). All praise to the Life Giver for giving me a bundle of joy, Shalom, who was born sixteen days after I passed my final defense. She is the most precious gift to our little family.    1  Chapter 1: Introduction Renewable energy plays an essential role for the sustainability and development of a society. In everyday life, humans use energy in the forms of electricity, heat, and a wide variety of chemical products (e.g., plastics, textiles, and other polymers) mainly composed of three basic elements, such as carbon (C), hydrogen (H), and oxygen (O). The search for clean and sustainable energy sources has been driven by major global issues, such as the overdependence on non-renewable petroleum sources and the environmental problems of greenhouse gas emissions. Since the Industrial Revolution (1760–1840), fossil fuels (coal, petroleum, and natural gas) have been the major energy sources. Over the last century, the global efforts to develop a variety of renewable energy resources have been intensified aiming to produce fuels, chemicals, and materials with environmental and economic benefits. Energy utilization from renewable resources (e.g., solar, wind, hydro, geothermal, and biomass energy) represents an eco-friendly approach for powering economic growth and securing future energy supply while mitigating the risks of climate change.1  1.1. Current trend of worldwide energy supply and demand According to the latest energy statistics report by the International Energy Agency (IEA), the world energy supply and demand still relies on the fossil fuels (oil, coal, and natural gas).2 In 2017, fossil energy comprised around 67% and 81% of the total energy consumption and supply, respectively (Figure 1.1). Bioenergy accounts for roughly 10% of world total primary energy supply today. Electricity has been increasing in demand, making up nearly 19% of the total world energy consumption.2 Global electricity demand is reported to grow at 2.1% per year, twice the rate of primary energy demand, and expected to reach 31% of final energy consumption in 2040.3 With the accelerated efforts on decarbonizing electricity through renewables, nuclear power, and carbon  2  capture technologies, it is estimated that renewable energy sources (including hydro, solar photovoltaic, and wind) will provide two-thirds (67%) of electricity supply worldwide by 2040.3 Renewable electricity consumption, along with natural resources and energy innovation, improve environmental quality and can have a positive role in controlling CO2 emissions related to economic growth.4 All these data show the enormous potential of electricity-driven processes, such as electrocatalysis, for sustainable energy production in the coming years.   Figure 1.1. World total primary energy supply and demand (consumption) by source. Adapted from Key World Energy Statistics 2019 by the International Energy Agency (IEA).2 1.2. Lignocellulosic biomass as a renewable carbon-based energy source As the Earth’s most abundant carbon source, lignocellulosic biomass represents a promising natural resource for energy and chemical production.5–7 Lignocellulosic materials are inexpensive (can be derived from organic wastes), environmentally friendly (biodegradable), and do not World total energy supply by sourceWorld total energy demand by source 3  compete with the feedstocks for food production. Three biopolymers which constitute biomass, cellulose (40–50%), hemicellulose (25–35%), and lignin (15–30%), can be transformed into smaller and upgradable bio-chemicals, which have versatile industrial applications, such as transportation fuels, fine chemicals, and polymeric materials.5,6,8 The chemistry and structure of cellulose, hemicellulose, and lignin have been reviewed extensively elsewhere.6,7,9–11 Lignocellulosic biomass structure is depicted in Figure 1.2, showing the three major biopolymer constituents.    Figure 1.2. Lignocellulosic biomass structure consists of cellulose (crystalline), hemicellulose, and lignin (amorphous). Cellulose and hemicellulose are broken down into carbohydrates (glucose and xylose), while lignin into aromatic compounds. Reproduced from ref. 7, with permission from the Royal Society of Chemistry, Copyright 2012.  The overall biomass conversion into bioproducts involves two major reactions: fractionation (including depolymerization) and valorization (including reductive or oxidative upgrading). In the fractionation step, lignocellulosic biomass is deconstructed into its constituents,10 either via biological process (e.g., enzymatic hydrolysis) or thermochemical process (e.g., acid hydrolysis, fast pyrolysis), whereas electrochemical routes (e.g., electrochemical delignification) remain  4  underexplored for this purpose. A carbohydrate-rich stream (hydrolysate) and a lignin-derived substrate (bio-oil) are produced from this fractionation–depolymerization step to be further catalytically upgraded into higher value products. An ideal strategy for biomass valorization should aim for energy densification (to maximize the energy content of the final products) and carbon preservation (to minimize carbon losses in the process) in order to attain the competitiveness of bioproducts toward petroleum. The integration of thermochemical (e.g., fast pyrolysis) and electrochemical pathways using renewable electricity is considered a promising carbon retentive scheme that could enhance the profitability of future biorefineries.12,13 A general scheme for the reductive biomass transformation pathways is illustrated in Figure 1.3.   Figure 1.3. General schematic illustration of lignocellulosic biomass transformation into bio-products (fuels, chemicals, and/or polymeric materials). The overarching goal of this process is to deconstruct the complex structure of biomass and upgrade the resultant substrates, i.e. carbohydrate-rich streams (hydrolysate) and lignin-derived streams (bio-oil) into higher value products. Conventional reductive upgrading pathways are done via thermocatalytic processes. As a novel approach, electrocatalytic reduction of lignin derivatives is the primary focus of this review. Note that the tree image serves for illustration purposes only and does not at all justify deforestation.  BIO-OIL• Low energy density• High functionalityBIO-PRODUCTS• High energy density• Low functionalityOxygen removal(Deoxygenation)Deconstruction (depolymerization)GOAL:• Fermentation (anaerobic)Biological• Hydrogenation• Hydrogenolysis• HydrodeoxygenationThermochemical• Hydrogenation• Hydrogenolysis• HydrodeoxygenationElectrochemicalEnergy densification, Carbon retentionConcerns:C-H bondsC-C bondsC-O bondsPROCESS:ReductivepathwaysFUELSCHEMICALSMATERIALSBiomass Bioproducts• Enzymatic hydrolysisBiological• Acid hydrolysis• Fast pyrolysisThermochemical(This area is still underexplored)Electrochemical 5  1.3. Lignin and bio-oil as natural sources for renewable chemicals and fuels Lignin, being the second most plentiful biopolymer after cellulose and the most abundant source of aromatic compounds in nature, accounts for 15–30% (by weight) and 40% (by energy) of the biomass.14,15 Lignin is considered important, but underutilized material derived from lignocelluloses-to-ethanol process and/or pulp and paper plants.9,14,16 The underutilization of lignin is due to its complex, irregular structure and variability in composition which depends on the biomass origin and the isolation methods. The representative lignin network is composed of three major monolignols: p-coumaryl alcohol, coniferyl alcohol, and sinapyl alcohol, which are commonly referred to as p-hydroxyphenyl (H), guaiacyl (G), and syringyl (S) units, respectively (Figure 1.4).9,14,15 Lignin convertibility is basically determined by the interunit linkages between monolignols; the most abundant linkage is -aryl ether (-O-4) which can be found in softwood (43–50%) and hardwood (50–65%).16 The lignin resistance toward chemical degradation is considerably dictated by the reactivity of -O-4 bonds.16 Factors affecting lignin solubility include the molecular weight, temperature, liquid to solid ratio, and the ionic strength of the aqueous solution.17 Lignin solubility is affected by the ratio of the number of phenolic hydroxyls to the number of phenylpropane units in the macromolecule (the higher ratio means the higher solubility).17 Lignin is a preferable feedstock for pyrolysis to produce bio-oil due to the lower oxygen content (0.32 < O/C < 0.46) compared to that of woody biomass (O/C > 0.61).15 In the scheme of a sustainable biorefinery, lignin valorization is essential to enhance the economic feasibility of the overall biomass conversion process.14–16 Development of genetic engineering, analytical chemistry and computational modeling, and biomass pretreatment technologies could improve lignin processing in the biorefinery, specifically for the biosynthesis, characterization, and recovery of lignin, respectively.18  6   Figure 1.4. Representative lignin structures: (a) the primary building blocks (monolignols), (b) the network with the interunit linkages. Adapted from ref. 16 with permission from the American Chemical Society, Copyright 2015.  Bio-oil is a highly oxygenated, multi-component liquid mixture containing acids, alcohols, sugars, aldehydes, esters, ketones, phenolics, aromatics, and oligomers.19–21 The exact chemical composition of bio-oil depends on the feedstock type and source (biomass or lignin) and the fractionation or depolymerization method.22 Compared to petroleum crude oil, pyrolysis bio-oil from wood has lower energy density (40 MJ/kg vs. 16–19 MJ/kg).19,20 Bio-oil is a potential source for renewable chemicals and fuels, however, its direct use is improbable owing to high contents of oxygen (35–40 wt.%) and water (15–30 wt.%),5 high viscosity and acidity, instability toward polymerization, and incompatibility with the petroleum fuels. The current industrial uses of bio-oil (or lignin) are limited mostly to low-grade fuels and power or heat generation via direct combustion.15,20 Thus, stabilization and upgrading of the bio-oil are necessary before it can be used a) Lignin primary building blocks (monolignols)b) Representative structure models of lignin(S)(H)(G) 7  as an alternative to petroleum fuels and chemicals feedstock.5,15,20 The main goal of bio-oil upgrading process is principally to optimize the energy density (C–H bonds) and molecular weight (C–C bonds) while reducing the oxygen content (C–O bonds) of the final products. Conventional bio-oil upgrading processes, including hydrocracking, decarbonylation, decarboxylation, hydrogenation, and hydrodeoxygenation (HDO), have been investigated using a variety of heterogeneous catalysts, such as noble metal (supported on carbon or metal oxides) and transition metal carbide, nitride, and phosphide catalysts.20,23,24 Most of these thermochemical processes, referred to as thermocatalytic hydrogenation-hydrogenolysis (TCH), have common requirements, such as high temperature (200–400 oC) and high H2 pressure (100–200 bar) to achieve high production rates. In such a harsh environment, bio-oil is not stable and its polymerization, which starts to accelerate at 80 oC, will cause substantial tar and coke formation, leading to reactor plugging and catalyst deactivation.25,26 The majority of hydrogen gas (~95%) is currently produced from fossil resources (via methane steam reforming or coal gasification).27 The high cost of hydrogen is considered one major impediment to the commercial success of this TCH approach, therefore hydrogen-lean or hydrogen-free approaches to bio-oil valorization are desirable.28 For these reasons, bio-oil upgrading via TCH process is yet to be cost-effective and energy-efficient, therefore further research and development is needed in this area.15 The complex nature of bio-oil and the limited operating conditions are among the critical factors that need to be addressed for large-scale implementation of this upgrading process.28 1.4. Renewable hydrogen from water electrolysis Water is indispensable feedstock in electrochemical valorization of biomass derivatives. While hydrogen can be made from a variety of feedstocks, including natural gas, coal, biomass, or  8  wastes,27 water electrolysis is the simplest technology for producing hydrogen.29 Electrolysis of water is essentially the conversion of electrical energy to chemical energy in the form of hydrogen, with oxygen as a useful by-product30 and is currently the only way to produce large quantities of hydrogen with no emissions of fossil carbon pollutants.27,29   Commercial electrolyzers have typical efficiencies about 56–73%.29 Alkaline electrolysis is the most common technology that employs potassium hydroxide (KOH), sodium hydroxide (NaOH), or sodium chloride (NaCl) as the electrolyte solution (25–30 wt.% concentration).29 Alternatively, proton exchange membrane (PEM) electrolysis, PEM-based acid electrolysis, and solid oxide electrolysis cells (SOEC) are also being developed.30,31 Commercial alkaline electrolyzers are typically operated at lower current densities (100–300 mA cm-2) than PEM electrolyzers (>1600 mA cm-2).29,30 Of all these technologies, SOEC electrolyzers are the most electrically efficient (85–90%),30 followed by PEM-based acid electrolyzers (65–82%),31,32 but they are the least developed and require costly materials and fabrication methods in addition to a heat source. PEM electrolyzers are more efficient (55–70%) than alkaline electrolyzers (50–60%) but have higher capital costs.30,32   A comprehensive review, with direct comparison of water electrolysis technologies, has been recently provided by Gür31 and a summary is presented here (Table 1.1) to highlight the potential applications of water electrolyzer infrastructures with the ranges of operating current density and electrolyte pH relevant for electrocatalytic conversion of biomass. In the context of electrochemical reduction for biomass valorization, hydrogen gas is not the target product; however, knowledge of electrolytic hydrogen technology is necessary for the overall process development, especially in regards to efforts to build accessible hydrogen energy infrastructure in remote sites that can be coupled with the other renewable energy sources.  9  Table 1.1. Typical structure and operational characteristics of the four major classes of electrolyzers for hydrogen production named after their electrolytes. Summarized from ref. 31 with permission from the Royal Society of Chemistry, Copyright 2020, combined with data from refs. 30,32,33.   Alkaline Acid PEM SOEC Electrolyte NaOH KOH H2SO4 H3PO3 Polymer  (Nafion™) Ceramic (YSZ, BYZ) Carrier ion OH- H+ H+ O2- (YSZ) H+ (BYZ) Electrodes Ni Pt/C, IrO2 Pt/C, IrO2 Ni, ceramic Operating temperature 80 oC 150 oC 80 oC 600–900 oC Current density 100–300 mA cm-2 >1000 mA cm-2 >1600 mA cm-2 300–1000 mA cm-2 Efficiency 50–60% 59–70%** 65–82%** 55–70% 40–60%* 85–90%Δ Strengths Low-cost materials Commercial technology High activity electrodes High activity electrodes Commercial technology Low-cost materials Lower barrier to split H2O Higher efficiency Short-comings High barrier to split H2O Lower efficiency High barrier to split H2O Expensive electrodes Cell durability Developing technology High barrier to split H2O Expensive electrodes Hydrogen crossover Cell durability High operating temperature Materials stability Fabrication costs Developing technology PEM: polymer exchange membrane, SOEC: solid oxide electrolysis cell. (YSZ: yttria-stabilized zirconia, BYZ: yttria-doped barium zirconate). High-temperature electrolysis efficiency depends on the operating temperature and the thermal energy source efficiency. Efficiency: **based on H2 yield, *including thermal energy input, Δbased on electrical input alone.30,32   1.5. Research scope, objective, and outline Biomass upgrading via electrocatalysis has been widely studied in recent years. The present research mainly focuses on the electrochemical reduction process known as electrocatalytic hydrogenation-hydrogenolysis (ECH) for the upgrading of lignin derivatives using a stirred slurry reactor. Process mechanisms and parameter effects are studied under potentiostatic and galvanostatic conditions. The predominant proton source is identified by a comprehensive investigation of different catholyte-anolyte pairs of aqueous electrolytes in the ECH of phenolic compounds, showing synergistic effects between electrocatalyst and electrolyte. Catalytic  10  performance of carbon-supported metals (Pt/C, Ru/C, and Pd/C) is comparatively investigated in the ECH of guaiacol and phenol. The influence of organic solvent is studied in the ECH of guaiacol and the electrolysis process is developed with the other lignin-relevant model compounds, such as cerulignol and creosol. Furthermore, ECH of bio-oil substrates is evaluated in the mixed organic and aqueous electrolyte at mild conditions. Overall, this research aims to contribute to the advancement of electrochemical routes for green synthesis of renewable bio-products, mainly commodity organic chemicals. This dissertation outline is given as follows:  ➢ Chapter 1 introduces the prospects of lignocellulosic biomass, particularly lignin and bio-oil, coupled with electrolytic hydrogen technology for the green synthesis of value-added chemicals via electrocatalytic reduction pathways. ➢ Chapter 2 reviews the fundamental concepts and theoretical aspects of electrocatalytic hydrogenation including the electron transfer mechanism. The recent progress in ECH of lignin derivatives and the knowledge gaps in this area are discussed as well. ➢ Chapter 3 describes the experimental methodology in this work, including the electrochemical cell configuration, electrochemical characterization and experimental procedures, product analysis, and reaction metrics calculations. Part of this chapter has been published together with Chapters 4 and 5. ➢ Chapter 4 presents the fundamental study of ECH of guaiacol and phenol in diverse aqueous electrolyte pairs using stirred Pt/C catalyst slurry under potentiostatic and galvanostatic controls. This chapter has been published as a full research article in ChemSusChem.  ➢ Chapter 5 provides a comparative study on the electrocatalytic performance of Pt/C, Ru/C, and Pd/C in the ECH of guaiacol and phenol in synergy with the electrolyte effects. Major part of this chapter has been published in the special issue of Journal of Applied Electrochemistry.   11  ➢ Chapter 6 reports the impact of organic solvents in the ECH of guaiacol and bio-oil substrates using acidic electrolytes, including mass transport and kinetic analyses and the process optimization. Part of this chapter is prepared for a journal publication.  ➢ Chapter 7 summarizes the conclusions and key findings from this study and highlights some potential future research directions. Major part of Chapters 1, 2, and 7 has been published as a review paper in Green Chemistry.  12  Chapter 2: State of the Art and Literature Review Electrocatalytic hydrogenation (ECH) has been studied for over a century and its applications for electrosynthesis of organic compounds have gained increasing interest in recent years.12,13,34–47 ECH is essentially an electrochemical reduction process that utilizes the chemisorbed hydrogen (Hads) resulting from proton and/or water reduction for the saturation of organic compounds on the catalyst. This ECH process could also involve hydrogenolysis and/or hydrodeoxygenation to remove oxygen from the organics (or decrease the O/C ratio), thereby storing higher chemical energy in the liquid products. 2.1. Prospects of electrocatalytic hydrogenation (ECH) technology In comparison to the classic TCH routes, the ECH process offers some advantages with respect to: (i) the greener hydrogen source (internally and continuously supplied from water splitting reactions, eliminating the requirement of external H2 supply), (ii) the milder reaction conditions (lower temperature and pressure using aqueous electrolytes, eliminating the need for organic solvents and the problem of catalyst deactivation due to coking), (iii) the cleaner process (resulting in higher carbon recovery and useful by-products, such as hydrogen and oxygen gases, which in a large-scale can be utilized for fuel cell applications), (iv) the simpler operation (controllable operating parameters, such as current/voltage input).48–51 Furthermore, this ECH process becomes more attractive with the possibility of integration with solar, wind, and/or hydro energy, which are renewable and becoming more economical sources of electricity.12,29,30 In this regard, the electrochemical reduction of biomass-derived substrates, particularly lignin derivatives, represents a sustainable process that could be integrated with renewable energy and promote the overall biorefinery process with electricity storage in a chemical form. An inherent limitation of the ECH  13  method comes from the competitive side reaction, that is the formation of molecular hydrogen (H2), either via electrochemical or thermal desorption.48 A general comparison between ECH and TCH with respect to the potentials and limitations of the processes is summarized in Table 2.1. Table 2.1. General comparison between ECH and TCH for biomass reductive upgrading pathways.  Potentials Limitations Electrocatalytic hydrogenation-hydrogenolysis (ECH) • Mild operating conditions (low temperatures and ambient pressures) • In situ H2 generation (reaction can be performed without external H2 supply) • Electrosynthesis in aqueous electrolyte (no organic solvents are required) • Higher carbon recovery (>80%) due to little coke/char formation  • Electrocatalyst is easily reusable and recoverable • Feasible product selectivity control by tuning the applied potential or current density • Integration with renewable electricity sources (wind, solar, hydro), more accessible infrastructure to produce H2 in remote areas  • Low energy efficiency due to the competing hydrogen evolution reaction • Complex reactions (heterogeneous reaction on the electrocatalyst surface, non-electrochemical homogeneous reaction in organic and aqueous phases) with limited organic substrate solubility • Lower conversion rates owing to the milder conditions • Limited studies on the continuous flow process • Large scale process with high substrate concentration is not yet proven viable • High reliance on noble metal catalyst • Purification of the final product may require extraction of the organics from aqueous phase and neutralization step to remove acidic/alkaline substances Thermocatalytic hydrogenation-hydrogenolysis (TCH) • Adaptable technology with the existing infrastructures in petroleum refineries • Large scale process is possible with continuous flow reactor configuration • Product recovery is easier if the reaction is performed using organic solvent and heterogeneous catalyst • Higher conversion rates, possible to achieve higher products yield • Non-precious metal catalysts have been widely investigated  • Requires external H2 supply • Severe reaction conditions (high pressures and temperatures), i.e. high energy consumption • High cost infrastructures  • Extensive coke formation, leading to low carbon recovery • Catalyst deactivation (coking, sintering, poisoning), thus thermal regeneration is required for the spent catalyst   2.2. Reaction mechanisms in ECH of organic compounds A simplified, plausible ECH reaction mechanism involves the following elementary steps: • Generation of chemisorbed hydrogen on the catalytic metal sites (M):  H3O+ + e- + M → H·M + H2O (2.1)  14  • Adsorption of the organic reactant on the catalyst adsorption sites (A):  (R=O) + A ↔ (R=O)·A (2.2) • Hydrogenation between the adsorbed organics and the chemisorbed hydrogen:  (R=O)·A + 2H·M ↔ (RH–OH)·A + 2M (2.3) (RH–OH)·A + 2H·M ↔ (R–H2)·A + H2O + 2M (2.4) • Desorption of the hydrogenated product: (RH–OH)·A ↔ RH–OH + A (2.5) (R–H2)·A ↔ R–H2 + A (2.6)  As the competitive pathway to ECH, the hydrogen evolution reaction (HER) usually proceeds via Volmer–Heyrovsky–Tafel reaction mechanism, depending on the electrolyte type, as follows: • Chemisorbed hydrogen generation via proton/water reduction Volmer reactions: H+ + e- + M → H·M … (in acid media) (2.7a) H2O + M + e- → H·M + OH- … (in neutral/base media) (2.7b) • Hydrogen (electrochemical) desorption via Heyrovsky reactions: H·M + (H+)aq + e- → H2 + M … (in acid media) (2.8a) H·M + H2O + e- → H2 + OH- + M … (in neutral/base media) (2.8b) • Hydrogen (thermal) desorption via Tafel reactions: H·M + H·M → H2 + 2M (2.9) Surface coverages of adsorbed organics and chemisorbed hydrogen are the key factors in the ECH process which determine the Faradaic efficiency. The studies of H2 adsorption into metals have confirmed that the Hads coverage increases with an increase in the negative overpotential.46 It is noteworthy that changing the cathode potential to control the surface Hads coverage is equivalent to changes of the hydrogen pressure in the TCH process.46   15  2.3. Electron transfer mechanisms in electrochemical hydrogenation Many important energy conversion processes in chemistry and biology are essentially redox reactions, in which both electrons and protons are transferred, and they occur by a variety of mechanisms.52 Electron transfer (ET), along with proton transfer (PT) and hydrogen atom transfer (HAT), are among the elementary steps for reactions involving proton-coupled electron transfer (PCET). The coupling of ET and PT influences both energetics and mechanisms, allowing for the buildup of multiple redox equivalents at single sites or clusters for multielectron reactions or providing reaction pathways that involve simultaneous ET and PT thus avoiding high-energy intermediates.52,53 The mechanisms in PCET reactions are dictated by the conditions, such as temperature, pH, and solvent (polarity).53 In PCET, protons and electrons are transferred from different orbitals on the donor to different orbitals on the acceptor, while in HAT, both electrons and protons are transferred from the same chemical bond.52 Comprehensive reviews on PCET are provided in the literatures.52,53  Two distinct electron transfer mechanisms in electrochemical hydrogenation are proposed based on the requirement of direct chemical interaction with the electrode surface (direct electroreduction followed by a protonation, i.e. electronation-protonation (EP) reaction)46 and the involvement of chemisorbed hydrogen on the electrode surface (electrocatalytic hydrogenation).51 ECH involves an inner-sphere ET that requires direct interaction with the electrode (or the charged catalyst surface) through specific adsorption of atomic hydrogen (Hads) as the proton source,51,54 whereas EP proceeds via either inner-sphere or outer-sphere ET whereby the protonation step may occur in the bulk solution46,51 and the electrode reactions are characterized by a large H2 overpotential and low Hads coverage.46 In contrast to the ECH mechanism described by Equations  16  (2.1) to (2.6), the EP mechanism is described as follows (where R denotes the organic reactant molecules):  R + e- ↔ R-  (2.10)  R- + H+ → RH  (2.11)  RH + e- ↔ RH-  (2.12)  RH- + H+ → RH2  (2.13) Examples of inner-sphere and outer-sphere reactions are provided in the literature.52,54–56 A general comparison on the inner-sphere and outer-sphere ET is summarized in Figure 2.1.  Figure 2.1. Electron transfer mechanism in electrochemical hydrogenation reactions, involving two distinct pathways: direct electroreduction followed by electronation-protonation (EP) and electrocatalytic hydrogenation (ECH). The inner-sphere and outer-sphere comparison is summarized from various literatures.51,54,55   Inner-sphereChemical species are connected via metal-ligand interactionET occurs within a primary bond systemSpecific adsorption of chemical species in the electrode reactionMore influenced by the nature and electronic states of the electrodeOuter-sphereChemical species remain intact in the activated complex  ET occurs from one primary bond system to anotherNo adsorption nor changes in the chemical bondsLess dependent on the electrode materialElectron Transfer MechanismMetal electrode H H H HR=ORRH–OHR–H2e-H+H+R–HR–H2Electrocatalytic hydrogenationElectronation-protonation 17  2.4. Electrocatalysis mechanism in organic electrosynthesis In organic electrosynthesis, there are three possible scenarios for the electron transfer from the electrode to the substrate, as summarized by Mӧhle et al.,57 including: (i) direct electron transfer via an inert electrode, (ii) electrocatalytic reaction over an active electrode, (iii) mediated electrolysis (Figure 2.2). In the first mode, the substrate reactivity at the electrode surface is not influenced by a surface layer thus selectivity control is possible by tuning the electrode potential. In the second mode, an electrochemically active species and electrically conductive layer is formed and regenerated in situ, thus the substrate electroconversion becomes less dominated by the applied potential. In the third mode, electrolyte-soluble redox-active species act as mediators between the electrode and the substrate, thus the electroconversion can be conducted at milder potentials than an electrolysis without mediator, avoiding large overpotentials.57  Figure 2.2. Different operation modes of electrodes in electrosynthesis applications. Reproduced from ref. 57 with permission from the John Wiley & Sons, Copyright 2018.  18  2.5. The roles of electrocatalyst and electrolyte in ECH of organic compounds The appropriate choice of electrocatalyst and electrolyte is important for an effective ECH. The electrocatalyst provides large active surface area for the reaction between the adsorbed organics and the chemisorbed hydrogen, whereas the electrolyte provides ionic conductivity for the water splitting reactions. Acidic electrolyte is favorable for ECH applications due to the requirement of high proton concentration as the source of hydrogen radicals.49 Acid-tolerant catalysts are therefore desirable for the effective ECH and, in this regard, the role of noble metals remains significant. The synergy between electrocatalyst and electrolyte aims to maximize the Faradaic efficiency (F.E.), which represents the predominant use of electrons for the ECH of organics over HER.50 High surface coverages of the adsorbed organics and the chemisorbed hydrogen on the catalytic active sites are hence pivotal for the efficient ECH process. Importantly, the electrolyte effect reveals that proton/water reduction and hydrogen evolution reactions proceed differently in acid, neutral, and base solutions50 (as shown in Equations 2.7–2.9). The formation of adsorbed hydrogen radicals (Volmer reactions, Equation 2.7a–b), as the first step in the electrocatalytic H2 evolution, plays an important role in the ECH of organics. Meanwhile, the side reactions that produce hydrogen gas can proceed electrocatalytically (via Heyrovsky reactions, Equation 2.8a–b) or thermally (via Tafel reaction, Equation 2.9). Hydrogen radicals are formed via the electroreduction of protons in acidic media (where proton is abundant) or via water reduction with cogeneration of hydroxide ions in neutral/alkaline media.  The HER kinetics in alkaline media is generally considered more sluggish than in acidic electrolytes because of an additional water dissociation step and the more complex mechanism where the reaction rate and pathway are influenced not only by hydrogen adsorption/desorption energies (as in case of acidic media), but also by water adsorption and dissociation and hydroxyl  19  ions affinity to catalyst surface (which could poison the surface by occupying the active sites). Consequently, the best performing catalysts in acidic media (even the noble metals) showed poor activity and stability in alkaline electrolytes, requiring large overpotentials to obtain reasonable current densities.58 Different catalyst and electrolyte properties are therefore expected to have different impacts on the ECH efficiency. 2.6. Recent progress in ECH of lignin derivatives and the knowledge gaps In the past two decades, there have been numerous studies on the upgrading of lignin-derived compounds via electrocatalytic reduction processes. Phenol is the most common lignin monomer for ECH studies36,41,43–45,59–62 and represents an important platform chemical for the synthesis of cyclohexanone and cyclohexanol as the industrial precursors for nylon polymers.63–65 A variety of phenolic and aromatic model compounds, such as p-cresol, 4-methoxyphenol, guaiacol, syringol, benzaldehyde, benzyl phenyl ether, phenolic ethers, and bio-oil substrates have also been investigated in the ECH under various conditions (Table 2.2). In the electrocatalytic hydrogenolysis of lignin model compounds, such as benzyl phenyl ether, using Raney nickel electrodes in aqueous ethanol, Mahdavi et al.42 reported that hydrogenolysis of the ether bonds must be faster than hydrogenation of aromatic rings for the efficient lignin reduction. Lignin degradation by ECH seemed feasible since the efficiency of hydrogenolysis of the C–O bond could be optimized under the proper substrate concentration, current density, and temperature.42 In another study, Li et al.34 reported the use of ruthenium supported on activated carbon cloth (Ru/ACC) as an efficient catalyst for ECH of phenolic compounds, such as phenol, guaiacol, and syringol, under mild conditions (≤80 oC and ambient pressure). The guaiacol conversion was found to be more favorable under acidic conditions and higher temperature (≈ 50 oC) with cyclohexanol and phenol as the major and  20  intermediate products, respectively.34 Moreover, high carbon recovery (>80%) into the liquid phase was obtained in the ECH of water-soluble bio-oil, showing that ECH can stabilize bio-oil against polymerization.34,66 More recently, a comparative study on ECH and TCH of phenol was reported by Song et al.36  using Pt/C, Rh/C, and Pd/C suspended catalysts in different acid electrolytes (such as acetic acid, phosphoric acid, and acetic acid) at a range of current inputs (-20 mA to -60 mA) and temperatures (5 oC to 50 oC). This work demonstrated that ECH and TCH were independent pathways as the electroreduction of protons to H2 did not contribute to the rates of thermally catalyzed reactions. Furthermore, the study of reaction mechanisms also revealed that ECH proceeded via a Langmuir-type mechanism where the adsorbed hydrogen radicals were produced on the catalyst metal particles by electroreduction of protons (through contact between the catalysts and the electrode), instead of dissociative chemisorption of H2 as in TCH.36  Most of the studies have reported ECH of lignin model compounds, mainly phenol, using low substrate concentration (<50 mM) and low current densities (<|50 mA cm-2|) that resulted in low Faradaic efficiencies (<50%). These conditions are not practically relevant for large-scale applications and these issues have also been recognized by Liu et al.67 in a recent study of electrocatalytic hydrodeoxygenation of phenolic compounds. Besides, the impact of different electrocatalyst configurations in ECH process has not been specifically addressed. In general, the electrocatalyst preparation and application can be categorized into three methods: electrodeposition, impregnation (mechanical and chemical deposition), and suspension (as illustrated in Figure 2.3). Most of the reported ECH studies used fixed bed electrodes prepared either by electrodeposition or impregnation methods. In this work, catalyst suspension technique is manifested in a stirred slurry electrochemical reactor (SSER) of an H-type cell configuration using diverse aqueous electrolyte pairs. Concerning the importance of electrolyte effects (with broad pH range), this work presents for  21  the first time a comprehensive investigation of different catholyte-anolyte pairs in the ECH of guaiacol. Insights from the previous works on the ECH of lignin derivatives are comparatively analyzed with the results from this study.  Figure 2.3. Three common techniques for the electrocatalyst preparation and application in the ECH of lignin derivatives, including electrodeposition, impregnation or spray deposition, and stirred suspension.  Guaiacol is regarded as a representative monomer of bio-oil owing to the prevalence of hydroxy and methoxy groups in lignin derivatives. As the primary focus of this work is initially on the ECH of guaiacol, the hydrogenation reaction mechanisms between the adsorbed guaiacol and the chemisorbed hydrogen can provide basic understanding of the process. Plausible elementary steps for the adsorption, surface reaction, and desorption steps in the guaiacol ECH (as the elaboration of Equations 2.2–2.6) involving demethoxylation and hydrogenation (aromatic ring saturation) reactions are illustrated in Figure 2.4. The organic solvent effect in ECH process has not been extensively studied. In biomass conversion, solvent improves substrate solubility, hence, to deal with real substrate (e.g., bio-oil) as the feedstock in electrocatalytic process, the presence of conductive organic electrolyte is critical. Here, the ECH of guaiacol and bio-oil substrates is evaluated using methanesulfonic acid electrolytes. Mass transport and kinetic aspects are also studied in conjunction - +ElectrodepositionH2PtCl6Graphite (G) Pt sheetImpregnation, Spray DepositionMetal precursor(e.g., Ru(NH3)6Cl3)Activated carbon cloth (ACC)Stirred suspensionSupported-metal catalyst (e.g., Pt/C, Pd/C, Pt/Al2O3)Pt/GRu/ACC- + 22  with process optimization using factorial analysis. This research focuses on addressing the practical feasibility of the ECH process for the green valorization of lignin derivatives.   Figure 2.4. Plausible elementary steps in ECH of guaiacol to cyclohexanol: Illustrative reactions between guaiacol, chemisorbed hydrogen, and the intermediate products.26   ECH of bio-oil has been increasingly studied in recent years, but thus far it works mainly for stabilization toward polymerization rather than hydrogenation purposes.68–70 Due to the inherent complexity of bio-oil, specific product selectivity and current efficiency quantifications are presently very difficult in order to be useful for measuring the process performance. Effective reduction of oxygenated aromatic and unsaturated compounds in the bio-oil via ECH and the subsequent selective product purification remains a challenge to be addressed in the future.Hydrogenation of guaiacol (Reaction between adsorbed guaiacol and chemisorbed hydrogen) Demethoxylation of guaiacol Hydrogenation of phenol into cyclohexanone Hydrogenation of cyclohexanone into cyclohexanol Desorption of the hydrogenated products  (2-methoxycyclohexanol) Desorption of the hydrogenated products  (cyclohexanol)  23  Table 2.2. Summary of the electrocatalytic reduction of lignin derivatives from various publications. Substrate / Reactant WE | CE | RE Membrane Electrolyte Conditions ProductsΔ Ref. Phenol (C = 50 mM) Pt (0.1–5% loading) on Vulcan XC-72R (bound by Teflon) | Platinized Pt screen | n.s. Nafion-324 Catholyte = Anolyte: H2SO4 (0.05 M)  I = 40 mA (A = 8 cm2) T = 60 oC, t = n.s. Cyclohexanol  (F.E. = 0.5–85%, X = n.s.) 41 Phenol (C = 8.5 mM) Pt/Pt, Rh/Ni, Ru/Ni, RaNi | Glassy C plate or Pt grid cylinder | n.s.  Nafion-324 Catholyte = Anolyte: H2SO4 (0.1 M) or H3BO3 (0.5 M) + NaCl (5–50 mM), Na2SO4 (0.5 M) j = 1 to 15 mA cm-2  (at room T), t = n.s. Cyclohexanol, Cyclohexanone (F.E. = 10–72%, X = 2–94%) 43 Phenol (C = 50 mM)  Pt (2–60% loading) on C (Vulcan XC-72R), Pd/C, Ru/C, Rh/C, Pt alloys: Cr, V, Co, Ir | Pt/Pt screen | n.s. Nafion-324 Catholyte = Anolyte: H2SO4 (0.05 M)  I = 40 mA (A = 8 cm2) T = 60 oC, t = n.s. Cyclohexanol, Cyclohexanone (F.E. = 10–72%, X = n.s.) 44 Phenol (C = 50 mM) Pt on C (Vulcan XC-72R) or Pt/Pt or Pt/C (rod) | Pt/Pt screen | n.s. Nafion-324 Catholyte = Anolyte: H2SO4 (0.05 M) I = 40 mA (A = 8 cm2) T = 60 oC, t = 4–5 h. Cyclohexanol, Cyclohexanone (F.E. = 8–72%, yield = 93–99%) 45 Phenol (C = 8.85 mM, added with 10%-Pd/Al2O3) RVC (reticulated vitreous carbon, A = 17.7 cm2) supported on a carbon rod | Pt mesh | n.s. [in a dynamic cell equipped with a variable flow pump] Nafion-117 Catholyte: Organic buffer + NaOH (pH 5) (Organics: methanol, acetic acid, propionic acid, butyric acid) Anolyte: Acetate buffer solution (1 M) I = 20 mA, flow rate = 1.072 L/min, T = n.s., t = n.s. Cyclohexanol, Cyclohexanone (F.E. = 8–72%, X = 10–100%, yield = n.s.) 71  24  Substrate / Reactant WE | CE | RE Membrane Electrolyte Conditions ProductsΔ Ref. Phenol (C = 50 mM) 0.5–2%Pt on graphite (G), Rh/G, Pd/G / Pt sheet (1 cm2) / SCE DuPont® Nafion-117 Catholyte: H2SO4, HCl, HClO4, NaOH, or NaCl (0.2 M)  Anolyte: H2SO4 (0.1 M) I = 10 to 90 mA (A = 9.5 cm2) Ecell = 4–6 V  T = 20–60 oC, t = 15 h Cyclohexanol, Cyclohexanone, Cyclohexane (F.E. = 9–27%, X = 18–98%) 59  Phenol (C = 17.7 mM, added with  carbon-supported Pt, Pd, Rh catalysts) RVC (100 pores per inch, area of 66 cm2 per cm3) connected to a graphite rod | Pt wire | Ag/AgCl Nafion-117, pretreated in H2SO4 (2 M) Acetic acid solution, H3PO4, H2SO4 (pH 5) E = -0.65 to -0.75 V, I = -20 to -60 mA, T = 5–80 oC, t = 2 h, SR = 500 rpm (under N2 flow) Cyclohexanol, Cyclohexanone (F.E. = 26–63%, X ≈ 100%) 36 Phenol  (C = 18 mM) Rh/C on a carbon felt | Pt wire | Ag/AgCl Nafion-117 Catholyte = Anolyte: Acetate buffer (50 mM) E = -0.15 to -0.45 V I = n.s., T = 23–60 oC, t = 60–120 min Cyclohexanol, Cyclohexanone (F.E. = n.s., X = 55–95%) 60 Phenol  (C = 16 mM, added with Rh/C catalyst) Carbon felt connected to a graphite rod | Pt wire | Ag/AgCl Nafion-117, pretreated in H2SO4 (2 M) Catholyte = Anolyte: Acetate buffer (50 mM) E = -0.4 to -0.9 V j = -0.02 to -0.25 mA cm-2, T = 23 oC, t = 210 min (under He flow) Cyclohexanol, Cyclohexanone  (F.E. = 20–66%, X ≈ 100%) 61 4-Methylphenol, 4-Methoxyphenol  (C = 16 mM, added with Rh/C catalyst) Carbon felt connected to a graphite rod | Pt wire | Ag/AgCl Nafion-117, pretreated in H2O2 (3 vol%) and H2SO4 (2 M) Catholyte = Anolyte: Acetate buffer (50 mM) E = -0.6 V j = -0.05 mA cm-2, T = 23 oC, t = 180–210 min (under He flow) 4-Methylcyclohexanol, 4-Methylcyclohexanone; 4-Methoxycyclohexanone, 4-Methoxycyclohexanol (F.E. = 31–35%, X ≈ 80%) 61 Benzyl phenyl ether, Diphenyl ether, p-Tolyl ether (added with Rh/C catalyst) Carbon felt connected to a graphite rod | Pt wire | Ag/AgCl Nafion-117, pretreated in H2O2  (3 vol%) Catholyte = Anolyte: Acetate buffer (50 mM) E = -0.9 V j = -0.11 mA cm-2, T = 23 oC, t = 180 min (under He flow) Dicyclohexyl ether, Cyclohexyl phenyl ether; 4-Methylphenyl-4-methylcyclohexyl ether, 61  25  Substrate / Reactant WE | CE | RE Membrane Electrolyte Conditions ProductsΔ Ref. and H2SO4 (2 M) etc. (F.E. = 18–36%, X ≈ 70–100%) Phenol, Benzaldehyde (C = 20 mM, added with Pt/C, Pd/C, Rh/C catalysts) Carbon felt connected to a graphite rod | Pt wire | Ag/AgCl Nafion-117 Catholyte = Anolyte: Acetate buffer (50 mM) E = -0.7 V j = -0.11 mA cm-2, T = 25 oC, t = 180 min (under He flow) Cyclohexanol, Cyclohexanone; Benzyl alcohol (F.E. = 1–70% for phenol, 50–99% for benzaldehyde, TOF reported elsewhere39) 62 Benzaldehyde (C = 20 mM ± 25%, added with Rh/C, Pt/C, Pd/C, or Ni/C catalyst) Carbon felt (3 × 1.5 × 0.6 cm) connected to graphite rod | Pt mesh | Ag/AgCl Nafion-117, pretreated in H2O2 (3 vol.%) and H2SO4 (2M)  Acetate buffer (pH 5) E = -0.7 to -0.9 V, T = 12–34 oC, t = 120 min, SR = 500 rpm (under 1 bar N2) Benzyl alcohol (F.E. = 11–99.7%, X = 5–100 %, TOF (×103 h-1) = 0.5–2.2 (Rh/C), 1.0–2.2 (Pt/C), 1.0–4.0 (Pd/C), 0.675–14.2 (Ni/C)) 39 Benzaldehyde  (C = 20–180 mM) Pd (4 wt.%) on graphitic carbon felt (3 cm × 6 cm) | Pt paper | Ag/AgCl [in a fixed-bed continuous flow cell] Nafion-117 Catholyte: deionized (DI) water and acetic acid (32:1 by mole) added with alcohols Anolyte: 1 M KOH in methanol and DI water solution (10:90 by mass) I = -50 to -150 mA,  T = 25 oC, Catholyte and anolyte feed rates = 2 cm3 min-1, Alcohols: methanol, ethanol, isopropanol Benzyl alcohol (F.E. = 25–100%, TOF = 100–900 h-1) 72 Benzaldehyde  (C = 80 mM) and other oxygenated organics  Metal (Pd, Rh, Ru, Cu, Ni, Zn, or Co) on carbon felt | Pt paper | Ag/AgCl  Nafion-117 A mixture of isopropanol (47.5 wt.%), H2O (47.5 wt.%), and acetic acid (5 wt.%) E = -0.6 to -1.6 V, I = -25 to -250 mA, T = 25 oC, Catholyte and anolyte feed rates = 2 cm3 min-1 Benzyl alcohol, hydrobenzoin (F.E. = 25–100%, TOF = 125–650 h-1 (over Pd), 50 – 300 h-1 (over Cu) 73  26  Substrate / Reactant WE | CE | RE Membrane Electrolyte Conditions ProductsΔ Ref. [in a fixed-bed continuous flow cell] Benzaldehyde  (C = 80 mM), in some cases mixed with bio-oil (10 wt.%) obtained from fast pyrolysis of blended feedstock Metal (Pd, Rh, Ru, Cu, Ni, Zn, or Co) on carbon felt | Pt paper | Ag/AgCl  [in a fixed-bed continuous flow cell] Nafion-117 Catholyte: A mixture of isopropanol (47.5 wt.%), H2O (47.5 wt.%), and acetic acid (5 wt.%), Anolyte: 1 M KOH in 10 wt.% methanol solution E = -0.6 to -1.4 V, I = -50 to -700 mA, T = 25 oC, t = 0–100 min, Catholyte and anolyte feed rates = 2 cm3 min-1 Benzyl alcohol, hydrobenzoin (F.E. = 7–53%, ECH rate (mmol g-1cat h-1) = ~4.5 (over Pd), ~3 (over Cu) 70 Benzyl phenyl ether, -phenoxyethyl-benzene, -phenoxy-acetophenone  (C = 9–26 mM) RaNi alloy (4:1, w/w) | Glassy carbon | n.s. Nafion-324 NaCl (0.1 M), Ethanol-water (75:25, v/v) j = 0.2 to 1.6 mA cm-2,  T = 25–40 oC, t = n.s. Hydrogenolysis products not specified. In some cases, small amounts of cyclohexanol and phenol were reported (F.E. = 20–100%, X = 9–124%) 42 Guaiacol, phenol, syringol  (C = 20 mM) Ru/ACC (activated carbon cloth) | Pt wire | n.s. DuPont® Nafion-117 Catholyte: HCl (0.2 M), NaCl (0.2 M), NaOH (0.2 M) Anolyte: phosphate buffer (0.2 M) I = 40 to 160 mA (A = n.s.), T = 25–80 oC,  t = 2 h Cyclohexanol, Cis-2-Methoxycyclohexanol, Trans-2-Methoxycyclohexanone, Phenol, Cyclohexanone (F.E. = 8–31%, X = 13–89%) 34 Guaiacol, 2-ethoxyphenol, 2-isopropoxyphenol, 3-methoxyphenol, 4-RaNi (trapping Ni-Al alloy particles in an electrodeposited nickel matrix) | Co-P-coated Nafion-117 Catholyte: Potassium borate, BK3O3 (0.1 M, pH 8) I = 50 mA (j = 8 mA cm-2), T = 75 oC, t = 6 h Cyclohexanol, 3-Methoxycyclohexanol, 4-Methoxycyclohexanol, Phenol (F.E. = 18–26%, X < 100%) 35  27  Substrate / Reactant WE | CE | RE Membrane Electrolyte Conditions ProductsΔ Ref. methoxyphenol  (C = 10–20 mM) stainless-steel mesh | n.s. Anolyte: Potassium phosphate, K3PO4 (0.1 M, pH 7) Guaiacol, Phenol  (C = 106 mM, added with Pt/C catalyst) Pt gauze, Ti gauze, or Ni gauze | Pt mesh | Ag/AgCl Nafion-117 Catholyte: H2SO4 (0.2–1 M), HClO4 (0.2–0.5 M), NaCl (0.5 M), NaOH (0.2 M), Anolyte: H2SO4 (0.2 M), HClO4 (0.2–0.5 M), NaCl (0.5 M), NaOH (0.2 M) E = -1.0 to -2.5 V,  j = -109 to -255 mA cm-2, T = 23–60 oC, t = 2–5 h, SR = 350–500 rpm Cyclohexanol, Cyclohexanone, 2-Methoxycyclohexanol, 2-Methoxycyclohexanone, Phenol, Methanol (F.E. = 0–94%, X = 0–100%) 49 Guaiacol, Phenol  (C = 106 mM, added with Pt/C, Ru/C, Pd/C catalysts)  Pt gauze | Pt wire | Ag/AgCl Nafion-117 Catholyte: H2SO4 (0.2–0.5 M), NaCl (0.2–0.5 M), Anolyte: H2SO4 (0.2 – 0.5 M)   j = -109 mA cm-2, T = 50 oC, t = 4 h, SR = 240 rpm Cyclohexanol, Cyclohexanone, 2-Methoxycyclohexanol, 2-Methoxycyclohexanone, Phenol, Methanol (F.E. = 20–80%, X = 18–98%) 50 Phenolics and aromatics (diverse compounds and functionalities, C = 5, 10, 20, 40, 60 mM) Ru/ACC | Pt wire | Nafion-117 Catholyte: HCl (0.2 M) Anolyte: Phosphate buffer (0.2 M) I = 100 mA (j = 22.22 mA cm-2), T = 80 oC, t = 2–10 h Cyclohexanol, alkylcyclohexanols, methoxycyclohexanols, Cyclohexanone, etc. (F.E. = 4–46%, X = 50–100%) 13 Phenolic monomers (e.g., Guaiacol, C = 10 mM)  PtNiB/CMK-3 | IrO2/C | n.s. (ECH electrolyzer cell)  Nafion-117 (catalyst-HClO4 (0.2 M) I = 40 mA, E = 1.7–2.0 V, T = 60 oC, t = 1–5 h, Flow rate = 50 mL/min Cyclohexanol, cyclohexanone, etc. (X = 74  28  Substrate / Reactant WE | CE | RE Membrane Electrolyte Conditions ProductsΔ Ref. coated membrane) 95–100%, F.E. = 80–93%) Phenolic compounds (phenol, guaiacol, and other substituted phenols, C = 20 mM, added with Pt/C and silicotungstic acid, SiW12)  Graphite rod | Pt mesh | Ag/AgCl Nafion-117 Catholyte: SiW12 (0.1 M), methanol/water (1:9, v:v) added in some cases (for more complex reactants) Anolyte: H3PO4 (1 M) j = 100–800 mA cm-2, T = 35–75 oC, t = 20–25 min, SR = 800 rpm (the cathode was purged by N2 to remove air) Cyclohexanol, Cyclohexanone, Cyclohexane  (F.E. = 30–99%, X = 77–100%) 67 WSBO (water-soluble bio-oil), from pyrolysis of poplar, extracted with water (C = 1857 mM in total organic carbon or 15 wt.% of the aqueous solution) Ru/ACC | Pt wire | Ag/AgCl DuPont® Nafion-117 Catholyte: NaCl (0.2 M)  Anolyte: H2SO4 (1 M) E = -7.5 V, I = n.s., T = 27 oC, t = 6.5 h Ethanol, 1-Propanol, 1-Butanol, Tetrahydrofurfuryl alcohol, Ethylene glycol, Propylene glycol (Carbon recovery = 80%) 66 WSBO, from microwave-induced fast pyrolysis of corn stover Ru@OMC (ordered mesoporous carbon) coated on Ni foam | Graphite felt | Ag/AgCl Nafion-115, DuPont Catholyte: NaCl (0.25 M) Anolyte: FeCl2 (1 M) + HCl (1 M) I = 100 mA, T = 25 oC, t = 3 h Carbon content (wt.%): Acids (~18), alcohols (~28), carbonyls (~15), phenols (7.5) 75 Surrogate oil [mixed acetic acid (5 wt.%) and formic acid (3 wt.%)], pine pyrolysis oil Vulcan XC-72R | Ti sheets with a Pt coating | n.s.  [in a continuous flow cell] Dual membrane (Tokuyama AMX AEM, Catholyte: Oil Center compartment: Na2SO4 (1 M) Anolyte: Purified water  I = 10 to 50 mA, Ecell = 4 to >10 V, T = 35 oC, t = 1 to 15.5 h, Flow rate = 2 mL min-1 Total acid number (TAN) and carboxylic acid number (CAN) reduced, phenolic acid number 68  29  Substrate / Reactant WE | CE | RE Membrane Electrolyte Conditions ProductsΔ Ref. Nafion 117 CEM) (PhAN) unchanged, pH increased from 2.6 to >4,  Whole bio-oil, from fast pyrolysis of rice husk (C = 5 wt.%) dissolved in a mixture of CH3OH and CH2Cl2 (4:1, w/w) Pt foil | Pt wire | n.s. [An undivided cell was used] n.s.  LiCl (0.1 M) as supporting electrolyte E = 10 V, t = 12 h, room temperature, ambient pressure Phenol, 2,3-dimethyl-phenol, Creosol, 2-methoxy-phenol, 2-methoxy-3-propenyl-phenol (Peak area decreases observed) 76 Deoxidized bio-oil, from microwave-assisted catalytic fast pyrolysis of corn stover  Ru/ACF (activated carbon fiber) | n.s. | n.s. PEM (pretreated with 5% H2SO4 at 80 oC for 2 h) n.s. I = 0.5 A, t = 9 h, room temperature, atmospheric pressure Acids, Esters, Carbonyls, Phenols, Sugars, Furans (relative contents decreased), Alcohols (relative content increased) 77 Note: C = initial reactant concentration, WE = working electrode, CE = counter electrode, RE = reference electrode, I = current input (j = current density, A = geometric area), E = cathode potential, T = temperature, t = time, SR = stirring rate, F.E. = Faradaic efficiency, X = reactant conversion, TOF = turnover frequency, ΔMajor products in the liquid phase and the experimental results, n.s. = not specified.   30  Chapter 3: Experimental Methodology This chapter exclusively provides the experimental methodology in this research, which, unless otherwise stated, has been applied in Chapters 4–6. Electrochemical cell setup, cyclic voltammetry and electrolysis experimental procedures, product analysis and catalyst characterization, reaction metrics calculation, materials and reagents are presented herein.  3.1. Electrochemical cell: Stirred slurry configuration The experimental setup for ECH is schematically displayed in Figure 3.1. Two H-cells were employed: non-jacketed cell (without temperature control) and jacketed cell (with temperature control). The former was used for potentiostatic electrolysis (including preliminary screening and control experiments), while the latter for galvanostatic electrolysis experiments. The H-cells were equipped on the cathode side with a Luggin probe for the reference electrode (filled with 3 M KCl) and a proton exchange membrane (Nafion®117). A high-surface area Pt wire cylindrical mesh was used as anode, while Pt gauze served as cathode current feeder. The geometric Pt electrode areas were similar (ca. 2.75 cm2). The inter-electrode gap was ca. 6 cm. Typically, guaiacol (1.0–1.3 g) or phenol (0.8–1.0 g) was dissolved in the catholyte (80 mL for the non-jacketed cell or 100 mL for the jacketed cell) to prepare the initial substrate concentration of 0.1 M. In addition, the catalyst (5 wt.% Pt/C, 0.10–0.13 g) was also dispersed in the catholyte. The H-cell setup is well suited for easy testing of different ECH reaction conditions, product distributions, and electrode polarizations in the cathode compartment. However, owing to the high inter-electrode gap and batch mode of operation with static anolyte the H-cell configuration cannot be used to draw industrially relevant conclusions about the overall cell potential and energy efficiency of the system. Note that the  31  reduction currents are expressed with negative sign throughout this paper: “higher” current means “more negative” current.  Figure 3.1. A sketch of electrochemical cell setup for ECH of guaiacol and phenol: (1) cathode, (2) anode, (3) reference electrode (Ag/AgCl/3M KCl), (4) fritted side compartment (Luggin probe) for reference electrode filled with 3M KCl, (5) cation exchange membrane (Nafion-117), (6) catalyst slurry. A magnetic stirrer was used in the cathode. The cathode jacket of the actual size also has water inlet and outlet. 3.2. Cyclic voltammetry and electrolysis experimental protocols Cyclic voltammetry (CV) tests were performed in three different electrolytes: acid (HClO4, H2SO4), neutral (NaCl, KCl) and base (NaOH, KOH), at the same concentration (0.2 M) using Pt gauze as the working electrode and the cylindrical Pt mesh as the counter electrode in the H-cell. Initially, CV scans were run for 50 cycles with a scan rate of 100 mV s-1 to clean the Pt working electrode surface and obtain stable voltammograms. Guaiacol or phenol was then added to the electrolyte such that to obtain a concentration range between 0.02 M to 0.1 M. The CV tests were run for 50 cycles with a scan rate of 100 mV s-1. The potential window for the acid, neutral, and VA- +H+(1) (2)(3)(4)(5)e-e-(6)Jacketed H-cellNon-jacketed H-cell 32  base solution was -0.60 to 2.00 V, -1.50 to 2.00 V, and -1.50 to 1.25 V, respectively. Note that all the potentials herein are referenced vs. Ag/AgCl with saturated KCl (3 M). Electrocatalytic hydrogenation experiments were conducted in the H-cell using two modes of operation: potentiostatic and galvanostatic, respectively. In case of the former, a potentiostat (CH Instruments Electrochemical Workstation, Model 1100 C) was used, whereas for the latter a power supply (BK Precision 9110) was employed. For temperature control, a jacketed H-cell (CANSCI Glass Products Ltd.) was connected to a recirculating water bath (Fisher Scientific Isotemp 3016HS). Initially, the H-cell was filled with the catholyte and anolyte, respectively, and the cell equipped with the cathode and anode was polarized for 10 min without the addition of either the organic substrate or the catalyst slurry. The organic substrate (e.g., guaiacol) was then dissolved in the catholyte and stirred for 30 min without the cell being polarized before adding the catalyst. Afterwards the catalyst was added to the catholyte and then ECH was performed at either constant potential or constant current under constant stirring (ca. 350 rpm) of the catholyte. Before and after experiment, the pH of the electrolytes was measured using a pH meter (Thermo Scientific Orion Star A2110). The electrolyte conductivity was also measured using a conductivity meter (Thermo Orion 105) at room temperature. At the end of the experiment, the catalyst was recovered by vacuum filtration. The H-cell was washed thoroughly with a mixture of water and acetone after each use, while the current feeders were cleaned ultrasonically for 60 min in distilled water. Bio-oil ECH experiments were performed under galvanostatic control using mixed electrolytes, which consisted of methanesulfonic acid (MSA) solution and organic solvents (with equal volume ratio). Bio-oil density was measured (ca. 1.2 g cm-3) and typically 5–10 mL bio-oil was added in the mixed catholyte solution (100 mL) paired with MSA anolyte solution in each experiment. The electrolysis experiments were run over 5 wt.%-Pt/C catalyst at constant temperatures (50–60 oC).  33  3.3. Product analysis and catalyst characterization   Sample aliquots (1 mL) of catholyte were taken before reaction and periodically over the course of the reaction, while the sample aliquots of anolyte were only taken after each reaction to check if there was any substrate crossover from cathode to anode. All the samples were filtered to separate the catalyst and then extracted with n-butanol (2 mL). Any acid-containing sample typically required saturation with inorganic salt (NaCl) and the mixture was left overnight to ensure a complete phase separation. Qualitative (identification) and quantitative analyses of the products were performed by gas chromatography (GC, Agilent 7820A) with a mass selective detector (MSD, Agilent 5975). The organic solution was filtered using a 0.45 m syringe filter prior to each analysis. The separation of compounds was performed in an HP-INNOWax column (30 m×250 m×0.25 m) with the injector temperature was set at 250 oC with a split ratio of 50:1. The GC oven was initiated at 50 oC for 5 min, heated to 150 oC at 5 oC/min and then to 260 oC at 15 oC/min before being held at 260 oC for 5 min. The products were quantitatively determined using calibration curves and response factors of each compound with standard chemicals.  Sample aliquots of catholyte from the bio-oil ECH were taken before and after reaction: a half portion (2 mL) was dried by Na2SO4 and another half (2 mL) was extracted by n-decane. Compound analysis (identification and quantification) was performed based on GC-MS results. The dried sample revealed the overall compounds detected in the bio-oil from the catholyte (including carbohydrate and lignin derivatives), whereas the extracted sample revealed mostly lignin-derived compounds from the bio-oil sample. The detected major compounds were classified based on the functional group and calculated based on the relative content of the compound in the sample normalized by the weight of bio-oil (on the solvent- and oligomer-free basis). The GC oven program included initiation at 40 oC for 5 min and ramp up at 5 oC/min to 260 oC, and equilibration  34  for 5 min. The average molecular weight of bio-oil samples was analyzed using a gel permeation chromatography (GPC) instrument (Shimadzu) equipped with a UV-vis detector and a refractive index detector (RID). Tetrahydrofuran (THF, ≥99.9%, Sigma-Aldrich) was used as the eluent at a flow rate of 1 mL min-1 and a system calibration was performed with polystyrene standards. The dried bio-oil samples from ECH (50–100 L, depending on the initial bio-oil concentration) were diluted in THF (2 mL) for each analysis (with 20 L injection volume). All the samples were syringed filtered (0.45 m filter paper pore size) prior to each chromatographic analysis. Catalyst characterizations include N2 physisorption, CO chemisorption, and transmission electron microscopy (TEM). N2 physisorption measurements were performed in a surface area and porosity analyzer (Micromeritics ASAP 2020) at 77 K to determine Brunauer-Emmett-Teller (BET) surface areas and pore volumes of the catalysts, along with the average pore diameter by Barrett-Joyner-Halenda (BJH) method. The sample was degassed at 200 oC for 2 h before measurement. CO chemisorption was performed in Micromeritics AutoChem II equipped with thermal conductivity detector (TCD) using a pulsed injection of 5% CO/He to measure the metal surface area. Prior to measurement, the sample was reduced under H2 flow (50 cm3/min) at 300 oC for 1 h. TEM analyses were performed in UBC Bioimaging Facility to qualitatively examine the surface morphology of the fresh, acid-treated, and alkali-treated catalysts. The TEM micrographs were obtained on a FEI Tecnai G2 TEM apparatus equipped with 200 kV LaB6 filament.  3.4. Figure of merit calculations Reaction metrics, such as conversion (X), normalized selectivity (S), yield (Y), carbon balance (C.B.), and Faradaic efficiency (F.E.), were calculated as follows, assuming the reactant (R) is converted to product (P).  35  Conversion is given by: 𝑋𝑅(%) =𝑛𝑅,0−𝑛𝑅,𝑡𝑛𝑅,0× 100  (3.1) {𝑛𝑅,0 = initial moles of reactant (guaiacol/phenol); 𝑛𝑅,𝑡 = moles of reactant after t hours reaction}. Normalized selectivity is expressed as: 𝑆𝑃(%) =𝑎𝑃∙𝑛𝑃,𝑡∑𝑎𝑃∙𝑛𝑃,𝑡× 100  (3.2) {𝑎𝑃 = the number of carbon atom in a product, 𝑛𝑃,𝑡 = moles of a product after t hours reaction}.  The selectivity calculations were based on carbon atom (C mol%) and normalized including all the identified products. In other words, Equation (3.2) represents the molar ratio of carbon in a particular reaction product versus the total moles of carbon in all the identified products. This approach for selectivity calculation is advantageous because puts less weight on the methanol by-product contribution, whereas it emphasizes the contributions of desired high carbon atom hydrogenation products.  Yield was calculated as follows, expressing the carbon yield: 𝑌𝑃(%) =𝑎𝑃∙𝑛𝑃,𝑡𝑎𝑅∙𝑛𝑅,0× 100  (3.3) All the calculations were verified in terms of carbon balance closure: 𝐶. 𝐵. (%) =∑𝑎𝑃∙𝑛𝑃,𝑡+𝑎𝑅∙𝑛𝑅,𝑡𝑎𝑅∙𝑛𝑅,0× 100 =6(𝑛1+𝑛2+𝑛6)+7(𝑛3+𝑛4+𝑛0)+𝑛57𝑛0× 100  (3.4) {∑𝑎𝑃 ∙ 𝑛𝑃,𝑡 = the sum of moles of carbon in the products after t hours reaction; 𝑎𝑅 = the number of carbon atom in the reactant; 𝑛𝑅,𝑡 = moles of the reactant after t hours reaction; 𝑎𝑅 ∙ 𝑛𝑅,0 = initial moles of carbon in the reactant; 0 = guaiacol, 1 = cyclohexanol, 2 = cyclohexanone, 3 = 2-methoxycyclohexanol, 4 = 2-methoxycyclohexanone, 5 = methanol, 6 = phenol}  36  A general expression of F.E. can be written as: 𝐹. 𝐸. (%) =𝑄𝑎𝑐𝑡𝑢𝑎𝑙𝑄𝑡𝑜𝑡𝑎𝑙× 100 =𝑧∙𝑛∙𝐹𝐼∙𝑡× 100 (3.5) For the present case Equation (3.5) is rewritten as: 𝐹. 𝐸. (%) =(8𝑛1+6𝑛2+6𝑛3+4𝑛4+2𝑛5)𝐹𝐼∙𝑡× 100 (3.6) {𝑛𝑥  = the moles of compound x: 0 = guaiacol, 1 = cyclohexanol, 2 = cyclohexanone, 3 = 2-methoxycyclohexanol, 4 = 2-methoxycyclohexanone, 5 = phenol; z = number of electrons} 3.5. Kinetic parameter estimation methods Reaction order for each individual sub-reaction in the guaiacol ECH was estimated by graphical kinetic analysis based on the experimental data using the corresponding standard chemical (e.g., guaiacol, phenol, cyclohexanone, 2-methoxycyclohexanone, and 2-methoxycyclohexanol). The experimental results are presented in Chapter 6. The rate constants were approximated using Levenberg-Marquardt algorithm in MATLAB® solving the ordinary differential equation (ODE) models. The confidence interval (CI) was determined using MATLAB routine (nlparci) that computes 95% confidence intervals with the Jacobi method to fit a nonlinear regression model and get the coefficient estimates. Activation energy and pre-exponential factor were determined based on linear regression of the Arrhenius equation. 3.6. Design of experiment and factorial analysis methods Factorial experiments of guaiacol ECH were conducted at fixed current and temperature in MSA electrolyte using 5 wt.%-Pt/C catalyst. The objective is to determine the interaction between the selected parameters and its significance using two-level factorial design and optimize the conditions that give maximum response variables. Design-Expert v12 software (statease.com) was  37  used to perform the factorial analysis. A full-factorial design was chosen which requires 2k runs (k = the number of factors). Initially, Shapiro-Wilk test was run to evaluate the normality of the experimental data population, followed by the statistical analysis of variance (ANOVA) to determine the significant correlations. In the model, only significant terms (p-value < 0.05) were included (except those required to maintain hierarchy) in order to obtain the better responses. The significant interaction between the factors was interpreted and modelled by 3D surface graphs. Finally, numerical optimization was done through the program by adjusting the criteria for the desired goals. Further discussion can be found in Chapter 6.   3.7. Materials and reagents All materials and reagents used in this study were purchased from various suppliers as listed below. Table 3.1. List of materials and reagents used in this work. Material Supplier Reagent Supplier Pt gauze (100 mesh) Nafion-117 membrane Ag/AgCl reference electrode Pt/C (1, 3, 5, 10 wt.% metal loading) Ru/C (5 wt.% metal loading) Pd/C (5 wt.% metal loading) C (carbon or activated charcoal) Pt/Al2O3 (5 wt.% metal loading) Sigma-Aldrich Guaiacol (≥99%), Phenol (≥99%), 4-Propylguaiacol (≥99%), Creosol (≥98%), Perchloric acid (HClO4, 70%), Potassium chloride (KCl, 99%), Methanesulfonic acid (≥99%), Acetone (≥99.7%), Isopropanol (>70%), Acetonitrile (≥99.7%), Ethanol (reagent grade) Sigma-Aldrich  Pt wire, Pt foil sheet Ti gauze (80 mesh) Ni gauze (100 mesh) Alfa Aesar 1-Butanol (99%) n-Decane (99+%) Acetic acid (≥99.7%) Alfa Aesar  Magnetic stirrer bar Stirrer A (small): 2.4 cm, 3.9 g Stirrer B (big): 3.6 cm, 9.6 g  Fast pyrolysis oil (FPO) VWR   BTG Bioliquid (Netherlands) Sulfuric acid (H2SO4, 95–98%) Sodium chloride (NaCl, 99.8%) Sodium chloride (NaOH, 98.9%) Potassium hydroxide (KOH, 99%) Fisher Scientific / Fisher Chemical  38  Chapter 4: Electrocatalytic Reduction of Phenolics in a Stirred Slurry Reactor In this work, electrocatalytic hydrogenation (ECH) of guaiacol (and phenol) was performed in a stirred slurry electrochemical reactor (SSER) using 5 wt.% Pt/C catalyst. Different electrolyte pairs in catholyte-anolyte combinations including acid (H2SO4), neutral (NaCl), and alkaline (NaOH) were investigated under potentiostatic and galvanostatic conditions. This approach aimed to probe the electrolyte and proton concentration effects on guaiacol conversion, product distribution, and Faradaic efficiency. The impacts of electrolyte conductivity, cathode materials, and cathode potentials were also evaluated in the potentiostatic electrolysis experiments. Synergistic effects between temperature and potential toward reaction pathways in the ECH of guaiacol were identified in the galvanostatic electrolysis experiments. This work demonstrates a feasibility of mild synthesis of industrially important chemicals (e.g., cyclohexanol and cyclohexanone) from biomass via electrocatalytic reduction pathways, as illustrated in Scheme 4.1 with the representative ECH of guaiacol reaction networks.  Scheme 4.1. Conceptual routes for synthesis of renewable chemicals from biomass, involving the mild reaction pathways represented by electrocatalytic reduction of guaiacol. Electrocatalytic hydrogenation-hydrogenolysis (ECH) of guaiacol:BiomassLigninBio-oilGuaiacol, PhenolFractionationHydrodeoxygenationCylohexanol, CyclohexanoneHydrogenation-HydrogenolysisOxidationAdipic acidNylon 66PolycondensationCaprolactamBeckmanrearrangementNylon 6Ring-opening polymerizationPyrolysis 39  4.1. Electrochemical characterizations: cyclic voltammetry analysis Cyclic voltammetry (CV) experiments were initially performed to: (i) investigate the potential domains for hydrogen evolution (HER), hydrogen oxidation (HOR), oxygen evolution (OER), and oxygen reduction (ORR) reactions in different electrolytes; (ii) provide insights on the operating cathode potential for the ECH experiments; (iii) observe how the presence of organics (guaiacol, phenol) would affect the Pt voltammograms in different potential domains due to the substrate adsorption, (iv) demonstrate how the presence of catalysts slurry (Pt/C) could enhance the reduction of protons. Figure 4.1 shows the effect of guaiacol and phenol on the Pt electrode cyclic voltammograms in acid, neutral, and alkaline electrolytes. In the absence of organics, the HER, HOR, OER, and ORR waves are clearly identified and labelled (Figure 4.1a). Distinct spectrums for guaiacol and phenol ECH were not observed, suggesting that the ECH reactions likely occur within the overpotential ranges of HER. Comparing Figure 4.1a and 4.1b, the presence of phenol in acid and neutral electrolytes dramatically suppressed HER, HOR, OER, and ORR, as shown by a more than four times lower HER current density at around -0.6 V (vs. Ag/AgCl). This was due to the strong adsorption of phenol on the Pt surface. It was reported that phenol acts as a scavenger of adsorbed hydrogen radicals,36 thereby decreasing the HER current density at a given overpotential. In contrast, the presence of guaiacol at the same 20 mM concentration as phenol, suppressed only slightly those reactions (compare Figure 4.1c and 4.1d), which suggests that the adsorption of guaiacol on Pt is weaker (i.e. lower surface coverage) compared to phenol. When the guaiacol concentration was increased from 40 to 100 mM, the HER, HOR, OER, and ORR reactions eventually faded (Figure 4.1e). The addition of the Pt/C catalyst (i.e. catalyst slurry configuration) resulted in more negative cathodic currents in the H2 evolution region (Figure 4.1f).   40   Figure 4.1. Effect of guaiacol and phenol on the Pt gauze cyclic voltammograms in different electrolytes: (a) and (c) acidic, neutral, and basic electrolytes without any organics, (b) phenol-containing electrolytes, (d) guaiacol-containing electrolytes, (e) acidic electrolytes with the increasing guaiacol concentration, (f) guaiacol-containing acidic electrolytes before and after the addition of dispersed catalyst (5 wt.%-Pt/C). Concentration of all electrolytes: 0.2 M, temperature: 25 oC. Reproduced from ref. 49 with permission from John Wiley & Sons, Copyright 2019.-500-400-300-200-1000100200300400-2 -1.5 -1 -0.5 0 0.5 1 1.5 2 2.5Current Density (A m-2)Potential, E (V) vs. Ag/AgClHClO₄NaClNaOH-500-400-300-200-1000100200300400-2 -1.5 -1 -0.5 0 0.5 1 1.5 2 2.5Current Density (A m-2)Potential, E (V) vs. Ag/AgClHClO₄KClKOH-500-400-300-200-1000100200300400-2 -1.5 -1 -0.5 0 0.5 1 1.5 2 2.5Current Density (A m-2)Potential, E (V) vs. Ag/AgClHClO₄ + Phenol (20 mM)NaCl + Phenol (20 mM)NaOH + Phenol (20 mM)-500-400-300-200-1000100200300400-2 -1.5 -1 -0.5 0 0.5 1 1.5 2 2.5Current Density (A m-2)Potential, E (V) vs. Ag/AgClHClO₄ + Guaiacol (20 mM)KCl + Guaiacol (20 mM)KOH + Guaiacol (20 mM)-600-400-2000200400600-1 -0.5 0 0.5 1 1.5 2 2.5Current Density (A m-2)Potential, E (V) vs Ag/AgClH₂SO₄ H₂SO₄ + Guaiacol (40 mM)H₂SO₄ + Guaiacol (60 mM)H₂SO₄ + Guaiacol (80 mM)H₂SO₄ + Guaiacol (100 mM)-600-400-2000200400600-1 -0.5 0 0.5 1 1.5 2 2.5Current Density (A m-2)Potential, E (V) vs Ag/AgClH₂SO₄ + Guaiacol (100 mM)H₂SO₄ + Guaiacol (100 mM) + Pt/C(a) (b)(c) (d)(e) (f)HEROERHORORRPt-O formationHEROERHORORR41  This demonstrates that the reduction of protons was enhanced by the presence of the slurry catalyst. These CV results also show that acidic electrolytes are more appropriate for guaiacol reduction (due to the least negative potentials for HER54). For screening purpose, the acid (H2SO4, HClO4), neutral (NaCl, KCl), and base (NaOH, KOH) electrolytes with the same concentrations (0.2 M) were used and the cyclic voltammograms show no stark differences caused by the different anions in acidic media and likewise by the different cations in alkaline media. 4.2. Thermodynamic analysis of ECH of guaiacol  Guaiacol conversion to cyclohexanol proceeds via two parallel routes: (i) demethoxylation–ring saturation and (ii) ring saturation–demethoxylation (Scheme 4.1).49 The first route is marked with the formation of phenol and cyclohexanone while the second route can be identified by the production of 2-methoxycyclohexanone and 2-methoxycyclohexanol. Both routes generate methanol as a by-product. The calculations of free energy, enthalpy, and equilibrium reduction potentials for each specific reaction at standard conditions are summarized in Table 4.1. These data reveal that all the direct electrocatalytic reductions of guaiacol are thermodynamically favorable and the standard potentials are all above that of the H2 evolution reaction. Kinetically, however, as shown by the cyclic voltammograms (Figure 4.1) there is no separate reduction wave for guaiacol. Hence, guaiacol reduction can only occur in the potential domain of the HER.  As shown by Table 4.1 and Scheme 4.1, the expected final product of guaiacol ECH is cyclohexanol. Further  hydrogenolysis of cyclohexanol to cyclohexane (C–O cleavage), which involves dehydration of cyclohexanol to cyclohexene (C6H12O → C6H10 + H2O, ∆𝐻𝑅0 = +25.6 kJ mol-1) and the subsequent hydrogenation of cyclohexene to cyclohexane (C6H10 + 2H+ + 2e- → C6H12, ∆𝐻𝑅0 = -117.1 kJ mol-1) is not feasible because of equilibrium limitations, since the former  42  reaction is inhibited by water as solvent (i.e. aqueous electrolyte) and is endothermic in nature, thus, much higher temperatures and/or overpotentials than those employed in this study would be required for complete deoxygenation via hydrogenolysis.36,78 Stoichiometric analysis of the guaiacol ECH (see Appendix A.1) showed that theoretically, at complete guaiacol conversion, the maximum yields of cyclohexanol and methanol based on the carbon balance would be 85.71% and 14.29%, respectively, either via the first or the second routes. In this guaiacol ECH, 100% cyclohexanol selectivity cannot be achieved since methanol is always formed as the by-product. 4.3. Control experiments in ECH of guaiacol Prior to electrolysis, some control experiments were conducted. Three different conditions were tested in a guaiacol-containing catholyte solution: (1) under electric potential without the addition of dispersed catalyst, (2) under electric potential and with the addition of catalyst but in the absence of stirring, (3) with the stirred catalyst but without electric potential. The first test was performed in 0.5 M HClO4 at E = -2.0 V vs. Ag/AgCl for 4 h; the second test in 0.2 M H2SO4 at E = -2.0 V vs. Ag/AgCl for 2 h where Pt/C settled at the bottom of the solution; and the third test in a stirred slurry solution of 0.2 M H2SO4 for 2 h. None of these experiments resulted in guaiacol conversion products, which confirm at least three requirements for an effective electrocatalytic reduction: (1) availability of electrons to reduce protons into hydrogen radicals, (2) contact between catalyst particles and electrode for the protons to be reduced into adsorbed hydrogen on the catalyst surface, (3) contact between reactant (organic) molecules and catalyst particles under applied potential. Moreover, the electrolysis of guaiacol using only activated charcoal resulted in no products (Table 4.2, Figure 4.2), suggesting that the presence of dispersed metal catalyst on the carbon support is essential for ECH in which the reaction involves Hads formed on the metal sites.      43  Table 4.1. Standard Gibbs free energy, enthalpy, standard potential and temperature coefficient of the standard potential for reactions in ECH of guaiacol. Reaction Equation ∆𝐺𝑅0 (kJ mol-1) ∆𝐻𝑅0 (kJ mol-1) ∆𝑆0 (J mol-1 K-1) 𝐸𝑟𝑒𝑑0  (VSHE) 𝜕𝐸𝑇0𝜕𝑇 (mV K-1) Route 1: Demethoxylation–Ring saturation      Guaiacol to Phenol C7H8O2 + 2H+ + 2e- → C6H6O + CH4O -73.07 -70.15 9.80 0.38 0.05 Phenol to Cyclohexanone  C6H6O + 4H+ + 4e- → C6H10O -57.85 -120.94 -211.71 0.15 -0.55 Cyclohexanone to Cyclohexanol C6H10O + 2H+ + 2e- → C6H12O -21.94 -75.90 -181.07 0.11 -0.94 Route 2: Ring saturation–Demethoxylation     Guaiacol to 2-Methoxycyclohexanone C7H8O2 + 4H+ + 4e- → C7H12O2  -55.93 -118.26 -209.16 0.15 -0.54 2-Methoxycyclohexanone to 2-Methoxycyclohexanol C7H12O2 + 2H+ + 2e- → C7H14O2 -34.87 -47.00 -40.70 0.18 -0.21 2-Methoxycyclohexanol to Cyclohexanol C7H14O2 + 2H+ + 2e- → C6H12O + CH4O -74.99 -101.73 -89.73 0.39 -0.47 Cathode side reactions        Hydrogen evolution reaction (HER) (a) 2H+ + 2e- → H2                       0.00 0.00 0.00 0.00 0.00  (b) 2H2O + 2e- → H2 + 2OH-  474.28 571.66 326.77 -0.83 1.69 Anode side reactions       Oxygen evolution reaction (OER) (a) 2H2O → O2 + 4H+ + 4e-      -474.28 -571.66 -326.77 1.23 -0.85  (b) 4OH- → O2 + 2H2O + 4e-    474.28 571.66 326.77 0.40 0.85 ∆𝐺𝑅0 = standard Gibbs free energy of the reaction; ∆𝐻𝑅0 = standard enthalpy of the reaction; ∆𝑆0 = standard entropy of the reaction; 𝐸𝑟𝑒𝑑 0  = standard reduction potential of the reaction; 𝜕𝐸𝑇0𝜕𝑇 = temperature coefficient of the standard equilibrium electrode potential; Reaction (a) in acidic media, (b) in neutral or alkaline media. All the thermodynamic data were obtained based on NIST and Joback method at standard condition (298 K and 1 atm).  44  Table 4.2. Summary of the control experiments in the ECH of guaiacol resulting in no hydrogenation products.49 Condition Potential Catalyst Electrolyte Stirring Products 1 Yes (E = -2 V) No HClO4 (0.5 M) Yes (350 rpm) No 2 Yes (E = -2 V) Yes (Pt/C, 0.1 g) H2SO4 (0.2 M) No No 3 No Yes (Pt/C, 0.1 g) H2SO4 (0.2 M) Yes (350 rpm) No 4 Yes (E = -1.25 V) Yes (AC, 0.15 g)  HClO4 (0.2 M) Yes (350 rpm) No The experiments were performed in acidic electrolyte pair for 2–4 h using a non-jacketed H-cell. E = cathode potential (vs. Ag/AgCl), Pt/C = carbon-supported platinum (5 wt.% Pt content), AC = activated charcoal (the same type of carbon used in the Pt/C). Cathode (Pt gauze) surface area = 1.47×10-3 m2 g-1.    Figure 4.2. Comparison on the ECH of guaiacol over activated charcoal (AC) vs. over carbon-supported platinum (Pt/C). Conditions: E = -1.25 V (vs. Ag/AgCl), T ≈ 35 oC, j ≈ -160 mA cm-2, t = 4 h. No hydrogenation products were identified over AC. This result shows the importance of dispersed metal (e.g., Pt) on the catalyst for an effective ECH.  0102030405060708090100AC Pt/C(%)Guaiacol Conversion Cyclohexanol SelectivityCarbon Balance Faradaic Efficiency 45  4.4. Potentiostatic ECH of guaiacol and phenol  4.4.1. Effect of different catholyte-anolyte pairs at controlled cathode potential These screening experiments aimed to determine the most effective electrolyte combination for ECH. Different pairs of electrolytes were tested for ECH of guaiacol using the Pt/C slurry cathode operated at -2.0 and -2.5 V vs. Ag/AgCl using chronocoulometry. The corresponding pH and temperature corrected overpotentials were calculated and are presented in Table 4.3, showing that the electrolysis experiments for the effective ECH were carried out at relatively comparable overpotentials (-2.0 ± 0.1 V, based on the average standard deviation). Each pair of electrolytes is denoted by the catholyte-anolyte combination, for example “neutral-acid” represents the combination of NaCl (catholyte) and H2SO4 (anolyte). The acidic, neutral, and basic electrolytes are represented by H2SO4 (0.2 M), NaCl (0.5 M), and NaOH (0.2 M), respectively. For the sake of comparison, similar experiments were also performed using phenol, an important intermediate in the reaction pathway of guaiacol ECH. The time-dependent conversion and selectivity profiles for the ECH of guaiacol and phenol are shown in Figure 4.3.    The chronocoulometric plot (Figure 4.3a) revealed that at comparable overpotential the charge transfer rates decreased in the following order: acid-acid > neutral-acid > base-acid > acid-neutral > neutral-neutral > acid-base ≈ base-base. This order is due to the cathode polarizations and the availability of protons in the different electrolytes associated with the transport of protons across the membrane. The guaiacol conversion (X) and cyclohexanol yield (Y1) decreased as: acid-acid > neutral-acid > acid-neutral > acid-base > base-acid ≈ neutral-neutral ≈ base-base (see Table 4.3). Figures 4.3b and 4.3c show the time profile of the guaiacol conversion and the various product selectivities for the two best case scenarios: acid-acid and neutral-acid, respectively. Furthermore,  46  this trend shows a good agreement with that obtained in the parallel experiments for ECH of phenol (see Figure 4.3d-f and Table 4.4).   Figure 4.3. Effect of catholyte-anolyte pairs on ECH of guaiacol (a-c) and phenol (d-f) under controlled cathode potential at either -2.0 or -2.5 V vs. Ag/AgCl. Chronocoulometric plot (a, d), conversion and product distribution over time in the acid-acid pair (b, e) and neutral-acid pair (c, f). Acid: H2SO4 (0.2 M) and neutral: NaCl (0.5 M). Initial guaiacol (or phenol) concentration: 0.1 M. Catalyst: 5 wt.%-Pt/C (0.10 g), cathode: Pt gauze, anode: a wired Pt mesh.  -12000-10000-8000-6000-4000-200000 1 2 3 4Charge (C)Time (h)Acid-Acid (E = -2.0 V)Neutral-Acid (E = -2.5 V)Acid-Neutral (E = -2.5 V)Neutral-Neutral (E = -2.5 V)Base-Acid (E = -2.5 V)Acid-Base (E = -2.5 V)Base-Base (E = -2.5 V)(a)Catholyte-Anolyte01020304050607080901000 1 2 3 4Conversion, Selectivity (%)Time (h)GuaiacolCyclohexanolCyclohexanone2-Methoxycyclohexanol2-MethoxycyclohexanonePhenolMethanol(b)01020304050607080901000 1 2 3 4Conversion, Selectivity (%)Time (h)GuaiacolCyclohexanolCyclohexanone2-Methoxycyclohexanol2-MethoxycyclohexanonePhenolMethanol(c)-12000-10000-8000-6000-4000-200000 1 2 3 4Charge (C)Time (h)Acid-Acid (E = -2.0 V)Neutral-Acid (E = -2.5 V)Acid-Neutral (E = -2.5 V)Neutral-Neutral (E = -2.5 V)Base-Acid (E = -2.5 V)Acid-Base (E = 2.5 V)Base-Base (E = -2.5 V)(d)01020304050607080901000 0.5 1 1.5 2Conversion, Yield (%)Time (h)PhenolCyclohexanolCyclohexanone(e)Catholyte-Anolyte01020304050607080901000 1 2 3 4Conversion, Yield (%)Time (h)PhenolCyclohexanolCyclohexanone(f) 47  In both cases (acid-acid and neutral-acid pairs), the major products were cyclohexanol and 2-methoxycyclohexanol, suggesting that demethoxylation and full ring hydrogenation (i.e. saturation) occurred simultaneously. Under the operating conditions, the yields of phenol and cyclohexanone were very low (<2%). High guaiacol conversion (90%) with similar major products (cyclohexanol and 2-methoxycyclohexanol) was also reported for the ECH process in HCl (0.2 M) at 0.1 A and 80 oC using Ru/ACC cathode.34  The results from both experiments demonstrate that ECH of guaiacol (or phenol) proceeds more effectively in acidic electrolytes and the choice of anolyte has a significant impact on the electron transfer and the hydrogenation rates. Protons for ECH are continuously supplied from water oxidation on the anode in the presence of acid, hence acidic anolyte is preferred for an effective ECH. The impact of different anolytes was very noticeable when comparing the pairs of neutral and acidic electrolytes (Table 4.3, Entry 2 vs. 3). The neutral-acid (catholyte-anolyte) pair resulted in higher (more negative) currents than the acid-neutral (catholyte-anolyte) pair, and thus, higher guaiacol conversion (80% vs. 56%) and higher yields of the hydrogenated products were obtained, such as cyclohexanol (34% vs. 23%) and 2-methoxycyclohexanol (36% vs. 12%). It is therefore apparent that acidic anolyte is preferable to facilitate the guaiacol ECH at the cathode. High anolyte proton concentration is necessary to provide steady flux of protons to in case when neutral catholyte is used. The OER at the anode producing protons compensated the proton transfer across the Nafion membrane to the cathode.  Carrying out the ECH of guaiacol with NaCl anolyte, either in the acid-neutral or neutral-neutral catholyte-anolyte combinations, is not advantageous owing also to the formation of chlorine gas (Cl2) and hypochlorous acid (HClO). Interestingly, when 0.5 M NaCl was employed as both catholyte and anolyte, no hydrogenation products were identified and the current input was  48  low (Table 4.3, Entry 4). The catholyte turned strongly alkaline (pH = 12.9) after 4 h of reaction, caused by formation of NaOH resulting from the reduction of H2O generating OH- and H2. Guaiacol is unstable under alkaline conditions due to deprotonation, which hinders its adsorption onto the catalyst.34 Moreover, it was recently reported that Pt/C suffers from destabilization in alkaline medium, which modifies the anchoring sites of the Pt nanoparticles on the carbon surface, resulting in detachment/loss of the metal.79  To further prove the prominent effect of acidic media in ECH of guaiacol, the pairs of basic and acidic electrolytes were tested as well. The H2SO4-NaOH catholyte-anolyte pair gave better guaiacol conversion and product yield as compared to the NaOH-H2SO4 catholyte-anolyte combination, despite the lower current input (Table 4.3, Entry 5 vs. 6). When NaOH solution was used as both catholyte and anolyte (Table 4.3, Entry 7), the lowest current was obtained and virtually no hydrogenation reactions took place. The color change of the catholyte sample (i.e. brownish color, Figure 4.4), is indicative of guaiacol deprotonation and oxidation under alkaline conditions. With these findings, the pairs of neutral and basic electrolytes were not tested further, as such combination would predictably result in poor electron transfer and hydrogenation rates.   The ECH experiments in the non-jacketed H-cell obviously resulted in temperature variation depending on the current and catholyte-anolyte pair. In other words, temperature difference was simply the natural consequence of electrolyte pairing in an H-cell without temperature control. Table 4.3 shows that when non-acidic anolyte was used, the temperature was close to the room temperature; when acidic anolyte was used, temperature increases were observed. The most dramatic increase was found in the acid-acid pair (T ≈ 53 oC), which can be attributed to Joule heating at high current and exothermic ECH reactions. The heating effect of an electric current in the ECH depends on the operating current, electrolyte resistance (related to the ionic species and  49  concentration), and electrolysis time. Despite the temperature variation, it can be inferred that: (i) acid-acid and neutral-acid are the two most effective pairs, (ii) acidic anolyte is necessary for the effective ECH because protons are mostly supplied from anode.  In addition to the mechanistic aspects of guaiacol ECH at the cathode, the effect of different pairs of electrolytes is also manifested in the specific ionic conductivities (Tables 4.5–4.6). To confirm the effect of anolyte proton concentration, different H2SO4 anolyte concentrations were used in the acid-acid and neutral-acid pairs, showing that guaiacol conversion and F.E. increased with the anolyte concentration (Figure 4.5). Generally, there were no organics found in the anolyte samples, which indicated that the membrane was impermeable to organics during the reported experimental conditions. After prolonged use of the membrane (180–200 h), guaiacol cross-over (~5%) was observed as shown by the anolyte solution color turning dark orange or brownish, presumably as a result of guaiacol oxidation. At this point, membrane replacement was required. The observed guaiacol losses due to diffusion across the membrane and oxidation at the anode were also reported elsewhere.80 In the present work it was observed that the crossover of organics was more significant at low current densities, whereas at high superficial current densities (e.g., > |150 mA cm-2|) the crossover was minimized by the counter flux of electro-osmotic water associated with proton migration from anode to cathode. Material balance analyses estimated that the average carbon balances from the ECH of guaiacol (or phenol) were 90±5%. In most cases, carbon balances from phenol ECH were slightly higher than those from guaiacol ECH, possibly because phenol has better solubility in water (guaiacol solubility in water is about 23 g L-1 at 25 oC), which could be attributed to its higher polarity than guaiacol. 50  Table 4.3. Summary of the results from ECH of guaiacol in different pairs of aqueous electrolytes under potentiostatic control. Entry Catholyte  (c) Anolyte  (a) E  (V) 𝜂 cathode  (V) j  (mA cm-2) T  (oC) pH  (c) pH  (a) X  (%) S1  (%) S2  (%)  S3  (%) S4  (%)  S5  (%)  S6 (%) F.E. (%)  1 H2SO4  H2SO4  -2.0 -1.96 -284 53 0.69 0.51 92.32 47.48 2.89 25.41 14.48 8.58 1.16 39.50 2 NaCl  H2SO4 -2.5 -2.00 -164 38 8.09 0.77 80.36 42.65 1.32 44.87 2.46 7.48 1.22 61.70 3 H2SO4 NaCl  -2.5 -2.42 -65 29 1.37 1.43 55.95 40.39 5.90 20.77 24.81 5.02 3.11 24.42 4 NaCl  NaCl -2.5 -1.91 -59 24 12.92 1.40 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 5 NaOH  H2SO4 -2.5 -1.68 -109 35 13.36 0.60 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 6 H2SO4  NaOH -2.5 -2.45 -18 23 0.80 6.46 26.24 17.23 20.43 21.91 18.60 10.75 11.09 13.13 7 NaOH NaOH -2.5 -1.71 -15 22 13.58 10.65 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 E = cathode potential (vs. Ag/AgCl), 𝜂  = calculated cathode overpotentials (based on H2 evolution reaction), j = superficial current density, T = temperature, X = guaiacol conversion, S = normalized selectivity (C mol%) for cyclohexanol (1), cyclohexanone (2), 2-methoxycyclohexanol (3), 2-methoxycyclohexanone (4), methanol (5), and phenol (6), F.E. = Faradaic efficiency. Catalyst = 5 wt.%-Pt/C (0.10 g); Reaction time = 4 h; Concentration of all electrolytes: 0.2 M, except NaCl (0.5 M); Reactant to metal molar ratio (R/M) = 315; pH at the end of reaction in the catholyte and anolyte, respectively. All the experiments started at room temperature using the non-jacketed H-cell.     51  Table 4.4. Summary of the results from ECH of phenol in different pairs of aqueous electrolytes under potentiostatic control. Entry Catholyte  (c) Anolyte  (a) E  (V)  𝜂 cathode  (V) j  (mA cm-2) T (oC) t (h)  pH  (c) pH  (a) X (%) S1 (%) S2 (%)  C.B. (%) F.E. (%)  1 H2SO4  H2SO4  -2.0 -1.97 -276 48 1 0.53 0.39 59.23 72.80 27.18          2   100 100 0.00 88.07 90.31 2 NaCl H2SO4  -2.5 -2.32 -175 42 2 2.87 0.84 80.39 53.46 46.55          4   100 100 0.00 94.16 70.82 3 H2SO4 NaCl  -2.5 -2.41 -91 32 2 1.42 1.40 60.52 49.83 50.15          4   70.90 64.17 35.84 97.35 56.42 4 NaCl NaCl  -2.5 -1.72 -69 27 2 13.11 1.41 6.67 100 0.00          4   8.16 100 0.00 81.63 6.34 5 NaOH  H2SO4  -2.5 -1.72 -98 33 2 12.84 0.92 6.29 100 0.00          4   10.50 100 0.00 97.02 13.51 6 H2SO4  NaOH  -2.5 -2.44 -18 23 2 1.03 9.28 15.42 33.14 66.86          4   21.20 36.79 63.21 93.36 16.74 7 NaOH  NaOH  -2.5 -1.73 -18 22 2 13.21 12.13 0.00 0.00 0.00          4   0.00 0.00 0.00 90.04 0.00 E = cathode potential (vs. Ag/AgCl), 𝜂  = calculated cathode overpotentials, j = current density, T = temperature, t = reaction time (h), X = phenol conversion, S = normalized product selectivity (C mol%) for cyclohexanol (1), cyclohexanone (2), C.B. = carbon balance, F.E. = Faradaic efficiency; pH was measured after the reaction. Concentration of all electrolytes: 0.2 M, except NaCl (0.5 M). Reactant to metal molar ratio (R/M) = 332. Catalyst: 5 wt.%-Pt/C (0.10 g).  52   Figure 4.4. Color changes of the catholyte product sample after 4 h guaiacol ECH in 0.2 M H2SO4 (a), 0.5 M NaCl (b), 0.2 M NaOH (c) paired with acidic anolyte. The anolyte sample after 4 h of reaction in 0.2 M H2SO4 (d) shows also orange/brown caused by oxidation of guaiacol (due to crossover through the membrane). The anolyte sample from the reaction in 0.2 M H2SO4 (c) turning colorless (e), which looks similar to (a), after reducing the oxidized solution, indicating the possibility to recycle unreacted guaiacol-containing solution in the ECH to further obtain the hydrogenated products, e.g., cyclohexanol. Table 4.5. Conductivity and pKa of the electrolytes at RT (± 25 oC).  No. Solution (c) pKa  1 H2SO4 (0.1 M) 0.90 8.67 2  H2SO4 (0.2 M) 0.84 8.73 3 H2SO4 (0.5 M) 0.55 19.33 4 HClO4 (0.2 M) 0.89 7.52 5 HClO4 (0.5 M) 0.59 16.85 6 NaCl (0.5 M) 7.43 4.76 7 KCl (0.5 M) 6.10 5.96 8 KH2PO4 (0.2 M) 7.34 0.30 9 NaOH (0.2 M) 13.66 4.02 10 KOH (0.2 M) 13.56 4.09 c = electrolyte concentration (M); pKa ≈ pH of the electrolyte measured experimentally;   = measured conductivity (specific conductance) of the electrolyte [S m-1].  (a) (b) (c) (d) (e)pH = 0.84 3.89 13.36 0.62 0.83+ H+Phenolate  53   Figure 4.5. Acid concentration effect on guaiacol conversion and hydrogenation product yields after 4 h reaction in: (a) the pair of acid-acid electrolytes at E = -1.0 VAg/AgCl (j = -120 to -227 mA cm-2) and (b) the pair of neutral-acid electrolytes at E = -2.5 VAg/AgCl (j = -150 to -176 mA cm-2) using 5 wt.%-Pt/C catalyst (0.10 g) in the non-jacketed H-cell. Acid: H2SO4 (0.1–0.5 M), neutral: NaCl (0.5 M).    010203040506001020304050607080900.1 0.2 0.5Faradaic Efficiency (%)Conversion, Yield (%)Acid concentration (M)(a)01020304050607001020304050607080901000.1 0.2 0.5Faradaic Efficiency (%)Conversion, Yield (%)Acid concentration (M)Guaiacol Cyclohexanol Cyclohexanone2-Methoxycyclohexanol 2-Methoxycyclohexanone F.E. (%)(b) 54  4.4.2. Impacts of electrolyte conductivity, cathode potentials, and cathode materials The effect of different pairs of electrolytes are also attributed to the ionic conductivity of each electrolyte (listed in Table 4.5). The ECH of guaiacol was further tested in the pairs of buffer solution (KH2PO4) and perchloric acid (HClO4) to prove this hypothesis (hereinafter referred to as buffer-acid or acid-buffer). Monopotassium phosphate is known as weak electrolyte and thus, has much lower conductivity as compared to acids, such as H2SO4 and HClO4. In the pair of HClO4 (0.2–0.5 M), high guaiacol conversion (85–91%) and cyclohexanol yield (33–49%) were obtained, confirming the predominance of acid over neutral and alkaline electrolytes (Table 4.6, Entries 1–2). When the ECH of guaiacol was performed in the pair of buffer-acid or acid-buffer (both of 0.2 M) at -2.0 V, no guaiacol conversion products were observed, and this slow reaction was indicated by the very low currents (Table 4.6, Entries 4–5). Much higher voltage should be applied to drive this reaction (Table 4.6, Entries 6–7). To verify the reliability of reaction pathways for ECH of guaiacol shown in Scheme 4.1, some additional tests were conducted in the pair of H2SO4 (0.2 M) electrolytes over 5wt.%-Pt/C with the different starting materials, such as phenol and 2-methoxycyclohexanone (Table 4.4, Entry 1 and Table 4.6, Entries 8–9). The results show that phenol was first hydrogenated to cyclohexanone before fully converted into cyclohexanol (after 2 h), while 2-methoxycyclohexanone was directly hydrogenated to 2-methoxycyclohexanol (52–76%) and cyclohexanol (4–9%) at different temperatures (30–60 oC). The conversion route of 2-methoxycyclohexanone to cyclohexanone was not observed, therefore the reaction network in Scheme 4.1 can be justified. Additional potentiostatic electrolysis experiments were performed to evaluate the effects of cathodic potentials (Tables 4.6–4.7, Figure 4.6) and cathode current feeder materials (Table 4.7, Figure 4.14a) including the polarization tests (Figure 4.7).  55  In electrocatalysis, the limitations of the operating temperature for aqueous solutions can be compensated by the possibility to enhance the reaction rates by tuning the applied potential.48 In a recent work on ECH of benzaldehyde to benzyl alcohol, it was reported that the rates of ECH are affected by the coverages of reactants, the nature of the metal catalysts, and cathode potential; the increasing cathode potential promoted the hydrogenation rates because Hads coverages were increased and sustained by electric potential.39 Further ECH experiments were conducted by varying the cathodic potentials. In the acid-acid pair (Table 4.7, Entries 1–2), by increasing the overpotentials twofold, guaiacol conversion (73–92%), the yields of cyclohexanol (35–44%) and 2-methoxycyclohexanol (18–24%) were all increased after 4 h reactions; however, Faradaic efficiency decreased (from 46% to 40%). The increasing applied potentials resulted in the increasing currents and temperatures, thereby accelerating the reduction of guaiacol at the expense of the efficiencies (Figure 4.6) due to the accelerated hydrogen gas evolution. By the fourfold potential increase (from -0.5 V to -2.0 V), guaiacol conversion and cyclohexanol yield were increased threefold and fourfold, respectively. Temperature and current increased linearly over the lower potential (E < -1.0 V) but exponentially over the higher potential (E > -1.0 V).  Different working electrodes (Ti and Ni gauzes) were also tested in the acid-acid pair and the results show high guaiacol conversion (81–89%) and cyclohexanol yield (39–42%) could be obtained at the same conditions (Table 4.7, Entries 3–4). Polarization tests (Figure 4.7) revealed that Ti and Ni gauze can be an alternative for the expensive Pt gauze as both exhibited a comparably good polarizability in the acid-acid and neutral-acid pairs, especially at the high overpotentials (E < -2.5 V). Graphite rod can also be used for the electrode (cathode/anode), however, it is less stable owing to carbon corrosion issue, especially when employed as anode in acidic solution; therefore, it was not evaluated further in this study.  56  In the neutral-acid pair (Table 4.7, Entries 5–10), the increases in guaiacol conversion (37–80%), cyclohexanol (10–34%) and 2-methoxycyclohexanol (12–36%) yields were also clearly observed with the increasing overpotentials (-1.5 V to -2.5 V), either using Pt/Ti/Ni gauze. It was found experimentally that the increasing applied potential led to temperature increases in the both pairs; this can be attributed to Joule heating effect, in which the passage of electric charges through a conductive material generates heat, in addition to the exothermic heat of reactions. These effects may be decoupled using a robust cell with temperature control to better understand the correlation between current and temperature increases and their individual effects on the reaction rates. Some insights can be drawn from the electrolysis experiments using the non-jacketed H-cell: (i) the different pairs of electrolytes induced different current inputs leading to different temperatures, (ii) temperature increases were predominantly caused by Joule heating effect owing to potential increases which might accelerate HER, resulting in lower F.E. Overall, by changing the cathode, the hydrogenation reaction still proceeded with comparable conversion, yield, and F.E. results (only the currents slightly differed due to the nature of the cathode materials). This may suggest that the role of cathode in this case was mainly to provide electrons (current feeder) to be used for the reduction of protons into adsorbed H on the catalyst surface. The protons might be reduced on the charged catalyst particles before reaching to the cathode surface while the solution being agitated. The presence of stirred catalyst might interfere the diffusion of protons to the cathode surface to form H2, and the metal particles (Pt), having good affinity toward hydrogen, attracted hydrogen radicals (protons being reduced) resulting in adsorbed hydrogen (Hads) that readily reacted with the adsorbed organics. This way, the ECH may be promoted over HER. This also explains the advantage of SSER over fixed bed electrode configuration, in which the organic molecules should collide with the chemisorbed hydrogen at  57  the electrode surface to favor ECH over HER. In the SSER, the ECH proceeds at the well-dispersed catalyst slurry with more favorable liquid-solid mass transport and electron transfer through collisions between the reacting molecules and the catalyst particles.  Figure 4.6. Guaiacol conversion (X), product yield (Y), and Faradaic efficiency (F.E.) as a function of applied potential (a); Current density and temperature as a function of applied potential (b). Reactions were performed in the pair of acid – acid H2SO4 (0.2 M) electrolytes using 5 wt.%-Pt/C (0.10 g) for 4 h. Note that the current density is expressed in absolute (positive) value. Product yield: cyclohexanol (Y1), cyclohexanone (Y2), 2-methoxycyclohexanol (Y3), and 2-methoxycyclohexanone (Y4). Temperature increases were mainly attributed to the Joule heating effect. 01020304050607080900102030405060708090100-2.0 -1.0 -0.8 -0.5Faradaic Efficiency (%)Conversion, Yield (%)Potential (V vs. Ag/AgCl)X Y1 Y2 Y3 Y4 F.E. (a)0102030405060050100150200250300-2.0 -1.0 -0.8 -0.5Temperature (oC)Current Density (mA cm-2)Potential (V vs. Ag/AgCl)j (mA/cm²)T (⁰C)(b) 58   Figure 4.7. Polarization curves for three different cathode current feeders (Pt, Ti, and Ni gauze) in: (a) the pair of acid-acid electrolytes and (b) the pair of neutral-acid electrolytes. Acid: H2SO4 (0.2 M), neutral: NaCl (0.5 M). Cathode geometrical size: 2.5 cm × 1.1 cm.    -1.00-0.90-0.80-0.70-0.60-0.50-0.40-0.30-0.20-0.100.00-3.5 -3 -2.5 -2 -1.5 -1 -0.5 0Current, I (A)Potential, E (V) vs. Ag/AgClPt gauzeTi gauzeNi gauze-0.70-0.60-0.50-0.40-0.30-0.20-0.100.00-3.5 -3 -2.5 -2 -1.5 -1 -0.5 0Current, I (A)Potential, E (V) vs. Ag/AgClPt gauzeTi gauzeNi gauze(a)(b) 59  Table 4.6. Summary of the results from ECH of guaiacol (or 2-methoxycyclohexanone) in different pairs of aqueous electrolytes under potentiostatic control. Entry Catholyte  (c) Anolyte  (a) E (V) 𝜂 cathode  (V) j  (mA cm-2) T (oC)  pH  (c) pH  (a) X (%) S1 (%) S2 (%)  S3 (%)  S4 (%)  S5 (%)  F.E. (%)  1 HClO4 (0.2 M) HClO4 (0.2 M) -2.0 -1.94 -291 54 0.89 0.69 84.90 38.42 16.01 17.03 18.49 6.84 33.40 2* HClO4 (0.5 M) HClO4 (0.5 M) -1.0 -0.97 -219 34 0.46 0.31 91.35 53.92 0.00 30.95 6.46 8.41 53.91 3** HClO4 (0.5 M) HClO4 (0.5 M) -1.0 -0.97 -218 33 0.53 0.38 80.95 53.09 0.00 31.08 5.34 9.93 48.34 4 Buffer (0.5 M) HClO4 (0.2 M) -2.0 -1.87 -10 23 2.25 n.d. 0.00 0.00 0.00 0.00 0.00 0.00 0.00 5 HClO4 (0.2 M) Buffer (0.5 M) -2.0 -1.95 -8 23 0.80 n.d. 0.00 0.00 0.00 0.00 0.00 0.00 0.00 6 Buffer (0.5 M) HClO4 (0.5 M) -5.0 -4.89 -73 36 1.77 0.39 52.38 34.88 10.92 21.57 20.48 10.65 69.41 7 HClO4 (0.5 M) Buffer (0.5 M) -5.0 -4.97 -29 25 0.47 1.63 26.97 19.35 18.46 11.35 13.53 33.19 66.32 8Δ H2SO4 (0.2 M) H2SO4 (0.2 M) -2.0 -1.96 -348 60 0.67 0.56 94.47 9.40 0.00 79.95 n.a. 10.65 23.03 9Δ H2SO4 (0.2 M) H2SO4 (0.2 M) -1.0 -0.96 -128 30 0.74 0.67 64.25 6.47 0.00 80.76 n.a. 12.77 28.26 E = cathode potential (vs. Ag/AgCl), 𝜂  = calculated cathode overpotentials (based on H2 evolution reaction), j = current density, T = temperature, X = guaiacol conversion, S = normalized selectivity (C mol%)  for cyclohexanol (1), cyclohexanone (2), 2-methoxycyclohexanol (3), 2-methoxycyclohexanone (4), and methanol (5), F.E. = Faradaic efficiency. Catalyst = 5wt.%-Pt/C (0.10 g, *0.15 g, **spent); Reaction time = 4 h; pH was measured at the end of reaction; Buffer: potassium phosphate (KH2PO4); Δapplied for reaction with 2-methoxycyclohexanone (Entry 8 was performed for 2 h); n.d. = not determined, n.a. = not applicable.    60  Table 4.7. Summary of the results from ECH of guaiacol using different cathode materials at various applied potentials. Entry C-A pair Cathode E (V) 𝜂 cathode  (V) j  (mA cm-2) T  (oC) pH  (c) pH  (a) X (%) S1 (%) S2  (%) S3 (%)  S4  (%) S5  (%) S6 (%) F.E. (%)  1 Acid-acid Pt gauze -2.0 -1.96 -284 53 0.69 0.51 92.32 47.48 2.89 25.41 14.48 8.58 1.16 39.50 2 Acid-acid Pt gauze -1.0 -0.95 -138 32 0.88 0.84 72.74 48.27 4.51 24.13 12.35 9.77 0.98 46.08 3 Acid-acid Ti gauze -2.0 -1.94 -265 50 0.88 0.70 81.03 47.99 6.41 19.28 15.75 9.32 1.26 37.50 4 Acid-acid Ni gauze -2.0 -1.95 -269 51 0.84 0.72 89.01 47.14 6.98 20.67 15.52 7.93 1.76 39.90 5 Neutral-acid Pt gauze -2.5 -2.00 -164 38 8.09 0.77 80.36 42.65 1.32 44.87 2.46 7.48 1.22 61.70 6 Neutral-acid Ti gauze -2.0 -1.84 -109 30 2.63 0.58 66.89 35.22 1.70 43.82 13.38 5.61 0.27 77.62 7 Neutral-acid Ti gauze -1.5 -1.40 -55 24 1.72 0.64 39.04 32.35 3.84 35.27 20.88 6.97 0.69 85.32 8 Neutral-acid Ni gauze -2.0 -1.87 -116 31 2.17 0.68 69.39 33.62 3.06 41.49 14.96 6.59 0.29 73.05 9 Neutral-acid Ni gauze -1.5 -1.40 -47 24 1.76 0.76 37.38 27.88 5.08 32.16 24.29 9.07 1.52 95.31 10* Neutral-acid Pt gauze -2.5 -2.36 -184 40 2.25 0.64 79.00 36.32 4.28 41.65 10.97 6.43 0.35 35.83 E = cathode potential (vs. Ag/AgCl), 𝜂  = calculated cathode overpotentials (based on H2 evolution reaction), j = current density, T = temperature, X = guaiacol conversion, S = normalized selectivity (C mol%) for cyclohexanol (1), cyclohexanone (2), 2-methoxycyclohexanol (3), 2-methoxycyclohexanone (4), methanol (5), and phenol (6), F.E. = Faradaic efficiency. C-A (catholyte-anolyte) pair: Acid = H2SO4 (0.2 M), Neutral = NaCl (0.5 M). Catalyst = 5 wt.%-Pt/C (0.10 g), *over spent catalyst (~6 h); Reaction time = 4 h; pH of catholyte (c) and anolyte (a) were measured at the end of reaction.   61  4.5. Galvanostatic ECH of guaiacol and phenol 4.5.1. Effect of different catholyte-anolyte pairs at constant current and temperature The effects of different catholyte-anolyte pairs (i.e. acid-acid, neutral-acid, and base-acid) on the guaiacol ECH were evaluated under constant superficial current density (-109 mA cm-2) and temperature (50 oC) conditions (selected results presented in Table 4.8 and Figure 4.8). Guaiacol conversion decreased as: acid-acid (38%) > neutral-acid (36%) > acid-base (16%) > base-acid (0 %). Faradaic efficiency (F.E.) decreased as: neutral-acid (94%) > acid-acid (82%) > acid-base (27%) > base-acid (0%). The base-acid pair resulted in poor performance (zero guaiacol conversion to hydrogenation products), consistent with the results from potentiostatic experiments (Table 4.3, Entry 5). Instantaneous guaiacol deprotonation was also observed here as the catholyte color turned dark yellowish (Figure 4.4). The protons from the anode compartment generated also by water oxidation (OER) and transported through the Nafion membrane were fast neutralized by the hydroxide ions at the cathode (produced also by water reduction), thereby hampering the reduction of protons and ECH reactions, as confirmed by the constant high pH of catholyte (13.2). Under alkaline conditions, guaiacol is prone to deprotonation, which hinders its adsorption onto the catalyst.34 Another possible explanation may also be due to the instability of Pt/C in alkaline medium, which modifies the anchoring sites of the Pt nanoparticles on the carbon surface, resulting in detachment/loss of the metal.79    The current was stable whenever acidic anolyte was used, however, in the acid-base pair the current dropped significantly (Table 4.8, Entry 4). Under this condition, it was not possible to maintain the current constant, hence implying that the alkaline anolyte was not favorable for ECH purposes since proton transport across the membrane was limited. Nevertheless, marginal guaiacol  62  conversion (17%) was achieved, suggesting the protons resulting from acid dissociation on the cathode were also involved in the reaction. The highest F.E. obtained with the neutral-acid pair, indicating that the protons for ECH reactions were mostly supplied from the anode side. The proton flux from the anode to the cathode through the membrane is related to the applied current density. Thus, under galvanostatic operation a controlled flux of protons is assured. Similar trends were also observed in the ECH of phenol (Table 4.9) and the ECH under potentiostatic conditions (Tables 4.3–4.4, Figure 4.3), implying the consistent electrolyte effect on the overall ECH process.   Furthermore, the conversion and selectivity profiles over extended reaction time (4 h) show differences between galvanostatic and potentiostatic ECH, respectively (compare Figure 4.8a-b vs. 4.3b-c). The stark contrast can be seen especially in the acid-acid pairs, caused by the large differences in the cathodic potential (-0.7 V vs. -2.0 V) between the two types of experiments. In galvanostatic experiments due to lower cathode potential of -0.7 V, higher selectivities of cyclohexanone (24–32%) and 2-methoxycyclohexanone (22–25%) were obtained (Figure 4.8a). This could be attributed to the lower adsorbed hydrogen (Hads) surface coverages, hence the full hydrogenation reactions generating cyclohexanol and 2-methoxycyclohexanol, respectively were hampered. In contrast, under potentiostatic reduction at -2.0 V, the cyclohexanol and 2-methoxycyclohexanol selectivities were higher (Figure 4.8a vs. 4.3b). Thus, these results indicate that more negative cathodic potentials increase the Hads coverages, as also reported by Song et al.81 in the ECH of benzaldehyde over carbon-supported metals, favoring complete hydrogenation (ring saturation). Note that, in addition to cathodic potential, temperature also synergistically affected the product distribution, for instance as shown by the remarkably higher cyclohexanone selectivities in the galvanostatic conditions (24–32% in acid-acid, 5–32% in neutral-acid) as  63  compared to those obtained in the potentiostatic conditions (2–6% in acid-acid and 4–13% in neutral-acid).   The time-dependent profiles for the guaiacol ECH suggest clear evidence that each route is a series reaction, as indicated by maxima in the intermediates (Figure 4.8a-b). Comparing the product distribution for ECH of guaiacol in acid and neutral catholytes (Table 4.8, Entries 1–2 and Figure 4.8a-b), it is noteworthy that the reaction pathway was also affected by the catholyte pH. The catholyte pH profiles were dramatically different between the acid-acid and neutral-acid pairs (Figure 4.8c vs. 4.8d). In the former case, the pH was relatively constant (0.7–0.8), however, in the latter case, the pH of NaCl (0.2 M) initially dropped to 2.1 due to proton diffusion from the anolyte during dissolution of guaiacol without current applied (at t = 0). Once the electrolysis started, the pH increased over time and after 4 h, the final pH was 9.2 due to water reduction to H2 and OH-. The F.E. gradually decreased from near 100% to 70% in 4 h, but it was still higher than in the case of acid-acid pair.   For the sake of comparison, similar general trends were also observed for phenol ECH (Figure 4.9). Phenol conversion, which proceeds in a series reaction, was found to be higher than guaiacol in either of the electrolyte pair (Figure 4.8 vs. 4.9, see also Figure 4.3). At the fixed current density (j = -109 mA cm-2) and temperature (50 oC), the acid-acid pair gave lower cathode potential (-0.7 V) than the neutral-acid pair (-2.3 V), thus cyclohexanone selectivity was higher than cyclohexanol (Figure 4.9a vs. 4.9b). It is noteworthy that phenol conversion was always higher than guaiacol under galvanostatic (Table 4.8 vs. 4.9) as well as potentiostatic conditions (Table 4.3 vs. 4.4). This applies also for the alkaline catholyte where a marginal phenol conversion (~10%) was still obtained (Table 4.4, Entries 4–5). Remarkably, phenol was fully converted to cyclohexanol in the acid-acid pair (only after 2 h) and the neutral-acid pair (after 4 h), clearly showing that phenol is  64  more reactive (than guaiacol) toward hydrogenation (Figure 4.3). This observation is also supported by phenol scavenging effect on hydrogen radicals.36 Obviously, the higher functionalities in the aromatic compounds render more difficulties in converting the substrates. It was recently reported by Garedew et al.13 regarding the effects of functional group types on the reactivity, suggesting that the more complex the functional group, the lower the conversion.   Figure 4.8. Galvanostatic ECH of guaiacol in the two most effective catholyte-anolyte pairs: acid-acid and neutral-acid. Guaiacol conversion and product normalized selectivity (C mol%) in (a) acid-acid and (b) neutral-acid. Catholyte pH and Faradaic efficiency in (c) acid-acid and (d) neutral-acid; at t = 0 the pH is determined by proton diffusion from the anolyte during the pre-electrolysis (mixing) period. Acid: H2SO4 0.2 M, Neutral: NaCl 0.2 M. Conditions: j = -109 mA cm-2, T = 50 oC, t = 4 h. Initial guaiacol concentration: 0.1 M. Catalyst: 5 wt.%-Pt/C (0.133 g). All the experiments were performed in the jacketed H-cell. Other conditions are given in Table 4.8. 010203040500 1 2 3 4Conversion, Selectivity (%)Time (h)GuaiacolCyclohexanolCyclohexanone2-Methoxycyclohexanol2-MethoxycyclohexanonePhenolMethanol01020304050607080901000123456789100 1 2 3 4Faradaic Efficiency (%)pH of catholyteTime (h)pH of catholyteF.E. (%)010203040500 1 2 3 4Conversion, Selectivity (%)Time (h)GuaiacolCyclohexanolCyclohexanone2-Methoxycyclohexanol2-MethoxycyclohexanonePhenolMethanol01020304050607080901000123456789100 1 2 3 4Faradaic Efficiency (%)pH of catholyteTime (h)pH of catholyteF.E. (%)(a)(b)(c)(d) 65   Figure 4.9. Galvanostatic ECH of phenol in the two most effective catholyte-anolyte pairs: acid-acid and neutral-acid. Phenol conversion and product selectivity over time in acid-acid (a) and neutral-acid (b). Time-dependent profiles of catholyte pH and Faradaic efficiency in acid-acid (c) and neutral-acid (d). Acid: H2SO4 (0.2 M), Neutral: NaCl (0.2 M). Conditions: I = -0.3 A, T = 50 oC, t = 4 h. Initial phenol concentration: 0.1 M. Catalyst: 5 wt.%-Pt/C (0.125 g). All the experiments were performed in the jacketed H-cell. Other conditions are given in Table 4.9.0102030405060700 1 2 3 4Conversion, Yield (%)Time (h)PhenolCyclohexanolCyclohexanone(a)01020304050607080901000123456789100 1 2 3 4Faradaic Efficiency (%)pH of catholyteTime (h)pH of catholyteF.E. (%)010203040506070800 1 2 3 4Conversion, Yield (%)Time (h)PhenolCyclohexanolCyclohexanone01020304050607080901000123456789100 1 2 3 4Faradaic Efficiency (%)pH of catholyteTime (h)pH of catholyteF.E. (%)(b)(c)(d) 66  Table 4.8. ECH of guaiacol in different catholyte-anolyte pairs under galvanostatic control. Entry Catholyte  (c) Anolyte  (a) E  (V) pH  (c) pH  (a) X  (%) S1  (%) S2 (%)  S3  (%) S4 (%)  S5  (%)  S6 (%) C.B. (%) F.E. (%)  rx,0 (mol s-1 g-1) 1 H2SO4  H2SO4  -0.7 0.66 0.54 37.88 19.55 30.44 10.79 25.46 9.52 4.23 93.07 82.02 1.67×10-4 2 NaCl H2SO4  -2.4 8.18 0.64 36.36 28.00 16.56 26.79 18.55 8.78 1.32 94.44 93.99 1.58×10-4 3 NaOH H2SO4  -2.3 13.21 0.70 0.00 0.00 0.00 0.00 0.00 0.00 0.00 96.22 0.00 0.00 4* H2SO4 NaOH  -0.2 0.87 6.01 16.39 5.96 29.65 6.93 20.29 9.46 27.71 95.33 27.33 8.64×10-5  E = cathode potential (vs. Ag/AgCl), X = guaiacol conversion, S = normalized product selectivity (C mol%) for cyclohexanol (1), cyclohexanone (2), 2-methoxycyclohexanol (3), 2-methoxycyclohexanone (4), methanol (5), and phenol (6), F.E. = Faradaic efficiency, rx,0 = initial reaction rates (mol s-1 gmetal-1) by t = 30 min. Conditions: T = 50 oC, I = -0.3 A (j = -109 mA cm-2), *except for Entry 4 (I = -0.01 A); Catalyst = 5 wt.%-Pt/C (0.133 g); Reactant to metal molar ratio (R/M) = 315; Reaction time = 2 h; Concentration of all electrolytes: 0.2 M; pH was measured at the end of reaction.      67  Table 4.9. ECH of phenol in different catholyte-anolyte pairs under galvanostatic control. Entry Catholyte  (c) Anolyte  (a) E  (V) pH  (c) pH  (a) X (%) S1  (%) S2  (%)  C.B. (%) F.E. (%)  rx,0 (mol s-1 g-1) 1 H2SO4  H2SO4  -0.7 0.71 0.63 38.95 23.73 76.27 95.37 82.74 1.74×10-4 2 NaCl H2SO4  -2.3 8.05 0.65 39.52 25.67 74.33 99.89 92.39 1.62×10-4 3 NaOH H2SO4  -2.0 13.18 0.65 0.00 0.00 0.00 95.48 0.00 0.00 4* H2SO4 NaOH  -0.2 0.94 8.39 17.80 15.33 84.67 97.83 25.49 8.85×10-5  E = cathode potential (vs. Ag/AgCl), X = phenol conversion, S = normalized product selectivity (C mol%) for cyclohexanol (1), cyclohexanone (2), C.B. = carbon balance, F.E. = Faradaic efficiency, rx,0 = initial conversion rates (mol s-1 gmetal-1), after 30 mins. Conditions: T = 50 oC, I = -0.3 A (j = -109 mA cm-2), *except for Entry 4 (I = -0.01 A), Catalyst = 5 wt.%-Pt/C (0.125 g); Reactant to metal molar ratio (R/M) = 332; Reaction time = 2 h; Concentration of all electrolytes = 0.2 M; pH was measured after the reaction.   68  4.5.2. Impact of acid concentration, current density, and temperature The ECH of guaiacol was further investigated in the pair of acid (H2SO4) electrolytes using a jacketed cell under galvanostatic control to evaluate the effects of acid concentration (0.2–1.0 M), cathode superficial current density (-109 to -255 mA cm-2 corresponding to current loads of -60 to -140 A g-1Pt) and temperature (30–60 oC).  Increasing the cathode superficial current density to -182 mA cm-2 requires increased catholyte acid concentrations (e.g., 1 M) to sustain the F.E. at (or above) 40% after 5 h of electrolysis (Figure 4.10a). The positive effect of acid concentration on guaiacol conversion and Faradaic efficiency was also observed under potentiostatic conditions (Figure 4.5). Enhanced guaiacol conversion and cyclohexanol selectivity was obtained when the superficial current density was increased at a constant acid concentration and temperature (Figure 4.10b). At the highest current density (-255 mA cm-2), cyclohexanol was the most dominant product (selectivity ~38%) and the guaiacol conversion was 77% after 5 h reaction (Figure 4.10b). However, these improvements were obtained at the expense of F.E. decrease (from 48% to 37% after 5 h), presumably due to the enhanced hydrogen evolution reaction (HER). The optimal combination of current density and acid concentration must provide sufficient surface coverage of adsorbed hydrogen radicals (Hads) generated by the Volmer step (H+ + e- → Hads) to sustain high guaiacol conversion while minimizing either the Tafel (Hads + Hads → H2,(g)) or Heyrovsky steps (H+ + Hads + e-  → H2,(g)) leading to hydrogen gas evolution. The undesirable shift toward HER as the organic reactants being consumed was also reported elsewhere.13  Higher initial guaiacol concentration would then expectedly result in higher F.E. because more organic substrates were available (i.e. higher reactant surface coverage) and this was confirmed either by galvanostatic or potentiostatic experiments (Figure 4.11). As the conversion increased,  69  F.E. decreased because fewer catalytic active sites were occupied by the organics. This can be clearly seen in the guaiacol conversion and Faradaic efficiency time-dependent profiles from the selected experiments (Figure 4.10d-f). Surface coverage of both organic molecules and atomic hydrogen is crucial to promote the ECH over HER.  Interestingly, at higher temperature (60 oC), as shown by Figure 4.10c (see also Figure 4.10f), the demethoxylation route was more favored than the ring saturation as indicated by the higher cyclohexanone selectivity (35%) and much lower 2-methoxycyclohexanol selectivity (7.5%) compared to the results obtained at lower temperatures (30–40 oC). The higher temperature promoted the desorption of Hads, thus, lowering the surface coverage and changing the reaction pathway. Moreover, the increased temperature (∆T = 30 oC) decreased the cathode potential by 100 mV (in absolute value) at a constant current density, which also affected the Hads coverage. Temperature effect can also be explained by reaction kinetics of the guaiacol ECH (discussed separately in Section 6.2.4). Both cyclohexanone and cyclohexanol, the mixture of which is known as ketone-alcohol (KA) oil, represent important chemicals for the synthesis of nylon precursors, such as caprolactam for nylon 6 and adipic acid for nylon 66.63,64 Overall, the galvanostatic experiments demonstrated that the reactor can be efficiently operated at high superficial current density and catalyst mass activity of -109 to -225 mA cm-2 and -60 to -140 A g-1Pt, respectively. Temperature control is evidently required to enable selectivity control as the current/potential and temperature could synergistically affect the Hads coverage, leading to the higher selectivity for cyclohexanone. Without temperature control, the product distribution from guaiacol ECH was dominated by cyclohexanol and 2-methoxycyclohexanol, regardless of the cathode potential and electrolyte type (Figures 4.3, 4.5, and 4.6, see also Tables 4.6–4.7).   70   Figure 4.10. The effect of (a) acid concentration, (b) superficial cathode current density, and (c) temperature on the ECH of guaiacol using the catholyte-anolyte pair of H2SO4 electrolytes after 5 h of electrolysis. Guaiacol conversion, product selectivity, and Faradaic efficiency profiles as function of reaction time from the corresponding experiments (d, e, f). Operating conditions: (a) j = -182 mA cm-2 and T = 50 oC, (b) [H2SO4] = 0.5 M and T = 50 oC, (c) [H2SO4] = 0.5 M and j = -182 mA cm-2. All the experiments were performed in the jacketed H-cell using 5 wt.%-Pt/C (0.10 g) under galvanostatic control. Initial guaiacol concentration: 0.1 M.  0102030405060010203040506070800.2 0.5 1Faradaic Efficiency (%)Conversion, Selectivity (%)Acid concentration (M)(a)0102030405060708090100010203040506070800 50 100 150 200 250 300Faradaic Efficiency (%)Conversion, Selectivity (%)Time (min)(d)0102030405060708090100010203040506070800 50 100 150 200 250 300Faradaic Efficiency (%)Conversion, Selectivity (%)Time (min)(e)0102030405060708090100010203040506070800 50 100 150 200 250 300Faradaic Efficiency (%)Conversion, Selectivity (%)Time (min)Guaiacol CyclohexanolCyclohexanone 2-Methoxycyclohexanol2-Methoxycyclohexanone MethanolPhenol F.E. (%)(f)01020304050600102030405060708030 40 50 60Faradaic Efficiency (%)Conversion, Selectivity (%)Temperature (oC)Cyclohexanol Cyclohexanone2-Methoxycyclohexanol 2-MethoxycyclohexanoneMethanol PhenolConversion F.E. (%)(c)010203040506001020304050607080109 182 255Faradaic Efficiency (%)Conversion, Selectivity (%)Current density (mA cm-2)(b) 71   Figure 4.11. The dependence of guaiacol conversion rates on the initial substrate concentration (CGUA). Conversion, selectivity, and Faradaic efficiency over different guaiacol concentration. (a) Galvanostatic conditions: j = -182 mA cm-2, Ecathode = -0.6 V (vs. Ag/AgCl), T = 50 oC, Electrolyte: H2SO4 (0.5 M), Catalyst: 5 wt.%-Pt/C (0.10 g), t = 5 h.  (b) Potentiostatic conditions: Ecathode = -1.25 V (vs. Ag/AgCl), I = -0.5 to -0.6 A (corresponding to j = -182 to -218 mA cm-2), T = 39 oC (average), Electrolyte: H2SO4 (0.2 M), Catalyst: 5 wt.%-Pt/C (0.10 g), t = 4 h.   051015202530354045010203040506070809071 89 106Faradaic Efficiency (%)Conversion, Selectivity (%)Initial CGUA (mM)010203040506070010203040506070809010056 78 99 121Faradaic Efficiency (%)Conversion, Selectivity (%)Initial CGUA (mM)Guaiacol CyclohexanolCyclohexanone 2-Methoxycyclohexanol2-Methoxycyclohexanone MethanolPhenol F.E. (%)(a)(b) 72  Graphical kinetic analysis for ECH of guaiacol: Basic approach: for a reaction: 𝐴 → 𝐵, the rate of reaction: 𝑑𝐶𝐴𝑑𝑡= −𝑘𝐶𝐴𝛼  Linear form Rate constant T (oC) R2 Zeroth order 𝐶𝐴 = 𝐶𝐴0 − 𝑘𝑡 𝑘 =𝐶𝐴0 − 𝐶𝐴𝑡 ;  [𝑀 𝑠−1] 30 40 50 60 0.8973 0.9535 0.9304 0.8964 First order 𝑙𝑛𝐶𝐴 = 𝑙𝑛𝐶𝐴0 − 𝑘𝑡 𝑘 =𝑙𝑛(𝐶𝐴0 𝐶𝐴⁄ )𝑡 ; [𝑠−1] 30 40 50 60 0.9448 0.9896 0.9812 0.9634 Second order 1 𝐶𝐴⁄ = 1 𝐶𝐴0⁄ + 𝑘𝑡 𝑘 =1 𝐶𝐴⁄ − 1 𝐶𝐴0⁄𝑡 ; [𝑀−1𝑠−1] 30 40 50 60 0.9758 0.9970 0.9999 0.9954   Figure 4.12. Graphical kinetic analysis for determination of reaction order for ECH of guaiacol, showing profile for (a) zeroth order, (b) first order, and (c) second order. The best linear regression data fit is shown for the second order reaction. A linear plot (d) to estimate the activation energy (Ea) based on Arrhenius equation: k = A exp(-Ea/RT). Experiment conditions: [H2SO4] = 0.5 M, I = -0.5 A (j = -182 mA cm-2), Catalyst = 5 wt.%-Pt/C (0.10 g), Initial guaiacol concentration = 0.106 M.   0204060801001200 50 100 150 200 250 300C GUA(mM)Time (min)30 ⁰C40 ⁰C50 ⁰C60 ⁰C3.23.43.63.844.24.44.64.80 50 100 150 200 250 300ln[CGUA]Time (min)30 ⁰C40 ⁰C50 ⁰C60 ⁰C0.0000.0050.0100.0150.0200.0250.0300.0350 50 100 150 200 250 3001/CGUATime (min)30 ⁰C40 ⁰C50 ⁰C60 ⁰Cy = -2.8476x + 1.7909R² = 0.9996-7.7-7.6-7.5-7.4-7.3-7.2-7.1-7.0-6.9-6.8-6.72.95 3 3.05 3.1 3.15 3.2 3.25 3.3 3.35ln k1/T (×10-3)(a) (b)(c) (d) 73  Graphical kinetic analysis for ECH of phenol:  Figure 4.13. Graphical kinetic analysis for determination of reaction order for ECH of phenol. The best linear fit is shown for the zeroth order reaction. Experiment conditions: [H2SO4] = 0.2 M, E = -1 V (vs. Ag/AgCl), I = -0.36 A (j = -131 mA cm-2), T = 32 oC, Catalyst = 5 wt.%-Pt/C (0.10 g), Initial phenol concentration = 0.105 M.     01020304050607080901000 1 2 3 4Conversion, Yield (%)Time (h)PhenolCyclohexanolCyclohexanoney = -28.771x + 104.03R² = 0.99860204060801001200 1 2 3 4CP(mM) Time (h)0th ordery = -0.564x + 4.7745R² = 0.957201234560 1 2 3 4ln CPTime (h)1st ordery = 0.0139x + 0.0037R² = 0.83110.000.010.020.030.040.050.060 1 2 3 41/CPTime (h)2nd order 74  A graphical kinetic analysis revealed the ECH of guaiacol in this experiment appears to best fit a second order reaction with an activation energy of 23.7 kJ mol-1 (Figure 4.12). The activation energy is slightly lower than that reported for ECH of phenol on Pt/C (28.9 kJ mol-1).36 This difference is reasonable because the reaction conditions were also different, the phenol ECH was performed in acetic acid (pH = 5) at -0.75 V vs. Ag/AgCl, while the guaiacol ECH was carried out in 0.5 M H2SO4 (pH = 0.37) at around -0.65 V vs. Ag/AgCl corresponding to a superficial current density of -182 mA cm-2. The rate constant for guaiacol consumption (expressed in L mol-1 s-1) increased with temperature in the following order: 5.0×10-4 (30 oC) < 6.7×10-4 (40 oC) < 8.8×10-4 (50 oC) < 1.2×10-3 (60 oC). In contrast to guaiacol, phenol ECH was found to be a zeroth order reaction (with respect to phenol) at the applied conditions in this work (Figure 4.13) and reported elsewhere,36 implying that phenol conversion rate is independent of the reactant concentration because phenol is highly reactive toward hydrogenation (acts as the H scavenger, allowing faster adsorption and reduction of protons).  A plausible explanation for the second-order reaction of guaiacol is that adsorption of guaiacol on Pt/C could be the rate determining step. The adsorption rate equation may appear second-order with respect to the adsorbate surface concentration as proposed originally by Blanchard et al.82,83 Unlike phenol, guaiacol conversion rate is dependent on the initial guaiacol concentration (Figure 4.11). ECH of guaiacol may be interpreted as first- or second-order according to marginal differences in the correlation coefficient and depending on the operating conditions (i.e. initial reactant concentration, stirring rates, catalyst loading, etc.). The apparent reaction order is not an intrinsic parameter as it depends on the reaction conditions. Note that doubling reactant concentration did not simply result in a quadruple reaction rate because guaiacol ECH is adsorption-controlled reaction where the rate constant also varied with the reactant concentration.  75  Thermal hydrodeoxygenation (HDO) of guaiacol to phenol, catechol, and cyclopentanone over Pt/C at much higher temperatures (275–325 oC) was also reported to be second-order with respect to guaiacol.82 At elevated temperatures (350–400 oC) using external H2 (4 MPa), benzene and phenol were the most dominant products on carbon-supported metal catalysts such as Ru/C and Mo/C in a continuous pack-bed reactor.84 However, at 160 oC under 1.5 MPa H2, demethoxylation and ring saturation products (e.g., cyclohexanol, 2-methoxycyclohexanol and methanol) were obtained over Ru/C–MgO catalyst.85 These results clearly suggested that full deoxygenation of guaiacol to benzene proceeds more favorably at much higher temperatures (>200 oC) via a thermocatalytic process. This work, on the other hand, demonstrates that effective ring saturation and partial deoxygenation of guaiacol to cyclohexanol can be achieved via a mild ECH process.  4.6. Catalyst reusability tests in ECH of guaiacol Catalyst durability tests and characterization results are discussed separately here. Experiments with fresh catalyst were conducted in acid-acid and base-acid (catholyte-anolyte) pairs and the catalyst was recovered after 4 h reaction, denoted as spent A (acid-treated) and spent B (base-treated), respectively. After a simple treatment, i.e. being washed and dried overnight (70 oC), the spent catalysts were retested for the ECH of guaiacol and retained the activity (Figure 4.14b). The Pt/C catalysts underwent morphology and surface area changes during 4 h reaction in either acidic or alkaline conditions (Table 4.10, Figures 4.15–4.16). Compared to the results using the fresh catalyst, the spent A catalyst resulted in comparable guaiacol conversion (79% vs. 80%) and yields of cyclohexanol (29% vs. 34%), and 2-methoxycyclohexanol (33% vs. 36%) after slightly prolonged reaction time (6 h) as shown in Table 4.7 (Entry 10 vs. 5). The spent B catalyst showed a slightly decreased activity, resulting in lower guaiacol conversion (81% vs. 91%) and yields of  76  cyclohexanol (43% vs 49%) and 2-methoxycyclohexanol (25% vs. 28%) as shown in Table 4.6 (Entry 2 vs. 3). The spent Pt/C catalysts were reusable in spite of the morphological changes.  Figure 4.14. (a) Cathode current feeder effect on guaiacol conversion, product selectivity, and Faradaic efficiency. Conditions are shown in Table 4.7 (Entries 1, 3, 4); (b) Comparison on the electrocatalytic performance of the Pt/C catalyst: fresh vs. spent (A – in acid environment, B – in basic environment). Detailed conditions are shown in Table 4.6 (Entries 2–3) and Table 4.7 (Entries 5, 10). Figure (a) confirms that the role of the current feeder is mainly to provide electrons to the catalyst bed while ECH proceeds on the surface of Pt/C. Figure (b) shows that spent Pt/C catalyst retains ECH activity. 0102030405060708090100Pt Ni TiConversion, Selectivity, F.E. (%)Working ElectrodeConversion CyclohexanolCyclohexanone 2-Methoxycyclohexanol2-Methoxycyclohexanone MethanolPhenol F.E. (%)0102030405060708090100Fresh Spent B Fresh Spent AConversion, Selectivity, F.E. (%)Catalyst (5 wt.%-Pt/C)Acid-acid Neutral-acid(a)(b) 77    Figure 4.15. Adsorption-desorption isotherms for the carbon-supported Pt (5 wt.%) catalysts using N2 physisorption: fresh/untreated (●), after use for ECH in H2SO4 electrolyte (Spent A), (■), after use for ECH in NaOH (Spent B) (▲). Table 4.10. Characteristics of the carbon-supported Pt catalysts used in this study.  Catalyst SBETa  (m2/g) Vporeb (cm3/g) Dporec  (nm) φ  (%) Smetal  (m2/g)  dp (nm)  Pt/C (fresh) 1487 1.43 4.35 28.95 71.50 3.91 Pt/C (spent-A) 1025 1.07 4.37 11.49 28.38 9.86 Pt/C (spent-B) 1128 1.16 4.43 6.73 16.61 16.84 Spent A – catalyst used in acidic conditions, spent B – catalyst used in alkaline conditions. Pt metal content was estimated to be 10% of the original content after the ECH reactions, based on the literature79 and TEM results.  a BET surface area;  b Single point adsorption total pore volume (P/P0 = 0.99); c Average pore diameter by BJH desorption; φ (metal dispersion), Smetal (metal surface area), and dp (active particle diameter) determined by CO chemisorption.  010020030040050060070080090010000.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0Volume adsorbed (cm3g-1)Relative pressure (P/P0)FreshSpent ASpent B 78   Figure 4.16. TEM micrographs for the 5 wt.%-Pt/C samples: (a) original/fresh, before use in ECH reaction, (b) spent-A, after ECH in H2SO4 solution, (c and d) spent-B, after ECH in NaOH solution. The Pt particles in the fresh sample are well dispersed on the carbon support while agglomeration and nanoparticles loss are identified for the acid- and alkali-treated samples, which were also reported elsewhere.79 The spent catalysts were however still active in the subsequent ECH experiments using acid-acid and neutral-acid catholyte-anolyte pairs, likely by the in situ, continuous reduction that activates the catalyst.    (a) (b)(c) (d) 79  4.7. Summary of the chapter Electrocatalytic hydrogenation-hydrogenolysis (ECH) of guaiacol was successfully conducted at mild conditions (25–60 oC, 1 atm) using a membrane-divided stirred slurry electrochemical reactor (SSER) configuration with dispersed 5 wt.% Pt/C in the cathode compartment. Different pairs of catholytes and anolytes were studied using either potentiostatic or galvanostatic control, to determine the electrolyte effect on guaiacol conversion, product distribution, and Faradaic efficiency. The most effective pairs were the acid-acid and neutral-acid catholyte-anolyte combination, whereas in alkaline catholytes the rate of guaiacol ECH was virtually zero owing to deprotonation and extremely low catalytic activity.  Under galvanostatic control, the neutral-acid pair gave higher Faradaic efficiencies and cyclohexanol selectivities compared to the acid-acid pair after 4 h electrolysis at 50 oC and superficial cathode current density of -109 mA cm-2. The NaCl catholyte pH (between 2.1 and 9.2 after 4 h) was determined by the proton transport (diffusion and migration across the membrane) from the anolyte and proton electroreduction on Pt/C (or OH- generation by water electroreduction at higher pH), respectively.  The use of less acidic catholyte opens the possibility of employing non-precious metal catalysts even though Pt/C slurry could also be a viable option at low loadings. In this work, the molar ratio of guaiacol to Pt was typically 315, meaning that only very small amount of Pt (i.e. low Pt/C catalyst loading) was used. Furthermore, Pt/C has good durability in acidic environments and can be reused. Based on the graphical kinetic analysis, the ECH of guaiacol appeared to follow a first or second order reaction depending on the operating conditions. This interpretation is supported by the experimental calculations which showed that the reaction rates were dependent on the initial guaiacol concentration and controlled by the guaiacol adsorption step. The limitations of this  80  analysis are related to the complex reaction pathway of guaiacol ECH, involving parallel and series reactions, where it is arduous to model each elementary step in the intermediate reactions. Guaiacol conversion in the kinetic analysis was treated as an overall, single reaction without making a distinction between the first demethoxylation and ring saturation steps (see Figure 4.12). The kinetic parameters in this study (e.g., rate constant, reaction order, activation energy) describe only the apparent values, which depend on the operating conditions (e.g., reactant and/or catalyst concentration, stirring rates, electric current/potential).    The reaction pathways and product distributions were affected by synergistic interaction effects among the major variables such as proton concentration, temperature and cathode potential (or superficial cathode current density) influencing the Hads surface coverage, showing the possibility for product selectivity control. This work demonstrates that in a slurry reactor the ECH of lignin model compounds can be performed at high superficial cathode current densities and catalyst mass activities (e.g., -255 mA cm-2 and -140 A g-1Pt, respectively), therefore, contributing to the development of a mild process for catalytic conversion of lignin-derived substrates at industrially relevant operating conditions. Other advantages of the SSER configuration include: (i) ease of catalyst bed preparation eliminating the need for catalyst deposition on an electrically conductive substrate, (ii) favorable multi-phase mass and heat transfer conditions, and (iii) feasibility to operate the ECH at high current densities (> |100 mA cm-2|). Potential limitations of the SSER may be associated with catalyst erosion due to intense friction among the catalyst particles, possibility of non-uniform polarization of the electrocatalyst bed caused by loss of electric contact between the current feeder and particles, and need for separation and recovery of the catalyst from the liquid product. The future work will focus on further enhancing the practical feasibility of the electrochemical pathway. 81  Chapter 5: Synergy between Electrocatalyst and Electrolyte in Electrocatalytic Reduction of Phenolics Biomass substrate valorization via electrocatalytic hydrogenation-hydrogenolysis (ECH) is an attractive approach for selective production of organic chemicals. The electrocatalytic activity is strongly dependent on the surface coverage of adsorbed hydrogen radicals as a complex function of the catalytically active surface sites, electrolyte (pH and composition), and electrode potential. This work focused on investigating the performances of fresh carbon-supported metal catalysts (Pt/C, Ru/C, and Pd/C) in the ECH of guaiacol and phenol using a stirred slurry electrochemical reactor (SSER), as illustrated in Figure 5.1. The most effective catholyte-anolyte pairs (i.e. acid-acid and neutral-acid) were employed in the electrolysis at fixed current density and constant temperature. By pairing the neutral (NaCl) catholyte and the acidic (H2SO4) anolyte, the activity of acid intolerant catalyst, such as Ru/C, could be improved dramatically, suggesting the efficient synergy between electrocatalyst and electrolyte in the ECH of lignin model compounds.  Figure 5.1. Comparative study on the electrocatalytic performances of Pt/C, Ru/C, and Pd/C in ECH of phenolics. Activated carbon supportM M M M M M MH2OH+ OH-HH2 I(-) (+)Neutral/Acid AcidH+CatalystVElectrocatalytic reduction of guaiacolM = metal (Pt / Ru / Pd)e- 82  5.1. The synergistic effects between electrocatalyst and electrolyte In the SSER configuration with carbon supported metal catalysts, an effective ECH requires at least three conditions: (i) availability of electrons on the dispersed catalyst particles which implies good electric contact among the particles and current feeder, (ii) efficient reduction of protons on the catalyst particles to generate adsorbed hydrogen (Hads) on the surfaces of the catalytically active metal sites, and (iii) contact between reactant (organic) molecules and the negatively charged catalyst particles in the electric field. Our previous observations (in Table 4.2 and Figure 4.2) indicated that if one of the above-mentioned conditions was not met, the ECH could not proceed and virtually no hydrogenation products were identified.49  The synergy between electrocatalyst and electrolyte aims to maximize Faradaic efficiency (F.E.), which implies that the electrons are predominantly used for the ECH of organics over the hydrogen evolution reaction (HER). Concerning the electrolyte effect, it is noteworthy that the proton/water reduction and hydrogen evolution reactions proceed differently in acid, neutral, or base solutions (Table 5.1). The first step in the electrocatalytic H2 evolution is the formation of adsorbed hydrogen radicals (Volmer steps: R1 and R4), which play an important role in ECH of the organics. Hydrogen gas can be generated either via the electrocatalytic Heyrovsky step (R2 and R5), or via the thermocatalytic recombination of Hads, referred to as Tafel step (R3 or R6). In alkaline electrolytes, the HER kinetics is generally more sluggish than in acidic media due to an additional water dissociation step and the catalyst stability is poorer as well.58 Different catalyst and electrolyte properties would then expectedly determine the ECH efficiency. Experimental results on the electrocatalyst-electrolyte synergy are presented in the following sections.  83  Table 5.1. Plausible proton/water reduction and hydrogen evolution reactions in acid, neutral, or base solutions.26,86,87 Reaction Acid Neutral/Base Volmer H+ + e- + M → H·M (R1) H2O + M + e- → H·M + OH- (R4) Heyrovsky  H·M + (H+)aq + e- → H2 + M (R2) H·M + H2O + e- → H2 + OH- + M (R5) Tafel H·M + H·M → H2 + 2M (R3) H·M + H·M → H2 + 2M (R6)  5.2. Platinum group metals for hydrogenation and catalyst characterization results Platinum group metals (PGM), mainly palladium, platinum, rhodium, and ruthenium, have been widely used for catalytic hydrogenation reactions.88,89 Most functional groups can be reduced under mild conditions over the PGM catalysts.88 The PGM catalyst activity is determined by two distinct but related quantities: electronic and geometric factors.89 The former is related to the number of vacant d-orbitals which can accept electrons from the reactants, whereas the latter refers to the spatial arrangement of the surface metal atoms that gives the minimum activation energy of the transition-state complex of the reaction.89 Supported PGM hydrogenation catalysts have a number of advantages over the unsupported catalysts: (i) greater efficiency in the metal use by increasing the active metal surface and by facilitating metal recovery,88 (ii) greater resistance to deactivation due to poisoning or sintering,88,89 (iii) enabled control over selectivity. However, supports may affect catalyst activity as a result of agglomeration (catalyst clumping).88 Catalyst support also plays an important role, for example, in the hydrogenation of phenol over Pd/Al2O3 and Pd/MgO,90 which have different acid-base properties affecting the product selectivity. The reaction occurs mainly between phenol chemisorbed on the support and hydrogen activated on the metal sites.90  PGM catalysts are subject to partial or total deactivation by many substances (e.g., heavy metal ions, halides, sulfur compounds, carbon monoxide, amines, phosphine, etc.), depending on the quantity of the inhibitor.88 The catalyst inhibition may occur in various ways: (i) overly strong  84  adsorption that blocks the catalyst sites, (ii) inhibitor and substrate interaction that alters the nature of the reactant or prevents the conversion of the catalyst to an active form.88 In electrochemical process, the electrolyte plays an important role in the reaction affecting the catalyst stability, activity, and selectivity (product distribution). Ruthenium has been reported as one of the least active metals toward HER in acidic media, but increased ECH activity when supported on activated carbon.13,86 The activity of Ru/C was, however, inhibited by the presence of H2SO4 due to sulfate poisoning, similar to the sulfur poisoning in case of the liquid-phase hydrogenation of levulinic acid to gamma-valerolactone.91 On the other hand, ruthenium-based catalysts were recently reported to be more active than platinum toward HER in alkaline media. In this regard, the choice of electrocatalyst and electrolyte are crucial for the electrocatalytic process development.   Among the physical properties of a support that may influence catalyst performance are total surface area, average pore size, pore size distribution, and particle size, as these properties affect the metal dispersion and control the transport of reactants (adsorption) and products (desorption) to and from the catalyst surface.88 The presence of dispersed metal on carbon support has been shown to be pivotal for an effective ECH (see the control experiments in Section 4.3). N2 physisorption measurements were performed in a surface area and porosity analyzer (Micromeritics ASAP 2020) at 77 K to determine Brunauer-Emmett-Teller (BET) surface areas and pore volumes of the catalysts, along with the average pore diameter by Barrett-Joyner-Halenda (BJH) method. The sample was degassed at 200 oC for 2 h before measurement. CO chemisorption was performed in Micromeritics AutoChem II equipped with thermal conductivity detector (TCD) using a pulsed injection of 5% CO/He to measure the metal surface area. Prior to measurement, the sample was reduced under H2 flow (50 cm3/min) at 300 oC for 1 h. The catalyst characterization results are presented in Figure 5.2 and Table 5.2. Of the three catalysts, Pt/C was found to have the largest surface area (~1500 m2 g-1)  85  and pore volume (1.43 cm3 g-1) and the highest metal dispersion (~29%). Ru/C and Pd/C showed similar textural properties based on the BET measurements.   Figure 5.2. Adsorption-desorption isotherms for Pt/C (●), Ru/C (■), and Pd/C (▲) catalysts using N2 physisorption. Catalyst metal content = 5 wt.%. Table 5.2. Characteristics of the carbon-supported metal catalysts used in this study. Catalyst SBETa  (m2/g) Vporeb (cm3/g) Dporec  (nm) φ  (%) Smetal  (m2/g)  dp (nm)  Pt/C 1487 1.4 4.4 28.9 71.5 3.9 Ru/C 777 0.9 6.2 22.0 80.4 6.0 Pd/C 762 0.8 5.2 25.1 111.8 4.5 a BET surface area;  b Single point adsorption total pore volume (P/P0 = 0.99); c Average pore diameter by BJH desorption; φ (metal dispersion), Smetal (metallic surface area), and dp (active particle diameter) determined by CO chemisorption.  0200400600800100012000.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0Volume adsorbed (cm3g-1)Relative pressure (P/P0)Pt/CRu/CPd/C 86  5.3. Catalyst performance analysis in different electrolyte pairs 5.3.1. Comparative investigation in galvanostatic ECH of guaiacol and phenol The effects of different metal catalysts were investigated under the same operating conditions (i.e. j = -109 mA cm-2, T = 50 oC, and t = 4 h) with the same catalyst amount (0.5 g) for ECH of phenol and guaiacol. In all cases, Pt/C showed superior activity than Ru/C and Pd/C, either in the acid-acid or neutral-acid pairs, possibly because of its higher surface area and metal dispersion (see Figure 5.2 and Table 5.2). In the ECH of phenol under acidic conditions (pH = 0.7–0.8), the reaction rate (in mmol per mmol dispersed metal h-1) decreases as follows: Pt/C (64) > Pd/C (33) > Ru/C (16). In the acid-acid pairs, the highest conversion (72%) and F.E. (90%) were obtained with Pt/C (Figure 5.3a). With neutral-acid pairs, significant improvements were noticed in the activity of Ru/C and Pd/C (Figure 5.3b), resulting in higher rates [Pd/C (52), Ru/C (32)] and phenol conversions (70–75%), cyclohexanol selectivities (70–100%) and F.E. (>96%). Under these conditions, the catholyte pH increased from approximately 2 to 10 because of water reduction to hydroxide ions (beside H2), suggesting that the anodic protons transported through the cation exchange membrane were also effectively reduced on the catalyst surface.  Similar trends were observed in the ECH of guaiacol. In the acid-acid pair, the reaction rate decreases as Pt/C (54) > Pd/C (7) ≈ Ru/C (6). The best performing catalyst, Pt/C, resulted in higher guaiacol conversion (60%) and F.E. (69%) after 4 h reaction (Figure 5.4a). Interestingly, in the neutral-acid pair, the activity of Pt/C dropped while Ru/C and Pd/C showed increased activities. The lower Pt/C activity in ECH of guaiacol under high pH (>9) conditions had been reported previously,49 which could be attributed to the hampered guaiacol adsorption on the catalyst caused by the deprotonation34 as well as the loss of Pt nanoparticles due to modification of the anchoring  87  sites of the particles on the support.79 Ru/C showed the most dramatic increase in activity, resulting in over 5 times faster reaction rates (32 mmol h-1 vs. 6 mmol h-1, per mmol dispersed metal) with nearly 8 times higher conversion (48% vs. 6%) and 7 times higher F.E. (61% vs. 9%) compared to those obtained in the acid-acid pair. The higher activity of Ru-based catalysts than Pt toward HER was ascribed to the atomically dispersed Ru within the carbon matrix lowering the kinetic barrier for water dissociation.87 Ru activity in alkaline medium was also attributed to the appropriate metal-hydrogen binding energy allowing for a rapid proton adsorption, reduction, and product desorption as a highly efficient HER,92 where both Ru and neighboring carbon serve as the active sites.87 The remarkable stability of Ru dispersed on carbon catalysts was also exhibited by the unchanged HER polarization curves after 10,000 potential cycles in 0.1 M KOH and the absence of oxygen signal in TEM and electron energy loss spectroscopic (EELS) measurements, indicating that the catalysts were largely unoxidized.87 Meanwhile, Pd/C showed moderate activity in the guaiacol ECH, either in acid-acid or neutral-acid pair, which might be attributed to the instability (degradation and agglomeration) of its nanoparticles under acidic or alkaline conditions.93 As for the product distribution, Ru/C favored full hydrogenation of the benzene ring producing mainly cyclohexanol (in the phenol ECH) and 2-methoxycyclohexanol (in the guaiacol ECH) (Figures 5.3a-b and 5.4a-b). Carbon- and oxide-supported Ru were also recognized as efficient catalysts in the aqueous-phase hydrogenation of carbonyl compounds into the corresponding alcohols.94 Similarly, Pd/C also preferred ring saturation of guaiacol as shown by the dominant selectivity of 2-methoxycyclohexanol. This trend was also observed in the thermocatalytic hydrogenation of guaiacol over Pd/C at 200–260 oC, resulting in 2-methoxycyclohexanol as the major products (with only a small amount of cyclohexanol).95 Generally, the hydrogenolysis of the C–O bond from guaiacol was difficult in case Pd/C as shown by the lowest cyclohexanol  88  selectivity (Figure 5.4a-b). Even so, the improved activities of Ru/C and Pd/C in ECH of guaiacol and phenol in the neutral-acid pairs suggest the possibility for broader catalyst options (e.g., non-precious metals) paired with suitable electrolyte for electrosynthesis applications.   Figure 5.3. Catalytic performance of carbon-supported metals in ECH of phenol with different catholyte-anolyte pairs: (a) acid-acid, (b) neutral-acid. Electrolyte: acid (0.2 M H2SO4), neutral (0.2 M NaCl). Catalyst loading = 0.5 g (metal content: 5 wt.%). Reaction conditions: I = -0.3 A (j = -109 mA cm-2), T = 50 oC, t = 4 h. Phenol concentration = 0.105 M, Catholyte pH after 4 h: (a) 0.69–0.74, (b) 10–11. Reaction rate: [mmol of phenol / (h × mmol of dispersed metal)]. 010203040506070800102030405060708090100Pt/C Ru/C Pd/CReaction rate (mmol h-1mmol-1)Conversion, Selectivity, F.E. (%)CatalystPhenolCyclohexanolCyclohexanoneFaradaic EfficiencyRate(a)010203040506070800102030405060708090100Pt/C Ru/C Pd/CReaction rate (mmol h-1mmol-1)Conversion, Selectivity, F.E. (%)Catalyst(b) 89   Figure 5.4. Catalytic performance of carbon-supported metals in ECH of guaiacol with different catholyte-anolyte pairs: (a) acid-acid, (b) neutral-acid. Electrolyte: acid (0.2 M H2SO4), neutral (0.2 M NaCl). Catalyst loading = 0.5 g (metal content: 5 wt.%). Reaction conditions: I = -0.3 A (j = -109 mA cm-2), T = 50 oC, t = 4 h. Guaiacol concentration = 0.106 M, Catholyte pH after 4 h: (a) 0.68–0.71, (b) 10–11. Reaction rate: [mmol of guaiacol / (h × mmol of dispersed metal)].   01020304050600102030405060708090100Pt/C Ru/C Pd/CReaction rate (mmol h-1mmol-1)Conversion, Selectivity, F.E. (%)CatalystGuaiacolCyclohexanolCyclohexanone2-Methoxycyclohexanol2-MethoxycyclohexanoneMethanolPhenolFaradaic EfficiencyRate01020304050600102030405060708090100Pt/C Ru/C Pd/CReaction rate (mmol h-1mmol-1)Conversion, Selectivity, F.E. (%)Catalyst (b) 90  5.3.2. Catalyst loading effect and carbon balance analysis Interesting trends were observed when the catalyst loading was varied. In the acid-acid pair, higher Pt/C amount (0.13–0.50 g) resulted in higher guaiacol conversion (49–61%) and F.E. (65–69%) as expected with insignificant pH changes (Figure 5.5a). The counter-intuitive results, however, were noticed in the neutral-acid pair where higher Pt/C amount resulted in lower conversion (46% to 38%) and F.E. (69% to 46%). This negative trend was likely caused by guaiacol deprotonation and Pt/C instability in high pH conditions, supported by the increasing catholyte pH (9–10) after 4 h (Figure 5.5b). Higher catalyst amount would presumably increase Hads coverage as the proton reduction was accelerated, however, in the neutral catholyte, hydroxide ions were produced via Volmer step (Table 5.1, R4). If the protons supplied from anode were insufficient, catholyte pH would inevitably rise. This effect is undesirable when Pt/C was used for the guaiacol ECH, but in case of phenol ECH, higher catalyst loading gave positive results (i.e. higher conversion and F.E., see Figure 5.6). Compared to guaiacol, phenol is less prone to deprotonation and could react easier and faster with hydrogen radicals. It was confirmed from the catholyte solution after the reaction: in the guaiacol ECH, discolorations were very obvious while in the phenol ECH, the solution colors relatively unchanged.49 Different product distributions from the guaiacol ECH were obtained as follows: in the acid-acid pair, two parallel routes occurred simultaneously, as indicated by the substantial selectivities to cyclohexanone (~24%) and cyclohexanol (28–31%) relative to 2-methoxycyclohexanone (16–22%) and 2-methoxycyclohexanol (12–18%); in the neutral-acid pair, the aromatic ring saturation route prevailed, shown by the dominant selectivity to 2-methoxycyclohexanol (43–67%).  91   Figure 5.5. Effect of Pt/C catalyst loading in the ECH of guaiacol with different catholyte-anolyte pairs: (a) acid-acid, (b) neutral-acid. Electrolyte: acid (0.2 M H2SO4), neutral (0.2 M NaCl). Reaction conditions: I = -0.3 A (j = -109 mA cm-2), T = 50 oC, t = 4 h. R/M = reactant to metal (bulk) molar ratio. Catalyst concentration = mass of carbon-supported catalyst / (mass of carbon-supported catalyst + mass of guaiacol). Guaiacol concentration = 0.106 M, Catholyte pH was measured after 4 h.  0246810121401020304050607080901009 16 27pH of CatholyteConversion, Selectivity, F.E. (%)Pt/C catalyst concentration (wt.%)Guaiacol CyclohexanolCyclohexanone 2-Methoxycyclohexanol2-Methoxycyclohexanone MethanolPhenol Faradaic EfficiencypH0.00.10.20.30.40.50.60.70.80.901020304050607080901009 16 27pH of CatholyteConversion, Selectivity, F.E. (%)Pt/C catalyst concentration (wt.%)(a)(b) 92   Figure 5.6. Effect of Pt/C catalyst loading in the ECH of phenol with different catholyte-anolyte pairs: (a) acid-acid, (b) neutral-acid. Electrolyte: acid (0.2 M H2SO4), neutral (0.2 M NaCl). Reaction conditions: I = -0.3 A (j = -109 mA cm-2), T = 50 oC, t = 4 h. R/M = reactant to metal (bulk) molar ratio. Catalyst concentration = mass of carbon-supported catalyst / (mass of carbon-supported catalyst + mass of phenol). Phenol concentration = 0.105 M, Catholyte pH was measured after 4 h. Legend information corresponds to Figure 5.5 (shown in identical markers and colors).   0.00.10.20.30.40.50.60.70.80.9010203040506070809010011 20 33pH of CatholyteConversion, Selectivity, F.E. (%)Pt/C catalyst concentration (wt.%)(a)02468101214010203040506070809010011 20 33pH of CatholyteConversion, Selectivity, F.E. (%)Pt/C catalyst concentration (wt.%)(b) 93   Time-dependent profiles from ECH of guaiacol over low Pt/C loading are presented in Figure 5.7. In the acid-acid pair (Figure 5.7a-c), the product distribution was not affected by the higher acid concentration, in which cyclohexanol was obtained with the highest selectivity (28–30%) at reasonable guaiacol conversion (49–60%). The catholyte pH was relatively unchanged during the reaction (Figure 5.7d-f) and guaiacol conversion increased more significantly with the higher anolyte proton concentration (Table 5.3, Entry 1 vs. 2), but only slightly with the higher catholyte proton concentration (Table 5.3, Entry 2 vs. 3). In the latter case, the F.E. decreased, possibly owing to the enhanced Heyrovsky step (Table 5.1, R2). In the neutral-acid pair with the same pair concentration (0.2 M), guaiacol conversion reached a plateau after 4 h because pH increased dramatically from 2.1 (due to proton diffusion before the electrolysis) to 9.2 (due to predominant hydroxide ions formation). At this condition, guaiacol adsorption to the Pt/C catalyst was hindered, resulting from the co-occurrence of guaiacol deprotonation and Pt/C destabilization under the increasing pH (Figure 5.8a, d). Interestingly, at the higher anolyte concentration (0.5 M), guaiacol conversion steadily increased over time (Figure 5.8b) and the catholyte pH could be kept low (<3) with relatively higher F.E. after 4 h (Figure 5.8d vs. 5.8e). Moreover, both demethoxylation and aromatic ring saturation were facilitated in the presence of higher proton concentration, as shown by the higher selectivities to cyclohexanone (22–32%) and cyclohexanol (13–27%). The increase in cyclohexanone selectivity was further obtained at the higher NaCl catholyte concentration (Table 5.3, Entry 6), possibly because of less hydrogen radical produced from water reduction with less reducing environment (i.e. less negative cathode potential). Note that guaiacol conversion and F.E. did not increase dramatically either (Table 5.3, Entry 5 vs. 6), confirming that anolyte proton concentration plays more dominant role in promoting the ECH efficiency than the catholyte concentration.  94   Figure 5.7. ECH of guaiacol in acid (H2SO4)-acid (H2SO4) catholyte-anolyte pair with different concentrations. Guaiacol conversion and product selectivity (C mol%) in catholyte-anolyte concentration pairs: (a) 0.2 M–0.2 M (already reported in the previous work49), (b) 0.2 M–0.5 M, (c) 0.5 M–0.5 M. Catholyte pH and Faradaic efficiency profiles for the corresponding results (d, e, f). Reaction conditions: I = -0.3 A (j = -109 mA cm-2), T = 50 oC, t = 4 h. Catalyst: 5 wt%-Pt/C (0.133 g). Guaiacol concentration (initial) = 0.106 M, Catholyte pH was measured after 4 h. 0102030405060700 1 2 3 4Conversion, Selectivity (%)Time (h)GuaiacolCyclohexanolCyclohexanone2-Methoxycyclohexanol2-MethoxycyclohexanoneMethanolPhenol0102030405060708090100024681012140 1 2 3 4Faradaic Efficiency (%)pH of catholyteTime (h)pH of catholyteF.E. (%)(d)0102030405060700 1 2 3 4Conversion, Selectivity (%)Time (h)0102030405060708090100024681012140 1 2 3 4Faradaic Efficiency (%)pH of catholyteTime (h)(e)0102030405060700 1 2 3 4Conversion, Selectivity (%)Time (h)(c)0102030405060708090100024681012140 1 2 3 4Faradaic Efficiency (%)pH of catholyteTime (h)(f)(a)(b) 95   Figure 5.8. ECH of guaiacol in neutral (NaCl)-acid (H2SO4) catholyte-anolyte pair with different concentrations. Guaiacol conversion and product selectivity (C mol%) in catholyte-anolyte concentration pairs: (a) 0.2 M–0.2 M (already reported in the previous work49), (b) 0.2 M–0.5 M, (c) 0.5 M–0.5 M. Catholyte pH and Faradaic efficiency profiles for the corresponding results (d, e, f). Reaction conditions: I = -0.3 A (j = -109 mA cm-2), T = 50 oC, t = 4 h. Catalyst: 5 wt%-Pt/C (0.133 g), corresponding to catalyst concentration of 9 wt.%. Guaiacol concentration (initial) = 0.106 M, Catholyte pH was measured after 4 h.0102030405060700 1 2 3 4Conversion, Selectivity (%)Time (h)GuaiacolCyclohexanolCyclohexanone2-Methoxycyclohexanol2-MethoxycyclohexanoneMethanolPhenol(a)0102030405060708090100024681012140 1 2 3 4Faradaic Efficiency (%)pH of catholyteTime (h)pH of catholyteF.E. (%)0102030405060708090100024681012140 1 2 3 4Faradaic Efficiency (%)pH of catholyteTime (h)0102030405060700 1 2 3 4Conversion, Selectivity (%)Time (h)(b)0102030405060700 1 2 3 4Conversion, Selectivity (%)Time (h)(c)0102030405060708090100024681012140 1 2 3 4Faradaic Efficiency (%)pH of catholyteTime (h)(d)(e)(f) 96  Table 5.3. ECH of guaiacol in different combinations of electrolyte pair concentration using Pt/C in acid-acid and neutral-acid catholyte-anolyte pairs. Entry Catholyte (c)  Anolyte  (a) Ecathode (V) pH (c)  pH (a) X (%) S1 (%) S2 (%)  S3 (%)  S4 (%)  S5 (%)  S6 (%)  C.B. (%) F.E. (%)  1 H2SO4 (0.2 M) H2SO4 (0.2 M) -0.93 0.72 0.68 48.51 28.73 24.32 11.93 22.01 10.04 2.98 97.17 64.70 2 H2SO4 (0.2 M) H2SO4 (0.5 M) -0.82 0.58 0.40 56.28 28.74 23.37 15.05 20.46 9.87 2.52 94.55 72.78 3 H2SO4 (0.5 M) H2SO4 (0.5 M) -0.54 0.41 0.38 59.92 29.69 20.35 17.82 17.19 11.62 3.33 93.34 68.41 4 NaCl (0.2 M) H2SO4 (0.2 M) -2.29 9.21 0.63 45.91 37.25 5.89 43.22 4.47 7.89 1.28 96.85 69.20 5 NaCl (0.2 M) H2SO4 (0.5 M) -2.27 2.75 0.34 61.38 27.25 22.26 19.01 21.81 8.65 1.03 95.35 80.99 6 NaCl (0.5 M) H2SO4 (0.5 M) -1.80 1.93 0.27 62.40 21.11 28.01 20.31 20.95 7.53 2.09 97.24 81.52 E = cathode potential (vs. Ag/AgCl), X = guaiacol conversion, S = normalized product selectivity (C mol%) for cyclohexanol (1), cyclohexanone (2), 2-methoxycyclohexanol (3), 2-methoxycyclohexanone (4), methanol (5), phenol (6), C.B. = carbon balance, F.E. = Faradaic efficiency. Conditions: T = 50 oC, I =  -0.3 A (j = -109 mA cm-2), Catalyst = 5 wt.%-Pt/C (0.133 g); Reactant to metal molar ratio (R/M) = 315; Reaction time = 4 h; pH was measured after the reaction.  97  Having proven the good performance of Ru/C at high pH (i.e. prevalent in the neutral-acid electrolyte combination), increasing the Ru/C loading in the slurry (from 0.15 g to 1 g) generated a remarkable increase of conversion (from 13% to 73%) and F.E. (from 17% to 76%) after 4 h reaction time (Figure 5.9a). This trend showed an obvious difference in terms of Ru/C activity compared to Pt/C under alkaline conditions (Figure 5.5b). In contrast to Ru/C, however, Pd/C did not show good activity in the guaiacol ECH even at high catalyst concentrations (Figure 5.9b). In all cases with the neutral-acid pairs, the higher catalyst loading accelerated the catholyte pH increases, indicating that the protons were consumed faster, thereby favoring the water reduction to hydrogen and hydroxide ions (Figures 5.9 and 5.5b). The plausible reaction pathways for the ECH of guaiacol (and phenol) and the activity order of the catalysts based on the reaction rates are illustrated in Figure 5.10. Under similar conditions, phenol was converted faster than guaiacol regardless of the catalyst, showing its high reactivity in ECH. While phenol ECH proceeds in a series reaction to cyclohexanone and cyclohexanol, guaiacol ECH occurs in a parallel pathway involving demethoxylation and benzene ring saturation steps. In a recent publication, the phenol ECH has been reported to be a zeroth order reaction36 as opposed to the guaiacol, which may be first or second order reaction depending on the operating conditions.49 Therefore, the phenol reaction rate is independent of the initial reactant concentration with hydrogenation of the adsorbed phenol being the rate determining step (RDS),36 while the guaiacol ECH rate is dependent on the initial concentration in which guaiacol competitive adsorption with Hads was the likely RDS.49 Due to these mechanistic differences, different catalytic performances are expected.  98   Figure 5.9. Effect of catalyst loading in the ECH of guaiacol in neutral-acid catholyte-anolyte pairs with: (a) Ru/C, (b) Pd/C. Electrolyte: acid (0.2 M H2SO4), neutral (0.2 M NaCl). Reaction conditions: I = -0.3 A (j = -109 mA cm-2), T = 50 oC, t = 4 h. Catalyst concentration = mass of carbon supported catalyst / (mass of carbon supported catalyst + mass of guaiacol). Guaiacol concentration = 0.106 M. Catholyte pH was measured after 4 h. 02468101214010203040506070809010010 27 43pH of CatholyteConversion, Selectivity, F.E. (%)Ru/C catalyst concentration (wt.%)Guaiacol CyclohexanolCyclohexanone 2-Methoxycyclohexanol2-Methoxycyclohexanone MethanolPhenol Faradaic EfficiencypH(a)02468101214010203040506070809010010 27 43pH of CatholyteConversion, Selectivity, F.E. (%)Pd/C catalyst concentration (wt.%)(b) 99   Figure 5.10. Schematic of the reaction pathways for the guaiacol and phenol ECH and the activity order of the carbon-supported metal catalysts. Compounds: (a) guaiacol, (b) phenol, (c) cyclohexanone, (d) cyclohexanol, (e) 2-methoxycyclohexanone, (f) 2-methoxycyclohexanol, and methanol as the byproduct.  A secondary effect of the higher catalyst loading experiments was the lower carbon recovery due to adsorption of the organics on the supported catalyst and mainly on the activated charcoal support (see Table 5.4). Carbon balance analysis was further conducted in a series of blank experiments where the organic compounds in solution were mixed with the supported catalyst with no current applied. The results (Table 5.5) showed that adsorption of organics was mainly responsible for the decreasing carbon balance at the higher catalyst loading. Note that all conversion data reported in this study were calculated by excluding the contributions from carbon losses to avoid overestimations. Thus, all the values purely represent the conversion from reaction. Phenol ECHPt/C > Pd/C > Ru/CGuaiacol ECHPt/C > Ru/C > Pd/CCatalyst activity:Catalyst activity: 100  Table 5.4. Summary of carbon balance calculation results in the ECH of guaiacol using different catalysts and loadings, corresponding to the results shown in Figures 5.5b and 5.9 (in neutral-acid pair).  Pt/C Ru/C Pd/C R/M 315 167 84 145 43 22 152 46 23 CCAT (%) 9 16 27 10 27 43 10 27 43 Mass (g) 0.13 0.25 0.50 0.15 0.50 1.00 0.15 0.50 1.00 XT (%) 49.1 55.0 54.5 15.7 58.2 86.3 27.4 35.7 49.6 XR (%) 45.9 41.2 38.3 12.7 47.8 73.4 10.8 17.6 27.3 C.B. (%) 97 87 84 97 90 87 84 81 78 R/M = reactant to metal (bulk) molar ratio, CCAT (%) = catalyst mass concentration, XT = guaiacol total conversion, XR = guaiacol reaction conversion. The guaiacol total conversion represents the amount of guaiacol disappearance due to reaction, adsorption, and deprotonation, hence: XT – XR = 100 – C.B.  Table 5.5. Effect of catalyst loading on the organic carbon recovery in different solutions. Entry Solution A - Low loading (0.125 g) B - High loading (0.50 g) 1 Water 96 ± 0.534 84 ± 1.857 2 0.2 M H2SO4 92 ± 1.349 81 ± 1.291 3 0.2 M NaOH 98 ± 0.698 85 ± 1.129   A series of blank experiments were conducted using 5 wt.%-Pt/C in different pH solutions: water (pH ≈ 6–7), 0.2 M H2SO4 (pH ≈ 0.7–0.8), and 0.2 M NaOH (pH ≈ 13.45). Different catalyst loadings were employed (i.e. low loading: 0.125 g and high loading: 0.50 g). A mixture of individual compounds with equimolar amount was used in each test. Guaiacol, phenol, cyclohexanone, cyclohexanol, 2-methoxycyclohexanone, 2-methoxycyclohexanol, and methanol were mixed in the solution (100 mL). The operating conditions were identical with those used in the ECH experiments: stirring rate (240 rpm), temperature (50 oC), and mixing time (2 h). The organic carbon recovery from each experiment is summarized in Table 5.5, which confirms that higher catalyst loadings were inclined to decrease the carbon balance.   There are at least three possible causes of carbon loss in this experiment: adsorption, evaporation, and/or diffusion. The latter (diffusion) was found to be insignificant due to the  101  negligible amount of organics cross-over from cathode to anode (confirmed in GC-MS results). The evaporation might slightly contribute to the carbon losses as it was noticed that the reactor stopper was wet after each experiment. An additional test was then performed using activated charcoal (0.5 g) mixed with the identical amount of organic compounds in water at 25 oC to exclude the evaporation effect. The result showed the carbon recovery was only 84 ± 1.236% after 2 h with virtually no organic diffusion to the anode side. Therefore, it may be justified that adsorption on the catalyst is the most influential factor that contributes to the lower carbon retention of the organic compounds in the liquid phase. 5.4. Electrolyte concentration effect in ECH of guaiacol As the neutral-acid catholyte-anolyte pair was found to be advantageous for ECH,49 it is worthwhile to further investigate the impact of different electrolyte concentrations. Here, the ECH of guaiacol was performed using the combinations of neutral catholyte (NaCl) and acidic anolyte (H2SO4) with two different concentrations (0.2 M or 0.5 M) at a constant superficial current density (-109 mA cm-2) and temperature (50 oC). The same catalyst (Pt/C, Ru/C or Pd/C) loading was used (0.2 g, corresponding to a catalyst concentration of 13 wt.%) for all the experiments. Conversion, selectivity, F.E., and catholyte pH were monitored over the course of the reaction to better understand the influence of pH changes on the catalyst performance and the product distribution.   In case of Pt/C, the guaiacol conversion and F.E. increased with the anolyte proton concentration (Figure 5.11a and b), confirming that Pt/C works more effectively at the lower pH conditions (1.8) where higher guaiacol conversion (65%) is achieved in the NaCl (0.2 M) catholyte paired with H2SO4 (0.5 M) anolyte. Product distributions were affected by the pH as shown by the time-dependent profiles (Figure 5.11). At high pH (> 9), direct ring saturation route was  102  predominant with the highest selectivity to 2-methoxycyclohexanol (62–72%) obtained at moderate guaiacol conversion (39–47%) (Figure 5.11a and c). Meanwhile at lower pH (< 2), due to higher flux of proton across the membrane from the higher (0.5 M) H2SO4 concentration anolyte to the catholyte, the demethoxylation step gained significance as indicated by the fairly similar selectivity for cyclohexanol, cyclohexanone and 2-methoxycyclohexanol (Figure 5.11b and d). The latter trends were also evident when low Pt/C loading was used in the acid-acid and neutral-acid pairs (Figures 5.7–5.8, Table 5.3). Operation at higher catholyte NaCl concentration (i.e. 0.5 M vs. 0.2 M) lowered both the guaiacol conversion and F.E. especially after 2 hours, but enhanced the selectivity toward cyclohexanol (i.e. demethoxylation–ring saturation pathway) particularly in case of 0.5 M NaCl – 0.5 M H2SO4 catholyte–anolyte pair (Figure 5.11c and d). It is proposed that the interaction between surface pH and specific adsorption of Cl- on Pt could have altered the reaction pathways enhancing the competitiveness of demethoxylation–ring saturation. Promotional effects of chloride ions have been reported in acid-catalyzed dehydration of fructose to hydroxymethylfurfural (HMF) in polar aprotic solvents, whereby Cl- ions aid in the stabilization of oxocarbenium ion and protonated transition states, leading to enhanced rates and selectivities.96  In contrast, both Ru/C and Pd/C favored only the ring saturation pathway with 2-methoxycyclohexanol as the main product (selectivity ≥ 60%) under all the explored conditions (Figures 5.12 and 5.13). Thus, catholyte pH changes did not dramatically affect the product distribution in the guaiacol ECH over Ru/C or Pd/C (unlike in the Pt/C cases).  Furthermore, both the guaiacol conversion and F.E. were lower on Ru/C and Pd/C compared with Pt/C, with somewhat better performance under alkaline conditions for the former. The poor activity of Ru/C in acidic conditions was also found in the ECH of guaiacol using different acid electrolytes (0.2 M HClO4, 0.2 M HCl) whereby virtually no products were identified even after 12 h reaction  103  (results not shown). Ru atoms are prone to dissolution in acidic media owing to corrosion and changes of the Ru oxidation state.97 The highest guaiacol conversions with Ru/C (34%) and Pd/C (28%) were obtained using the pair of NaCl (0.5 M) catholyte and H2SO4 (0.2 M) anolyte which showed the highest pH after 4 h reaction. Comparing the catholyte pH profiles, it is clear that with higher anolyte acid concentration (0.5 M) the catholyte pH remained low regardless of the catalyst, (Figures 5.11f, 5.12f, and 5.13f). Moreover, the profiles in the first (Figures 5.11e, 5.12e, and 5.13e), third (Figures 5.11g, 5.12g, and 5.13g), and fourth pairs (Figures 5.11h, 5.12h, and 5.13h) reveal the fastest pH increases over Pt/C, followed by Ru/C and Pd/C (see the overlay profiles in Figure 5.14). This shows that Pt/C catalyzes water reduction to hydroxide ions faster than Ru/C and Pd/C. In neutral electrolyte, this reaction is important as the first step in the electrocatalytic reaction that forms Hads (Table 5.1, R4). The faster Hads formation is desirable to promote the ECH rates over HER as long as the organics are present.         The overall results of the guaiacol galvanostatic ECH using different concentrations of neutral-acid electrolyte pairs with Pt/C, Ru/C, and Pd/C are also summarized in Table 5.6. In general, it can be stated that the measured cathode potentials under galvanostatic conditions for the three investigated catalyst beds were fairly similar for identical electrolyte compositions. Therefore, the reaction rate results are reflective of the specific performance for the investigated catalyst beds. It is noted, however, that the measured cathode potentials cannot be taken as the true values on the catalyst surface due to two effects: (i) ohmic losses in the catholyte with H2 gas evolving slurry of dispersed catalyst particles, and (ii) non-uniform potential and current distribution in the slurry bed. In all cases, higher catholyte NaCl concentration (0.5 M) resulted in lower (less negative) measured cathode potentials at constant operating current density because of lower ohmic resistance. In terms of the reaction rates, Pt/C clearly showed superior activity (around 6-7 times  104  higher) than Ru/C and Pd/C (Table 5.6). The higher anolyte concentration positively affected the guaiacol conversion and F.E. over Pt/C, but it had a negative effect in case of Ru/C and Pd/C, implying that Pt/C works better for ECH of guaiacol at lower pH, while Ru/C (or Pd/C) is more effective at higher pH conditions. 5.5. Influence of catalyst slurry in water reduction During the ECH of guaiacol in neutral (NaCl) catholyte and acid (H2SO4) pair, water was reduced to H2 and OH- via Heyrovsky steps (Table 5.1, R5). The catholyte pH increase can be controlled by adjusting the electrolyte concentrations. To examine the influence of different metal catalyst on the water reduction during guaiacol ECH, catholyte pH was monitored every hour at four different concentrations of the catholyte-anolyte pairs for each catalyst (Pt/C, Ru/C, Pd/C). The increasing catholyte pH over time is indicative of the accelerated water reduction in the presence of catalyst. In all cases, Pt/C resulted in the fastest pH increases, followed by Ru/C and Pd/C (Figure 5.14). The high pH increase rates were noticed at the lower H2SO4 anolyte concentration (0.2 M) (Figure 5.14a and c). Meanwhile, the pH increases could be suppressed in the pair of NaCl (0.2 M) catholyte and H2SO4 (0.5 M) (Figure 5.14b) and delayed in the pair of NaCl (0.5 M) catholyte and H2SO4 (0.5 M) (Figure 5.14d).  To verify the metal catalyst impact, electrolysis experiments were conducted at the identical conditions, but without organic and catalyst. In the electrolyte pair with lower H2SO4 concentration (0.2 M), the catholyte pH increased slowly at the beginning (during the first hour), but then further increased dramatically after 1 h, reaching between 11–12 (Figure 5.14a and c). In the presence of guaiacol and catalyst, the catholyte pH increased faster within the first hour, but reached plateau (9–10) after 2 h. These differences were likely caused by the reaction between Hads and adsorbed  105  organics that competitively hindered the OH- formation via Heyrovsky reaction. The most obvious case for the accelerated water reduction by the presence of metal catalyst can be seen in the neutral-acid pair with 0.5 M concentrations (Figure 5.14d). After 4 h, the catholyte pH remained constant (1.9) in the absence of organics and catalyst, however increased to nearly 10, surpassing the equivalence point in acid-base titration curves, in the presence of organics and metal catalyst. Further electrolysis tests over the catalyst slurry (without guaiacol) confirmed the previous observations: (i) the metal catalyst accelerated water reduction in the NaCl catholyte, resulting in the pH increases over time, (ii) the catalyst reduction ability decreased in the order of Pt/C > Ru/C > Pd/C. In the absence of guaiacol, the catholyte pH increased faster as the Hads from Volmer steps reacted further to generate H2 and OH-, implying the non-existent competitive ECH reactions involving the chemisorbed hydrogen (Figure 5.15).  5.6. Summary of the chapter Electrocatalytic reduction of lignin model compounds (i.e. phenol and guaiacol) has been investigated in a stirred slurry electrochemical reactor (SSER) under mild conditions (50 oC, 1 atm). Three different catalysts (Pt/C, Ru/C, Pd/C, with the same metal content of 5 wt.%) were tested in acid-acid and neutral-acid catholyte-anolyte pairs at constant superficial current density (-109 mA cm-2). All the catalysts displayed good hydrogenation activity for phenol, where the activity decreased in the order of Pt/C > Pd/C > Ru/C, either in the acid-acid or neutral-acid pairs. In the ECH of guaiacol, Pt/C was the most active in the acidic electrolyte pair at low pH (<0.8), however, the activity of Ru/C improved in the neutral-acid pair when the catholyte pH increased owing to the formation of hydroxide ions via water reduction. Such improvements were observed  106  for either phenol or guaiacol ECH, implying that high pH (9–11) conditions were more favorable for Ru/C. The catalyst activity in the ECH of guaiacol decreased as Pt/C > Ru/C > Pd/C.  In terms of reaction pathways for guaiacol ECH, ring saturation leading to 2-methoxycyclohexanol was the dominant pathway, with the exception of Pt/C (operated in either 0.2 M or 0.5 M NaCl catholyte and 0.5 M H2SO4 anolyte). In the latter case demethoxylation–ring saturation producing cyclohexanol and cyclohexanone was equally competitive. Further fundamental studies are required to better understand these effects and their impact on the ECH pathways. Theoretical studies and molecular simulations may provide insights on how different metal surfaces affect the ECH mechanisms as well as the HER catalysis. In situ microscopic, structural, compositional characterizations could help identify the changes on the catalyst surface during the ECH and HER reactions.58   Pairing neutral (NaCl) catholyte with acidic (H2SO4) anolyte has been shown to improve the Faradaic efficiency with the possibility of pH and selectivity control, which reveals the electrocatalyst-electrolyte synergy and opens new opportunities for process design. With respect to the reactor design, the SSER could be implemented in a large-scale application as either a fluidized bed or moving bed electrocatalytic reactor. This work focuses on the comparative investigation of the fresh carbon-supported metal catalyst performance in different electrolyte pairs. Reusability tests of Pt/C have been performed in our previous work using acid (H2SO4) and neutral (NaCl) catholytes. The spent Pt/C catalysts remained active after simple recovery steps (filtration and oven drying), despite some morphological changes.49 Stability and characterization experiments for Ru/C and Pd/C are subject to further research. For a broader purpose, diverse electrolyte choices in catholyte-anolyte combinations could facilitate the electrocatalytic reduction and oxidation processes for lignin valorization using different metal electrocatalysts.   107   Figure 5.11. ECH of guaiacol in neutral (NaCl)-acid (H2SO4) catholyte-anolyte pair with different concentrations. Guaiacol conversion and product selectivity (C mol%) in catholyte-anolyte concentration pairs: (a) 0.2 M–0.2 M, (b) 0.2 M–0.5 M, (c) 0.5 M–0.2 M, (d) 0.5 M–0.5 M. Catholyte pH and Faradaic efficiency profiles for the corresponding results (e, f, g, h). Reaction conditions: I = -0.3 A (j = -109 mA cm-2), T = 50 oC, t = 4 h. Catalyst: 5 wt%-Pt/C (0.20 g). Guaiacol concentration (initial) = 0.106 M. 010203040506070800 1 2 3 4Conversion, Selectivity (%)Time (h)(a)0102030405060708090100024681012140 1 2 3 4Faradaic Efficiency (%)pH of catholyteTime (h)(e)010203040506070800 1 2 3 4Conversion, Selectivity (%)Time (h)(b)0.2 M – 0.5 M010203040506070800 1 2 3 4Conversion, Selectivity (%)Time (h)(c)0.5 M – 0.2 M010203040506070800 1 2 3 4Conversion, Selectivity (%)Time (h)GuaiacolCyclohexanolCyclohexanone2-Methoxycyclohexanol2-MethoxycyclohexanoneMethanol(d)0.5 M – 0.5 M0.2 M – 0.2 M0102030405060708090100024681012140 1 2 3 4Faradaic Efficiency (%)pH of catholyteTime (h)(f)0102030405060708090100024681012140 1 2 3 4Faradaic Efficiency (%)pH of catholyteTime (h)(g)0102030405060708090100024681012140 1 2 3 4Faradaic Efficiency (%)pH of catholyteTime (h)pH of catholyteF.E. (%)(h) 108   Figure 5.12. ECH of guaiacol in neutral (NaCl)-acid (H2SO4) catholyte-anolyte pair with different concentrations. Guaiacol conversion and product selectivity (C mol%) in catholyte-anolyte concentration pairs: (a) 0.2 M–0.2 M, (b) 0.2 M–0.5 M, (c) 0.5 M–0.2 M, (d) 0.5 M–0.5 M. Catholyte pH and Faradaic efficiency profiles for the corresponding results (e, f, g, h). Reaction conditions: I = -0.3 A (j = -109 mA cm-2), T = 50 oC, t = 4 h. Catalyst: 5 wt%-Ru/C (0.20 g). Guaiacol concentration (initial) = 0.106 M. 010203040506070800 1 2 3 4Conversion, Selectivity (%)Time (h)(a)0.2 M – 0.2 M010203040506070800 1 2 3 4Conversion, Selectivity (%)Time (h)(b)0.2 M – 0.5 M010203040506070800 1 2 3 4Conversion, Selectivity (%)Time (h)(c)0.5 M – 0.2 M010203040506070800 1 2 3 4Conversion, Selectivity (%)Time (h)GuaiacolCyclohexanolCyclohexanone2-Methoxycyclohexanol2-MethoxycyclohexanoneMethanol(d)0.5 M – 0.5 M0102030405060708090100024681012140 1 2 3 4Faradaic Efficiency (%)pH of catholyteTime (h)(f)0102030405060708090100024681012140 1 2 3 4Faradaic Efficiency (%)pH of catholyteTime (h)(e)0102030405060708090100024681012140 1 2 3 4Faradaic Efficiency (%)pH of catholyteTime (h)(g)0102030405060708090100024681012140 1 2 3 4Faradaic Efficiency (%)pH of catholyteTime (h)pH of catholyteF.E. (%)(h) 109   Figure 5.13. ECH of guaiacol in neutral (NaCl)-acid (H2SO4) catholyte-anolyte pair with different concentrations. Guaiacol conversion and product selectivity (C mol%) in catholyte-anolyte concentration pairs: (a) 0.2 M–0.2 M, (b) 0.2 M–0.5 M, (c) 0.5 M–0.2 M, (d) 0.5 M–0.5 M. Catholyte pH and Faradaic efficiency profiles for the corresponding results (e, f, g, h). Reaction conditions: I = -0.3 A (j = -109 mA cm-2), T = 50 oC, t = 4 h. Catalyst: 5 wt%-Pd/C (0.20 g). Guaiacol concentration (initial) = 0.106 M. 010203040506070800 1 2 3 4Conversion, Selectivity (%)Time (h)(a)0.2 M – 0.2 M0102030405060700 1 2 3 4Conversion, Selectivity (%)Time (h)(b)0.2 M – 0.5 M010203040506070800 1 2 3 4Conversion, Selectivity (%)Time (h)(c)0.5 M – 0.2 M010203040506070800 1 2 3 4Conversion, Selectivity (%)Time (h)GuaiacolCyclohexanolCyclohexanone2-Methoxycyclohexanol2-MethoxycyclohexanoneMethanol(d)0.5 M – 0.5 M0102030405060708090100024681012140 1 2 3 4Faradaic Efficiency (%)pH of catholyteTime (h)(e)0102030405060708090100024681012140 1 2 3 4Faradaic Efficiency (%)pH of catholyteTime (h)(f)0102030405060708090100024681012140 1 2 3 4Faradaic Efficiency (%)pH of catholyteTime (h)(g)0102030405060708090100024681012140 1 2 3 4Faradaic Efficiency (%)pH of catholyteTime (h)pH of catholyteF.E. (%)(h) 110   Figure 5.14. Catholyte pH profiles from the ECH of guaiacol experiments showing the catalyst effect on the water reduction reactions. These overlay profiles in blue (▲), green (♦), and grey (■) lines correspond to Figures 5.11, 5.12, 5.13 (e-h). Catholyte pH profiles from blank electrolysis tests (no organic, no catalyst) are shown by orange lines (●). Conditions: j = -109 mA cm-2, T = 50 oC, catholyte: NaCl, anolyte: H2SO4.  Figure 5.15. Catholyte pH profiles from three different experiments: (1) ECH of guaiacol, (2) Blank electrolysis, (3) Electrolysis over slurry catalyst (dashed lines). Conditions: j = -109 mA cm-2, T = 50 oC, catholyte: NaCl (0.5 M), anolyte: H2SO4 (0.5 M), catalyst: Pt/C (▲), Ru/C (♦), Pd/C (■). 024681012140 1 2 3 4pH of catholyteTime (h)Blank CPt/C + GUARu/C + GUAPd/C + GUA(c)024681012140 1 2 3 4pH of catholyteTime (h)Blank DPt/C + GUARu/C + GUAPd/C + GUA(d)024681012140 1 2 3 4pH of catholyteTime (h)Blank APt/C + GUARu/C + GUAPd/C + GUA(a)024681012140 1 2 3 4pH of catholyteTime (h)Blank BPt/C + GUARu/C + GUAPd/C + GUA(b)0.2 M – 0.2 M0.2 M – 0.5 M0.5 M – 0.2 M0.5 M – 0.5 M024681012140 1 2 3 4pH of catholyteTime (h)Blank DPt/C + GUARu/C + GUAPd/C + GUAPt/CRu/CPd/C 111  Table 5.6. ECH of guaiacol under galvanostatic conditions with different concentrations of neutral-acid catholyte-anolyte pairs using Pt/C, Ru/C, and Pd/C. Entry Catalyst C (M)  A  (M) Ecathode (VSSCE) pH (c)  pH (a) X (%) S1 (%) S2 (%)  S3 (%)  S4 (%)  S5 (%)  S6 (%)  C.B. (%) F.E. (%)  Rx  (h-1) 1 Pt/C 0.2  0.2 -2.5 9.9 0.5 47.4 24.6 0.0 71.9 0.7 2.2 0.6 92.9 72.3 130.1 2 0.2 0.5 -2.8 1.8 0.2 64.7 26.4 25.1 21.1 17.0 8.4 2.1 90.6 84.5 133.5 3 0.5 0.2 -1.8 9.5 0.6 39.1 32.2 0.0 61.8 0.0 3.7 2.3 87.6 51.7 109.3 4 0.5 0.5 -1.8 9.2 0.3 45.9 44.5 0.0 47.4 1.4 5.8 0.9 89.5 69.4 127.3 5 Ru/C 0.2  0.2 -2.5 9.3 0.5 25.7 25.7 0.0 67.8 1.7 2.1 2.7 92.5 34.0 23.1 6 0.2 0.5 -2.7 3.2 0.3 23.4 21.1 0.0 72.8 2.3 1.5 2.4 96.5 33.0 18.1 7 0.5 0.2 -1.7 9.4 0.5 34.3 25.0 0.0 70.0 1.5 1.9 1.7 91.0 46.9 25.0 8 0.5 0.5 -1.8 8.8 0.3 27.4 20.1 0.0 72.3 3.4 1.9 2.2 93.1 35.3 24.8 9 Pd/C 0.2  0.2 -2.5 8.7 0.5 22.6 18.1 0.0 64.8 12.6 1.5 3.1 90.2 28.3 17.5 10 0.2 0.5 -2.5 2.4 0.2 16.3 13.7 0.0 62.7 15.5 3.4 4.7 92.8 18.4 15.8 11 0.5 0.2 -1.8 8.9 0.4 27.9 16.0 0.0 66.3 12.4 2.0 3.4 89.2 33.8 24.0 12 0.5 0.5 -1.9 8.8 0.2 13.2 17.5 0.0 65.3 11.0 2.4 4.0 97.2 17.7 13.0 C (M) = catholyte concentration, A (M) = anolyte concentration, E = cathode potential (vs. Ag/AgCl), X = guaiacol conversion, S = normalized product selectivity (C mol%) for cyclohexanol (1), cyclohexanone (2), 2-methoxycyclohexanol (3), 2-methoxycyclohexanone (4), methanol (5), phenol (6), C.B. = carbon balance, F.E. = Faradaic efficiency, Rx = reaction rate (mmol of guaiacol h-1 mmol of dispersed metal-1). Conditions: T = 50 oC, I = -0.3 A (j = -109 mA cm-2), Catalyst loading = 0.20 g (catalyst concentration = 13.05 wt.%); Reaction time = 4 h; pH was measured after the reaction. Electrolyte: NaCl (catholyte), H2SO4 (anolyte).112  Chapter 6: Electrocatalytic Reduction of Phenolics and Bio-oil Using Mixed Organic and Aqueous Electrolytes Organic solvent plays an important role in biomass conversion. Electrocatalytic reduction pathways have been studied for upgrading biomass substrates, including carbohydrate and lignin derivatives, mostly in aqueous electrolytes. This approach, however beneficial with no requirements of additional organic solvent, has limitations associated with reactant solubility, especially for a large-scale process. Since most lignin-derived substrates are poorly soluble in water, organic electrolyte is necessary for the advancement of the ECH process. This chapter presents the applications of organic electrolyte in the ECH of lignin model compounds and bio-oil substrates. A rationale on the important role of organic solvent is discussed, focusing on the potential of methanesulfonic acid (MSA) solution as a green electrolyte. Mass transport and kinetic aspects in the ECH of guaiacol using stirred slurry electrocatalyst are also evaluated to better understand the reaction mechanism. Impact of additional organic solvent in mixed electrolyte configuration is studied, in conjunction with its applications in the ECH of bio-oil.  6.1. The importance of organic solvent in electrocatalytic reduction of lignin derivatives In biomass conversion process, organic solvent plays an important role in promoting the reactant solubility. Chemical reactivity, including reaction rate, reaction pathways, product selectivity could also be influenced by the solvent medium, which changes the solvent–solute interactions associated with hydrogen bonding and electric dipole moments.98 Organic solvent effects in catalytic hydrogenation have been discussed elsewhere, showing that thermodynamic interaction of solvent with reactants and products is likely the dominant factor which influences the rate of hydrogenation.99 The presence of solvent also affected the solubility of hydrogen and the  113  hydrogenation reaction energetics, as reflected by changes in the activation energy over different reaction media. Solvent system can modify the free energy of the reactants, transition states, products, and the catalyst, affecting the thermodynamic states of the substrates.98 Dissolution of lignin in medium-polarity solvents (e.g., acetone, ethanol, tetrahydrofuran) is preferable rather than in very polar solvent (e.g., water) or non-polar solvents (e.g., hexane) considering a medium polarity of lignin structure, which consists of both non-polar (e.g., aromatic rings, methoxy groups, and ether linkages) and polar functionalities (e.g., hydroxyl, carbonyl, and aldehyde groups).98   Organic solvent is particularly needed for reactions involving substrate with higher functionalities (or larger molecular weights). The presence of organic solvent in the electrolyte, however, tends to limit electrochemical reactivity because of the lower electrical conductivity. A water-soluble, highly conductive organic solvent is therefore necessary for further development of electrocatalytic process for valorization of lignin-derived substrates. Methanesulfonic acid (MSA), in this regard, is a potential organic electrolyte with environmental and economic advantages. MSA is a stronger organic acid (than citric acid, formic acid, and acetic acid) and a greener acid (i.e. less toxic and corrosive than mineral acids, such as HCl and H2SO4).100–102 MSA is considered to be a natural product as part of the natural sulfur cycle, which is readily biodegradable with sulfate and CO2 as the degradation products.101,102 MSA has also been used in various applications, such as electroplating, catalysis, solvolysis, and extractive metallurgy.101–103 6.2. Mass transport and kinetic study in ECH of guaiacol using MSA electrolyte 6.2.1. Investigating the effect of mass-transport related parameters Under potentiostatic control, several process parameters (e.g., type of electrolyte, cathode potential, cathode material) were previously investigated in the ECH of guaiacol using a dispersed catalyst  114  (5 wt.%-Pt/C).49 In this work, different catalytic and mass transport-related parameters were further evaluated using MSA electrolyte (0.2 M) for 4 h reactions (Figure 6.1).  Guaiacol conversion and Faradaic efficiency (F.E.) increased with the increasing Pt content (from 1 to 5 wt.%), however decreased dramatically at the higher loading of 10 wt.%-Pt (Figure 6.1a). The optimum catalytic activity of 5 wt.%-Pt/C can be attributed to its textural properties, including high surface area and large pore volume (Table 6.1). Remarkably, over 82% guaiacol was converted with cyclohexanol as the main product (45% selectivity) at low temperature (35 oC) and good F.E. (73%). Among the tested catalysts, the poorer performance of 10 wt.%-Pt/C was likely caused by its lower surface area, smaller pore volume, and lower metal dispersion. As a result, low guaiacol conversion (13%) was obtained at extremely low F.E. (4.7%) which indicates the current was largely used for HER rather than ECH; therefore, only first-step hydrogenation products (i.e. phenol, methanol, and 2-methoxycyclohexanone) were observed. Table 6.1. Characteristics of the Pt/C catalysts with different metal contents. Pt content (wt.%) SBETa  (m2/g) Vporeb (cm3/g) Dporec  (nm) φ  (%) Smetal  (m2/g)  dp (nm)  1 1010 0.82 4.36 36.98 91.3 3.1 3 1448 1.19 3.70 14.55 35.9 7.8 5 5* 1487 138 1.43 0.29 4.35 6.16 28.95 18.23 71.5 45.0 3.9 6.2 10 972 0.91 4.17 8.45 20.9 13.4 a BET surface area;  b Single point adsorption total pore volume (P/P0 = 0.99); c Average pore diameter by BJH desorption; φ (metal dispersion), Smetal (metallic surface area), and dp (active particle diameter) determined by CO chemisorption. *Exception case for Pt/Al2O3.   115   Interestingly, the stirring rates also significantly affected the reactivity of guaiacol ECH. A volcano-type profile was obtained (Figure 6.1b) showing the optimum stirring rates between 350–700 rpm for the cell used in this study, resulting in high guaiacol conversions (83–86%) and F.E. (70–73%). Thus, these stirring conditions may be considered under kinetic-controlled regime, excluding the external mass transport limitation. At the lowest stirring rates applied (125 rpm), only 25% guaiacol conversion was achieved with low F.E. (18%). Too slow agitation caused poor catalyst dispersion on the electrolyte solution, signifying the importance of physical contact between catalyst particles, reacting molecules, and the cathode current feeder. On the other extreme, too fast agitation (1100 rpm) induced vortex formation causing poor electron transfer, as indicated by the dramatic current drop (i.e. increased ohmic resistance), since most parts of the cathode were no longer fully submerged in the electrolyte. Obviously, no guaiacol was converted due to the mass transfer issues. Although it might not be a problem in a continuous flow reactor, or if a rotating electrode was used, these results importantly highlighted the necessity of thorough stirring for effective mass transfer in the SSER configuration. Moreover, the results also evidently show that the stirring rates in a batch reactor could discriminate the diffusion- or kinetic-controlled reaction regime. Under the applied conditions, 350–700 rpm is the agitation range that resulted in the kinetic-controlled reactions (no external diffusion limitations). Beyond this range, the reaction would be governed by external diffusion, while the internal diffusion effect is negligible because of the small catalyst particle sizes used (i.e. fine powder: <75 m, based on U.S. standard test sieve with 3–4 nm active particle diameter, based on CO chemisorption analysis, see Table 6.1). The effect of internal and external mass-transfer resistances was assessed by Weisz-Prater criterion and Sherwood number, respectively using our experimental data and other literature data,104–109 confirming the insignificant effect of diffusion limitations in this study (see Appendix A.2).   116   Figure 6.1. ECH of guaiacol in MSA electrolyte (0.2 M) under potentiostatic control showing the effects of (a) metal content, (b) stirring rates, and (c) catalyst concentration. Reaction conditions: E = -1.25 V (vs. Ag/AgCl), j ≈ -150 mA cm-2 (except j = -10 mA cm-2 at Rd = 1100 rpm), T ≈ 36 oC (except T = 24 oC at Rd = 1100 rpm), t = 4 h. Default parameters: Pt content = 5 wt.%, Rd = 350 rpm (Stirrer A), catalyst concentration = 9.10 wt.%. Guaiacol concentration = 0.1 M. Catholyte pH ≈ 0.8–0.9 (measured at the end of reaction). All the experiments started at room temperature and the temperature increases were attributed to the Joule heating effect and the exothermic nature of the ECH reaction.  0102030405060708001020304050607080901001 3 5 10Faradaic Efficiency (%)Conversion, Selectivity (%)Pt content (wt.%)GuaiacolCyclohexanolCyclohexanone2-Methoxycyclohexanol2-MethoxycyclohexanoneMethanolPhenolF.E. (%)(a)010203040506070809001020304050607080901004 (125) 6 (350) 8 (700) 10 (1100)Faradaic Efficiency (%)Conversion, Selectivity (%)Rd (rpm)(b)010203040506070809001020304050607080901004.77 9.10 13.05Faradaic Efficiency (%)Conversion, Selectivity (%)Catalyst concentration (wt.%)(c) 117   The presence of a well-dispersed catalyst is therefore required for an effective ECH. As expected, guaiacol conversion increased (from 48% to 100%) with the catalyst concentration (from 4.8 wt.% to 13.1 wt.%, corresponding to catalyst loading of 0.05 g to 0.15 g) (Figure 6.1c). The increasing F.E. (from 40% to 78%) shows that higher catalyst amount could effectively provide higher surface coverages of the adsorbed hydrogen and the organic molecules. In all cases, the product distribution was not altered whereby cyclohexanol (38–52%) and 2-methoxycyclohexanol (19–32%) were the most selective products. Interestingly, the activity of 10 wt.%-Pt/C catalyst was significantly enhanced at the higher concentration (10.7 wt.%, corresponding to catalyst loading of 0.15 g) and higher MSA concentration (0.5 M), resulting in remarkably high guaiacol conversion (97%) and F.E. (70%) after 4 h under similar potentiostatic conditions (E= -1.25 V, j = -224 mA cm-2, T = 37 oC). The two major products were again cyclohexanol (53%) and 2-methoxycyclohexanol (31%), suggesting that even at high guaiacol conversion (>90%), the parallel pathways involving demethoxylation and ring saturation steps remained prevalent under the mild conditions applied in this work.  Additionally, different catalyst supports (i.e. carbon vs alumina) which possess largely different electrical conductivity were compared in the guaiacol ECH using dilute H2SO4 electrolyte (0.2–0.5 M). With the same bulk catalyst amount (0.10 g, R/M = 314), Pt/C showed superior performance than Pt/Al2O3 (Figure 6.2), resulting in nearly 8 times higher guaiacol conversion and 10 times higher F.E, which may again be attributed to its surpassing textural properties (e.g., surface area, pore volume, and metal dispersion) (Table 6.1). Because the molar mass of Al2O3 is about 8.5 times higher than that of C, it required the corresponding amount of Pt/Al2O3 (0.85 g, R/M = 37) to increase the ECH reactivity (Figure 6.2a-b). At the higher electrolyte concentration (0.5 M), comparably high guaiacol conversion (92% vs. 96%) and cyclohexanol selectivity (49%  118  vs. 51%) were obtained despite the moderate F.E. (31% vs. 35%) in comparison with the low loading of Pt/C (Figure 6.2b). These results suggested that the less electrically conductive support material (Al2O3) can still provide good catalytic activity whereby the dispersed metal (Pt) remains to play a pivotal role for the hydrogenation. In a separate electrolysis experiment using mere support (e.g., activated charcoal), no hydrogenation products were formed at all showing the importance of dispersed metal sites.49,50 No stark differences were observed in terms of the product distribution, implying that the different catalyst supports did not influence the reaction pathways under the operating conditions.   Further potentiostatic ECH experiments revealed the comparable performance of MSA with benchmark acid electrolytes (H2SO4, HClO4) at the same concentrations (0.2 M and 0.5 M). Comparably high guaiacol conversions (83–96%) were achieved at interestingly higher F.E. (Figure 6.3), which could be attributed to the improved guaiacol solubility and lower H2 evolution rates for the reactions with MSA. Product distribution, however, was not affected by the strong acid types, again showing the predominant selectivities to cyclohexanol (45–53%) and 2-methoxycyclohexanol (20–26%). Overall, these results demonstrate that: (i) MSA can be applied as a strongly acidic, water-soluble organic electrolyte that can be a good alternative to inorganic and mineral acid electrolytes for the ECH purposes, (ii) optimum stirring is essential for an efficient ECH in SSER configuration to ascertain well dispersion of the catalyst particles and uniform contact with the reactant molecules; and this is true for all types of electrolytes (as discussed in Chapters 4–5).  119   Figure 6.2. ECH of guaiacol in H2SO4 electrolyte: (a) 0.2 M, (b) 0.5 M. Guaiacol conversion, product selectivity, and Faradaic efficiency profiles at different catalyst support materials. Reaction conditions: E = -1.25 V (vs. Ag/AgCl), T ≈ 42 oC, t = 4 h, Rd = 350 rpm (Stirrer A), (a) I ≈ -0.57 A (j = -206 mA cm-2), (b) I ≈ -0.95 A (j = -345 mA cm-2). Catalyst: 5 wt.%-Pt/C (0.10 g), 5 wt.%-Pt/Al2O3 (0.10 g and 0.85 g). Guaiacol concentration (initial) = 0.1 M. R/M = reactant/metal molar ratio. Molecular weight of the catalyst support: Mw = 12.01 (carbon), 101.96 (alumina). 0102030405060708090100Pt/C (314) Pt/Al₂O₃ (37) Pt/Al₂O₃ (314)Conversion, Selectivity, F.E. (%)Catalyst (R/M)(a)0102030405060708090100Pt/C (314) Pt/Al₂O₃ (37) Pt/Al₂O₃ (314)Conversion, Selectivity, F.E. (%)Catalyst (R/M)Guaiacol CyclohexanolCyclohexanone 2-Methoxycyclohexanol2-Methoxycyclohexanone MethanolPhenol F.E. (%)(b)[H2SO4] = 0.2 M [H2SO4] = 0.5 M  120   Figure 6.3. ECH of guaiacol in different acid electrolytes, including sulfuric acid (HSA), perchloric acid (HPA), and methanesulfonic acid (MSA) with different concentrations: (a) 0.2 M, (b) 0.5 M. Reaction conditions: E = -1.25 V (vs. Ag/AgCl), t = 4 h, Rd = 350 rpm (Stirrer A), for HSA/HPA: T ≈ 40–43 oC, I ≈ -0.53 A (j = -193 mA cm-2) to -0.90 A (j = -326 mA cm-2), for MSA: T ≈ 35–41 oC, I ≈ -0.39 A (j = -140 mA cm-2) to -0.77 A (j = -282 mA cm-2). Catalyst: 5 wt.%-Pt/C (0.10 g), R/M = 314. CGUA (initial) = 0.1 M. Catholyte pH: (a) 0.8–0.9, (b) 0.5–0.6. 0102030405060708090100HSA HPA MSAConversion, Selectivity, F.E. (%)Electrolyte (C = 0.5 M)Guaiacol CyclohexanolCyclohexanone 2-Methoxycyclohexanol2-Methoxycyclohexanone MethanolPhenol F.E. (%)(b)0102030405060708090100HSA HPA MSAConversion, Selectivity, F.E. (%)Electrolyte (C = 0.2 M)(a) 121  6.2.2. Reaction network and mechanism study in the ECH of guaiacol The rate-determining step (RDS) in the guaiacol ECH was first determined through comparison between the assumed reaction mechanism and the rate data. In formulating the rate law for guaiacol ECH, the experimental rate data were ensured to be governed by reaction kinetic rather than mass transport. Under the operating conditions, the fluid velocities were large enough at the optimum stirring rates while the catalyst particles were small enough such that neither external diffusion nor internal diffusion is limiting. The RDS was determined based on Langmuir–Hinshelwood mechanism and the derived rate law was then compared with the best fitting experimental data. As in the classic heterogeneous catalysis, three possible scenarios were evaluated to verify the RDS: adsorption-, surface reaction-, and desorption-limited reaction.110 The mathematical derivations are provided in the Appendices A.3–A.4. Adsorption of guaiacol was found to be the RDS as shown by the linear relationship between the initial reaction rate and the initial guaiacol concentration, either under potentiostatic or galvanostatic conditions (Figure 6.4), confirming our observation in the previous work.49 However, at low guaiacol concentrations, this linear relationship could also imply the surface reaction-controlled mechanism.  Guaiacol ECH mechanism is hence different than that of phenol ECH, indicating the different reactivity toward hydrogen radicals. Phenol ECH was found to be zeroth order reaction, implying that the reaction rate is independent of the initial phenol concentration.36,49 The RDS in phenol ECH could be the hydrogenation step (when phenol concentration is sufficiently abundant) or the desorption step (when both the adsorption and surface reaction are all fast due to the nature of phenol as hydrogen scavenger). On the other hand, it is proposed that guaiacol ECH rates are dependent on the mass transport-related parameters (e.g., reactant concentration, stirring rates, catalyst loading), thus the reaction may be interpreted as first- or second-order depending on the  122  experimental conditions.49 Again, it should be noted that the apparent reaction order is not an intrinsic parameter, which affected by the reaction conditions (discussed in Chapter 4). In contrast to phenol ECH which proceeds in a series reaction to cyclohexanone and cyclohexanol, guaiacol ECH occurs in a parallel pathway involving demethoxylation and aromatic ring saturation steps.  To verify the reaction mechanism with experimental data, guaiacol ECH experiments were carried out in acidic electrolytes (H2SO4 and MSA with the same concentration) under potentiostatic and galvanostatic conditions, respectively (Figure 6.4). Similar trends were observed in terms of initial reaction rate (-rA0) and Faradaic efficiency (F.E.), which increased with the guaiacol concentration. The higher guaiacol concentration promotes F.E. as the surface coverage of organic molecules increased. Consequently, at nearly complete guaiacol conversion (>99%), H2 evolution reaction (HER) became more dominant, thus lowering the F.E. Under potentiostatic control, cyclohexanol (~52%) and 2-methoxycyclohexanol (27–35%) were the most selective products (Figure 6.4a). However, under temperature-controlled galvanostatic conditions (T = 50 oC), demethoxylation of guaiacol was promoted over ring saturation by the increasing temperature, resulting in the higher cyclohexanone (18–28%) and the lower 2-methoxycyclohexanol (9–18%) selectivities (Figure 6.4b). In both cases, a linear relationship is obtained between the initial reaction rate and the guaiacol concentration (50–130 mM) under the applied conditions (Figures 6.4c–d). These results demonstrate that the ECH of guaiacol in this work is either adsorption-limited or surface reaction-limited (at low concentrations), consistent with the rate law formulation described earlier. The kinetic model was then validated by fitting the experimental and calculated rate data using nonlinear regression (see Appendix A.4). In the model, the active sites for organic reactant adsorption and proton reduction and the amount of Hads coverage for ECH and HER could not be distinguished. However, insights can still be drawn from the computational results.  123   Figure 6.4. ECH of guaiacol under potentiostatic and galvanostatic conditions in H2SO4 (0.2 M) and MSA (0.2 M) electrolytes, respectively. Upper panel (a–b): Guaiacol conversion, product selectivity, and Faradaic efficiency profiles at different initial guaiacol concentrations. Lower panel (c–d): Initial reaction rate as a function of initial guaiacol concentration showing a linear relationship, thus implying that adsorption is the RDS under the operating conditions. Experimental conditions: (a, c) E = -1.25 V (vs. Ag/AgCl), j ≈ -182 to -218 mA cm-2, T ≈ 40 oC, t = 4 h, Catalyst: 5 wt.%-Pt/C (0.10 g); (b, d) j = -182 mA cm-2, E ≈ -1.09 V to -1.29 V (vs. Ag/AgCl), T = 50 oC, t = 4 h, Catalyst: 5 wt.%-Pt/C (0.15 g). Initial reaction rates were measured after 1 h. 6.2.3. Impact of catalyst concentration and stirring profiles in a slurry reactor In the SSER configuration, catalyst loading and stirring profiles are influential toward the dispersion and solid-liquid mass transfer, affecting the frequency and intensity of collision between 0102030405060708090100010203040506070809010056 78 99 121Faradaic Efficiency (%)Conversion, Selectivity (%)Guaiacol concentration (mM)0102030405060708090100010203040506070809010053 80 106 132Faradaic Efficiency (%)Conversion, Selectivity (%)Guaiacol concentration (mM)Guaiacol CyclohexanolCyclohexanone 2-Methoxycyclohexanol2-Methoxycyclohexanone MethanolPhenol F.E. (%)y = 1.4338x + 380.08R² = 0.966630035040045050055060040 60 80 100 120 140-rA0(mmol h-1 g-1)CA0 (mM)y = 0.9935x + 305.78R² = 0.965230035040045050055060040 60 80 100 120 140-rA0(mmol h-1 g-1)CA0 (mM)Potentiostatic Galvanostatic(a) (b)(d)(c) 124  the reacting molecules and the catalyst particles. Interparticle collisions could affect adsorption/desorption and surface reaction rates on the catalyst. A well-stirred heterogeneous catalytic reactor is therefore used to eliminate interphase and interparticle transport gradients, such that the experimental kinetic data reflect only chemical events.104 In this work, sufficiently high stirring rates (240–500 rpm, depending on the stirrer size) and low catalyst loading ensured the ECH rates were not limited by external diffusion.60 Moreover, the cell reactor radius was also small enough, the solution was relatively dilute (i.e. low guaiacol concentrations of 50–130 mM corresponding to 0.6–1.6 wt.%), and the catalyst particles were sufficiently small that interparticle transport resistances could be minimized, based on the criteria proposed by Mears.104  The experimental results (Figure 6.5) show consistent trends with the previously reported ECH of guaiacol in H2SO4 (0.5 M) electrolyte pairs in terms of the product distribution.49 The higher temperature (60 oC) favored demethoxylation–ring saturation step leading to higher cyclohexanone selectivity (~42%) at high guaiacol conversion (76%), in contrast to the lower temperatures (40–50 oC) which resulted in cyclohexanol (32–43%) as the most selective product after 4 h. These reaction pathway shifts were ascribed to the potential-temperature synergistic effect which affected the Hads coverage on the catalyst surface. Interestingly, with lower catalyst concentration (7 wt.%), the profile trends are very similar (Figure 6.6) for all temperatures. For instance, the results at 60 oC also show comparably high cyclohexanone selectivity (43%) and guaiacol conversion (65%), implying that under the operating conditions the product distribution is not affected by the catalyst concentration. However, in all temperature conditions, the higher catalyst concentration (10 wt.%) promoted ECH rates as shown by the higher F.E. (59–62% vs. 44–51%). These galvanostatic results (compare Figures 6.5d-f and 6.6d-f) are in a good agreement with those obtained potentiostatically (Figure 6.1c) in terms of the effect of higher catalyst loading  125  on the F.E. It should be noted, however, that an excessive catalyst loading could negatively affect the carbon balance due to physical adsorption onto the porous support.50  Figure 6.5. ECH of guaiacol in MSA electrolyte (0.2 M). Guaiacol conversion and product selectivity profiles at different temperatures: (a) 40 oC (b) 50 oC, (c) 60 oC. Faradaic efficiency profiles for the corresponding results (d, e, f). Reaction conditions: I = -0.5 A (j = -182 mA cm-2), t = 4 h, Rd = 240 rpm (Stirrer B). Catalyst: 5 wt%-Pt/C (0.15 g, corresponding to concentration of 10 wt.%). Guaiacol concentration (initial) = 106 mM. Catholyte pH ≈ 0.8–0.9. Reactant/metal molar ratio (R/M) ≈ 279. The apparent reaction order = 1 (vs. guaiacol). 01020304050607080900 1 2 3 4Conversion, Selectivity (%)Time (h)GuaiacolCyclohexanolCyclohexanone2-Methoxycyclohexanol2-MethoxycyclohexanonePhenolMethanol(a)01020304050607080901000 1 2 3 4Faradaic Efficiency (%)Time (h)F.E. (%)(d)01020304050607080900 1 2 3 4Conversion, Selectivity (%)Time (h)(b)01020304050607080901000 1 2 3 4Faradaic Efficiency (%)Time (h)(e)01020304050607080900 1 2 3 4Conversion, Selectivity (%)Time (h)(c)01020304050607080901000 1 2 3 4Faradaic Efficiency (%)Time (h)(f) 126   Figure 6.6. ECH of guaiacol in MSA electrolyte (0.2 M). Guaiacol conversion and product selectivity profiles at different temperatures: (a) 40 oC, (b) 50 oC, (c) 60 oC. Faradaic efficiency profiles for the corresponding results (d, e, f). Reaction conditions: I = -0.5 A (j = -182 mA cm-2), t = 4 h, Rd = 240 rpm (Stirrer B). Catalyst: 5 wt%-Pt/C (0.10 g, corresponding to concentration of 7 wt.%), R/M ≈ 419. Guaiacol concentration (initial) = 106 mM. Catholyte pH ≈ 0.8–0.9. The apparent reaction order = 2 (vs. guaiacol). 0102030405060700 1 2 3 4Conversion, Selectivity (%)Time (h)GuaiacolCyclohexanolCyclohexanone2-Methoxycyclohexanol2-MethoxycyclohexanonePhenolMethanol(a)01020304050607080901000 1 2 3 4Faradaic Efficiency (%)Time (h)F.E. (%)(d)0102030405060700 1 2 3 4Conversion, Selectivity (%)Time (h)(b)01020304050607080901000 1 2 3 4Faradaic Efficiency (%)Time (h)(e)0102030405060700 1 2 3 4Conversion, Selectivity (%)Time (h)(c)01020304050607080901000 1 2 3 4Faradaic Efficiency (%)Time (h)(f) 127   Figure 6.7. ECH of guaiacol in MSA electrolyte (0.2 M). Guaiacol conversion and product selectivity profiles at different temperatures: (a) 40 oC, (b) 50 oC, (c) 60 oC. Faradaic efficiency profiles for the corresponding results (d, e, f). Reaction conditions: I = -0.5 A (j = -182 mA cm-2), t = 4 h, Rd = 500 rpm (Stirrer A). Catalyst: 5 wt%-Pt/C (0.15 g, corresponding to concentration of 10 wt.%), R/M ≈ 279. Guaiacol concentration (initial) = 106 mM. Catholyte pH ≈ 0.8–0.9. The apparent reaction order = 2 (vs. guaiacol). 0102030405060700 1 2 3 4Conversion, Selectivity (%)Time (h)GuaiacolCyclohexanolCyclohexanone2-Methoxycyclohexanol2-MethoxycyclohexanonePhenolMethanol(a)01020304050607080901000 1 2 3 4Faradaic Efficiency (%)Time (h)F.E. (%)(d)0102030405060700 1 2 3 4Conversion, Selectivity (%)Time (h)(b)01020304050607080901000 1 2 3 4Faradaic Efficiency (%)Time (h)(e)0102030405060700 1 2 3 4Conversion, Selectivity (%)Time (h)(c)01020304050607080901000 1 2 3 4Faradaic Efficiency (%)Time (h)(f) 128    Figure 6.8. Guaiacol conversion, product selectivity, and Faradaic efficiency at different temperatures and I = -0.5 A (j = -182 mA cm-2) after 4 h. Catalyst (5 wt%-Pt/C) with different loading (concentration): (a) 0.15 g (10 wt.%), (b) 0.10 g (7 wt.%), (c) 0.15 g (10 wt.%). Stirring rate (Rd) and stirrer size (L): (a) 240 rpm, 3.6 cm, (b) 240 rpm, 3.6 cm, (c) 500 rpm, 2.4 cm. Guaiacol concentration (initial) = 106 mM. In all cases, the increasing temperature resulted in the lower cathode potential (vs. Ag/AgCl) and thus affecting the product distribution. This potential-temperature synergistic effect has also been observed in the previous work using H2SO4 electrolyte.49  010203040506070800102030405060708040 50 60Faradaic Efficiency (%)Conversion, Selectivity (%)Temperature (⁰C)(b)010203040506070800102030405060708040 50 60Faradaic Efficiency (%)Conversion, Selectivity (%)Temperature (⁰C)Cyclohexanol Cyclohexanone2-Methoxycyclohexanol 2-MethoxycyclohexanoneMethanol PhenolGuaiacol F.E. (%)(c)010203040506070800102030405060708040 50 60Faradaic Efficiency (%)Conversion, Selectivity (%)Temperature (⁰C)(a)T (oC) Ecathode (V) 40 50 60 -1.193 -1.090 -1.001  T (oC) Ecathode (V) 40 50 60 -1.234 -1.093 -0.931  T (oC) Ecathode (V) 40 50 60 -1.322 -1.267 -1.210   129   Condition Reaction order Rate constant (k) Unit 40 oC 50 oC 60 oC (a) 1st  8.0 × 10-5 (7.20) 9.0 × 10-5 (8.09) 1.0 × 10-4 (8.98) s-1 (s-1 mol-1Pt) (b) 2nd 7.2 × 10-4 (97.3) 9.5 × 10-4 (128.3) 1.1 × 10-3 (150.6) M-1 s-1 (M-1 s-1 mol-1Pt) (c) 2nd  6.8 × 10-4 (60.7) 1.1 × 10-3 (101.7) 1.2 × 10-3 (106.8) M-1 s-1 (M-1 s-1 mol-1Pt) Figure 6.9. Graphical analysis for the apparent reaction order determination in ECH of guaiacol using MSA (0.2 M) electrolyte pairs with different catalyst concentrations, stirring rates, and stirrer sizes: (a) 10 wt.%, 240 rpm, 3.6 cm [Figures 6.5, 6.8a], (b) 7 wt.%, 240 rpm, 3.6 cm [Figures 6.6, 6.8b], (c) 10 wt.%, 500 rpm, 2.4 cm [Figures 6.7, 6.8c].  Reaction conditions Figures (a–c): I = -0.5 A (j = -182 mA cm-2), T = 60 oC, t = 4 h.  The table shows the rate constant for each temperature under the different conditions with the values in brackets are determined by normalization with the catalyst active sites (dispersed Pt molar amount). y = -19.619x + 98.333R² = 0.96030204060801001200 1 2 3 4CATime (h)0th orderLinear (0th order)(a)y = -0.3518x + 4.6629R² = 0.99720.00.51.01.52.02.53.03.54.04.55.00 1 2 3 4ln(CA)Time (h)1st orderLinear (1st order)y = 0.0073x + 0.007R² = 0.93830.000.010.010.020.020.030.030.040.040.050 1 2 3 41/CATime (h)2nd orderLinear (2nd order)y = -16.805x + 98.24R² = 0.93950204060801001200 1 2 3 4CATime (h)0th orderLinear (0th order)(b)y = -0.2599x + 4.6192R² = 0.98990.00.51.01.52.02.53.03.54.04.55.00 1 2 3 4ln(CA)Time (h)1st orderLinear (1st order)y = 0.0043x + 0.009R² = 0.9970.000.010.010.020.020.030.030 1 2 3 41/CATime (h)2nd orderLinear (2nd order)y = -15.974x + 95.397R² = 0.88810204060801001200 1 2 3 4CATime (h)0th orderLinear (0th order)(c)y = -0.2437x + 4.5727R² = 0.95730.00.51.01.52.02.53.03.54.04.55.00 1 2 3 4ln(CA)Time (h)1st orderLinear (1st order)y = 0.004x + 0.0098R² = 0.99450.000.010.010.020.020.030.030 1 2 3 41/CATime (h)2nd orderLinear (2nd order) 130   Figure 6.10. Arrhenius plots for the determination of activation energy (Ea) and pre-exponential factor (A) in ECH of guaiacol using MSA (0.2 M) electrolyte pairs at different temperatures (40–60 oC). Reaction conditions: I = -0.5 A (j = -182 mA cm-2), t = 4 h, corresponding to Figure 6.9: (a) 10 wt.%, 240 rpm, 3.6 cm (Stirrer B), (b) 7 wt.%, 240 rpm, 3.6 cm (Stirrer B), (c) 10 wt.%, 500 rpm, 2.4 cm (Stirrer A).    y = -1.156x + 5.6677R² = 0.99991.952.002.052.102.152.202.252.95 3.00 3.05 3.10 3.15 3.20 3.25ln kT-1 (× 10-3, K-1)(a)y = -2.276x + 11.85R² = 14.554.604.654.704.754.804.854.904.955.005.052.95 3.00 3.05 3.10 3.15 3.20 3.25ln kT-1 (× 10-3, K-1)(b)y = -2.9403x + 13.502R² = 0.99994.004.104.204.304.404.504.604.704.802.95 3.00 3.05 3.10 3.15 3.20 3.25ln kT-1 (× 10-3, K-1)(c)Ea  (kJ mol-1) A  (s-1 mol-1)  9.61 2.89 × 102  Ea  (kJ mol-1) A  (M-1 s-1 mol-1)  18.92 1.40 × 105  Ea (kJ mol-1) A  (M-1 s-1 mol-1) 24.45 7.31 × 105   131   Furthermore, the stirring profiles, combining the effects of stirring rate and stirrer size, were also investigated in the galvanostatic ECH of guaiacol to ensure the operation under kinetic-controlled regimes. Previously, the ECH of guaiacol in H2SO4 electrolyte at 30–60 oC appeared to be a second-order reaction using 5 wt.%-Pt/C (0.10 g, corresponding to 7 wt.% concentration) with the slurry stirred at 500 rpm using a small stirrer (Ls = 2.4 cm, ws = 3.9 g).49 Further ECH experiments and graphical kinetic analyses (Figures 6.5–6.9) showed changes in the apparent reaction order (between 1st and 2nd) at the higher catalyst loading using the bigger stirrer (Lb = 3.6 cm, wb = 9.6 g) with reasonable trends for the rate constants (directly proportional to the reaction temperature). The apparent activation energy (Ea) and pre-exponential factor (A) were also calculated from the experiments. Factors affecting Ea and A have long been discussed in the heterogeneous catalysis field and the kinetic analysis is complex because both parameters include contributions of adsorption and intrinsic activation steps of the reacting molecules.111 The Arrhenius equation k = Ae−EaRT is strongly associated with the collision theory and transition state theory. It has been understood that Ea represents the minimum energy reacting molecules must possess in order to react to form a product, while A is attributed to the frequency of collisions with proper orientation and sufficient energy.110,112,113 The molecules need energy to (i) distort or stretch their bonds to break and form new bonds, (ii) overcome the steric and electron repulsive forces as they come close together. This barrier to energy transfer (from kinetic energy to potential energy) that must be overcome by the reacting molecules is represented by the activation energy. The kinetic energy of the colliding molecules could increase potential energy of the reactants, thereby lowering the energy barrier.110 Higher temperature would affect the relative velocity between the colliding molecules which contributes to higher fraction of molecular collisions with enough energy to surmount the barrier and react, leading to the faster reactions. True activation energy of  132  a catalytic reaction is hence determined under diffusion-effect-free conditions.105 In this study, the experimental variation of catalyst concentration and stirring profiles resulted in the similar magnitude of apparent activation energy (10–25 kJ mol-1) for the guaiacol ECH, implying that the reaction occurred in the comparable kinetic regime (Figure 6.10). Slight differences in the resulting Ea and A values may be attributed to the adsorption-controlled guaiacol ECH rates. Relationship between Ea and A was also investigated in catalytic cracking of short chain alkanes on zeolites, showing that the pre-exponential factor tends to compensate the change in activation energy.111  6.2.4. Reaction order and rate constant determination in the guaiacol ECH pathways The purpose of this analysis is to evaluate the kinetic parameters of each sub-reaction in the reaction network of guaiacol ECH at different temperatures (40–60 oC). Individual experiments using the intermediate model compounds (e.g., phenol, cyclohexanone, 2-methoxycyclohexanone, and 2-methoxycyclohexanol) were conducted at the identical conditions to determine the reaction order of each reaction by graphical analysis (Figure 6.11). Under the operating conditions, the reaction order was found to be 1 (with respect to phenol and cyclohexanone) and 2 (2-methoxycyclohexanone) based on the best approximation results (R2 = 0.9957 for phenol, R2 = 0.9912 for cyclohexanone, and R2 = 0.9431 for 2-methoxycyclohexanone). It was observed in most cases that 2-methoxycyclohexanol was formed as cis and trans isomers. Due to the commercial unavailability of cis-2-methoxycyclohexanol, trans-2-methoxycyclohexanol was used in this work. Unfortunately, no products were formed from the ECH of trans-2-methoxycyclohexanol (thus the reaction order could not be determined experimentally). This implies that cyclohexanol in this work was formed via cyclohexanone hydrogenation and/or cis-2-methoxycyclohexanol demethoxylation (Figures 6.11b-c). A control experiment on 2-methoxycyclohexanol by Garedew  133  et al.13 also did not show any conversion to cyclohexanol, which suggested that demethoxylation step of guaiacol preceded ring saturation. The reaction order of 2-methoxycyclohexanol, owing to this limitation, was assumed to be the same as that of 2-methoxycyclohexanone.   Figure 6.11. Concentration and Faradaic efficiency profiles from ECH of intermediate reactant: (a) phenol, (b) cyclohexanone, (c) 2-methoxycyclohexanone. Reaction conditions:  I = -0.5 A (j = -182 mA cm-2), T = 50 oC, t = 2 h, Rd = 240 rpm (Stirrer B). Catalyst: 5 wt.%-Pt/C (0.15 g, corresponding to concentration of 10 wt.%). Initial reactant concentration = 0.1 M. Catholyte pH ≈ 0.8. The apparent reaction order: (a) 0th or 1st (vs. phenol), (b) 1st (vs. cyclohexanone), (c) 2nd (vs. 2-methoxycyclohexanone). 0204060801001200 0.5 1 1.5 2Concentration (mM)Time (h)PhenolCyclohexanolCyclohexanone(a)01020304050607080901000 0.5 1 1.5 2Faradaic Efficiency (%)Time (h)F.E. (%)(d)0204060801001200 0.5 1 1.5 2Concentration (mM)Time (h)CyclohexanoneCyclohexanol(b)01020304050607080901000 0.5 1 1.5 2Faradaic Efficiency (%)Time (h)F.E. (%)(e)01020304050607080901000 0.5 1 1.5 2Faradaic Efficiency (%)Time (h)F.E. (%)(f)0204060801001200 0.5 1 1.5 2Concentration (mM)Time (h)2-MethoxycyclohexanoneCyclohexanol2-MethoxycyclohexanolMethanol(c) 134   The rate constants were then approximated numerically to estimate the fastest and slowest reactions in the ECH of guaiacol. The model construction is illustrated in Figure 6.12, showing six ordinary differential equations with six rate constant parameters to be solved. Concentration data were taken from the galvanostatic ECH of guaiacol in MSA electrolyte at fixed temperatures (40–60 oC), which were already plotted in Figure 6.5. Model fitting was done using Levenberg-Marquardt method for the nonlinear least-squares algorithm in the MATLAB Optimization Toolbox solvers (see the codes in Appendix A.5). The program converged and the fitting results are displayed in Figure 6.13 – the numerical approximation results are summarized in Table 6.2.  Figure 6.12. Schematic procedure for kinetic analysis in the ECH of guaiacol to determine the reaction rate constants.  Overall, the model and actual data fit well, especially in guaiacol case, which suggested that the experimentally obtained reaction order values are acceptable and, hence, the predicted rate constant values are reliable. Data fitting for the other compound shows good trends with reasonable residuals. The discrepancies might be associated with the assumptions in the model, such as the Simplified:A B C DE Fk11 k12 k13k21 k22 k232e- 4e- 2e-4e- 2e- 2e-Demethoxylation – Ring SaturationRing Saturation – Demethoxylationa = b = c = 1e = f = 2Reaction order:Galvanostatic ECH experiments Numerical approximationRate constant:𝑘11 𝑘12 𝑘1 𝑘21 𝑘22 𝑘2  135  same order for 2-methoxycyclohexanone and 2-methoxycyclohexanol and the constant volume during reactions. With respect to the rate constant approximations, ECH of phenol to cyclohexanone was found to be the fastest step while 2-methoxycyclohexanol demethoxylation to cyclohexanol being the slowest (Table 6.2). The rate constants of phenol hydrogenation (3.31–6.85 h-1) were much higher than those of cyclohexanone hydrogenation (0.20–0.82 h-1). This trend is in accordance with the phenol ECH study by Song et al.,36 which reported that the rates of phenol hydrogenation are significantly faster than those of cyclohexanone hydrogenation, although the former step consumes twice as many protons/electrons. Demethoxylation of guaiacol was also favored by higher temperature as the rate constant increased proportionally with temperature. The opposite trend was seen in the aromatic ring saturation of guaiacol, which could explain the reaction pathway shift by temperature effect on the catalyst surface coverages leading to higher cyclohexanone selectivity (as discussed in Chapter 4).   Surprisingly, temperature negatively impacted the phenol hydrogenation step as the rate constant decreased over two-fold (6.85 h-1 to 3.31 h-1). This counter-intuitive effect of temperature was also reported by Singh et al.60 in the ECH of phenol over carbon-supported rhodium (Rh/C) at low cathode potentials (-0.15 to -0.45 V). The phenol conversion was nearly 100% at room temperature, however reached a plateau (60–75%) at higher temperature (60 oC). This was possibly caused by thermal dehydrogenation of phenol species that could block the active sites and/or desorption of the adsorbed species, resulting in the lower Hads coverage at elevated temperatures. In order to remove the site blockers, the applied potential (for ECH) or H2 pressure (for TCH) was increased, thus facilitating Hads coverage and resulting in enhanced phenol conversion at higher temperatures (60–100 oC).60 Decreases in phenol conversion at higher temperatures (160–200 oC) were also reported by Neri et al.90 over Pd/MgO or Pd/Al2O3, ascribed to the lower surface  136  coverage of the phenol reactants. Based on the temperature variation data (Figure 6.8), the increasing temperature contributed to positive changes in the cathode potential while the current remained constant. This lowered potential (i.e. turning less negative) could affect the surface Hads and organic coverages, resulting in the altered reaction pathway where phenol and cyclohexanone formation was favored. The surface coverage of Hads increases while the adsorption of organic species decreases with an increase in the negative overpotential.46 At constant potential chronocoulometric ECH of guaiacol, the reaction pathway changes were not observed (see Figures 4.3, 4.5, 4.6) and the dominant product was cyclohexanol, indicating that the Hads coverages were sustained by the fixed electric potential. Hence, temperature and electric potential could synergistically affect the surface Hads coverages and the reactivity of different organic molecules which have different activation energies on the catalyst. As a result, selectivity control in the ECH is possible through adjustment of the reaction parameters, such as current/potential, acid concentration, and/or pH, in synergy with temperature.  Meanwhile, the slow demethoxylation of 2-methoxycyclohexanol step could be attributed to the cis- and trans- isomerization. This effect is at present not able to be distinguished owing to the limitations in testing the ECH of pure cis-2-methoxycyclohexanol model compound. Here, all the calculations of 2-methoxycyclohexanol selectivity reflected the sum of both cis- and trans- isomers. In GC-MS analysis, they appeared at different retention times and typically the peak area ratios of cis- and trans- were approximately 8:2 or 9:1 under the experimental conditions. It was also frequently observed that cis-2-methoxycyclohexanol concentration changed more obviously over time, implying that the cis- form can be readily converted into cyclohexanol (as opposed to the seemingly inert trans- form). This different reactivity might be ascribed to the different adsorption caused by steric effect. Factors such as catalyst type, substrate structure, and reaction  137  conditions may contribute to the adsorption mode and orientation of the substrate on the catalyst surface.13 For instance, adsorption of aromatic ring parallel to the catalyst surface facilitates the ring saturation, whereas nonplanar adsorption facilitates the step-wise addition of hydrogen leading to the formation of intermediate ketone, which was likely the case in phenol hydrogenation to cyclohexanone.13,90 Phenol molecules chemisorption mode on the catalyst support depends on the catalyst acid-base properties.90 In the aforementioned case, Pd/Al2O3 favored co-planar adsorption, thus phenol was preferentially hydrogenated to cyclohexanol, whereas Pd/MgO (which has basic characteristic) favored non-planar adsorption enhancing the formation of cyclohexanone. The adsorption modes of 2-methoxycyclohexanone (which resulted in the cis- and trans-2-methoxycyclohexanol) and both 2-methoxycyclohexanol isomers are presently unknown. This can be the subject of further research to improve the kinetic understanding of the overall guaiacol ECH pathways. On a side note, 2-methoxycyclohexanone can be used as precursor for adipic acid, the monomer for nylon or polyurethane, via aerobic oxidation with phosphotungstic acid catalyst.114   Comparing the rate constant values for guaiacol conversion, the ratio of demethoxylation and ring saturation rates (k11/k21) was found to increase with temperature (Table 6.2), which could explain the increasing cyclohexanone and phenol selectivities along with the decreasing 2-methoxycyclohexanone and 2-methoxycyclohexanol selectivities (Figure 6.8). In addition, the total rate constant values for the overall guaiacol conversion (k11+k21) determined numerically are comparable to those calculated by graphical analysis (Figure 6.9a). The computed rate constant values lie reasonably between the 95% confidence interval (CI) lower and upper limits, with the standard deviation data in Appendix A.5. Overall, this work demonstrates the feasibility of numerical analysis for parameter estimation by nonlinear regression method, which delivered the decent results useful for quantitative interpretations of the kinetic trends in the ECH of guaiacol.    138   Figure 6.13. Concentration profiles for the actual and model data fitted by non-linear regression method as the rate constant approximation results from the galvanostatic ECH of guaiacol at different temperatures (40–60 oC). Compound notation: A – guaiacol, B – phenol, C – cyclohexanone, D – cyclohexanol, E – 2-methoxycyclohexanone, F – 2-methoxycyclohexanol. T = 40 oCT = 50 oCT = 60 oCT = 40 oCT = 50 oCT = 60 oC 139  Table 6.2. Guaiacol ECH rate constant approximation results from the galvanostatic experiments at different temperatures (40–60 oC). Reaction Index Rate constant (k) Unit 95% CI lower to upper limits   40 oC 50 oC 60 oC  40 oC 50 oC 60 oC Route 1: Demethoxylation – Ring Saturation Guaiacol to Phenol 11 0.174 0.210 0.242  h-1  0.163 – 0.185  0.202 – 0.218  0.233 – 0.252 Phenol to Cyclohexanone 12 6.848 5.656 3.314 h-1 1.942 – 11.755 4.125 – 7.186 2.744 – 3.883 Cyclohexanone to Cyclohexanol 13 0.819 0.376 0.199 h-1 0.720 – 0.918 0.340 – 0.412 0.175 – 0.223 Route 2: Ring Saturation – Demethoxylation         Guaiacol to 2-Methoxycyclohexanone 21 0.119 0.110 0.093 h-1 0.109 – 0.130 0.103 – 0.117 0.085 – 0.102 2-Methoxycyclohexanone to 2-Methoxycyclohexanol 22 0.022 0.017 0.017 mM-1 h-1 0.017 – 0.027 0.012 – 0.021 0.011 – 0.022 2-Methoxycyclohexanol to Cyclohexanol 23 0.008 0.013 0.033 mM-1 h-1 -0.085 – 0.024 -0.014 – 0.039 -0.023 – 0.089 Overall Guaiacol Conversion 11 + 21 0.293 0.320 0.336     Ratio of Guaiacol Conversion Pathways 11 / 21 1.46 1.92 2.60      140  6.3. Factorial analysis on the galvanostatic ECH of guaiacol  ECH of guaiacol is a complex process which involves parallel reaction pathways and heterogeneous phase, solid-liquid mass transfer in the presence of electric field. The ECH performance is also influenced by a variety of factors, such as guaiacol concentration, catholyte/anolyte proton concentration, temperature, current/potential, catalyst loading, electrolyte pH, stirring rates, etc. In this study, reaction conditions that give optimum conversion and efficiency for galvanostatic ECH are approximated by factorial analysis. The goal of factorial analysis is to reduce the large number of variables into fewer numbers of factors to interpret and explain the results. In general, this work aimed to find the most significant parameters (either as individual or synergistic effects) and the reaction conditions that give maximum guaiacol conversion at the highest Faradaic efficiency and maximum KA oil selectivities.  6.3.1. Design of experiments and the guaiacol ECH factorial experimental results A regular two-level factorial design was used with four selected reaction parameters, such as guaiacol concentration (CG), catholyte/anolyte proton concentration (CH,c or CH,a), temperature (T), and current (I). The responses were guaiacol conversion (X), Faradaic efficiency (F.E.), cyclohexanol selectivity (S1), and cyclohexanone selectivity (S2). The catholyte and anolyte proton concentration was decoupled to probe the individual effect, hence they were separated in the model. As a result, the number of experiments was 24 (the results are presented in Table 6.3). The overall lower and upper limit response values were obtained as follows: X (45.5–99.9%), F.E. (31.7–81.3%), S1 (25.9–66.2%), S2 (0–32.7%). Two designs were run in the Design-Expert software, including data from Entries 1–16 (for 1st Design) and Entries 1–4, 9 –12, 17–24 (for 2nd Design).    141  Table 6.3. Summary of the results from galvanostatic ECH of guaiacol in MSA electrolytes for factorial analysis. Entry  CG  (M) CH,c (M) CH,a  (M) T  (⁰C) I  (A) E  (V) X  (%) S1  (%) S2  (%) S3  (%) S4  (%) S5  (%) S6  (%) F.E.  (%) Rx   1 0.05 0.2 0.2 40 -0.3 -1.001 85.39 55.51 4.86 18.93 10.08 10.37 0.25 66.35 127 2 0.1 0.2 0.2 40 -0.3 -1.040 49.86 28.49 22.90 18.39 19.62 8.81 1.79 68.76 136 3 0.05 0.2 0.2 60 -0.3 -0.817 97.49 66.22 1.98 16.18 3.88 11.74 0.00 79.18 130 4 0.1 0.2 0.2 60 -0.3 -0.939 57.57 25.92 28.23 17.10 12.36 11.56 4.82 77.49 164 5 0.05 0.2 1 40 -0.3 -1.058 93.27 57.50 1.07 24.36 6.86 10.12 0.09 73.82 132 6 0.1 0.2 1 40 -0.3 -0.956 58.75 28.72 20.66 19.73 21.16 8.40 1.34 81.14 153 7 0.05 0.2 1 60 -0.3 -0.893 99.21 62.73 1.03 20.01 4.08 12.02 0.12 79.51 148 8 0.1 0.2 1 60 -0.3 -0.878 60.40 25.94 28.32 16.40 13.35 10.83 5.15 81.25 166 9 0.05 0.2 0.2 40 -0.7 -2.029 99.88 56.54 0.00 34.77 0.00 8.69 0.00 34.80 237 10 0.1 0.2 0.2 40 -0.7 -2.238 94.11 48.76 3.46 28.05 11.97 7.64 0.12 62.12 283 11 0.05 0.2 0.2 60 -0.7 -1.736 99.85 59.08 0.00 25.09 5.19 10.65 0.00 34.11 219 12 0.1 0.2 0.2 60 -0.7 -1.605 90.49 40.73 15.96 19.67 14.69 8.38 0.57 57.29 316 13 0.05 0.2 1 40 -0.7 -1.777 99.56 55.88 0.00 26.68 7.40 10.04 0.00 33.59 193 14 0.1 0.2 1 40 -0.7 -1.777 75.15 42.71 8.64 20.99 18.96 8.12 0.58 47.29 232 15 0.05 0.2 1 60 -0.7 -1.624 99.80 64.71 0.00 17.06 5.41 12.82 0.00 34.20 228 16 0.1 0.2 1 60 -0.7 -1.381 82.56 27.34 32.65 9.13 18.23 11.46 1.18 48.14 271 17 0.05 1 0.2 40 -0.3 -0.427 82.68 53.20 4.15 16.32 11.94 14.08 0.31 61.92 119 18 0.1 1 0.2 40 -0.3 -0.443 51.21 41.60 5.92 18.87 21.56 10.60 1.46 72.65 135 19 0.05 1 0.2 60 -0.3 -0.378 73.14 36.88 28.33 7.03 11.30 15.45 1.00 51.31 112 20 0.1 1 0.2 60 -0.3 -0.401 45.51 27.24 14.81 11.60 18.68 20.92 6.74 55.24 139 21 0.05 1 0.2 40 -0.7 -0.681 94.87 57.47 0.74 20.88 9.41 11.40 0.10 31.66 185 22 0.1 1 0.2 40 -0.7 -0.679 95.81 52.18 0.00 26.80 10.75 10.21 0.06 63.16 276 23 0.05 1 0.2 60 -0.7 -0.603 99.83 60.60 0.00 16.57 2.69 20.15 0.00 32.57 234 24 0.1 1 0.2 60 -0.7 -0.645 85.04 38.78 12.78 10.64 17.70 18.33 1.77 49.07 250 CG = initial guaiacol concentration, CH,c = catholyte proton concentration,  CH,a = anolyte proton concentration, T = temperature,  I = current, E =  cathode potential (vs. Ag/AgCl), X = guaiacol conversion, S = normalized selectivity (C mol%) for cyclohexanol (1), cyclohexanone (2), 2-methoxycyclohexanol (3), 2-methoxycyclohexanone (4), methanol (5), and phenol (6), F.E. = Faradaic efficiency. Rx = guaiacol conversion rate (after 2 h, in mmol / (mmol dispersed Pt ∙ h)). Catalyst = 5 wt.%-Pt/C (0.15 g). Reactant to metal molar ratio (R/M) = 140–279. Reaction time = 4 h. Catholyte pH = 0.1–0.9, measured after the reaction. 142  Note that each design consists of 16 runs. General trends can be observed from the experimental results (Table 6.3) and summarized as follows: • Higher CG resulted in higher Rx, lower X, and higher F.E. • Higher CH,a resulted in higher Rx and higher X (at lower I) • Higher T resulted in higher Rx, higher X, and higher F.E. (at lower I) • Higher I resulted in higher Rx, higher X, and lower F.E. • Higher CH,c resulted in lower Rx lower X, and lower F.E. Anolyte proton concentration was found to have better effect than catholyte proton concentration toward guaiacol conversion and Faradaic efficiency. This was likely attributed to the promoted H2 evolution reaction at the higher catholyte proton concentration under galvanostatic ECH conditions. The results could give qualitative insights on the interaction between factor variables, yet statistical analysis is required to better understand the factor-response correlations. 6.3.2. Individual and synergistic effects of the factor variables on the responses  The analysis of variance (ANOVA) results revealed that guaiacol concentration and current were overall the most affecting factors in this experiment. Both have significant effect (p-values < 0.05) on all the responses (X, F.E., S1, and S2). Temperature effect is significant toward cyclohexanone selectivity; it is consistent with the previous findings which showed that higher temperature (50–60 oC) promoted cyclohexanone selectivity under galvanostatic control. Meanwhile, catholyte proton concentration effect is significant toward F.E., which is reasonable owing to competitive proton reduction leading to ECH and HER. On the other hand, anolyte proton concentration did not significantly affect F.E., but its interaction with current affected X and F.E. The ANOVA results for all responses can be seen in Appendix A.6.  143   In the next analysis, interactions between significant factors for all responses were modelled by 3D surface graphs (Figures 6.14–6.17). Initial guaiacol concentration, catholyte/anolyte proton concentration, temperature, and current are denoted as A, B, C, D, respectively. Description of the individual factor and interacting factor effects on the responses is summarized in Table 6.4. Table 6.4. Summary of the factor-response interaction and correlation in the factorial ECH of guaiacol experiments. Response Significant factors Correlation (Coded Equation) Guaiacol conversion (X)  Faradaic efficiency (F.E.)     Cyclohexanol selectivity (S1)  Cyclohexanone selectivity (S2) A, D, AD, BD  A, D, AD, BD  A, B′, D, AD, B′C, B′D, B′CD  A, D, AC, AD   A, C, D, AC X = 83.98 – 12.84*A – 0.3688*B + 8.72*D + 5.74*AD – 3.03*BD (hierarchical model of 1st Design) F.E. = 59.94 + 5.49*A – 0.0813*B – 16.01*D + 4.27*AD – 3.06*BD (hierarchical model of 1st Design) F.E. = 56.11 + 7.11*A – 3.91*B′ – 1.58*C – 10.50*D + 5.20*AD – 3.58*B′C + 2.45*B′D – 0.7625*CD + 2.61*B′CD (hierarchical model of 2nd Design) S1 = 46.66 – 13.10*A – 0.10*C + 2.80*D – 3.51*AC + 3.51*AD (hierarchical model of 1st Design) S2 = 10.62 + 9.49*A + 2.91*C – 3.02*D + 3.28*AC (reduced model of 1st Design) Note: The Coded Equation can be used to make predictions about the response for given levels (+1 for high levels or -1 for low levels) of each factor and to identify the relative impact of the factors by comparing the factor coefficients. Differentiate between B (anolyte proton concentration) and B′ (catholyte proton concentration).  High guaiacol conversion was favored by the combination of lower guaiacol concentration, higher current, and lower anolyte proton concentration (Figure 6.14). Faradaic efficiency was promoted by the combination of higher guaiacol concentration, lower current, higher anolyte proton concentration, and lower catholyte proton concentration (Figure 6.15). Cyclohexanol selectivity was not directly influenced by temperature, but the combination of lower guaiacol concentration with higher temperature and higher current could boost cyclohexanol production (Figure 6.16). Cyclohexanone selectivity was otherwise directly impacted by temperature and guaiacol concentration in a positive way (Figure 6.17).  144   Figure 6.14. Model 3D surface graph visualizing the synergistic effects between guaiacol concentration and current (left) and anolyte proton concentration and current (right) on guaiacol conversion.  Overall, the analysis results demonstrated that Faradaic efficiency was response with the most number of significant factors. Substrate concentration (in this case: guaiacol) has the most positive impact on F.E., implying the importance of high surface coverage of reactant during ECH. On the contrary, current has the most negative impact on F.E., implying that HER predominates over ECH. It is hence reasonable that catholyte/anolyte proton concentration, in conjunction with current, also negatively affected F.E. since the HER was facilitated by the higher H+ concentration. Higher temperature, in conjunction with catholyte proton concentration, could also contribute to H2 thermal and electrochemical desorption, thereby leading to lower F.E. (this was the case especially when acidic electrolytes were used). When neutral-acid catholyte-anolyte pairs were used, anolyte proton concentration could positively affect F.E. because of the more efficient utilization of acidic protons for ECH reactions (see Chapters 4 and 5). Faradaic efficiency is not only a complex function which depends on many interacting factors, but also a unique response that characterizes electrochemical process. Hence, efforts and strategies to optimize F.E. are always critical in the process development of electrocatalytic reactions.    0.3  0.4  0.5  0.6  0.70.05  0.06  0.07  0.08  0.09  0.1  40  50  60  70  80  90  100  110  Conversion (%)A: Guaiacol Concentration (M)D: Current (A)3D SurfaceFactor Coding: ActualConversion (%)49.9 99.9X1 = AX2 = DActual FactorsB = 0.6C = 50  0.3  0.4  0.5  0.6  0.70.2  0.4  0.6  0.8  1  40  50  60  70  80  90  100  110  Conversion (%)B: Anolyte Proton Concentration (M)D: Current (A)3D SurfaceFactor Coding: ActualConversion (%)49.9 99.9X1 = BX2 = DActual FactorsA = 0.075C = 50  0.3  0.4  0.5  0.6  0.70.05  0.06  0.07  0.08  0.09  0.1  40  50  60  70  80  90  100  110  Conversion (%)A: Guaiacol Concentration (M)D: Current (A)3D SurfaceFactor Coding: ActualConversion (%)49.9 99.9X1 = AX2 = DActual FactorsB = 0.6C = 50 145   Figure 6.15. Model 3D surface graph visualizing the synergistic effects between guaiacol concentration and current (upper left), anolyte proton concentration and current (upper right), catholyte proton concentration and temperature (lower left), and catholyte proton concentration and current (lower right) on Faradaic efficiency.       0.3  0.4  0.5  0.6  0.70.2  0.4  0.6  0.8  1  20  30  40  50  60  70  80  Faradaic Efficiency (%)B: Catholyte Proton Concentration (M)D: Current (A)3D SurfaceFactor Coding: ActualFaradaic Efficiency (%)31.7 79.2X1 = BX2 = DActual FactorsA = 0.075C = 50  40  45  50  55  600.2  0.4  0.6  0.8  1  20  30  40  50  60  70  80  Faradaic Efficiency (%)B: Catholyte Proton Concentration (M)C: Temperature (deg C)3D SurfaceFactor Coding: ActualFaradaic Efficiency (%)31.7 79.2X1 = BX2 = CActual FactorsA = 0.075D = 0.5  40  45  50  55  600.2  0.4  0.6  0.8  1  20  30  40  50  60  70  80  Faradaic Efficiency (%)B: Catholyte Proton Concentration (M)C: Temperature (deg C)3D SurfaceFactor Coding: ActualFaradaic Efficiency (%)31.7 79.2X1 = BX2 = CActual FactorsA = 0.075D = 0.5  0.3  0.4  0.5  0.6  0.70.2  0.4  0.6  0.8  1  30  40  50  60  70  80  90  Faradaic Efficiency (%)B: Anolyte Proton Concentration (M)D: Current (A)3D SurfaceFactor Coding: ActualFaradaic Efficiency (%)33.6 81.3X1 = BX2 = DActual FactorsA = 0.075C = 50  0.3  0.4  0.5  0.6  0.70.05  0.06  0.07  0.08  0.09  0.1  30  40  50  60  70  80  90  Faradaic Efficiency (%)A: Guaiacol Concentration (M)D: Current (A)3D SurfaceFactor Coding: ActualFaradaic Efficiency (%)33.6 81.3X1 = AX2 = DActual FactorsB = 0.6C = 50  0.3  0.4  0.5  0.6  0.70.05  0.06  0.07  0.08  0.09  0.1  30  40  50  60  70  80  90  Faradaic Efficiency (%)A: Guaiacol Concentration (M)D: Current (A)3D SurfaceFactor Coding: ActualFaradaic Efficiency (%)33.6 81.3X1 = AX2 = DActual FactorsB = 0.6C = 50 146   Figure 6.16. Model 3D surface graph visualizing the synergistic effects between guaiacol concentration and temperature (left) and guaiacol concentration and current (right) on cyclohexanol selectivity.  Figure 6.17. Model 3D surface graph visualizing the synergistic effects between guaiacol concentration and temperature (left) and guaiacol concentration and current (right) on cyclohexanone selectivity.        40  45  50  55  600.05  0.06  0.07  0.08  0.09  0.1  20  30  40  50  60  70  Cyclohexanol Selectivity (%)A: Guaiacol Concentration (M)C: Temperature (deg C)3D SurfaceFactor Coding: ActualCyclohexanol Selectivity (%)25.9 66.2X1 = AX2 = CActual FactorsB = 0.6D = 0.5  0.3  0.4  0.5  0.6  0.70.05  0.06  0.07  0.08  0.09  0.1  20  30  40  50  60  70  Cyclohexanol Selectivity (%)A: Guaiacol Concentration (M)D: Current (A)3D SurfaceFactor Coding: ActualCyclohexanol Selectivity (%)25.9 66.2X1 = AX2 = DActual FactorsB = 0.6C = 50  40  45  50  55  600.05  0.06  0.07  0.08  0.09  0.1  20  30  40  50  60  70  Cyclohexanol Selectivity (%)A: Guaiacol Concentration (M)C: Temperature (deg C)3D SurfaceFactor Coding: ActualCyclohexanol Selectivity (%)25.9 66.2X1 = AX2 = CActual FactorsB = 0.6D = 0.540  45  50  55  60    0.05  0.06  0.07  0.08  0.09  0.1-10  0  10  20  30  40  Cyclohexanone Selectivity (%)A: Guaiacol Concentration (M)C: Temperature (deg C)3D SurfaceFactor Coding: ActualCyclohexanone Selectivity (%)0 32.7X1 = AX2 = CActual FactorsB = 0.6D = 0.50.3  0.4  0.5  0.6  0.7    0.05  0.06  0.07  0.08  0.09  0.1-10  0  10  20  30  40  Cyclohexanone Selectivity (%)A: Guaiacol Concentration (M)D: Current (A)3D SurfaceFactor Coding: ActualCyclohexanone Selectivity (%)0 32.7X1 = AX2 = DActual FactorsB = 0.6C = 500.3  0.4  0.5  0.6  0.7    0.05  0.06  0.07  0.08  0.09  0.1-10  0  10  20  30  40  Cyclohexanone Selectivity (%)A: Guaiacol Concentration (M)D: Current (A)3D SurfaceFactor Coding: ActualCyclohexanone Selectivity (%)0 32.7X1 = AX2 = DActual FactorsB = 0.6C = 50 147  6.3.3. Optimization of process conditions  The Design-Expert software enables numerical optimization with the possibilities to set the desired goals based on criteria from each factor and response. In this study, the goal was to maximize all the responses (guaiacol conversion, Faradaic efficiency, cyclohexanol selectivity, and cyclohexanone selectivity) with the same level of importance. The goals are combined into an overall desirability function, which ranges from zero to one for any given response. A value of one represents the ideal case, while a zero indicates that one or more responses fall outside desirable limits. The optimization results are displayed in contour plots (Figures 6.18–6.19) and summarized in Table 6.5 for each design with three possible goal scenarios.  Table 6.5. Optimization results of the guaiacol ECH based on the pre-determined goal scenarios. Goal Maximize all responses Maximize X only Maximize F.E. only  1st Design 2nd Design 1st Design 2nd Design 1st Design 2nd Design Factor:       CG (M) 0.063 0.050 0.051 0.050 0.100 0.100 CH (M) 1.00 0.20 0.29 1.00 1.00 0.20 T (oC) 60 60 50.61 40 60 60 I (A) 0.30 0.30 0.64 0.70 0.30 0.30 Response:       X (%) 86.66 84.68 100 98.63 59.34 51.05 F.E. (%) 78.35 76.44 42.35 35.14 80.15 80.26 S1 (%) 53.22 63.59 58.95 60.64 23.64 33.01 S2 (%) 10.54 13.65 < 0.00 < 0.00 29.32 21.65 Desirability 0.620 0.744 1.000 0.977 0.976 1.000 All the factor and response values are predicted by numerical optimization in the program. CH represents anolyte proton concentration (for 1st Design) and catholyte proton concentration (for 2nd Design). CG = initial guaiacol concentration, T = temperature, I = current, X = guaiacol conversion, F.E. = Faradaic efficiency, S1 = cyclohexanol selectivity, S2 = cyclohexanone selectivity.    148   The optimization results suggest that maximizing all response variables is less desirable than maximizing only a response (e.g., either guaiacol conversion or Faradaic efficiency). Nevertheless, at all maximum responses, high guaiacol conversions (85–87%) and Faradaic efficiencies (76–78%) could be achieved with good selectivities to cyclohexanol (53–64%) and cyclohexanone (11–14%). These predicted results would require low levels of guaiacol concentration, catholyte proton concentration, and current with high levels of anolyte proton concentration and temperature. At maximum guaiacol conversion scenario, lower Faradaic efficiencies (35–42%) with approximately 60% cyclohexanol selectivity were predicted at lower temperature (40–50 oC) and higher current (0.64–0.70 A) conditions. Maximum Faradaic efficiency scenario requires high levels of guaiacol concentration, anolyte proton concentration, and temperature with low levels of catholyte proton concentration and current. The highest possible F.E. (~80%) was predicted at the expense of X (51–59%), resulting in the similar selectivities to cyclohexanol (24–33%) and cyclohexanone (22–29%). It is noteworthy that in the multi-parameter process like ECH of guaiacol, the response variables (such as X and F.E. as well as S1 and S2) are interrelating competitively. Increasing one desired response would often expense another, thus numerical optimization is useful to find the best trade-offs to achieve multiple goals. Finally, the optimization results for the first goal scenario (i.e. to maximize all responses) are presented in contour plots (Figures 6.18–6.19) to see the “sweet spot” visualizing the conditions that satisfy the desired responses. Guaiacol concentration and current were chosen as term for the plots considering their highest significance among all factors investigated. These contour plots include the predicted response values and will vary depending on the criteria and goal of the optimization.   149   Figure 6.18. Contour plots for optimization of guaiacol ECH process conditions that give maximum values for all responses based on 1st Design of Experiment (including the factors of guaiacol concentration and current with high levels of anolyte proton concentration and temperature).   0.05 0.06 0.07 0.08 0.09 0.10.30.40.50.60.7DesirabilityA: Guaiacol Concentration (M)D: Current (A)0.10.10.20.20.30.30.40.40.50.6Desirability  0.620 0.05 0.06 0.07 0.08 0.09 0.10.30.40.50.60.7Conversion (%)A: Guaiacol Concentration (M)D: Current (A)708090Prediction  86.4027 0.05 0.06 0.07 0.08 0.09 0.10.30.40.50.60.7Faradaic Efficiency (%)A: Guaiacol Concentration (M)D: Current (A)4050607080Prediction  78.4952 0.05 0.06 0.07 0.08 0.09 0.10.30.40.50.60.7Cyclohexanol Selectivity (%)A: Guaiacol Concentration (M)D: Current (A)30405060Prediction  50.8234 0.05 0.06 0.07 0.08 0.09 0.10.30.40.50.60.7Cyclohexanone Selectivity (%)A: Guaiacol Concentration (M)D: Current (A)0102030Prediction  11.3972 0.05 0.06 0.07 0.08 0.09 0.10.30.40.50.60.7DesirabilityA: Guaiacol Concentration (M)D: Current (A)0.10.10.20.20.30.30.40.40.50.6Desirability  0.620 0.05 0.06 0.07 0.08 0.09 0.10.30.40.50.60.7Conversion (%)A: Guaiacol Concentration (M)D: Current (A)708090Prediction  86.4027 0.05 0.06 0.07 0.08 0.09 0.10.30.40.50.60.7Faradaic Efficiency (%)A: Guaiacol Concentration (M)D: Current (A)4050607080Prediction  78.4952 0.05 0.06 0.07 0.08 0.09 0.10.30.40.50.60.7Cyclohexanol Selectivity (%)A: Guaiacol Concentration (M)D: Current (A)30405060Prediction  50.8234 0.05 0.06 0.07 0.08 0.09 0.10.30.40.50.60.7Cyclohexanone Selectivity (%)A: Guaiacol Concentration (M)D: Current (A)0102030Prediction  11.3972  150   Figure 6.19. Contour plots for optimization of guaiacol ECH process conditions that give maximum values for all responses based on 2nd Design of Experiment (including the factors of guaiacol concentration and current with low level of catholyte proton concentration and high level of temperature).     0.05 0.06 0.07 0.08 0.09 0.10.30.40.50.60.7DesirabilityA: Guaiacol Concentration (M)D: Current (A)0.20.40.40.6Desirability  0.743715 0.05 0.06 0.07 0.08 0.09 0.10.30.40.50.60.7Conversion (%)A: Guaiacol Concentration (M)D: Current (A)60708090Prediction  84.675 0.05 0.06 0.07 0.08 0.09 0.10.30.40.50.60.7Faradaic Efficiency (%)A: Guaiacol Concentration (M)D: Current (A)4050607080Prediction  76.4375 0.05 0.06 0.07 0.08 0.09 0.10.30.40.50.60.7Cyclohexanol Selectivity (%)A: Guaiacol Concentration (M)D: Current (A)30405060Prediction  63.5875 0.05 0.06 0.07 0.08 0.09 0.10.30.40.50.60.7Cyclohexanone Selectivity (%)A: Guaiacol Concentration (M)D: Current (A)5101520Prediction  13.65 0.05 0.06 0.07 0.08 0.09 0.10.30.40.50.60.7DesirabilityA: Guaiacol Concentration (M)D: Current (A)0.20.40.40.6Desirability  0.743715 0.05 0.06 0.07 0.08 0.09 0.10.30.40.50.60.7Conversion (%)A: Guaiacol Concentration (M)D: Current (A)60708090Prediction  84.675 0.05 0.06 0.07 0.08 0.09 0.10.30.40.50.60.7Faradaic Efficiency (%)A: Guaiacol Concentration (M)D: Current (A)4050607080Prediction  76.4375 0.05 0.06 0.07 0.08 0.09 0.10.30.40.50.60.7Cyclohexanol Selectivity (%)A: Guaiacol Concentration (M)D: Current (A)30405060Prediction  63.5875 0.05 0.06 0.07 0.08 0.09 0.10.30.40.50.60.7Cyclohexanone Selectivity (%)A: Guaiacol Concentration (M)D: Current (A)5101520Prediction  13.65  151  6.4. Impact of different organic solvents in ECH of guaiacol Concerning the important role of organic solvent in catalytic hydrogenation, it is worthwhile to investigate the impact of organic electrolyte in ECH applications. Five different organic solvents (acetone, isopropanol, acetonitrile, ethanol, and acetic acid) were used as electrolyte mixed with the MSA solution (0.5 M) with 10% volume ratio in the potentiostatic ECH of guaiacol. Overall, the presence of organic solvent suppressed guaiacol ECH, as shown by the lower conversions (<54%) and F.E.s (<38%) after 4 h (Figure 6.20a). Based on polarization tests, the organic electrolyte current density decreased as: acetonitrile > acetone > isopropanol > acetic acid > ethanol (Figure 6.20b), which correspond to the conductivity measurement results (Table 6.6) at room temperature. However, it was counter-intuitive that acetonitrile, despite the highest conductivity, had the most undesirable effect in the guaiacol ECH, resulting in zero products. As a polar aprotic solvent, acetonitrile is known to have Lewis base characteristic (an electron pair donor), hence it could presumably interfere the adsorption chemistry of guaiacol and proton reduction on the catalyst surface and block the adsorption sites for Hads as reported elsewhere.115 Isopropanol and acetic acid showed the least detrimental effect, resulting in moderate guaiacol conversions (51–54%) and F.E.s (35–38%), while ethanol and acetone were similarly suppressive, resulting in the lower guaiacol conversions (30–34%) and F.E.s (22–29%). Hence, the reactivity of guaiacol ECH over the same catalyst (5 wt.%-Pt/C) was not proportionally related to the organic electrolyte conductivity. In all cases, cyclohexanol was the most selective product (32–46%) and the presence of organic solvent did not appear to influence guaiacol ECH product distribution. Interestingly, in a separate experiment using catholyte mixture of H2SO4 (0.2 M) and isopropanol (1:1 volume ratio), 40% guaiacol conversion was achieved after 2 h with Pt/C catalyst (R/M = 209), resulting in marginal selectivity to cyclohexane (3%) at mild conditions (j = -149 mA cm-2,  152  T = 45 oC, F.E. = 59%).  The addition of isopropanol to the acidic electrolyte, combined with increased temperature, could facilitate hydrodeoxygenation step in the ECH of guaiacol.   In the absence of organic solvent, higher guaiacol conversion (~75%) was achieved at reasonable F.E. (~50%). The suppressive effect of organic solvent was further investigated by varying the acetone concentration (5–40 vol.%). Guaiacol conversion decreased with the increasing acetone concentration (Figure 6.20c). With only 5 vol.% acetone added, guaiacol conversion was over 40% suppressed and the conversion further dropped to 15% in the presence of 40 vol.% acetone. This negative effect was likely attributed to: (i) the increased ohmic resistance by the higher acetone concentration (shown by the decreasing current density), (ii) the competitive catalyst surface active sites occupancy (indicated by the formation of isopropanol from acetone hydrogenation), (iii) unfavorable desorption of organic solvent molecules from the catalyst at low temperatures (27–45 oC). It is estimated that acetone converted in the catholyte was about 15% and acetone diffused to the anolyte was 3–9% forming acetic acid via electrooxidation. The acetone inhibition effect was also clearly observed at the different cathode potentials. With the increasing potential (from -1.25 V to -2.25 V), guaiacol conversion was not significantly improved (13–16%) and, consequently, F.E. decreased further (from 24% to 9%) because HER predominated over ECH as the current density increased (Figure 6.20d). Recently, the negative impact of alcohols (methanol, ethanol, and isopropanol) on benzaldehyde ECH rates was also reported by Lopez-Ruiz et al.72  using a fixed-bed continuous flow electrocatalytic cell with Pd supported on carbon felt electrode. The presence of alcohols diluted the H3O+ concentration (i.e. decreased the activity of H3O+ ions), thereby slowing down the ECH rates. Consequently, the higher alcohol concentrations resulted in the lower F.E. Increasing half-cell cathodic potentials also decreased F.E. due to the HER rates enhancement to a greater extent than ECH rates.   153   These results indicate another challenge in ECH processes using organic electrolyte, albeit the benefit of improved substrate solubility. Competitive adsorption between organic solute and solvent molecules on the catalyst is inevitable under mild conditions (low temperatures). Intermediate to high temperature conditions (80–150 oC) may instead promote product desorption and facilitate hydrodeoxygenation and/or dehydration rates. For this reason, an improved reactor design for a robust, high-temperature electrolysis will be necessary for the future development of this area. Until then, the use of organic electrolyte in mild ECH process seems more reasonable if the solvent becomes the desired product along with the upgraded substrates. For instance, conversion of acetone to isopropanol via ECH can be competitive with the conventional hydrogenation routes (100–300 oC). Hydrogenation of acetone to isopropanol, an industrially important commodity chemical, represents the reduction of ketones to the corresponding alcohols, which is very valuable in organic synthesis.116 Table 6.6. Summary of physical properties of the organic solvents used in this work.  Solvent Acetone Isopropanol Acetonitrile Ethanol Acetic acid Formula C3H6O C3H8O C2H3N C2H6O C2H4O2 BP (oC) 56.05 82.5 81.65 78.5 118 MW (g mol-1) 58.08 60.10 41.05 46.07 60.05  (kg m-3) 784 786 786 789 1045 pKa 19.16 16.50 25 16 4.75   21.01 18.30 36.64 24.60 6.20  (mS cm-1) 140.5 132.7 147.4 113.8 128.7 BP = boiling point, MW = molecular weight,  = density (at 25 oC), pKa = acidity,  = dielectric constant,  = measured conductivity (specific conductance) of the solvent mixed with 0.5 M MSA solution (volume ratio = 1:9). Physical properties data (except ) were adapted from Division of Organic Chemistry, American Chemical Society: https://organicchemistrydata.org/solvents/     154   Figure 6.20. Impact of different organic solvents in the ECH of guaiacol under potentiostatic conditions. (a) Conversion, selectivity, and Faradaic efficiency comparison, (b) Polarization test results for the different solvents mixed with MSA (0.5 M) electrolyte, (c) Effect of acetone volumetric concentration, (d) Effect of cathode potential in the ECH of guaiacol in the presence of acetone (40 vol.%). Typical conditions: E = -1.25 V (vs. Ag/AgCl), t = 4 h, CMSA = 0.5 M, CGUA = 0.1 M, Temperature (T): (a) 32–35 oC, (b) room T, (c) 27–36 oC, (d) 27–45 oC. Current density (average) for Experiment (a): -197 mA cm-2. Catalyst: 5 wt.%-Pt/C (0.125 g, R/M = 314), Rd = 350 rpm (Stirrer A). Notation: ACT = acetone, IPA = isopropanol, ANI = acetonitrile, EtOH = ethanol, AcA = acetic acid. -400-350-300-250-200-150-100-500-3.00 -2.50 -2.00 -1.50 -1.00 -0.50 0.00Current Density (mA cm-2)Potential, E (V) vs. Ag/AgClEthanolAcetoneAcetonitrileIsopropanolAcetic Acid0102030405060708090100ACT IPA ANI EtOH AcAConversion, Selectivity, F.E. (%)Organic Electrolyte (10 vol.%)Guaiacol CyclohexanolCyclohexanone 2-Methoxycyclohexanol2-Methoxycyclohexanone MethanolPhenol Faradaic Efficiency05010015020025001020304050607080901000 5 10 25 40-j(mA cm-2)Conversion, Selectivity, F.E. (%)Acetone Concentration (vol.%)0501001502002500102030405060708090100-1.25 -1.75 -2.25-j(mA cm-2)Conversion, Selectivity, F.E. (%)Cathode Potential (V vs. Ag/AgCl)GuaiacolCyclohexanolCyclohexanone2-Methoxycyclohexanol2-MethoxycyclohexanoneMethanolPhenolFaradaic EfficiencyCurrent Density(a) (b)(c) (d) 155  6.5. Electrocatalytic upgrading of bio-oil substrates Attempts to electrocatalytically upgrade pyrolysis bio-oil substrates have been made in recent years.66,68–70,75–77 Mostly, the ECH experiments of bio-oil were done using fixed bed electrode configurations at room temperatures and low currents (<100 mA). The main goal was to demonstrate the viability of ECH as a bio-oil stabilization strategy, mainly through reduction of carbonyl contents (aldehydes and ketones), which could cause polymerization.69 High temperature may promote condensation polymerization of bio-oil, low-temperature ECH is therefore considered a promising approach to stabilize bio-oil. However, recent advances in ECH of bio-oil have shown the limited reductive upgrading chemistry (hydrogenation, hydrogenolysis, or hydrodeoxygenation) through this approach. This was mainly attributed to the complexity of bio-oil and the restricted electrolysis operating conditions. Most of the published works in ECH of pyrolysis bio-oil reported the reduction of carbonyl content into alcohols in the substrate rather than the hydrodeoxygenation of aromatics or phenolics.68–70,77 More than 300 oxygenated organic compounds in pyrolysis oil, including carbohydrate- and lignin-derivatives, complicate the study of its reaction mechanisms and kinetics.117 There are at least 12 types of compounds in bio-oil with the composition based on the chemical functional groups summarized in Table 6.7.  In this initial study, ECH of fast pyrolysis oil (FPO) in acidic electrolytes using SSER configuration was conducted for the first time. A compositional analysis suggested that the FPO sample contained water (29 wt.%) and detected monomers (38.5 wt.%), which comprise of carbohydrate derivatives (35.2 wt.%) and lignin derivatives (3.3 wt.%).118 Three organic solvents (e.g., ethanol, isopropanol, and acetone) were tested in a mixed aqueous and organic electrolyte with MSA solution. The electrolysis experiments were carried out for 4–30 h at constant currents (I = -0.6 to -0.7 A) and temperatures (50–60 oC) with detailed conditions are provided in Table 6.8.    156  Table 6.7. General composition of bio-oils categorized by the chemical functional groups, adapted from ref. 69.  Compound Relative content  (wt.%) Compound Relative content  (wt.%) Acids formic acid acetic acid propionic acid levulinic acid 5–10 0.3–9.1   0.5–12  0.1–1.8 0.1–0.3 Phenolics phenol 2-ethyl-phenol 1,4-dihydroxybenzene methyl phenol 20–30  0.1–3.8 0.1–1.3 0.1–1.9 0.1–5 Alcohols methanol ethanol ethylene glycol 0–5 0.4–2.4 0.6–1.4 0.7–2  Guaiacols 2-methoxy phenol 4-methyl guaiacol isoeugenol eugenol 2–14  0.1–1.1 0.1–1.9 0.1–7.2 0.1–2.3 Ketones acetone hydroxyacetone 0–10 2.8 0.7–7.4  Syringols 2,6-dimethoxyphenol propyl syringol syringaldehyde 2–8  0.7–4.8 0.1–1.5 0.1–1.5 Aldehydes formaldehyde acetaldehyde ethanedial hydroxyacetaldehyde 5–20  0.1–3.3 0.1–8.5 0.9–4.6 0.9–13 Furans furanone furfural furfural alcohol 5-hydroxymethylfurfural 0–12  0.1–1.1 0.1–1.1 0.1–5.2 0.3–2.2 Sugars D-xylose levoglucosan glucose fructose cellobiosan 1,6-anhydroglucofuranose 5–30  0.1–3.2 0.4–1.4 0.4–1.3 0.7–2.9 0.6–3.2 0.1–3.1 Others methyl cyclopentenolone 3-methoxybenzaldehyde  Oligomers Water  0.1–1.9 0.1–1.1  0–20  15–30    In all cases, decreases in the weight average molecular weight (Mw) and the number average molecular weight (Mn) were observed with prolonged reaction times. The molecular weight distribution, as indicated by polydispersity index (PDI), indicates molecular weight compositions (heterogeneity) of the bio-oil samples.119 The bio-oil mixture has perfect homogeneity in case of  157  PDI = 1, hence, the greater PDI implies the more diverse content of the bio-oil. Increased temperature and catalyst loading contributed to faster depolymerization. Using different organic solvents (e.g., ethanol, isopropanol, acetone), the degree of depolymerization was about 33–37% after nearly 20 h reactions (Figure 6.21). Color changes in the catholyte bio-oil samples were observed in all cases after 18–25 h, turning from dark brown to light yellow. This observation supports the GPC results showing the molecular weight reduction after the ECH. In electrochemical depolymerization of kraft lignin, Marino et al.120 also reported a correlation between size exclusion chromatography (SEC) results and the color of the extracted solution before and after the reaction, suggesting that the lighter color indicated the lower molecular weight distribution.  Compound identification and the composition quantification were determined by GC-MS analysis. GC-MS detectable compounds typically account for 30–40 wt.% of the whole bio-oil while the undetected substances might include water-soluble oligomeric fractions, such as pyrolytic humins and hybrid oligomers (products of carbohydrate and lignin reaction).121 It is well-known that the quantitative analysis of bio-oil samples is very difficult due to the intricate nature of bio-oil. Consequently, the chemistry during the upgrading reaction is hard to predict, let alone the reaction pathway and mechanism. A reasonable approach is then taken by classifying the detected compounds based on the functional group. In this study, compound distributions were determined based on ten functional groups, such as acids, alcohols, aldehydes, ketones, esters, furans, alkanes, aromatics, phenols, and guaiacols. The compound distributions before and after the ECH of bio-oil using different solvents are presented in Figure 6.22 along with the list of specific compounds for each group in Table 6.9. In all cases, the reduction of ketones, aromatics, and guaiacols was noticed, while the alkanes content slightly increased. The most dramatic  158  increases in esters content were observed when ethanol (23–30%) or isopropanol (19–31%) was used. This was attributed to esterification of carboxylic acids (mainly acetic acid) in the bio-oil with alcohols in the presence of MSA, which catalyzed the reaction. When acetone was used, the initial bio-oil sample did not contain esters; however, after ECH, some esters were formed, likely because some of the acetone were hydrogenated to isopropanol. These results suggested the reactivity of some compounds in bio-oil, especially those having carbonyl group (e.g., carboxylic acids, aldehydes, and ketones). Hydrolysis of esters eventually resulted in the higher formation of carboxylic acids, hence the organic acids content significantly increased after the reactions. The acidity of bio-oil samples in the catholyte was, however, determined by the MSA concentration (pH < 1). This presence of acidic protons could be both profitable and detrimental. High proton concentrations increase the electrolyte conductivity and the nature of Brønsted acid could facilitate dehydration reactions (aside from esterification). On the other hand, high acidity is generally not favorable due to the corrosiveness. Hence, depending on the target chemical product, the acid concentration and organic solvent type need to be adjusted carefully in the ECH of bio-oil. If the target product is liquid fuel, it should be noted that acetic acid, which is contained in substantial amount in bio-oil, can also be hydrogenated to ethanol via thermochemical process.122,123  Evolution of the lignin-relevant compounds (i.e. phenols, aromatics, and guaiacols) can be seen from the compound distributions (Figures 6.23–6.24) and evidently shown in the GC-MS chromatogram examples (Figures 6.25–6.26). Reduction of p-eugenol to cerulignol (4-propylguaiacol) was the most noticeable in all cases, implying that the unsaturated bond in the allyl group is easily reduced under the ECH conditions. Significant increases in the cerulignol contents were achieved with all the different solvents: ethanol (0.5–5.2%), isopropanol (1.4–6.9%), and acetone (1.4–5.4%), based on the composition calculation normalized by the lignin-relevant  159  compounds detected in the catholyte samples (Figure 6.24). Note that the compound compositions were approximated in such a way to eliminate the contributions from carbohydrate and solvent derivatives. Both approximations (Figures 6.23–6.24) resulted in similar magnitude of the chemical compositions compared with those of bio-oil shown in Table 6.7. Interactions among bio-oil compounds during electrochemical treatment were observed in a recent work by Deng et al.76 The electrolysis of the whole bio-oil, the water-soluble (aromatic-poor) fraction, and the water-insoluble (aromatic-rich) fraction resulted in different trends. The aromatic-rich fraction tended to undergo condensation, forming large-molecular-weight compounds, while this aromatic condensation was suppressed in the whole bio-oil. This trend was attributed to the presence of light components (carbohydrate-derived compounds) in bio-oil which were reactive and enhanced the hydrogenation reaction. The aromatic and phenolic contents were reduced in all the bio-oil substrates with a prolonged processing time (~12 h).76 Mild ECH of deoxidized bio-oil from microwave-assisted catalytic fast pyrolysis of corn stover could also significantly increase the relative content of alcohols and decrease the relative contents of acids, esters, carbonyls, phenols, sugars, and furans;77 however the reaction mechanisms and specific product yields were not yet discussed in detail in all the aforementioned examples. Due to the inherent intricacy of bio-oil, specific product selectivity and current efficiency quantifications are presently very difficult to measure the process performance.  As part of the process development, ECH of larger phenolic compounds relevant to bio-oil (e.g., cerulignol and creosol) was explored using the slurry Pt/C catalyst. Cerulignol is poorly miscible with aqueous solution at ambient temperature (while creosol is slightly miscible), thus isopropanol was added into MSA (0.2 M) electrolyte with equal ratio (1:1, v/v) to fully dissolve the phenolic reactants. In the absence of organic solvent, two separate layers were formed like oil  160  (hydrophobic) and water (hydrophilic) phases and the catalyst particles were agglomerated by the oil layer from the phenolic reactant molecules (Figure 6.27a). The ECH simply could not proceed under this condition, resulting in no products. By contrast, in the presence of organic solvent (isopropanol), the catalyst particles were well-dispersed in the solution (Figure 6.27b) and thus ECH could proceed, despite the higher applied potential was required, and products were identified in the GC-MS. Under the experimental conditions (see Table 6.11), cerulignol and creosol conversions (20–35%) were achieved after 2 h reactions. The identified major products were 4-propylphenol, 4-propylcyclohexanone, 3-propylcyclohexene, and propylcyclohexane (from cerulignol) and 4-methylphenol, 4-methylcyclohexanone, 4-methylcyclohexanol, and methylcyclohexane (from creosol). Possible reaction pathways are depicted in Figure 6.27c–d, showing that electrocatalytic hydrodeoxygenation (ECHDO) of alkyl guaiacols to alkylcyclohexane requires 10 mol of electrons and involves demethoxylation, ring saturation, hydrogenation, and dehydration (or dehydroxylation) steps. A different reaction pathway was reported by Joshi et al.117 in the hydrodeoxygenation (HDO) of cerulignol using a packed-bed microreactor with presulfided NiMo/Al2O3 catalyst. The most selective product was 4-propylphenol followed by 4-propylbenzene, 4-ethylphenol, phenol, and cresol under high temperature (200–450 oC) and pressure (1.65–3.30 MPa) conditions. The cerulignol conversion is highly influenced by temperature, marked by the increasing yields of 4-propylphenol (~70%) and propylbenzene (~7.5%) at 450 oC and 2.07 MPa.117 Interestingly, the fully saturated and deoxygenated product (i.e. propylcyclohexane) was only slightly formed under these harsh conditions. In this study, fully deoxygenated products were obtained with noticeable selectivities, including propylcyclohexane (33–35%) and methylcyclohexane (12–34%) from ECH of cerulignol and creosol, respectively (Table 6.11).  161   Insights from guaiacol ECH factorial experiments were applied to the ECH of cerulignol and creosol. All the experiments were conducted under conditions that were expected to maximize the Faradaic efficiency. The catholyte mixture of MSA (0.2 M) and isopropanol was paired with MSA anolyte (1 M) and the ECH were performed for 2 h at fixed current (-0.4 A, -0.5 A) and constant temperature (50 oC, 60 oC). The factors of interest, including reactant concentration, current density, and temperature, were evaluated. As expected, higher reactant concentration resulted in lower conversion but higher F.E. (Table 6.11, Entries 1 vs. 2 and 5 vs. 6). Interestingly, reactant conversions were not increased significantly by temperature and/or current density, hence the efficiencies were decreased, presumably owing to the promoted HER (Table 6.11, Entries 1 vs. 3, 4 and Entries 5 vs. 7, 8). This sluggish reactivity may be attributed to: (i) the equilibrium limitations associated with water removal step in the water-containing reaction medium, (ii) the presence of organic solvent covering up the catalyst active sites which hindered the reactant adsorption and surface reaction, (iii) low desorption rates owing to insufficient thermal energy at the low temperatures. Even so, complete deoxygenation pathways (through dehydroxylation and/or dehydration) were identified in the ECH of alkyl guaiacols. Note that the reaction pathways (Figure 6.27c-d) are proposed based on the major products identified in the GC-MS analysis. Unknown products classified as ‘others’ (in Table 6.11) include, but are not limited to, dehydroxylation products, such as 1-methoxy-3-propylbenzene (from cerulignol) and 1-methoxy-3-methylbenzene (from creosol), recognized by the GC-MS fragmentation spectrum (i.e. m/z patterns).  The model compound study indicates that demethoxylation occurred easily in ECH of cerulignol and creosol (including guaiacol, which has been extensively discussed), producing methanol as the by-product. Dehydration of cyclic alcohols, which occurred in the guaiacol ECH, was also observed in the ECH of cerulignol and creosol under the experimental conditions. All the  162  reactions in ECH of cerulignol to propylcyclohexane (as well as creosol to methylcyclohexane) are thermodynamically favorable and, particularly, conversions of 4-propylcyclohexanone into 3-propyl-1-cyclohexene (∆𝐻𝑅0 = -86.4 kJ mol-1) and 4-methylcyclohexanol to methylcyclohexane (∆𝐻𝑅0 = -104.1 kJ mol-1) are exothermic, thus can proceed at low temperatures. Moreover, the presence of alkyl chain (propyl or methyl) could presumably weaken the C–O bond in the hydroxyl group or allow the phenyl ring saturation more easily, thereby leading to the bond cleavage via hydrogenolysis even at mild conditions. Interestingly, the first aromatic ring saturation step, which occurred in guaiacol ECH, was neither noticeable in cerulignol nor creosol ECH, possibly due to the steric effect associated with the additional alkyl group. Different adsorption mechanisms of the phenolic reactants with different functionalities and their effect on the substrate and catalyst interactions will be the subject of further study. Thermodynamic analysis for ECH of cerulignol and creosol is presented in Table 6.10, based on the proposed reaction pathways. In both ECH of cerulignol and creosol, diffusion of isopropanol from cathode to anode was clearly noticed as acetone and acetic acid were detected in the anolyte owing to the isopropanol oxidation; even so, diffusion of the compounds of interest is negligible over the course of the reaction, signified by the high carbon balances (>95%). The role of isopropanol is not only important for improving the reactant solubility, but also for promoting the dehydration reaction; it has been suggested that the presence of polar solvents could affect the stabilization of acidic protons and protonated transition states, thereby accelerating the rates of acid-catalyzed dehydration reactions for biomass conversion.96,98,124 The reactivity of a strong Brønsted acid catalyst in the liquid phase depends on the extent of proton solvation in the solvent, described by the Gibbs free energy for proton solvation (G(H+)solv);124 it is possible that the acidic protons solvated in isopropanol have higher reactivity to dehydration, resulting from the lowered energy barrier, particularly at higher  163  temperatures. In thermocatalytic routes, selective aqueous-phase HDO pathway of phenolic monomers demands dual-functional catalysis, such as metal-catalyzed hydrogenation and acid-catalyzed dehydration. Catalytic HDO of cerulignol and creosol in water has been reported by Zhao et al.78,125 using Pd/C and phosphoric acid (H3PO4) or zeolite (H-ZSM-5) at 200–250 oC using external H2 (50 bar) for 0.5–2 h with the major products of methylcyclohexane and propylcyclohexane, respectively. Aqueous-phase HDO of phenolic monomers to produce cycloalkanes is also reported by Zhang et al.126 using acid-metal bifunctional catalyst by supporting a transition metal on a solid acid support (e.g., Ru/HZSM-5). In light of these studies and experimental results, the electrocatalytic HDO in organic solvent-mixed acidic electrolytes would be a reasonable approach for mild conversion of phenolics into cycloalkanes.  Further experiments using the reactant mixture of cerulignol, creosol, and guaiacol (or phenol) could verify the occurrence of HDO routes to produce fully deoxygenated products, i.e. 4-propylcyclohexane, methylcyclohexane, and cyclohexane, respectively. The product distribution results suggest that the reaction of each compound proceeded independently at low conversion rates (Figure 6.28). After 2 h, the reactant conversion decreased in the order of phenol (26%) > guaiacol (20%) > cerulignol (16%) > creosol (12%). Competitive adsorption and surface reaction on the catalyst can be clearly observed from these results as compared to those obtained using individual reactant (Table 6.11, Entries 3 and 7). In the presence of the other phenolic reactant (i.e. guaiacol or phenol), the cerulignol and creosol conversions were retarded (approximately reduced by half). The deoxygenation product selectivities were also lower, i.e. propylcyclohexane (13% vs. 35%) and methylcyclohexane (9% vs. 17%). Meanwhile, demethoxylation was dominant as shown by high selectivities to 4-propylphenol (54%) and 4-methylphenol (59%), respectively. From the guaiacol and phenol conversions, cyclohexanone was the most selective product (39% and 82%,  164  respectively). As expected, phenol was converted faster than the other reactants (with F.E. = 43%), possibly due to its higher reactivity with chemisorbed hydrogen. In a recent study by Liu et al.,67 direct electrolytic deoxygenation of phenol to cyclohexane was performed using a dual-catalyst system consisting of suspended Pt/C catalyst and soluble silicotungstic acid (SiW12), also known as polyoxometalate (POM). While Pt/C is active for hydrogenation/ring saturation, POM is a superacid with high Brønsted acidity co-catalyzing dehydration reactions. The POM enhances the protonation of the –OH group and H2O removal in the target molecules to from a carbon cation, thus C–O bond cleavage is enabled at Pt/C particle surfaces. In the ECH of phenol, the POM also acts as the charge storage and transfer catalyst which can transfer the electrons from the cathode to the catalyst particle surface, thereby improving the charge transfer efficiency.67 Fully deoxygenated products (e.g., cyclohexane, methylcyclohexane) were obtained from the ECHDO of substituted phenols (e.g., creosol, guaiacol, catechol, diphenyl ether, etc.) using catholyte mixture of SiW12 (0.2 M) in methanol/water (1:9, v/v) solution at 55 oC. This work demonstrated that high current densities (100–800 mA cm-2) can be applied for the ECH of phenol without Faradaic efficiency loss if phenol concentration or catalyst amount is increased.67     Overall, insights from the ECH of cerulignol and creosol can be relevant to the bio-oil ECH for better understanding the reactivity of diverse compounds in the bio-oil. Individual experiments of phenolic model compounds have shown the potential applications of mixed aqueous and organic electrolytes for the mild reductive upgrading of bio-oil. The H-cell SSER configuration is particularly useful for the ECH experimental studies using mixed electrolytes because the glass reactor allows better visibility on the process phenomena (e.g., mixing and phase separation), which would not be observable in a typical closed flow cell reactor, to give better understanding and intuition on the process. The experimental results, combined with literature study, clearly  165  suggest that the larger phenolic monomers can be electrocatalytically reduced, either partially or fully deoxygenated, under benign conditions, albeit the lower conversions and yields than those resulting from the simpler phenolics (i.e. phenol and guaiacol). An effective and efficient ECH of bio-oil with high product yield or selectivity has not been remarkably achieved, implying the limitations of this approach for real substrates at present, despite the overall data have suggested the feasibility of ECHDO in a stirred slurry reactor to increase the energy density of liquid products by complete oxygen removal. The effect of compound mixture in bio-oil (including oligomers) toward the substrate reactivity on the catalyst is presently very difficult to be distinguished. Under low temperatures, competitive adsorption is favored while product desorption is slow, hence there is a possibility that some reactant molecules were adsorbed without being reacted, blocking substantial active sites of the catalyst. Detailed mechanistic study of the ECH of bio-oil substrates will be required to further advance this process, addressing the possibility of decoupling the random, competing surface reactions, as well as adsorption and desorption steps, which might not be all favorable at low-temperature conditions. For this purpose, ECH studies using surrogate bio-oil (e.g., mixture of phenolic reactant and organic acid) will be important contributions in the future. A reasonable strategy for improving the bio-oil reductive upgrading technology would be by using an intermediate-to-high temperature electrolyzer (80–150 oC) to promote the reaction rates and energy efficiency. This approach, although emulates the traditional TCH process, can still be favorable because the hydrogen remains internally supplied, thermally driven catalyst deactivation (e.g., via coking or sintering) is ruled out, less organic solvent is used, and less undesired by-products are formed. In addition, ECH process also offers flexibility in terms of the choice of electrolyte (with broad pH range). Strategies to upgrade bio-oil via ECH with diverse electrolyte pairs assisted by organic solvent or surfactant will be implemented in the future study. 166  Table 6.8. Summary of the galvanostatic ECH of bio-oil experimental results. Entry Catholyte (M) Solvent Anolyte (M) Bio-oil (mL) Catalyst: Pt/C (g) T  (oC) I  (A) t  (h) Ecathode  (VAg/AgCl) Mw  (i→f) Mn  (i→f) PDI overall (i→f) 1 MSA (0.5) Ethanol MSA (1) 5 0.5 50 -0.6 22 -2.13 646 → 474 214 → 199 3.0 → 2.4 2 MSA (0.5) Ethanol MSA (1) 10 1 1 1 50 50 50 -0.6 -0.6 -0.6 4 6 5 -3.36 -3.15 -3.37 611 → 513 513 → 495 495 → 490 231 → 230 230 → 215 215 → 208 2.6 → 2.3 3 MSA (0.5) Ethanol MSA (2) 10 1 50 -0.7 22 -2.27 612 → 453 232 → 199 2.6 → 2.3 4 MSA (0.5) Ethanol MSA (2) 5 1 1 1 60 60 60 -0.7 -0.7 -0.7 4 5 6 -2.26 -2.08 -1.80 619 → 550 550 → 441 441 → 415 234 → 247 247 → 210 210 → 220 2.6 → 1.9 5 MSA (1) Ethanol MSA (2) 5 1 2 2 50 50 60 -0.7 -0.7 -0.7 18 7 5 -2.06 -1.88 -1.54 608 → 409 409 → 324 324 → 307 247 → 214 214 → 189 189 → 187 2.5 → 1.6 6 MSA (0.5) Isopropanol MSA (2) 5 1 2 2 50 50 60 -0.7 -0.7 -0.7 15 5 6 -3.15 -2.70 -1.74 633 → 485 485 → 399 399 → 385 242 → 227 227 → 211 211 → 209 2.6 → 1.8 7 MSA (1) Acetone  MSA (2) 5 1 1 1 50 60 60 -0.7 -0.7 -0.7 18 7 5 -1.85 -1.54 -2.64 604 → 395 395 → 377 377 → 257 229 → 187 187 → 182 182 → 130 2.6 → 2.0 MSA = methanesulfonic acid, T = temperature, I = current, t = time, Mw = weight average molecular weight, Mn = number average molecular weight, PDI = polydispersity index.    167   Figure 6.21. Average molecular weight analysis results by GPC for bio-oil samples from ECH experiments at constant current (I = -0.7 A) and mixed aqueous and organic electrolyte (volume ratio of 1:1), with the following conditions: T = 50–60 oC; Catholyte solution mixture: (a)  MSA (0.5 M)–Ethanol, (b) MSA (1 M)–Ethanol, (c)  MSA (0.5 M)–IPA, (d) MSA (1 M)–Acetone; Anolyte: MSA (2 M), Catalyst: 5 wt.%-Pt/C (1–2 g). Inset picture shows color changes in the bio-oil samples from the catholyte over time (~25 h), observed in all the electrolyte–solvent combinations. (a) (b)(c) (d)S0-C (t = 0 h) S1-C (t = 4 h)S2-C (t = 9 h) S3-C (t = 15 h)S0-C (t = 0 h) S1-C (t = 18 h)S2-C (t = 25 h) S3-C (t = 30 h)S0-C (t = 0 h)S1-C (t = 18 h)S2-C (t = 25 h)S0-C (t = 0 h) S1-C (t = 15 h)S2-C (t = 20 h) S3-C (t = 25 h)S0-C S1-C S2-C 168   Figure 6.22. Compound distributions in the samples from bio-oil ECH experiments for the MSA catholyte solution mixed with: (A) ethanol, (B) isopropanol, (C) acetone before and after reactions (~20 h). Compositions (wt.%) were calculated based on the detected compounds in GC-MS analysis of the catholyte samples on a solvent- and oligomer-free basis.   Guaiacols9%Alkylated aromatics29%Phenols1%Aldehydes2%Esters3%Alkanes14%Furans2%Ketones6%Acids30%Alcohols4%Guaiacols15%Alkylated aromatics37%Phenols5%Aldehydes3%Esters0%Alkanes13%Furans5%Ketones9%Acids11%Alcohols2%Guaiacols8%Alkylated aromatics16%Phenols2%Aldehydes7%Esters31%Alkanes9%Furans3%Ketones7%Acids15%Alcohols2%Guaiacols12%Alkylated aromatics27%Phenols4%Aldehydes8%Esters19%Alkanes7%Furans5%Ketones11%Acids5%Alcohols2%Guaiacols6%Alkylated aromatics19%Phenols1%Aldehydes7%Esters30%Alkanes11%Furans3%Ketones7%Acids14%Alcohols2%Guaiacols10%Alkylated aromatics27%Phenols2%Aldehydes8%Esters23%Alkanes4%Furans4%Ketones11%Acids8%Alcohols3%BeforeAfterBeforeAfterBeforeAfter 169   Figure 6.23. Compound distributions in stack columns showing the evolution of lignin derivatives (including phenols, aromatics, and guaiacols) during the ECH of bio-oil, based on the overall detected compounds in the catholyte samples (on a solvent- and oligomer-free basis). Compositions of guaiacols and phenols are magnified (right panel) showing the reduction of the phenolic compounds, mainly p-eugenol to 4-propylguaiacol. Note: A, B, C indicates the different solvent used (ethanol, isopropanol, acetone, respectively) before (0) and after (1) reactions.   0102030405060708090100A0 A1 B0 B1 C0 C1Composition (wt.%)AcidsAlcoholsAldehydesKetonesEstersFuransAlkanesAromaticsPhenolsp-Eugenol4-Propylguaiacol4-Ethylguaiacolp-CreosolGuaiacol 1.95 1.453.212.003.632.172.701.904.672.395.452.671.251.071.851.301.961.500.24 1.930.712.370.962.833.460.001.410.002.760.002.31.54.12.25.11.00510152025A0 A1 B0 B1 C0 C1Composition (wt.%) 170   Figure 6.24. Compound distributions in stack columns showing the evolution of lignin derivatives (including phenols, aromatics, and guaiacols) during the ECH of bio-oil, based on the lignin-relevant compounds detected in the catholyte samples (on a solvent- and oligomer-free basis). Compositions of guaiacols and phenols are magnified (right panel) showing the reduction of the phenolic compounds, mainly p-eugenol to 4-propylguaiacol. Note: A, B, C indicates the different solvent used (ethanol, isopropanol, acetone, respectively) before (0) and after (1) reactions.      62.350.353.745.453.554.510.028.614.325.017.826.30102030405060708090100A0 A1 B0 B1 C0 C1Composition (wt.%)AlkanesAromaticsPhenolsp-Eugenol4-Propylguaiacol4-Ethylguaiacolp-CreosolGuaiacol 4.5 3.96.4 5.8 5.3 4.16.35.19.36.9 7.95.12.92.93.73.7 2.82.90.5 5.21.4 6.91.45.48.00.02.80.04.00.05.44.08.36.37.41.8010203040A0 A1 B0 B1 C0 C1Composition (wt.%) 171   Figure 6.25. GC-MS chromatogram of the dried sample from bio-oil ECH experiment in the MSA electrolyte mixed with acetone before and after 25 h reactions. The amount of cerulignol (t = 33.2 min) dramatically increased, as opposed to the amount of p-eugenol (t = 34.3 min).       Before ECHAfter ECH 172   Figure 6.26. GC-MS chromatogram of the extracted sample from bio-oil ECH experiment in MSA electrolyte mixed with acetone before and after 25 h reactions. The amount of cerulignol (t = 33.2 min) dramatically increased, as opposed to the amount of p-eugenol (t = 34.3 min).      Alkylated aromaticsAlkylated aromaticsBefore ECHAfter ECH 173  Table 6.9. List of specific compounds identified in the bio-oil samples by GC-MS analysis. Carbohydrate Derivatives Lignin Derivatives Type Compound Formula Type Compound Formula Acids acetic acid  C2H4O2 Alkanes 2-methylpropyl-cyclohexane C7H16  formic acid CH2O2  butyl-cyclohexane C10H20  propanoic acid C3H6O2  ethyl-cyclopentane C7H14 Alcohols methanol  CH4O Aromatics 1-methyl-3-propyl-benzene C10H14     benzene, 1,2,4-trimethyl- C10H14 Aldehydes acetaldehyde  C2H4O  benzene, 1-ethyl-3-methyl-  C10H14  diethoxymethane  C5H12O2 Phenols phenol C6H6O  diisopropoxymethane C7H16O2  2-ethylphenol C8H10O Ketones 4-methyl-2-pentanone C6H12O Guaiacols 2-methoxyphenol (guaiacol) C7H8O2  1-hydroxy-2-propanone C3H6O2  2-methoxy-4-methyl-phenol (p-creosol) C8H10O2  2-hydroxy-3-methyl-2-cyclopentenone C6H8O2    Esters isopropyl formate C4H8O2  4-ethyl-2-methoxy-phenol (4-ethylguaiacol) C9H12O2  methyl acetate C7H12O3  2-methoxy-4-propyl-phenol (4-propylguaiacol) C10H14O2  methyl formate C2H4O2  2-methoxy-4-(2-propenyl)-phenol (p-eugenol) C10H12O2  ethyl acetate C4H8O2    Furans 2,5-diethoxytetrahydrofuran C6H12O3     3-methyl-2(5H)-furanone, C5H6O2     2(5H)-furanone  C4H4O2     2-furanmethanol C5H6O2     furfural (2-furancarboxaldehdye) C5H4O2    Note: Compound identification was based on the following criteria: (i) high quality (>80) and/or (ii) relatively large peak area (>0.5%).   174  Table 6.10. Thermodynamic data for ECH of bio-oil-relevant phenolic compounds (cerulignol and creosol). Reaction Equation ∆𝐺𝑅0 (kJ mol-1) ∆𝐻𝑅0 (kJ mol-1) ∆𝑆0 (J mol-1 K-1) 𝐸𝑟𝑒𝑑0  (VSHE) 𝜕𝐸𝑇0𝜕𝑇 (mV K-1) ECH of cerulignol       4-Propylguaiacol to 4-Propylphenol C10H14O2 + 2H+ + 2e- → C9H12O + CH4O -73.1 -68.8 14.4 0.38 0.08 4-Propylphenol to 4-Propylcyclohexanone  C9H12O + 4H+ + 4e- → C9H16O -55.9 -132.0 -255.2 0.14 -0.66 4-Propylcyclohexanone to 3-Propylcyclohexene 3-Propylcyclohexene to Propylcyclohexane C9H16O + 2H+ + 2e- → C9H16 + H2O C9H16 + 2H+ + 2e- → C9H18  -154.1 -30.0 -86.4 -81.9 227.3 -174.2 0.80 0.16 1.18 -0.90 ECH of creosol      Creosol to p-Cresol C8H10O2 + 2H+ + 2e- → C7H8O + CH4O -73.1 -68.6 15.0 0.38 0.08 p-Cresol to 4-Methylcyclohexanone C7H8O + 4H+ + 4e- → C7H12O -55.9 -114.0 -195.0 0.14 -0.51 4-Methylcyclohexanone to 4-Methylcyclohexanol C7H12O + 2H+ + 2e- → C7H14O -21.9 -64.9 -144.2 0.11 -0.75 4-Methylcyclohexanol to Methylcyclohexane C7H14O + 2H+ + 2e- → C7H14 + H2O -162.2 -104.1 194.8 0.84 1.01  ∆𝐺𝑅0 = standard Gibbs free energy of the reaction; ∆𝐻𝑅0 = standard enthalpy of the reaction; ∆𝑆0 = standard entropy of the reaction; 𝐸𝑟𝑒𝑑 0  = standard reduction potential of the reaction; 𝜕𝐸𝑇0𝜕𝑇 = temperature coefficient of the standard equilibrium electrode potential. All the thermodynamic data were obtained based on NIST and Joback method at standard condition (298 K and 1 atm).     175  Table 6.11. Summary of the results from galvanostatic ECH of bio-oil representative monomers (cerulignol and creosol). Entry Reactant E  (V) j  (mA cm-2) T  (oC) pH  (c) pH  (a) X  (%) S1  (%) S2  (%)  S3  (%)  S4  (%) S5 (%)  Sothers (%) C.B. (%) F.E. (%)  1 Cerulignol  -3.0 -146 50 0.86 0.32 30.73 19.15 4.88 27.66 35.26 8.15 4.90 99.8 91.6 2 Cerulignol*  -2.4 -146 50 1.12 0.37 24.05 31.16 7.19 13.36 33.09 8.23 6.98 >99.9 96.4 3 Cerulignol  -2.5 -146 60 1.33 0.35 30.91 28.50 7.43 14.73 35.17 8.91 5.27 99.4 84.0 4 Cerulignol  -2.8 -182 60 1.34 0.33 35.11 24.59 8.35 17.76 34.99 9.55 4.76 96.1 72.8 5 Creosol -2.4 -146 50 1.26 0.42 23.24 28.60 5.16 7.62 34.00 15.70 8.92 95.2 62.9 6 Creosol*  -3.8 -146 50 1.28 0.41 18.34 40.45 4.50 7.27 19.65 13.60 14.53 >99.9 78.0 7 Creosol -2.3 -146 60 1.41 0.72 24.41 39.01 7.30 7.53 17.30 17.32 11.54 97.9 51.4 8 Creosol -3.2 -182 60 1.28 0.43 23.75 47.19 6.97 7.52 11.98 15.46 10.89 98.2 35.4 E = average cathode potential (vs. Ag/AgCl), j = superficial current density, T = temperature, t = 2 h, X = cerulignol (or creosol) conversion, S = normalized selectivity (C mol%) of the major products: (1) 4-propylphenol (or 4-methylphenol), (2) 4-propylcyclohexanone (or 4-methylcyclohexanone) (3) 3-propylcyclohexene (or 4-methylcyclohexanol), (4) propylcyclohexane (or methylcyclohexane), (5) methanol, C.B. = carbon balance, F.E. = Faradaic efficiency. Catalyst = 5 wt.%-Pt/C (0.2 g); Initial reactant concentration = 0.1 M, except *0.2 M. Catholyte: MSA (0.2 M) + Isopropanol (1:1 v/v ratio), Anolyte: MSA (1 M); pH of catholyte (c) and anolyte (a) measured at the end of reaction. Sothers include unknown products, which have distinguished peaks, but are not specifically recognized by GC-MS. These compounds were calculated based on the response factor of 4-propylphenol and 4-methylphenol, respectively, and include dehydroxylation products, such as 1-methoxy-3-propylbenzene (from cerulignol) and 1-methoxy-3-methylbenzene (from creosol), based on the GC-MS fragmentation spectrum.    176   Figure 6.27. Visual appearances of catholyte mixture in the ECH of cerulignol using acidic electrolyte (H2SO4, 0.2 M): (a) without the addition of isopropanol, (b) with the addition of isopropanol (at 1:1 volume ratio). The former case shows catalyst clumping (agglomeration) due to the immiscible reactant, whereas the latter case shows the catalyst (Pt/C) well-dispersion during the reaction. Possible reaction pathways are shown for (c) ECH of cerulignol and (d) ECH of creosol under the operating conditions in this study.  cerulignol(4-propyl-guaiacol)4-propyl-phenol 3-propyl-1-cyclohexenepropyl-cyclohexane-CH3OH -H2O2H+4H+2H+4-propyl-cyclohexanone2H+-CH3OH2H+ 4H+ 2H+-H2O2H+creosol(4-methyl-guaiacol)4-methyl-phenol 4-methyl-cyclohexanone4-methyl-cyclohexanolmethyl-cyclohexane(a) (b)(c)(d)Overall reaction: C10H14O2 + 10H+ + 10e- → C9H18 + CH3OH + H2OOverall reaction: C8H10O2 + 10H+ + 10e- → C7H14 + CH3OH + H2O 177   Figure 6.28. Product distributions from the ECH of mixed phenolic reactants, including cerulignol, creosol, and guaiacol (or phenol). Conditions: j = -146 mA cm-2, T = 60 oC, t = 2 h, Rd = 240 rpm. Catholyte: MSA (0.2 M)–IPA (1:1, v/v), Anolyte: MSA (1 M). Catalyst: 5 wt.%-Pt/C (0.2 g). Reactant concentration = 0.1 M (for each reactant, with respect to the catholyte volume). Note: X = conversion, S = product selectivity, F.E. = Faradaic efficiency, C.B. = carbon balance.15.6254.337.304.0512.646.1515.5329.910102030405060708090100X S1 S2 S3 S4 S5 S6 F.E. C.B.%(a)11.7758.662.62 2.089.2512.93 14.4620.100102030405060708090100X S1 S2 S3 S4 S5 S6 F.E. C.B.%(b)CH3OHCH3OHOthersOthers19.6221.7639.416.28 4.8210.2713.124.3434.370102030405060708090100X S1 S2 S3 S4 S5 S6 S7 F.E. C.B.%(c)25.6981.6010.13 8.2842.980102030405060708090100X S1 S2 S3 F.E. C.B.%(d)CH3OH 178  6.6. Summary of the chapter Mass transport study in the ECH of guaiacol explores the benefits of stirred slurry electrochemical reactor configuration. The SSER design allows the cell to operate at the industrially relevant current densities (> |100 mA cm-2|) and improves the liquid–solid mass and heat transfer between the reacting molecules and the negatively charged catalyst particles during the reaction. In the conventional ECH using fixed bed electrode, organic molecules should collide with chemisorbed hydrogen at the electrode surface to favor ECH over HER, whereas in the SSER, the hydrogenation reactions proceed on the large surface area of the charged metal active sites of the dispersed catalyst in the electrolyte solution. In the former case, the hydrogenation reactions are localized at the fixed bed electrode, whereby the reaction rate is limited by diffusion of unsaturated reactants to the electrode surface.67 In the latter case, the reactions could proceed entirely at the well-dispersed electrocatalyst (stirred slurry electrode) as a result of the neighboring effect in which electrons are conductively transmitted to the surrounding metal particles through the collisions. Stirring rate is thus a critical parameter in the SSER that significantly affects the ECH efficiency.  Reaction mechanism and kinetic analyses of the guaiacol ECH revealed that adsorption of guaiacol was likely the rate determining step under the experimental conditions, based on Langmuir-Hinshelwood model. The ECH of guaiacol can follow first- or second-order kinetics depending on the mass transport-related factors, such as substrate concentration, stirring rates, and catalyst loading. Considering the catalyst particle size, low catalyst loading, sufficiently high stirring rates, and the cell reactor dimension, diffusion limitations are insignificant in this work. Stirring profile modifications did not affect the activation energy, confirming that the reactions were under similar kinetic-controlled regime. The rate constant parameter estimation by numerical approximations implies that phenol hydrogenation to cyclohexanone is the fastest step, consistent  179  with the phenol characteristic as a hydrogen scavenger. Meanwhile, demethoxylation of 2-methoxycyclohexanol is the slowest step of all the reactions in guaiacol ECH network, presumably caused by the additional isomerization step forming cis- and trans-2-methoxycylohexanol.  Factorial study of the guaiacol ECH implies that guaiacol concentration and current are the most significant factors, either individually or synergistically, among all the other factors (e.g., catholyte/anolyte proton concentration and temperature). High surface coverage of organics facilitates ECH over HER, therefore Faradaic efficiency can be promoted by optimizing the guaiacol concentration in conjunction with current and proton concentration. Under galvanostatic conditions in this study, reaction temperature is not a significant factor toward Faradaic efficiency, but does affect the product selectivity (i.e. cyclohexanone over cyclohexanol). Anolyte proton concentration positively influenced ECH rates at low current levels, in contrast to the effect of catholyte proton concentration. Finding an optimum condition for ECH of guaiacol is not a simple, straightforward process because of the competitive correlations between the response variables (i.e. guaiacol conversion vs. Faradaic efficiency, cyclohexanone vs. cyclohexanol selectivities). Nevertheless, insights from the predicted factor-response relationships can be useful for designing the optimized ECH experiments in future studies.   Organic solvent can be used as supporting electrolyte in the ECH process. It is mainly beneficial if the organic substrates are immiscible with water (i.e. for highly non-polar compounds), however the presence of organic solvent imposes additional energy requirement to compensate for the dramatically reduced ionic activities in the electrolyte solution. Organic solvent could also induce inhibitory effect on the ECH rates, owing to the competitive adsorption on catalyst active sites. This was clearly observed in the ECH of guaiacol in the acetone-mixed electrolytes. A small amount of acetone (i.e. 5 vol.%) was enough to drastically reduce the guaiacol conversion (-30%).  180  Higher acetone concentration (40 vol.%) suppressed ECH more severely, as guaiacol conversions remained low (<20%) even if higher cathode potentials were applied. The competing reactivity of acetone was verified by the formation of isopropanol, its hydrogenation product, which is also an industrially important solvent. These results suggested that the use of organic electrolyte can be practically justified if the solvent is also the target product of the reaction besides the upgraded chemicals of interest. High-temperature electrolysis might improve the feasibility of this process, possibly increasing the product and solvent desorption rates and enhancing the efficiency.   The ECH of bio-oil substrates using stirred slurry catalyst was conducted for the first time using mixed aqueous and organic electrolytes. This work, albeit at the preliminary stage, demonstrates the potential of ECH in a stirred slurry reactor for mild depolymerization and synthesis of hydrocarbon fuels (e.g., propylcyclohexane, which is a component of the surrogate biokerosene model fuel) from lignin derivatives. The presence of polar organic solvent not only improves the reactant solubility, but also favors HDO routes via dehydration and/or dehydroxylation steps, mainly at higher temperatures (≥50 oC), whereby the acidic electrolyte provides high proton concentration as a homogeneous catalytic function. Hence, for the scaled-up process, it would be possible to target feedstocks with higher substrate concentrations as well as larger reactant molecules that more closely resemble bio-oil. Cerulignol and creosol, the representative monomers of bio-oil, can be hydrodeoxygenated into propylcyclohexane and methylcyclohexane, respectively, under mild conditions (50–60 oC, 1 atm), albeit the limited conversions (20–35%) at low Pt/C catalyst concentration (~10 wt.%). In order to accomplish complete deoxygenation purposes at substantially higher rates, elevated temperature electrolysis with dual catalytic functions that favor hydrogenation and dehydration reactions might be worth investigating, thereby expanding the scope of electrosynthesis work in the future.  181  Chapter 7: Conclusions and Recommendations 7.1. Conclusive summary and key findings In summary, electrocatalytic reduction of lignin derivatives is a promising approach for mild synthesis of renewable value-added chemicals from biomass. It is essentially a chemical energy storage technology with a promise of economic and environmental benefits for future biorefineries. The potency of redox reactions in water electrolysis, which internally produce hydrogen and oxygen as the natural reducing and oxidizing agent, is harnessed to catalytically upgrade the organic substrates. The sustainability of this process could also be promoted via synergistic integration with renewable electricity sources, further improving the hydrogen economy. Diverse electrolyte choices, including mineral acid (H2SO4, HClO4, HCl), organic acid (CH3COOH), inorganic salt (NaCl), and mineral base (NaOH) render the ECH process operations more flexible adapting with the water electrolysis technologies. Recent applications of stirred catalyst suspension with different electrolyte pairs enable high current density electrolysis over the extended electrocatalyst options, thereby improving the practical feasibility of the ECH process. From the engineering standpoint, the use of acid/neutral/base catholyte paired with acidic anolyte offers an auspicious approach, since neutral solution represents eco-friendliness of the process, basic (alkaline) solution may assist the substrate solubilization, while acidic solution (such as H2SO4) could be recyclable from the acid hydrolysis of lignocellulosic biomass. The attractiveness of the ECH process is attributed to minimal, less toxic, and less hazardous waste generation since the by-products are mostly oxygen and hydrogen, which can be useful for fuel cell applications.  At present, ECH has been, however, applied mostly for lignin model compound upgrading and limited bio-oil stabilization rather than for lignin depolymerization. ECH of lignin is not available  182  to date, to the best of our knowledge, because lignin is not soluble in acidic electrolytes. Conceptually, it is possible to depolymerize lignin in a mixed organic and aqueous electrolyte via ECH pathways. However, high temperature and super acid catalysts might be required to facilitate hydrodeoxygenation (HDO) reactions, which usually involve dehydration steps. These conditions are not preferable for the electrocatalytic reduction pathways because at least three components are required: (i) organic solvent, (ii) high-temperature reactor, (iii) bifunctional catalyst, which already characterized thermochemical routes. Hence, it is understandable that lignin depolymerization is carried out via electrocatalytic oxidation (ECO) rather than ECH. On the other hand, ECO of lignin model compounds is less prevalent than ECO of real lignin, presumably because the ECO process targets pulp delignification and lignin degradation rather than chemical synthesis. In this regard, ECH is potentially more useful approach for synthesis purposes. Electrochemical pathways are, however, still attractive with the possibility of performing simultaneous redox reactions in a single cell. For example, ECH of model compounds (or bio-oil) in a cathode coupled with ECO of lignin in an anode. Moreover, if the target product is monomeric chemical (e.g., cyclohexanone/cyclohexanol), ECH can be very competitive versus TCH, due to its milder reaction conditions with greener H2 source and no need for organic solvent. On the industrial scale, cyclohexanol and cyclohexanone are produced by either phenol hydrogenation (140–170 oC, 1 atm) or cyclohexane oxidation (140–180 oC, 0.8–2 MPa),63 which require high temperatures and external gas (H2 or O2) supply. Phenol hydrogenation is the preferred route for cyclohexanone production, so as to avoid the endothermic step of cyclohexanol dehydrogenation, therefore the former process is more beneficial in terms of investment and energy saving.90 Synthesis of commodity organic chemicals via ECH and/or ECO offers greener approaches which can be competitive to the petroleum-based conventional processes.   183  7.2. Recommendations for future work  Possible pathways forward can be identified based on the current understanding of potentials and challenges in this field.127 Three major areas for future development include the design of (i) electrolyte system that improves organic substrate solubility without losses in conductivity and reactivity, (ii) robust electrochemical reactor that can operate efficiently at harsh conditions (elevated temperature and pressure), (iii) electrocatalyst that alleviates reliance on noble metals. Further explanations are given below: (i) Conductive, organic electrolyte to enhance substrate solubility While lignin model compounds, including phenolics, aromatics, and dimers, have been widely explored, the ECH of real substrates (e.g., pyrolysis bio-oil) has not been yet evaluated in detail. Reaction mechanism in the ECH of complex mixture like bio-oil remains unclear. Most of the ECH studies have used aqueous electrolytes for model reactants with low concentrations, however, higher molecular weight organic substrates are poorly soluble in water. The solubility issues would also occur with the increasing oxygen removal from the substrates, leading to the formation of hydrophobic layers in the reaction media. This phase separation would dramatically cause mass transport and electron transfer limitations. Organic solvent addition, such as alcohols or acetone, facilitates the reactant dissolution, however, their presence could suppress the ECH rates, either by decreasing the activity of H3O+ ions or by competitively occupying the active sites of the catalyst. A conductive, water-soluble organic solvent, such as methanesulfonic acid, might be a useful electrolyte for this purpose. Besides the requirements for high current density (> |100 mA cm-2|), higher substrate concentrations are also necessary toward large scale process. The presence of the electrically conductive organic solvent as supporting electrolyte could hence play an  184  important role in the ECH of larger organic substrates. Likewise, a conductive, organic base electrolyte may also be desirable for advancing the lignin electrochemical depolymerization.  (ii)  Improved reactor design for a robust, high temperature electrosynthesis Some challenges in the electrochemical upgrading process are also related to the reactor design. Most ECH studies were carried out in a glass H-type cell configuration with a batch mode of operation. This reactor type, while convenient for laboratory scale testing of different ECH conditions, normally employs a large inter-electrode gap and a small membrane, which contribute to high ohmic resistance for electron transfer, resulting in voltage losses. This technical issue has been recognized in some literatures49,61,66 and a solid polymer electrolyte ECH cell has been recommended to reduce the distance between electrodes (and hence the cell voltage), thus enhancing the energy efficiency.66 Intermediate to high temperature electrochemical reactor may also be necessary for hydrodeoxygenation reactions, particularly if the target product is liquid alkane (hydrocarbon). Conversion of oxygenated compounds to alkanes typically involves dehydration of alcohols, which is endothermic by nature and thermodynamically limited by the reaction equilibrium in the presence of water,49 thus high temperatures (>100 oC) are necessary for high yield production. The durability and cost of the membrane should also be taken into consideration in the reactor design with the critical opportunity to employ dual membrane configuration for simultaneous redox reactions. A good membrane for electrolyzer has to be mechanically stable (able to resist elevated temperature up to 100–120 oC and pressure up to 30 bar) and chemically stable (inert toward reducing and oxidizing chemicals, e.g., H2 and O2).128 A recent study of Nafion-117 stability at elevated temperature (110–150 oC) and pressure (5–7 bar) showed the possibility of operating water electrolysis at those conditions with sufficient membrane  185  hydration.129 Online measurement of H2 and O2 generated from the water electrolysis will also be an important addition to the electrochemical reactor system in order to evaluate the electrical efficiency more precisely and better understand the process mechanisms. (iii) Electrocatalyst design and development  ECH processes still rely on noble metals for stability and effectivity owing to the requirements of high proton concentration, which can be supplied by acidic electrolytes. Platinum-based catalysts (e.g., Pt/C) are most effective in acidic media while ruthenium has been increasingly used in alkaline media considering its stability.13,34,49,50 Non-precious metals (e.g., Ni, Cu, Co, Zn)35,70,73 have also been evaluated in the ECH of lignin model compounds and some of them exhibited good catalytic activity even though the stability and selectivity under different pH conditions are still questionable. These catalysts were mostly used for hydrogenation, ring saturation, or partial deoxygenation purposes, however, the applications of bifunctional catalysts (as in the thermocatalytic processes) for electrocatalytic hydrodeoxygenation reactions are still underexplored. The recent applications of heteropoly acid in the ECH of phenolics67 could facilitate the future electrocatalytic upgrading of bio-oil to produce higher yields of fully deoxygenated products in a dual catalyst system. For better understanding the reaction and electron transfer mechanisms, impacts of different support materials (e.g., conductive vs. non-conductive, hydrophilic vs. hydrophobic) need to be investigated in a comprehensive manner, including evaluation of the electrocatalyst durability and reusability after reduction and/or oxidation reactions, which is currently underexplored. To the best of our knowledge, detailed studies on electrode reusability are at present non-existent for the biomass upgrading purposes. Development of an active, selective, and stable electrocatalyst material is essential to advance this ECH process.  186   In a broader context, the feasible integration of electrocatalytic valorization of lignin derivatives with renewable electricity sources and thermocatalytic processes (for the fractionation stage) offers a promising future biorefinery scheme and many research opportunities. The contrasting nature and environment for effectively degrading and upgrading lignin represents intrinsically interesting challenges toward the future realization of this process. The alkaline water and PEM-acid electrolysis infrastructures could potentially be applied for the electrochemical depolymerization of lignin and the electrocatalytic reduction of lignin derivatives, respectively. The following key research areas are highlighted for further investigation and development toward the practical applications of this process:  1) Simultaneous reduction and oxidation for lignin valorization, for instance, by using a dual (or bipolar) membrane cell configuration with different catholyte and anolyte pair,  2) Continuous electrocatalytic reduction in a flow cell reactor using a slurry mode, such as fluidized bed or moving bed electrode, for a large-scale electrosynthesis,  3) Electrocatalytic upgrading of bio-oil substrates for producing higher aromatic and alkane yields using a robust, high temperature-enabled electrochemical reactor design with bifunctional catalyst,  4) Selective and efficient electrosynthesis of industrially important chemicals (e.g., cyclohexanol and cyclohexanone) under mild conditions at high working current densities,  5) Theoretical studies and molecular simulations combined with in situ microscopic, structural, compositional characterizations regarding the interactions between different organic molecules and different metal catalyst surfaces and their effects on the ECH as well as HER catalysis, 6) Techno-economic assessment on the feasibility of electrocatalytic technologies for prospective biorefineries.130  187  Knowledge from the batch ECH process in this study can be useful for the design and development of a continuous reactor. Different aqueous electrolyte pairs configuration can be implemented for simultaneous redox process using a dual membrane flow cell, consisting of a cation exchange membrane and anion exchange membrane, or a bipolar membrane,131 with the appropriate electrocatalysts. Electrochemical flow reactors are primarily intended to overcome mass transport limitations inherent to the conventional batch electrolysis process,132,133 which can be achieved by increasing the flow rate of the reactant solution and by using turbulence promoters (e.g., mesh stack) in the flow channel with a three dimensional electrode (e.g., RVC).132 Mass transport in H-cell electrolysis using fixed bed electrode limits the operation at low current densities (<100 mA cm-2).133 Large electrode surface area in a flow cell reactor is needed to enhance the rate of electrolysis and energy efficiency. Flow electrosynthesis using a catalyst slurry mode, which remains unexplored to date, can be operated using fluidized bed or moving bed electrode configuration, whereby the catalyst slurry is advected in close proximity to the electrode while the electrolyte solution being circulated through the cell. Alternatively, this flow cell design can also employ membrane-electrode assembly (MEA) prepared using a well-dispersed slurry by a hydrodynamic cavitation method.134 Flow cell electrocatalysis has been increasingly applied for CO2 reduction into fuels and commodity chemicals (e.g., CO, formic acid, methanol).133 Overall, the electrocatalytic process for lignin valorization represent promising pathways that currently serve three major purposes: (i) synthesis of renewable chemicals and fuels, (ii) stabilization of biomass or lignin derived oils, (iii) degradation of lignin substrates. Aqueous-phase ECH in the stirred slurry reactor represents a green approach for the electrosynthesis of value-added chemicals from water-soluble biomass derivatives, including lignin-derived phenols Insights from this research could be applicable to the electrocatalytic upgrading pathways of  188  biomass derivatives, for either lignin- or carbohydrate-derived substrates. The integration of pyrolysis, ECH, and hydrotreatment processes for bio-oil production, stabilization, and upgrading powered by renewable electricity from solar or wind in a distributed biomass processing station has been proposed for the production of sustainable carbon-neutral fuels and chemicals (Figure 7.1).69,76 In spite of all the advantages and advances in the electrochemical routes for lignin valorization, the remaining technological barriers need to be addressed before an economically feasible and scaled-up process can be established.135 Challenges and opportunities for bio-oil refining schemes, including progress in the fractionation and purification techniques, are recently reviewed by Pires et al.121 Product purification from bio-oil upgrading can be done using various methods, such as extraction, distillation, or membrane separation, depending on the product characteristics (e.g., polarity, acid-base, boiling point, molecular weight).121 Vapor pressure and solubility data for the organic compounds and solvents used in this study (see Appendix A.8) indicate the possibility of product recovery using distillation. Interdisciplinary and collaborative efforts are imperative to accelerate the progress in this area, aiming for the integration of renewable energy and biorefinery to provide our society with clean and sustainable chemical products.  Figure 7.1. Concept design of a distributed biomass processing station. Reproduced from ref. 69 with permission from American Chemical Society, Copyright 2020. 189  References 1. Chu, S. & Majumdar, A. Opportunities and challenges for a sustainable energy future. Nature 488, 294–303 (2012). 2. The International Energy Agency IEA. Key World Energy Statistics 2019. OECD Publishing (2019). 3. The International Energy Agency IEA. World Energy Outlook 2019. OECD Publishing (2019). 4. Balsalobre-Lorente, D., Shahbaz, M., Roubaud, D. & Farhani, S. How economic growth, renewable electricity and natural resources contribute to CO2 emissions? Energy Policy 113, 356–367 (2018). 5. Huber, G. W., Iborra, S. & Corma, A. Synthesis of Transportation Fuels from Biomass:  Chemistry, Catalysts, and Engineering. Chem. Rev. 106, 4044–4098 (2006). 6. Alonso, D. M., Bond, J. Q. & Dumesic, J. A. Catalytic conversion of biomass to biofuels. Green Chem. 12, 1493–1513 (2010). 7. Alonso, D. M., Wettstein, S. G. & Dumesic, J. A. Bimetallic catalysts for upgrading of biomass to fuels and chemicals. Chem. Soc. Rev. 41, 8075–8098 (2012). 8. Wijaya, Y. P., Putra, R. D. D., Widyaya, V. T., Ha, J.-M., Suh, D. J. & Kim, C. S. Comparative study on two-step concentrated acid hydrolysis for the extraction of sugars from lignocellulosic biomass. Bioresour. Technol. 164, 221–231 (2014). 9. Pandey, M. P. & Kim, C. S. Lignin Depolymerization and Conversion: A Review of Thermochemical Methods. Chem. Eng. Technol. 34, 29–41 (2011). 10. Mosier, N., Wyman, C., Dale, B., Elander, R., Lee, Y. Y., Holtzapple, M. & Ladisch, M. Features of promising technologies for pretreatment of lignocellulosic biomass. Bioresour. Technol. 96, 673–686 (2005). 11. Stöcker, M. Biofuels and biomass-to-liquid fuels in the biorefinery: Catalytic conversion of lignocellulosic biomass using porous materials. Angew. Chemie - Int. Ed. 47, 9200–9211 (2008).  190  12. Lam, C. H., Das, S., Erickson, N. C., Hyzer, C. D., Garedew, M., Anderson, J. E., Wallington, T. J., Tamor, M. A., Jackson, J. E. & Saffron, C. M. Towards sustainable hydrocarbon fuels with biomass fast pyrolysis oil and electrocatalytic upgrading. Sustain. Energy Fuels 1, 258–266 (2017). 13. Garedew, M., Young-Farhat, D., Jackson, J. E. & Saffron, C. M. Electrocatalytic Upgrading of Phenolic Compounds Observed after Lignin Pyrolysis. ACS Sustain. Chem. Eng. 7, 8375–8386 (2019). 14. Zakzeski, J., Bruijnincx, P. C. A., Jongerius, A. L. & Weckhuysen, B. M. The Catalytic Valorization of Lignin for the Production of Renewable Chemicals. Chem. Rev. 110, 3552–3599 (2010). 15. Saidi, M., Samimi, F., Karimipourfard, D., Nimmanwudipong, T., Gates, B. C. & Rahimpour, M. R. Upgrading of lignin-derived bio-oils by catalytic hydrodeoxygenation. Energy Environ. Sci. 7, 103–129 (2014). 16. Li, C., Zhao, X., Wang, A., Huber, G. W. & Zhang, T. Catalytic Transformation of Lignin for the Production of Chemicals and Fuels. Chem. Rev. 115, 11559–11624 (2015). 17. Evstigneev, E. I. Factors affecting lignin solubility. Russ. J. Appl. Chem. 84, 1040–1045 (2011). 18. Ragauskas, A. J., Beckham, G. T., Biddy, M. J., Chandra, R., Chen, F., Davis, M. F., Davison, B. H., Dixon, R. a, Gilna, P., Keller, M., Langan, P., Naskar, A. K., Saddler, J. N., Tschaplinski, T. J., Tuskan, G. a & Wyman, C. E. Lignin valorization: improving lignin processing in the biorefinery. Science 344, 1246843 (2014). 19. Zhang, Q., Chang, J., Wang, T. & Xu, Y. Review of biomass pyrolysis oil properties and upgrading research. Energy Convers. Manag. 48, 87–92 (2007). 20. Xiu, S. & Shahbazi, A. Bio-oil production and upgrading research: A review. Renew. Sustain. Energy Rev. 16, 4406–4414 (2012). 21. Basu, P. Production of Synthetic Fuels and Chemicals from Biomass. Biomass Gasification, Pyrolysis and Torrefaction (Elsevier Inc., 2013).  22. Bridgwater, A. V. Review of fast pyrolysis of biomass and product upgrading. Biomass and Bioenergy 38, 68–94 (2012).  191  23. Mortensen, P. M., Grunwaldt, J.-D., Jensen, P. A., Knudsen, K. G. & Jensen, A. D. A review of catalytic upgrading of bio-oil to engine fuels. Appl. Catal. A Gen. 407, 1–19 (2011). 24. Ruddy, D. A., Schaidle, J. A., Ferrell III, J. R., Wang, J., Moens, L. & Hensley, J. E. Recent advances in heterogeneous catalysts for bio-oil upgrading via “ex situ catalytic fast pyrolysis”: catalyst development through the study of model compounds. Green Chem. 16, 454–490 (2014). 25. Routray, K., Barnett, K. J. & Huber, G. W. Hydrodeoxygenation of Pyrolysis Oils. Energy Technol. 5, 80–93 (2016). 26. Saffron, C. M., Li, Z., Miller, D. J. & Jackson, J. E. Electrocatalytic Hydrogenation and Hydrodeoxygenation of Oxygenated and Unsaturated Organic Compounds. US Patent, US 20150008139 A1 (2015). 27. Ogden, J. M. Prospects for building a hydrogen energy infrastructure. Annu. Rev. Energy Environ. 24, 227–279 (1999). 28. Mettler, M. S., Vlachos, D. G. & Dauenhauer, P. J. Top ten fundamental challenges of biomass pyrolysis for biofuels. Energy Environ. Sci. 5, 7797 (2012). 29. Turner, J., Sverdrup, G., Mann, M. K., Maness, P.-C., Kroposki, B., Ghirardi, M., Evans, R. J. & Blake, D. Renewable hydrogen production. Int. J. Energy Res. 32, 379–407 (2008). 30. Holladay, J. D., Hu, J., King, D. L. & Wang, Y. An overview of hydrogen production technologies. Catal. Today 139, 244–260 (2009). 31. Gür, T. M. Review of electrical energy storage technologies, materials and systems: Challenges and prospects for large-scale grid storage. Energy Environ. Sci. 11, 2696–2767 (2018). 32. Zeng, K. & Zhang, D. Recent progress in alkaline water electrolysis for hydrogen production and applications. Prog. Energy Combust. Sci. 36, 307–326 (2010). 33. Schiller, G., Ansar, A., Lang, M. & Patz, O. High temperature water electrolysis using metal supported solid oxide electrolyser cells (SOEC). J. Appl. Electrochem. 39, 293–301 (2009). 34. Li, Z., Garedew, M., Lam, C. H., Jackson, J. E., Miller, D. J. & Saffron, C. M. Mild electrocatalytic hydrogenation and hydrodeoxygenation of bio-oil derived phenolic  192  compounds using ruthenium supported on activated carbon cloth. Green Chem. 14, 2540 (2012). 35. Lam, C. H., Lowe, C. B., Li, Z., Longe, K. N., Rayburn, J. T., Caldwell, M. A., Houdek, C. E., Maguire, J. B., Saffron, C. M., Miller, D. J. & Jackson, J. E. Electrocatalytic upgrading of model lignin monomers with earth abundant metal electrodes. Green Chem. 17, 601–609 (2015). 36. Song, Y., Gutiérrez, O. Y., Herranz, J. & Lercher, J. A. Aqueous phase electrocatalysis and thermal catalysis for the hydrogenation of phenol at mild conditions. Appl. Catal. B Environ. 182, 236–246 (2016). 37. Jung, S. & Biddinger, E. J. Electrocatalytic Hydrogenation and Hydrogenolysis of Furfural and the Impact of Homogeneous Side Reactions of Furanic Compounds in Acidic Electrolytes. ACS Sustain. Chem. Eng. 4, 6500–6508 (2016). 38. Cardoso, D. S. P., Šljukić, B., Santos, D. M. F. & Sequeira, C. A. C. Organic Electrosynthesis: From Laboratorial Practice to Industrial Applications. Org. Process Res. Dev. 21, 1213–1226 (2017). 39. Song, Y., Sanyal, U., Pangotra, D., Holladay, J. D., Camaioni, D. M., Gutiérrez, O. Y. & Lercher, J. A. Hydrogenation of benzaldehyde via electrocatalysis and thermal catalysis on carbon-supported metals. J. Catal. 359, 68–75 (2018). 40. Miller, L. L. & Christensen, L. Electrocatalytic Hydrogenation of Aromatic Compounds. J. Org. Chem. 43, 2059–2061 (1978). 41. Amouzegar, K. & Savadogo, O. Electrocatalytic Hydrogenation of Phenol on Highly Dispersed Pt Electrodes. Electrochim. Acta 39, 557–559 (1994). 42. Mahdavi, B., Lafrance, A., Martel, A., Lessard, J., Ménard, H. & Brossard, L. Electrocatalytic hydrogenolysis of lignin model dimers at Raney nickel electrodes. J. Appl. Electrochem. 27, 605–611 (1997). 43. Martel, A., Mahdavi, B., Lessard, J., Brossard, L. & Menard, H. Electrocatalytic hydrogenation of phenol on various electrode materials. Can. J. Chem. 75, 1862–1867 (1997). 44. Amouzegar, K. & Savadogo, O. Electrocatalytic hydrogenation of phenol on dispersed Pt:  193  Effect of metal electrochemically active surface area and electrode material. J. Appl. Electrochem. 27, 539–542 (1997). 45. Amouzegar, K. & Savadogo, O. Electrocatalytic hydrogenation of phenol on dispersed Pt : reaction mechanism and support effect. Electrochim. Acta 43, 503–508 (1998). 46. Chapuzet, J. M., Lasia, A. & Lessard, J. Electrocatalytic hydrogenation of organic compounds. Electrocatalysis 155–159 (1998). 47. Cyr, A., Chiltz, F., Jeanson, P., Martel, A., Brossard, L., Lessard, J. & Ménard, H. Electrocatalytic hydrogenation of lignin models at Raney nickel and palladium-based electrodes. Can. J. Chem. 78, 307–315 (2000). 48. Lessard, J. Electrocatalytic Hydrogenation. Encyclopedia of Applied Electrochemistry 443–448 (2014). 49. Wijaya, Y. P., Grossmann-Neuhaeusler, T., Dhewangga Putra, R. D., Smith, K. J., Kim, C. S. & Gyenge, E. L. Electrocatalytic Hydrogenation of Guaiacol in Diverse Electrolytes Using a Stirred Slurry Reactor. ChemSusChem 13, 629–639 (2020). 50. Wijaya, Y. P., Smith, K. J., Kim, C. S. & Gyenge, E. L. Synergistic effects between electrocatalyst and electrolyte in the electrocatalytic reduction of lignin model compounds in a stirred slurry reactor. J. Appl. Electrochem. 51, 51–63 (2021). 51. Li, K. & Sun, Y. Electrocatalytic Upgrading of Biomass‐Derived Intermediate Compounds to Value‐Added Products. Chem. – A Eur. J. 24, 18258–18270 (2018). 52. Huynh, M. H. V. & Meyer, T. J. Proton-coupled electron transfer. Chem. Rev. 107, 5004–5064 (2007). 53. Weinberg, D. R., Gagliardi, C. J., Hull, J. F., Murphy, C. F., Kent, C. A., Westlake, B. C., Paul, A., Ess, D. H., McCafferty, D. G. & Meyer, T. J. Proton-coupled electron transfer. Chem. Rev. 112, 4016–4093 (2012). 54. Bard, A. J. & Faulkner, L. R. Electrochemical Methods: Fundamentals and Applications. (John Wiley & Sons, Inc., 2001). 55. He, Z., Chen, Y., Santos, E. & Schmickler, W. The Pre-exponential Factor in Electrochemistry. Angew. Chemie 57, 7948–7956 (2018).  194  56. Ramaswamy, N. & Mukerjee, S. Influence of inner- and outer-sphere electron transfer mechanisms during electrocatalysis of oxygen reduction in alkaline media. J. Phys. Chem. C 115, 18015–18026 (2011). 57. Mçhle, S., Zirbes, M., Rodrigo, E., Gieshoff, T., Wiebe, A. & Waldvogel, S. R. Modern Electrochemical Aspects for the Synthesis of Value-Added Organic Products Angewandte. Angew. Chemie Int. Ed. 57, 6018–6041 (2018). 58. Mahmood, N., Yao, Y., Zhang, J.-W., Pan, L., Zhang, X. & Zou, J.-J. Electrocatalysts for Hydrogen Evolution in Alkaline Electrolytes: Mechanisms, Challenges, and Prospective Solutions. Adv. Sci. 5, 1700464 (2018). 59. Zhao, B., Guo, Q. & Fu, Y. Electrocatalytic Hydrogenation of Lignin-Derived Phenol into Alkanes by Using Platinum Supported on Graphite. Electrochemistry 82, 954–959 (2014). 60. Singh, N., Song, Y., Gutiérrez, O. Y., Camaioni, D. M., Campbell, C. T. & Lercher, J. A. Electrocatalytic Hydrogenation of Phenol over Platinum and Rhodium: Unexpected Temperature Effects Resolved. ACS Catal. 6, 7466–7470 (2016). 61. Song, Y., Chia, S. H., Sanyal, U., Gutiérrez, O. Y. & Lercher, J. A. Integrated catalytic and electrocatalytic conversion of substituted phenols and diaryl ethers. J. Catal. 344, 263–272 (2016). 62. Singh, N., Sanyal, U., Ruehl, G., Stoerzinger, K. A., Gutiérrez, O. Y., Camaioni, D. M., Fulton, J. L., Lercher, J. A. & Campbell, C. T. Aqueous phase catalytic and electrocatalytic hydrogenation of phenol and benzaldehyde over platinum group metals. J. Catal. 382, 372–384 (2020). 63. Musser, M. T. & Du, E. I. Cyclohexanol and Cyclohexanone; in: Ullmann’s Encyclopedia of Industrial Chemistry. (2012) doi:10.1002/14356007.a08_217.pub2. 64. Liu, H., Jiang, T., Han, B., Liang, S. & Zhou, Y. Selective phenol hydrogenation to cyclohexanone over a dual supported Pd-Lewis acid catalyst. Science 326, 1250–2 (2009). 65. Talukdar, A. K., Bhattacharyya, K. G. & Sivasanker, S. Hydrogenation of phenol over supported platinum and palladium catalysts. Appl. Catal. A, Gen. 96, 229–239 (1993). 66. Li, Z., Kelkar, S., Raycraft, L., Garedew, M., Jackson, J. E., Miller, D. J. & Saffron, C. M. A mild approach for bio-oil stabilization and upgrading: electrocatalytic hydrogenation  195  using ruthenium supported on activated carbon cloth. Green Chem. 16, 844–852 (2014). 67. Liu, W., You, W., Gong, Y. & Deng, Y. High-efficiency electrochemical hydrodeoxygenation of bio-phenols to hydrocarbon fuels by a superacid-noble metal particle dual-catalyst system. Energy Environ. Sci. 13, 917–927 (2020). 68. Lister, T. E., Diaz, L. A., Lilga, M. A., Padmaperuma, A. B., Lin, Y., Palakkal, V. M. & Arges, C. G. Low-Temperature Electrochemical Upgrading of Bio-oils Using Polymer Electrolyte Membranes. Energy and Fuels 32, 5944–5950 (2018). 69. Lam, C. H., Deng, W., Lang, L., Jin, X., Hu, X. & Wang, Y. Minireview on Bio-Oil Upgrading via Electrocatalytic Hydrogenation: Connecting Biofuel Production with Renewable Power. Energy & Fuels 34, 7915–7928 (2020). 70. Andrews, E., Lopez-Ruiz, J. A., Egbert, J. D., Koh, K., Sanyal, U., Song, M., Li, D., Karkamkar, A. J., Derewinski, M. A., Holladay, J., Gutiérrez, O. Y. & Holladay, J. D. Performance of Base and Noble Metals for Electrocatalytic Hydrogenation of Bio-Oil-Derived Oxygenated Compounds. ACS Sustain. Chem. Eng. 8, 4407–4418 (2020). 71. Cirtiu, C. M., Hassani, H. O., Bouchard, N., Rowntree, P. A. & Me, H. Modification of the Surface Adsorption Properties of Alumina-Supported Pd Catalysts for the Electrocatalytic Hydrogenation of Phenol. 6414–6421 (2006). 72. Lopez-Ruiz, J. A., Sanyal, U., Egbert, J., Gutiérrez, O. Y. & Holladay, J. Kinetic Investigation of the Sustainable Electrocatalytic Hydrogenation of Benzaldehyde on Pd/C: Effect of Electrolyte Composition and Half-Cell Potentials. ACS Sustain. Chem. Eng. 6, 16073–16085 (2018). 73. Lopez-Ruiz, J. A., Andrews, E., Akhade, S. A., Lee, M. S., Koh, K., Sanyal, U., Yuk, S. F., Karkamkar, A. J., Derewinski, M. A., Holladay, J., Glezakou, V. A., Rousseau, R., Gutiérrez, O. Y. & Holladay, J. D. Understanding the Role of Metal and Molecular Structure on the Electrocatalytic Hydrogenation of Oxygenated Organic Compounds. ACS Catal. 9, 9964–9972 (2019). 74. Zhou, Y., Gao, Y., Zhong, X., Jiang, W., Liang, Y., Niu, P., Li, M., Zhuang, G., Li, X. & Wang, J. Electrocatalytic Upgrading of Lignin-Derived Bio-Oil Based on Surface-Engineered PtNiB Nanostructure. Adv. Funct. Mater. 29, 1–11 (2019).  196  75. Zhang, B., Zhang, J. & Zhong, Z. Low-Energy Mild Electrocatalytic Hydrogenation of Bio-oil Using Ruthenium Anchored in Ordered Mesoporous Carbon. ACS Appl. Energy Mater. 1, 6758–6763 (2018). 76. Deng, W., Xu, K., Xiong, Z., Chaiwat, W., Wang, X., Su, S., Hu, S., Qiu, J., Wang, Y. & Xiang, J. Evolution of Aromatic Structures during the Low-Temperature Electrochemical Upgrading of Bio-oil. Energy and Fuels 33, 11292–11301 (2019). 77. He, T., Zhong, Z. & Zhang, B. Bio-oil Upgrading via Ether Extraction, Looped-Oxide Catalytic Deoxygenation, and Mild Electrocatalytic Hydrogenation Techniques. Energy & Fuels 34, 9725–9733 (2020). 78. Zhao, C., He, J., Lemonidou, A. A., Li, X. & Lercher, J. A. Aqueous-phase hydrodeoxygenation of bio-derived phenols to cycloalkanes. J. Catal. 280, 8–16 (2011). 79. Zadick, A., Dubau, L., Sergent, N., Grégory, G., Berthomé, B. & Chatenet, M. Huge Instability of Pt/C Catalysts in Alkaline Medium. ACS Catal. 5, 4819–4824 (2015). 80. Lam, C. H., Lowe, C. B., Li, Z., Longe, K. N., Rayburn, J. T., Caldwell, M. A., Houdek, C. E., Maguire, J. B., Saffron, C. M., Miller, D. J. & Jackson, J. E. Electrocatalytic upgrading of model lignin monomers with earth abundant metal electrodes. Green Chem. 17, 601–609 (2014). 81. Song, Y., Sanyal, U., Pangotra, D., Holladay, J. D., Camaioni, D. M., Gutiérrez, O. Y. & Lercher, J. A. Hydrogenation of benzaldehyde via electrocatalysis and thermal catalysis on carbon-supported metals. J. Catal. 359, 68–75 (2018). 82. Gao, D., Xiao, Y. & Varma, A. Guaiacol Hydrodeoxygenation over Platinum Catalyst: Reaction Pathways and Kinetics. Ind. Eng. Chem. Res. 54, 10638–10644 (2015). 83. Liu, Y. & Shen, L. From Langmuir Kinetics to First- and Second-Order Rate Equations for Adsorption. Langmuir 24, 11625–11630 (2008). 84. Chang, J., Danuthai, T., Dewiyanti, S., Wang, C. & Borgna, A. Hydrodeoxygenation of guaiacol over carbon-supported metal catalysts. ChemCatChem 5, 3041–3049 (2013). 85. Nakagawa, Y., Ishikawa, M., Tamura, M. & Tomishige, K. Selective production of cyclohexanol and methanol from guaiacol over Ru catalyst combined with MgO. Green Chem. 16, 2197–2203 (2014).  197  86. Dubé, P., Kerdouss, F., Laplante, F., Proulx, P., Brossard, L. & Mé Nard, H. Electrocatalytic hydrogenation of cyclohexanone: Simple dynamic cell design. J. Appl. Electrochem. 33, 541–547 (2003). 87. Lu, B., Guo, L., Wu, F., Peng, Y., Lu, J. E., Smart, T. J., Wang, N., Finfrock, Y. Z., Morris, D., Zhang, P., Li, N., Gao, P., Ping, Y. & Chen, S. Ruthenium atomically dispersed in carbon outperforms platinum toward hydrogen evolution in alkaline media. Nat. Commun. 10, 1–11 (2019). 88. Rylander, P. N. Platinum Metal Catalysts. Catal. Hydrog. Over Platin. Met. 61, 3–29 (1967). 89. G. C. Bond. Platinum Metals as Hydrogenation Catalysts. Platin. Met. Rev. 1, 87–93 (1957). 90. Neri, G., Visco, A. M., Donato, A., Milone, C., Malentacchi, M. & Gubitosa, G. Hydrogenation of phenol to cyclohexanone over palladium and alkali-doped palladium catalysts. Appl. Catal. A, Gen. 110, 49–59 (1994). 91. Ftouni, J., Genuino, H. C., Muñoz-Murillo, A., Bruijnincx, P. C. A. & Weckhuysen, B. M. Influence of Sulfuric Acid on the Performance of Ruthenium-based Catalysts in the Liquid-Phase Hydrogenation of Levulinic Acid to γ-Valerolactone. ChemSusChem 10, 2891–2896 (2017). 92. Mahmood, J., Li, F., Jung, S., Okyay, M. S., Ahmad, I., Kim, S., Park, N., Jeong, H. Y. & Baek, J. An efficient and pH-universal ruthenium-based catalyst for the hydrogen evolution reaction. Nat. Nanotechnol. 12, 441–447 (2017). 93. Zadick, A., Dubau, L., Demirci, U. B. & Chatenet, M. Effects of Pd nanoparticle size and solution reducer strength on Pd/C electrocatalyst stability in alkaline electrolyte. J. Electrochem. Soc. 163, F781–F787 (2016). 94. Michel, C. & Gallezot, P. Why Is Ruthenium an Efficient Catalyst for the Aqueous-Phase Hydrogenation of Biosourced Carbonyl Compounds? ACS Catal. 5, 4130–4132 (2015). 95. Lu, M., Du, H., Wei, B., Zhu, J., Li, M., Shan, Y. & Song, C. Catalytic Hydrodeoxygenation of Guaiacol over Palladium Catalyst on Different Titania Supports. Energy and Fuels 31, 10858–10865 (2017). 96. Mellmer, M. A., Sanpitakseree, C., Demir, B., Ma, K., Elliott, W. A., Bai, P., Johnson, R. L., Walker, T. W., Shanks, B. H., Rioux, R. M., Neurock, M. & Dumesic, J. A. Effects of  198  chloride ions in acid-catalyzed biomass dehydration reactions in polar aprotic solvents. Nat. Commun. 10, 1–10 (2019). 97. Hodnik, N., Jovanovič, P., Pavlišič, A., Jozinović, B., Zorko, M., Bele, M., Šelih, V. S., Šala, M., Hočevar, S. & Gaberšček, M. New insights into corrosion of ruthenium and ruthenium oxide nanoparticles in acidic media. J. Phys. Chem. C 119, 10140–10147 (2015). 98. Shuai, L. & Luterbacher, J. Organic Solvent Effects in Biomass Conversion Reactions. ChemSusChem 9, 133–155 (2016). 99. Rajadhyaksha, R. A. & Karwa, S. L. Solvent effects in catalytic hydrogenation. Chem. Eng. Sci. 41, 1765–1770 (1986). 100. BASF. Lutropur® – the friendly acidthe friendly acid. The purest form of MSA methanesulfonic acid made by BASF. (2011). 101. Palden, T., Onghena, B., Regadío, M. & Binnemans, K. Methanesulfonic acid: a sustainable acidic solvent for recovering metals from the jarosite residue of the zinc industry. Green Chem. 21, 5394–5404 (2019). 102. Gernon, M. D., Wu, M., Buszta, T. & Janney, P. Environmental benefits of methanesulfonic acid : Comparative properties and advantages. Green Chem. 1, 127–140 (1999). 103. Rackemann, D. W., Bartley, J. P. & Doherty, W. O. S. Methanesulfonic acid-catalyzed conversion of glucose and xylose mixtures to levulinic acid and furfural. Ind. Crops Prod. 52, 46–57 (2014). 104. Mears, D. E. Tests for Transport Limitations in Experimental Catalytic Reactors. Ind. Eng. Chem. Process Des. Dev. 10, 541–547 (1971). 105. Weisz, P. B. & Prater, C. D. Interpretation of Measurements in Experimental Catalysis. Adv. Catal. 6, 143–196 (1954). 106. Reddy, S. & Krishnan, V. Electrochemical Oxidation of Guaiacol at Platinum Electrode. Indian J. Chem. -Section A 16, 684–687 (1978). 107. Yadav, G. D. & Pathre, G. S. Selectivity Engineering of Cation-Exchange Resins over Inorganic Solid Acids in C-Alkylation of Guaiacol with Cyclohexene. Ind. Eng. Chem. Res. 46, 3119–3127 (2007).  199  108. Kumbhar, P. S. & Yadav, G. D. Catalysis by Sulfur-Promoted Superacidic Zirconia: Condensation Reactions of Hydroquinone with Aniline and Substituted Anilines. Chem. Eng. Sci. 44, 2535–2544 (1989). 109. Masende, Z. P. G., Kuster, B. F. M., Ptasinski, K. J., Janssen, F. J. J. G., Katima, J. H. Y. & Schouten, J. C. Platinum catalysed wet oxidation of phenol in a stirred slurry reactor: A practical operation window. Appl. Catal. B Environ. 41, 247–267 (2003). 110. Fogler, H. S. Essentials of Chemical Reaction Engineering, 2nd Edition. (Pearson Prentice Hall, 2017). 111. Katada, N., Sota, S., Morishita, N., Okumura, K. & Niwa, M. Relationship between activation energy and pre-exponential factor normalized by the number of Brønsted acid sites in cracking of short chain alkanes on zeolites. Catal. Sci. Technol. 5, 1864–1869 (2015). 112. Johns, D. & Hutton, A. The Arrhenius Law: Arrhenius Plots. Chemistry LibreTexts 1–16 https://chem.libretexts.org/ (2017). 113. Chorkendorff, I. & Niemantsverdriet, J. W. Concepts of Modern Catalysis and Kinetics. Synthesis vol. 2005 (John Wiley & Sons, Inc., 2003). 114. Hatakeyama, K., Nakagawa, Y., Tamura, M. & Tomishige, K. Efficient production of adipic acid from 2-methoxycyclohexanone by aerobic oxidation with a phosphotungstic acid catalyst. Green Chem. 22, 4962–4974 (2020). 115. Rudnev, A. V., Molodkina, E. B., Danilov, A. I., Polukarov, Y. M., Berna, A. & Feliu, J. M. Adsorption behavior of acetonitrile on platinum and gold electrodes of various structures in solution of 0.5 M H2SO4. Electrochim. Acta 54, 3692–3699 (2009). 116. Rahman, A. Catalytic Hydrogenation of Acetone to Isopropanol : An Environmentally Benign Approach. Bull. Chem. React. Eng. Catal. 5, 113–126 (2010). 117. Joshi, N. & Lawal, A. Hydrodeoxygenation of 4-propylguaiacol (2-methoxy-4-propylphenol) in a microreactor: Performance and kinetic studies. Ind. Eng. Chem. Res. 52, 4049–4058 (2013). 118. Putra, R. D. D. Studies on Lignin Model Compounds Upgrading with In-situ Glycerol Aqueous Phase Reforming and the Application for Upgrading of Ligneous Material. (The University of British Columbia, 2019).  200  119. Ertaş, M. & Hakki Alma, M. Pyrolysis of laurel (Laurus nobilis L.) extraction residues in a fixed-bed reactor: Characterization of bio-oil and bio-char. J. Anal. Appl. Pyrolysis 88, 22–29 (2010). 120. Di Marino, D., Stöckmann, D., Kriescher, S., Stiefel, S. & Wessling, M. Electrochemical depolymerisation of lignin in a deep eutectic solvent. Green Chem. 18, 6021–6028 (2016). 121. Pires, A. P. P., Arauzo, J., Fonts, I., Domine, M. E., Garcia-Perez, M. E., Montoya, J., Chejne, F., Ferna, A., Pfromm, P. & Garcia-Perez, M. Challenges and Opportunities for Bio-oil Refining: A Review. Energy & Fuels 33, 4683–4720 (2019). 122. Rachmady, W. & Vannice, M. A. Acetic Acid Hydrogenation over Supported Platinum Catalysts. J. Catal. 192, 322–334 (2000). 123. Kumar, P., Kumar, R. & Upadhyayula, S. Acetic acid hydrogenation to ethanol over supported Pt-Sn catalyst : E ff ect of Bronsted acidity on product selectivity. Mol. Catal. 448, 78–90 (2018). 124. Mellmer, M. A., Sener, C., Gallo, J. M. R., Luterbacher, J. S., Alonso, D. M. & Dumesic, J. A. Solvent Effects in Acid-Catalyzed Biomass Conversion Reactions. Angew. Chemie Int. Ed. 53, 11872–11875 (2014). 125. Zhao, C. & Lercher, J. A. Selective Hydrodeoxygenation of Lignin-Derived Phenolic Monomers and Dimers to Cycloalkanes on Pd/C and HZSM-5 Catalysts. ChemCatChem 4, 64–68 (2012). 126. Zhang, W., Chen, J., Liu, R., Wang, S., Chen, L. & Li, K. Hydrodeoxygenation of Lignin-Derived Phenolic Monomers and Dimers to Alkane Fuels over Bifunctional Zeolite-Supported Metal Catalysts. ACS Sustain. Chem. Eng. 2, 683–691 (2014). 127. Wijaya, Y. P., Smith, K. J., Kim, C. S. & Gyenge, E. L. Electrocatalytic hydrogenation and depolymerization pathways for lignin valorization: Toward mild synthesis of chemicals and fuels from biomass. Green Chem. 22, 7233–7264 (2020). 128. Cassir, M., Jones, D., Ringuedé, A. & Lair, V. Electrochemical devices for energy: Fuel cells and electrolytic cells. Handbook of Membrane Reactors vol. 2 (2013). 129. Mališ, J., Mazúr, P., Paidar, M., Bystron, T. & Bouzek, K. Nafion 117 stability under conditions of PEM water electrolysis at elevated temperature and pressure. Int. J. Hydrogen  201  Energy 41, 2177–2188 (2016). 130. Dang, Q., Wright, M. M. & Li, W. Technoeconomic Analysis of a Hybrid Biomass Thermochemical and Electrochemical Conversion System. Energy Technol. 6, 178–187 (2018). 131. Thiele, S., Mayerhöfer, B., McLaughlin, D., Böhm, T., Hegelheimer, M. & Seeberger, D. Bipolar membrane electrode assemblies for water electrolysis. ACS Appl. Energy Mater. 3, 9635–9644 (2020). 132. Folgueiras-Amador, A. A., Teuten, A. E., Pletcher, D. & Brown, R. C. D. A design of flow electrolysis cell for ‘Home’ fabrication. React. Chem. Eng. 5, 712–718 (2020). 133. Weekes, D. M., Salvatore, D. A., Reyes, A., Huang, A. & Berlinguette, C. P. Electrolytic CO2 Reduction in a Flow Cell. Acc. Chem. Res. 51, 910–918 (2018). 134. Kuroki, H., Onishi, K., Asami, K. & Yamaguchi, T. Catalyst Slurry Preparation Using a Hydrodynamic Cavitation Dispersion Method for Polymer Electrolyte Fuel Cells. Ind. Eng. Chem. Res. 58, 19545–19550 (2019). 135. Du, X., Zhang, H., Sullivan, K., Gogoi, P. & Deng, Y. Electrochemical Lignin Conversion. ChemSusChem 13, 4318–4343 (2020). 136. Yaws, C. L. & Satyro, M. A. Vapor Pressure – Organic Compounds. The Yaws Handbook of Vapor Pressure (Elsevier Inc., 2015).    202  Appendices A.1. Stoichiometric analysis of the ECH of guaiacol The stoichiometric analysis estimates that the product distribution from the ECH of guaiacol will vary depending on the ratio of demethoxylation step to the first hydrogenation (ring saturation) step. Since, stoichiometrically, 100% conversion of 1 mol guaiacol would result in 1 mol phenol and 1 mol methanol (via demethoxylation), then the maximum (theoretical) yields of cyclohexanol and methanol would be 85.71% and 14.29%, respectively. The same yields are true for 100% conversion of guaiacol merely via the ring saturation step. In other words, it is unlikely to obtain 100% selectivity to cyclohexanol from guaiacol based on the carbon balance because methanol is always formed as the by-product. Profiles of the product yield over different conversion of intermediates (Figure A.1) depict the different product distributions if the guaiacol were converted via either of the two extreme pathways (100% demethoxylation–ring saturation or 100% ring saturation–demethoxylation). In this study, the products from ECH of guaiacol were found between these two extremes, meaning that both pathways occurred simultaneously at different proportions because all the intermediates (i.e. phenol, cyclohexanone, 2-methoxycyclohexanone, 2-methoxycyclohexanol) are identified. In most cases, under experimental conditions without temperature control, aromatic ring saturation is more dominant as indicated by the higher yields of cyclohexanol and 2-methoxycyclohexanol than those of cyclohexanone and phenol. This trend, however, could be altered via synergistic effect of temperature and potential, which affects coverage of hydrogen on the catalyst surface.   203   Figure A.1. Product distribution of a complete guaiacol conversion via: (a) 100% demethoxylation–ring saturation and (b) 100% ring saturation–demethoxylation pathways as a function of the conversion of intermediates based on stoichiometric analysis.   01020304050607080900 10 20 30 40 50 60 70 80 90 100Yield (%)Conversion of Intermediates (%)PhenolMethanolCyclohexanoneCyclohexanol2-Methoxycyclohexanone2-Methoxycyclohexanol(a)01020304050607080901000 10 20 30 40 50 60 70 80 90 100Yield (%)Conversion of Intermediates (%)PhenolMethanolCyclohexanoneCyclohexanol2-Methoxycyclohexanone2-Methoxycyclohexanol(b) 204  A.2. Mass-transfer limitation assessment in the ECH of guaiacol Internal mass-transfer resistance (Weisz-Prater criterion) In heterogeneous catalytic reactions, the Weisz-Prater (W–P) criterion is used to estimate the influence of pore diffusion on reaction rates. If the criterion is satisfied, pore diffusion limitations (internal mass-transfer resistance) is negligible. The W–P criterion104 is given below:  𝑵𝑾−𝑷 =−𝒓𝑨𝝆𝒑𝑹𝒑𝟐𝑪𝒔𝑫𝒆𝒇𝒇≤𝟏𝒏 (A.1) where −𝒓𝑨 = reaction rate per mass of catalyst, 𝝆𝒑 = density of catalyst, 𝑹𝒑 = catalyst particle radius, 𝑪𝒔 = reactant concentration at the catalyst surface, 𝑫𝒆𝒇𝒇 = effective diffusivity, n = reaction order (1). In a stirred slurry reactor, 𝑪𝑨𝟎 ≈ 𝑪𝒔 (the boundary layer thickness is negligible).  Guaiacol ECH experimental data (𝑪𝑨𝟎, −𝒓𝑨), catalyst properties data (𝝆𝒑,𝑷𝒕 = 21.45 g cm-3, 𝒅𝒑,𝑷𝒕 = 3.91 nm from Table 6.1) and guaiacol diffusivity data (𝑫𝒆𝒇𝒇 = 6.04×10-10 m2 s-1, obtained from literature106) can be used to estimate 𝑵𝑾−𝑷 as follows:  𝑪𝑨𝟎 (mM) −𝒓𝑨 (mmol s-1 gPt-1) 𝑵𝑾−𝑷 53 80 106 132 9.88×10-2 1.07×10-1 1.17×10-1 1.20×10-1 2.52×10-7 1.92×10-10 1.49×10-10 1.23×10-10  In all cases,  𝑁𝑊−𝑃  << 1, thus internal mass-transfer resistance is negligible. In other words, reaction rates are not affected by guaiacol diffusion rate to the catalyst pore. 𝑁𝑊−𝑃 << 1  205  External mass-transfer resistance (Sherwood number) Sherwood number represents the ratio of the convective mass transfer to the rate of diffusion:  𝑺𝒉 =𝒌𝑺−𝑳  𝒅𝒑𝑫𝒆𝒇𝒇 (A.2) The mass-transfer coefficient from liquid to solid (𝒌𝑺−𝑳) is calculated from Sherwood number as follows, with the catalyst geometry is assumed to be spherical:109 𝑺𝒉 = 𝟐 + 𝟎. 𝟒𝑹𝒆𝟏 𝟒⁄ 𝑺𝒄𝟏 𝟑⁄  (A.3) Reynolds number (𝑹𝒆) for a stirred vessel can be approximated as: 𝑹𝒆 =𝝆 𝑵 𝒓𝟐𝝁  (A.4) Schmidt number (𝑺𝒄) is the ratio of viscous diffusion rate and mass (molecular) diffusion rate: 𝑺𝒄 =𝝁𝝆𝑫  (A.5) where 𝝆 = density of the solution, 𝝁 = viscosity of the solution, N = rotational speed, 𝒓 = radius of the stirrer, D = mass diffusivity (= 𝑫𝒆𝒇𝒇), 𝒅𝒑 = catalyst particle diameter. Equations A.2–A.5 can be solved using the following experimental data: 𝝆 = 1.01 g cm-3, 𝝁 = 5.5×10-4 Pa. s (at 50 oC), N = 240 rpm, r = 1.8 cm, resulting in: 𝑺𝒄 = 904, 𝑹𝒆 = 2374, 𝑺𝒉 = 29, and 𝒌𝑺−𝑳 = 2.34×10-4 m s-1.  If external mass-transfer resistance is not significant (i.e. surface reaction is very fast), this correlation applies:107,108 (𝟏−𝒓𝑨≪𝟏𝒌𝑺−𝑳 𝒂𝒑 𝑪𝑨𝟎) or (−𝒓𝑨 ≫ 𝒌𝑺−𝑳 𝒂𝒑 𝑪𝑨𝟎) (A.6)  206  where the particle surface area per unit volume is given by: 𝒂𝒑 =𝟔𝒘𝝆𝒑𝒅𝒑 for spherical catalyst (𝒘 = catalyst loading per unit volume of liquid phase = 1.5 kg m-3, 𝝆𝒑,𝐏𝐭/𝐂 = 2.97 g cm-3, 𝒅𝒑,𝑷𝒕/𝑪 =𝟕𝟓 m), thus 𝒂𝒑 = 40.37 m-1 and Equation A.6 can be calculated further to obtain the following: 𝑪𝑨𝟎 (mM) −𝒓𝑨 (mol s-1 m-3) 𝒌𝑺−𝑳 𝒂𝒑 𝑪𝑨𝟎 53 80 106 132 2120 2415 2503 2569 0.50 0.75 1.00 1.25  In all cases, −𝑟𝐴 ≫ 𝑘𝑆−𝐿 𝑎𝑝 𝐶𝐴0, thus external mass-transfer resistance is negligible. In other words, reaction rates are not affected by guaiacol diffusion rate to the catalyst surface. Overall, this mass-transfer limitation assessment demonstrates that the guaiacol ECH rates are kinetically controlled under the operating conditions in this study.    −𝑟𝐴 ≫ 𝑘𝑆−𝐿 𝑎𝑝 𝐶𝐴0  207  A.3. Catalytic reaction steps in ECH of guaiacol  Figure A.2. Plausible reaction network in the ECH of guaiacol under the experimental conditions in this work. Guaiacol ECH (Overall Reaction):  𝐶7𝐻8𝑂2 + 8𝐻+ + 8𝑒− → 𝐶6𝐻12𝑂 + 𝐶𝐻4𝑂  (1) Two Parallel Routes in the Guaiacol ECH: 1. Demethoxylation – Ring Saturation 𝐶7𝐻8𝑂2 + 2𝐻+ + 2𝑒− → 𝐶6𝐻6𝑂 + 𝐶𝐻4𝑂 (Guaiacol to Phenol and Methanol) (2) 𝐶6𝐻6𝑂 + 4𝐻+ + 4𝑒− → 𝐶6𝐻10𝑂  (Phenol to Cyclohexanone) (3) 𝐶6𝐻10𝑂 + 2𝐻+ + 2𝑒− → 𝐶6𝐻12𝑂  (Cyclohexanone to Cyclohexanol) (4) 2. Ring Saturation – Demethoxylation  𝐶7𝐻8𝑂2 + 4𝐻+ + 4𝑒− → 𝐶7𝐻12𝑂2  (Guaiacol to 2-Methoxycyclohexanone) (5) 𝐶7𝐻12𝑂2 + 2𝐻+ + 2𝑒− → 𝐶7𝐻14𝑂2  (2-Methoxycyclohexanone to 2-Methoxycyclohexanol) (6) 𝐶7𝐻14𝑂2 + 2𝐻+ + 2𝑒− → 𝐶6𝐻12𝑂 +  𝐶𝐻4𝑂  (2-Methoxycyclohexanol to Cyclohexanol and Methanol) (7)  208  A.4. Formulating rate law and rate-determining step This approach is based on Langmuir–Hinshelwood kinetic mechanism.110 All the elementary steps are assumed as first-order, reversible, dual-site reaction with competitive adsorption and uniform surface activity. The organic reactants are adsorbed on the catalyst surface through molecular or non-dissociative adsorption. The organic compounds in Figure A.2 are denoted as follows: Guaiacol (A), Phenol (B), Cyclohexanone (C), Cyclohexanone (D), 2-Methoxycyclohexanone (E), 2-Methoxycyclohexanol (F), and Methanol (G).  All the elementary steps involved in the ECH of guaiacol can then be written as follows (with S denoting the catalyst surface active sites): 𝐴 + 𝑆 ↔ 𝐴 ∙ 𝑆  (Guaiacol adsorption) (8) 𝐻+ + 𝑒− + 𝑆 ↔ 𝐻 ∙ 𝑆  (Proton chemisorption) (9) The surface reaction is derived from the following elementary steps: Route 1: Demethoxylation – Ring Saturation 𝐴 ∙ 𝑆 + 2𝐻 ∙ 𝑆 ↔ 𝐵 ∙ 𝑆 + 𝐺 ∙ 𝑆 + 𝑆  (Guaiacol hydrogenolysis) (10) 𝐵 ∙ 𝑆 + 4𝐻 ∙ 𝑆 ↔ 𝐶 ∙ 𝑆 + 4𝑆 (Phenol hydrogenation) (11) 𝐶 ∙ 𝑆 + 2𝐻 ∙ 𝑆 ↔ 𝐷 ∙ 𝑆 + 2𝑆 (Cyclohexanone hydrogenation) (12) Route 2: Ring Saturation – Demethoxylation 𝐴 ∙ 𝑆 + 4𝐻 ∙ 𝑆 ↔ 𝐸 ∙ 𝑆 + 4𝑆  (Guaiacol hydrogenation) (13) 𝐸 ∙ 𝑆 + 2𝐻 ∙ 𝑆 ↔ 𝐹 ∙ 𝑆 + 2𝑆 (2-Methoxycyclohexanone hydrogenation) (14)  209  𝐹 ∙ 𝑆 + 2𝐻 ∙ 𝑆 ↔ 𝐷 ∙ 𝑆 + 𝐺 ∙ 𝑆 + 𝑆 (2-Methoxycyclohexanol hydrogenation) (15) Note that the sum of all elementary steps in Route 1 or 2 generates the overall reaction as follows:  𝐴 ∙ 𝑆 + 8𝐻 ∙ 𝑆 ↔ 𝐷 ∙ 𝑆 + 𝐺 ∙ 𝑆 + 7𝑆 (Overall surface reaction) (16) The ECH of guaiacol taking place in the catalyst consists of consecutive reactions in the parallel pathways rather than a single-step reaction. In such complex chemical reaction, the relative reaction rate of the intermediate products can become dependent on diffusive conditions.105 In this study, the model is simplified by focusing on guaiacol as the reference point for the surface reaction. The active sites for organic reactant adsorption and proton reduction are not distinguished. Product desorption steps can be written as follows: 𝐵 ∙ 𝑆 ↔ 𝐵 + 𝑆 (Phenol desorption) (17) 𝐶 ∙ 𝑆 ↔ 𝐶 + 𝑆 (Cyclohexanone desorption) (18) 𝐷 ∙ 𝑆 ↔ 𝐷 + 𝑆 (Cyclohexanol desorption) (19) 𝐸 ∙ 𝑆 ↔ 𝐸 + 𝑆 (2-Methoxycyclohexanone desorption) (20) 𝐹 ∙ 𝑆 ↔ 𝐹 + 𝑆 (2-Methoxycyclohexanol desorption) (21) 𝐺 ∙ 𝑆 ↔ 𝐺 + 𝑆 (Methanol desorption) (22) Possible scenarios for the RDS are derived step-by-step to propose the rate law that best fits the experimental data: Case 1: Adsorption of guaiacol is the RDS  The rate expression is derived from Equation 8: 𝑟𝑎 = 𝑘𝑎𝐶𝐴𝐶𝑣 − 𝑘−𝑎𝐶𝐴∙𝑆 = 𝑘𝑎 (𝐶𝐴𝐶𝑣 −𝐶𝐴∙𝑆𝐾𝑎)   (23)  210  Note: 𝐾𝑎 = 𝑘𝑎 𝑘−𝑎⁄  is the adsorption equilibrium constant, 𝐶𝑣 is the concentration of vacant sites. Analogously, the rate expressions for the other elementary steps (Equations 9, 16, 19, 22) can be derived as follows: 𝑟𝐻 = 𝑘𝐻𝐶𝐻𝐶𝑣 − 𝑘−𝐻𝐶𝐻∙𝑆 = 𝑘𝐻 (𝐶𝐻𝐶𝑣 −𝐶𝐻∙𝑆𝐾𝐻)   (24) 𝑟𝑠 = 𝑘𝑠𝐶𝐴∙𝑆𝐶𝐻∙𝑆8 − 𝑘−𝑠𝐶𝐷∙𝑆𝐶𝐺∙𝑆𝐶𝑣7 = 𝑘𝑠 (𝐶𝐴∙𝑆𝐶𝐻∙𝑆8 −𝐶𝐷∙𝑆𝐶𝐺∙𝑆𝐶𝑣7𝐾𝑠) (25) Note that the overall surface reaction is used to formulate the rate expression: the power of 8 (𝐶𝐻∙𝑆) refers to the number of protons/electrons involved while the power of 7 (𝐶𝑣) refers to the number of compounds involved in the overall guaiacol ECH reaction. For simplification purposes, the rate expressions for desorption steps exclusively take into account the overall products (cyclohexanol and methanol): 𝑟𝑑,𝐷 = 𝑘𝑑,𝐷𝐶𝐷∙𝑆 − 𝑘−𝑑,𝐷𝐶𝐷∙𝑆𝐶𝑣 = 𝑘𝑑,𝐷(𝐶𝐷∙𝑆 − 𝐾𝑑,𝐷𝐶𝐷𝐶𝑣) (26) 𝑟𝑑,𝐺 = 𝑘𝑑,𝐺𝐶𝐺∙𝑆 − 𝑘−𝑑,𝐺𝐶𝐺∙𝑆𝐶𝑣 = 𝑘𝑑,𝐺(𝐶𝐺∙𝑆 − 𝐾𝑑,𝐺𝐶𝐺𝐶𝑣) (27) Since the guaiacol adsorption is the RDS, proton chemisorption, surface reaction, and product desorption proceed fast, giving large 𝑘𝐻 , 𝑘𝑠 , and 𝑘𝑑  by comparison, thus 𝑟𝐻/𝑘𝐻 , 𝑟𝑠/𝑘𝑠 , and 𝑟𝑑/𝑘𝑑 in Equations 24–27 will be approximate to zero.  The equilibrium concentrations of adsorbed reactants and products can then be obtained as follows:  𝐶𝐻∙𝑆 = 𝐾𝐻𝐶𝐻𝐶𝑣  (28) 𝐶𝐴∙𝑆 =𝐶𝐷∙𝑆𝐶𝐺∙𝑆𝐶𝑣7𝐾𝑠𝐶𝐻∙𝑆8   (29) 𝐶𝐷∙𝑆 = 𝐾𝑑,𝐷𝐶𝐷𝐶𝑣  (30)  211  𝐶𝐺∙𝑆 = 𝐾𝑑,𝐺𝐶𝐺𝐶𝑣  (31) Equations 28–31 can be combined and rearranged to obtain 𝐶𝐴∙𝑆: 𝐶𝐴∙𝑆 =𝐾𝑑,𝐷𝐶𝐷𝐾𝑑,𝐺𝐶𝐺𝐶𝑣𝐾𝑠(𝐾𝐻𝐶𝐻)8  (32) Site balance:  𝐶𝑡 = 𝐶𝑣 + 𝐶𝐴∙𝑆 + 𝐶𝐻∙𝑆 + 𝐶𝐷∙𝑆 + 𝐶𝐺∙𝑆 (33) Equations 28, 30–32 can be substituted into Equation 33 to obtain 𝐶𝑣: 𝐶𝑣 = 𝐶𝑡 (1 +𝐾𝑑,𝐷𝐶𝐷𝐾𝑑,𝐺𝐶𝐺𝐾𝑠(𝐾𝐻𝐶𝐻)8+ 𝐾𝐻𝐶𝐻 + 𝐾𝑑,𝐷𝐶𝐷 + 𝐾𝑑,𝐺𝐶𝐺)⁄  (34) Finally, Equations 32 and 34 are substituted into Equation 23 to obtain the rate expression for guaiacol adsorption:  𝑟𝑎 =𝐶𝑡𝑘𝑎(𝐶𝐴 − 𝐾𝑑,𝐷𝐶𝐷𝐾𝑑,𝐺𝐶𝐺𝐾𝑎𝐾𝑠(𝐾𝐻𝐶𝐻)8 )(1 + 𝐾𝑑,𝐷𝐶𝐷𝐾𝑑,𝐺𝐶𝐺𝐾𝑠(𝐾𝐻𝐶𝐻)8  + 𝐾𝐻𝐶𝐻 + 𝐾𝑑,𝐷𝐶𝐷 + 𝐾𝑑,𝐺𝐶𝐺)  (35) The initial rate of reaction (𝑟𝑎0) as a function of guaiacol concentration (𝐶𝐴0) is now given by: 𝑟𝑎0 =𝐶𝑡𝑘𝑎𝐶𝐴01 + 𝐾𝐻𝐶𝐻 = 𝑘𝐶𝐴0    (36) (Initially, no products are present, thus 𝐶𝐷0 = 𝐶𝐺0 = 0) Assuming the proton concentration is constantly abundant (as continuously supplied from the water splitting reactions), 𝑟𝑎,0  becomes linearly dependent on 𝐶𝐴0  and can be written as:  𝑟𝑎0 = 𝑘𝐶𝐴0 (where 𝑘 =𝐶𝑡𝑘𝑎1+𝐾𝐻𝐶𝐻 ). Case 2: Surface reaction is the RDS The rate expression is derived from Equation 25:  212  𝑟𝑠 = 𝑘𝑠 (𝐶𝐴∙𝑆𝐶𝐻∙𝑆8 −𝐶𝐷∙𝑆𝐶𝐺∙𝑆𝐶𝑣7𝐾𝑠)  (25) In the similar manner as in Case 1 (to derive Equations 28–31), 𝐶𝐴∙𝑆, 𝐶𝐻∙𝑆, 𝐶𝐷∙𝑆, and 𝐶𝐺∙𝑆 can be obtained to solve for Equation 25, in combination with 𝐶𝑣 expression from the site balance: 𝐶𝐴∙𝑆 = 𝐾𝑎𝐶𝐴𝐶𝑣  (37) 𝐶𝑣 = 𝐶𝑡 (1 + 𝐾𝑎𝐶𝐴 + 𝐾𝐻𝐶𝐻 + 𝐾𝑑,𝐷𝐶𝐷 + 𝐾𝑑,𝐺𝐶𝐺)⁄  (38) Thus, the rate expression for surface reaction can be written as follows: 𝑟𝑠 =𝐶𝑡9𝑘𝑠(𝐾𝑎𝐶𝐴 (𝐾𝐻𝐶𝐻)8− 𝐾𝑑,𝐷𝐶𝐷𝐾𝑑,𝐺𝐶𝐺𝐾𝑠)(1 + 𝐾𝑎𝐶𝐴 + 𝐾𝐻𝐶𝐻 + 𝐾𝑑,𝐷𝐶𝐷 + 𝐾𝑑,𝐺𝐶𝐺)  (39) The initial rate of reaction (𝑟𝑠0) is obtained when 𝐶𝐷0 = 𝐶𝐺0 = 0: 𝑟𝑠0 =𝐶𝑡9𝑘𝑠(𝐾𝑎𝐶𝐴0 (𝐾𝐻𝐶𝐻)8)1 + 𝐾𝑎𝐶𝐴0 + 𝐾𝐻𝐶𝐻 =𝑘(𝐶𝐴0)1 + 𝐾𝑎𝐶𝐴0 + 𝐾𝐻𝐶𝐻  {where 𝑘 = 𝐶𝑡9𝑘𝑠(𝐾𝐻𝐶𝐻)8𝐾𝑎} (40) This expression implies two possible consequences if the RDS is the surface reaction: • At low concentrations of A: (1 + 𝐾𝐻𝐶𝐻 ≫ 𝐾𝑎𝐶𝐴0 ), hence: 𝑟𝑠0 =𝑘(𝐶𝐴0)1 + 𝐾𝐻𝐶𝐻 = 𝑘′𝐶𝐴0  → The initial reaction rate is linearly dependent on guaiacol concentration. • At high concentrations of A: (𝐾𝑎𝐶𝐴0 ≫ 1+ 𝐾𝐻𝐶𝐻 ), hence: 𝑟𝑠0 =𝑘(𝐶𝐴0)𝐾𝑎𝐶𝐴0= 𝑘′′ → The initial reaction rate is independent of guaiacol concentration. Case 3: Desorption of the product is the RDS  The rate expression derivation starts with Equation 26 or 27 with two possible scenarios: a. If cyclohexanol desorption is limiting:  𝑟𝑑,𝐷 = 𝑘𝑑,𝐷(𝐶𝐷∙𝑆 − 𝐾𝑑,𝐷𝐶𝐷𝐶𝑣) (26) b. If methanol desorption is limiting:  𝑟𝑑,𝐺 = 𝑘𝑑,𝐺(𝐶𝐺∙𝑆 − 𝐾𝑑,𝐺𝐶𝐺𝐶𝑣) (27)  213  In order to solve for 𝐶𝐷∙𝑆 and 𝐶𝐺∙𝑆, 𝐶𝐴∙𝑆 is first determined when 𝑟𝑠/𝑘𝑠 ≈ 0 (see Equation 32) and then rearranged to obtain:   𝐶𝐷∙𝑆 =𝐾𝑎𝐶𝐴𝐾𝑠(𝐾𝐻𝐶𝐻)8𝐶𝑣𝐾𝑑,𝐺𝐶𝐺  (41) 𝐶𝐺∙𝑆 =𝐾𝑎𝐶𝐴𝐾𝑠(𝐾𝐻𝐶𝐻)8𝐶𝑣𝐾𝑑,𝐷𝐶𝐷  (42) From the site balance, 𝐶𝑣 can be obtained for each case above: For Case (a): 𝐶𝑣 = 𝐶𝑡 (1 + 𝐾𝑎𝐶𝐴 + 𝐾𝐻𝐶𝐻 +𝐾𝑎𝐶𝐴𝐾𝑠(𝐾𝐻𝐶𝐻)8𝐾𝑑,𝐺𝐶𝐺+ 𝐾𝑑,𝐺𝐶𝐺)⁄  (43) For Case (b): 𝐶𝑣 = 𝐶𝑡 (1 + 𝐾𝑎𝐶𝐴 + 𝐾𝐻𝐶𝐻 + 𝐾𝑑,𝐷𝐶𝐷 +𝐾𝑎𝐶𝐴𝐾𝑠(𝐾𝐻𝐶𝐻)8𝐾𝑑,𝐷𝐶𝐷)⁄  (44) Finally, Equations 41 and 43 are substituted into Equation 26 (in the same manner, Equations 42 and 44 into Equation 27) to obtain the rate expressions for desorption control as follows:   𝑟𝑑,𝐷 =𝐶𝑡𝑘𝑑,𝐷(𝐾𝑎𝐶𝐴𝐾𝑠(𝐾𝐻𝐶𝐻)8𝐾𝑑,𝐺𝐶𝐺 − 𝐾𝑑,𝐷𝐶𝐷)1+𝐾𝑎𝐶𝐴+𝐾𝐻𝐶𝐻+𝐾𝑎𝐶𝐴𝐾𝑠(𝐾𝐻𝐶𝐻)8𝐾𝑑,𝐺𝐶𝐺+𝐾𝑑,𝐺𝐶𝐺=𝐶𝑡𝑘𝑑,𝐷(𝐾𝑎𝐶𝐴𝐾𝑠(𝐾𝐻𝐶𝐻)8𝐾𝑑,𝐺 − 𝐾𝑑,𝐷𝐶𝐷𝐶𝐺)𝐶𝐺+𝐾𝑎𝐶𝐴𝐶𝐺+𝐾𝐻𝐶𝐻𝐶𝐺+𝐾𝑎𝐶𝐴𝐾𝑠(𝐾𝐻𝐶𝐻)8𝐾𝑑,𝐺+𝐾𝑑,𝐺𝐶𝐺2 (45) 𝑟𝑑,𝐺 =𝐶𝑡𝑘𝑑,𝐺(𝐾𝑎𝐶𝐴𝐾𝑠(𝐾𝐻𝐶𝐻)8𝐾𝑑,𝐷𝐶𝐷 − 𝐾𝑑,𝐺𝐶𝐺)1+𝐾𝑎𝐶𝐴+𝐾𝐻𝐶𝐻+𝐾𝑑,𝐷𝐶𝐷+𝐾𝑎𝐶𝐴𝐾𝑠(𝐾𝐻𝐶𝐻)8𝐾𝑑,𝐷𝐶𝐷=𝐶𝑡𝑘𝑑,𝐺(𝐾𝑎𝐶𝐴𝐾𝑠(𝐾𝐻𝐶𝐻)8𝐾𝑑,𝐷 − 𝐾𝑑,𝐺𝐶𝐺𝐶𝐷)𝐶𝐷+𝐾𝑎𝐶𝐴𝐶𝐷+𝐾𝐻𝐶𝐻𝐶𝐷+𝐾𝑑,𝐷𝐶𝐷2+𝐾𝑎𝐶𝐴𝐾𝑠(𝐾𝐻𝐶𝐻)8𝐾𝑑,𝐷 (46) The initial rate of reaction (𝑟𝑑0) is obtained when 𝐶𝐷0 = 𝐶𝐺0 = 0, thereby simplifying the above equations into: 𝑟𝑑,𝐷0 =𝐶𝑡𝑘𝑑,𝐷(𝐾𝑎𝐶𝐴0𝐾𝑠(𝐾𝐻𝐶𝐻)8𝐾𝑑,𝐺 )𝐾𝑎𝐶𝐴0𝐾𝑠(𝐾𝐻𝐶𝐻)8𝐾𝑑,𝐺= 𝐶𝑡𝑘𝑑,𝐷   (47)  214  𝑟𝑑,𝐺0 =𝐶𝑡𝑘𝑑,𝐺(𝐾𝑎𝐶𝐴0𝐾𝑠(𝐾𝐻𝐶𝐻)8𝐾𝑑,𝐷 )𝐾𝑎𝐶𝐴0𝐾𝑠(𝐾𝐻𝐶𝐻)8𝐾𝑑,𝐷= 𝐶𝑡𝑘𝑑,𝐺   (48) The bracketed terms on numerator and the denominator terms are cancelled out. Thus, if desorption were the RDS, the initial reaction rate would be constant for different concentration values (i.e. independent of the initial guaiacol concentration, and the relationship between 𝑟𝑑0 and 𝐶𝐴0 would be plotted as a flat line with zero gradient).  In summary, the following trends will be observed if the RDS is: • Adsorption: the initial reaction rate is linearly dependent on guaiacol concentration. • Surface reaction: the initial reaction rate is linearly dependent on guaiacol concentration (at low concentrations) but independent of guaiacol concentration (at high concentrations). • Desorption: the initial reaction rate would be independent of guaiacol concentration.  Assessment of the model parameters to estimate surface sites: Suppose the rate expression for guaiacol ECH based on the rate-determining step (i.e. surface reaction) is given as follows: 𝑟𝑠 =𝑘(𝐾𝑎𝐶𝐴 (𝐾𝐻𝐶𝐻)8− 𝐾𝑑,𝐷𝐶𝐷𝐾𝑑,𝐺𝐶𝐺𝐾𝑠)(1 + 𝐾𝑎𝐶𝐴 + 𝐾𝐻𝐶𝐻 + 𝐾𝑑,𝐷𝐶𝐷 + 𝐾𝑑,𝐺𝐶𝐺)  (39) To verify if the rate expression is valid under the reaction conditions, experimental data and model need to be fitted using non-linear regression (in this work, a built-in Solver in Microsoft Excel is used). The model parameters, including 𝑘, 𝐾𝑎, 𝐾𝐻, 𝐾𝑑,𝐷, 𝐾𝑑,𝐺, 𝐾𝑠 can be estimated by minimizing the sum of (𝑟𝑒𝑥𝑝 − 𝑟𝑐𝑎𝑙𝑐)2 where 𝑟𝑒𝑥𝑝 is actual rate of guaiacol conversion (from experimental data) and 𝑟𝑐𝑎𝑙𝑐 is model rate of guaiacol conversion (from calculated data).   215  To solve the model parameters, initial guess of 0.001 was used for all and the following results were obtained after 5 iterations:  𝑘 = 32.80 ,  𝐾𝑎 = 9.86 × 10−4 ,  𝐾𝐻 = 1.29 × 10− , 𝐾𝑑,𝐷 =2.31 × 10−5, 𝐾𝑑,𝐺 = 4.70 × 10−6, 𝐾𝑠 = 7.87 × 10−  with the sum of square (SS) = 1.35×10-6. Note that the experimental and calculated rate data show a good fit (Figure A.3a).  Figure A.3. (a) Correlation between experimental and calculated rate data for ECH of guaiacol using the kinetic model based on Langmuir-Hinshelwood mechanism where surface reaction is the rate-determining step. (b) Profiles of the surface coverage ratio between adsorbed guaiacol and chemisorbed hydrogen compared to guaiacol concentration over time. Experiment conditions: [MSA] = 0.2 M, I = -0.5 A (j = -182 mA cm-2), Catalyst: 5wt.%-Pt/C (0.15 g). Once the model parameters were solved, the surface sites could be estimated. Here, the ratio between concentration of adsorbed guaiacol (CA.S) and chemisorbed hydrogen (CH.S) was determined using these correlations:  𝐶𝐴∙𝑆 = 𝐾𝑎𝐶𝐴𝐶𝑣  and 𝐶𝐻∙𝑆 = 𝐾𝐻𝐶𝐻𝐶𝑣  cancelling out the concentration of vacant sites. Plotting 𝐶𝐴∙𝑆𝐶𝐻∙𝑆 over time clearly shows that the surface coverage of guaiacol decreased over time, consistent with the decreasing trend of guaiacol concentration (Figure A.3b), which indicates that the assumption of excess proton supply can be justified. The limitations of this approach include inability to distinguish: (i) the amount of Hads coverage for ECH and HER, (ii) the active sites for organic reactant adsorption and proton reduction, (iii) the amount of surface sites for each different pathway in the guaiacol ECH. 0.00000.00200.00400.00600.00800.01000.01200 20 40 60 80 100 120Rate (mM s-1)Concentration (mM)Exp CalcR2 = 0.97(a)0204060801001200.0000.0200.0400.0600.0800.1000.1200.1400.1600.1800.2000 0.5 1 1.5 2 2.5 3 3.5 4Concentration (mM)CA.S/ CH.STime (h)[A.S]/[H.S][A](b) 216  A.5. MATLAB codes for the ECH of guaiacol kinetics % kinfit.m % This program evaluates the parameter estimation method to determine rate % constants of each reaction in the ECH of guaiacol pathways. % This program uses Levenberg-Marquardt method for the nonlinear regression of % the model and actual data from the ECH experiments.  clear all clc clf   % Initialization and input specification Data = readtable('ch-data60.xlsx'); % Experimental concentration data  % Declare the variables to all the functions global CA0 CB0 CC0 CD0 CE0 CF0 Cex C tspan sse SD  CA0 = 106.03; CB0 = 0; CC0 = 0; CD0 = 0; CE0 = 0; CF0 = 0; Cex = table2array(Data(:,2:7)); % Convert table to homogeneous array tspan = table2array(Data(:,1)); k0 = [0.01 0.01 0.01 0.01 0.01 0.01]; % Initial values in a row vector lb = zeros (1,6); % Lower bounds ub = 1000*ones(1,6); % Upper bounds  % Optimization options structure using 'lsqnonlin' to solve nonlinear % least-squares (nonlinear data-fitting) problems and the default algorithm % is a so-called 'trust-region reflective' that requires the number of % equations (i.e. the row dimension of F) to be at least as great as the  % number of variables. The alternative algorithm is 'levenberg-marquardt' % which uses unbound constraints. options = optimoptions(@lsqnonlin,'Display','Iter'); [k,resnorm,res, exitflag,output,lambda,J]=lsqnonlin(@optim,k0,lb,ub,options)  % nonlinear regression parameter confidence intervals to compute 95% % confidence intervals Ci = nlparci(k,res,'Jacobian',J)     Cex; % Measured dependent variables  C; % Calculated dependent variables  t=tspan'; % Independent variables  % Plot of guaiacol concentration (real vs. model) figure(1) plot(t,C(:,1),'b-'); xlabel('Time (h)'); ylabel('Concentration (mM)'); xticks([0:0.5:4]); hold on; plot(t,Cex(:,1),'ob'); legend({'A model','A real'}); hold off   217  % Plot of product concentration (real vs. model) figure(2) plot(t,C(:,2),'r--',t,C(:,3),'k-.',t,C(:,4),'k--',t,C(:,5),'m-.',t,C(:,6),'k:'); xlabel('Time (h)'); xticks([0:0.5:4]); ylabel('Concentration (mM)'); hold on; plot(t,Cex(:,2),'dr',t,Cex(:,3),'ok',t,Cex(:,4),'sk',t,Cex(:,5),'*m',t,Cex(:,6),'^k'); legend({'B model','C model','D model','E model','F model','B real','C real','D real','E real','F real'}); hold off;  % Output declaration sse SD k   % Integrate the ODEs using Runge-Kutta 4 method function [sse, SD] =optim(k); global CA0 CB0 CC0 CD0 CE0 CF0 Cex C tspan sse SD  [t,C] = ode45(@balance,tspan,[CA0 CB0 CC0 CD0 CE0 CF0]);  % Function to be integrated function dCdt=balance(t,C) dCdt=zeros(6,1); % Initialization  % Material balance expression CT=C(1)+C(2)+C(3)+C(4)+C(5)+C(6);    CA = CA0*(C(1)/CT); CB = CA0*(C(2)/CT); CC = CA0*(C(3)/CT); CD = CA0*(C(4)/CT); CE = CA0*(C(5)/CT); CF = CA0*(C(6)/CT);  % ODEs model: multiple equations, multiple parameters dCdt(1) = -k(1)*CA-k(4)*CA; dCdt(2) = k(1)*CA-k(2)*CB; dCdt(3) = k(2)*CB-k(3)*CC; dCdt(4) = k(3)*CC+k(6)*CF^2; dCdt(5) = k(4)*CA-k(5)*CE^2; dCdt(6) = k(5)*CE^2-k(6)*CF^2;  end  nt=length(tspan); Cmod = C; sse = (Cmod-Cex).^2; % sum of squares of the residual SD = sqrt(sum(sse)/nt); % standard deviation end  218  Standard deviation between model and actual data: 𝑆𝐷 = √1𝑁∑|𝐶𝑖,𝑎 − 𝐶𝑖,𝑚|2𝑁𝑖=1  40 oC 50 oC 60 oC A 2.76 1.65 1.79 B 1.12 1.32 1.52 C 1.59 1.55 1.84 D 1.95 1.36 1.46 E 1.67 1.61 1.72 F 2.11 1.52 1.28 Note: SD = standard deviation, N = number of data, 𝐶𝑖,𝑎 = actual concentration data, 𝐶𝑖,𝑚 = model concentration data. Compound label: A = guaiacol, B = phenol, C = cyclohexanone, D = cyclohexanol, E = 2-methoxycyclohexanone, F = 2-methoxycyclohexanol.      219  A.6. Statistical analysis results from the ECH of guaiacol factorial experiments ANOVA for selected factorial model, fit statistics, and equation in terms of actual factors are provided herein for all response variables (i.e. guaiacol conversion, Faradaic efficiency, cyclohexanol selectivity, and cyclohexanone selectivity), from the sample data of 1st Design.   Response 1: Guaiacol Conversion Source Sum of Squares df Mean Square F-value p-value Model 4593.93 6 765.65 37.89 < 0.0001 A-Guaiacol Concentration 2639.39 1 2639.39 130.62 < 0.0001 B-Anolyte Proton Concentration 2.18 1 2.18 0.1077 0.7503 C-Temperature 61.23 1 61.23 3.03 0.1157 D-Current 1216.27 1 1216.27 60.19 < 0.0001 AD 527.85 1 527.85 26.12 0.0006 BD 147.02 1 147.02 7.28 0.0245 Residual 181.86 9 20.21   Cor Total 4775.78 15    Std. Dev. 4.50 R² 0.9619 Mean 83.98 Adjusted R² 0.9365 C.V. % 5.35 Predicted R² 0.8797   Adeq Precision 17.8593  Actual Equation:  X = 123.20 – 1088.125*CG + 18.02*CH,a + 0.196*T – 19.83*I + 1148.75*CG*I – 37.89*CH,a*I  Note: The actual equation can be used to make predictions about the response for given levels of each factor (which should be specified in the original units of each factor). However, this equation, in contrast to the coded equation, should not be used to determine the relative impact of each factor.   220  Response 2: Faradaic Efficiency  Source Sum of Squares df Mean Square F-value p-value Model 5023.21 5 1004.64 45.12 < 0.0001 A-Guaiacol Concentration 482.90 1 482.90 21.69 0.0009 B-Anolyte Proton Concentration 0.1056 1 0.1056 0.0047 0.9464 D-Current 4099.20 1 4099.20 184.10 < 0.0001 AD 291.56 1 291.56 13.09 0.0047 BD 149.45 1 149.45 6.71 0.0269 Residual 222.67 10 22.27   Cor Total 5245.88 15    Std. Dev. 4.72 R² 0.9576 Mean 59.94 Adjusted R² 0.9363 C.V. % 7.87 Predicted R² 0.8913   Adeq Precision 16.9961  Actual Equation:  F.E. = 104.155 – 207.125*CGUA + 18.90*CH,a –121.14*I + 853.75*CGUA*I – 38.20*CH,a*I     221  Response 3: Cyclohexanol Selectivity Source Sum of Squares df Mean Square F-value p-value Model 3266.17 5 653.23 37.14 < 0.0001 A-Guaiacol Concentration 2745.76 1 2745.76 156.12 < 0.0001 C-Temperature 0.1600 1 0.1600 0.0091 0.9259 D-Current 125.44 1 125.44 7.13 0.0235 AC 197.40 1 197.40 11.22 0.0074 AD 197.40 1 197.40 11.22 0.0074 Residual 175.87 10 17.59   Cor Total 3442.04 15    Std. Dev. 4.19 R² 0.9489 Mean 46.66 Adjusted R² 0.9234 C.V. % 8.99 Predicted R² 0.8692   Adeq Precision 15.6730  Actual Equation:  S1 = 53.119 – 172.75*CGUA + 1.044*T – 38.69*I – 14.05*CGUA*T + 702.50*CGUA*I     222  Response 4: Cyclohexanone Selectivity   Source Sum of Squares df Mean Square F-value p-value Model 1895.31 4 473.83 17.89 < 0.0001 A-Guaiacol Concentration 1442.10 1 1442.10 54.45 < 0.0001 C-Temperature 135.14 1 135.14 5.10 0.0452 D-Current 145.81 1 145.81 5.51 0.0387 AC 172.27 1 172.27 6.50 0.0270 Residual 291.31 11 26.48   Cor Total 2186.62 15    Std. Dev. 5.15 R² 0.8668 Mean 10.62 Adjusted R² 0.8183 C.V. % 48.46 Predicted R² 0.7181   Adeq Precision 10.9801  Actual Equation:  S2 = 10.169 – 87.125*CGUA – 0.694*T + 13.3125*I + 13.125*CGUA*T    223  A.7. GC-MS chromatogram results from the ECH of phenolics    Figure A.4. GC-MS chromatograms from the ECH of guaiacol in the pair of NaCl (0.5 M) and H2SO4 (0.2 M) electrolytes over 5 wt.% Pt/C catalyst for: (a) catholyte sample before reaction, (b) catholyte sample after 4 h reaction, (c) anolyte sample after 4 h reaction. Note that guaiacol and other hydrogenated products were not found in the anolyte, suggesting the good membrane stability. Acetone was only used as the injection washing solvent. Inset: compound information from the library search report in the GC-MS software corresponding to each of the spectrum. GC analysis was performed using HP-INNOWax column.   (a)(c)CH3OH(b)Sample RT Area Pct Library/ID Ref CAS Qual(a) 1.862 0.246 2-Propanone 689 000067-64-1 909.364 92.327 1-Butanol 2596 000071-36-3 9128.112 4.776 Phenol, 2-methoxy- 28590 000090-05-1 97(b) 1.863 0.216 2-Propanone 689 000067-64-1 919.672 93.345 1-Butanol 2596 000071-36-3 9113.153 0.097 Cyclohexanone 9202 000108-94-1 9616.747 0.904 Cyclohexanol 10982 000108-93-0 9518.359 0.829 trans-2-Methoxycyclohexanol 36876 999036-93-1 8018.876 0.332 2-Methoxycyclohexanone 33708 007429-44-9 9419.954 0.361 6-Methoxy-(E,Z)-5-hexen-1-ol 36876 999036-87-6 7628.099 1.155 Phenol, 2-methoxy- 28590 000090-05-1 9730.142 0.011 Phenol 7618 000108-95-2 81(c) 1.863 0.239 Acetone 686 000067-64-1 909.569 96.248 1-Butanol 2607 000071-36-3 9128.106 0.035 Phenol, 2-methoxy- 28597 000090-05-1 94 224   Figure A.5. GC-MS chromatograms from the ECH of cerulignol in the isopropanol-mixed H2SO4 (0.2 M) catholyte over 5 wt.% Pt/C catalyst: (a) before reaction, (b) after 2 h reaction. Major product peaks were identified, such as propylcyclohexane (RT = 15.38 min), 3-propyl-cyclohexene (27.53 min), 4-propylcyclohexanone (28.08 min), 4-propylphenol (30.40 min) from the cerulignol reactant (32.38 min). GC analysis was performed using HP-5ms column.(a)(b) 225   Figure A.6. GC-MS chromatograms from the ECH of mixed cerulignol, creosol, and guaiacol in the isopropanol-mixed MSA (0.2 M) catholyte over 5 wt.% Pt/C catalyst: (a) before reaction, (b) after 2 h reaction. Major products from the corresponding reactant were detected with high quality (>90) and the results indicate that the phenolic reactant was converted independently. GC analysis was performed using HP-INNOWax column.CH3OHOthers(a)(b) 226  A.8. Physical properties data for lignin-relevant model compounds and organic solvents used in this study Table A.1. Physical properties of lignin-relevant model compounds and organic solvents used in the ECH experiments. Compound Structure Formula H/C O/C MW (g/mol)  (kg/m3) B.P. (oC) P*  (mmHg) Solubility in water Guaiacol      C7H8O2 1.14 0.29 124.14 1110 205 0.25 (25 oC), 1.39 (50 oC) 23.3 g/L  (at 25 oC) Phenol  C6H6O  1.00 0.17 94.11 1070 181.7 0.30 (25 oC), 2.11 (50 oC) Soluble Cyclohexanone   C6H10O    1.67 0.17 98.15 948 155.6 3.76 (25 oC), 15.1 (50 oC) 8.6 g/100 mL  (at 20 oC) Cyclohexanol  C6H12O  2.00 0.17 100.16 962 161.8 0.75 (25 oC), 5.15 (50 oC) 3.6 g/100 mL  (at 20 oC) 2-Methoxy-cyclohexanone  C7H12O2  1.71 0.29 128.17 1020 185 0.6±0.4  (25 oC)Δ Slightly soluble 2-Methoxy-cyclohexanol  C7H14O2  2.00 0.29 130.18 1000 198 0.1±0.8  (25 oC)Δ Slightly soluble Cyclohexane  C6H12 2.00 0.00 84.16 779 80.74 92.1 (25 oC), 264 (50 oC) Immiscible  227  Compound Structure Formula H/C O/C MW (g/mol)  (kg/m3) B.P. (oC) P*  (mmHg) Solubility in water Creosol  C8H10O2 1.25 0.25 138.16 1097 221 0.1±0.4  (25 oC)Δ Slightly soluble 4-Methylphenol (p-Cresol)  C7H8O 1.14 0.14 108.13 1035 202 0.15 (25 oC), 1.07 (50 oC) 2.4 g/100 mL (at 40 oC) 4-Methyl-cyclohexanone  C7H12O 1.71 0.14 112.17 914 170 1.5±0.3  (25 oC)Δ Insoluble 4-Methyl-cyclohexanol  C7H14O 2.00 0.14 114.19 917 173 0.54 (25 oC), 3.32 (50 oC) Poor Methyl-cyclohexane  C7H14 2.00 0.00 98.19 770 101 46.6 (25 oC), 138 (50 oC) Insoluble Cerulignol (4-Propyl-guaiacol)  C10H14O2 1.40 0.20 166.22 1038 125 0.1±0.6  (25 oC)Δ Insoluble 4-Propyl-phenol  C9H12O 1.33 0.11 136.19 983 120 0.02 (25 oC), 0.19 (50 oC) Insoluble 4-Propyl-cyclohexanone  C9H16O 1.78 0.11 140.22 907 203 0.11 (25 oC), 0.87 (50 oC) 1.96 g/L (at 20 oC) 3-Propyl-cyclohexene  C9H16 1.78 0.00 124.22 800 155.2 3.3±0.1  (25 oC)Δ Insoluble  228  Compound Structure Formula H/C O/C MW (g/mol)  (kg/m3) B.P. (oC) P*  (mmHg) Solubility in water Propyl-cyclohexane  C9H18 2.00 0.00 126.24 793 155.8 5.2 (25 oC), 19.4 (50 oC) Insoluble Methanol  CH4O 4.00 1.00 32.04 792 64.7 126 (25 oC), 414 (50 oC) Miscible Methane- sulfonic acid  CH4O3S 4.00 1.00 96.10 1480 167 1 (at 20 oC) Miscible Acetone   C3H6O 2.00 0.33 58.08 785 56.1 230 (25 oC), 614 (50 oC) Miscible Isopropanol  C3H8O 2.67 0.33 60.10 786 82.6 25 (25 oC), 101 (50 oC) Miscible Acetonitrile  C2H3N 1.50 0.00 41.05 776 82.1 88.5 (25 oC), 252 (50 oC) Miscible Ethanol  C2H6O 3.00 0.50 46.07 789 78.4 65.6 (25 oC), 232 (50 oC) Miscible Acetic acid  C2H4O2 2.00 1.00 60.05 1050 117.9 8.38 (25 oC), 40 (50 oC) Miscible Note: H/C = hydrogen-to-carbon ratio, O/C = oxygen-to-carbon ratio, MW = molar mass,  = density, B.P. = boiling point, P* = vapor pressure, determined by the Antoine equation from The Yaws Handbook of Vapor Pressure (2015) in ref. 136 and Δobtained from www.chemspider.com. Solubility data can also be obtained from https://iupac.org/ in various published volumes of the solubility data series. 229  Table A.2. Vapor pressure data for the compounds used in this study predicted by Antoine equation.136  Antoine Equation: log P = A - B / (T + C); P [=] mmHg, T [=] °C Compound A B C T min T max Guaiacol 7.89776 2203.8 234.222 85.27 233.09 Phenol 7.36683 1629.404 181.369 40.91 207.98 Cyclohexanone 7.64127 1894.117 243.036 42.17 181.71 Cyclohexanol 7.19276 1415.266 168.37 60.17 184.48 Cyclohexane 6.88938 1200.826 218.815 -14.92 105.07 p-Cresol 7.57676 1886.66 199.914 86.95 229.37 4-Methylcyclohexanol 7.87231 1898.258 208.248 67.97 196.46 Methylcyclohexane 7.00206 1375.133 232.819 -3.71 127.14 4-Propylphenol 7.64762 2016.363 191.151 112.17 260.36 4-Propylcyclohexanone 7.83019 1948.109 196.85 88.37 222.25 Propylcyclohexane 6.95859 1537.008 221.073 36.88 185.89 Methanol 8.08404 1580.459 239.096 -15.99 199.45 Acetone 7.31742 1315.674 240.479 -32.22 234.95 Isopropanol 7.77658 1518.796 213.076 11.05 263.63 Acetonitrile 7.54423 1583.4 257.887 -43.83 272.35 Ethanol 8.12875 1660.871 238.131 -5.15 240.75 Acetic acid 7.27594 1327.163 183.913 16.66 318.8 Water 8.05573 1723.643 233.08 0.01 373.98  

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