Applied Science, Faculty of
Chemical and Biological Engineering, Department of
DSpace
UBCV
Setiawan, Felix Arie
Doctor of Philosophy - PhD
University of British Columbia
Bio-oil upgrading by electrocatalysis requires significant technical innovations so as to compete with thermocatalytic processes that have significantly higher catalytic activity. In the present study, the primary advantage of in-situ hydrogen supply from water electrolysis is combined with the development of non-noble metal catalysts for the bio-oil electrocatalytic hydrodeoxygenation (ECH-DO) reaction. Furthermore, the well-established technology of paired electrosynthesis is applied to bio-oil ECH-DO with anodic coupling electrosynthesis (ACE), utilizing the glycerol electrooxidation reaction, creating product intensification and increasing process energy efficiency. Firstly, the study assessed candidate anodic reactions using multi-objective optimization (MOO) based on six key elements: safety, environment, energy, economics, separation, and technology readiness level. Secondly, the selected ACE and bio-oil ECH-DO were optimized individually in different electrolyte pairs using noble metal catalysts in a divided stirred slurry electrocatalytic reactor (SSER). Thirdly, the paired electrosynthesis of guaiacol (as bio-oil model compound) electrocatalytic hydrodeoxygenation (GECH-DO) and the ACE was assessed to find the best operating conditions in terms of agitation speed, initial concentration, current density, temperature, and metal loading. Lastly, tailored binary (PtNi/C and PtCo/C) and ternary (PtNiCo/C) electrocatalysts were introduced and compared with the existing reference electrocatalyst (Pt/C). Glycerol electrooxidation (GEOR) to glyceraldehyde was the best candidate for the anodic electrosynthesis based on the MOO analysis. Results showed that 25% isopropanol (IPA) in the catholyte triggered the dehydration reaction of guaiacol towards cyclohexane. Furthermore, the Pt/C catalyst had the highest catalytic activity for GECH-DO and GEOR for glyceraldehyde production, compared to Ru/C and Pd/C. The paired electrosynthesis was favored in acid-acid electrolyte pairs. Compared with the half-cell, the paired cell electrosynthesis decreased the required energy and increased faradaic efficiency. The best operating condition of the paired cell (400 RPM, 100 mM guaiacol and 300 mM glycerol initial concentration, 66 mA cm⁻² (0.4 A), and 50 °C) with the Pt/C catalyst resulted in 93.7% guaiacol conversion. The tailored PtNi/C decreased the dependency on Pt by as much as 40% while maintaining performance of the paired synthesis.
PAIRED ELECTROSYNTHESIS IN A STIRRED SLURRY ELECTROCHEMICAL REACTOR FOR BIO-OIL UPGRADING AND GLYCEROL ELECTROOXIDATION by Felix Arie Setiawan M.Eng., University of Gadjah Mada, 2015 B.Eng., University of Gadjah Mada, 2010 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) April 2025 © Felix Arie Setiawan, 2025 ii The following individuals certify that they have read and recommend to the Faculty of Graduate and Postdoctoral Studies for acceptance, the dissertation entitled: Paired electrosynthesis in a Stirred Slurry Electrochemical Reactor for Bio-Oil Upgrading and Glycerol Electrooxidation submitted by Felix Arie Setiawan 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 Dan Bizzotto, Professor, Chemistry Department, UBC Supervisory Committee Member Heather Trajano, Associate Professor, Chemical and Biological Engineering, UBC University Examiner Edouard Asselin, Professor, Materials Engineering, UBC University Examiner Elizabeth J. Biddinger, Associate Professor, Chemical Engineering, City College of New York External Examiner iii Abstract Bio-oil upgrading by electrocatalysis requires significant technical innovations so as to compete with thermocatalytic processes that have significantly higher catalytic activity. In the present study, the primary advantage of in-situ hydrogen supply from water electrolysis is combined with the development of non-noble metal catalysts for the bio-oil electrocatalytic hydrodeoxygenation (ECH-DO) reaction. Furthermore, the well-established technology of paired electrosynthesis is applied to bio-oil ECH-DO with anodic coupling electrosynthesis (ACE), utilizing the glycerol electrooxidation reaction, creating product intensification and increasing process energy efficiency. Firstly, the study assessed candidate anodic reactions using multi-objective optimization (MOO) based on six key elements: safety, environment, energy, economics, separation, and technology readiness level. Secondly, the selected ACE and bio-oil ECH-DO were optimized individually in different electrolyte pairs using noble metal catalysts in a divided stirred slurry electrocatalytic reactor (SSER). Thirdly, the paired electrosynthesis of guaiacol (as bio-oil model compound) electrocatalytic hydrodeoxygenation (GECH-DO) and the ACE was assessed to find the best operating conditions in terms of agitation speed, initial concentration, current density, temperature, and metal loading. Lastly, tailored binary (PtNi/C and PtCo/C) and ternary (PtNiCo/C) electrocatalysts were introduced and compared with the existing reference electrocatalyst (Pt/C). Glycerol electrooxidation (GEOR) to glyceraldehyde was the best candidate for the anodic electrosynthesis based on the MOO analysis. Results showed that 25% isopropanol (IPA) in the catholyte triggered the dehydration reaction of guaiacol towards cyclohexane. Furthermore, the Pt/C catalyst had the highest catalytic activity for GECH-DO and GEOR for glyceraldehyde iv production, compared to Ru/C and Pd/C. The paired electrosynthesis was favored in acid-acid electrolyte pairs. Compared with the half-cell, the paired cell electrosynthesis decreased the required energy and increased faradaic efficiency. The best operating condition of the paired cell (400 RPM, 100 mM guaiacol and 300 mM glycerol initial concentration, 66 mA cm-2 (0.4 A), and 50 °C) with the Pt/C catalyst resulted in 93.7% guaiacol conversion. The tailored PtNi/C decreased the dependency on Pt by as much as 40% while maintaining performance of the paired synthesis. v Lay Summary Bio-oil upgrading using an electrosynthesis process, rather than the established thermo-catalytic processes, has the potential to decrease society’s dependency on fossil fuels for energy, fuels and chemicals production. Although electrosynthesis processes have lower catalytic activity than their thermocatalytic counterpart, other advantages warrant their further development. This study implemented paired electrosynthesis, combining bio-oil electrocatalytic hydrodeoxygenation (ECH-DO) and glycerol electrooxidation (GEOR) to increase the product intensification and decrease energy demand, while valorizing the waste or by-product feedstock into valuable chemicals. The study also reduced the process dependency of noble metal catalysts (Pt/C) by introducing binary metal (PtNi/C) electrocatalysts with comparable activity. vi Preface The dissertation report consists of eight chapters. The research design such as the literature review, reactor setup, and data analysis and interpretation, were performed by Felix Arie Setiawan under Professor Kevin J. Smith, Professor Elӧd L. Gyenge, and Professor Chang Soo Kim supervisions. All laboratory work was conducted at the Chemical and Biological Engineering Department and KIST-UBC Biorefinery on-site Laboratory at the University of British Columbia. A publication included in this dissertation is given below: 1. “Effect of Isopropanol on the Electrocatalytic Hydrodeoxygenation of Guaiacol to Cyclohexane in a Stirred Slurry Reactor: Factorial Experimental Design.” ACS Sustainable Chem. Eng. 2024, 12, 51, 18466–18474. https://doi.org/10.1021/acssuschemeng.4c08404. A version of the manuscript is included in Chapter 3, 5, and 7. Beini Shen helped a part of the experimental works. Several manuscripts were prepared in accordance with the Ph.D. work entitled: 1. “Intensification process of bio-oil electrocatalytic hydrogenation-hydrodeoxygenation with potential anodic coupling electrosynthesis (ACE): a semi-quantitative review.” A version of the manuscript is included in Chapters 1, 2, 3 and 4. 2. “Paired electrosynthesis using Bio-Oils: Guaiacol Hydrodeoxygenation coupled with Glycerol Oxidation in a Stirred Slurry Reactor.” A version of the manuscript is included in Chapter 1, 2, 3, 6, and 7. 3. “Highly Selective Cyclohexane Production by PtNi/C on Guaiacol Electrocatalytic Hydrodeoxygenation Coupled with Glycerol Electrooxidation.” A version of the manuscript is included in Chapter 1, 2, 3, 6, and 7. Bradford Ross at Bioimaging Facility (BIF), UBC, performed TEM measurement. Earth, Ocean and Atmospheric Sciences vii Department of UBC conducted the XRD analysis. High-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) were conducted in the 4D LABS core facility at Simon Fraser University (SFU) and supported by the Canada Foundation for Innovation (CFI), British Columbia Knowledge Development Fund (BCKDF) and Pacific Economic Development Canada (PacifiCan). All of the manuscripts were prepared and written by the author under insight and intellectual contributions from Prof. E. L. Gyenge, Prof. K. J. Smith, and Prof. C. S. Kim. The experimental work and data analysis were performed by the author. A fruitful discussion was conducted with Prof. I.S.Y. Louise regarding the manuscript frame. Meanwhile, regarding the artificial intelligence used during the thesis drafting, AI was mainly used on the paraphrasing the sentences. The author also presented the work as conference abstracts in the proceedings of international conferences: The 245th Electrochemical Society Meeting in San Fransisco, United States (2024). Felix Arie Setiawan, Beini Shen, Kevin J Smith, Chang Soo Kim, Elod Lajos Gyenge. “Isopropyl Alcohol Effects on the Electro-Catalytic Hydrogenation of Guaiacol.” viii Table of Contents Abstract ......................................................................................................................................... iii Lay Summary .................................................................................................................................v Preface ........................................................................................................................................... vi Table of Contents ....................................................................................................................... viii List of Tables .............................................................................................................................. xiii List of Figures ............................................................................................................................ xvii List of Symbols ........................................................................................................................ xxvii List of Abbreviations ............................................................................................................... xxxi Acknowledgements ................................................................................................................ xxxvi Dedication ............................................................................................................................. xxxviii Chapter 1: Introduction ................................................................................................................1 1.1 Biomass and bio-oil potential ......................................................................................... 2 1.2 Electrochemical versus thermochemical processes ........................................................ 8 1.3 Electrochemical parameters and improvements ........................................................... 12 1.4 Research scope, objective, and outline ......................................................................... 15 Chapter 2: Literature Review of Electrocatalytic Upgrading of Bio-oil ................................19 2.1 Paired electrocatalysis ................................................................................................... 19 2.2 The thermodynamic consideration ................................................................................ 23 2.3 Reactor design ............................................................................................................... 25 2.4 Electron transfer and electrocatalyst mechanism in organic electrosynthesis .............. 29 2.5 Electrosynthesis component.......................................................................................... 31 ix 2.5.1 Electrolyte ............................................................................................................. 31 2.5.2 Electrocatalyst ....................................................................................................... 33 2.6 Recent progress in ECH of bio-oil derivatives and the knowledge gaps ...................... 36 Chapter 3: Experimental Methodology .....................................................................................44 3.1 Material and reagents .................................................................................................... 44 3.2 Experimental reactor cell set-up ................................................................................... 44 3.3 Cyclic voltammetry experiment .................................................................................... 46 3.4 Product analysis and catalyst characterization .............................................................. 47 3.5 Catalyst characterization ............................................................................................... 47 3.6 Figure of merit calculations .......................................................................................... 49 3.7 Least significant difference analysis ............................................................................. 50 3.8 Design of experiments and factorial analysis in response surface methodology.......... 51 3.9 The van Krevelen analysis ............................................................................................ 52 Chapter 4: Potential Anodic Coupling Electrosynthesis Selection..........................................53 4.1 Potential ACEs .............................................................................................................. 55 4.2 Reaction ........................................................................................................................ 58 4.3 ACEs selection for MOO analysis ................................................................................ 61 4.4 Multi-objective Optimization calculation of ACEs ...................................................... 63 4.4.1 Economic aspect ................................................................................................... 65 4.4.2 Environmental aspect ............................................................................................ 68 4.4.3 Energy aspect ........................................................................................................ 71 4.4.4 Safety aspect ......................................................................................................... 74 4.4.5 Technology Readiness Level ................................................................................ 77 x 4.4.6 Separation technology impacts ............................................................................. 80 4.5 The total MOO analysis and sensitivity analysis .......................................................... 83 4.6 Summary of the chapter ................................................................................................ 88 Chapter 5: Half-cell Optimization of ECH-DO and GEOR ....................................................91 5.1 Isopropanol effect on the ECH-DO of guaiacol in half-cell electrosynthesis .............. 92 5.1.1 RSM analysis ........................................................................................................ 95 5.1.2 Energy-related issues ............................................................................................ 96 5.1.3 Product distribution analysis ................................................................................. 98 5.1.4 Solvent recovery and compatibility .................................................................... 100 5.2 Electrolyte pairs selection ........................................................................................... 100 5.3 Catalysts screening...................................................................................................... 104 5.4 Summary of the chapter .............................................................................................. 109 Chapter 6: Paired Electrosynthesis Optimization Process ....................................................112 6.1 Electrolyte pairs selection ........................................................................................... 114 6.2 Paired cell optimization .............................................................................................. 118 6.3 Optimized paired cell experiment ............................................................................... 124 6.4 Tailoring electrocatalyst in electrosynthesis of guaiacol ECH-DO and glycerol electrooxidation....................................................................................................................... 126 6.4.1 Catalyst synthesis ................................................................................................ 127 6.4.2 XRD analysis ...................................................................................................... 129 6.4.3 CO-uptake and TEM analysis ............................................................................. 134 6.5 Tailored metal electrocatalytic activity ....................................................................... 138 6.5.1 Half-cell electrosynthesis .................................................................................... 138 xi 6.5.2 Paired cell electrosynthesis ................................................................................. 142 6.6 Summary of the chapter .............................................................................................. 146 Chapter 7: Kinetics of guaiacol electrocatalytic hydrodeoxygenation (GECH-DO) and glycerol electrooxidation (GEOR) ............................................................................................149 7.1 Catalytic reaction kinetics and pathways .................................................................... 149 7.1.1 Guaiacol electrocatalytic hydrodeoxygenation (GECH-DO) ............................. 151 7.1.2 Glycerol electrocatalytic oxygenation reaction (GEOR) .................................... 156 7.2 Reaction pathways: reaction order and rate constant determination for GECH-DO and GEOR ..................................................................................................................................... 160 7.3 Isopropanol and MSA concentration variation ........................................................... 166 7.4 Temperature variation on 5 wt% Pt/C catalyst in 1.0 M acidic electrolyte pairs ....... 168 7.5 Temperature variation on PtNi/C catalyst in 1.0 M acidic electrolyte pairs ............... 171 7.6 Designing an optimum reactor from experimental kinetic data .................................. 173 7.7 Summary of the chapter .............................................................................................. 177 Chapter 8: Conclusion ...............................................................................................................178 8.1 Conclusive summary and key findings ....................................................................... 178 8.2 Future recommendation .............................................................................................. 181 References ...................................................................................................................................187 Appendices ..................................................................................................................................232 Appendix A : Fundamental study and half-cell experiment ................................................... 232 A.1 Mass-transfer limitation assessment ....................................................................... 236 A.2 GC-MS chromatogram results from the ECH-DO of guaiacol .............................. 237 A.3 HPLC standard solution .......................................................................................... 239 xii A.4 Cyclic voltammetry analysis of half-cell optimization ........................................... 240 A.5 RSM and data analysis of half-cell optimization process ....................................... 241 A.6 Energy analysis of half-cell optimization process .................................................. 244 A.7 MATLAB codes for the ECH-DO and GEOR ....................................................... 245 Appendix B : Paired cell electrosynthesis .............................................................................. 253 B.1 Catalyst characterization ......................................................................................... 253 B.2 Cyclic voltammetry on half-cell to paired cell comparison and optimization ........ 253 B.3 Tafel slope analysis of binary and ternary metal catalyst on carbon support ......... 255 B.4 Paired cell optimization .......................................................................................... 259 B.5 Kinetics analysis ..................................................................................................... 262 xiii List of Tables Table 1.1. Lignin building blocks (black color) and platform (blue color) chemicals.27 ............... 6 Table 1.2. Pyrolysis method type and operating conditions.29 ....................................................... 7 Table 1.3. The hydrodeoxygenation of guaiacol into cyclohexane. ............................................... 9 Table 1.4. The advantages and limitations of flow and batch electrocatalytic reactors. .............. 13 Table 2.1. The thermodynamic data derived from ChemDraw 18.1 software and the chemical price from several suppliers of the first reported commercial paired electrosynthesis................. 22 Table 2.2. Paired electrocatalytic process in laboratory scale experiments .................................. 23 Table 2.3. The pros and cons of coated and suspended catalyst in the electrosynthesis process. 25 Table 2.4. Hydrogenation of bio-oil model compounds. .............................................................. 38 Table 3.1. The materials and reagents. ......................................................................................... 44 Table 3.2. Experimental matrix design for ECH of guaiacol with isopropyl alcohol variations. . 51 Table 3.3. The H/C and O/C ratios of involved chemicals related to guaiacol ECH-DO. ........... 52 Table 4.1. The inorganic industrial electrosynthesis in anode chamber with standard hydrogen potential.50 ..................................................................................................................................... 57 Table 4.2. The organic industrial electrosynthesis in anode chamber with standard hydrogen potential......................................................................................................................................... 58 Table 4.3. The organic electrosynthesis in pilot plant scale. ........................................................ 59 Table 4.4. The multi-objective optimization weighting scale. ...................................................... 65 Table 4.5. The MOO score evaluation on the economic aspect. .................................................. 67 Table 4.6. The MOO score evaluation on the environmental aspect. ........................................... 70 Table 4.7. The MOO score evaluation on energy. ........................................................................ 74 Table 4.8. The MOO score evaluation on safety. ......................................................................... 76 xiv Table 4.9. The yields of ACEs from several studies. .................................................................... 78 Table 4.10. TRL in the National Aeronautics and Space Administration (NASA), Department of Defense (DOD), and chemical industry classification with MOO score of ACEs selection. ....... 79 Table 4.11. The guide of separation technology selection based on the stream phase.370 ............ 81 Table 4.12. The MOO score for separation technology................................................................ 83 Table 4.13. The ACE MOO score with the criteria and weight factor. ........................................ 84 Table 4.14. The glycerol oxidation products of the study and their applications. ........................ 85 Table 5.1. The RSM output parameters of MSA and IPA variations. .......................................... 95 Table 5.2. Graphical representation of the effects of the independent variables on guaiacol conversion, product yields and energy requirement. 0.1 M guaiacol ECH-DO in SSER with 5 g/L 5%wt-Pt/C, 60 oC and -66 mA cm-2.............................................................................................. 96 Table 5.3. The effect of electrolyte pairs in half-cell electrosynthesis of GEOR. ...................... 101 Table 5.4. The effect of electrolyte pairs in half-cell electrosynthesis of GECH-DO. ............... 104 Table 5.5. Galvanostatic glycerol electrooxidation results in the SSER anodic half-cell using PGMs. ......................................................................................................................................... 108 Table 6.1. The effect of electrolyte pairs in paired cell of GECH-DO or GEOR. ...................... 115 Table 6.2. The effect of temperature in paired cell of GECH and GEOR using Pt/C catalyst ... 123 Table 6.3. The catalyst synthesis preparation using the variation of metal weight. ................... 128 Table 6.4. The XRD analysis of Pt/C catalyst. ........................................................................... 132 Table 6.5. The XRD analysis of PtNi/C catalyst. ....................................................................... 132 Table 6.6. The XRD analysis of PtCo3/C catalyst. ..................................................................... 133 Table 6.7. The analysis of PtNi0.5Co0.5/C catalyst. ..................................................................... 133 Table 6.8. The CO-uptake and TEM analysis of the tailored catalyst. ....................................... 134 xv Table 6.9. The expected catalyst chemical composition on weight basis of the tailored catalyst...................................................................................................................................................... 137 Table 7.1. Guaiacol ECH-DO rate constant analysis results in SSER at 0.5 g Pt/C, and -66 mA cm-2. ............................................................................................................................................ 167 Table 7.2. Rate constant of GECH-DO and GEOR on the paired cell electrosynthesis application in SSER using Pt/C (5wt%) catalyst. .......................................................................................... 169 Table 7.3. The kinetics constant of GECH-DO and GEOR in SSER using PtNi/C-Pt/C catalyst pair. ............................................................................................................................................. 173 Table A.1. Upgrading methods advantages and disadvantages .................................................. 232 Table A.2. Glycerol oxidation catalyst based selection. ............................................................. 233 Table A.3. Thermodynamics data of cathode chamber on the GECH-DO and HER. ................ 234 Table A.4. Thermodynamics data of anode chamber on the GEOR and OER. .......................... 235 Table A.5. The internal mass transfer resistance evaluation using Weiz-Prater criteria in paired electrosynthesis of GECH and GEOR ........................................................................................ 236 Table A.6. The external mass transfer resistance evaluation using Sherwood number in paired electrosynthesis of GECH and GEOR ........................................................................................ 237 Table B.1. Characterization of noble metal catalyst with various metal loading content on carbon support.105.................................................................................................................................... 253 Table B.2. The Tafel slope calculation from the linear voltammetry experiments. ................... 256 Table B.3. Agitation speed and platinum loading effects in paired cell of GECH-DO and GEOR...................................................................................................................................................... 259 Table B.4. Curret density and initial reactant concentration effects in GECH-DO and GEOR paired electrosynthesis with 5wt.% Pt/C cathode and anode catalyst. ....................................... 260 xvi Table B.5. Half-cell reaction of GECH-DO and GEOR using binary metal catalysts. .............. 261 Table B.6. Half-cell reaction of GECH-DO and GEOR using ternary metal catalysts. ............. 262 Table B.7. Reaction order determination using graphical analysis ............................................ 263 Table B.8. Standard deviation value of experiment and kinetics model concentration for isopropanol variation experiment. .............................................................................................. 263 Table B.9. Langmuir-Hinshelwood and Eley-Rideal mechanism comparison for guaiacol ECH-DO and glycerol EOR ................................................................................................................. 264 Table B.10. Standard deviation value of experiment and kinetics model concentration for paired electrosynthesis on Pt/C catalyst. ................................................................................................ 266 Table B.11. Arrhenius parameter of GECH reaction in 1.0 M MSA electrolyte pairs at different temperatures (30–60 °C). Reaction condition: j = 66 mA cm-2, N = 600 rpm, t = 4 h. Catalyst: 5 wt%-Pt/C (5 g/L), all electrolyte also containing 0.25 M NaCl with 25% v/v IPA addition in the cathode chamber. ........................................................................................................................ 266 Table B.12. Arrhenius parameter of GEOR reaction in 1.0 M MSA electrolyte pairs at different temperatures (30–60 °C). Reaction condition: j = 66 mA cm-2, N = 600 rpm, t = 4 h. Catalyst: 5 wt%-Pt/C (5 g/L), all electrolyte also containing 0.25 M NaCl with 25% v/v IPA addition in the cathode chamber. ........................................................................................................................ 267 Table B.13. Standard deviation value of experiment and kinetics model concentration of paired cell electrosynthesis using PtNi/C and Pt/C in cathode and anode chamber, respectively......... 267 xvii List of Figures Figure 1.1. The historical and future carbon emission concentration and carbon-climate feedbacks in the Earth system, along with the key compartment, processes, and pathways. Reproduced from ref. 1 with permission from the Cambridge University Press, Copyright 2023. ............................ 2 Figure 1.2. The primary building blocks (monolignols) of lignin with the complex cross-linked phenolic polymer. ........................................................................................................................... 3 Figure 1.3. The lignin utilization concept to support sustainability and a circular economy. ........ 5 Figure 1.4. The electricity price from the generation of renewable energy source and fossil gas.48....................................................................................................................................................... 10 Figure 2.1. Schematic illustration of paired electrosynthesis types (a) parallel (PPE), (b) divergent (DPE), (c) linear (LPE), (d) convergent (CPE), and (e) tandem (TPE). ....................................... 20 Figure 2.2. The first commercial paired electrosynthesis process. ............................................... 21 Figure 2.3. The relationship among criteria for thermodynamic spontaneity............................... 24 Figure 2.4. The PMR application is on (a) thermocatalytic and (c) electrocatalytic processes with (b) gas phase hydrogen permeation and (d) H+ permeation from proton/water reduction. Reprint from ref. 93 with permission from the American Chemical Society, Copyright 2024. ................. 28 Figure 2.5. The reaction pathway involves single electron transfer in the electrosynthesis process. Reproduced from ref. 49 with permission from the American Chemical Society, Copyright 2021........................................................................................................................................................ 29 Figure 2.6. The possible charge transfer process of organic hydrogenation in a stirred slurry electrocatalytic reactor. ................................................................................................................. 30 Figure 2.7. Molar electrolytic conductivity of aqueous methanesulfonic acid133 and other electrolytes134 at 25 °C. ................................................................................................................. 32 xviii Figure 2.8. Volcano plot for different metals of (a) HER145 and (b) OER146. .............................. 34 Figure 2.9. ECH reaction pathway of lignin bio-oil monomer to produce cyclohexanol and cyclohexane................................................................................................................................... 36 Figure 2.10. The possible reaction pathways of ECH-HDO of phenol in dual catalyst system. Reproduced from ref. 42 with permission from the Royal Society of Chemistry, Copyright 2020........................................................................................................................................................ 37 Figure 2.11. The upgrading of lignin bio-oil through electrocatalytic in a divided cell proton exchange membrane (PEM) summarized from the literature.42, 43, 105-107, 111, 113, 122, 124, 144, 171-191 42 Figure 3.1. The jacketed reactor set-up of (a) cathode half-cell, (b) anode half-cell, and (c) paired cell stirred slurry electrocatalytic reactor (SSER) setup for guaiacol ECH-DO and glycerol EOR with similar size of the chambers. (1) Reference electrode (Ag/AgCl/3M KCl), (2) fritted side compartment (Luggin probe) for reference electrode filled with 3M KCl, (3) suspended catalyst, (4) cation exchange membrane (Nafion-117), (5) magnetic stirrer. ............................................. 45 Figure 4.1. The paired electrosynthesis selection to be coupled with lignin bio-oil ECH-HDO. 54 Figure 4.2. The end use of chlorine gas and sodium hydroxide. .................................................. 55 Figure 4.3. The potential ACE reactions with the number of electrons involved of the study. Note: renewable feedstock (green dash), inorganic chemicals (blue dash), fossil fuel feedstock (red dash), chemical reaction (solid arrow), fermentation (dash arrow), and Electrooxidation reaction (orange arrow). ............................................................................................................................. 60 Figure 4.4. Standard potential (open symbol) and reported onset (closed symbol) potential of the ACEs illustrated from Table 4.7. .................................................................................................. 72 Figure 4.5. Relative energy use from several energy-intensive separation technologies.371 ........ 82 Figure 4.6. The sensitivity analysis plot of the glycerol electrooxidation into glyceraldehyde. .. 86 xix Figure 4.7. The tornado plot of the (a) weighting aspect shift and (b) the ACEs score toward the weighting aspect shifting toward the economic aspect. ................................................................ 87 Figure 5.1. The ECH-DO results of 0.1 M guaiacol with IPA addition using SSER. (a) 0.2 M; (b) 0.5 M; and (c) 1.0 M MSA concentration. Reaction condition: I = - 0.4 A (j = -66 mA cm-2), T = 60 °C, t = 4 h. Catalyst: 5 wt%-Pt/C (0.5 g/L). ............................................................................. 94 Figure 5.2. The required energy per mole of converted guaiacol with different IPA and MSA concentrations. Reaction condition: I = - 0.4 A (j = -66 mA cm-2), T = 60 °C, t = 4 h. Catalyst: 5 g/L Pt (5 wt%)/C. .......................................................................................................................... 97 Figure 5.3. Energy efficiency in terms of (a) Voltage efficiency (V.E.) and (b) Faradaic efficiency (F.E.) with different IPA and MSA concentrations on ECH of guaiacol. Reaction condition: I = - 0.4 A (j = -66 mA cm-2), T = 60 °C, t = 4 h. Catalyst: 5 wt%-Pt/C (0.5 g/L). ............................. 98 Figure 5.4. The van Krevelen plot of experimental results with 0.2 M (black), 0.5 M (blue), and 1.0 M MSA (red). Reaction condition: I = - 0.4 A (j = -66 mA cm-2), T = 60 °C, t = 4 h. Catalyst: 5 wt%-Pt/C (0.5 g/L)..................................................................................................................... 99 Figure 5.5. The XRD spectra of commercial (a) Pt/C, (b) Pd/C and (c) Ru/C catalysts with 5 wt% of the metal. ................................................................................................................................ 106 Figure 5.6. The cyclic voltammetry of half-cell glycerol Electrooxidation in stirred slurry electrocatalytic reactor at (a) 25 °C and (b) 60 °C with several potential catalysts.................... 107 Figure 5.7. Glycerol oxidation mechanism on Pt(100) and Pt(111) in acidic condition. Reproduced from ref. 408 with permission from the American Chemical Society, Copyright 2016. .............. 109 Figure 6.1. Raw material to product of the proposed paired electrosynthesis system in this study...................................................................................................................................................... 113 xx Figure 6.2. The required energy per mole of converted guaiacol or glycerol with different electrolyte pairs. Reaction condition: j = 66 mA cm-2, N = 600 rpm, T = 60 °C, t = 4 h. Catalyst: 5 wt%-Pt/C (5 g/L). Electrolyte: 0.25 M NaCl with 25% v/v IPA addition in the cathode chamber. Calculated based on Equation 3.10. ............................................................................................ 116 Figure 6.3. Acid-acid (1.0 M) electrolyte pairs of GECH-DO (a and c) and GEOR (b and d) in half-cell (left side) and paired cell (right side) system. Reaction condition: j = 66 mA cm-2, N = 600 rpm, T = 60 °C, t = 4 h. Catalyst: 5 wt%-Pt/C (5 g/L. Anolyte and catholyte consist of 0.25 M NaCl with 25% v/v IPA addition in the cathode chamber. ......................................................... 117 Figure 6.4. The required energy per mole of reactant converted with the varied reaction condition (a) agitation speed, (b) catalyst metal loading, (c) current density, (d) initial reactant concentration, and (e) reaction temperature. ...................................................................................................... 120 Figure 6.5. The optimized condition compared to the typical initial reaction condition in paired electrosynthesis. Initial condition: N = 600 rpm, T = 60 °C, and C0_glycerol = 0.1 M. Optimum condition: N = 400 rpm, T = 50 °C, C0_glycerol = 0.3 M. Reaction condition: t = 4 h, catalyst: 5 wt%-Pt/C (5 g/L), anolyte and catholyte consist of 1.0 M MSA and 0.25 M NaCl with 25% v/v IPA addition in the cathode chamber. ......................................................................................... 125 Figure 6.6. Activated carbon pretreatment. ................................................................................ 127 Figure 6.7. Binary and ternary catalyst synthesis on activated carbon support. The number in the first step described the mixing order of the chemicals. .............................................................. 127 Figure 6.8. The acid pretreatment effect on the XRD profile of (a) the activated carbon support and (b) the PtNi/C catalyst. ......................................................................................................... 130 Figure 6.9. The XRD pattern of the tailored catalyst with platinum JCPDS card as reference. . 131 xxi Figure 6.10. The TEM images with the catalyst size distribution of (a) Pt/C, (b) PtNi/C, and (c) PtCo3/C, and (d) PtNi0.5Co0.5/C................................................................................................... 135 Figure 6.11. The images of PtNi/C catalyst from (a) TEM with (b) selected area electron diffraction (SAED), and (c) selected HAADF area followed by (d) – (i) the elemental mapping by EDX analysis. ....................................................................................................................................... 136 Figure 6.12. The voltammogram profile of tailored (a) binary and (b) ternary metals catalyst on carbon support catalyst in paired electrosynthesis of GECH-DO and GEOR. ........................... 139 Figure 6.13. The GECH-DO performance in half-cell electrosynthesis derived from Table B.5 and B.6 (Appendices) to compare tailored catalysts in regard of (a) guaiacol conversion and (b) cyclohexane selectivity. .............................................................................................................. 140 Figure 6.14. The van Krevelen diagram of tailored catalyst. M belongs to Ni or Co. ............... 141 Figure 6.15. The GEOR performance in half-cell electrosynthesis derived from Table B.5 and B.6 (Appendices) to compare tailored catalysts in regard of (a) glycerol conversion and (b) glyceraldehyde selectivity. .......................................................................................................... 142 Figure 6.16. The cathode cell reaction activity result from different catalyst pairs in paired electrosynthesis system. Reaction condition: j = 66 mA cm-2, N = 400 rpm, t = 4 h. Electrolyte: 1.0 M MSA, 0.25 M NaCl, and 25% v/v IPA addition in the cathode chamber with C0 guaiacol = 0.1 M, C0 glycerol = 0.3 M. ................................................................................................................... 143 Figure 6.17. The effect of current density in the paired electrosynthesis of (a) PtNi/C-Pt/C and (b) Pt/C-Pt/C catalyst pairs. Reaction condition: T= 50 °C, N = 400 rpm, t = 4 h. Electrolyte: 1.0 M MSA, 0.25 M NaCl, and 25% v/v IPA addition in the cathode chamber with C0 guaiacol = 0.1 M, C0 glycerol = 0.3 M. ............................................................................................................................. 144 xxii Figure 6.18. The effect of isopropanol in the cathode chamber on the paired electrosynthesis of PtNi/C-Pt/C catalyst pairs. Reaction condition: T= 50 °C, j = -66 mA cm-2, N = 400 rpm, t = 4 h. Electrolyte: 1.0 M MSA, and 0.25 M NaCl with C0 guaiacol = 0.1 M, C0 glycerol = 0.3 M. ............. 145 Figure 6.19. The effect of glycerol initial concentration in the paired electrosynthesis of PtNi/C-Pt/C catalyst pairs for (a) cathode and (b) anode chambers. Reaction condition: T= 50 °C, j = -66 mA cm-2, N = 400 rpm, t = 4 h. Electrolyte: 1.0 M MSA, 0.25 M NaCl, and 25% v/v IPA addition in the cathode chamber with C0 guaiacol = 0.1 M. .......................................................................... 146 Figure 7.1. The ECH of guaiacol under galvanostatic conditions at (a) lower and (b) higher guaiacol concentration with at 1.0 M MSA and 25% v/v IPA. Reaction condition: I = - 0.4 A (j = -66 mA cm-2), T = 60 °C, t = 4 h. Catalyst: 5 wt%-Pt/C (5 g/L). Initial reaction rates were measured by Equation 3.4. .......................................................................................................................... 151 Figure 7.2. Rate of GECH-DO (a) data and model plot based on Langmuir-Hinshelwood mechanism to obtain kinetic rate constant (see Equation 7.16) where surface reaction is the rate-determining step and the effect of (b) proton and (c) IPA addition in the cathode chamber according to Equation 7.16. ......................................................................................................................... 155 Figure 7.3. The GEOR under galvanostatic conditions at (a) lower and (b) higher glycerol concentration in acidic condition (1.0 M MSA). Reaction condition: I = 0.4 A (j = 66 mA cm-2), T = 60 °C, t = 4 h. Catalyst: 5 wt%-Pt/C (5 g/L). Initial reaction rates were measured by Equation 3.4................................................................................................................................................ 158 Figure 7.4. Reaction rate of GEOR data and model plot based on Langmuir-Hinshelwood mechanism to obtain the rate constant where glycerol adsorption is the rate-determining step.159 xxiii Figure 7.5. Proposed reaction pathway of GECH-DO with IPA addition in SSER using Pt/C (5 wt%) catalyst with isopropanol oxidation reaction toward acetone as a proton donor in the inserted figure provided with the kinetic rate equations used to estimate the reaction rate constant. ...... 161 Figure 7.6. The ECH of (a) phenol and (b) anisole kinetic model vs. experiment using SSER with 1.0 M MSA concentration and 25 v/v % IPA content. Reaction condition: I = - 0.4 A (j = -66 mA cm-2), T = 60 oC, t = 4 h, N = 600 rpm. Catalyst: 5 wt%-Pt/C (0.5 g/L). ................................... 162 Figure 7.7. Parity plot of isopropanol effect 0 – 90 vol.% on the Guaiacol ECH-DO kinetics constants determination in Table 7.1. I = - 0.4 A (j = -66 mA cm-2), N = 600 rpm, T = 60 oC, t = 4 h. Catalyst: 5 wt%-Pt/C (0.5 g/L). .............................................................................................. 163 Figure 7.8. The electrooxidation of (a) glyceraldehyde and (b) dihydroxyacetone kinetic model vs. experiment using SSER with 1.0 M MSA concentration. Reaction condition: I = 0.4 A (j = 66 mA cm-2), T = 60 oC, t = 2 h. Catalyst: 5 wt%-Pt/C (0.5 g/L). ................................................... 164 Figure 7.9. Proposed reaction pathway of GEOR in SSER using Pt/C (5 wt%) catalyst with the kinetics rate equation to estimate the reaction rate constant. ...................................................... 165 Figure 7.10. Parity plot of the temperature effect on (a) guaiacol ECH-DO and (b) glycerol EOR kinetics constants determination in Table 7.2. I = - 0.4 A (j = -66 mA cm-2), N = 600 rpm, t = 4 h. Catalyst: 5 wt%-Pt/C (0.5 g/L). .................................................................................................. 166 Figure 7.11. Arrhenius plots for the determination of activation energy (Ea) and pre-exponential factor (A) in total kinetics rate of (a) GECH-DO (k11+k21+k31) and (b) GEOR (k1+k2) using 1.0 M MSA electrolyte pairs at different temperatures (30–60 °C). Reaction condition: j = 66 mA cm-2, N = 600 rpm, t = 4 h. Catalyst: 5 wt%-Pt/C (5 g/L). Electrolyte: 0.25 M NaCl with 25% v/v IPA addition in the cathode chamber. ................................................................................................ 170 xxiv Figure 7.12. The effect of temperature and kinetics rate evaluation in the paired electrosynthesis of PtNi/C-Pt/C catalyst pairs. Reaction condition: j = 66 mA cm-2, N = 400 rpm, t = 4 h. Electrolyte: 1.0 M MSA, 0.25 M NaCl, and 25% v/v IPA addition in the cathode chamber with CA0 guaiacol = 0.1 M, CA0 glycerol = 0.3 M. ............................................................................. 172 Figure 7.13. Modelled (a) glyceraldehyde and (b) cyclohexane yield with (c) van Krevelen diagram of cathode chamber on the paired electrosynthesis of GECH-DO and GEOR using PtNi/C and Pt/C catalyst in stirred slurry electrocatalytic reactor. Reaction condition: j = 66 mA cm-2, N = 400 rpm, t = 20 h. Electrolyte: 1.0 M MSA, 0.25 M NaCl, and 25% v/v IPA addition in the cathode chamber with CA0 guaiacol = 0.1 M, CA0 glycerol = 0.3 M. ..................................................................... 174 Figure 7.14. The ideal reactor design for the paired electrosynthesis of GECH-DO and GEOR with Gantt chart of semi-batch SSER. ................................................................................................ 176 Figure 8.1. The plausible industry implementation of the paired electrosynthesis on the bio-oil ECH-HDO and glycerol electrooxidation toward glyceraldehyde in the blue square with the comparison of the CO2RR technology432 in the red square. ....................................................... 179 Figure A.1. The jacketed reactor set-up of (a) cathode half-cell, (b) anode half-cell, and (c) paired cell stirred slurry electrocatalytic reactor (SSER) setup for guaiacol ECH-DO and glycerol EOR with similar size of the chambers................................................................................................ 237 Figure A.2. GC-MS chromatograms of (a) standard solution equal to 0.05 M of organic compounds and (b) 0.1 M GECH-DO half-cell at 0.2 M MSA, 0.25 NaCl, and 50% IPA for 2 h of reaction and j = -66 mA cm-2. ......................................................................................................................... 238 Figure A.3. HPLC chromatograms of standard solution equal to 0.0125 M of organic compounds in anode chamber (a) RI detection and (b) UV detection. .......................................................... 239 xxv Figure A.4. Cyclic voltammograms on Pt: (a) the stability test, (b) guaiacol addition, (c) guaiacol and 5 g/L dispersed Pt (5 wt%)/C slurry addition, (d) IPA addition, (e) and (f) IPA, guaiacol, and Pt/C addition at 25 oC and 60 oC respectively in 1.0 M MSA and 0.25 M NaCl. ...................... 240 Figure A.5. The concentration profile of guaiacol ECH-DO with 50% IPA addition using SSER at (a) 0.2 M MSA; (b) 0.5 M MSA; and (c) 1.0 M MSA concentration. Reaction condition: I = - 0.4 A (j = -66 mA cm-2), T = 60 °C, t = 4 h. Catalyst: 5 wt%-Pt/C (5 g/L). .................................... 241 Figure A.6. Pareto chart of RSM analysis of XA (guaiacol conversion, %), YB – YJ (product yield, %), Y1 (required power, Wh), and Y2 (H+ concentration, M) for (a) – (l) respectively. ............ 242 Figure A.7. The contour plot of (a) XA (guaiacol conversion, %) and YF (cyclohexane yield, %)...................................................................................................................................................... 243 Figure A.8. The comparison of experiment and model data obtained from RSM of (a) CA – CJ and (b) Y1. .......................................................................................................................................... 243 Figure A.9. The faradaic efficiency profile of guaiacol ECH-DO using SSER at (a) 0.2 M; (b) 0.5 M; and (c) 1.0 M of MSA concentration. Reaction condition: I = - 0.4 A (j = -66 mA cm-2), T = 60 °C, t = 4 h. Catalyst: 5 wt%-Pt/C (5 g/L). .............................................................................. 244 Figure A.10. The salt addition effect on half-cell glycerol electrooxidation (0.25 M) using SSER at 1.0 M of MSA concentration in regards of (a) glycerol conversion and (b) glyceraldehyde selectivity. Reaction condition: I = - 0.4 A (j = -66 mA cm-2), N = 600 rpm, T = 60 °C, t = 4 h. Catalyst: 5 wt%-Pt/C (5 g/L). ..................................................................................................... 245 Figure B.1. The Cyclic voltammetry of (a-b) 25 °C and (c-d) 60 °C for half-cell and paired electrosynthesis of guaiacol eletrocatalytic hydrogenation (GECH) and GEOR. Electrolyte: 1.0 M Methanesulfonic acid (MSA) or NaOH with 0.25 M NaCl for both chamber and 25% v/v IPA addition in the cathode chamber using 5wt%-Pt/C (5 g/L). ........................................................ 254 xxvi Figure B.2. The effect of (a) Pt loading content and (b) temperature on the cyclic voltammetry of paired electrosynthesis of GECH and GEOR. Electrolyte: 1.0 M MSA with 0.25 M NaCl for both chamber and 25% v/v IPA addition in the cathode chamber using 5wt%-Pt/C (5 g/L). ............ 255 Figure B.3. Tafel slope analysis plot log current density versus overpotential. ......................... 256 Figure B.4. The effect of (a) 0.5 M acid-acid, (b) 0.2 M acid-acid, (c) 1.0 M acid-alkaline, and (d) 1.0 M acid-neutral electrolyte pairs on half-cell GECH (left) and paired cell GECH (right). ... 257 Figure B.5. The effect of (a) 0.5 M acid-acid, (b) 0.2 M acid-acid, (c) 1.0 M acid-alkaline, and (d) 1.0 M acid-neutral electrolyte pairs on half-cell GEOR (left) and paired cell GEOR (right). ... 258 Figure B.6. Concentration profile of the experiment and kinetics model at 30 °C for GECH (left) and GEOR (right). Reaction condition: j = 66 mA cm-2, N = 600 rpm, t = 4 h. Electrolyte 1.0 M MSA with 0.25 M NaCl for both chamber and 25% v/v IPA addition in the cathode chamber using 5 wt%-Pt/C (5 g/L). Cathode: guaiacol (A), phenol (B), cyclohexanone (C), cyclohexanol (D), anisole (E), cyclohexane (F), 2-methoxycyclohexanone (G), 2-methoxycyclohexanol (H), methoxycylohexane (I), and methanol (J). Anode: glycerol (A), glyceraldehyde (B), glyceric acid (C), glycolic acid (D), tartronic acid (E), DHA (F), lactic acid (G), Formic acid (H), and acetic acid (I). ........................................................................................................................................ 265 Figure B.7. The kinetics rate constant comparison of glyceraldehyde and DHA formation in glycerol electrooxidation. Reaction condition: j = 66 mA cm-2, N = 600 rpm, t = 4 h. Catalyst: 5 wt%-Pt/C (5 g/L), electrolyte also containing 0.25 M NaCl with 25% v/v IPA addition in the cathode chamber. ........................................................................................................................ 268 xxvii List of Symbols 𝐴𝐴𝑤𝑤𝑤𝑤𝑤𝑤𝑤𝑤ℎ𝑡𝑡𝑤𝑤𝑡𝑡 𝑠𝑠𝑠𝑠𝑠𝑠 Overall objective function (-) 𝐴𝐴𝑦𝑦 Individual objective function (-) 𝐶𝐶𝑤𝑤𝑒𝑒𝑒𝑒𝑤𝑤𝑒𝑒𝑤𝑤𝑠𝑠𝑤𝑤𝑒𝑒𝑡𝑡 Experimental concentration (mM) 𝐶𝐶𝑠𝑠𝑚𝑚𝑡𝑡𝑤𝑤𝑚𝑚 Kinetics model concentration (mM) 𝐷𝐷𝑤𝑤𝑒𝑒𝑒𝑒 Effective diffusivity (m2 s-1) 𝐸𝐸𝑒𝑒𝑤𝑤𝑡𝑡0 Standard reduction potential of the reaction (V vs. SHE) 𝑃𝑃𝑒𝑒𝑤𝑤𝑟𝑟𝑠𝑠𝑤𝑤𝑒𝑒𝑤𝑤𝑡𝑡 Required energy (Wh) 𝑅𝑅𝑒𝑒 Catalyst particle radius (nm) 𝑎𝑎𝑦𝑦 Weighting factor (-) 𝑚𝑚𝑃𝑃𝑡𝑡 Amount of platinum (g) 𝜌𝜌𝑒𝑒 Catalyst density (g cm-3) 𝜕𝜕𝐸𝐸𝑇𝑇0𝜕𝜕𝜕𝜕 Temperature coefficient of standard equilibrium electrode potential (mV K-1) ∆𝐺𝐺𝑅𝑅0 Standard Gibbs free energy of the reaction (kJ mol-1) ∆𝐻𝐻𝑅𝑅0 Standard enthalpy of the reaction (kJ mol-1) ∆𝑆𝑆0 Standard entropy of the reaction (J mol-1 K-1) A Pre-exponential factor (h-1) an Number of carbon atom in each product (-) aP Particle surface area per unit volume (m-1) C.B. Carbon balance (%) CA0 Initial reactant concentration (mM) Cs Chemical stability aspect (-) xxviii dF Degree of freedom (-) dp Active particle diameter (nm) Dp Average pore diameter (nm) Dpore Average pore diameter (nm) E Cathode / anode potential of the reaction (V vs. RHE) E0 Standard potential of Ag/AgCl (0.197 V vs. NHE) Ea Activation energy (kJ mol-1) Eanode Anode potential of the reaction (V vs. SHE) Ecell cell potential (V vs. SHE) Ex Explosiveness aspect (-) F Faraday’s constant (96,485 C mol e-) F.E. Faradaic efficiency (%) Fl Flammability aspect (-) Hads Adsorbed (chemisorbed) hydrogen I Operating current (A or C s-1) In Inventory aspect (-) j Superficial current density (mA cm-2) k Kinetics constant (h-1 or M-1 h-1) K Equilibrium constant (-) kS-L Mass-transfer coefficient at solid-liquid interface (m s-1) 𝑀𝑀𝑀𝑀𝑃𝑃𝑡𝑡 Molecular weight of Platinum (195.084 g/mol) N Agitation speed (rpm) N Sample size (unit) xxix nA,0 The initial moles of reactant, guaiacol or glycerol (mol) nA,t Moles of reactant at specified time, guaiacol or glycerol (mol) nc Moles of supported catalyst (mol) nn,t Moles of chemicals at specified time (mol) NW-P Weisz-Prater criterion (-) Ox Oxidation state aspect (-) P Pressure of operation (atm) pH Potential of hydrogen (-) pKa Potential of acid dissociation constant (-) R Gas constant (J mol-1 K-1) -rA Reaction rate per mass of catalyst (mmol s-1 gPt-1) Re Reynold number (-) rx,0 Initial reaction rate at 1 h (mol of reactant s−1 gPt−1) R/M Reactants to metal (bulk) molar ratio (mol/mol) S Selectivity (%) 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) SS Safety score (-) t Reaction time (h) T Temperature of the reaction (°C) To Toxicity aspect (-) xxx T Phase-transition temperature (°C) V.E. Voltage efficiency (%) Vpore Total pore volume of catalyst (cm3 g-1) X Conversion of reactant (%, on molar basis) zi Involved electron on the reaction (mol e-) φ Metal dispersion (%) 𝐻𝐻/𝐶𝐶 𝑟𝑟𝑎𝑎𝑟𝑟𝑟𝑟𝑟𝑟 The ratio of H and C in the organic chemicals (-) 𝑀𝑀𝑆𝑆 Mean square error of data (-) 𝑂𝑂/𝐶𝐶 𝑟𝑟𝑎𝑎𝑟𝑟𝑟𝑟𝑟𝑟 The ratio of O and C in the organic chemicals (-) 𝑆𝑆𝐷𝐷 Standard deviation (-) 𝑟𝑟(𝛼𝛼, 𝑑𝑑𝑑𝑑) The t-value from the 𝑟𝑟-distribution (-) 𝛼𝛼 Level of significance (-) 𝜇𝜇 Electrolyte viscosity (Pa s-1) xxxi List of Abbreviations ACC Activated carbon ACE Anode coupling electrosynthesis 2-MCHO 2-methoxycyclohexanone 2-MCHOL 2-methoxycyclohexanol AD Acceptorless dehydrogenation Ag/AgCl Silver-silver chloride AGA 2,3,4,5-tetra-O-acetylgalactaric acid AGEs Advanced glycation end-products ANOVA Analysis of variance BCKDF British Columbia Knowledge Development Fund BET Brunauer-Emmett-Teller BJH Barrett-Joyner-Halenda BPE Benzyl phenyl ether CAPEX Capital cost CC Carbon cloth CFI Canada Foundation for Innovation CFP Catalytic fast pyrolysis CH Cyclohexane CHO Cyclohexanone CHOL Cyclohexanol CMK-3 Ordered mesoporous carbon CO2RR Electrochemical CO2 reduction reaction xxxii CPE Convergent paired electrosynthesis CSTR Continuous stirred tank reactor CV Cyclic voltammetry DFT Density functional theory DHA Dihydroxyacetone DOD Department of Defense DPE Diphenyl ether DPE Divergent paired electrosynthesis ECH Electrocatalytic hydrogenation ECH-DO Electrocatalytic hydrodeoxygenation ECSA Electrochemically active surface area EDX Energy dispersive X-ray ePMR Electrocatalytic palladium membrane ETA Event tree analysis FDCA 2,5-furandicarboxylic acid FTA Fault tree analysis FTIR Fourier Transform Infrared Spectroscopy GA Glycolic acid GC Glassy carbon GC-MS Gas chromatography-mass spectrometry GECH-DO Guaiacol electrocatalytic hydrodeoxygenation GEOR Glycerol electrooxidation GEP Gross economic potential xxxiii GLA Glyceric acid GLAD Glyceraldehyde GUA Guaiacol GWP Global warming potential HAADF-STEM High-angle annular dark-field scanning transmission electron microscopy HDO Hydrodeoxygenation HER Hydrogen evolution reaction HMF Hydroxymethylfurfural HPA Hydroxypyruvic HPLC High-performance liquid chromatography IPA Isopropanol LCA Life cycle analysis LCC Levelized cost of chemicals LPE Linear paired electrosynthesis LSD Least significant difference MCH Methylcyclohexane MeOH Methanol MOF Metal–organic framework MOO Multi-objective optimization MSA Methanesulfonic acid MSDS Material safety data sheet NASA National Aeronautics and Space Administration OER Oxygen evolution reaction xxxiv OPEX Operational cost OXA Oxalic acid P&ID Process and instrumentation diagram PasifiCan Pacific Economic Development Canada PEM Proton exchange membrane PGM Platinum group metals PHE Phenol PMMA Poly methyl methacrylate POM Polyoxometalate PPE Phenyl ethyl phenyl ether PPE Parallel paired electrosynthesis QRA Quantitative risk assessment RDS Rate-determining step RHE Reversible hydrogen evolution RI Refractive index RSM Response surface methodology RT Room temperature SFU Simon Fraser University SHE Standard hydrogen evolution SSER Stirred slurry electrocatalytic reactor TCD Thermal conductivity detector TCH Thermocatalytic hydrogenation TCH-DO Thermocatalytic hydrodeoxygenation xxxv TEM Transmission electron microscopy TOF Turnover frequency TPE Tandem paired electrosynthesis TRL Technology readiness level UBC University of British Columbia UV-vis Ultraviolet visible light WE Working electrode XRD X-ray Diffraction xxxvi Acknowledgements Primary and foremost, I would like to express my sincere gratitude to my supervisors, Professor Kevin J. Smith, Professor Elӧd L. Gyenge, and Professor Chang Soo Kim, whose expertise, patience, generous and continuous financial and academic support guided me throughout my study and research life. I remember that when I arrived at UBC four years ago, I had no knowledge of biomass and the electrosynthesis process. I should be thankful to all the guidance of my supervisors who helped me to criticize my own insight and paved my way to the current dissertation. During my worst health condition, my supervisors have enhanced my courage and confidence to finish my study with all of their support. Specially, I would like to thank The Indonesia Endowment Fund for Education Agency during my doctoral program (4 years) with additional financial support from Professor Kevin J. Smith, Professor Elӧd L. Gyenge, and Professor Chang Soo Kim through the research assistant program. I would like to thank Beini Shen for her assistance in my research, especially in conducting the experimental work with a great effort. My deepest gratitude goes to Dr. Yanuar Philip Wijaya, Dr. Robertus D. D. Putra, and Dr. Kwang Ho Kim, the members of laboratory group on KIST-UBC Biorefinery, for the technical support, discussion, and equipment sharing during the experiment and research activities. I would like to thank CHBE colleagues Dr. Yu Pei, Dr. Ray Bi, Dr. Shahid Bahar, Dr. Majed Alamoudi, and Dr. Samira Shirvani for their cooperation in lab management and guidance with nice conversations for life and scientific matters. I also would like to thank the CHBE staff: Marlene Chow, Brittany Ji, Kristi Chow, and Doug Yuen, for their assistance during my research and study. Special thank goes to my parents, Lukas Ari Nuryanto, Isana Supiah Yosephine Louise, Yohanes Mudjilan, and Christina Sri Nuryanti, who always pray, love, and support me in xxxvii Indonesia. Moreover, my mother, Professor Isana Supiah Yosephine Louise, gives me knowledge in the electrosynthesis process and meaningful discussion to solve my dissertation study and experiments. Finally, yet importantly, I would like to send thanks and love for my greatest woman, my wife Valentina Kunthi Bratadewi, who is not blood related but she always supports and prays for me during this past doctoral program. Without your support, dedication, fruitful discussion, and long last pray, I would not be able to finish the doctoral program. My son’s smile, Rafael Arjuna Dewanata, always guides me towards the light and cheers me when I am down. I am also grateful to all the participants, organizations, and individuals who contributed to this research. This work would not have been possible without your cooperation and willingness to share your experiences. Thank you all for being a part of this academic endeavor. xxxviii Dedication Praise to the Almighty Lord Jesus Christ for all the blessing and help during my works of PhD life due to my heart condition. The work is dedicated to everyone who believed in my potential, even during the times when I doubted myself. Thank you, Lord Jesus Christ, for your endless love for my family, carrying us through challenges and for providing us with the strength and wisdom to overcome the burden. “And now these three remain: faith, hope and love. But the greatest of these is love.” (1 Corinthians 13:13) 1 Chapter 1: Introduction Since the Industrial Revolution, energy has been a priority sector, powering the world economy. To fulfill human needs and maximize economic output, the exploitation of energy, and especially fossil fuels, has resulted in a substantial amount of carbon emissions over the past 250 years, with 46% distributed in the atmosphere, 23% to the ocean, and 31% stored by vegetation and plants on land.1 Fossil fuels are a slow domain of the carbon cycle because of the long period of formation, which leads to an imbalance in carbon emissions to the atmosphere when fossil fuels are exploited. As shown schematically in Figure 1.1, the carbon cycle of fossil fuels leads to climate change, global warming, water cycle change, and extreme climate events. As plants and vegetation provide a balanced carbon cycle and minimize land use change, substituting fossil fuels with biomass is crucial to support human activities. Biomass as an energy source has been known for centuries. With a worldwide trend towards prioritizing green, sustainable, and renewable processes and products, biomass as a source of energy and intermediate chemicals, is seen as possibly meeting this need. Conversion of solid biomass into liquid fuels that could be used in combustion engines or produce intermediate organic chemicals, would provide a renewable, sustainable and carbon-neutral resource due to carbon adsorption during the growth cycle of the solid biomass.2 Therefore, novel technological applications are the foundation for developing biomass utilization as the solution that balances the carbon cycle and contributes to future green and sustainable chemicals production and energy consumption. 2 Figure 1.1. The historical and future carbon emission concentration and carbon-climate feedbacks in the Earth system, along with the key compartment, processes, and pathways. Reproduced from ref. 1 with permission from the Cambridge University Press, Copyright 2023. 1.1 Biomass and bio-oil potential The carbon, hydrogen, and oxygen content of biomass means that it could be used in various applications including biofuel3, intermediate chemicals, bioplastics4, nanoparticles5, adhesives and binders6, 7 and carbon fibers 8. Biofuel, in particular, has drawn a lot interest because of its versatility in industrial applications. Biofuels may be in the form of biodiesel, bioethanol, vegetable oil, syngas, solid fuel, biogas and bio-oil, depending on the source of the biomass. Lignocellulosic biomass is promising because it contains lignin (10 – 25 wt %), hemicelluloses 3 (20 – 35%), and cellulose (35 – 50%).9 Lignin derived from agricultural waste, forest residue and industrial by-products such as straw, husks10, 11, sugarcane bagasse10, 12, wood chips, sawdust, and pulp and paper industry waste13, has potential as an inexpensive and plentiful energy resource, without competing with the food supply chain. Compared to the vast utilization of hemicellulose and cellulose, lignin is still an underdeveloped feedstock. However, hemicellulose and cellulose typically compete with the food supply chain, which has raised ethical questions regarding food security and price. Moreover, biomass products such as bio-oil and intermediate chemicals derived from residual biomass or waste has additional benefits in terms of price, availability, sustainability, and a circular economy. Figure 1.2. The primary building blocks (monolignols) of lignin with the complex cross-linked phenolic polymer. 4 Lignin utilization is not straightforward because of its complex chemical structure, as reflected by an underdeveloped, efficient and consistent technology for lignin use. Lignin consists of a three-dimensional heterogeneous macromolecule composed of numerous phenylpropanoid units formed through the oxidative polymerization of three types of monolignols.14-16 The monolignols are p-hydroxyphenyl (H), guaiacyl (G), and syringyl (S), illustrated in Figure 1.2. The amount and type of monolignols (H-G-S) are used to classify lignocellulosic biomass as softwood (0-90-10 wt%), hardwood (0-50-50 wt%), and grass (5-75-25 wt%).15 The macrostructure polymer of lignin combines monolignols with the dominant linkage of β-O-4 aryl ether between the β-carbon of the aliphatic chain and the O-atom from the aromatic moiety.17 Thus, the breakage of β-O-4-aryl ether linkages is a critical step in lignin depolymerization, as it is responsible for around 50% of the ether linkages, firstly degrading lignin to a shorter carbon chain. Thus, niche and advanced technologies should assume that lignin pre-treatment processes will yield functional monomers and oligomers via the summarized technologies presented in Figure 1.3. Chemicals derived from the shorter carbon chains of lignin can be related to the building blocks and platform molecules listed in Table 1.1. Given the wide-ranging potential applications of lignin-derived building blocks and platforms, which are predominantly associated with aromatic compounds, the lignocellulosic bio-refinery concept aims to produce green aromatic compounds sustainably. On the other hand, lignin has a high oxygen-to-carbon ratio (O/C) and, therefore, subsequent processes such as hydrogenolysis or hydrodeoxygenation to reduce the O content are required. As the most common chemicals derived from lignin pretreatment, phenol monomers derived from methoxy phenols and dimeric structures play a significant role in the lignin upgrading process. 5 Figure 1.3. The lignin utilization concept to support sustainability and a circular economy. The reduction of phenol to cyclohexanol in a cathode chamber using a platinized platinum working electrode was first accomplished in 1914 by Fichter and Stocker.18 Hereinafter, many scholars employed lignin-derived aromatic model compounds (platform chemicals) in their studies, such as guaiacol, phenol, diphenyl ether (DPE), phenyl ethyl phenyl ether (PPE), benzyl phenyl ether (BPE), and m-cresol. The lignin-derived model molecules in electrocatalytic studies focused on utilizing lignin through upgrading processes.19, 20 Despite the different properties of the lignin polymer, utilizing model compounds derived from lignin aromatics serves as a crucial starting point for the subsequent transformation into bio-based chemicals.21 Acids, alcohols, aldehydes, esters, ketones, cycloalkanes, phenolic and aromatic compounds22-24 can all be produced from bio-oils. To produce bio-oil, catalytic fast pyrolysis (CFP) has been 6 recommended among nascent thermochemical conversion pathways due to its high yield and good bio-oil quality.25, 26 Table 1.1. Lignin building blocks (black color) and platform (blue color) chemicals.27 Chemicals B.P. (°C) Application Chemicals B.P. (°C) Application Guaiacol 205 Expectorant 4-hydroxybenzaldehyde 191 Pharmaceutical Phenol 181.7 Resin and nylon xylene 140 Solvent Toluene 110.6 Solvent Benzene 80.1 Plastic, nylon, and fibers diphenyl ether (DPE) 258 Soap perfumes Vanillic acid 209 Flavoring phenyl ethyl phenyl ether (PPE) 319 fragrances Vanillin 285 Flavorings, perfumes, and pharmaceuticals benzyl phenyl ether (BPE) 286 Synthesis of amide compounds Syringaldehyde 192 Flavoring m-cresol 203 Intermediates of vitamin E and pesticides Coniferol 332 Antifungal Syringic acid 379.5 Therapeutic applications coniferyl aldehyde 338.8 Antimicrobial Eugenol 252 Antiseptic and an anesthetic 4-Hydroxybenzoic acid 213.5 Cosmetics Caffeic acid 416.8 Supplements Synapyl alcohol 384.7 Anti-inflammatory Terephthalic acid Decomposes Saturated polyesters Benzoic acid 250 Food industry and antibacterial Ferulic acid 250.62 Cosmetics Styrene 145 Plastics, synthetic rubbers, and polyesters Cinnamic acid 300 Fragrances, cosmetics, and flavor Catechol 245.5 Antioxidant and reagent Cinnamaldehyde 248 Fragrances and flavor p-coumaric acid 346.1 Therapeutic applications p-Coumaric acid 231.61 Cosmetics Naphthalene 217.97 Pharmaceutical and resin Compared to the complicated biomass pretreatment processes required by other utilization processes, the pyrolysis process typically only requires physical pre-treatment of the biomass, such as chipping, grinding, or water content removal as shown in Figure 1.3. Biomass pyrolysis not only generates bio-oil but also produces several other products such as charcoal, gas, char, and chemicals, depending on the type of pyrolysis applied in the biomass processing, as presented in Table 1.2. 7 Bio-oil derived from lignin is a potential solution to convert solid biomass into liquid form. Since bio-oil is a renewable energy source with some properties similar to fossil fuels, it is seen as a potential future green and sustainable source of fuels and chemicals. Bio-oil combined with heavy fuel oil can be directly applied as fuel for a furnace or boiler. However, standalone bio-oil combustion is difficult due to the high water and oxygen content (0.32 < O/C < 0.46)28, which results in a low flame temperature and low energy density of the bio-oil. Table 1.2. Pyrolysis method type and operating conditions.29 Type Reaction time T (°C) Heating rate Products Carbonation days 400 very low charcoal Conventional 5-30 min 600 low oil, gas, char Fast 0.5-5 s 650 very high bio-oil Flash-liquida <1 s <650 high bio-oil Flash-gasb <1 s <650 high chemicals, gas Hydropyrolysisc <10 s <500 high bio-oil Methanopyrolysisd <10 s >700 high chemicals Ultra pyrolysise <0.5 s 1000 very high chemicals, gas Vacuum pyrolysis 2-30 s 400 medium bio-oil aFlash-liquid: Liquid obtained from flash pyrolysis accomplished in <1 s. bFlash-gas: Gaseous material obtained from flash pyrolysis in <1 s. cHydropyrolysis: Pyrolysis with water. dMethanopyrolysis: Pyrolysis with methanol. eUltra pyrolysis: Pyrolysis with very high degradation rate. Bio-oil also has high acidity and viscosity which creates instability during storage. Moreover, the composition of bio-oil varies significantly because of the chemical complexity of the solid biomass and the lack of mechanistic insight into catalytic fast pyrolysis (CFP) reactions. Thus, a subsequent process is mandatory to upgrade the quality of bio-oil for transportation fuel and intermediate chemicals purposes.23, 30, 31 Some bio-oil upgrading methods are presented in the Appendices (Table A.1.) with the corresponding advantages and limitations of each method. Bio-oil upgrading 8 must accommodate energy density optimization (hydrogenation) and oxygen minimization (deoxygenation). 1.2 Electrochemical versus thermochemical processes The hydrodeoxygenation (HDO) of bio-oil is typically conducted through a thermochemical (TCH) process. Thermochemical hydrodeoxygenation (TCH-DO) offers several advantages including high conversion, established technology, scale-up feasibility, and the possibility of non-precious metal catalyst use. Thermochemical methods lead the development of lignin derived bio-oil upgrading processes. Many studies utilize guaiacol, a typical lignin-derived phenolic compound, as a model reactant in thermochemical upgrading, as summarized in Table 1.3. Cyclohexane is the desired product of guaiacol TCH-DO, such that when applied to bio-oil, the product will consist of higher saturated C-H bonds with limited oxygen bonds, thus increasing the upgraded bio-oil stability and quality. Many TCH processes utilize non-noble metal catalysts in their systems, contrary to the electrocatalytic hydrogenation (ECH) process that uses noble metals to obtain significant cyclohexane yield. TCH requires an external pressurized hydrogen supply. The hydrogen is mostly produced from fossil fuels (around 96%)32 through steam methane reforming, with large associated CO2 emissions33, making the development of lignocellulose conversion to bio-oil very dependent on fossil fuels. Additionally, the challenges associated with hydrogen gas transport and storage increase the operational costs of TCH processes. TCH also operates under harsh reaction conditions (high T and P), and extensive coke formation leads to rapid catalyst deactivation, requiring regeneration. Thus, a substitute method of upgrading is pivotal to support lignocellulose biomass utilization based on green and sustainable processes. 9 Table 1.3. The hydrodeoxygenation of guaiacol into cyclohexane. No Process (Volume) Reactant Catalyst (metal+support) Solution P | T | t (MPa) | (°C) | (h) Yield (%) Ref 1 TCH (25 mL) Guaiacol (0.16 mol/L) RuRe/C (4 g/L) 2-propanol 2 | 240 | 5 56.4 34 2 TCH (150 mL) Guaiacol (0.6 mol/L) NiZr-SBA-TS (5.7 g/L) Hexadecane 5 | 300 | 8 63.5 35 3 TCH (150 mL) Guaiacol (0.6 mol/L) NiZr-KIT-TS (5.7 g/L) Hexadecane 5 | 300 | 8 71.6 35 4 TCH (25 mL) Guaiacol (0.6 mol/L) NiZr–I/CMK-3 (5/7 g/L) Hexadecane 5 | 300 | 8 30.3 36 5 TCH (500 mL) Guaiacol (0.6 mol/L) Ni5Cu/SZ-3 (11.2 g/L) Deodecane 5 | 300 | 8 80.8 37 6 TCH (50 mL) Guaiacol (0.28 mol/L) Ni/HZSM-5 (1.35 g/L) 0.1 M NaOH 5 | 250 | 4 79.0 38 7 TCH (500 mL) Guaiacol (0.6 mol/L) Ni/SiO2-ZrO2 (11.2 g/L) Deodecane 5 | 300 | 8 96.8 39 8 TCH (75 mL) Guaiacol (0.2 mol/L) Pt/H-MFI-90 (5 g/L) n-hexadecane 5 | 180 | 5 93.0 40 9 TCH (100 mL) Guaiacol (0.225 mol/L) Fe10Ni1/h-BN (10 %) decaline 3 | 300 | 2 80.0 41 10 ECH (20 mL) Guaiacol (0.02 mol/L) Pt/C (1.6 g/L) 1.0 M H3PO4 and 0.1 M SiW12 0.1| 35 | 0.42 16.3 42 11 ECH (20 mL) Guaiacol (0.015 mol/L) PtNiB/CMK-3 (Coated electrode) 0.2 M HClO4 0.1 | 60 | 1 32.5 43 12 ECH (100 mL) Guaiacol (0.1 mol/L) Pt/C (5 g/L) 1.0 M MSA (25% IPA) 0.1 | 60 | 4 6.7 44 ECH overcomes the limitations associated with TCH with in situ proton reduction which facilitates in situ and continuous generation of chemisorbed hydrogen, mild reaction conditions, minimal coke formation, and tunable selectivity using the electrode potential.45-47 Moreover, the oxygen of the oxygenated compounds in the bio-oil electrocatalytic hydrodeoxygenation (ECH-DO) are released greener products as water and methanol instead of CO and CO2, as in the TCH-DO process. With progress in electrocatalysis, preferably using non-precious metal catalysts, ECH has great potential as an alternative to TCH of bio-oil derived from lignin. This approach fits well 10 with the growing focus on sustainable and green chemical practices, including more efficient use of resources, making it a promising option for the future of bio-oil upgrading and related processes. Meanwhile, the rapid progress of renewable energy such as geothermal, wind, and solar in the electricity sector creates competitive prices compared with the fossil gas electricity, as seen in Figure 1.4. Thus, coupling electrosynthesis together with renewable energy resources seems feasible in the future to drive a synergistic green chemical synthesis. Figure 1.4. The electricity price from the generation of renewable energy source and fossil gas.48 A review by Lucas et al., 202149 stated that the electrocatalytic hydrogenation (ECH) process has several limitations, including (i) low faradaic efficiency, (ii) high cost of membranes and product separation, (iii) sluggish conversion rates, (iv) poor reaction mechanism understanding, (v) scale-up reactor feasibility, (vi) reactant cross-over and (vii) noble metals dependency. Furthermore, the fully hydrogenated product is not formed during most ECH experiments with bio-oil because of the aqueous electrolyte and thermodynamic limitations. Consequently, 11 significant improvements are necessary to make electrocatalytic processes competitive with conventional thermochemical methods. In the present study, a paired electrosynthesis and the tailoring of selected non-noble metal catalysts are investigated to address some of the limitations associated with current electrosynthesis technologies. ECH upgrading of bio-oil is typically conducted on the cathode side, referenced as a half-cell electrosynthesis, with the anode chamber producing oxygen, an undesired product with sluggish kinetics. Many scholars have focused on the cathode chamber as the reduction process, however, the electrosynthesis process consists of two chambers which could dictate each other’s system kinetically and thermodynamically. Thus, maximizing each chamber’s effectiveness is pivotal to fully exploit the reduction and oxidation reactor chamber in the electrosynthesis process. Hence, the cathode and anode chamber could perform a valuable reduction and oxidation reaction simultaneously. However, managing both reactors simultaneously and the limited understanding of the electrosynthesis process complicates the paired electrosynthesis. Addressing these challenges will require significant effort to evaluate and optimize both chambers concurrently. Paired electrosynthesis is an innovative approach to simultaneously utilize both chambers for reactants of the electrochemical processes. The oxidation and reduction reactions can produce valuable chemicals, improving productivity and process efficiency instead of producing undesirable materials such as oxygen and water. The simultaneous production of valuable chemicals reduces operational costs and improves economic viability, contributing to sustainability by lowering energy consumption and reducing the environmental impact, compared to half-cell electrosynthesis. Moreover, the versatility of the oxidation reaction offers a wide range of viable chemical reactions, making it applicable across various industries, including pharmaceuticals, fine chemicals, and renewable energy. Paired electrosynthesis has already been 12 implemented industrially in a few cases.50, 51 The selection of reactants for the paired electrosynthesis should consider several factors such as economy, energy demand, safety, and environment. 1.3 Electrochemical parameters and improvements The electrosynthesis process has several fundamental parameters such as the cell configuration (flow, batch, divided and undivided cell, and electrode distance), electrolyte composition, electrocatalyst or electrode selection, current density, and overpotential which dictate and influence the efficiency, selectivity, and the reaction kinetics. These parameters must be carefully controlled and optimized during the electrosynthesis process. In an electrochemical reactor, electrons actively charge or discharge the electrode and/or electrocatalyst to trigger the chemical reaction. The movement of electrons between the electrodes is facilitated by an external power supply in electrocatalytic cell, while the electrolyte allows the ion movement to maintain the charge balance in the system. Previous studies report improving the reactor configuration by closing the electrode gap using a flow reactor to eliminate the ohmic losses and maximize energy efficiency.52, 53 By contrast, the batch electrocatalytic reactor relies on diffusion which needs agitation to assist the mass transfer from the bulk to the electrode surface.53 Thus, the selection of reactor configuration is mainly based on the study preference because each reactor has advantages and limitations. Multi-step reactions and longer reaction times are typically required for organic reactions; thus, the batch reactor is an ideal reactor configuration. Both flow and batch reactors have unique advantages and limitations which are presented in Table 1.4. The electrolyte pair selection is another crucial design parameter since the electrolyte provides ion conductivity, solubility of the reactants and products, reaction media, and 13 electrode/electrocatalyst compatibility. Selecting the right electrolyte pair can significantly affect the reaction efficiency, control the reaction environment, and enhance the overall performance of the electrochemical system. The electrolyte selection also dictates the electron transfer mechanism since acid and alkaline conditions will have different electron transfer mechanisms which lead to different reaction activation energies. In the hydrogen evolution reaction (HER) for instance, the process prefers an acidic rather than alkaline condition due to the lower thermodynamic potential (0 V vs. -0.83 V), lower overpotential, abundant proton availability, and high exchange current density. By understanding these relationships, the electrolyte pair selection can be optimized for the specific reaction. Table 1.4. The advantages and limitations of flow and batch electrocatalytic reactors. Feature Flow Reactor Batch Reactor Mass Transfer High, better mixing and mass transfer Relying on diffusion with assisted by agitation Energy efficiency Lower ohmic losses with greater heat transfer Higher ohmic losses with assisted heat transfer by agitation Scalability High, more suitable for scale up for industrial applications Low, more challenging to scale up efficiently Control Over Reaction Precise control over reaction conditions Limited, less control during reaction Cost and Complexity Higher initial cost, complex design Lower cost, simpler design Suitability for Long Reactions Limited, shorter residence times for reactants Good, suitable for longer or multi-step reactions Maintenance and Fouling Prone to clogging, requires maintenance Lower risk, generally easier maintenance Furthermore, electrocatalyst or electrode selection plays a pivotal role in the electrocatalytic reaction. With higher reaction activity and better properties, the electrocatalyst can optimize the efficiency and decrease intermediates chemicals with the possibility of lowering the reaction energy barrier. Thus, many electrocatalytic reactions rely on noble metal catalysts such as Pt, Pd, 14 Ir, and Ru because of their high stability, activity, and temperature tolerance. The platinum group metals (PGM) have been widely used in catalytic hydrogenation54 and oxidation55 reactions. However, PGM catalyst exploitation is hindered by the high cost, scarcity, and the environmental impact of the mining and production process. The PGMs have significant global reserves of at least 100,000 metric tons56 which is considered sufficient for current industrial demands, with the exception of Rh.57 Replacing PGM catalysts with non-noble metal catalysts such as transition metal oxides, perovskites, or metal–organic frameworks (MOFs) is an important goal of current research activities.58, 59 The non-noble transition metals such as Ni, Fe, Co, Cu, and Ti are abundant with sustainable supply and cost-effective prices. Moreover, the non-noble catalysts are less toxic for humans and the environment.60 However, the activity of single non-noble metals (Ni, Co, Mo, and W) showed much lower activity toward HER in acidic condition compared to the binary metallic ceramics (Ni5P4, Co2P, MoS2, and WC) with higher dissolution degree on the stability test.61 In terms of the oxygen evolution reaction (OER), non-noble metals frequently perform well in alkaline conditions but suffer from poor durability performance in acidic media.62 Tailoring noble and non-noble metal catalysts has the potential to provide improved catalyst properties in terms of chemical reaction efficiency, economics, energy consumption, and environmental impact. The intermetallic phase is advantageous in catalyst synthesis because it often shows improved activity and stability.63 The superior intermetallic properties are caused by the active site produced from the arrangement of the intermetallic phase.64-66 Moreover, thermodynamics suggests that the intermetallic phase is stable67-69 and less prone to degradation in the harsh acid / alkaline electrolyte condition compared with the standalone non-noble metal62. Thus, this study investigated reducing the noble metal 15 catalyst content by tailoring a series of non-noble and noble metal binary and ternary metal catalysts for ECH. 1.4 Research scope, objective, and outline The ECH of bio-oil model compounds has mostly been studied in the cathode chamber, without exploiting the anode chamber as an isolated oxidation reactor of the membrane-divided electrocatalytic reactor. This study evaluated the potential anode coupling electrosynthesis (ACE) for process intensification to reduce the energy and cost of bio-oil upgrading in a stirred slurry electrocatalytic reactor (SSER). The criteria for potential ACE reactions include the economics, environmental impact, energy requirement, safety, separation process, and technology readiness level (TRL). Synergistic reduction and oxidation reactions could optimize each reaction process through a continuous hydrogen and hydroxide ion supply. Each chamber was optimized in terms of electrolyte selection, polar protic solvent addition (isopropanol), and commercial catalyst selection, before applying them in the paired electrosynthesis process. The SSER reactor used suspended electrocatalysts in the electrolyte, which differs from most studies that have implemented a coated electrode with a catalyst. To limit noble metal dependency of the electrosynthesis process, suitable bi- or tri-metallic electrocatalysts for the paired electrocatalyst were investigated, with appropriate electrolyte selection. The overall goal of the study is to contribute to the development of biomass electrosynthesis as a promising green and sustainable technology. To achieve this goal, several sub-objectives were developed: a. To develop the paired electrosynthesis of bio-oil upgrading and potential ACE in cathode and anode chambers. The pairing system is expected to increase cell faradaic efficiency and product intensification so that the electrocatalytic process's high production costs are reduced. 16 b. Understanding the reaction pathway and kinetics in paired electrosynthesis is important for both bio-oil upgrading and potential ACE reaction selection. Developing an understanding of the reaction mechanism, kinetics and thermodynamics supports scale-up process development. c. To investigate the electrolyte selection, such as acid, neutral, and alkaline, with other substances, such as organic solvents. The optimized electrolyte pairs eventually decrease the energy demand and increase the kinetics of the paired electrosynthesis process. d. To design binary and ternary non-noble and noble metal electrocatalysts for bio-oil upgrading and potential ACE process reaction. The tailored catalyst will reduce noble metal dependency of the electrocatalytic process by substituting non-noble metals in the catalyst formulation. The outline of the dissertation is summarized below: Chapter 1 introduces the potential of bio-oil upgrading by electrochemical hydrodeoxygenation and its advantages and limitations. Several parameters are also examined to improve shortcomings of the electrochemical upgrading method. Chapter 2 reviews the fundamental and theoretical aspects of the bio-oil upgrading process using model compounds, including recent findings and knowledge gaps. Chapter 3 consists of the methodology of the experiments, including the selected materials and reagents, reactor cell setup, cyclic voltammetry experiments, product analysis and catalyst characterization, the figure of merit calculations, design of experiments and factorial analysis, the van Krevelen analysis, and the kinetic parameter estimation method. Part of this chapter has been prepared for journal publication. 17 Chapter 4 examines the potential anodic reactions to be paired with the guaiacol electrocatalytic hydrodeoxygenation (ECH-DO) reaction using a multi-objective optimization (MOO) approach. Part of this chapter has been prepared for a journal publication entitled “Intensification process of bio-oil electrocatalytic hydrogenation-hydrodeoxygenation with potential anodic coupling electrosynthesis (ACE): a semi-quantitative review”. Chapter 5 presents the half-cell optimization process of the cathode and anode chamber, focused on electrolyte selection, polar protic solvent addition (isopropanol), and commercial catalyst selection. Part of this chapter has been published in ACS Sustainable Chemistry and Engineering (2025) with the title “Effect of Isopropanol on the Electrocatalytic Hydrodeoxygenation of Guaiacol to Cyclohexane in a Stirred Slurry Reactor: Factorial Experimental Design”. Part of this chapter has also been prepared for journal publication under the title: “Paired electrosynthesis using Bio-Oils: Guaiacol Hydrodeoxygenation coupled with Glycerol Oxidation in a Stirred Slurry Reactor”. Chapter 6 compares the half-cell and paired cells and further optimizes the paired cells in terms of temperature, current density, agitation speed, initial reactant concentration, and catalyst metal loading. The chapter also reports on the tailored non-noble and noble metal catalysts used in the paired electrosynthesis. Part of this chapter has been prepared for journal publications entitled “Paired electrosynthesis using Bio-Oils: Guaiacol Hydrodeoxygenation coupled with Glycerol Oxidation in a Stirred Slurry Reactor” and “Highly Selective Cyclohexane Production by PtNi/C on Guaiacol Electrocatalytic Hydrodeoxygenation Coupled with Glycerol Electrooxidation”. Chapter 7 reports the kinetics of half-cell electrosynthesis and paired electrosynthesis of guaiacol electrocatalytic hydrodeoxygenation (GECH-DO) and glycerol electrooxidation (GEOR). Part of this chapter has been published in ACS Sustainable Chemistry and Engineering (2024) Effect of 18 Isopropanol on the Electrocatalytic Hydrodeoxygenation of Guaiacol to Cyclohexane in a Stirred Slurry Reactor: Factorial Experimental Design”, and part has been prepared for journal publications entitled “Paired electrosynthesis using Bio-Oils: Guaiacol Hydrodeoxygenation coupled with Glycerol Oxidation in a Stirred Slurry Reactor” and “Highly Selective Cyclohexane Production by PtNi/C on Guaiacol Electrocatalytic Hydrodeoxygenation Coupled with Glycerol Electrooxidation”. Chapter 8 summarizes the key findings and conclusion of the study with some potential future research remarks. 19 Chapter 2: Literature Review of Electrocatalytic Upgrading of Bio-oil As demand for renewable biofuel increases, use of lignin as a renewable feedstock becomes critically important.70 Upgrading bio-oil derived from lignin through electrocatalytic processes is a green and sustainable approach to support biomass utilization towards bio-energy and bio-chemicals. Moreover, the simultaneous utilization of both chambers of an electrochemical reactor, through paired electrosynthesis, to produce valuable products from renewable resources will intensify the electrochemical process while maintaining or improving the ECH of bio-oil derivatives, thereby improving process economics. In this chapter, previous literature that addresses the issues pertinent to this study are reviewed and critically assessed to guide further studies. 2.1 Paired electrocatalysis Paired electrosynthesis is classified into five distinct configurations based on how the reactants and products interact, as shown in Figure 2.1. These configurations are parallel (PPE), divergent (DPE), linear (LPE), convergent (CPE), and tandem (TPE). The categorization is determined by the nature of reactions involving raw materials. For instance, in parallel paired electrosynthesis (PPE), different raw materials are used and react simultaneously but independently in the cathode and anode chambers. Meanwhile the divergent paired electrosynthesis (DPE) uses similar raw materials to produce different products through a reduction and oxidation process. Although paired electrosynthesis holds great potential and promise, there remains several challenges. The electrode compatibility to catalyze the reduction or oxidation reaction is critical and needs to be investigated. The reaction conditions such as pH, temperature, and electrolyte pair selection need to be evaluated, which affects both the oxidation and reduction reaction 20 simultaneously. The product separation of the many valuable chemicals from both chambers, leading to process and operation complexity, is another consideration. Scale-up requires sophisticated engineering judgment, design, and economic hurdles and is difficult even though the process is a feasible chemical process. As research progresses, addressing the current challenges is essential to realize the full potential of paired electrosynthesis. Figure 2.1. Schematic illustration of paired electrosynthesis types (a) parallel (PPE), (b) divergent (DPE), (c) linear (LPE), (d) convergent (CPE), and (e) tandem (TPE). The first reported commercial paired electrosynthesis was developed by BASF to produce phthalide and 4 tert-butylbenzaldehyde dimethylacetal (Figure 2.2) with 90% yield for each of the products in an undivided cell with graphite electrode and 0.1 to 1 kA.m-2 current density at 4 to 7 V cell voltage.50 The reaction of 4 tert-butyltoluene to 4 tert-butylbenzaldehyde dimethylacetal (anode chamber) and phthalic acid methyl ester to phthalide (cathode chamber) involves equivalent two moles of methanol produced and used in the cathode and anode chambers, 21 respectively. The proton balance of both reactions is in the stoichiometric state. Fortunately, the BASF electrochemists have overcome the purification limitation of the process using a layer crystallization process. Figure 2.2. The first commercial paired electrosynthesis process. Assessing the initial commercial paired electrosynthesis marks a pivotal stage in determining the viability of electrooxidation reactions in the paired electrosynthesis process of lignin bio-oil ECH-HDO. Initially, utilizing an undivided cell for electrosynthesis offers cost advantages, ease of design, reduced internal resistance, and prolonged lifespan. The costly membrane expense must be factored into these paired systems. The 90% product yield made the BASF paired system ideal for the further commercial development. Furthermore, favorable thermodynamic data (Table 2.1) for the cathode (-2.9 V vs. 0 V) and anode (0.4 V vs. 1.23 V) electrosynthesis, enhances the system's superiority compared with the OER. In terms of current density, the system applies a medium current density (10 – 100 mA cm-2). Ultimately, economic considerations serve as a crucial parameter for scale-up production with such substantial raw material and product price differences. The well-established technology of commercial industrial electrooxidations should first be proposed when considering product intensification through the paired electrosynthesis for lignin OOOOHOOO O+ +Phthalic acid methylester4 tert butyltoluene phthalide 4 tert-butylbenzaldehydedimethylacetal22 bio-oil ECH-HDO. Commercial technologies are characterized by high efficiency and selectivity, scalability, economic viability, compatibility with existing industries, and integration with renewable energy. Considering these benefits, commercial industrial electrooxidations stand out as a practical and strategic choice for intensifying the production of lignin bio-oil ECH-HDO in paired electrosynthesis processes. However, one of the inhibiting factors of commercial industrial electrooxidation is the patent fee and associated intellectual property constraints that limit the ability to modify and improve the technology or use complementary innovations. In a recent review, Chen et al.71 stated that replacing OER with anodic coupling electrosynthesis (ACE) requires (i) lower onset potential or (ii) higher economic value than OER either by producing more valuable chemical substances or by mitigating the adverse effects of harmful chemicals. However, the review by Chen et al. did not evaluate ACE, which has been applied commercially, eliminating some evaluation of the possibility of more sustainable and comprehensive lignin bio-oil ECH-HDO coupling. Table 2.1. The thermodynamic data derived from ChemDraw 18.1 software and the chemical price from several suppliers of the first reported commercial paired electrosynthesis. The study focused solely on the levelized cost of chemicals (LCC) without considering other aspects, such as the separation process and environmental factors associated with the ACE. The few studies that have implemented paired electrosynthesis are summarized in Table 2.2, and include hydroxymethylfurfural (HMF), p-nitrophenol, 1,2 propanediol, and CO2. Unfortunately, Chamber Reaction Yield (%)E°red(VSHE) (m V K-1)Cathode phthalic acid methylester1 to phthalide2 1 50 90 -2.9 -1.6Anode 4 tert butyltoluene1 to 4 tert-butyl benzaldehyde dimethyl acetal1 - 2 C.D. 90 0.4 5.61 Career Henan Chemical Co and 2 Ouhuang Engineering Materials (Hubei) Co., LtdPrice raw to product ($/kg)CD : could not determined𝝏𝑬𝑬𝑻°𝝏𝑻23 many of these reactants are expensive or require gas storage facilities and were therefore eliminated as potential anodic coupling electrosynthesis (ACE) reactions to be paired with the ECH-DO of the bio-oil model compound. Table 2.2. Paired electrocatalytic process in laboratory scale experiments No Cathode | Anode Catalyst (WE | CE) Electrolyte Year Ref 1 CO2 | 1,2-Propanediol Au/Carbon Cloth 0.5 M KHCO3 2019 72 2 p-nitrophenol | HMF NiBx 1 M KOH 2019 73 3 HMF | HMF Ag/C | ACT, TEMPO 0.5 M Na3BO3 2019 74 4 Furfural | Furfural Cu3P/ Carbon Cloth | Ni2P/ Carbon Cloth 1 M KOH 2018 75 5 HMF | HMF Pd/ vanadium nitride coated on a bipolar membrane 1 M KOH 2019 76 6 CO2 | glycerol Pt/C 2 M KOH 2019 77 2.2 The thermodynamic consideration Thermodynamic analysis is a fundamental step in assessing the feasibility of the electrosynthesis reaction compared to the HER and OER. Thermodynamic analysis helps one to understand the energy landscape and guide catalyst and electrolyte selection by optimizing reaction conditions. By carefully evaluating thermodynamic data on the reduction and oxidation reaction, the energy efficiency of electrosynthesis is revealed. In the electroreduction of guaiacol, the thermodynamic data reveals favorable conditions due to higher standard potential compared to the HER78-80, indicating an energetically more favorable reaction than HER. It is noteworthy to evaluate the overall electrosynthesis process, which depends on the electrooxidation reaction involved, compared to the OER. Therefore, a pre-examination of the thermodynamic data of electrooxidation should be conducted, ensuring a comprehensive understanding of the energy requirement. Paired electrosynthesis is an equilibrium cell redox reaction with the following response with n stoichiometric involved electrons: 24 𝑎𝑎 Ox1 + 𝑏𝑏 Red2 ⇌ 𝑎𝑎 Red1 + 𝑏𝑏 Ox2 (2.1) The standard free-energy change and the equilibrium constant are related by the following equation: ∆𝐺𝐺° = −𝑛𝑛𝑑𝑑𝐸𝐸𝑐𝑐𝑤𝑤𝑚𝑚𝑚𝑚° (2.2) ∆𝐺𝐺° = −𝑅𝑅𝑅𝑅 𝑙𝑙𝑛𝑛(𝐾𝐾) (2.3) Given the relationship between Equation 2.2 and 2.3, we can write: −𝑛𝑛𝑑𝑑𝐸𝐸𝑐𝑐𝑤𝑤𝑚𝑚𝑚𝑚° = −𝑅𝑅𝑅𝑅 𝑙𝑙𝑛𝑛(𝐾𝐾) 𝐸𝐸𝑐𝑐𝑤𝑤𝑚𝑚𝑚𝑚° = 𝑅𝑅𝜕𝜕𝑒𝑒𝑛𝑛 𝑙𝑙𝑛𝑛(𝐾𝐾) (2.4) Where ∆𝐺𝐺° is the standard free energy change (J mol-1), 𝐸𝐸𝑐𝑐𝑤𝑤𝑚𝑚𝑚𝑚° is the standard equilibrium cell potential at actual temperature in the system (V), R is the gas constant (J mol-1 K-1), T is the standard temperature (298 K), and F is the faradaic constant (96,485 C mol-1). At equilibrium, the reaction quotient Q is the equilibrium constant, K. 𝐾𝐾 = �𝑎𝑎𝑂𝑂𝑂𝑂2�𝑒𝑒𝑒𝑒𝑏𝑏 �𝑎𝑎𝑅𝑅𝑒𝑒𝑅𝑅1�𝑒𝑒𝑒𝑒𝑎𝑎�𝑎𝑎𝑂𝑂𝑂𝑂1�𝑒𝑒𝑒𝑒𝑎𝑎�𝑎𝑎𝑅𝑅𝑒𝑒𝑅𝑅2�𝑒𝑒𝑒𝑒𝑏𝑏 (2.5) Meanwhile, the thermodynamic and kinetic relationship is illustrated in Figure 2.3. These three properties (∆𝐺𝐺°,𝐸𝐸𝑐𝑐𝑤𝑤𝑚𝑚𝑚𝑚° , and K) could be used to predict the spontaneity of a redox reaction under standard conditions. Figure 2.3. The relationship among criteria for thermodynamic spontaneity. 25 2.3 Reactor design In a batch electrosynthesis reactor, the reactor configuration focuses on the coated and suspended catalyst electrodes. Coated electrodes are prepared by electrodeposition and impregnation methods. The selection of coated and suspended electrocatalysts considers the pros and cons of the system, as presented in Table 2.3. Table 2.3. The pros and cons of coated and suspended catalyst in the electrosynthesis process. Catalyst Pros Cons Coated ⋅ High-charged surface area ⋅ Improved mass transfer ⋅ Ease of separation ⋅ Minimize the competition with the HER ⋅ Limited Catalyst Loading ⋅ Potential Degradation ⋅ Difficult Scale-up Suspended ⋅ High catalyst loading, efficiency, and stability ⋅ Flexibility in catalyst selection ⋅ Simpler catalyst preparation ⋅ Ease of handling ⋅ Potential for homogeneous catalysis ⋅ Better scalability ⋅ Diffusion limitations ⋅ Catalyst separation ⋅ Potential contamination ⋅ Stability concerns due to collision ⋅ Higher overpotential Nevertheless, the primary components employed in the system are noble metal catalysts, including Pt, Pd, Ru, and Rh. Due to scale-up feasibility, the suspended catalyst system is preferable in the ECH-HDO of the lignin bio-oil model compound. Water electrolysis encounters significant scientific and economic challenges because it relies on precious and rare metals (such as Pt, Ru, and Ir) and requires high energy consumption (5 kWh Nm−3 H2 at 10 kA m−2).81, 82 Additionally, applying noble metal catalysts and membranes increases the operational cost (OPEX) in the lignin bio-oil ECH-HDO, which could be reduced with product intensification without significantly impacting the capital cost (CAPEX). Due to the gap between electrodes in membrane applications, the electrolyte solution is crucial for maintaining ionic conductivity during paired electrosynthesis. The potential window is also constrained by the hydrogen and oxygen evolution reactions, which are influenced by the 26 overpotentials in the aqueous solution. A stable electrolyte without any response or degradation is another consideration for electrolyte selection within the applied potential range. The electrolyte conceivably aids the extraction, separation, and purification of the chemicals for the next steps. Thus, the concentration should be at least 0.1 M83, although a lower concentration might be applied in certain cases. The reaction performance is optimal at a pH close to the pKa (dissociation constant) in the seminal (electro) catalytic studies of the organic molecule.84-89 For instance, in aqueous media, CO2 is typically studied at a pH around 8 (pKa of CO2(aq) = 6.3 and 10.3), formic acid at approximately pH 4 (pKa = 3.77), and alcohols at pH levels above 12 (very weak acids with pKa > 12) glucose (12.23), glycerol (14.15), ethylene glycol (14.77), and ethanol (15.9). Therefore, alkoxides, the anions of dissociated alcohols, are the active species in alkaline media that drive the reaction,84-89 making pKa the crucial property for describing the reactivity and indicates the alkoxide ion concentrations in the system.90 Excess protons facilitate the ECH-HDO process91, enhancing the kinetics and improving the electrocatalytic efficiency. However, the high proton concentration should be accompanied by an optimized applied potential and specified catalyst selection to minimize HER. As the overpotential increases, the surface coverage of organic species decreases, and hydrogen coverage increases.92 Achieving significant hydrogenation rates often requires a highly negative potential, exacerbating HER.93 The adsorption strength of organic species relative to the nature of the electrode or catalyst compound should also be considered. Thus, ECH-HDO and HER competition is usually evaluated using faradaic efficiency. In the presence of the organic compound in the acidic cathode chamber, several reaction mechanisms are plausible as shown below: (2.6) the chemisorption of hydrogen via the proton/water reduction (Volmer reaction) on the catalytic metal site (M), (2.7) the 27 adsorption of the organic compound on the catalyst adsorption site (A), (2.8) ECH-HDO of the organic compound and hydrogen evolution reaction (2.8c and 2.8d via Heyrovsky and Tafel reactions, respectively), and (2.9) desorption of the organic-hydrogenated product: 𝐻𝐻3𝑂𝑂+ + 𝑒𝑒− + 𝑀𝑀 → 𝐻𝐻 ∙ 𝑀𝑀 + 𝐻𝐻2𝑂𝑂 (2.6) (𝑅𝑅 = 𝑂𝑂) + 𝐴𝐴 ↔ (𝑅𝑅 = 𝑂𝑂) ∙ 𝐴𝐴 (2.7) (𝑅𝑅 = 𝑂𝑂) ∙ 𝐴𝐴 + 2𝐻𝐻 ∙ 𝑀𝑀 ↔ (𝑅𝑅𝐻𝐻 − 𝑂𝑂𝐻𝐻) ∙ 𝐴𝐴 + 2𝑀𝑀 (2.8a) (𝑅𝑅𝐻𝐻 − 𝑂𝑂𝐻𝐻) ∙ 𝐴𝐴 + 2𝐻𝐻 ∙ 𝑀𝑀 ↔ (𝑅𝑅 − 𝐻𝐻2) ∙ 𝐴𝐴 + 𝐻𝐻2𝑂𝑂 + 2𝑀𝑀 (2.8b) 𝐻𝐻 ∙ 𝑀𝑀 + 𝐻𝐻2𝑂𝑂 + 𝑒𝑒− → 𝐻𝐻2 + 𝑀𝑀 (2.8c) 𝐻𝐻 ∙ 𝑀𝑀 + 𝐻𝐻 ∙ 𝑀𝑀 → 𝐻𝐻2 + 2𝑀𝑀 (2.8d) (𝑅𝑅𝐻𝐻 − 𝑂𝑂𝐻𝐻) ∙ 𝐴𝐴 ↔ (𝑅𝑅𝐻𝐻 − 𝑂𝑂𝐻𝐻) + 𝐴𝐴 (2.9a) (𝑅𝑅 − 𝐻𝐻2) ∙ 𝐴𝐴 ↔ (𝑅𝑅 − 𝐻𝐻2) + 𝐴𝐴 (2.9b) The ECH-HDO half-cell reaction mainly occurs in a cell divided by a membrane such as Nafion® 117 in an acidic condition, improving ion transport while preserving the cell's high conductivity.94, 95 However, a recent study utilized an electrocatalytic Pd membrane (ePMR) reactor that acted as a hydrogen sponge, which provide a controllable flow of adsorbed hydrogen atoms for the hydrogenation reaction and different chambers for supplying the hydrogen from the water splitting reaction.93 Therefore, the hydrogen supply could be provided by the in situ water splitting process producing adsorbed hydrogen radicals from proton electroreduction instead of the fossil fuel based steam reforming of methane. Furthermore, the hydrogenation reaction from dissociative chemisorption of H2 could be coupled in the thermocatalytic reactor to increase the reaction kinetics. The thermocatalytic and electrocatalytic application of PMR, illustrated in Figure 2.4, significantly reduces the competition in hydrogen evolution reactions. The ePMR has several 28 advantages, such as the possibility of solvent selection, high selectivity, and efficiency, while also streamlining downstream product purification and electrolyte recycling, thereby providing operational flexibility and marking a notable breakthrough in the field.96 Moreover, the separated reactor concepts are readily provided in the divided cell electrosynthesis by the membrane, which liberates the selection of product intensification to support the ECH-HDO of lignin bio-oil through paired electrosynthesis. Figure 2.4. The PMR application is on (a) thermocatalytic and (c) electrocatalytic processes with (b) gas phase hydrogen permeation and (d) H+ permeation from proton/water reduction. Reprint from ref. 93 with permission from the American Chemical Society, Copyright 2024. 29 2.4 Electron transfer and electrocatalyst mechanism in organic electrosynthesis The electrosynthesis process starts with the electron removal or addition of a substance (RX), which transforms it into a reactive intermediate, subsequently converting it to the product through possible reaction pathways (Figure 2.5).97, 98 A reduction occurs when electrons are transferred from the electrode surface to the unoccupied molecular orbital (LUMO), forming the 𝑅𝑅𝑋𝑋•− compound. Conversely, the removal of electrons from the highest occupied molecular orbital (HOMO) leads to oxidation, resulting in the 𝑅𝑅𝑋𝑋•+ compound. The intermediate states are very reactive due to instability and form a further substance depending on the electrolytic condition: nucleophilic if anodic, electrophilic if cathodic, and dimerization with excess radicals. The electron-transfer driven approach is more advantageous in more negative and positive potential for reduction and oxidation, respectively.51 Thus, the paired electrosynthesis should determine the efficient reaction potential window for both the reduction and oxidation electrosynthesis. Figure 2.5. The reaction pathway involves single electron transfer in the electrosynthesis process. Reproduced from ref. 49 with permission from the American Chemical Society, Copyright 2021. 30 In Figure 2.5, the interaction between the electrode and the substance is categorized into three types: the direct electron transfer via an inert electrode, the electrocatalytic reaction over an active electrode, and the mediated electron transfer by redox mediators in the electrolyte.94, 99-101 Each mode has intrinsic properties: selectivity control through adjustment of electrode potential in the first mode, less dependence on the applied potential in the second mode, and milder applied potential enabled by the mediator in the third mode.100 Consequently, many reactions might occur on the electrodes, with the most common being direct oxidation or reduction, addition, substitution, and cleavage.51, 97 Figure 2.6. The possible charge transfer process of organic hydrogenation in a stirred slurry electrocatalytic reactor. In the stirred slurry reactor, the electron transfer mechanism occurs in several ways, as illustrated in Figure 2.6, because the catalyst particles have intermittent contact with the electrode as the electron source.102 The rate of electrochemical reaction depends on the charge transfer between the electrocatalyst and the reactant. The electrocatalyst charge starts with the contact between the electrocatalyst and the electrode (Figure 2.6a), therefore, the electrocatalyst is activated and ready for either the organic (R) hydrogenation reaction (Figure 2.6b) or the HER. 31 Charge transfer between the electrocatalyst particles is also possible due to a high agitation speed (Figure 2.6c). Desorption of the product (P) or intermediate product (I) occurs in the bulk solution (Figure 2.6d). Meanwhile, a direct reaction of HER at the electrode surface (Figure 2.6e) is also feasible, which reduces the faradaic efficiency of the organic hydrogenation process. The selection of proper material at the electrode reduces the undesired electrochemical HER reaction. 2.5 Electrosynthesis component The electrosynthesis process, which comprises three major components, electrode, electrolyte, and electrocatalyst, is a simultaneous reduction-oxidation reaction through the direct transfer of electrons, which involves electrocatalysts to lower the reaction activation energy.103 The electrode materials must have important properties, such as conductivity and corrosion resistance. Pt is frequently selected for the electrode. The electrolyte plays an important role in the electrocatalyst stability, activity, and selectivity, while maintaining the system's conductivity.104 Therefore, the selection of electrolytes should be investigated to choose the best condition in terms of activity and selectivity with specific electrocatalysts. Increasing the efficiency of electrocatalysis requires a thorough understanding of the reaction mechanism. The electrocatalytic process employs many efficiency terms, including faradic, thermodynamic, operating, and electrochemical conversion. The efficiencies are related to the electrocatalytic process's operational cost, which becomes the primary issue for electrocatalytic scale-up. Numerous scientists work to improve the electrocatalytic efficiency by adjusting the electrolytes 80, 105, 106, designing the electrocatalysts 43, 67, 107-126, and designing the reactor80, 127, 128. 2.5.1 Electrolyte On the cathode side, the acidic electrolyte also facilitates the protonation of the organic hydrogenation and does not need an additional water dissociation step from HER.129 Therefore, 32 acid-tolerant properties of the noble metal electrocatalysts are a system requirement. In the anodic chamber, the alkaline electrolyte is preferable to provide excess radical hydroxyl ions. The conductivity value from several common electrolytes is shown in Figure 2.7. In the organic hydrogenation and electrooxidation reaction process, acid electrolytes (H2SO4 and HCl) and alkaline electrolytes (NaOH and KOH), respectively, have been mostly employed. Sulfuric acid has a higher conductivity than methanesulfonic acid (MSA); however, MSA is an organic acid that is considered safer and less toxic than sulfuric acid.105, 130, 131 Moreover, MSA is not an oxidant compared to sulfuric acid at a similar pH. As Wijaya et al.105 reported, MSA enhanced the faradaic efficiency for guaiacol hydrogenation while improving the system's green factor, compared to sulfuric and hydrochloric acid utilization. MSA has also been used in various applications, such as electrodeposition, metal leaching agents, catalysis, and solvolysis. 130-132 Figure 2.7. Molar electrolytic conductivity of aqueous methanesulfonic acid133 and other electrolytes134 at 25 °C. 0 2 4 60102030405060708090 NaCl KCl NaOH HCl H2SO4 MSAConductivity, κ / S.m-1Concentration, C / mol.dm-333 Salts such as NaCl and KCl can be added to the system, as well as the acid/base solution, to increase conductivity. A higher conductivity reduces the ohmic voltage drop. Moreover, electrolyte and electrocatalyst interaction should also be considered to optimize the electrocatalytic process.106 As a consideration, a solvent might be added to the system to increase the organic substrate solubility, but this suppresses electrocatalytic activity due to higher ohmic resistance and competitive adsorption on the electrocatalyst surface. 2.5.2 Electrocatalyst An electrocatalyst is an important component of the electrochemical reactor that lowers the activation energy of the reactions. The catalyst provides active sites where the reaction occurs following several preceding steps, such as diffusion, adsorption-desorption, and chemical reaction. The chemical components of the electrocatalyst are unique to specified reactions. The selection of a particular electrocatalyst is determined by the catalyst activity, selectivity, and price. The activity and selectivity of electrocatalysts depend on several factors, including particle size, morphology, composition, interaction, support, acidity, and molecular structure.121 Noble metals, especially Pd, Pt, Rh, and Ru, have exceptional electrocatalytic activity and durability for the HER within different pH conditions.135 The electrocatalytic hydrodeoxygenation (ECH-DO) of guaiacol in a stirred slurry electrocatalytic reactor (SSER) using noble metals as electrocatalysts has been investigated by Wijaya et al.80, 105, 106 which showed that Pt/C catalyst is superior to Pd/C and Rh/C catalysts. Regarding the electrocatalytic oxidation reaction (EOR), Pt/C based catalyst have been widely used to oxidize ethanol136, 137, methanol138, and glycerol139. The application of non-noble metals has also been studied in the ECH of lignin model compounds.113, 140, 141 Andrews et al.140 investigated base metals (Cu, Co, and Ni) on several biomass-derived organic molecules. Cu had the highest ECH-DO activity for aromatic aldehydes 34 and HER activity, followed by Ni and Co, but Co excelled in the ECH of cyclohexanecarboxaldehyde and 1-heptanal activity. The same research group compared the experimental results with density functional theory (DFT) analysis to show the optimal binding energy of metals on hydrogen (Figure 2.8a), aromatic aldehydes, cyclohexanecarboxaldehyde, and 1-heptanal141. Figure 2.8b shows a volcano plot for several metals and their OER activity. Figure 2.8. Volcano plot for different metals of (a) HER142 and (b) OER143. A superior ECH electrocatalyst candidate should accommodate the ECH and HER with intermediate binding energies associated with hydrogen ions and bio-oil model compounds such as guaiacol. Meanwhile, Lam et al.113 successfully generated cyclohexanol from guaiacol with high selectivity (~93%) using Raney-Nickel and a Co-P-coated stainless steel mesh cathode and anode, respectively. However, the Raney-Nickel performance decreased within longer reaction times of 16 h. The metal with oxophilic property, such as Ni, promotes C-O cleavage on the demethylation of guaiacol144 and gives a synergistic effect when combined with noble metals.145 Meanwhile, adding Co to Pt/C in phenol ECH increased the activity and selectivity towards cyclohexanol.146 A Co catalyst was also employed in the C=O hydrogenation of acetophenone, aryl-alkyl, diaryl, and aliphatic carbonyl compounds147 and C=C hydrogenation148, 149. Co assists (a) (b) 35 in the breakage of C – C bonds150, which is very useful in the hydrodeoxygenation of larger molecules of bio-oil model compounds. On the other hand, a superior electrooxidation catalyst should have intermediate binding energies towards oxygen or hydroxyl ions. Instead of noble metal catalysts, Ag, Cu, Ni, and Co might be suitable based on the volcano plot of Figure 2.8b. In terms of methanol electrooxidation, AgNi/C has higher activity and stability than AgCo/C and Ag/C electrocatalysts.151 Cu, Ni, and Co were also used in binary metals with Pt for methanol electrooxidation, resulting in a Pt(Cu)/glassy carbon (GC) having the highest activity during potentiostatic tests for short and medium-term, compared to pure Pt, Pt(Ni)/GC, and Pt(Co)/GC electrodes.152 Ni has been extensively investigated for GEOR using alkaline media.110, 153-155 Ni/C and NiCo/C had 60% and 35% glycerol conversion, respectively at 1.9 V vs RHE for 8 h of reaction.153 The modification of Ni with Fe or Co on carbon support showed that CoNi/C had the highest glycerol conversion (17.9%), with FeCoNi/C having the highest selectivity (34.1%) toward formate.155 Moreover, the addition of Ni to the noble metal as a modifier or co-catalyst could improve the glycerol electrooxidation process.156 Tailoring noble and non-noble metal catalysts to improve electrocatalysts in terms of chemical reaction efficiency, economics, energy consumption, and environmental impact is an important goal of current electrocatalyst research. Thus, this study has investigated tailored noble (Pt) and non-noble metal (Ni and Co) catalysts on carbon supports to be applied in bio-oil ECH-DO and potential ACE. As stated in the literature, both Ni and Co can provide an intermetallic phase with Pt in combination.157, 158 Hence, the tailoring method could lower the catalyst price and dependency on the noble metal. The co-impregnation method was used in this study because it is facile and a widely used method to prepare supported metal catalysts.159-161 The carbon support was selected 36 due to its hydrophobicity and high porosity that can enhance the adsorption of the substrates onto the catalyst surface, leading to high reaction activity. In strong acidic or alkaline environments, the carbon support is chemically resistant and inert.162-164 Moreover, carbon support can be produced from low-cost and abundant biomass resources of industrial or agricultural waste. 2.6 Recent progress in ECH of bio-oil derivatives and the knowledge gaps Several review papers discuss lignin electrocatalytic upgrading processes.45-47, 78, 165-169 These reviews mainly considered upgrading lignin bio-oil through electrooxidation and lignin model compounds through electroreduction. Meanwhile, cyclohexanol and cyclohexanone are the primary products derived from lignin bio-oil model compound monomers, such as guaiacol, phenol, and syringol, with the reaction pathway illustrated in Figure 2.9. Figure 2.9. ECH reaction pathway of lignin bio-oil monomer to produce cyclohexanol and cyclohexane. A few papers have reported on the formation of cyclohexane, the full HDO product of bio-oil model compound monomers.42, 43 Cyclohexane is formed by cyclohexanol dehydration to cyclohexene, followed by hydrogenation to cyclohexane, which is inhibited by water in the aqueous ECH-HDO process.80 Minor amounts of cyclohexane were produced on a PtNiB/ordered mesoporous carbon (CMK-3) catalyst43, which suggests the catalyst was active for C–O bond cleavage. Meanwhile, increased cyclohexane was produced with the addition of SiW12, silicotungstic acid, also known as polyoxometalate (POM) by Liu et al. 42 that reduced the C–O bond cleavage energy (1.80 eV to 0.74 eV). This dual catalyst system enhanced the HDO via 37 specific mechanisms, as described in their study and shown in Figure 2.10. Density functional theory (DFT) showed that phenol hydrogenation was the preferred reaction pathway. Meanwhile, cyclohexanol deoxygenation is challenging in low-temperature aqueous electrolysis because of the high activation energy, 1.80 eV.42 Figure 2.10. The possible reaction pathways of ECH-HDO of phenol in dual catalyst system. Reproduced from ref. 42 with permission from the Royal Society of Chemistry, Copyright 2020. The impact of POM, categorized as a superacid, is mainly caused by its superior properties compared to sulfuric acid, even in diluted solutions.170 The electron transfer from the electrode onto the catalyst surface is facilitated by SiW12, which allows for charge transfer and storage. Due to the higher Brønsted acidity of SiW12, it is more corrosive, making it challenging to handle and store, complicating the operational procedure that also requires expensive materials of construction. SiW12 also has a higher price than other acids, such as sulfuric and hydrochloric acids. A summary of ECH-HDO for bio-oil-derived lignin compounds is presented in Figure 2.11 and Table 2.4 with some parameter modifications reported in these studies, such as the cathode material, catalyst, and electrolyte selections. 38 Table 2.4. Hydrogenation of bio-oil model compounds. Substrate Catalyst | WE Electrolyte j (mA cm-2) Temp (°C) X (%) Time Selectivity (%) F.E. (%) Ref Guaiacol Pt-C | C Catholyte: M SiW2; MeOH 10 v/v percent Anolyte: 1.0 M H3PO4 250 55 94.6 9.2 min 56.00% CHOL; 7.60% CHO; 11.70% 2-MCHO; 11.60% CH; 8.30% MCH 95.60 42 Guaiacol Pt-C | Pt 0.2 M H2SO4 109 50 37.88 2 h 19.55% CHOL; 30.44% CHO; 10.79% 2-MCHOL; 25.46% 2-MCHO; 9.52% MeOH; 4.23% PHEN 82.02 80 Guaiacol 20% Pt/C | Carbon cloth 0.2 M HClO4 100 60 89.34 10 h 85.32% CHOL; 13.66% CHO n.s. 171 Guaiacol Ra-Ni electrode 0.1 M B4K2O7 buffer (pH = 8) 8 75 ~ 70 3 h ~ 80% CHOL ~ 70 113 Guaiacol Ru/ACC 0.2 M HCl 22.22 80 95 5 h 52% CHOL; 48% 2-MCHOL 39 172 Guaiacol Ru/ACC 0.2 M HCl 40 80 75 3 h 53.0% CHOL; 36.0% cis-MCHOL; 11.0% trans-MCHOL 30 173 Guaiacol PtRhAu 0.2 M HClO4 116 25 95 5 h 76% 2-MCHO 58 174 Guaiacol Pd-C | Pt Catholyte: 0.5 M NaCl Anolyte: 0.2 M H2SO4 109 50 27.9 4 h 16.00% CHOL; 66.30% 2-MCHOL; 12.40% 2-MCHO; 2.00% MeOH; 3.40% PHEN 33.80 106 Guaiacol Ru-C | Pt Catholyte: 0.5 M NaCl Anolyte: 0.2 M H2SO4 109 50 34.3 4 h 25.00% CHOL; 70.00% 2-MCHOL; 1.50% 2-MCHO; 1.90% MeOH; 1.70% PHEN 46.90 106 39 Substrate Catalyst | WE Electrolyte j (mA cm-2) Temp (°C) X (%) Time Selectivity (%) F.E. (%) Ref Guaiacol Pt-C | Pt Catholyte: 0.2 M NaCl; Anolyte: 0.5 M H2SO4 109 50 64.7 4 h 26.40% CHOL; 25.10% CHO; 21.10% 2-MCHOL; 17.00% 2-MCHO; 8.40% MeOH; 2.10% PHEN 84.50 106 Phenol Rh/C 50 mM Acetate buffer 40 23 – 80 > 99.9 1 h n.s. ns 12 Phenol Rh/C Acetate buffer (pH = 5) 0.16 23 100 2.5 h 100% CHOL 70 14 Phenol Pt/C Acetate buffer (pH = 5) 23 n.s. 4 h 95% CHO and CHOL ~40 15 Phenol Pt/C | C Catholyte: 0.1 M SiW2; MeOH 10 v/v % Anolyte: 1.0 M H3PO4 800 35 > 99.9 11.3 min 80.20% CHOL; 1.20% CHO; 18.60% CH 95.30 42 Phenol Pt/C | Pt 0.2 M H2SO4 109 50 38.95 2 h 23.73% CHOL; 76.27% CHO 82.74 80 Phenol Pt/C | Graphite rod Acetic acid at pH 5 40 RT ~ 85 7 h 40% CHOL; 60% CHO 62.1 175 Phenol Pd/Al2O3 | RVC foam Phosphate buffer 1 21 97.8 4.5 min 78.1% CHOL; 19.7% CHO 26.3 176 Phenol 20% Pt/C | Carbon cloth 0.2 HClO4 100 60 69.24 10 h 35.73% 2-MCHO; 44.85% CH n.s. 171 Phenol 1.5%Pt | Graphite 0.1 M H2SO4 30 20 95 12.5 h 30.4% CH; 58.9% CHOL 20.3 177 Phenol 1.5%Pt | Graphite 0.2 M HClO4 30 20 98 12.5 h 44.1% CH; 53.9% CHOL 27.4 177 Phenol 5% Rh/C | Carbon felt acetate buffer and isopropanol 100 RT n.s. 2 h n.s. 68 178 40 Substrate Catalyst | WE Electrolyte j (mA cm-2) Temp (°C) X (%) Time Selectivity (%) F.E. (%) Ref Phenol Pt 2% | Carbon rod 0.05 M H2SO4 40 60 n.s. 4 h 65% CHOL; 35% CHO 72 146 Phenol Pt 2%, Co 0.6% | Carbon rod 0.05 M H2SO4 40 60 n.s. 4 h 95% CHOL; 5% CHO 71 146 Phenol Pt/Pt 0.05 M H2SO4 40 60 n.s. 4 h n.s. 8 179 Phenol Pt/Pt 0.5 M H3BO4 1 RT 59 4 h 36% CHOL; 23% CHO 23 180 Phenol RaNi 58% 0.5 M H3BO4; 0.05 M NaCl 10 60 92.00 17 h 100% CHOL 11 180 Phenol Pt/CC 0.2 M H2SO4 100 50 90.2 100 min 88.8% CHOL; 10.5% CHO 34.6 181 Phenol Ru/CC 0.2 M H2SO4 100 50 73.4 100 min 85% CHOL; 14.1% CHO 28.2 181 Phenol Pt3Ru/CC 0.2 M H2SO4 100 50 96.3 100 min 91.4% CHOL; 7.4% CHO 37.6 181 Phenol Pt3RuSn/CC 0.2 M H2SO4 100 50 91.5 100 min 96.8% CHOL; 2.5% CHO 39.5 181 Phenol Ru/ACC 0.2 M HCl 40 80 89 2.3 h 100% CHOL 29 173 Diphenyl ether Rh/C Acetate buffer (pH = 5) 0.11 23 ~80 3 h 16-23% hydrolysis; 69-73% hydrogenation; 8-12% hydrogenolysis 25 14 Benzyl phenyl ether Rh/C Acetate buffer (pH = 5) 0.11 23 ~100 3 h 1-2% hydrolysis; 36-39% hydrogenation; 59-63% hydrogenolysis 36 14 41 Substrate Catalyst | WE Electrolyte j (mA cm-2) Temp (°C) X (%) Time Selectivity (%) F.E. (%) Ref Benzaldehyde Pd/C | Pt wire Acetate buffer (pH 5) n.s. RT 35.9 45 min 95% Benzyl Alcohol 99 107 Syringol PtNiB/CMK-3 0.2 M HClO4 5 60 98.6 1.5 h 81.6% CHOL; 11.8% CHO 80.2 43 Syringol Ru/ACC 0.2 M HCl 40 80 58 35.0% CHOL; 27.0% cis-2MCHOL; 9.0% trans-2MCHOL; 13.0% 2-MCHOL; 16.0% GUA 29 173 Note: WE = working electrode, j = current density input, X = conversion, F.E. = faradaic efficiency, n.s. = not specified. Chemicals: CHOL = cyclohexanol, CHO = cyclohexanone, 2-MCHOL = 2-methoxycyclohexanol, 2-MCHO = 2-methoxycyclohexanone, MeOH = Methanol, PHE = Phenol, MCH = methylcyclohexane, CH = cyclohexane, GUA = Guaiacol. 42 Most of the studies illustrated in Figure 2.11 and presented in Table 2.4 still depend on noble metals catalysts such as Pt, Ru, Rh, and Pd. Moreover, only a few generated cyclohexane from the bio-oil model compound ECH-DO. Some studies also reported low faradaic efficiency with longer reaction time. All studies focused on the cathode chamber instead of observing the electrosynthesis as the whole oxidation-reduction reactor. Figure 2.11. The upgrading of lignin bio-oil through electrocatalytic in a divided cell proton exchange membrane (PEM) summarized from the literature.42, 43, 105-107, 111, 113, 122, 124, 146, 171-191 43 Based on the summary of previous studies presented above, several factors need to be investigated further to achieve the goal of improving the bio-oil upgrading process through electrosynthesis: (i) the low solubility of bio-oil model compounds, (ii) the unutilized anode chamber, (iii) low energy efficiency with high energy demands, (iv) noble metal dependency, (v) high membrane cost, (vi) and not full hydrogenated product generated. To overcome these limitations in the electrosynthesis process, the present study aims to pair bio-oil ECH in the cathode chamber with an anode coupling electrosynthesis (ACE). The pairing system could synergistically assist the cathode chamber in terms of energy efficiency and proton supply while intensively producing valuable products. The study focuses on the potential ACEs that enhance the efficiency and product spectrum of current ECH-HDO processes for bio-oil in terms of economics, environment, energy, safety, separation technology, and technology readiness level (TRL). In particular, the potential electrooxidation reaction in the anode chamber beneficial to the electrosynthesis process was evaluated using the multi-objective optimization (MOO) method. Applying literature studies and expert judgment, the study aims to comprehensively analyze the process intensification strategies for ECH of bio-oil model compounds. Various potential ACEs were also investigated, based on use of renewable resources, the potential benefits of the integration, and the potential challenges. To the best of the author's knowledge, no research combines bio-oil ECH in a cathode chamber with a potential ACE. The study will investigate and optimize the paired electrosynthesis of bio-oil model compound, guaiacol, and the potential anode reaction in a stirred slurry electrochemical reactor (SSER) to yield useful fuels and chemicals derived from renewable resources. The study also develops a tailored binary and ternary metal catalyst supported on carbon from the combination of noble and non-noble metals to decrease the dependency on noble metal catalysts. 44 Chapter 3: Experimental Methodology Chapter 3 provides details regarding the experimental methodology used throughout the study. It presents the materials and reagents, experimental reactor cell setup, cyclic voltammetry experiment, product analysis and catalyst characterization, the figure of merit calculations, the van Krevelen analysis, kinetic parameter estimation methodology, design of experiment, and factorial analysis method. 3.1 Material and reagents All materials and reagents were purchased from various suppliers as listed below Table 3.1. The materials and reagents. Material Supplier Reagent Supplier Pt gauze (100 mesh) Nafion-117 membrane Ag/AgCl reference electrode Pt/C (1, 3, 5, 10 wt% metal loading) Pd/C (5% wt% metal loading) Ru/C (5% wt% metal loading) C (Activated carbon) Pt/Al2O3 (5 wt% metal loading) Ni/SiO2-Al2O3 (Al-Ni 50:50 wt%) Raney-Nickel (50 wt% in H2O) Sigma-Aldrich Guaiacol (≥99%), Glycerol ((≥99%), Methanesulfonic acid (≥99%), Isopropyl alcohol (>99.7%), 1-Butanol (99%), Potassium chloride (99%), H2PtCl6.xH2O (≥99.9%), Ni(NO3)2.6H2O (≥99.9%), Co(NO3)2.6H2O (≥99.9%) Sigma-Aldrich Pt wire Alfa Aesar Sodium chloride (≥99%) Sodium hydroxide (NaOH, 98.9%) Fisher Chemical Magnetic stirrer bar (2.54 cm) VWR 3.2 Experimental reactor cell set-up Two jacketed half-cells (CANSCI Glass Product Ltd.) separated by a Nafion-117 membrane and similar chamber capacity (150 mL) with controlled recirculating water bath were used in this study as seen in Figure 3.1, while the laboratory reactors are illustrated in Figure A.1. The power supply (B&K Precision 9110 Multi range DC Power Supply, 100W) was utilized to supply constant 45 current with Pt gauze wire and mesh (ca. 6.07 cm2) combined and installed on both the electrodes, cathode and anode, with ca. 8.00 cm inter-electrode gap. An Ag/AgCl, KCl std. electrode immersed in a Luggin capillary filled with 3M KCl was employed as a reference electrode. The reactor temperature was maintained using a recirculating water bath (Fisher Scientific Isotemp 3016HS). Figure 3.1. The jacketed reactor set-up of (a) cathode half-cell, (b) anode half-cell, and (c) paired cell stirred slurry electrocatalytic reactor (SSER) setup for guaiacol ECH-DO and glycerol EOR with similar size of the chambers. (1) Reference electrode (Ag/AgCl/3M KCl), (2) fritted side compartment (Luggin probe) for reference electrode filled with 3M KCl, (3) suspended catalyst, (4) cation exchange membrane (Nafion-117), (5) magnetic stirrer. (a) (b) (c) 46 A 100 ml mix of isopropyl alcohol (IPA), typically 25% v/v, and 1.0 M methanesulfonic acid (MSA) was added as catholyte, while the anode compartment was filled with 100 mL of acid (1.0 M MSA solution). 0.25 M NaCl was added in both chambers to increase the electrolyte conductivity. The power supply was turned on after the water bath reached 60 ℃ or a specified temperature. Before the electrosynthesis reaction, the electrocatalytic cell was polarized for 10 min. Reactant guaiacol (1.13 mL – 5.65 mL) and glycerol (0.98 g – 4.90 g) and dispersed catalyst (5 wt% Pt/C, 5 g/L) were added into the catholyte and anolyte, respectively. A 5 g/L catalyst concentration with a low reactant to metal molar ratio (R/M) of 78, was used in this study to provide sufficient catalyst surface for the reaction, increasing the faradaic efficiency (F.E.) and conversion.192 A constant current of 0.4 A (j = -66 mA cm-2) was applied during the electrosynthesis reaction under constant agitation speed (600 rpm, to provide sufficient mixing without vortex formation) or stated differently. The superficial current density (j) was simply calculated from the current divided by the geometric surface area of the Pt wire and mesh (6.07 cm2). 3.3 Cyclic voltammetry experiment Cyclic voltammetry was conducted to investigate the cathodic and anodic peaks under electrosynthesis conditions. The experiment used Pt gauze and Pt wire as the working and counter electrode in an half-cell jacketed reactor with a potentiostat (CH Instruments Electrochemical Workstation, Model 1100C). To initiate the test, 20 cycles with 100 mV s-1 scan rate of CV were performed to clean the Pt electrode surface to acquire a stable voltammogram. Reactants and catalysts were added into the electrolyte solution for the cathode or anode chamber. The potential window was recorded at -1.00 to 2.00 V using 3 M KCl (Ag/AgCl) as a reference with a scan rate of 100 mV s-1. 47 3.4 Product analysis An aliquot sample (1.5 mL) of the anolyte was taken before the reaction, and a 3 mL sample of the catholyte was taken every hour during the reaction. Other samples of the anolyte and catholyte were taken immediately after the reaction for pH testing and substrate cross-over checks. The cathode samples were filtered to separate the catalyst and then extracted with n-butanol (≥ 99%, 1 mL). For the acid-containing samples, saturation with inorganic salt (NaCl) was required, and the mixture was left overnight to ensure complete separation of the organic and aqueous phases. The organic phase was then analyzed qualitatively and quantitatively by gas chromatography-mass spectrometry (GC-MS) using a Shimadzu GCMS-QP2010 with an auto-injector (AOC-20i). Separation of compounds was performed on a Stabilwax column (30 m x 0.25 mm x 25 μm), with the injector temperature set at 250°C and a split ratio of 20:1. The GC oven was programmed to start at 45°C for 5 minutes, then heated to 150°C at 5°C/min, and finally to 250°C at 15°C/min, where it was held for 5 minutes. On the anode side, 1.5 mL of glycerol electrooxidation product was filtered than analyzed using the Ultimate 3000 high-performance liquid chromatograph (HPLC) series using an Aminex HPX-87H column, 300 x 7.8 mm. The column temperature was set at 65°C with 0.3 mL/min effluent of 7 mM sulfuric acid (40%) and ultrapure water (60%). The UV-vis and refractive index (RI) detectors were utilized to detect the glycerol electrooxidation product at 210 nm and 10 Hz, respectively. All products with standard chemicals were quantitatively determined using each compound's calibration curves and response factors. 3.5 Catalyst characterization Several analyses, such as N2 physisorption, CO chemisorption, X-ray Diffraction (XRD), and transmission electron microscopy (TEM) were carried out to characterize the catalysts. The N2 physisorption analysis was conducted using a Micromeritics ASAP 2020 operated at 77 K to 48 determine Brunauer-Emmett-Teller (BET) surface areas and pore volumes of the catalysts, along with the average pore diameter by the Barrett-Joyner-Halenda (BJH) method. The catalyst was degassed at 200 °C for 2 h before the measurement. CO chemisorption was performed using a Micromeritics AutoChem II equipped with a thermal conductivity detector (TCD) using a pulsed injection of 5% CO/He at 50 °C, with prior reduction under H2 flow (50 cm3/min) at 300 °C for 1 h. The morphology of the catalyst was assessed using TEM analysis in the UBC Bioimaging Facility. The TEM micrographs were obtained on a FEI Tecnai G2 TEM apparatus equipped with 200 kV LaB6 filament. XRD was conducted in the Earth, Ocean and Atmospheric Sciences Department of the University of British Columbia. The samples were mounted as is on a top-mounting poly methyl methacrylate (PMMA) sample holder. Continuous-scan X-ray powder-diffraction data were collected over a range of 4-80° of 2θ with CoKα radiation on a Bruker D8 Advance Bragg-Brentano diffractometer equipped with a 0.6 mm (0.3°) divergence slit, incident- and diffracted-beam Soller slits and a LynxEye-XE-T detector. The long fine-focus Co X-ray tube was operated at 35 kV and 40 mA. High-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) was also conducted to analyze the morphology along with the element distributions and crystallographic information using energy dispersive X-ray analysis (EDX). The HAADF-STEM and EDX were conducted in the 4D LABS core facility at Simon Fraser University (SFU) and supported by the Canada Foundation for Innovation (CFI), British Columbia Knowledge Development Fund (BCKDF), and Pacific Economic Development Canada (PacifiCan). The sample was diluted with isopropanol ACS Plus grade (99.5%), and sonicated for two minutes. The diluted sample was cast a few droplets onto a 200 mesh Cu - lacey carbon grid (Ted Pella). The 49 FEI Tecnai Osiris (Serial number: 9921542) S/TEM system is equipped with Bruker (Serial number: NANO B-2201-0000-000 0254) SuperX EDX detectors and operated at 200 keV. 3.6 Figure of merit calculations Some parameters such as the conversion (X), normalized selectivity (S), yield (Y), reaction rate (r), carbon balance (C.B.), and faradaic efficiency (F.E.) are calculated based on the following equation. Conversion: 𝑋𝑋𝐴𝐴(%) = 𝑒𝑒𝐴𝐴,0−𝑒𝑒𝐴𝐴,𝑡𝑡𝑒𝑒𝐴𝐴,0 × 100 (3.1) Selectivity: 𝑆𝑆𝑒𝑒(%) = 𝑎𝑎𝑛𝑛×𝑒𝑒𝑛𝑛,𝑡𝑡∑𝑎𝑎𝑛𝑛×𝑒𝑒𝑛𝑛,𝑡𝑡 × 100 (3.2) Yield: 𝑌𝑌𝑒𝑒(%) = 𝑋𝑋𝐴𝐴×𝑆𝑆𝑛𝑛100 (3.3) Reaction rate: 𝑟𝑟0 = 𝑒𝑒𝐴𝐴,0−𝑒𝑒𝐴𝐴,𝑡𝑡𝑡𝑡 × 𝜑𝜑 ×𝑒𝑒𝑐𝑐 or 𝑟𝑟𝑒𝑒,0 = 𝑘𝑘𝐶𝐶𝐴𝐴0(1−𝑋𝑋𝐴𝐴)𝑡𝑡 × 𝑠𝑠𝑃𝑃𝑡𝑡 (3.4) Carbon balance: 𝐶𝐶.𝐵𝐵. (%) = ∑𝑎𝑎𝑛𝑛∙𝑒𝑒𝑛𝑛,𝑡𝑡+𝑎𝑎𝐴𝐴∙𝑒𝑒𝐴𝐴,𝑡𝑡𝑎𝑎𝐴𝐴∙𝑒𝑒𝐴𝐴,0 × 100 (3.5) Faradaic efficiency: 𝑑𝑑.𝐸𝐸. (%) = 𝑄𝑄𝑎𝑎𝑐𝑐𝑡𝑡𝑎𝑎𝑎𝑎𝑎𝑎𝑄𝑄𝑡𝑡𝑡𝑡𝑡𝑡𝑎𝑎𝑎𝑎× 100 = (∑ 𝑧𝑧𝑛𝑛𝑒𝑒𝑛𝑛,𝑡𝑡𝑁𝑁𝑖𝑖=0 )𝑛𝑛𝐼𝐼∙𝑡𝑡× 100 (3.6) Total F.E.: 𝑅𝑅𝑟𝑟𝑟𝑟𝑎𝑎𝑙𝑙 𝑑𝑑.𝐸𝐸. (%) = 𝑑𝑑.𝐸𝐸. 𝑐𝑐𝑎𝑎𝑟𝑟ℎ𝑟𝑟𝑑𝑑𝑒𝑒 + 𝑑𝑑.𝐸𝐸.𝑎𝑎𝑛𝑛𝑟𝑟𝑑𝑑𝑒𝑒 (3.7) Voltage efficiency: 𝑉𝑉.𝐸𝐸. (%) = 𝐸𝐸𝑎𝑎𝑛𝑛𝑡𝑡𝑅𝑅𝑒𝑒−𝐸𝐸𝑐𝑐𝑎𝑎𝑡𝑡ℎ𝑡𝑡𝑅𝑅𝑒𝑒𝐸𝐸𝑐𝑐𝑒𝑒𝑎𝑎𝑎𝑎 × 100 (3.8) E (RHE) 𝐸𝐸(𝑅𝑅𝐻𝐻𝐸𝐸) = 𝐸𝐸(𝐴𝐴𝐴𝐴/𝐴𝐴𝐴𝐴𝐶𝐶𝑙𝑙) + 𝐸𝐸0(𝐴𝐴𝐴𝐴/𝐴𝐴𝐴𝐴𝐶𝐶𝑙𝑙) + 0.059 × 𝑝𝑝𝐻𝐻 (3.9) Required energy (per moles of converted reactant): 𝑀𝑀𝐸𝐸 �𝑀𝑀ℎ 𝑚𝑚𝑟𝑟𝑙𝑙 𝐶𝐶𝐴𝐴−1� = 𝐼𝐼×𝐸𝐸𝑐𝑐𝑒𝑒𝑙𝑙𝑙𝑙×𝑟𝑟�𝑛𝑛𝐴𝐴,0−𝑛𝑛𝐴𝐴,𝑟𝑟�𝐴𝐴𝑔𝑔𝑎𝑎𝑟𝑟𝑎𝑎𝑐𝑐𝑟𝑟𝑙𝑙+�𝑛𝑛𝐴𝐴,0−𝑛𝑛𝐴𝐴,𝑟𝑟�𝐴𝐴𝑙𝑙𝑔𝑔𝑐𝑐𝑒𝑒𝑟𝑟𝑟𝑟𝑙𝑙 = 𝐼𝐼×𝐸𝐸𝑐𝑐𝑒𝑒𝑙𝑙𝑙𝑙×𝑟𝑟×V(𝐶𝐶𝐴𝐴0𝑋𝑋𝐴𝐴)𝐴𝐴𝑔𝑔𝑎𝑎𝑟𝑟𝑎𝑎𝑐𝑐𝑟𝑟𝑙𝑙+(𝐶𝐶𝐴𝐴0𝑋𝑋𝐴𝐴)𝐴𝐴𝑙𝑙𝑔𝑔𝑐𝑐𝑒𝑒𝑟𝑟𝑟𝑟𝑙𝑙 (3.10) Turnover frequency: TOF (s-1) = 𝑒𝑒𝐴𝐴,0−𝑒𝑒𝐴𝐴,11 ×3600 × 𝜑𝜑 × 𝑚𝑚𝑃𝑃𝑡𝑡𝑀𝑀𝑀𝑀𝑃𝑃𝑡𝑡 (3.11) Nomenclature: 𝑎𝑎𝑒𝑒 : The number of carbon atoms in each product (unit) 𝑛𝑛𝑐𝑐 : Moles of supported catalyst (mol) 50 𝐶𝐶𝐴𝐴0 : Initial reactant concentration, guaiacol or glycerol (mM) 𝑛𝑛𝑒𝑒,𝑡𝑡 : Moles of chemicals product (mol) at specified time 𝐸𝐸 : Reported potential (V vs. RHE or Ag/AgCl) 𝑃𝑃 : Required energy per converted reactant (𝑀𝑀ℎ 𝑚𝑚𝑟𝑟𝑙𝑙 𝐶𝐶𝐴𝐴−1) 𝐸𝐸0 : 0.197 V (Ag/AgCl (3 M KCl) vs. NHE) 𝑘𝑘 : Rate constant of reactant, guaiacol of glycerol (h-1 or mM h-1) 𝐸𝐸𝑐𝑐𝑤𝑤𝑚𝑚𝑚𝑚 : Applied cell potential (V) 𝑆𝑆𝑒𝑒 : Product selectivity at specified time (%) 𝑑𝑑 : Faradaic constant (96,500 C/mol e) t : Reaction time (h) 𝐼𝐼 : Applied current (A) V : Electrolyte volume (L) 𝑚𝑚𝑃𝑃𝑡𝑡 : Amount of platinum (gram) 𝑋𝑋𝐴𝐴 : Conversion at specified time, guaiacol or glycerol (%) 𝑀𝑀𝑀𝑀𝑃𝑃𝑡𝑡 : Molecular weight of platinum (195.084 g/mol) 𝑌𝑌𝑒𝑒 : Product yield at specified time (%) 𝑛𝑛𝐴𝐴,0 : Initial moles of reactant, guaiacol or glycerol (mol) 𝑧𝑧𝑒𝑒 : Involved electron on the reaction (mol e) 𝑛𝑛𝐴𝐴,𝑡𝑡 : Moles of reactant at specified time, guaiacol or glycerol (mol) φ : Metal dispersion (%) 3.7 Least significant difference analysis The statistical comparison using Fisher’s least significant difference (LSD) analysis helps the readers to evaluate the experimental data easily. It is a well-known two-step approach to compare the expected means of treatment groups. Letter symbols such as a, ab, b, bc, c, d, and e, are used to indicate the significance of within-group differences between treatments among the experimental data, as determined by one-way ANOVA followed by post-hoc tests. A post-hoc test was conducted to identify exactly which groups differ from each other following a significant result in an ANOVA. This technique can be used only if the ANOVA F omnibus or P-value is significant (P < 0.05).193 Therefore, these tests are also known as multiple comparison tests. 𝐿𝐿𝑆𝑆𝐷𝐷 = 𝑟𝑟(𝛼𝛼,𝑑𝑑𝑑𝑑)�𝑀𝑀𝑆𝑆 �1𝑁𝑁+ 1𝑁𝑁� (3.12) One-way ANNOVA analysis was conducted by Microsoft Excel software resulting in the dF, N, and MS. Meanwhile, t denotes the t-value from the 𝑟𝑟-distribution, based on the desired level of significance (𝛼𝛼) and degrees of freedom (dF). MS is the mean square error and N is the sample size for the individual treatment levels. The obtained value was then used to calculate LSD in Equation 3.12 by initially calculating the two-tailed Student's t-distribution (t). N is the number of 51 data points used in the ANOVA calculation. The calculated LSD was used to categorize the group number, starting with the highest number. 3.8 Design of experiments and factorial analysis in response surface methodology The dependent variables in this research were: XA (guaiacol conversion, %), YB (phenol yield, %), YC (cyclohexanone yield, %), YD (cyclohexanol yield, %), YE (anisole yield, %), YF (cyclohexane yield, %), YG (2-metoxycyclohexanone yield, %), YH (2-metoxycyclohexanol yield, %), YI (methoxycyclohexane yield, %), YJ (methanol yield, %), Y1 (required-energy, Wh), and Y2 (H+ concentration, M). The analysis was conducted using response surface methodology (RSM), using Minitab 19. In the RSM, the non-linear polynomial equation was generated as follows:194 𝑌𝑌 = 𝛽𝛽0 + ∑ 𝛽𝛽𝑤𝑤3𝑤𝑤=1 𝑥𝑥𝑤𝑤 + ∑ 𝛽𝛽𝑤𝑤𝑤𝑤3𝑤𝑤=1 𝑥𝑥𝑤𝑤𝑤𝑤2 + ∑ 𝛽𝛽𝑤𝑤𝑖𝑖31<𝑤𝑤<𝑖𝑖 𝑥𝑥𝑤𝑤𝑥𝑥𝑖𝑖 (3.13) where Y denotes the dependent variable, 𝛽𝛽0 and βI represent the factors of the main (linear) effect, βii is the curvature effect, and βij is the interaction effect, meanwhile, x represents the independent variables (IPA content, MSA concentration, and time, Table 3.2). Table 3.2. Experimental matrix design for ECH of guaiacol with isopropyl alcohol variations. Independent variable Range of level -1 -0.6 0 1 2 2.6 MSA concentration, M (X1) - 0.2 0.5 1.0 - - IPA content, % v/v (X2) 0 10 25 50 75 90 Time, h (X3) 1 - 2 3 4 - The experimental design matrix and levels of independent variables are shown in Table 3.2. A wide concentration range (e.g., from 0 to 90% v/v for IPA) was selected to improve the validity of the response surface methodology. The exploration of unequal level ranges for both IPA content and MSA concentration was undertaken to provide a more nuanced understanding of how these independent variables influence the dependent variables. 52 3.9 The van Krevelen analysis The van Krevelen plot diagram was calculated by: 𝐻𝐻/𝐶𝐶 𝑟𝑟𝑎𝑎𝑟𝑟𝑟𝑟𝑟𝑟 = 𝑒𝑒𝐴𝐴×𝐻𝐻/𝐶𝐶𝐴𝐴+𝑒𝑒𝐵𝐵×𝐻𝐻/𝐶𝐶𝐵𝐵+⋯+𝑒𝑒𝐽𝐽×𝐻𝐻/𝐶𝐶𝐽𝐽�𝑒𝑒𝐴𝐴+𝑒𝑒𝐵𝐵+⋯+𝑒𝑒𝐽𝐽� (3.14) 𝑂𝑂/𝐶𝐶 𝑟𝑟𝑎𝑎𝑟𝑟𝑟𝑟𝑟𝑟 = 𝑒𝑒𝐴𝐴×𝑂𝑂/𝐶𝐶𝐴𝐴+𝑒𝑒𝐵𝐵×𝑂𝑂/𝐶𝐶𝐵𝐵+⋯+𝑒𝑒𝐽𝐽×𝑂𝑂/𝐶𝐶𝐽𝐽�𝑒𝑒𝐴𝐴+𝑒𝑒𝐵𝐵+⋯+𝑒𝑒𝐽𝐽� (3.15) where n is the number of moles of a chemical component in the ECH product mixture, H/C is the ratio of H-to-C atoms in the chemical formula of a given component in the product mixture, and O/C is the ratio of O-to-C atoms in the chemical formula of a given component in the product mixture. The application of van Krevelen has been applied in coal characterization, denoting the diverse materials with relevant properties such as biomass, biodegradable waste, various chemicals, both pre and post reactions.195 The product mole distribution was obtained from the experiment, while the chemical formulae H/C and O/C ratios are shown in Table 3.3. Table 3.3. The H/C and O/C ratios of involved chemicals related to guaiacol ECH-DO. Chemicals Symbol C H O H/C O/C Guaiacol A 7 8 2 1.14 0.29 Phenol B 6 6 1 1.00 0.17 Cyclohexanone C 6 10 1 1.67 0.17 Cyclohexanol D 6 12 1 2.00 0.17 Anisole E 7 8 1 1.14 0.14 Cyclohexane F 6 12 - 2.00 0.00 2-metoxycyclohexanone G 7 12 1 1.71 0.14 2-metoxycyclohexanol H 7 14 1 2.00 0.14 methoxycyclohexane I 7 14 1 2.00 0.14 methanol J 1 4 1 4.00 1.00 Due to the many intermediate chemicals in the GECH-DO in the cathode chamber, this study introduced simple product distribution analysis, such as the van Krevelen analysis. In the GECH-DO, the reaction decreases the O/C ratio (deoxygenation) while increasing the H/C ratio (hydrogenation). 53 Chapter 4: Potential Anodic Coupling Electrosynthesis Selection Acknowledging the drawbacks of electrocatalytic hydrogenation-hydrodeoxygenation (ECH-HDO) in a half-cell system, the potential anode coupling electrosyntheses (ACEs) for process intensification on the ECH of lignin-derived bio-oil by coupling the readily-available anode chamber as an oxidation reactor, is investigated in this chapter. The integration of ACE optimizes the reactor efficiency and aligns with green chemistry principles by reducing fossil fuel dependency, while minimizing environmental impact. Hence, the selection of the prospective ACE should consider primarily several factors, including (i) the extensive application of renewable feedstock, (ii) low-carbon economy consideration, and (iii) ensuring resource efficiency. This consideration could be achieved through the electrosynthesis process using by-products and low-value materials from renewable resources. On the other hand, the selection of ACEs to support the lignin bio-oil ECH-HDO should consider several additional factors: (i) existing design compatibility, which consists of the electrode material, electrolyte, temperature, current density, and membrane application; (ii) thermodynamic consideration, to ensure a compatible standard potential to reduce energy consumption and the Gibbs free energy; (iii) reaction mechanisms such as the stoichiometric electron flow, intermediate stability, and mechanistic understanding; (iv) the separation and purification of the product, solvent recycle and by-product management; (v) economic considerations such as cost of materials, energy efficiency, and scale-up feasibility; and (vi) environment and safety considerations. 54 The paired selection of lignin bio-oil ECH-HDO is summarized in Figure 4.1, with a pre-selection process for the raw material origin and the reactor configuration, before the MOO analysis was conducted. The evaluation was carried out in three steps. Figure 4.1. The paired electrosynthesis selection to be coupled with lignin bio-oil ECH-HDO. First, the preliminary selection was evaluated by rapidly screening the raw material origin (sustainability) followed by the reaction mechanism screening. The reactor configuration was also investigated before conducting the second step. In the second step, the weighted sum method on the MOO was carried out, which must align with the product intensification purposes. Lastly, a 55 sensitivity analysis assessed the weight factor affecting the optimization outcome. Therefore, the robustness of the chosen weights and the solution's sensitivity to these weights were evaluated. 4.1 Potential ACEs Table 4.1 summarizes the inorganic ACEs with the possibility of organic and inorganic process production in one simultaneous system, with chlor-alkali as the pivotal process. Moreover, the chlor-alkali and aluminum industries are the most applied electrolysis industries, with the highest tonnage and electricity used, respectively.50 Due to the high temperature (1000 °C) and current density (1000 mA cm-2), aluminum production is excluded from this study. Moreover, the application of chlor-alkali products is responsible for 50% of all chemical industrial processes.196-199 While the chlor-alkali industry is essential for producing chemicals vital to numerous other industries, its high energy consumption and potential for polluting emissions make it a target for sustainability improvements and cleaner technologies. Thus, the simultaneous production process of Cl2 gas and lignin bio-oil ECH-HDO would be an ideal solution to balance the energy consumption (~2500 kWH/ton Cl2) of the chlor-alkali industry. Figure 4.2. The end use of chlorine gas and sodium hydroxide. 56 Moreover, caustic soda production which has economic viability in chlor-alkali industry might disrupt the pairing system of GECH-DO. The alkaline condition in the cathode process has a low kinetics for lignin bio-oil ECH-DO. Due to environmental and safety considerations, the chlor-alkali industry development is exacerbated by more popular non-chlorinating bleaches such as H2O2, O3, and persulphate.200 In an electrolysis system with organic chlorides as reactants, OH attack releases chloride ions, which can be further oxidized to hypochlorous acid (HOCl) or chloride ions (Cl−), thus promoting the decomposition of pollutants201, 202 as HOCl and Cl− are highly corrosive. They can aggressively attack the materials used, which leads to material degradation, reduced efficiency, and contamination. Some of the end-use industries of chlorine gas and caustic soda are shown in Figure 4.2. Since the inorganic ACEs have the potential to produce harsh inorganic oxidants, which lead to environmental and safety concerns, the organic ACEs offer alternatives to address this challenge. Moreover, the bio-oil electrocatalytic hydrogenation-hydrodeoxygenation process in the cathode chamber also utilizes an organic synthesis, which induces simplicity in the process of storage and purification. As a renewable resource to produce platform chemicals203-207, biomass processing is commonly called the biochemical, biorefinery, or bio-based chemicals industry. However, only a few studies have been successfully commercialized, even though extensive research has been conducted over a long period. Consequently, only a few processes have progressed beyond the research and development phase to be implemented on a full industrial scale or a smaller pilot scale. Tables 4.2 and 4.3 detail these commercialized processes at the industrial and pilot scales. The main obstacles to widespread commercialization are technological maturity and economic considerations. On the other hand, some syntheses have no issues with technological aspects. 57 Table 4.1. The inorganic industrial electrosynthesis in anode chamber with standard hydrogen potential.50 Chemical Reaction 𝑬𝑬𝟎𝟎 (V) Anode (A) | Cathode (C) Conditions Chlorine/ Caustic soda 2 𝑁𝑁𝑎𝑎𝐶𝐶𝑙𝑙 (𝑎𝑎𝑎𝑎) + 2 𝐻𝐻2𝑂𝑂 → 𝐶𝐶𝑙𝑙2(𝐴𝐴) + 𝐻𝐻2(𝐴𝐴) + 2 𝑁𝑁𝑎𝑎𝑂𝑂𝐻𝐻 1.36 Noble Metal Oxide/Ti | Steel or Hg Aqueous NaCl; asbestos or ion-exchange membrane; 80-95 ºC; 200-1000 mA cm–2 Chloride 2 𝐶𝐶𝑙𝑙− → 𝐶𝐶𝑙𝑙2 + 2𝑒𝑒− 2 𝐻𝐻2𝑂𝑂 + 2𝑒𝑒− → 2 𝑂𝑂𝐻𝐻− + 𝐻𝐻2 2 𝐻𝐻𝑂𝑂𝐶𝐶𝑙𝑙 + 𝑂𝑂𝐶𝐶𝑙𝑙− → 𝐶𝐶𝑙𝑙𝑂𝑂3− + 2 𝐶𝐶𝑙𝑙− + 2𝐻𝐻+ 1.36 Noble Metal Oxide/Ti | Steel Aqueous NaCl (310 g L-1); Na2Cr2O7 (1-6 g.L-1); 60-80 ºC; 150-400 mA cm–2 Perchlorate 𝑁𝑁𝑎𝑎+,𝑁𝑁𝐻𝐻4+ 𝐶𝐶𝑙𝑙𝑂𝑂3− + 𝐻𝐻2𝑂𝑂 → 𝐶𝐶𝑙𝑙𝑂𝑂4− + 2𝐻𝐻+ + 2𝑒𝑒− 1.19 Pt/Ti, PbO2/Graphite | Steel or Ni Aqueous ClO3− ; Na2Cr2O7; 35-50 ºC; 150-500 mA cm–2 Persulphate 𝑁𝑁𝑎𝑎+,𝑁𝑁𝐻𝐻4+,𝐻𝐻+ 2𝑆𝑆𝑂𝑂42− → 𝑆𝑆2𝑂𝑂82− + 2𝑒𝑒− 2.01 Pt or Pt/Ti (A) Conc. H2SO4, cold; 500-1000 mA cm–2; divided cell Permanganate 𝐾𝐾+ 𝑀𝑀𝑛𝑛𝑂𝑂42− → 𝑀𝑀𝑛𝑛𝑂𝑂4− + 𝑒𝑒− 0.54 Ni, Monel | Fe or Steel K2MnO4 (25-100 g.L-1) 1-4 mol L-1 aq. KOH at 60 ºC; 5-150 mA cm–2; undivided cell Fluorine 2 𝑑𝑑− → 𝑑𝑑2 + 2𝑒𝑒− 2.87 Carbon | Mild steel KF-2HF (eutectic) at 82 ºC; 13 mA cm–2; undivided cell (skirt) Manganese Dioxide 𝑀𝑀𝑛𝑛2+ + 2 𝐻𝐻2𝑂𝑂 → 𝑀𝑀𝑛𝑛𝑂𝑂2 + 4𝐻𝐻+ + 2𝑒𝑒− 1.23 Graphite, Pb or Ti (A) MnSO4 (0.5-1.2 mol L-1) in aq. H2SO4 at 90-100 ºC; 7-12 mA cm–2; undivided cell Ozone 𝑂𝑂2 + 𝐻𝐻2𝑂𝑂 → 𝑂𝑂3 + 2𝐻𝐻+ + 2𝑒𝑒− 2.07 Vitreous carbon (A) Conc. aq. HBF4, cold; 500 mA; divided cell 58 Table 4.2. The organic industrial electrosynthesis in anode chamber with standard hydrogen potential. Reactant | Product Reaction 𝑬𝑬𝟎𝟎 (V)* Anode (A) | Cathode (C) Conditions Ref. Anthracene | Anthraquinone 𝐶𝐶14𝐻𝐻10 + 2𝐻𝐻2𝑂𝑂 → 𝐶𝐶14𝐻𝐻8𝑂𝑂2 + 6𝐻𝐻+ + 6𝑒𝑒− 1.03 Pt | Pt H2SO4-CHCl3-V4+; 30 ºC; 3-5 mA cm–2; undivided cell 208 p-t-Butyltoluene | p-t-Butyl benzaldehyde 𝐶𝐶11𝐻𝐻16 + 𝐻𝐻2𝑂𝑂 → 𝐶𝐶11𝐻𝐻14𝑂𝑂 + 4𝐻𝐻+ + 4𝑒𝑒− 0.71 PbO2 on lead alloy | Metcote ES1 1.0 M H2SO4; 33 – 70 ºC; 468-900 mA cm–2; Undivided cell 209 Glucose | Calcium gluconate 𝐶𝐶6𝐻𝐻12𝑂𝑂6 + 𝐻𝐻2𝑂𝑂 → 𝐶𝐶6𝐻𝐻12𝑂𝑂7 + 2𝐻𝐻+ + 2𝑒𝑒− 𝐶𝐶6𝐻𝐻12𝑂𝑂7 + 𝐶𝐶𝑎𝑎𝐶𝐶𝑂𝑂3 → (𝐶𝐶6𝐻𝐻12𝑂𝑂7)2𝐶𝐶𝑎𝑎 + 𝐶𝐶𝑂𝑂2 + 𝐻𝐻2𝑂𝑂 -0.04 Graphite | Graphite NaBr-CaCO3; 30 ºC; 10-20 mA cm–2; Undivided cell 210 Lactose | Calcium lactobionate 𝐶𝐶12𝐻𝐻22𝑂𝑂11 + 𝐻𝐻2𝑂𝑂 → 𝐶𝐶12𝐻𝐻22𝑂𝑂12 + 2𝐻𝐻+ + 2𝑒𝑒− 𝐶𝐶12𝐻𝐻22𝑂𝑂12 + 𝐶𝐶𝑎𝑎𝐶𝐶𝑂𝑂3 → (𝐶𝐶12𝐻𝐻22𝑂𝑂12)2𝐶𝐶𝑎𝑎 + 𝐶𝐶𝑂𝑂2 + 𝐻𝐻2𝑂𝑂 -0.12 Gold/Pt/Pb | Pt-IrO2 sheet Na2CO3-NaHCO3; Nafion® 423 211 Diacetone-L-sorbose | Diacetone-2-keto-L-gulonic acid 𝐶𝐶12𝐻𝐻20𝑂𝑂6 + 𝐻𝐻2𝑂𝑂 → 𝐶𝐶12𝐻𝐻18𝑂𝑂7 + 4𝐻𝐻+ + 4𝑒𝑒− 0.16 nickel oxide or foam (A) Nickel salt-KOH; 50-70 ºC; 120– 470 mA cm–2 212, 213 Monomethyladipate | Dimethyl sebacate 2𝐶𝐶7𝐻𝐻12𝑂𝑂4 + 4𝐻𝐻2𝑂𝑂 → 𝐶𝐶12𝐻𝐻22𝑂𝑂4 + 2𝐶𝐶𝑂𝑂2 + 10𝐻𝐻+ + 10𝑒𝑒− 1.57 Pt | Pt KCl-Methanol; RT 214 Glucose | Gluconic acid 𝐶𝐶6𝐻𝐻12𝑂𝑂6 + 𝐻𝐻2𝑂𝑂 → 𝐶𝐶6𝐻𝐻12𝑂𝑂7 + 2𝐻𝐻+ + 2𝑒𝑒− -0.04 Graphite | Graphite NaBr; 30 ºC; 10-20 mA cm–2; Undivided cell 210, 215-217 Hexafluoropropylene | Hexafluoro propylene oxide 𝐶𝐶3𝑑𝑑6 + 𝐻𝐻2𝑂𝑂 → 𝐶𝐶3𝑑𝑑6𝑂𝑂 + 2𝐻𝐻+ + 2𝑒𝑒− 0.67 Boron Doped Diamond | Pt sheet Na2SO4 (pH = 7); 25 ºC; Undivided cell 218, 219 p-Methoxytoluene | p-Anisaldehyde 𝐶𝐶8𝐻𝐻10𝑂𝑂 + 3𝐻𝐻2𝑂𝑂 → 𝐶𝐶8𝐻𝐻8𝑂𝑂2 + 𝐶𝐶𝑂𝑂2 + 8𝐻𝐻+ + 8𝑒𝑒− 1.13 Pt | Pt/Pb 0.3 M Ce(III) in 1 M H2SO4; 50-192.3 mA cm–2; 36-50 ºC; Nafion® 423 220 Galacturonic acid | Mucic acid 𝐶𝐶6𝐻𝐻12𝑂𝑂7 + 𝐻𝐻2𝑂𝑂 → 𝐶𝐶6𝐻𝐻12𝑂𝑂8 + 2𝐻𝐻+ + 2𝑒𝑒− -0.04 Pt-Ti NaBr-HBr; 30-50 ºC; 10-150 mA cm–2; polyethylene membrane 221-223 *Calculated based Joback method from ChemdDraw 18.1© data59 However, process economics are not viable because of the high costs associated with implementation, operation, and maintenance, which make the process presented in Table 4.3 financially impractical to adopt on a large scale. Moreover, the choice of reactants in these processes (Table 4.3) includes a wide range of renewable and non-renewable feedstocks. This diversity in feedstock sources is an essential factor in the selection process for ACE's applications. Renewable feedstocks, such as biomass or bio-based materials, offer sustainability benefits and align with environmental goals by reducing reliance on finite resources and minimizing carbon footprints. Table 4.3. The organic electrosynthesis in pilot plant scale. Reactant | Product Reaction 𝑬𝑬𝟎𝟎 (V)* Naphthalene | 1-Acetoxynaphthalene 𝐶𝐶10𝐻𝐻8 + 𝐶𝐶𝐻𝐻3𝐶𝐶𝑂𝑂𝑂𝑂𝐻𝐻 → 𝐶𝐶12𝐻𝐻10𝑂𝑂2 + 2𝐻𝐻+ + 2𝑒𝑒− 0.39 Naphthalene, butadiene | Naphthaquinone 𝐶𝐶10𝐻𝐻8 + 2𝐻𝐻2𝑂𝑂 + 𝐶𝐶4𝐻𝐻6 → 𝐶𝐶14𝐻𝐻8𝑂𝑂2 + 10𝐻𝐻+ + 10𝑒𝑒− 0.18 Dimethyl maleate | 1,2,3,4-Butanetetracarboxylic acid 𝐶𝐶6𝐻𝐻8𝐻𝐻4 + 2𝐻𝐻2𝑂𝑂 + 2𝐶𝐶𝐻𝐻3𝑂𝑂𝐻𝐻 → 𝐶𝐶8𝐻𝐻10𝑂𝑂8 + 8𝐻𝐻+ + 8𝑒𝑒− 0.01 p-Aminophenol | Nitrobenzene 𝐶𝐶6𝐻𝐻7𝑁𝑁𝑂𝑂 + 𝐻𝐻2𝑂𝑂 → 𝐶𝐶6𝐻𝐻5𝑁𝑁𝑂𝑂 + 4𝐻𝐻+ + 4𝑒𝑒− 0.93 1-Nitro naphthalene | 5-Nitronaphthoquinone 𝐶𝐶10𝐻𝐻7𝑁𝑁𝑂𝑂2 + 2𝐻𝐻2𝑂𝑂 → 𝐶𝐶10𝐻𝐻5𝑂𝑂4 + 4𝐻𝐻+ + 4𝑒𝑒− 0.22 Propargyl alcohol | Propiolic acid 𝐶𝐶3𝐻𝐻4𝑂𝑂 + 𝐻𝐻2𝑂𝑂 → 𝐶𝐶3𝐻𝐻2𝑂𝑂2 + 4𝐻𝐻+ + 4𝑒𝑒− 0.08 Propylene | Propylene oxide 𝐶𝐶3𝐻𝐻6 + 𝐻𝐻2𝑂𝑂 → 𝐶𝐶3𝐻𝐻6𝑂𝑂 + 2𝐻𝐻+ + 2𝑒𝑒− 0.64 *Calculated based Joback method from ChemdDraw 18.1© data In contrast, fossil fuels, the non-renewable feedstocks, often offer advantages regarding technology maturity, established infrastructure, and economic factors. Undoubtedly, the environmental and sustainability aspects are of long-term significance. Thus, the first step of ACE screening should consider the appropriate feedstock, considering the availability, economic viability, and environmental implications of each type of reactant. The study ensures that the chosen feedstock, organic and inorganic, is derived from renewable and sustainable sources that align with the objective and interest of this research. 60 Figure 4.3. The potential ACE reactions with the number of electrons involved of the study. Note: renewable feedstock (green dash), inorganic chemicals (blue dash), fossil fuel feedstock (red dash), chemical reaction (solid arrow), fermentation (dash arrow), and Electrooxidation reaction (orange arrow). 61 Besides the previously mentioned ACE at the industrial and pilot plant scale, several promising organic electrooxidation processes have attracted significant interest among scholars. A comprehensive summary of organic and inorganic ACE processes is presented in Figure 4.3. As shown in Figure 4.3, each conceivable ACE was assigned a number code for reactant or product for ease of reference, for example NaCl (1) produced Cl2 (1a) and methanol (3) and formic acid (3a). This study favors renewable feedstocks primarily because they meet the specific requirements of the cathode electrocatalytic hydrogenation process used for converting lignin bio-oil compounds 4.2 ACEs selection for MOO analysis Methanol (3), a key commodity chemical, can be produced as a by-product of bio-oil electrochemical hydrogenation (ECH) to generate formic acid (3a). Methanol oxidation proceeds further, producing carbon dioxide and releasing protons, which are crucial for the hydrogenation process in the cathode chamber. Meanwhile, cyclohexanol (4), one of the intermediate products of syringol or guaiacol ECH, could be re-oxidized in an anode chamber to produce cyclohexanone (4a). One of the lignocellulose biomass-derived chemicals, galacturonic acid (5), is extracted from pectin-rich biomass such as citrus peels, apple pomace, sugar beet pulp, and mango peels. The abundance, low value, and sustainability aspects offer a significant advantage for this raw material. Furthermore, galacturonic acid is oxidized into mucic acid (5a) to obtain more beneficial chemicals, and it is known for its versatile applications in the chemical, pharmaceutical, cosmetic, and food sectors.224 For instance, the application of mucic acid derivatives in pharmaceuticals is being explored for drug development and as chelating agent. Moreover, as an intermediate chemical, mucic acid produces various specialty and fine chemicals such as 2,3,4,5-tetra-O-acetylgalactaric acid (AGA), which is used for the synthesis of copolyanhydrides, muconic acid, 62 and adipic acid synthesis as a monomer unit along with adipic acid.225, 226 Mucic acid is also a precursor for producing bio-based building blocks such as pyrone227, 228 and polyethylene furanoate229, 230. Mucic acid is also used in the food industry as a pH adjuster or an additive precursor.231 The electrosynthesis of mucic acid has been successfully implemented on an industrial scale,221 proving economically viable and supporting the development and implementation of sustainable and environmentally friendly products and processes. Furfural (6), and 5-hydroxymethylfurfural/HMF (7) from dehydration and cyclization of d-xylose and d-galactose, respectively, serve as promising feedstocks for producing valuable chemicals from lignocellulose biomass. As furfural oxidation produces 2-furoic acid and the anode chamber could be employed to serve the reaction. Meanwhile, HMF generates various compounds in the oxidation reaction, such as 2,5-diformylfuran (7a) and different forms of 5-hydroxymethyl-2-furan-carboxylic acid (7b/7c). The reactions might proceed through the acceptorless dehydrogenation (AD)232-234 or a two-electron (2e-) oxidation pathway. The reaction pathway determines the hydrogen adsorbed onto the surface of the catalyst, forming hydrogen or oxidizing into water. The wide applications of cellulose, composed of d-glucose (8) units, for both direct and indirect oxidation, are beneficial to support a sustainable and renewable chemical platform. The direct oxidation of d-glucose generates glucaric acid (8a) and gluconic acid (8b). D-glucose requires the oxidation of multiple hydroxyl groups to produce glucaric acid. Gluconic acid, on the other hand, is yielded by the oxidation of the aldehyde group in the glucose. The products significantly impact industrial applications, including pharmaceuticals and biodegradable polymers. In addition, d-glucose is indirectly used as a feedstock through fermentation processes to produce intermediate products, such as ethanol (9), isopropanol (10), 1,3-propanediol (11), and 1,2-propanediol (12). 63 Ethanol oxidation produces either acetaldehyde (9a) or acetic acid (9b) and has been validated in the electrosynthesis process. Meanwhile, isopropanol (10) electrooxidation generates acetone (10a). Even glycerol (13) is possibly converted into 1,3-propanediol (11) and 1,2-propanediol (12) through fermentation and chemical processes, respectively. The glycerol fermentation by Clostridia and Enterobacteriaceae microorganisms produces 1,3-propanediol.235 Meanwhile, the hydrogenolysis of glycerol can produce 1,2-propanediol.236, 237 The oxidation of 1,3-propanediol and 1,2-propanediol yield acrylic acid (11a) and lactic acid (11b), respectively. In addition, glycerol oxidation generates valuable compounds such as glyceraldehyde (13a), dihydroxyacetone (13b), glyceric acid (13c), and tartronic acid (13d). 4.3 Multi-objective Optimization calculation of ACEs Optimizing industrial process systems often requires changes to the design to improve performance, reduce costs, increase profitability, and ensure safety and reliability. In particular, optimizing chemical processes is a complex task due to the systems' nonlinear and large-scale nature.238 The processes frequently contain many constraints, equality and inequality, and a mix of continuous and discrete decision process variables. To date, chemical process optimization research has focused on theoretical and algorithmic advances to develop reliable and efficient strategies to solve critical and challenging problems in this field. Most research activities focus on optimization problems with a single objective function, such as process economics, yield, or efficiency. However, real-world optimization problems in chemical engineering typically involve multiple objectives, such as economics, performance, and reliability that must be optimized at the same time. These objectives often conflict with each other, partially or entirely, across the range of possible solutions. MOO is important for determining the optimal solution when trade-offs exist 64 between two or more conflicting objectives. MOO is broadly applied in process design and control, chemical reaction, refinery, petrochemicals, polymerization, power generation, renewable energy, hydrogen production, and fuel cells.238 MOO analysis is an ideal solution to be implemented in the ACEs evaluation with the bio-oil ECH-HDO process due to many optimization goals such as economic, environment, energy, safety, technology readiness level, and separation process. On the other hand, MOO analysis is relatively straightforward in assessing a complicated system where expert opinion plays an important role. The weighted sum method is the most commonly used approach in the MOO problem as an accessible and applicable approach.208 𝐴𝐴𝑤𝑤𝑤𝑤𝑤𝑤𝑤𝑤ℎ𝑡𝑡𝑤𝑤𝑡𝑡 𝑠𝑠𝑠𝑠𝑠𝑠 = 𝑎𝑎1𝐴𝐴1 + 𝑎𝑎2𝐴𝐴2 + 𝑎𝑎3𝐴𝐴3 + ⋯+ 𝑎𝑎𝑦𝑦𝐴𝐴𝑦𝑦 (4.1) where the 𝐴𝐴𝑤𝑤𝑤𝑤𝑤𝑤𝑤𝑤ℎ𝑡𝑡𝑤𝑤𝑡𝑡 𝑠𝑠𝑠𝑠𝑠𝑠 is the overall objective function while 𝑎𝑎𝑦𝑦 and 𝐴𝐴𝑦𝑦 are the weighting factor and the individual objective function, respectively. Moreover, the approach assigns weight to distinguish the relative importance of each objective in the aggregation of the overall objective function. As a straightforward method, the weighted sum factor in MOO requires decision-makers (DM) who can provide references and are sufficiently familiar with the problem. The weighting factor and consideration is presented in Table 4.4. Six key variables are considered in evaluating the selection of ACEs: Economic, environment, energy, safety, technology readiness level (TRL), and separation technology. As outlined in Table 4.4, economic factors are the most critical aspect of system development. Ensuring profitability makes it possible to attract investment to develop any system. Consequently, the economic criterion was assigned to the highest scale, 0.25, in the MOO analysis of ACEs. On the other hand, the environmental aspect has become vital in contemporary research and in the development of 65 such technologies or processes. Thus, the environmental aspect has the second highest scale in the MOO study analysis, 0.20. Other factors such as energy efficiency, safety, and TRL are crucial considerations, each receiving an equal weight of 0.15. Finally, while important, separation technology is given a slightly lower emphasis with a weighting of 0.10 in the MOO analysis. Table 4.4. The multi-objective optimization weighting scale. No Factor Judgment scale 1 Economic • Regardless of how innovative a system is, it may only be worthwhile pursuing if it is economically viable. • Securing that their investments yield the highest returns • Ensure that the developed product or technology can compete in the market. • Attracting the funds and investment required to move the project forward. 0.25 2 Environment • Emphasizing sustainable practices to reduce carbon footprints. • Complying with regulations that are stringent across industries. • Sustainable practices promote long-term environmental stewardship. • Increasingly prioritizing eco-friendly products and services of consumers and businesses. • Improving a company's reputation and setting it apart from competitors. 0.20 3 Energy • Highest ongoing costs in any process or product lifecycle. • Directly related to environmental impact, especially when it comes from nonrenewable sources such as fossil fuels. • As global resources become scarcer and energy costs fluctuate, developing energy-efficient processes and products is critical to long-term sustainability. 0.15 4 Safety • Ensuring human health and well-being. • Safety regulations. • The company responsible may be held liable for any resulting damage. • Ensuring safety during development is also essential and costly in protecting the workforce involved in product manufacturing, testing, and use. 0.15 5 TRL • Indicating how matures technology is allows organizations to assess the feasibility and risks of further development. • Guiding investment decisions, both within a company and among external investors. • Contributing to the planning and road-mapping of the development process • TRL assists in identifying specific gaps and challenges that must be addressed to advance technology to the next level. 0.15 6 Separation technology • Effective separation technologies significantly improve process efficiency and performance. • Costly, especially when dealing with large quantities or complex mixtures. • Essential to maintaining the quality and consistency of products. • Significantly reduce the environmental impact of industrial processes by reducing waste generation and controlling emissions. 0.10 4.3.1 Economic aspect Economically viable and highly beneficial chemical processes are the primary goal of system design in any industrial process establishment. A simple economic analysis that explores all key 66 variables is introduced in this study while considering all available data. Accurate economic evaluation is essential to identify potential cost drivers, risks and opportunities that may affect the profitability of the overall process. A comprehensive economic evaluation might reduce financial uncertainty and help to make informed decisions, ultimately paving the way for a successful and scalable production process. 𝐺𝐺𝑟𝑟𝑟𝑟𝐺𝐺𝐺𝐺 𝑚𝑚𝑎𝑎𝑟𝑟𝐴𝐴𝑟𝑟𝑛𝑛 = (𝑃𝑃𝑒𝑒𝑚𝑚𝑡𝑡𝑠𝑠𝑐𝑐𝑡𝑡 𝑣𝑣𝑎𝑎𝑚𝑚𝑠𝑠𝑤𝑤−𝑅𝑅𝑎𝑎𝑤𝑤 𝑠𝑠𝑎𝑎𝑡𝑡𝑤𝑤𝑒𝑒𝑤𝑤𝑎𝑎𝑚𝑚 𝑐𝑐𝑚𝑚𝑠𝑠𝑡𝑡)𝑒𝑒𝑒𝑒𝑚𝑚𝑡𝑡𝑠𝑠𝑐𝑐𝑡𝑡 𝑠𝑠𝑎𝑎𝑠𝑠𝑠𝑠 × 0.1 𝑝𝑝𝑟𝑟𝑟𝑟𝑒𝑒𝑛𝑛𝑟𝑟𝑟𝑟𝑎𝑎𝑙𝑙 𝑚𝑚𝑎𝑎𝑟𝑟𝑘𝑘𝑒𝑒𝑟𝑟 (4.2) 𝑃𝑃𝑟𝑟𝑟𝑟𝑑𝑑𝑔𝑔𝑐𝑐𝑟𝑟 𝑣𝑣𝑎𝑎𝑙𝑙𝑔𝑔𝑒𝑒 = 𝑅𝑅𝑎𝑎𝑅𝑅 𝑚𝑚𝑎𝑎𝑟𝑟𝑒𝑒𝑟𝑟𝑟𝑟𝑎𝑎𝑙𝑙 × 𝑔𝑔𝑟𝑟𝑒𝑒𝑙𝑙𝑑𝑑 × 𝑝𝑝𝑟𝑟𝑟𝑟𝑑𝑑𝑔𝑔𝑐𝑐𝑟𝑟 𝑚𝑚𝑎𝑎𝑟𝑟𝑘𝑘𝑒𝑒𝑟𝑟 𝑣𝑣𝑎𝑎𝑙𝑙𝑔𝑔𝑒𝑒 (4.3) 𝑅𝑅𝑎𝑎𝑅𝑅 𝑚𝑚𝑎𝑎𝑟𝑟𝑒𝑒𝑟𝑟𝑟𝑟𝑎𝑎𝑙𝑙 𝑐𝑐𝑟𝑟𝐺𝐺𝑟𝑟 = ∑ (𝑅𝑅𝑎𝑎𝑅𝑅 𝑚𝑚𝑎𝑎𝑟𝑟𝑒𝑒𝑟𝑟𝑟𝑟𝑎𝑎𝑙𝑙 × 𝑚𝑚𝑎𝑎𝑟𝑟𝑘𝑘𝑒𝑒𝑟𝑟 𝑣𝑣𝑎𝑎𝑙𝑙𝑔𝑔𝑒𝑒)𝑤𝑤𝑤𝑤1 (4.4) To simplify the calculation, the method only compares the product and raw material prices and product yields. A profitable output is only obtained with adequate positive price difference.239 The method is called the gross economic potential (GEP) to compare the price difference between product and reactants. The introduced GEP in this study also considered the potential market as the plant production capacity (corresponding to 10% of market demand) in Equation 4.2. A simplified economic model is developed in this study due to several factors: (i) the maturity of the discussed technology, (ii) multiple possible yields and outcomes, and (iii) scalable possibility. Furthermore, raw materials typically cost 40 – 60% of total production costs, which is usually the largest component of the plant design economic analysis, with utilities (energy) coming in second place. Energy demand in cell electrosynthesis reactors is evaluated in the energy sub section 4.3.3. In general, the electrolyte make-up with a 90% recycle ratio had 13 – 56% of operating expense (OPEX) for the anode potential assessment.239 This study also used a 90% recycle ratio as a consideration. Meanwhile, the fixed cost only accounts for 15 – 25% of the total production. In previous studies, fixed and capital costs typically were 14 – 21% and 79 – 86%, with 47 – 74% 67 raw material contribution.71, 240, 241 Thus, the introduced method represents a preliminary gross economic evaluation without neglecting economic assessment importance. Table 4.5. The MOO score evaluation on the economic aspect. Code* Potential market (Million ton/year)a Market value ($/kg)b Gross margin (in million $) MOO Score Raw material Product 1a 29.323 0.11 0.65 0.65 0.65 2a 0.368 0.56 2.23 2.23 2.23 3a 2.500 0.34 0.96 0.96 0.96 4a 5.552 0.76 1.54 1.54 1.54 5a 0.291 10.36 13.04 13.04 13.04 6a 0.000 1.10 17.78 17.78 17.78 7a 0.167 5.68 7.46 7.46 7.46 7b 1.976 5.68 7 7 7 7c 0.047 5.68 10.99 10.99 10.99 8a 0.067 0.51 14.43 14.43 14.43 8b 0.099 0.51 1.89 1.89 1.89 9a 0.227 0.85 7.19 7.19 7.19 9b 12.802 0.85 1.26 1.26 1.26 10a 5.050 0.67 1.2 1.2 1.2 11a 3.742 1.17 3.02 3.02 3.02 12a 1.672 0.97 1.98 1.98 1.98 13a 0.211 0.52 10 10 10 13b 0.003 0.52 61.21 61.21 61.21 13c 0.156 0.52 12.83 12.83 12.83 13d 1.818 0.52 1.1 1.1 1.1 Electrolyteb: NaOH: $0.41/kg; HBr: $1.66/kg; NaBr: $1.62/kg; Na2Cr2O7: $1.59/kg *Refer to Figure 4.3 for the chemical assigned to each code a https://www.chemanalyst.com/; bhttps://www.made-in-china.com/, https://www.chemicalbook.com/, https://dir.indiamart.com/ As shown in Table 4.5, the chlorine-alkali industry undoubtedly achieves the highest gross profit despite the relatively small price difference between the raw materials and the final product. Compared with the oxygen price, $0.024 – 0.04242, the ACE products have higher value and potential for price differences. Most of the chemicals' market value was obtained from the bulk supplier. The high gross profit margin is mainly due to the large potential market demand for chlorine gas, 29.323 million tons/year. In contrast, glycerol electrooxidation to dihydroxyacetone, 68 has low market demand, 0.003 Million tons/year, despite a relatively high market value compared to the raw material cost. However, if the demand for these products from glycerol electrooxidation increases in the future, the possibility of combining this process with the bio-oil ECH system might be promising. Surprisingly, 1,3 propanediol electrooxidation (11) has no economic benefit due to low electrooxidation yield (25.2%) toward acrylic acid (11a), as stated in Table 4.5. The low yield is presumably caused by the thermodynamic barrier of the dehydration reaction of 1,3 propanediol to acrylic acid (11a) in aqueous electrolyte. Instead of 1,3 propanediol, another study evaluated glycerol (13) oxidation to acrylic acid (11a) through electrochemical, photochemical, biochemical, and thermochemical methods, with yields of 4.84, 2.62, 7.60, and 84.95%, respectively.243 The data also showed the limitation of the electrochemical method in converting glycerol into acrylic acid compared with the superior thermochemical method. 4.3.2 Environmental aspect Environmental impact is considered one of the three pillars of sustainability alongside economic and social factors and can play a vital role in the development process. Moreover, reducing pollutants and maintaining the environment have become a higher priority in the decision-making process in modern industrial practice. Incorporating environmental objectives into optimization models is critical for achieving economic targets and developing processes that satisfy global sustainability standards. The most common environmental analysis and evaluation tool, life cycle analysis (LCA), is not used in this study due to its complexities. Thus, the study introduces a simplified technique by analyzing the product's CO2-eq and material ecological data. The environmental impact was evaluated using the existing CO2-eq of the product, based on commercially available production processes. The higher the CO2-eq of the product, the greater the potential for replacement by the production process of ACEs. This may lower the CO2-eq 69 generated, except for some commercial processes utilized in the anode chamber and chlor-alkali industry. The CO2-eq reduction potential was calculated for formic acid, as an example in Table 4.6, produced by hydrolysis of methyl formate and oxidation of biomass.244 The formic acid generates 1.17 kg CO2-eq/kg product. With the potential reduction of 10 – 35% using the anode electrosynthesis process, global warming potential (GWP) of formic acid would have 0.91±0.15 kg CO2-eq/kg product. On the other hand, the utilization of electrocatalysis demonstrated a 31% reduction in GWP compared to the benchmark (bio- and thermocatalytic) processes of lactic acid production from crude glycerol.245 In another study, 40% of CO2-eq was reduced from the conversion of HMF to 2,5-furandicarboxylic acid (FDCA) by electrochemical versus thermochemical methods.246 To significantly reduce the GWP of electrocatalytic processes, it is important that the energy required for this process comes from renewable sources. GWP varies greatly depending on the energy source. The reduction of organic carbon (TOC) was evaluated using different energy sources for electrosynthesis of glyphosate removal.247 For coal-fired power plants, the GWP is 5.7 kg CO2-eq/kg TOC. For natural gas, the value drops to 2.2 kg CO2-eq/kg TOC, further to 1.3 kg CO2-eq/kg TOC when combined with hydroelectric power, and even to 0.3 kg CO2-eq/kg TOC when using wind or solar energy.247 Therefore, switching to renewable energy sources such as hydropower, wind energy, solar power, and bioenergy is essential to minimize the environmental impact of electrocatalytic processes. Ecological information provided in Safety Data Sheets (SDS) is also a critical consideration in comparing substances' environmental effects and the ease of access to SDS simplifies the evaluation. Environmental effects are regularly assessed on a 0 – 4 scale, with 0 indicating minimum effect and 4 representing excessive environmental threat, as seen in Table 4.6. 70 Table 4.6. The MOO score evaluation on the environmental aspect. Code GWP (kg CO2-eq/kg)a Commercial production technology Estimated reduced CO2 Ecological informationb MOO score 1a 0.71 Electro-synthesis248 0 – 10% 3 1.9 2a 0.37 Electro-synthesis248, 249 0 – 10% 0 3.5 3a 1.17 Hydrolysis of methyl formate and oxidation of biomass244 10 – 35% 2 4.1 4a 1.54 Vapor phase dehydrogenation of cyclohexanol250 10 – 35% 3 4.2 5a 2.50251 Nitric acid oxidation of galactose252 10 – 35% 1 7.2 6a 1.15253 Cannizzaro disproportionation reaction254 10 – 35% 2 3.9 7a 1.18255 HMF oxidation256 10 – 35% 1 4.7 7b 1.60257 HMF oxidation256 10 – 35% 1 6.1 7c 1.60257 HMF oxidation256 10 – 35% 2 5.5 8a 1.68258 oxidation of glucose with nitric acid259 10 – 35% 1 5.7 8b 1.37 sodium gluconate uses submerged state fermentation with A. niger260 10 – 35% 1 5.4 9a 0.39 Oxidation of propylene261 10 – 35% 2 2.3 9b 0.64262 the carbonylation of methanol and Acetic acid produced via fermentation route263 10 – 35% 2 3.2 10a 0.90262 Cumene process264 10 – 35% 2 4.1 11a 0.81 High cost genetically modified microorganisms265 5 – 15% 2 2.7 12a 1.85251 Hydrolysis of propylene oxidete266 10 – 35% 1 5.7 13a 3.80267 Osmium tetroxide-catalyzed or conversion of acrolein 268 10 – 35% 1 9.6 13b 2.80269 microbial conversion from glycerol using bacteria of the genus Gluconobacter270 5 – 15% 1 6.1 13c 3.45271 By product of DHA270 5 – 15% 1 6.8 13d 3.80267 Oxidation of glycerol270 10 – 35% 1 9.6 aOpen LCA database except stated; bObtained from MSDS of raw material, electrolytes, and the product Ecological information gives essential insights into the dangers a substance poses to numerous environmental components, including air, water, soil, and living organisms. Cautiously studying those details gives extra perspective on the secure use, handling, and disposal of substances. SDS is also designed to assist businesses in evaluating one product against another. In this study, the ecological information data assessed the reactant, product, and electrolytes as the main components of each ACE selection. 71 With the combination of CO2-eq potential and ecological information, the MOO scale on the environmental impact was easily examined using 0.7 and 0.3 weighting factors, respectively, as reported in Table 4.6. For instance, the Cl2 gas (1a) production has the lowest GWP potential reduction (GWP score = 1) and the ecological information score of 3 (ecological score = 4), scale 0 – 4. 𝑀𝑀𝑂𝑂𝑂𝑂 𝐺𝐺𝑐𝑐𝑟𝑟𝑟𝑟𝑒𝑒 𝑟𝑟𝑛𝑛 𝑒𝑒𝑛𝑛𝑣𝑣𝑟𝑟𝑟𝑟𝑟𝑟𝑛𝑛𝑟𝑟𝑚𝑚𝑒𝑒𝑛𝑛𝑟𝑟 = 0.70 × 𝐺𝐺𝑀𝑀𝑃𝑃 𝐺𝐺𝑐𝑐𝑟𝑟𝑟𝑟𝑒𝑒 + 0.30 × 𝐸𝐸𝑐𝑐𝑟𝑟𝑙𝑙𝑟𝑟𝐴𝐴𝑟𝑟𝑐𝑐𝑎𝑎𝑙𝑙 𝐺𝐺𝑐𝑐𝑟𝑟𝑟𝑟𝑒𝑒 (4.5) The glycerol electrooxidation seems promising because of the highest MOO score followed by the mucic acid production in terms of environmental impact. Moreover, these two raw materials are the by-products of biodiesel (glycerol) and pectin (galacturonic acid) industries, making both options promising for further exploration. Another advantage in terms of the environmental impact of the glycerol (13) and galacturonic acid (5) electrooxidations, is that of waste utilization to produce valuable chemicals. 4.3.3 Energy aspect From the point of view of production costs and environmental sustainability, energy aspects are a critical factor that must be thoroughly evaluated in a MOO assessment. Energy consumption directly affects both the operating costs and the environmental footprint of production. Therefore, optimizing energy efficiency is essential, especially when selecting ACE. As electrosynthesis is highly dependent on electricity, a comprehensive energy evaluation is mandatory. This evaluation ensures that the selected energy source minimizes costs, reduces environmental impact, and achieves sustainability goals. Energy evaluation is, therefore, a critical factor in the decision-making process to optimize ACE and achieve a balanced result between economic and environmental goals. 72 The standard thermodynamic potential often differs significantly from the reported onset potentials in various experimental studies, as shown in Figure 4.4. In this study, the standard (thermodynamic) potential values were calculated using the Joback method, available through ChemDraw 18.1® software. Meanwhile, the onset potentials, which represent the experimental thresholds at which the reactions begin, have been recorded in many studies. The discrepancy between theoretical and experimental values highlights the complexity of translating the thermodynamic data into practical applications of electrosynthesis. Figure 4.4. Standard potential (open symbol) and reported onset (closed symbol) potential of the ACEs illustrated from Table 4.7. Many anode chamber reactions provide lower electrical energy consumption compared to the OER, further reducing the total energy demand of electrosynthesis. Thus, the anode onset potential solely becomes the main consideration in ACEs selection regarding electrosynthesis energy 0.0 0.5 1.0 1.5 2.01a2a3a4a5a6a7a7b7c8a8b9a9b10a11a12a13a13b13c13dOERPotential (V vs RHE) Standard OnsetIdeal target73 consumption. The difference between the onset and thermodynamic potentials leaves room for improving electrosynthesis performance. However, the present study only used the onset potential to evaluate the required electrical energy of ACE selection. The standard potential of methanol (3) electrooxidation to formic acid (3a) is relatively low at 0.11 V. However, the reported onset potentials are significantly higher, ranging from 1.29 to 1.36 V.272-274 The difference, also known as the overpotential, is primarily caused by the activation energy of the catalyst or electrode, the inherent resistance to electronic and ionic current, and the mass transport limitations due to the concentration gradients within the electrocatalytic reactor. All factors collectively contribute to the high overpotential observed during the process. On the other hand, cyclohexanol (4), one of the intermediate products of syringol or guaiacol ECH, could be re-oxidized in an anode chamber to produce cyclohexanone (4a) with standard thermodynamic potential around 0.11 V. Since 1,2-propanediol (12) and glycerol (13a-13b) electrooxidation have the lowest onset potentials (0.61 V vs. RHE), the ACEs are expected to consume less electrical energy compared to the other options. Furthermore, compared with the OER onset potential (1.58 V vs. RHE), the electrooxidation of 1,2-propanediol and glycerol are superior and energy efficient. On the other hand, other ACEs exhibited onset potentials above 1.1 V, with the exception of methanol to formic acid (3), ethanol to acetic acid (8b), and isopropanol to acetone (10). The reported onset potential of methanol electrooxidation (3) are significantly higher, ranging from 1.29 to 1.36 V.272-274 There is another possible ACE that has lower onset potential, such as urea electrooxidation, 0.37 V vs. RHE. However, it does not provide a significant economic benefit to the system. The chlor-alkali anode reaction (1) has quite a similar onset potential (1.55 V vs. RHE) compared with the OER. Thus, the data justifies the claim that 51 – 58% of the chlor-alkali industry's cost comes 74 from electricity.275 The summarized data on the MOO score for the energy aspect is presented in Table 4.7. Table 4.7. The MOO score evaluation on energy. Code Onset potential (V vs. RHE) MOO score Code Onset potential (V vs. RHE) MOO score 1a 1.55 ± 0.10276-283 3.8 8b 0.97 ± 0.19217, 284, 285 7.6 2a 1.81 ± 0.00286 2.1 9a 1.30 ± 0.01287-289 5.5 3a 0.95 ± 0.16272, 273, 290-294 8.1 9b 1.46 ± 0.06289, 295, 296 4.4 4a 1.40 ± 0.04297-299 4.8 10a 0.88 ± 0.17294, 297, 300-302 8.2 5a 1.21 ± 0.00221 6.1 11a 1.16 ± 0.27288, 303, 304 6.4 6a 1.35 ± 0.05305, 306 5.1 12a 0.61 ± 0.24294, 303 10.0 7a 1.44 ± 0.20307 4.6 13a 0.61 ± 0.08108, 290, 308-316 10.0 7b 1.10 ± 0.14317-319 6.8 13b 0.61 ± 0.11117, 118, 320 10.0 7c 1.46 ± 0.04307, 317, 321-328 4.4 13c 1.06 ± 0.09241, 308, 311, 329-332 7.0 8a 1.17 ± 0.06284, 285, 333 6.3 13d 0.82 ± 0.19331, 334, 335 8.6 OER: 1.58 ± 0.01 V vs RHE 336-343 4.3.4 Safety aspect Because of primarily similar electrosynthesis conditions, the safety aspect of the MOO analysis was evaluated using the Safety Data Sheets (SDS) of the reactants, products, and electrolytes. The safety concerns are closely tied to human exposure in the workplace, making it critical to evaluate the potential hazards early. The evaluation was conducted to identify the hazards associated with all the ACE chemicals. SDS, a technical safety document, provides information on health, fire, reactivity, and potential environmental hazards of each chemical as well as information on how to handle, store, and use the chemicals more safely. It also provides essential information on chemical properties, health risks, and recommended safety precautions, enabling a thorough safety assessment tailored to the specific conditions of the electrosynthesis reaction. Since the ACE selection occurred at the initial stages of the chemical process development, a simplified approach was introduced for hazard assessment and inherent safety analysis. Several studies have implemented the inherent safety analysis for early design stages of the conceptual of 75 chemical process plants.344-347 Lawrence (1996)348 employed seven critical parameters in their study: inventory, toxicity, chemical stability, temperature, pressure, flammability, and explosiveness. Most of the critical parameters were duplicated in this study to assess the safety aspects of the Multi-Objective Optimization (MOO) process. The potential hazards associated with ACE could be systematically examined by evaluating the SDS data, however, the study excluded temperature and pressure as factors since ACEs generally occur under mild conditions. Meanwhile, the oxidizing level was evaluated in terms of safety considerations as oxidizers pose a potentially fatal fire hazard. Oxidizers might not be combustible, but they enhance the combustion and increase the flammable range of substances, allowing them to burn more easily. Although the risk is typically calculated as a matrix of severity and likelihood, the likelihood of the ACEs process could not be determined solely based on the plant capacity. The likelihood is typically evaluated and calculated from the final P&ID diagram with additional data on the historical incident and accident rates in the chemical industry, equipment failure rates, human error probabilities, environmental factors, and operational conditions. Therefore, quantitative methods such as Quantitative Risk Assessment (QRA), Fault Tree Analysis (FTA), and Event Tree Analysis (ETA) could not be carried out at the plant development stage to model the probability of hazardous events. These approaches are critical for accurate risk estimation and developing robust safety measures in industrial settings. However, the P&ID flow diagrams were not available for the ACEs selected for this study because most of the ACEs still have a low technology maturity level, which limits the application of more rigorous quantitative safety methods. As a result, the study focused on identifying hazards based on the available MSDS data rather than conducting a full quantitative risk analysis. This 76 simplified approach helps streamline early-stage safety evaluations, even though a more detailed risk assessment would be necessary as the process design progresses. Table 4.8. The MOO score evaluation on safety. Code Inventory Reactivity Flammability Explosiveness Health Oxidizing MOO safety score 1a 0.0 8.75 10.00 10.00 3.33 5 5.9 2a 9.9 7.50 10.00 10.00 3.33 0 7.2 3a 9.5 8.33 5.83 9.45 3.33 10 7.7 4a 8.9 8.33 6.67 9.84 4.17 10 8.0 5a 8.4 8.75 8.75 10.00 5.00 5 7.8 6a 10.0 8.33 8.33 9.86 2.50 10 8.2 7a 0.0 8.33 8.33 10.00 5.83 10 6.7 7b 0.0 8.33 8.33 10.00 4.17 10 6.4 7c 10.0 8.33 8.33 10.00 4.17 10 8.5 8a 9.6 9.17 10.00 10.00 5.83 10 9.1 8b 10.0 9.17 9.17 10.00 7.50 10 9.3 9a 10.0 7.50 4.17 9.41 3.33 10 7.4 9b 7.5 9.17 5.83 8.55 2.50 10 7.1 10a 9.0 9.17 5.00 9.83 4.17 10 7.8 11a 9.3 6.67 7.50 9.95 4.17 10 7.9 12a 9.7 8.33 8.33 10.00 5.00 10 8.6 13a 10.0 9.17 8.33 10.00 6.67 10 9.0 13b 10.0 9.17 9.17 10.00 6.67 10 9.2 13c 10.0 9.17 9.17 10.00 5.83 10 9.0 13d 9.6 9.17 9.17 10.00 5.00 10 8.8 The MOO score in Table 4.8 was calculated based on the critical safety metrics as shown below and adjusted for the study:346, 347 𝑆𝑆𝑆𝑆 = 0.21 × 𝐼𝐼𝑛𝑛 + 0.17 × 𝑅𝑅𝑟𝑟 + 0.18 × 𝐶𝐶𝐺𝐺 + 0.15 × 𝑑𝑑𝑙𝑙 + 0.13 × 𝐸𝐸𝑥𝑥 + 0.15 × 𝑂𝑂𝑥𝑥 (4.6) With SS, In, To, Cs, Fl, Ex, and Ox denote the safety score, inventory, toxicity, chemical stability, flammability, explosiveness, and oxidation state, respectively. The inventory was calculated by the reaction hold-up time, a one-hour assumption344, multiplied by the chemical flow rate. The higher the inventory, the lower the inventory score. 77 The chemical stability, toxicity, flammability, and oxidative state data were obtained directly from the instability, health, flammability, and oxidation data (scale 0 – 4) on the MSDS of the materials involved in the system. Meanwhile, the explosiveness was calculated using the explosiveness index from the upper and lower explosive limit (scale 0 – 1). A wider explosive limit indicates a higher chance of explosion with a substantial potential hazard. The MOO score on safety represented the inherent safety analysis calculation, as presented in Table 4.8. The electrooxidation of glucose (8) to gluconic acid (8a) or glucaric acid (8b) had quite an excellent safety score. Meanwhile, glycerol electrooxidation (13) held the second position on the safety evaluation. The lowest safety score is that of the chlor-alkali industry (1), which has a high inventory and produces chlorine gas, oxidizers, and health issues. 4.3.5 Technology Readiness Level As the interest in evaluating technology maturity continues to grow among stakeholders, applying the technology readiness level (TRL) becomes a crucial tool in the assessment process. TRL serves as a metric to measure the maturity of a particular technology under consideration. As a chemical technology typically takes up to 10 years to be industrialized 349, evaluating a lab-scale process for scale-up should be comprehensively conducted to improve its TRL. Higher yield and efficiency are the primary goals of improving the TRL from the lab scale to the larger scale. On the other hand, process intensification, such as the pairing system of bio-oil ECH with a potential ACE, enhances the efficiency and economic viability; thus, it could boost the TRL of the process. In addition to TRL, other assessment tools like techno-economic analysis and life cycle assessment, as explored by Sorunmu et al.350, can be used in conjunction to provide a more comprehensive evaluation. In the context of the chemical industry, improving TRL is closely tied to the process's selectivity and yield. Higher selectivity and yield lead to easier purification and 78 separation, which is critical in scaling up industrial processes. Commercial ACEs have 80% – 93% yields compared to 25 – 99% yields for the ACEs with lower TRL levels, as listed in Table 4.9 and Table 4.10. Although technologies with higher TRLs can be modeled with lower uncertainty 351, lower TRLs still hold significant potential for future development and are being considered. Table 4.9. The yields of ACEs from several studies. Code Reactant Product Yield (%) 1a NaCl Cl2 82.5% 276-283 2a NaClO3 NaClO4 95.4% 286, 352 3a Methanol Formic acid 62.5% 353 4a Cyclohexanol Cyclohexanone 97.0% 354 5a Galacturonic acid Mucic acid 80.0% 221 6a Furfural 2-furoic acid 98.0% 305 7a HMF 2,5-diformylfuran 78.0% 355, 356 7b HMF 5-hydroxymethyl-2-furan-carboxylic acid 98.0% 234, 357 7c HMF 2,5-furandicarboxylic acid (FDCA) 99.0% 317, 319, 328 8a Glucose glucaric acid 83.0% 284 8b Glucose gluconic acid 83.2% 215 9a Ethanol Acetaldehyde 79.0% 137 9b Ethanol Acetic acid 65.0% 137 10a Isopropanol Acetone 90.0% 358 11a 1,3-Propanediol Acrylic acid 25.2% 303 12a 1,2-Propanediol Lactic acid 75.0% 72 13a Glycerol Glyceraldehyde 80.0% 359 13b Glycerol Dihydroxyacetone (DHA) 61.4% 118 13c Glycerol Glyceric acid 50.0% 360-362 13d Glycerol Glycolic acid 47.0%363, 364 79 Table 4.10. TRL in the National Aeronautics and Space Administration (NASA), Department of Defense (DOD), and chemical industry classification with MOO score of ACEs selection. TRL NASA and DOD practice 365 Chemical Industry 366, 367 1a 2a 3a 4a 5a 6a 7a 7b 7c 8a 8b 9a 9b 10a 11a 12a 13a 13b 13c 13d OER 1 Basic Principles observed and reported Idea X X X X X X X X X X X X X X X X X X X X X 2 Concept formulated Concept X X X X X X X X X X X X X X X X X X X X X 3 Proof-of-concept Proof of concept X X X X X X X X X X X X X X X X X X X X X 4 Lab-scale demonstration (“low fidelity”) Preliminary process development X X X X X X X X X X X X X X X X X X X X X 5 Lab-scale demonstration (“high fidelity”) Detailed process development X X X X X X X X X X X X X X X 6 Prototype system designed Pilot trails X X X X X X X 7 Prototype system tested in operational environment Demonstration and full-scale engineering X X X X X 8 Actual system completed Commissioning X X X X X 9 Actual system proved successful Production X X X X X MOO Score 10 10 3.9 4.4 10 4.4 3.9 4.4 5.6 5.6 10 4.4 3.9 4.4 3.9 3.9 4.4 3.9 3.9 3.9 – 80 The TRL evaluation and MOO scores are shown in Table 4.10, with the commercially available ACEs achieving the highest score. On the contrary, the lowest score is at 3 – 4 level of TRL, due mostly to less selective reactions. Thus, the increase of TRL for reactions with many side or intermediate reactions, such as in organic electrosynthesis, is quite challenging. Moreover, only a few ACEs are in the commercialization stage. 4.3.6 Separation technology impacts The last impact to be considered in the MOO evaluation is separation technology. Product separation and purification are costly due to their significant energy consumption, prohibitively expensive new technology, and importance in meeting product specifications. For example, nearly half of the total production cost of fuels and chemicals derived from biomass is attributed to separations. In the United States, industrial separations can consume up to 15% of total energy consumption of the domestic and industrial sector.368, 369 Distillation contributed 49% of the total industrial separation energy followed by drying (20%) and evaporation (11%), with the remaining assigned to non-thermal separation such as membrane-based separation.369 Consequently, the impact of separation technology should be included in the MOO analysis. At the initial design stage, much information on separation technology design is usually not available, thus making the evaluation quite challenging. A lack of information on the applied separation of each ACE process leads to difficulty in assessing the operational aspect of the system. Moreover, the low TRL of most ACEs means that the process flow diagram information is not available. However, the separation technology selection could be evaluated using the simple guide shown in Table 4.11. In this study, the separation technology for each ACE was selected for a similar industrial process. The 81 commercially established separation, recovery, and purification step will probably be as least as costly with similar technical risk for implementation.370 Table 4.11. The guide of separation technology selection based on the stream phase.370 COMPONENT MINOR Solid Liquid Gas / Vapour MAJOR Solid Sorting. Screening, Hydrocyclones, Classifiers, Jigs, Tables, Centrifuges, Dense media, Flotation, Magnetic, Electrostatic Pressing, Drying Liquid Thickeners, Clarifiers, Hydrocyclones, Filtration, Centrifuges, Crystallizers, Evaporators, Precipitation, Membranes, Reverse osmosis, Ion exchange, Adsorption Decanters, Coalescers, Solvent extraction, Leaching, Chromatography, Distillation Stripping Gas / Vapour Gravity settlers, Impingement separators, Cyclones, Filters, Wet scrubbers, Electrostatic, Precipitators Separating vessels, Demisting pads, Cyclones, Wet scrubbers, Electrostatic, precipitators Adsorption, Absorption, Membranes, Cryogenic distillation, Condensation The MOO score was considered based on the energy used, operating and capital cost for each component, and operation ease. Energy comparison is provided in Figure 4.5 for various separation technologies, with the distillation process serving as a benchmark. Compared to the distillation process, the energy consumption of crystallization is ten times lower, which means it lowers the operating cost and CO2-eq value. Distillation is generally more expensive and complex due to the high energy demands and the intricate design and control required, particularly for large-scale or high-purity applications. The distillation process accounts for 10–15% of the world’s energy consumption of domestic and industrial use.371, 372 82 Figure 4.5. Relative energy use from several energy-intensive separation technologies.371 Crystallization can be less expensive and complex, especially for smaller operations or when energy costs are a significant concern. However, it may present challenges in terms of process control and scalability. Based on Dietz373, the distillation process has the most expensive technology, approximately $ 0.0127/kg product, compared to high-pressure crystallization at $ 0.00533/kg product and $ 0.0116/kg product, depending on the pressurized system. The utilization of separation technology with the absence of heat would drastically lower global energy demand, emissions, and pollution without neglecting the product market specifications. For instance, the application of membrane-based separation requires 90% less energy compared with distillation technology.369 As stated in Table 4.12, the chlor-alkali industry (1) needs liquefaction technology to store chlorine gas as a product. On the other hand, other ACEs need either crystallization or distillation for product separation and purification. As the liquefaction process of chlorine gas requires low temperatures and compression units, it is energy-intensive to operate at cryogenic temperature condition (-34 °C). Chlorine gas is categorized as a toxic, highly corrosive, and hazardous material. 83 Therefore, the liquefaction pipe and equipment require high-quality materials to control corrosion and advanced inherent safety systems to ensure low risk. The crystallization process holds the highest MOO score due to its lower energy use, operational simplicity, and lower cost. Higher boiling points lead to higher energy usage and operational cost, so the MOO score has a lower value than for lower temperature product streams. Therefore, crystallization, distillation, and liquefaction rank from the highest to the lowest. Table 4.12. The MOO score for separation technology. Code Major - minor phase Separation technology Tb (°C) reactant - product MOO score 1a Liquid - gas Liquefaction 5 2a Liquid - solid Crystallization 10 3a Liquid - liquid Distillation 64.7 – 100.8 9 4a Liquid - liquid Extractive distillation 161.8 – 155.6 6 5a Liquid - solid Crystallization 10 6a Liquid - liquid Distillation 161.7 – 230 to 232 8 7a Liquid - liquid Distillation 114 to 116 – 276.8 8 7b Liquid - liquid Distillation 114 to 116 – 349.4 8 7c Liquid - liquid Distillation 114 to 116 – 420 8 8a Liquid - solid Crystallization 10 8b Liquid - solid Crystallization 10 9a Liquid - liquid Distillation 78.37 – 20.2 10 9b Liquid - liquid Distillation 78.37 – 117.9 9 10a Liquid - liquid Distillation 82.3 – 56 9 11a Liquid - liquid Distillation 211 to 217 – 141 8 12a Liquid - liquid Distillation 188.2 – 122 8 13a Liquid - liquid Distillation 290 – 228 7 13b Liquid - liquid Distillation 290 – 213.7±15.0 7 13c Liquid - liquid Distillation 290 – 412 7 13d Liquid - liquid Distillation 290 – 471.4±30.0 7 4.4 The total MOO analysis and sensitivity analysis As the MOO score is evaluated from previous sections, the overall score is presented in Table 4.13 for each ACE with a Fisher's Least Significant Difference (LSD) analysis to differentiate the group significance of each ACE. 84 Table 4.13. The ACE MOO score with the criteria and weight factor. Code Economic, A1 (a1 = 0.25) Environment, A2 (a2 = 0.20) Energy, A3 (a3 =0.15) Safety, A4 (a4 =0.15) TRL, A5 (a5 =0.15) Separation technology, A6 (a6 =0.10) Total* (1.00) 1a 10.0 1.9 3.8 5.9 10.0 5.0 6.3ab 2a 0.7 3.5 2.1 7.2 10.0 10.0 4.8e 3a 1.7 4.1 8.1 7.8 3.9 9.0 5.1d 4a 4.9 4.2 4.8 8.1 4.4 6.0 5.3d 5a 0.6 7.2 6.1 8.0 10.0 10.0 6.2b 6a 0.0 3.9 5.1 8.2 4.4 8.0 4.2f 7a 0.0 4.7 4.6 6.7 3.9 8.0 4.0f 7b 4.3 6.1 6.8 6.4 4.4 8.0 5.7c 7c 0.4 5.5 4.4 8.5 5.6 8.0 4.8e 8a 1.1 5.7 6.3 9.2 5.6 10.0 5.6cd 8b 0.2 5.4 7.6 9.3 10.0 10.0 6.2b 9a 1.6 2.3 5.5 7.4 4.4 10.0 4.5ef 9b 3.9 3.2 4.4 7.6 3.9 9.0 4.9e 10a 2.6 4.1 8.2 8.0 4.4 9.0 5.5d 11a 0.0 2.7 6.4 8.1 3.9 8.0 4.1f 12a 1.8 5.7 10.0 8.6 4.4 8.0 5.8c 13a 2.3 9.6 10.0 9.0 4.4 7.0 6.7a 13b 0.2 6.1 10.0 9.2 3.9 7.0 5.4d 13c 2.2 6.8 7.0 9.0 3.9 7.0 5.6cd 13d 0.5 9.6 8.6 8.9 3.8 7.0 5.9bc *Calculated using Equation 4.1, prior to significant difference analysis. The letter denotes the group’s significance. The analysis begins with the null hypothesis test. If the global null hypothesis can be rejected (α < 0.05), the Fisher's LSD test could be carried out in the next step. In this study, the ANOVA test was conducted using the difference weighting factor for the sensitivity analysis, which rejected the null hypothesis (α = 2.3E-81). Thus, the Fisher's LSD test could be conducted and is also presented in Table 4.13. The glycerol electrooxidation into glyceraldehyde (13a) holds the best MOO score in this study, with the chlor-alkali industry (1) coming in second place with a different significance group. The glycerol (13) electrooxidation needs careful attention because different oxidation products produce lower MOO scores, caused by the low economic value and small market demand, even though glycerol oxidation products have great potential. 85 Table 4.14. The glycerol oxidation products of the study and their applications. No Products Application 1 Glyceraldehyde (13a) Crystallization solution modifier 374, anti-aging agent of AGEs (advanced glycation end-products), measuring aldose reductase activity 375, starting material in D, L-serin synthesis 376, in the preparation of polyesters and adhesives; as a cellulose modifier; and in the tanning of leather 377. 2 1,3-Dihydroxyacetone (13b) Pharmaceutical intermediate 376, as a self-tanning agent and browning ingredient in the cosmetic industry 378, 379, monomers in polymer biomaterials 380, a metabolite for human, Escherichia coli and Saccharomyces cerevisiae metabolite 377. 3 Glyceric acid (13c) An anionic monomer of packaging material for exothermic and volatile agents 381, used in cosmetic and pharmaceutical products to treat skin diseases 382, anti-cholesterol activity, and esters of glyceric acid that have antitrypsin activity 377 4 Tartronic acid (13d) Oxygen scavenger 381, derivatives used in the treatment of osteoporosis 382 and diabetes 377, intermediate compound to generate mesoxalic acid 377 The glycerol electrooxidation in this study and their applications are listed in Table 4.14. Glycerol oxidation is commonly used in the pharmaceutical, cosmetics, medicine, polymer synthesis, and food industries, improving its market demand potential. As sustainable and green factors improve in the chemical industry synthesis, glycerol electrooxidation is expected to have a bright future in the coming years. A sensitivity analysis was also conducted in this study to evaluate the weighting factor of each process variable, as presented in Figure 4.6. The sensitivity analysis examines the robustness of the primary data and assumptions in the MOO criteria. The sensitivity analysis showed that only the economic factor has a medium sensitivity class (0.05 ≤ diff < 0.20), while the other aspects have a small or negligible sensitivity class (diff < 0.05). The separation process changes have not significantly shifted the MOO value due to the low MOO weighting factor of this process variable. 86 Figure 4.6. The sensitivity analysis plot of the glycerol electrooxidation into glyceraldehyde. However, changing the economic weighting factor significantly impacted the MOO analysis result. While the economic factor reflects market demand and the value of the chemical product, the volatility of the economic factor process variables should be considered. In economics, the law of demand dictates that the price goes up while the demand goes down and vice versa. However, the demand for this substance remains low due to the very volatile glyceraldehyde market price.383 If the robust and practical synthesis of glyceraldehyde has been established with great potential from glycerol electrooxidation, the market demand for glyceraldehyde is expected to increase. Because of the medium sensitivity class of the economic process variable, the study tried to analyze the effect of each MOO analysis of ACE. The data in Figure 4.7 shows that most of the ACE had a medium-sensitive index toward the economic aspect, except for furfural electrooxidation to 2-furoic acid (4), HMF electrooxidation to 5-hydroxymethyl-2-furan-carboxylic acid (7b), and ethanol electrooxidation to acetic acid (9b). The increasing economic 87 impact led to a shift in the highest MOO score from glycerol electrooxidation to glyceraldehyde (13a) to the chlor-alkali industry (1). Figure 4.7. The tornado plot of the (a) weighting aspect shift and (b) the ACEs score toward the weighting aspect shifting toward the economic aspect. However, as the industry and consumers' preferences toward green and sustainable products have rapidly increased due to the rising concern for the environment384, we assume that sustainability, such as the environmental impact and safety, would replace the economic consideration in the future. On the other hand, the chlor-alkali industry has grown slowly over the past ten years and slow growth is expected to remain in the next decade. Moreover, there are general guidelines for selecting an appropriate process location in chemical engineering plant design. Initially, economists viewed plant location decisions primarily through the lens of cost minimization. The ideal location was determined to be the one that minimized transportation costs for both raw materials to the plant and finished products to the market. From the early 1900s until the post-World War II period, the high cost of transportation was the primary consideration for most industries.385 As transportation costs decreased, the cost-(a) (b) 88 effectiveness consideration shifted to other factors, such as wages, energy, and local regulations. For instance, if a potential location has lower wages, compensating for the higher transportation costs, it is a more feasible and attractive option, even though it is located in a remote area. Nowadays, the plant design location should consider several factors to support the circular economy (chemicals and materials industry) and sustainability trends such as (i) proximity to the raw materials and the market, (ii) availability of waste streams for resource recovery and recycle, (iii) access to renewable energy source, (iv) infrastructure and transportation, (v) environmental and regulatory factors, (vi) safety and risk factors, (vii) social and community impact, (viii) political and economic stability, and (ix) future expansion and scalability. However, all considerations should align with the economic and financial factors to attract investments. Therefore, the integration of paired electrosynthesis for bio-oil upgrading and glycerol electrooxidation represents a promising approach that aligns with the principles of the circular economy and supports sustainable development. 4.5 Summary of the chapter The paired electrosynthesis has excellent potential for implementation in the electrosynthesis process. As the divided-cell electrosynthesis allows two junction reduction-oxidation reactors to operate simultaneously, the efficiency of each reactor is dictated by the comparison to the HER and OER for the reduction and oxidation reaction, respectively. The HER produces highly flammable hydrogen gas that needs cryogenic storage. Meanwhile, the OER produces oxygen gas, which is not valuable and has sluggish kinetics. These two reactions could be substituted by the direct utilization of proton reduction as an in-situ supply of adsorbed hydrogen of bio-oil in the cathode chamber and the potential anodic coupling electrosynthesis reviewed in this study. This 89 substitution is usually called product diversification, which produces many valuable chemicals in a similar system. Yet, electrosynthesis still requires improvement in organic synthesis, with mostly lower yields compared to conventional thermochemical synthesis methods. Due to the high CO2 contribution of many existing chemical industries, electrosynthesis has emerged as a solution to reduce GHG emissions. Thus, the chemical building block generated from biomass, a renewable and sustainable resource processed by electrosynthesis, would reduce fossil fuel dependency. The most promising biomass feedstock, such as glycerol, glucose, and galacturonic acid, have the most potential as biomass resources to be processed in the anode chamber, reducing the reliance on high-carbon footprint synthesis. However, the glycerol and glucose electrooxidation in the anode chamber needs partial oxidation instead of complete oxidation into CO2 in the fuel cell research area. Therefore, the complex oxidation process relies on the selectivity and high yield toward the desired product, such as glyceraldehyde (13a), dihydroxyacetone (13b), glyceric acid (13c), and tartronic acid (13d) for glycerol and glucaric acid (8a) and gluconic acid (8b) for glucose electrooxidation. The catalyst type 386, morphology 387, electrolyte, applied potential window, and pH 388 dictated the product distribution in many experimental studies. The high faradaic efficiency toward a particular product instead of generating CO2, the full oxidation product, requires more insight into the reaction mechanism on a specific catalyst and the reaction conditions. On the other hand, glycerol and glucose fuel cells are quite a niche technology and are widely operated nowadays. In addition, glucose could produce hydroxymethylfurfural, HMF (7), as the platform chemical under acidic conditions through a hydrolyzed reaction. This study concluded that glycerol electrooxidation to glyceraldehyde (13a) has the most potential based on the MOO evaluation of six key process variables: economics, environment, 90 energy, safety, TRL, and separation technology. The glycerol electrooxidation evidently increased the hydrogen production with high energy density, non-toxicity, and non-volatilization.389 In consideration, the market penetration of glyceraldehyde generated by the electrosynthesis process would accelerate the electrosynthesis process development 390 and improve the economic value dramatically. On the other hand, the integration of paired electrosynthesis for bio-oil upgrading and glycerol electrooxidation represents a promising approach that aligns with the principles of the circular economy and supports sustainable development. 91 Chapter 5: Half-cell Optimization of ECH-DO and GEOR Organic paired electrosynthesis, which involves producing numerous intermediate chemicals, is inherently complex due to factors such as reaction pathways, reaction conditions, purification and separation issues, and scale-up challenges. Optimizing the process involves fine-tuning parameters like electrolyte pairs, catalyst type and loading, solvent addition, and temperature. Initially, the half-cell optimization was performed to obtain the best condition before pairing the system. The study also focused on selecting various noble metal catalysts as a preliminary step in identifying the benchmark catalyst. On the other hand, in a previous study by Wijaya et al.106, the control experiment to distinguish the reaction mechanism on the suspended catalyst (Pt/C) in the stirred slurry electrochemical reactor was reported. The experiments of guaiacol electrocatalytic hydrodeoxygenation (ECH-DO) without dispersed catalyst, without electric potential, and without stirring concluded that at least three requirements for guaiacol reaction in the cathode chamber, were necessary: (i) the availability of electrons to reduce protons into hydrogen radicals, (ii) charge between electrode and catalyst surface for the protons to be reduced into adsorbed hydrogen, and (iii) contact between organic molecules and catalyst particles under applied potential.106 Furthermore, the substitution of Pt/C with activated charcoal resulted in no product from guaiacol ECH-DO, suggesting that the presence of dispersed metal is crucial for the adsorption of Hads and guaiacol.106 The half-cell experiments were conducted either in cathode or anode chamber with specified reaction conditions in the SSER configuration: CA0 = 0.1 M (guaiacol or glycerol), I = 0.4 A (66 mA.cm-2), N = 600 rpm, T = 60 °C, t = 4 h, NaCl = 0.25 M (catholyte and anolyte), and Pt/C = 5 wt.% (0.5 g/L). Meanwhile, the isopropanol (IPA) effect was investigated by varying the amount 92 of IPA in the cathode chamber (0, 10, 25, 50, 75, and 90 % vol.) accompanied by the variation of methanesulfonic acid (MSA) concentration (0.2, 0.5, and 1.0 M) using an experimental design. On the other hand, the half-cell experiment on the anode chamber was investigated using electrolyte selection (acid, neutral, and alkaline) followed by the noble metal catalyst selection (Pt/C, Pd/C, and Ru/C). 5.1 Isopropanol effect on the ECH-DO of guaiacol in half-cell electrosynthesis The primary purpose of the ECH-DO reaction is to increase the H/C ratio and decrease the O/C ratio of the oil. Hence, in ECH-DO of guaiacol, cyclohexane is the preferred product of the reaction due to (1) Molecular simplicity (no oxygen-containing groups), making it a clean, single hydrocarbon product which is applicable in many applications such as fuel blending or chemical synthesis; (2) Higher energy density compared to cyclohexanol and cyclohexanone; (3) The flexibility in downstream processing, versatile intermediate in organic synthesis; and (4) Its compatibility with existing infrastructure. However, only limited success has been achieved in the ECH of bio-oil model compounds to yield cyclohexane, with most studies showing that cyclohexanol and cyclohexanone are the main products. The process is limited by cyclohexanol dehydration to cyclohexene equilibrium, which is inhibited by the water solvent. Liu et al.42 stated that the C–O and C=O cleavage has higher activation energy and lower rate in aqueous electrolytes compared with the hydrogen evolution reaction. Thus, higher temperature and/or overpotential is required to initiate the HDO reaction. Meanwhile, the addition of ethanol in sub- or supercritical conditions can donate hydrogen which removes the oxygen by forming water.391 Alternatively, isopropyl alcohol (IPA) converts into acetone under milder conditions392 and donates hydrogen, making it potentially suitable for the 93 electrolyte system. Most studies of ECH-DO have focused on catalyst development with limited attention being placed on the potential impact of an organic solvent acting in conjunction with the electrolyte. Introducing an organic solvent in electrocatalytic bio-oil conversion processes can have multiple effects such as: i) enhanced solubilization of the organic reactant and products compared to a purely aqueous electrolyte, ii) changes in the ionic conductivity and hence, the required cell voltage and iii) possibility of acting as a co-reactant or catalyst. The desired product of this investigation is cyclohexane, which is generated by complete ECH-DO of guaiacol. Figure 5.1 represents the guaiacol conversion and selectivity for different products as a function of IPA content and MSA concentration. The letter symbols on each bar or line indicate the significance of within-group differences between treatments, as formulated in Equation 3.12. For instance, Figure 5.1a shows that the selectivity of phenol with 50% (ab) and 90% (b) IPA addition are significantly different compared to other IPA contents (a). As seen in Figure 5.1, cyclohexane was not obtained without IPA addition at any MSA concentrations. The maximum cyclohexane selectivity (SF) of 11.7% was obtained at 50% IPA with 0.5 M MSA (Figure 5.1a – 5.1c). The guaiacol conversion generally decreased when the IPA content in the catholyte exceeded 50%. The highest conversion (approximately 76%) was obtained with 0.5 M and 1.0 M MSA and 50% IPA in the catholyte. The higher MSA concentrations were also beneficial in preventing the pH from abruptly increasing during the 4 h experiment when the IPA content was greater than 50% and to provide the hydronium ions necessary for the reaction. Several cyclic voltammetry experiments were also conducted to verify the kinetics with and without IPA addition as presented in Figure A.4 (Appendices).94 Figure 5.1. The ECH-DO results of 0.1 M guaiacol with IPA addition using SSER. (a) 0.2 M; (b) 0.5 M; and (c) 1.0 M MSA concentration. Reaction condition: I = - 0.4 A (j = -66 mA cm-2), T = 60 °C, t = 4 h. Catalyst: 5 wt%-Pt/C (0.5 g/L). 95 5.1.1 RSM analysis The RSM analysis results are summarized in Table 5.1. The magnitude and sign of the coefficients for main (β0, β1, β2, and β3), interaction (β12, β13, β23), and curvature (β11, β22, and β33) effects express the relationship given by Equation 3.12. The Pareto chart in Figure A.6 (Appendices) showed that most of the main, interaction, and curvature effects are statistically significant. The countor plot of guaiacol conversion and cyclohexane yield are presented in Figure A.7. The generalized model can be used to predict the product distribution and energy requirement. The comparison of experimental and model data is presented in Figure A.8 (Appendices). The effects of IPA, MSA and reaction time are also graphically summarized in Table 5.2. Table 5.1. The RSM output parameters of MSA and IPA variations. Dependent variable β0 β1 β2 β3 β11 β22 β33 β12 β13 β23 F-value R2 (%) XA -6.6 42.4 0.2 26.1 -35.7 0.004 -2.3 0.1 0.8 -0.02 112.5 94.2 YB 8.6 -2.5 -0.08 8.0 -1.3 -0.0002 -1.3 0.07 -0.3 -0.005 17.0 71.2 YC -2.7 1.5 0.01 5.8 -2.5 -0.0004 -0.4 0.03 -0.05 -0.01 89.8 92.9 YD -1.3 1.4 0.009 1.2 -1.5 -0.00007 0.2 0.01 -0.1 -0.01 140.0 95.3 YE -11.7 25.2 0.4 2.7 -14.9 -0.004 -0.6 -0.05 1.3 0.02 21.6 75.8 YF -0.4 1.9 0.09 -1.4 -1.9 -0.001 0.5 0.006 0.3 0.01 28.9 80.7 YG 3.2 7.2 -0.2 5.7 -6.8 0.002 -0.7 0.007 -0.3 -0.004 36.6 84.2 YH -1.6 4.2 -0.05 1.6 -3.3 0.0006 0.1 0.004 -0.4 -0.01 56.0 89.0 YI -1.6 3.2 0.05 0.2 -2.5 -0.0006 -0.06 -0.008 0.6 0.006 23.1 77.1 YJ 0.8 0.3 -0.008 2.3 -1.1 -0.0001 -0.1 0.02 -0.1 -0.006 53.1 88.5 Y1 6.1 -8.2 -0.05 0.04 4.3 0.002 0.005 0.0002 -0.1 0.008 34.0 83.2 XA (guaiacol conversion, %), YB (phenol yield, %), YC (cyclohexanone yield, %), YD (cyclohexanol yield, %), YE (anisole yield, %), YF (cyclohexane yield, %), YG (2-metoxycyclohexanone yield, %), YH (2-metoxycyclohexanol yield, %), YI (methoxycyclohexane yield, %), YJ (methanol yield, %), Y1 (required energy, Wh). The analysis shows that the MSA concentration has a strong positive main effect on the guaiacol conversion (β1), which is coupled with a significant curvature effect (β11) indicating an 96 overall non-linear effect as shown also schematically in Table 5.2. In terms of interaction effects, there is a positive interaction effect between the MSA concentration and time on both cyclohexane yield and guaiacol conversion, reflected by β13 (Table 5.1). Furthermore, concerning the cyclohexane yield, the interaction between IPA content and time β23, is also positive. Thus, longer reaction times with higher MSA and IPA concentrations favor cyclohexane production. The optimization using RSM showed that the cyclohexane yield was 7.67% at 50% IPA and 1.0 M MSA at 4 h. Table 5.2. Graphical representation of the effects of the independent variables on guaiacol conversion, product yields and energy requirement. 0.1 M guaiacol ECH-DO in SSER with 5 g/L 5%wt-Pt/C, 60 oC and -66 mA cm-2. 5.1.2 Energy-related issues The required energy (Y1) and conversion from Table 5.1 were used to calculate the energy requirement per mole of guaiacol converted as presented in Figure 5.2. Generally, increased MSA concentration lowers the energy requirement. The effect of MSA concentration is more significant in the 0.2 M to 0.5 M range than in the 0.5 M to 1 M range. The reduction in energy requirement with the increase in acid concentration is primarily attributed to the higher electrolyte conductivity and lower surface-activation overpotential for H+ discharge forming Hads. Conversely, an increase in IPA content particularly above 50% by volume, leads to higher energy requirements. However, adding 10% or 25% IPA does not produce a significant difference in energy requirement compared to the case without IPA (Figure 5.2). X X A , % Y B , % Y C , % Y D , % Y E , % Y F ,% Y G , % Y H , % Y I , % Y J , % Y 1 , WhMSA, MIPA, %time, h97 Figure 5.2. The required energy per mole of converted guaiacol with different IPA and MSA concentrations. Reaction condition: I = - 0.4 A (j = -66 mA cm-2), T = 60 °C, t = 4 h. Catalyst: 5 g/L Pt (5 wt%)/C. In addition, there are a few other figures of merit for evaluating the efficiency of electric energy use in an electrosynthesis process. The most well-known is the Faradaic efficiency (F.E., Equation 3.6) whereas some research also used the voltage efficiency (V.E., Equation 3.8). The voltage efficiency expresses the fraction of the total cell voltage applied at the electrodes. Hence, the higher the V.E. the lower the ohmic voltage loss in the cell. Figure 5.3 presents the two-energy efficiency-related terms to examine the effect of IPA and MSA on the ECH-DO of guaiacol in the SSER. The design requires a highly conductive electrolyte because the V.E. falls below 50% when the acid concentration, MSA, is below 0.5 M (Figure 5.3a). Consequently, the introduction of the organic solvent IPA generally lowers the voltage efficiency. A higher V.E. (57%) occurred with the combination of 1.0 M MSA and 50% IPA addition. On the other hand, the Faradaic efficiency is mainly related to the competition between the main reaction (ECH of guaiacol) and the side reaction (hydrogen evolution reaction or HER). The 98 MSA concentration had a positive effect on F.E., as shown in Figure 5.3b, reaching a value of up to 73% at 1 M MSA and 50% or 75% IPA. Generally, high IPA concentration does not have a significant impact on the F.E. because the proton could be donated by the IPA to hydrogenate guaiacol or intermediates instead of competing with HER. Meanwhile, the complete faradaic efficiency profile could be seen in Figure A.9 of guaiacol ECH-DO using SSER at 0.2 M, 0.5 M, and 1.0 M of MSA concentration. Figure 5.3. Energy efficiency in terms of (a) Voltage efficiency (V.E.) and (b) Faradaic efficiency (F.E.) with different IPA and MSA concentrations on ECH of guaiacol. Reaction condition: I = - 0.4 A (j = -66 mA cm-2), T = 60 °C, t = 4 h. Catalyst: 5 wt%-Pt/C (0.5 g/L). 5.1.3 Product distribution analysis Examining the product distribution is essential for assessing the effectiveness of the guaiacol conversion process. One of the research aims is to increase the H/C ratio while decreasing the O/C ratio in the biofuel product. Thus, in this study, we investigated the H/C and O/C ratio using the van Krevelen plot, shown in Figure 5.4. The van Krevelen diagrams provide an easy method to evaluate, compare, and investigate the biomass conversion process.393 The IPA addition generally (a) (b) 99 decreased the O/C ratio through C–O hydrogenolysis compared to the case without IPA. However, the maximum IPA content should not be higher than 50% by volume because it is not advantageous for either further lowering the O/C ratio or increasing the H/C ratio (Figure 5.4) and may introduce product separation issues at a later stage. The insert picture in Figure 5.4 gives a brief perspective of the electrocatalytic hydrodeoxygenation process of bio-oil model compound within hour to produce color change. Figure 5.4. The van Krevelen plot of experimental results with 0.2 M (black), 0.5 M (blue), and 1.0 M MSA (red). Reaction condition: I = - 0.4 A (j = -66 mA cm-2), T = 60 °C, t = 4 h. Catalyst: 5 wt%-Pt/C (0.5 g/L). Literature reports on relevant thermocatalytic reactions performed in the presence of IPA, showed that the addition of IPA could convert cyclohexanol to cyclohexane with ~60% selectivity over RuRe/C bimetallic catalyst in Catalytic Transfer Hydrogenation/Hydrogenolysis (CTH) of guaiacol.34 In another experiment, the chemically transferred hydrogen from IPA for aromatic hydrogenation was shown to be more selective compared to molecular hydrogen (84% vs. 56%).394 Additionally, IPA might affect the stability of the protonated transition states, consequently 100 accelerating the rates of the dehydration reaction.192, 395 Meanwhile, IPA undergoes dehydrogenation during the reaction to donate its hydrogen and produce acetone, detected by GC-MS in this study. 5.1.4 Solvent recovery and compatibility In addition to the reaction kinetic benefits of IPA for the ECH-DO of guaiacol as demonstrated here, IPA is also one of the recommended solvents according to the selection guide for green solvents.396 As a consideration for process design, solvent recovery is another issue that strongly affects the overall process economics. Fortunately, IPA forms a homogeneous minimum boiling azeotrope with water at 353 K and 1 atm with 87.4 – 87.7 mass % of IPA.397, 398 Since the ECH process takes place in aqueous media, breaking the water-IPA azeotrope to obtain higher purity IPA might not be necessary. Furthermore, cyclohexane, the desired product in the ECH, is a suitable entrainer agent to separate up to 99.96 wt% of IPA by azeotropic distillation which lowers the original azeotrope boiling point.398 Based on this preliminary assessment, IPA is also compatible with process design and product recovery. 5.2 Electrolyte pairs selection The evaluation of the selection of half-cell electrolyte pairs was performed under different electrolyte pairs (cathode – anode), namely acid-acid, acid-neutral, and acid-alkaline. In this study, NaCl is also introduced in the catholyte and anolyte. Not only increasing the electrolyte conductivity, NaCl increased the glycerol conversion (45% vs. 60%) while maintaining relatively broad distribution of oxidation products.399 This study also demonstrated a similar trend with the addition of NaCl in the anode chamber, enhancing glycerol conversion from 22% to 48%, as shown in Figure A.10 (Appendices). CO2 formation was also suppressed by NaCl addition in the anode chamber, reducing the over-oxidation of glycerol.399 Moreover, another study revealed that 101 HClO4 has lower adsorption strength order compared to H2SO4 and H3PO4 on platinum electrodes400, which is beneficial in catalyst surface competition while maintaining the conductivity. The half-cell GEOR in alkaline conditions generated higher activity (XA) after 4 h compared to acidic and neutral conditions (85.7% > 34.8% > 29.6%) with significant difference level on the initial reaction rate value (1.56a > 1.02b > 0.85b ×105 mol of reactant s−1 gPt−1) as presented in Table 5.3. Table 5.3. The effect of electrolyte pairs in half-cell electrosynthesis of GEOR. Parameter 1.0 M MSA (c) - (a) 0.5 M MSA (c) - (a) 0.2 M MSA (c) - (a) MSA (c) - NaOH (a) (1.0 M) 1.0 M MSA (c) - DI water (a) Ec (V) -0.92 -0.71 -1.88 -1.20 -0.33 Ea(V) 1.84 2.83 2.04 2.67 4.84 Ecell (V) 9.0 12.3 21.7 14.5 15.3 pH 0.2 0.4 0.1 12.1 0.4 XA (%) 34.8 24.4 23.8 85.7 29.6 SB (%) 33.5 27.3 14.2 3.2 28.6 SC (%) 1.3 4.1 6.0 28.7 21.7 SD (%) 4.3 6.9 11.0 5.6 5.5 SE (%) 5.6 21.6 31.1 11.6 10.4 SF (%) 0.2 0.1 0.2 0.1 0.2 SG (%) 2.5 19.6 25.3 3.5 20.5 SH (%) 21.4 14.8 5.8 24.2 6.7 SI (%) 31.2 5.6 6.4 23.1 6.4 SJ (%) 0.0 0.0 0.0 0.0 0.0 C.B. (%) 98.2 96.7 97.7 87.2 97.2 F.E. (%) 35.4 17.1 16.1 99.5 17.6 rx,0 × 105* 1.02b 0.69b 0.66b 1.56a 0.85b *Significance difference level between groups as formulated in Equation 3.12. Conditions: T = 60 °C, j = 66 mA cm−2, N = 600 rpm, Catalyst loading = 0.50 g (5 g/L), Reaction time = 4 h, all electrolytes also containing 0.25 M NaCl and 25% v/v IPA in the cathode chamber. E is either cathode or anode potential (vs. RHE) measured by reference electrode (Ag/AgCl), Ecell was recorded from the potentiostat, pH was measured after the reaction. C.B. carbon balance, F.E. faradaic efficiency, rx,0 initial reaction rate (mol of reactant s−1 gPt−1). XA glycerol conversion, S normalized product selectivity (C mol%) for glyceraldehyde (B), glyceric acid (C), glycolic acid (D), tartronic acid (E), dihydroxyacetone, DHA (F), lactic acid (G), Formic acid (H), and acetic acid (I). 102 In alkaline conditions, the major products were the salt of glyceric acid, formic acid, acetic acid, and tartronic acid, indicating a higher degree of oxidation with formic acid and acetic acid as the primary products. Due to simplicity, the acid form was reported in Table 5.3. Meanwhile, in acidic anolyte (e.g., 1.0 M MSA), the selectivity was much higher toward glyceraldehyde, formic acid, and acetic acid (Table 5.3). However, by lowering the acid concentration (from 1.0 M to 0.2 M) selectivity shifted to glycolic acid, tartronic acid, and lactic acid. The lower acidity of the anolyte favored the formation of more oxidized products rather than stopping at intermediates like glyceraldehyde. Few studies have tried to use acidic conditions to produce valuable chemicals in GEOR as presented in Table A.2 (Appendices). Meanwhile, some studies also tried to utilize non-noble metal (Ni/C and NiCo/C) in alkaline conditions, as stated in Table A.2. Glyceraldehyde was selectively generated by introducing Pt/C catalyst instead of Ni/C and NiCo/C catalysts. Moreover, glyceraldehyde is considered a reactive intermediate401, thus targeting glyceraldehyde as the main product is challenging. Additionally, the anode side carbon balance (C.B.) closure was much higher with acid anolyte (Table 5.3). The low C.B. of 87.2% with alkaline anolyte (Table 5.3 suggests the formation of CO2 in the gas phase, further substantiating the more complete electrooxidation of glycerol under alkaline conditions. Lee et al.118 have reported that glycerol produced the selectivity of glyceraldehyde (63%), glyceric acid (22%), and dihydroxyacetone (6%) with 76% overall conversion in 0.5 M H2SO4 electrolyte, at 0.8 V vs. SHE, 60 °C, and using Pt/C catalyst-coated on the carbon paper electrode after 4 h of reaction.118 In another study reported by Kim et al.309, glyceraldehyde and glyceric acid were produced with selectivities of 42.1% and 57.8%, respectively, and a conversion of 20%.309 The study employed 0.5 M H2SO4 electrolyte, at 1.1 V vs. SHE, 60 °C, using Pt/C catalyst-coated carbon paper electrode for 7 h. 103 The electrooxidation of glycerol in alkaline electrolytes is typically preferred over acidic media due to several significant advantages that enhance both the efficiency and durability of the electrochemical process. Firstly, alkaline conditions provide a more favorable environment for the oxidation reaction, resulting in higher reaction rates402 caused by hydroxide ions (OH⁻) that facilitate the deprotonation of glycerol. Secondly, the stability of anode materials is markedly improved in alkaline media. Noble metal catalysts such as Pt and Pd, which are commonly used for glycerol oxidation, suffer from corrosion in acidic environments.341 This condition is aggravated with non-noble metal catalysts.155, 403-405 On the other hand, metal catalysts are less prone to degradation in basic solutions, thus ensuring a longer operational lifespan and consistent performance.341 Collectively, these factors underscore the superiority of alkaline electrolytes for the electrooxidation of glycerol, offering improved kinetics, enhanced catalyst performance, and greater electrode stability, thereby making them the preferred choice for this process in both research and industrial applications. However, in the present study we seek optimum conditions for both chambers, cathode and anode. Thus, using electrolyte pairs is the best approach to compare the product distribution and efficiency. In the cathode chamber, the acid – acid electrolyte pairs had the best condition for the ECH-DO of guaiacol in regards to conversion, initial reaction rate, and faradaic efficiency as seen in Table 5.4. The highest cyclohexane yield (7.1%) was obtained with 1.0 M MSA acid – acid electrolyte pairs. As higher acid concentration provides higher proton availability in the electrosynthesis system, the ECH-DO degree of guaiacol must be improved compared to the other conditions. The lower acid concentration (1.0 M to 0.2 M) tends to produce higher anisole yield of ECH-DO of guaiacol due to the higher possibility of IPA as hydrogen donor instead of the water. Furthermore, alkaline conditions are preferred for the glycerol fuel cell application due to 104 the C–C bond cleavage and high reaction rate to facilitate the hydrogen production in the cathode chamber.399, 403 On the other hand, the acid electrolyte offers the value added chemical products such as the dihydroxyacetone, glyceraldehyde, glyceric acid, and tartronic acid. Therefore, the present study will focus on the acid – acid electrolyte pairs to preserve the C–C bonds and produce valuable intermediates chemicals. Table 5.4. The effect of electrolyte pairs in half-cell electrosynthesis of GECH-DO. Parameter 1.0 M MSA (c) - (a) 0.5 M MSA (c) - (a) 0.2 M MSA (c) - (a) MSA (c) - NaOH (a) (1.0 M) 1.0 M MSA (c) - DI water (a) Ec (V) -0.37 -1.06 -1.27 -1.52 -0.35 Ea(V) 2.68 3.65 2.35 2.71 3.13 Ecell (V) 12.9 15.7 20.6 15.0 12.7 pH -0.1 0.2 1.0 1.4 0.4 XA (%) 73.1 74.6 63.8 61.3 61.6 SB (%) 7.5 5.4 5.3 6.1 3.7 SC (%) 30.6 22.9 19.3 27.0 19.2 SD (%) 11.0 8.5 8.3 10.3 8.2 SE (%) 16.7 24.1 20.2 1.7 20.1 SF (%) 7.1 6.0 4.5 6.1 5.1 SG (%) 10.3 14.3 22.7 17.1 17.7 SH (%) 2.8 6.0 12.1 7.9 8.3 SI (%) 4.8 5.5 1.5 16.3 12.1 SJ (%) 9.2 7.1 6.2 7.9 5.6 C.B. (%) ~99.9 ~99.9 ~99.9 ~99.9 100.0 F.E. (%) 87.2 81.6 64.3 84.7 66.7 rx,0 × 105* 2.54a 2.65a 2.13b 1.06c 2.05b *Significance difference level between groups as calculated by Equation 3.12. Conditions: T = 60 °C, j = 66 mA cm−2, N = 600 rpm, Catalyst loading = 0.50 g (5 g/L), Reaction time = 4 h, 0.25 M NaCl, 25% v/v IPA in the cathode chamber. E is either cathode or anode potential (vs. RHE) measured by reference electrode (Ag/AgCl), Ecell was recorded from the potentiostat, pH was measured after the reaction. C.B. carbon balance, F.E. Faradaic efficiency, rx,0 initial reaction rate (mol of reactant s−1 gPt−1). XA guaiacol conversion, S normalized product selectivity (C mol%) for phenol (B), cyclohexanone (C), cyclohexanol (D), anisole (E), cyclohexane (F), 2-methoxycyclohexanone (G), 2-methoxycyclohexanol (H), methoxycylohexane (I), and methanol (J). 5.3 Catalyst screening The acid – acid (1.0 M MSA) electrolyte pair was retained in the anode catalyst screening as it yielded the best results in half-cell of GECH-DO and GEOR. Platinum group metals (PGM) have 105 been widely used in hydrogenation54 or oxidation55 reactions. For electrocatalytic hydrogenation of guaiacol and phenol, Wijaya et al.106 screened a series of 5 wt% Pt/C, Pd/C, and Ru/C catalysts. The study evaluated electrolyte pairs with the reaction activity order of Pt/C > Ru/C > Pd/C, with the Ru/C and Pd/C catalysts as much as ~6 times and ~8 times lower compared to the Pt/C, respectively, at reaction condition of 50 °C and -109 mA cm-2 for 4 h of reaction. Therefore, based also on our own previous study of GECH-DO, Pt/C was chosen as the catalyst for the cathodic reaction.44 From XRD analysis of commercial noble catalyst Ru/C, Pd/C and Pt/C with 5 wt% resulted in the typical fingerprint of Ru (01-088-1734), Pd (01-087-0643), and Pt/C (01-070-2057), respectively. Ru/C catalyst had (100) and (101) face-centered-cubic (fcc) facet at 2θ of 36.57° and 43.03° as shown in Figure 5.5c. Meanwhile, Pd/C had (111) and (220) fcc at 39.82° and 68.07° (Figure 5.5b). The commercial Pt/C reported in Figure 5.5a, on the other hand, corresponds to the planes (111), (200), and (220). The facet (111) of Pt/C and Pd/C corresponds to the hydrogenation reaction because the (111) facet is the most compact and energetically stable face, which leads to an abundance of this facet on industrial catalysts. The Ru/C only offers facet (100) for the hydrogenation reaction. Thus, the activity of guaiacol and phenol electrocatalytic hydrogenation was in order of Pt/C > Pd/C > Ru/C in the acid-acid pair electrolyte at reaction conditions of 50 °C and -109 mA cm-2 for 4 h of reaction. Due to the carbon support and low metal loading, the XRD spectra showed amorphous phases in all of the commercial PGM catalysts. 106 Figure 5.5. The XRD spectra of commercial (a) Pt/C, (b) Pd/C and (c) Ru/C catalysts with 5 wt% of the metal. As a preliminary experiment, cyclic voltammetry analysis was conducted to evaluate the oxidation activity peaks of the PGM catalysts. Compared to an Al2O3 support, the carbon support performed better both in the cathodic and anodic regions, as seen in Figure 5.6b, due to higher conductivity of the carbon support. The electrical conductivity of Al2O3 and carbon black are 10-16 S/cm406 and 0.1 – 102 S/cm407, respectively, at ambient temperature. All PGM catalysts showed comparable HER and OER activities, thus, the glycerol electrooxidation experiments were performed to obtain the best PGM catalyst performance. On the other hand, the higher temperature (25 °C to 60 °C) in Figure 5.6 multiply the activity of OER ~ 2 times. 10 20 30 40 50 60 70Intensity (a.u.)2θ (Degrees)Pd (01-087-0643)(111) (220)C(002) 5% wt. Pd/C10 20 30 40 50 60 70Intensity (a.u.)2θ (Degrees)Ru (01-088-1734)(100)(101)C(002) 5% wt. Ru/C(a) (b) (c) 107 Figure 5.6. The cyclic voltammetry of half-cell glycerol Electrooxidation in stirred slurry electrocatalytic reactor at (a) 25 °C and (b) 60 °C with several potential catalysts. Table 5.5 compares the performance of carbon supported PGM catalysts for the GEOR with 5wt.% metal loading. As seen in Table 5.5, the Ru/C catalyst produced the highest conversion for glycerol oxidation (58.7%), followed by Pd/C and Pt/C. However, the higher conversion on Ru/C was not accompanied by higher selectivity toward glyceraldehyde. Meanwhile, both the selectivity towards glyceraldehyde (35.6%) and the initial reaction rate were highest for the Pt/C catalyst in the anodic SSER. The Ru/C and Pd/C catalysts had lower liquid phase carbon balances (C.B.) due to complete GEOR producing CO2. This observation is supported by the lower formic acid (H) and acetic acid (I) selectivity on Ru/C and Pd/C catalysts. The undetected CO2 formation also leads to the conversion not being aligned with the Faradaic Efficiency (F.E.) on Ru/C and Pd/C when compared to reactant conversion. In terms of catalyst properties, the Pt/C is considered to have the highest catalytic activity for glyceraldehyde formation per site of metallic surface area compared to the other PGMs (Ru/C and Pd/C), even though the Pt/C has lower metallic surface area as seen in Table B.1 (Appendices), because of the higher selectivity of the Pt/C catalyst in acidic condition. -1 0 1 2-2500250500Potential, E(V) vs. Ag/AgCl Pt/C Pt/Al2O3 Ni/SiO2-Al2O3 Ru/C Pd/C-1 0 1 2-2500250500Potential, E(V) vs. Ag/AgCl Pt/C Pt/Al2O3 Ni/SiO2-Al2O3 Ru/C Pd/C(b) (a) 108 Moreover, Pt/C has better properties compared to the other PGM (Ru/C and Pd/C) in regards of surface area (~1500 m2/g), pore volume (1.4 cm3/g), and metal dispersion (28.9%). Table 5.5. Galvanostatic glycerol electrooxidation results in the SSER anodic half-cell using PGMs. Catalyst Pt/C Ru/C Pd/C Ea (V) 1.7 1.92 1.92 pHa 0.02 -0.26 -0.24 XA (%) 36.2 ± 1.4 58.7 ± 3.3 46.7 ± 3.4 SB (%) 35.6 ± 1.3 10.3 ± 1 8.0 ± 0.8 SC (%) 1.2 ± 0.1 12.3 ± 0.8 33.6 ± 2.7 SD (%) 2.7 ± 1 5.6 ± 0.4 16.8 ± 0.2 SE (%) 2.9 ± 1.4 16.5 ± 1.1 12.7 ± 1.3 SF (%) 0.2 ± 0 0.1 ± 0 0.2 ± 0 SG (%) 2.9 ± 0 39.6 ± 1.7 18.3 ± 0.1 SH (%) 22.5 ± 0.6 11.9 ± 1.4 6.1 ± 0.2 SI (%) 32.0 ± 0.6 3.6 ± 0.2 4.3 ± 0 C.B. (%) 98.9 ± 3.8 84.7 ± 0.3 86.5 ± 2.4 F.E. (%) 57.4 ± 1.3 52.7 ± 3.5 43.0 ± 2.5 r0 × 105 7.14 ± 0.09 6.20 ± 0.10 6.04 ± 0.04 Conditions: T = 60 °C, j = 66 mA cm−2, N = 600 rpm, Catalyst loading = 0.50 g (5 g/L), Reaction time = 4 h. Electrolyte: 1.0 M MSA and 0.25 NaCl (catholyte-anolyte). Ea is anode potential (vs. RHE) measured by reference electrode (Ag/AgCl), pH was measured after the reaction, XA glycerol conversion, S normalized product selectivity (C mol%) for glyceraldehyde (B), glyceric acid (C), glycolic acid (D), tartronic acid (E), dihydroxyacetone, DHA (F), lactic acid (G), Formic acid (H), and acetic acid (I), C.B. carbon balance, F.E. Faradaic efficiency, r0 initial reaction rate at 1 h (mol of glycerol h−1 mmol of dispersed metal−1). The lower pH of the anode chamber indicates a higher GEOR activity which produces hydrogen ions as shown by the thermodynamic data (Table A.4 in Appendices). In terms of Faradaic Efficiency (F.E.), Pt/C is the most efficient catalyst for the valuable intermediate products of GEOR. Garcia et al.408 reported that a Pt (100) electrode leads to glyceraldehyde product only while Pt (111) electrode generates glyceraldehyde, glyceric acid and dihydroxyacetone, which is in line with the present study using polycrystalline Pt facet (111), (200), and (220). 109 Figure 5.7. Glycerol oxidation mechanism on Pt(100) and Pt(111) in acidic condition. Reproduced from ref. 408 with permission from the American Chemical Society, Copyright 2016. 5.4 Summary of the chapter The half-cell optimization process was conducted in several steps. First, isopropanol was introduced in the cathode chamber to trigger the deoxygenation reaction of guaiacol. The addition of IPA has a positive effect on the ECH-DO process of guaiacol in SSER by facilitating the production of cyclohexane. The RSM optimization process revealed a maximum cyclohexane yield of 7.67% with 50% by volume IPA and 1.0 M MSA in the catholyte during 4 h reaction at a cathode superficial current density of -66 mA cm-2 and 60 oC. Both the product distribution analysis and the energy efficiency analysis showed that a higher than 50% IPA addition is not preferable. The product distribution analysis coupled with reaction network modeling revealed that 110 in the presence of IPA the anisole pathway (i.e. guaiacol dehydration pathway), is operative as well. A guaiacol conversion of 69.3% was obtained at 25% by volume IPA under these conditions and with 1.0 M MSA, a cyclohexane yield of 6.7% was obtained with low energy demand (15.76 Wh mol-1). To conclude, the IPA effects are attributed to (i) increased reactant solubility as isopropanol is a protic organic solvent, (ii) additional source of protons and/or adsorbed hydrogen on catalytic active sites, (iii) lower guaiacol dehydration activation energy leading to anisole intermediate, and (iv) projected ease of product separation and solvent recovery. Secondly, the electrolyte pairs were evaluated in the half-cell optimization. In the cathode chamber, the higher proton concentration in acid – acid electrolyte pairs was preferred due to the higher conversion, initial reaction rate, and faradaic efficiency. The higher the MSA concentration, 1.0 M, produced higher cyclohexane selectivity (7.1%) with lower cell potential (12.9 V). Presumably, the acid – alkaline electrolyte pair electrolyte is the best option in the anode chamber due to higher hydroxyl (OH-) ions availability and lower cell overpotential. The presence of OH- ions could facilitate the reaction kinetics by electron transfer and improving the oxidation process. Surprisingly, compared to the acid – alkaline electrolyte pairs, our study concluded that 1.0 M MSA electrolyte pairs (acid – acid) were best in terms of glyceraldehyde (12% vs. 3%) yield and carbon balance (98% vs. 87%). The significant decrease of cell potential in acid – acid electrolyte pairs makes the process more energy-efficient, which is crucial in scaling up electrocatalytic processes for industrial applications. Lastly, the noble metal catalysts were screened through two methods, namely, cyclic voltammetry and glycerol electrooxidation. The carbon supported catalyst showed better properties due to higher conductivity compared to the Al2O3 support. In the cathode chamber, the PGM catalysts of Pt, Pd, and Ru had been evaluated by Wijaya et al.106. The study concluded that 111 the catalytic activity increased in the order Pt/C > Pd/C > Ru/C in acid-acid pair electrolyte at reaction condition 50 °C and -109 mA cm-2 for 4 h of reaction. In the present study, the PGM catalysts were evaluated in the anode chamber. In terms of glyceraldehyde selectivity (35.6%) and carbon balance (98.9%), Pt/C was superior compared to the Pd/C and Ru/C catalysts. Pd/C and Ru/C generated CO2, an undesirable product, inferred from the low carbon balance value, 86.5% and 84.7%, respectively. Therefore, Pt/C catalyst was used in the cathode and anode chamber as the reference commercial noble metal catalyst in this study. 112 Chapter 6: Paired Electrosynthesis Optimization Process In laboratory-scale experiments, the paired electrosynthesis method for upgrading lignin bio-oil typically concentrates on individual reactants, such as the reduction and oxidation of hydroxymethylfurfural (HMF)74, 76 and furfural409. However, other studies have explored combinations such as CO2–1,2 Propanediol410, p-nitrophenol–HMF73, and CO2–glycerol77 for cathode and anode reactants, respectively. Unfortunately, many of these reactants are costly or require gas storage facilities. In contrast, this study explores, for the first time, the combination of guaiacol and glycerol as the reactants for the cathode and anode chambers, respectively due to their significant potential and cost-effectiveness. Guaiacol, a lignin bio-oil model compound, and glycerol, a by-product of the biodiesel industry (e.g., derived from palm oil), can be converted simultaneously to value-added products as shown in Figure 6.1. The higher price and wide industrial application of selected products shown by Figure 6.1, are the main reason for coupling the guaiacol electrocatalytic hydrodeoxygenation (GECH-DO) and glycerol electrooxidation reactions (GEOR) in the paired electrosynthesis study. Moreover, the guaiacol or bio-oil electrocatalytic hydrodeoxygenation produces methanol which is required in the biodiesel industry as raw materials. Therefore, the paired electrosynthesis of GECH-DO and GEOR creates synergistically effect on the raw materials proximity and supports the circular economy of the chemicals and materials. In the present study, cyclohexane has been generated by introducing isopropanol in the stirred slurry electrocatalytic reactor (SSER) and enabling the dehydration reaction, which improves the guaiacol hydrodeoxygenation process.44 Compared to cyclohexanol ($1.48/kg) and cyclohexanone ($1.25/kg), the cyclohexane ($0.99/kg) price is quite low.411 However, higher energy density and greener hydrocarbon products are obtained by higher degrees of hydrogenation and 113 deoxygenation.44 Cyclohexane is regarded as the best chemical to exemplify the successful hydrodeoxygenation of guaiacol, as it serves as the final product of the reaction. Regarding anodic products, glyceraldehyde is a high value product (approximately $20,000/kg383) because a suitable technology for producing glyceraldehyde on large scale does not yet exist. Moreover, glyceraldehyde is broadly applied in many industries as summarized in Table 4.14. Figure 6.1. Raw material to product of the proposed paired electrosynthesis system in this study. To the best of the author’s knowledge, there has been no prior exploration into the paired electrosynthesis of GECH-DO and GEOR. Therefore, this study aims to investigate the paired electrosynthesis for producing valuable chemicals such as glyceraldehyde and cyclohexane. The paired-cell experiments were conducted simultaneously in the cathode and anode chambers with typical reaction conditions in the SSER configuration: CA0 = 0.1 M (guaiacol and glycerol), I = 0.4 A (66 mA.cm-2), N = 600 rpm, T = 60 °C, t = 4 h, NaCl = 0.25 M (catholyte and anolyte), 25% vol. IPA addition in catholyte, and Pt/C = 5 wt.% (0.5 g/L). Meanwhile, several parameters are explored including the electrolyte pairs, reactor agitation speed, metal loading of the catalyst, the 114 initial concentration of reactants, temperature, and the current density, to obtain an optimal set of conditions for the paired electrosynthesis. A tailored catalyst is also introduced in this chapter to decrease dependency on noble metals as the electrocatalysts. 6.1 Electrolyte pairs selection The effect of paired cell configurations using similar electrolyte pairs in half-cell electrosynthesis was investigated to highlight the advantages of paired cell electrosynthesis over half-cell electrosynthesis. As seen in Table 6.1, the acid – acid electrolyte pairs (1.0 M) were the best electrolyte with higher conversion and kinetics in the cathode chamber and high glyceraldehyde yield along with the highest initial reaction rate in the anode chamber. In terms of energy required per converted mole of reactant (guaiacol and glycerol in paired cells), the concentrated acid-acid (1.0 M) and acid-base (1.0 M) electrolyte pairs were the best at 13.9 𝑀𝑀ℎ 𝑚𝑚𝑟𝑟𝑙𝑙 𝐶𝐶𝐴𝐴−1 and 18.1 𝑀𝑀ℎ 𝑚𝑚𝑟𝑟𝑙𝑙 𝐶𝐶𝐴𝐴−1, respectively, according to Figure 6.2. Meanwhile, the paired cell electrosynthesis yielded values quite similar to the half-cell electrosynthesis, as seen in Table 5.3 and Table 6.1. This indicates that the paired cell method did not affect the product output while reducing the required energy per reactant converted into product, as clearly shown in Figure 6.2. To illustrate the product distribution, the results from half-cell and paired cell electrosynthesis using the 1.0 M acid-acid electrolyte pair are shown in Figure 6.3. Significant differences between the half-cell and paired electrosynthesis are observed in terms of guaiacol conversion and faradaic efficiency. The faradaic efficiency in the cathode chamber increased from 87.2% to 97.4% on average for the paired cell electrosynthesis, which aligns with the higher guaiacol conversion in the latter system (79% vs. 73%). Furthermore, the phenol selectivity decreased from the half-cell (29.1%) to paired cell (20.8%) system, as seen in Figures 6.3a and 6.3c. This indicates that instead of demethoxylation through 115 phenol, the ring saturation pathway was preferable in the paired systems, as evidenced by an increase in 2-methoxycyclohexanone selectivity from 12.9% to 33.3%. The paired electrosynthesis offered lower cell potential (9.4 V) compared with the half-cell electrosynthesis (12.9 V); hence, it might affect the reaction pathway shift to the ring saturation pathway. Based on the thermodynamics data of GECH-DO from Table A.3 (Appendices), the ring saturation (0.14 V vs. SHE) prefers lower potential compared with the demethoxylation pathway (0.38 V vs. SHE). Table 6.1. The effect of electrolyte pairs in paired cell of GECH-DO or GEOR. Parameter 1.0 M MSA (c) - (a) 0.5 M MSA (c) - (a) 0.2 M MSA (c) - (a) MSA (c) - NaOH (a) (1.0 M) 1.0 M MSA (c) - DI water (a) Cathode Anode Cathode Anode Cathode Anode Cathode Anode Cathode Anode E (V) -0.42 1.51 -0.62 2.25 -1.90 2.26 -1.15 2.85 -0.58 2.45 Ecell (V) 9.4 12.3 22.2 13.7 14.9 pH 0.1 0.0 0.5 0.4 1.3 0.5 1.4 12.0 0.3 0.3 XA (%) 79.1 27.6 71.7 21.8 71.4 23.1 60.4 77.0 64.8 26.8 SB (%) 4.3 12.5 3.7 30.2 3.7 11.2 5.1 3.3 4.3 17.8 SC (%) 23.5 4.0 18.7 2.9 20.2 10.1 21.5 38.5 20.4 14.5 SD (%) 11.5 9.5 9.2 11.0 9.0 9.3 9.1 7.6 9.1 9.2 SE (%) 8.5 11.2 14.6 20.3 15.2 25.1 17.9 23.7 20.6 25.4 SF (%) 6.9 0.2 4.8 0.1 4.8 0.1 5.5 0.0 5.7 0.1 SG (%) 19.3 4.4 23.1 7.5 22.6 29.5 19.5 0.8 18.2 20.5 SH (%) 14.0 32.8 14.4 22.4 13.3 6.7 8.7 15.0 9.6 6.6 SI (%) 4.7 25.4 5.9 5.7 5.5 7.9 6.3 11.2 6.1 5.9 SJ (%) 7.1 0.0 5.6 0.0 5.9 0.0 6.4 0.0 6.0 0.0 C.B. (%) 99.7 96.2 ~99.9 96.4 99.9 98.0 ~99.9 93.2 ~99.9 97.0 F.E. (%) 97.4 34.7 70.0 17.8 69.7 15.4 60.2 89.2 76.2 17.4 rx,0 × 105 2.84a 0.75b 2.49b 0.60c 2.45b 0.64c 0.77d 1.73a 2.19b 0.71b *Significance difference level between group of Cathode and Anode as formulated in Equation 3.12. Conditions: T = 60 °C, j = 66 mA cm−2, N = 600 rpm, Catalyst loading = 0.50 g (5 g/L), Reaction time = 4 h, all electrolytes also containing 0.25 M NaCl and 25% v/v IPA in the cathode chamber. E is either cathode or anode potential (vs. RHE) measured by reference electrode (Ag/AgCl), Ecell was recorded from the potentiostat, pH was measured after the reaction. C.B. carbon balance, F.E. faradaic efficiency, rx,0 initial reaction rate (mol of reactant s−1 gPt−1). Cathode: XA guaiacol conversion, S normalized product selectivity (C mol%) for phenol (B), cyclohexanone (C), cyclohexanol (D), anisole (E), cyclohexane (F), 2-methoxycyclohexanone (G), 2-methoxycyclohexanol (H), methoxycylohexane (I), and methanol (J). Anode: XA glycerol conversion, S normalized product selectivity (C mol%) for glyceraldehyde (B), glyceric acid (C), glycolic acid (D), tartronic acid (E), dihydroxyacetone, DHA (F), lactic acid (G), Formic acid (H), and acetic acid (I). 116 The conversion of glycerol decreased from 34.8% to 27.6% as seen in Figure 6.3b and 6.3d. The glyceraldehyde selectivity also decreased from 33.2% to 12.8% at 4 h of reaction. In terms of faradaic efficiency, the paired cell electrosynthesis (34.7%) had a value comparable with the half-cell electrosynthesis (35.4%) average value. Thus, the rate of glycerol consumption was not hampered by pairing the electrosynthesis cell with GECH-DO at the cathode. However, it negatively affects the product distribution of GEOR toward glyceraldehyde production. On the other hand, the intermediate products are evenly distributed to generate glyceric acid (3.6%), glycolic acid (10.2%), and tartronic acid (11.6%) at 4 h of reaction compared to the half-cell electrosynthesis; 1.5%, 4.6%, and 5.7% for glyceric acid, glycolic acid, and tartronic acid, respectively. Figure 6.2. The required energy per mole of converted guaiacol or glycerol with different electrolyte pairs. Reaction condition: j = 66 mA cm-2, N = 600 rpm, T = 60 °C, t = 4 h. Catalyst: 5 wt%-Pt/C (5 g/L). Electrolyte: 0.25 M NaCl with 25% v/v IPA addition in the cathode chamber. Calculated based on Equation 3.10.117 Figure 6.3. Acid-acid (1.0 M) electrolyte pairs of GECH-DO (a and c) and GEOR (b and d) in half-cell (left side) and paired cell (right side) system. Reaction condition: j = 66 mA cm-2, N = 600 rpm, T = 60 °C, t = 4 h. Catalyst: 5 wt%-Pt/C (5 g/L. Anolyte and catholyte consist of 0.25 M NaCl with 25% v/v IPA addition in the cathode chamber. (a) (c) (b) (d) 118 Organic synthesis, which involves producing numerous intermediate chemicals, is inherently complex due to factors such as reaction pathways, reaction conditions, purification and separation issues, and scale-up challenges. Optimizing the process involves fine-tuning of parameters like electrolyte pairs, catalyst type and loading, solvent addition, and temperature. Therefore, the paired electrosynthesis system was optimized by adjusting parameters such as agitation speed, platinum loading, current density, initial reactant concentration, and reaction temperature to achieve the best conditions for GECH-DO and GEOR by employing 1.0 M MSA concentration and Pt/C catalyst in the stirred slurry electrocatalytic reactor (SSER). 6.2 Paired cell – operational enhancements To assess mass transfer limitations in the SSER, agitation speed was varied at 400, 500, and 600 rpm and evaluated using the Weisz-Prater criteria412 and Sherwood number (Table A.5 and A.6 in Appendices). Results showed no significant internal and external mass transfer limitations. A summary of the effect of agitation speeds is presented in Table B.3 (Appendices), showing minimal differences in results across the variations, as all speeds fall within the same turbulent regime. Since the shaft energy demand, typically used in the industrial reactor, is proportional to the cube of the agitation speed (P ∝ N³) as described by Hall413, reducing the agitation speed would decrease the required power. Consequently, the optimal condition for paired electrosynthesis was achieved at 400 rpm. As mentioned by Wijaya105, achieving optimal agitation speed is crucial for efficient electrosynthesis in the SSER to guarantee an effective dispersion of the catalyst particles with uniform contact of the organic molecules and the current collector. Table B.3 also presents the carbon support Pt loading effect on the paired electrosynthesis conversion and product distribution in the range of 1 to 10 wt.%Pt/C. For both GECH-DO and GEOR, the highest conversions were obtained for 5 wt.%Pt, 79% for guaiacol and 27.6% for 119 glycerol, at 60°C and 66 mA cm−2, respectively. The highest conversions obtained with 5 wt.%Pt/C correlate with the highest BET surface area (1487 m² g-1 Table B.1). The total conversions after 4 h of reaction for 1 wt.% Pt/C in GECH-DO and GEOR were only 23.3% and 12.8%, respectively, attributed to the limited surface area (0.45 m² per gram of Pt) compared to 1.78 m² for the 5 wt.% Pt/C. With 5% Pt/C, conversions of 79.1% guaiacol (-0.62 V vs. RHE) and 27.6% conversion of and glycerol (1.78 V vs. RHE), respectively, at 60°C and 66 mA cm−2 gave superior performance (Table B.3). Additionally, both the conversion and faradic efficiency for GECH-DO and GEOR decreased at 10 wt.%-Pt/C. These findings are consistent with Wijaya's research105, which also identified 5 wt.% Pt/C as the optimal Pt loading in regards of GECH-DO. The effect of superficial current density and initial reactant concentration on the GECH-DO and GEOR are shown in Table B.4 (Appendices). The highest glycerol conversion was obtained at 40.9% applying 99 mA cm-2 current density, due to the favorable partial oxidation toward intermediate products rather than C–C bond cleavage at higher current density (132 mA cm-2).414 Moreover, higher current density provides more electrons in the chamber, driving the hydrogenation and oxidation processes more effectively. However, there is an optimal range for current density. Beyond this range, the benefits may plateau or even reverse due to mass transport limitations or excessive heat generation. However, the glyceraldehyde selectivity at 99 mA cm-2 was low only 2.9%. Generally, the glyceraldehyde selectivity decreased with the increase in current density (Table B.4). This study demonstrated that higher current densities (66, 99 and 132 mA.cm-2) result in lower cumulative (cathodic plus anodic) faradaic efficiencies (132.0% > 116.4% > 71.6%) as calculated with Equation 3.7 in Section 3. While the highest glyceraldehyde selectivity was 18.6% and it was obtained at 66 mA cm-2 with 0.3 M glycerol initial concentration. Furthermore, higher current density affects electrode stability and increases energy consumption 120 Figure 6.4. The required energy per mole of reactant converted with the varied reaction condition (a) agitation speed, (b) catalyst metal loading, (c) current density, (d) initial reactant concentration, and (e) reaction temperature. (a) (c) (b) (d) (e) 121 In terms of energy demand, the effect of several parameters in the optimization process is presented in Figure 6.4. Agitation speed did not affect energy demand, resulting in quite similar value (~ 14.0 𝑀𝑀ℎ 𝑚𝑚𝑟𝑟𝑙𝑙 𝐶𝐶𝐴𝐴−1), as shown in Figure 6.4a. The lower energy is required for higher Pt loading, as shown in Figure 6.4b, due to the lower cell potential (Table B.3) and potentially enhancing the reaction kinetics. Meanwhile, higher current density requires more energy per converted reactant (13.9 < 22.9 < 59.4 𝑀𝑀ℎ 𝑚𝑚𝑟𝑟𝑙𝑙 𝐶𝐶𝐴𝐴−1) as shown in Figure 6.4c. Therefore, the optimum current density within the range investigated in this study for the paired electrosynthesis of GECH-DO and GEOR is 66 mA cm-2. The variation of initial reactant concentration ensures an effective reaction while minimizing side reactions. As initial reactant concentration increased from 0.1 to 0.5 M, the required energy value decreased from 13.9 𝑀𝑀ℎ 𝑚𝑚𝑟𝑟𝑙𝑙 𝐶𝐶𝐴𝐴−1 to 8.5 𝑀𝑀ℎ 𝑚𝑚𝑟𝑟𝑙𝑙 𝐶𝐶𝐴𝐴−1 as shown in Figure 6.4d. Equation 3.10 directly explains the decreased value in the required energy due to quite a similar value of applied cell potential (~ 9 V) and current (0.4 A). Therefore, Equation 3.10 resulted in a similar numerator value on variation of initial reactant concentration, with the denominator (𝐶𝐶𝐴𝐴0𝑋𝑋𝐴𝐴) affecting the required energy. The F.E. was not involved in the required energy calculation because the total energy from the potentiostat required to electrify the whole reaction process. On the other hand, higher temperature hypothetically increases the reaction kinetics, therefore, a lower energy demand is expectedly shown in Figure 6.4e. As shown in Table B.4, both the guaiacol and glycerol conversion decrease with an increase in the initial reactant concentration, due to the inability of the catalyst slurry to provide sufficient surface area to facilitate the reaction in both GECH-DO and GEOR processes. This is also indicated by lower faradaic efficiency with increasing initial reactant concentration. Thus, maintaining the catalyst loading at 78 reactants to metal (bulk) molar ratio (R/M) is considered the 122 optimum condition in GECH-DO and GEOR with increasing initial reactant concentration. Meanwhile, the selectivity of GECH-DO for cyclohexanol, cyclohexanone, and cyclohexane is the highest at 0.2 M guaiacol initial concentration. However, due to the lower conversion (40%), an initial concentration of 0.1 M guaiacol was selected, using 5 g/L of 5 wt.% Pt/C catalyst. The highest selectivity for glyceraldehyde, glyceric acid, and tartronic acid was observed at an initial glycerol concentration of 0.3 M. At this concentration, the yield of glyceraldehyde, glyceric acid, and tartronic acid was also the highest compared to other initial glycerol concentrations. This trend occurs due to the limited active surface area available for further glycerol oxidation, resulting in C3 oxidation products remaining as the major intermediates. Saturating the bulk solution with glycerol could interfere with the formation of oxygenated species on the Pt surface, inhibiting the production of further oxidation product such as CO2 via CO oxidation.415 Thus, optimizing the initial reactant concentration should be considered in regard to kinetics and energy demand in the electrosynthesis reaction, making it a crucial factor in the scale-up process. The effect of temperature between 30 °C and 60 °C was also investigated to develop a kinetic model of the paired electrosynthesis. The experimental results and product distributions are presented in Table 6.2, whereas Table 7.2 summarizes the calculated rate constants based on the kinetic model described in the next chapter. While increasing the temperature increased both the guaiacol and glycerol overall conversions, the latter was more sensitive to the temperature increase reaching a conversion of 27.6% at 60 oC vs. 15.6% at 30 oC. However, the selectivity toward the most valuable product glyceraldehyde decreased with increasing temperature from 36.8% at 30 oC to 12.5% at 60 oC, as the higher temperature favors the more complete oxidation of glycerol. On the cathode side, decreases in selectivity with increasing temperature were observed for anisole, methoxycyclohexane, and to a lesser extent, for cyclohexanone (Table 6.2). Even though, higher 123 reaction temparature (60 oC) provides higher guaiacol and glycerol conversion, decreasing the reaction temperature reduce the heating and cooling energy demand for the reactor. Moreover, higher glyceraldehyde yield (5.6 %) also obtained at 50 oC of reaction temperature. Therefore, the optimum reaction temperature was obtained at 50 oC for paired electrosynthesis of GECH-DO and GEOR. Table 6.2. The effect of temperature in paired cell of GECH and GEOR using Pt/C catalyst Parameter Temperature 30 °C 40 °C 50 °C 60 °C (C) (A) (C) (A) (C) (A) (C) (A) E (V) -1.65 1.83 -1.07 1.92 -0.89 1.31 -0.62 1.78 Ecell (V) 12.8 11.2 9.4 9.4 pH 0.14 -0.07 0.16 0.08 0.09 0.02 0.08 -0.01 XA (%) 70.3 15.6 71.5 16.6 74.5 22.4 79.1 27.6 SB (%) 3.5 36.8 4.1 32.1 4.3 25.0 4.3 12.5 SC (%) 25.9 5.8 25.2 5.7 24.1 3.6 23.5 4.0 SD (%) 4.6 1.9 7.9 1.9 9.2 1.0 11.5 9.5 SE (%) 24.5 22.4 15.2 24.1 9.5 19.2 8.5 11.2 SF (%) 3.2 0.1 4.3 0.3 5.5 0.8 6.9 0.2 SG (%) 17.8 27.8 18.1 18.7 20.1 4.0 19.3 4.4 SH (%) 4.0 2.6 12.8 6.5 13.2 24.0 14.0 32.8 SI (%) 10.2 2.5 5.8 10.7 7.1 22.5 4.7 25.4 SJ (%) 6.2 0.0 6.6 0.0 7.0 0.0 7.4 0.0 C.B. (%) ~99.9 99.4 ~99.9 99.4 ~99.9 97.0 99.7 96.2 F.E. (%) 86.8 8.8 88.4 11.3 89.0 25.3 97.3 34.7 rx,0 × 105 2.49a 0.45b 2.56a 0.48b 2.69a 0.65a 2.84a 0.75a *Significance difference level between group of Cathode and Anode as formulated in Equation 3.12. Conditions: T = 60 °C, j = 66 mA cm−2, Catalyst loading = 0.50 g (5 g/L), Reaction time = 4 h, 0.25 M NaCl, all electrolytes also containing 0.25 M NaCl and 25% v/v IPA in the cathode chamber. (C) and (A) denote the cathode and anode chamber. E is either cathode or anode potential (vs. RHE) measured by reference electrode (Ag/AgCl), Ecell was recorded from the potentiostat, pH was measured after the reaction, η overpotential (based on H2 and O2 evolution reaction), pH was measured after the reaction. C.B. carbon balance, F.E. Faradaic efficiency, rx,0 initial reaction rate at 1 h (mol of reactant s−1 gPt−1). Cathode: XA guaiacol conversion, S normalized product selectivity (C mol%) for phenol (B), cyclohexanone (C), cyclohexanol (D), anisole (E), cyclohexane (F), 2-methoxycyclohexanone (G), 2-methoxycyclohexanol (H), methoxycylohexane (I), and methanol (J). Anode: XA glycerol conversion, S normalized product selectivity (C mol%) for glyceraldehyde (B), glyceric acid (C), glycolic acid (D), tartronic acid (E), DHA (F), lactic acid (G), Formic acid (H), and acetic acid (I). 124 6.3 Paired cell – operational enhancements experiment The present study successfully implemented paired electrosynthesis of GECH-DO and GEOR in the cathode and anode chambers, respectively, using an SSER configuration with MSA electrolyte and Pt/C catalyst. This setup did not significantly interfere with GECH-DO activity and selectivity. The paired electrosynthesis contributed to improved energy efficiency by raising faradaic efficiency, lowering cell potential, and the required energy per mole of converted reactant, whether guaiacol or glycerol. Additionally, this approach could simultaneously convert by-products from the biomass and biodiesel industries into valuable chemicals. Several key parameters namely agitation speed (400 rpm), catalyst loading (5%), initial reactant concentration (100 mM guaiacol and 300 mM glycerol), current density (66 mA cm-2), and temperature (50 °C) have been optimized from previous section. The optimized parameter was experimentally conducted in the laboratory to give clear comparison with the other result. The results showed that the paired electrosynthesis of GECH-DO and GEOR in SSER configuration was optimum to produce desired products (Figure 6.5) at the optimum condition. The guaiacol conversion was boosted from 79.1% at the initial reaction condition to 93.7% (Figure 6.5a) at the optimum condition. Hypothetically, the paired electrosynthesis tried to balance the system toward favorable conditions. The equal initial reactant concentrations of guaiacol and glycerol (0.1 M) achieves a moderate concentration of protons and electrons, limiting the extent of guaiacol conversion. Higher glycerol concentrations increases the protons and electrons involved in the reaction, which likely accelerates the HDO reaction in the cathode chamber, leading to higher guaiacol conversion. In addition, the increased glycerol concentration may slightly increase the overall ionic strength of the electrolyte, improving conductivity and cell voltage (9.4 V vs. 9.1 V, Table B.4 in Appendices). Moreover, a comparable guaiacol conversion (95%) was also found in 125 another study using Pt catalyst modified with Rh and Au in 0.2 M HClO4 for 5 h resulting 76% of 2-methoxycyclohexanone with low faradaic efficiency (58%).174 The turnover frequency (TOF), based on Equation 3.11, at the optimized condition (0.0093 s-1) in the cathode chamber was superior to the initial typical reaction condition (0.0061 s-1). In terms of the degree of hydrodeoxygenation in the cathode chamber, the optimized condition yielded lower O/C ratio and higher H/C ratio compared to the initial typical reaction condition, as shown in Figure 6.5b. Furthermore, the O/C ratio at the optimized condition is similar to the target value and only needs more reaction time for the hydrogenation of the bio-oil model compound. Figure 6.5. The optimized condition compared to the typical initial reaction condition in paired electrosynthesis. Initial condition: N = 600 rpm, T = 60 °C, and C0_glycerol = 0.1 M. Optimum condition: N = 400 rpm, T = 50 °C, C0_glycerol = 0.3 M. Reaction condition: t = 4 h, catalyst: 5 wt%-Pt/C (5 g/L), anolyte and catholyte consist of 1.0 M MSA and 0.25 M NaCl with 25% v/v IPA addition in the cathode chamber. On the other hand, the anode side generated lower conversion compared to the initial reaction condition. The conversion is still comparable to the other study utilizing higher Pt loading (20wt.%), resulting 20.3% glycerol conversion in 0.5 M H2SO4.309 The glycerol conversion (> 70%), experimentally, could be boosted by increasing the Pt loading (~40wt.%) for 4 h in acidic (a) (b) 126 condition.118 Meanwhile, the turnover frequency (TOF) at the optimum condition (0.0023 s-1) was higher than at the typical initial process (0.0009 s-1), since TOF is linearly dependent on the initial concentration for 1st order reactions. The glyceraldehyde yield was improved almost 2 times compared to the initial typical reaction condition, due to the saturated electrolyte solution with the glycerol. Meanwhile, the system energy improvement was observed in terms of faradaic efficiency and total required energy. The total energy of the optimized condition required less than ~50% energy (𝑀𝑀ℎ 𝑚𝑚𝑟𝑟𝑙𝑙 𝐶𝐶𝐴𝐴−1) compared to the initial typical reaction condition at Figure 6.5b. Therefore, the study contributes to electrosynthesis applications for greener and sustainable processes to produce fuel and valuable chemicals by maximizing electrocatalytic reactor utilization. As lignocellulose biomass and glycerol are considered waste or by-products of the agro-industry and biodiesel industry, both renewable stocks hold significant potential for value-added applications in a circular economy. As Wijaya et al.105 stated, the SSER configuration is suitable for industrial applications by enhancing mass and heat transfer between negatively charged catalyst particles and the reacting molecules during electrosynthesis. The results of this study support the sustainable and continuous development of SSER for GECH-DO and GEOR applications. 6.4 Tailoring electrocatalyst in electrosynthesis of guaiacol ECH-DO and glycerol electrooxidation The increasing interest in the sustainability of chemical processes leads to innovative improvement in product and process design with the goals of: i) to minimize energy consumption, ii) enhance the use of environmentally friendly chemicals, iii) effectively manage material life cycle, and iv) improve process efficiency. Catalysts play a pivotal role in trying to achieve these goals, by 127 controlling the chemical reaction. The present study was aimed at reducing the amount of Pt used in electrocatalysts, without neglecting its importance in determining catalytic activity. Herein, the combination of binary (PtNi and PtCo) and ternary (PtNiCo) metals catalyst on carbon support to assist the paired electrosynthesis of GECH-DO and glycerol electrooxidation was investigated. 6.4.1 Catalyst synthesis Figure 6.6. Activated carbon pretreatment. The catalysts were prepared using a two-step co-impregnation method via activated carbon pretreatment and catalyst synthesis as shown in Figure 6.6 and 6.7. As much as 3 g of the 100 mesh activated carbon (AC) was initially pretreated using 15 mL nitric acid (3 M) in a glass beaker with 400 rpm agitation for 4 h at room temperature. The treated carbon was rinsed and filtered using distillated water until the filtrate was neutral (pH ~ 7). The treated AC was dried in an oven at 60 °C overnight. Figure 6.7. Binary and ternary catalyst synthesis on activated carbon support. The number in the first step described the mixing order of the chemicals. 128 The objectives of the AC pretreatment was to (i) remove impurities such as N, P, Ca, K, Al, Si, Fe from the surface, (ii) introduce hydrophilic functional groups to improve the surface chemistry of the AC, and (iii) increase the surface area by enlarging the surface pore diameter.416 The excess solution co-impregnation method was used to prepare the catalysts. Accordingly. a 0.05 M Pt solution was prepared by mixing H2PtCl6.xH2O precursor with distillated water. The H2PtCl6.xH2O solution was mixed with the treated AC, and appropriate amounts of the metal precursors Ni(NO3)2.6H2O and Co(NO3)2.6H2O, and isopropanol at 400 rpm of agitation for 30 min at room temperature, with component concentrations reported in Table 6.3. The addition of isopropanol in the catalyst synthesis improved the catalyst properties by controlling the surface wettability and stabilizing the dispersion of metal ions within the carbon particle. Table 6.3. The catalyst synthesis preparation using the variation of metal weight. No Catalyst name Pretreated AC (g) Pt solution (mL) Ni salt (g) Co salt (g) IPA (mL) 1 PtNi/C 2.500 7.688 0.1118 - 7.688 2 Pt3Ni/C 2.500 7.688 0.0373 - 7.688 3 PtNi3/C 2.500 7.688 0.3353 - 7.688 4 PtCo/C 2.500 7.688 - 0.1119 7.688 5 Pt3Co/C 2.500 7.688 - 0.0373 7.688 6 PtCo3/C 2.500 7.688 - 0.3356 7.688 7 PtCo0.5Ni0.5/C 2.500 7.688 0.0559 0.0559 7.688 8 Pt3Co0.5Ni0.5/C 2.500 7.688 0.0186 0.0186 7.688 9 PtNi2Co/C 2.500 7.688 0.2236 0.1119 7.688 10 PtNiCo2/C 2.500 7.688 0.1118 0.2237 7.688 The dispersion was then sonicated for 30 min to ensure complete dissolution of the metal ions before recovering the solid by filtration. The recovered catalyst was then heated overnight at 60 °C followed by reduction under 10 mL H2/N2 (10% /90%) flow at 400 °C for 4 h. The Pt loading was maintained at 3 wt% for all the formulations, while the content of the second metal varied as 1:1, 1:2, and 1:3 molar ratio Pt to second metal. 129 6.4.2 XRD analysis The carbon support plays a crucial role in the electrocatalyst, because of its conductivity, harsh acid and alkaline stability, and large surface area and porosity that also provide high metal dispersion.417 Due to higher value of electronegativity of C (2.55) compared to Pt (2.28), an electron transfer from Pt onto the carbon support can occur, thus altering the electronic structure and physicochemical properties of the electrocatalyst.418-420 The electron transfer lowers the Fermi level of Pt 5d electrons and establishes more active interaction of the adsorbates, according to the d-band theory.421, 422 Moreover, carbon surface chemistry plays an important role in determining the metal dispersion. As the surface oxygen groups of activated carbon determine the hydrophilic/hydrophobic properties423, 424 and stabilize the pore walls, acid pretreatment alters the surface properties by introducing oxygen-containing functional groups (such as carboxyl, hydroxyl, and carbonyl) onto the surface of the carbon material. Moreover, nitric acid pretrement increases the surface area, pore volume and pore size distribution of the activated carbon. As reported in Figure 6.8a, the nitric acid pretreatement showed the addition of several peaks to the amorphous structure of the untreated activated carbon. The nitric acid pretreatement leads to removal of disordered amorphous carbon which in turn can expose more crystalline structures. Thus, several peaks represent the carbon support (2θ = 10 – 50°) and need to be excluded for the XRD peak analysis of the tailored catalyst. For instance, Figure 6.8b reports the XRD spectra of the PtNi/C catalyst, with the carbon treated support (black dotted line) peaks and the PtNi peaks (red dotted line) clearly identified. Since the reported XRD data from the Earth Science Laboratory were measured using Co K-alpha radiation (λ = 0.179 nm) a conversion to Cu K-alpha radiation (λ = 0.154 nm), was carried out such that the peaks shifted accordingly, as shown in Figure 6.8 and 6.9. The fcc of Pt (01-070-130 2057) corresponds to the planes (111), (200), (220), and (311) that also appeared on the bimetallic (PtNi and PtCo) tailored catalyst. The carbon support was detected by the fcc reflection of C (01-075-1621) for planes (002) at 2θ of 26.4° as reported in Table 6.4. Figure 6.8. The acid pretreatment effect on the XRD profile of (a) the activated carbon support and (b) the PtNi/C catalyst. 10 20 30 40 50Intensity (a.u.)2θ (Degrees) Ctreated support PtNi/C(a) (b) 131 Figure 6.9. The XRD pattern of the tailored catalyst with platinum JCPDS card as reference. However, the addition of nickel (Ni) or cobalt (Co) with platinum (Pt) on carbon support showed small peak shifts for the Pt planes (111) and (200). The Pt (111) and (200) planes seem to get sharper for the alloyed catalysts (PtNi/C and PtCo3/C), suggesting these particles are larger than for the Pt alone. On the other hand, the ternary metal catalyst PtNiCo on carbon support showed significantly different reflections as reported in Table 6.7. The d-spacing, FWHM, and the lattice parameter of each catalyst are reported in Tables 6.4 – 6.7 for Pt/C, PtNi/C, PtCo3/C, and PtNiCo/C, respectively. The tailored catalyst peak analysis was evaluated by carefully excluding the carbon support XRD spectra as reported in the Figure 6.9. As reported in Table 6.4, the lattice parameters of the commercial Pt/C catalyst fit to the standard Pt (α = 3.923 Å) for all planes (111), (200), (222) and (311). 132 Table 6.4. The XRD analysis of Pt/C catalyst. No. 2θ [°] FWHM [°] h k l d-spacing [Å] αcalc (Å) αref (Å) JCPDS 1 26.4 3.99 0 0 2 3.37 6.743 6.790 C (01-075-1621) 2 39.8 1.08 1 1 1 2.26 3.923 3.923 Pt (01-070-2057) 3 46.2 2.46 2 0 0 1.97 3.923 3.923 Pt (01-070-2057) 4 67.5 2.15 2 2 0 1.38 3.923 3.923 Pt (01-070-2057) 5 81.3 1.84 3 1 1 1.18 3.923 3.923 Pt (01-070-2057) The PtNi/C catalyst showed different lattice parameters compared to the Pt fingerprints for all planes of Pt at 2θ of 39.8, 46.3, 67.5, and 81.3°. The PtNi (111) facet (2θ = 39.7°), shown in Figure 6.9, is between the characteristic peaks of Pt (111) (2θ = 39.8°) and Ni (111) (2θ = 44.3°). The peak (2θ = 33.3°) also produced a typical fingerprint of the NiPt (03-065-2797) standard. Ni metal was not identified in the PtNi/C XRD spectra, as shown in Table 6.5. From the XRD analysis, the PtNi alloy exists in the PtNi/C bimetallic catalyst. Table 6.5. The XRD analysis of PtNi/C catalyst. No. 2θ [°] FWHM [°] h k l d-spacing [Å] αcalc (Å) αref (Å) JCPDS 1 26.4 0.92 0 0 2 3.38 6.754 6.736 C (00-008-0415) 2 33.3 1.84 1 1 0 2.69 3.797 3.823 NiPt (03-065-2797) 3 39.8 0.61 1 1 1 2.27 3.926 3.923 Pt (01-070-2057) 4 42.4 1.23 1 0 0 2.13 2.129 2.464 C (00-008-0415) 5 46.3 0.77 2 0 0 1.96 3.916 3.923 Pt (01-070-2057) 6 67.5 0.61 2 2 0 1.39 3.921 3.923 Pt (01-070-2057) 7 81.3 0.61 3 1 1 1.18 3.922 3.923 Pt (01-070-2057) On the other hand, the addition of Co yielded a peak typical of the CoPt3 (96-152-4399) (311) facet (2θ = 83.1°), as reported in Table 6.6. The PtCo3/C catalyst also had (111) peak facet (2θ = 40.3°) that lies between Pt (111) facet (2θ = 39.8°) and Co (111) facet (2θ = 45.9°). Another (220) and (311) facet of the Pt had different lattice parameter values compared to the standard Pt. Again, 133 the single Co metal was not present in the PtCo/C XRD spectra. Thus, the PtCo3/C catalyst also shows PtCo3 alloy formation in the PtCo3/C bimetallic catalyst. Table 6.6. The XRD analysis of PtCo3/C catalyst. No. 2θ [°] FWHM [°] h k l d-spacing [Å] αcalc (Å) αref (Å) JCPDS 1 26.2 1.84 1 1 1 3.40 6.799 6.790 C (00-008-0415) 2 40.3 0.77 1 1 1 2.24 3.871 3.923 Pt (01-070-2057) 3 46.4 1.23 1 1 1 1.95 3.910 3.923 Pt (01-070-2057) 4 50.0 0.92 1 1 0 1.82 2.579 2.470 C (00-008-0415) 5 67.4 0.77 2 2 0 1.39 3.924 3.923 Pt (01-070-2057) 6 81.4 1.23 3 1 1 1.18 3.920 3.923 Pt (01-070-2057) 7 83.1 0.92 3 1 1 1.16 3.853 3.854 CoPt3 (00-029-0499) The ternary metal of PtNiCo on carbon support dramatically changed the observed reflections. Only the Pt (111) plane at 39.4° (2θ) was observed, as shown in Figure 6.9. Other peaks showed different fingerprints of single, binary, and ternary metals as shown in Table 6.7. Single, binary, and ternary metals were detected in the spectra with several JCPDS card such as Pt, PtCo, PtNi, and PtNiCo peaks. Single metals of Ni and Co were not detected in Figure 6.9. Table 6.7. The analysis of PtNi0.5Co0.5/C catalyst. No. 2θ [°] FWHM [°] h k l d-spacing [Å] αcalc (Å) αref (Å) JCPDS 1 26.9 0.92 0 0 2 3.32 6.633 6.790 C (00-008-0415) 2 33.9 0.77 1 0 1 2.64 3.733 3.795 CoPt (96-152-4153) 3 39.4 0.92 1 1 1 2.29 3.959 3.923 Pt (01-070-2057) 4 42.4 1.23 1 0 0 2.13 2.129 2.470 C (00-008-0415) 5 44.8 0.92 0 0 2 2.02 4.039 4.065 Co1.5Ni0.5 (96-152-5375) 6 46.6 0.92 1 0 1 1.95 2.751 2.570 Pt0.4Co1.6 (96-152-4155) 7 53.2 0.92 2 0 0 1.72 3.849 3.854 CoPt3 (00-029-0499) 8 55.7 0.61 1 1 1 1.65 2.855 2.697 Co0.5PtNi0.5 (96-152-5469) 9 72.2 0.61 2 0 2 1.31 3.700 3.823 NiPt (03-065-2797) 10 74.4 0.92 3 0 0 1.27 3.821 3.823 NiPt (03-065-2797) 11 83.2 1.84 3 1 1 1.16 3.848 3.854 CoPt3 (00-029-0499) 12 89.2 0.92 1 1 3 1.10 3.639 3.589 NiPt (03-065-2797) 134 From the XRD data, it can be concluded that (i) the shifting peak of Pt (111), (200) (220), and (311) planes of the bimetallic catalyst compared to the Pt/C catalyst, (ii) the trimetalic catalyst had a completely different fingerprint compared to the Pt/C catalyst, and (iii) the lattice parameter showed that the catalysts formed binary or ternary metal alloys. 6.4.3 CO-uptake and TEM analysis The TEM analysis of Figure 6.10 showed the particle size distribution of the catalyst with well dispersed metals on the carbon support. The particle size distribution analysis was conducted by imageJ® software to capture each particle size. The PtCo/C catalyst showed the largest particle size due to sintering and lower mobility of the metal atom, as Pt-Co has higher phase-transition temperature (TPT of ~ 830 °C) compared to Pt-Ni (TPT of ~630 °C), as seen in Table 6.8. The particle size data estimated from the XRD peak broadening using the Scherrer equation are in good agreement with the TEM analysis, whereas the CO dispersion data also follow the same trend in particle size of the 4 catalysts. Table 6.8. The CO-uptake and TEM analysis of the tailored catalyst. Catalyst CO-uptake TEM XRD φ (%) Smetal (m2/g) dp (nm) dp (nm) dp (nm) Pt/C 29.0 71.5 3.45 3.26 ± 1.27 3.56 ± 0.06 PtNi/C 28.0 86.9 3.57 3.05 ± 1.32 3.72 ± 0.07 PtCo3/C 3.4 11.6 29.41 12.54 ± 1.19 12.74 ± 0.19 PtNi0.5Co0.5/C 19.3 78.6 5.18 5.45 ± 1.27 6.10 ± 0.73 Higher temperature below the phase-transition temperature may determine the particle size of PtM/C binary catalyst due to the inter and intra particle diffusion for alloying Pt with M to form disorder-to-order phase transition.425 135 Figure 6.10. The TEM images with the catalyst size distribution of (a) Pt/C, (b) PtNi/C, and (c) PtCo3/C, and (d) PtNi0.5Co0.5/C.1 2 3 4 5051015202530 Count (%)Particle size (nm)DO = 3.05 nmσSD = 1.32 nm2 3 4 5 6 7 8 9 10 1102468101214161820 Count (%)Particle size (nm)DO = 5.45 nmσSD = 1.27 nm(a) (b) (c) (d) 136 Figure 6.11. The images of PtNi/C catalyst from (a) TEM with (b) selected area electron diffraction (SAED), and (c) selected HAADF area followed by (d) – (i) the elemental mapping by EDX analysis. Therefore, the optimization study on reduction temperature below the phase-transition temperature of PtCo/C catalyst could highlight the key role of the alloy composition and intermetallic order of the tailored catalyst.426 Selected area electron diffraction (SAED) shown in Figure 6.11b indicates the amorphous phase of carbon support due to the halo ring pattern from the low content of the metals. Meanwhile, (a) (b) (c) (d) (e) (f) (g) (h) (i) 137 the dotted white ring pattern shows the single crystalline materials and randomly dotted ring (almost unclear) indicates polycrystalline materials from the PtNi/C catalyst, illustrated in Figure 6.11b. The polycrystalline materials were also detected by the elemental analysis in Figure 6.11d – 6.11g. The EDX elemental mapping images of Pt (Figure 6.11d), Ni (Figure 6.11e), and PtNi (Figure 6.11f) confirm uniform a distribution of Pt and Ni metals albeit with some minor agglomeration of Pt on the carbon support (Figure 6.11g). Interestingly, some bright dots in Figure 6.11c do not refer to Pt (Figure 6.11d) or Ni (Figure 6.11e), however, it appears in the PtNi (Figure 6.11f), indicating formation of the bimetallic PtNi phase. EDX elemental analysis was performed for several catalysts as presented in Table 6.9. The EDX analysis showed that the Pt, Ni, and Co content of the catalysts correspond to the nominally expected value. Table 6.9. The expected catalyst chemical composition on weight basis of the tailored catalyst. No Catalyst name Chemical composition (wt%) Nominal Analyzed by EDX Pt Ni Co Total metal Pt Ni Co Total metal 1 Pt/C commercial 5.00 - - 5.00 4.92 ± 0.12 - - 4.92 ± 0.12 2 PtNi/C 2.89 0.87 - 3.76 2.73 ± 0.09 0.56 ± 0.02 - 3.29 ± 0.09 3 PtCo3/C 2.84 - 2.57 5.41 1.64 ± 0.25 - 3.81 ± 0.73 5.45 ± 0.89 4 PtCo0.5Ni0.5/C 2.89 0.43 0.44 3.76 2.70± 0.88 0.43 ± 0.29 0.44 ± 0.30 3.56 ± 1.02 5 Pt3Ni/C 2.90 0.29 - 3.19 n.a. n.a. n.a. n.a. 6 PtNi3/C 2.84 2.56 - 5.40 n.a. n.a. n.a. n.a. 7 PtCo/C 2.89 - 0.87 3.76 n.a. n.a. n.a. n.a. 8 Pt3Co/C 2.90 - 0.29 3.19 n.a. n.a. n.a. n.a. 9 Pt3Co0.5Ni0.5/C 2.90 0.15 0.15 3.20 n.a. n.a. n.a. n.a. 10 PtNi2Co/C 2.84 1.71 0.86 5.41 n.a. n.a. n.a. n.a. 11 PtNiCo2/C 2.84 0.85 1.71 5.40 n.a. n.a. n.a. n.a. *n.a. = not analyzed. 138 6.5 Tailored metal electrocatalytic activity As a preliminary study, the half-cell electrocatalytic reactions either in the cathode or anode chamber was conducted using binary (PtNi or PtCo) and ternary (PtNiCo) metal catalysts on the carbon support in acidic conditions. The commercial 5 wt% Pt/C catalyst data was used as a reference. The best catalyst from the half-cell electrosynthesis reaction was then introduced in the paired electrosynthesis of GECH-DO and GEOR. 6.5.1 Half-cell electrosynthesis Cyclic voltammetry analysis was conducted to observe the anodic and cathodic peak of the reaction. The cyclic voltammetry experiment was conducted at 60 °C with Ag/AgCl as the reference electrode. The catholyte and anolyte consisted of 1.0 M MSA, 0.1 M reactant (guaiacol or glycerol), 0.25 M NaCl, and 5 g/L catalyst. The catholyte also had 25% v/v. of isopropanol to increase the guaiacol solubility and acted as polar protic solvent. During the cyclic voltammetry, the cathode chamber acted as the working electrode while the anode chamber served as counter electrode. From Figure 6.12, PtNi catalysts on carbon support have better activities in the half-cell electrocatalytic reaction of cathode and anode chambers, with higher cathodic and anodic peaks. The best binary metal catalyst performance occurred with the Pt3Ni/C catalyst in terms of cathodic and anodic peaks, which refer to the HER and OER activities. It has superior properties compared to the reference noble metal catalyst, Pt/C, with ~1.3 and ~1.8 times on cathodic and anodic peak activities, respectively. The ternary metal catalyst also showed superior anodic peak activity with the PtNi0.5Co0.5/C catalyst having the highest activity (~1.6 times). However, the peaks only show the reduction and oxidation activities without the specified reaction of GECH-DO or GEOR. Thus, the application of the catalyst in the GECH-DO or GEOR should be carried out to determine the best catalyst for the system. 139 Figure 6.12. The voltammogram profile of tailored (a) binary and (b) ternary metals catalyst on carbon support catalyst in paired electrosynthesis of GECH-DO and GEOR. The binary metal catalysts were introduced in the half-cell reaction of GECH-DO and GEOR with the resulting data reported in Table B.5. The reaction conditions of half-cell tailored catalyst application were quite similar with the initial reaction condition using 66 mA cm-2 of current density, 600 rpm of agitation speed, and 60 °C as the initial screening process. Compared with the commercial noble metal catalyst, the PtNi/C catalyst has the best initial rates (4.79a vs. 2.54c ×105 mol s-1 gmetals-1) in GECH-DO half-cell reaction. The result also showed that PtNi/C had higher cyclohexanone (C), cyclohexanol (D), and cyclohexane (F) selectivity. The initial rates have significance letters (a and c) to show the level of difference. The anisole formation route (dehydration) is claimed to be more feasible in the order of Fe (110) > Pt > Ni > Pd > Co due to higher activation barrier.144 Thus, tailoring the Pt surface with Ni metal leads to two main reaction routes of guaiacol, namely the demethoxylation and ring saturation, which produce phenol and 2-methoxycyclohexanone, respectively. Figure 6.13 shows a comparison of GECH-DO performance in regards to guaiacol conversion and cyclohexane selectivity. PtNi/C and PtCo/C had comparable values to Pt/C as the reference (a) (b) 140 catalyst in terms of guaiacol conversion, however, PtNi/C seems superior in producing cyclohexane compared to PtCo/Cand Pt/C. Therefore, the PtNi could be introduced in the cathode chamber for guaiacol ECH-DO. Meanwhile, PtCo3/C catalyst has a broad particle size distribution as seen from the TEM image (Figure 6.10c), which leads to lower GECH-DO (XA = 36.3%) and GEOR (XA = 16.3%) activities. On the other hand, the result of the ternary metal catalyst reported in Table B.6, shows poor performance for both GECH-DO and GEOR, in terms of the reactant conversion, at 31.6% and 21.3%, respectively. The Pt3Ni0.5Co0.5/C and PtNiCo2/C could not trigger the dehydration reaction toward cyclohexane as seen in Figure 6.13b. Figure 6.13. The GECH-DO performance in half-cell electrosynthesis derived from Table B.5 and B.6 (Appendices) to compare tailored catalysts in regard of (a) guaiacol conversion and (b) cyclohexane selectivity. As seen from the XRD spectra at Figure 6.9, the loss of Pt planes (111) occurred due to Co1.5Ni0.5 (002) and Pt0.4Co1.6 (101) planes formation at 2θ of 44.8° and 46.8°. Moreover, the initial kinetic rate of guaiacol and glycerol are decreased at different levels of significance compared with the reference catalyst (Pt/C). Thus, the introduction of the ternary metal catalyst showed inferior performance compared with the noble metal catalyst (5 wt% Pt/C). (a) (b) 141 In accordance with the product distribution, the van Krevelen diagram was constructed to provide a brief representation of GECH-DO in the cathode chamber, as illustrated in Figure 6.14. PtNi/C catalyst exhibits a higher H/C molar ratio and a lower O/C molar ratio in comparison to the commercial Pt/C (5 wt%) catalyst. The binary and ternary metal catalyst on the carbon support, with the exception of PtNi/C, might be implemented in the OER and HER as the cathodic and anodic peaks showed highest activity compared to the commercial Pt/C catalyst, as seen in Table B.2 (Appendices). To find the optimum conditions of the study, the paired electrosynthesis with the PtNi/C-Pt/C and Pt/C-Pt/C catalyst pairs was conducted with current density and temperature variations in the GECH-DO and GEOR. Figure 6.14. The van Krevelen diagram of tailored catalyst. M belongs to Ni or Co. In this study, glyceraldehyde production is one of the desired products, thus, the evaluation was carried out to obtain high glyceraldehyde yield and decrease the Pt metal loading content. However, Pt/C catalyst remains a superior catalyst in the GEOR compared to the binary and ternary catalysts according to Figure 6.15 for the glycerol conversion (6.15a) and glyceraldehyde 142 selectivity (6.15b). Meanwhile, several catalysts were quite promising for producing specified chemicals such as lactic acid (Pt3Ni/C, 66.4% selectivity) and glyceric acid (PtNi2Co/C, 68.3% selectivity), even though the binary and ternary catalysts have lower glycerol reaction activity compared to Pt/C. The Pt3Ni/C catalyst facilitates the selective oxidation of the secondary hydroxyl group in glycerol, forming dihydroxyacetone (F) toward lactic acid (G). Figure 6.15. The GEOR performance in half-cell electrosynthesis derived from Table B.5 and B.6 (Appendices) to compare tailored catalysts in regard of (a) glycerol conversion and (b) glyceraldehyde selectivity. 6.5.2 Paired cell electrosynthesis To begin the PtNi/C-Pt/C catalyst application, the paired electrosynthesis with PtNi/C-Pt/C and Pt/C-Pt/C catalyst pairs were conducted at 50 and 60 °C. From Figure 6.16, cyclohexane formation with the PtNi/C-Pt/C pair showed superior performance compared to the commercial noble metal catalyst pair. The faradaic efficiency showed an improvement at 50 °C. In terms of conversion, the PtNi/C-Pt/C catalyst pair had lower activity compared to the commercial noble metal catalyst due to reduced platinum content. However, the initial rate with the PtNi/C catalyst was significantly different compared to the Pt/C catalyst at 50 °C (3.96b to 3.88c ×105 mol s-1 gmetals-1) and 60 °C (3.91b to 4.45a ×105 mol s-1 gmetals-1). Moreover, the PtNi/C catalyst generated fewer intermediate (a) (b) 143 chemicals without the presence of anisole and methoxycyclohexane, reducing the complexity of later separation processes. Higher cyclohexane selectivity was also observed by PtNi/C utilization compared to the Pt/C catalyst as the reference either for 50 or 60 °C. Figure 6.16. The cathode cell reaction activity result from different catalyst pairs in paired electrosynthesis system. Reaction condition: j = 66 mA cm-2, N = 400 rpm, t = 4 h. Electrolyte: 1.0 M MSA, 0.25 M NaCl, and 25% v/v IPA addition in the cathode chamber with C0 guaiacol = 0.1 M, C0 glycerol = 0.3 M. A higher current density with the PtNi/C-Pt/C catalyst pair was also investigated and is presented in Figure 6.17. In general, the conversion was higher for both chambers because more electrons were supplied to the electrocatalytic reactor. However, the higher current density lowers the faradaic efficiency. The GEOR results were quite similar for the pair catalyst combination with 144 different orders of magnitude on the intermediate chemicals because both systems have Pt/C catalyst in the anode chamber. Even though higher current density leads to higher cyclohexane formation and conversion, the faradaic efficiency and glyceraldehyde formation become other considerations to select the optimum condition. The current density of 66 mA cm-2 is still considered the best condition in the paired electrosynthesis system of GECH-DO and GEOR. Figure 6.17. The effect of current density in the paired electrosynthesis of (a) PtNi/C-Pt/C and (b) Pt/C-Pt/C catalyst pairs. Reaction condition: T= 50 °C, N = 400 rpm, t = 4 h. Electrolyte: 1.0 M MSA, 0.25 M NaCl, and 25% v/v IPA addition in the cathode chamber with C0 guaiacol = 0.1 M, C0 glycerol = 0.3 M. To confirm the isopropanol (IPA) role, different MSA-IPA solutions were introduced to the cathode chamber with the PtNi/C-Pt/C paired catalyst, as shown in Figure 6.18. The GEOR had (a) (b) 145 no significant effect due to similar applied condition with few changes of the intermediate chemicals. Meanwhile, the addition of IPA still triggers the dehydration reaction of guaiacol which leads to the formation of anisole and cyclohexane. Thus, IPA still plays a significant role in the deoxygenation process of the bio-oil model compound, even though it also has a detrimental effect on the electrolyte conductivity, which lowers the activity. Figure 6.18. The effect of isopropanol in the cathode chamber on the paired electrosynthesis of PtNi/C-Pt/C catalyst pairs. Reaction condition: T= 50 °C, j = -66 mA cm-2, N = 400 rpm, t = 4 h. Electrolyte: 1.0 M MSA, and 0.25 M NaCl with C0 guaiacol = 0.1 M, C0 glycerol = 0.3 M. An increase in the glycerol initial concentration increased the GECH-DO conversion from 62.4% to 82.3% (Figure 6.19). On the other hand, the glyceraldehyde selectivity increased from 3.7% to 8.7% in the anode chamber of GEOR. The F.E. also improved in the anode chamber from 146 37.6% to 86.0%. Therefore, the higher glycerol concentration positively affects the paired electrosynthesis of GECH-DO and GEOR. Figure 6.19. The effect of glycerol initial concentration in the paired electrosynthesis of PtNi/C-Pt/C catalyst pairs for (a) cathode and (b) anode chambers. Reaction condition: T= 50 °C, j = -66 mA cm-2, N = 400 rpm, t = 4 h. Electrolyte: 1.0 M MSA, 0.25 M NaCl, and 25% v/v IPA addition in the cathode chamber with C0 guaiacol = 0.1 M. 6.6 Summary of the chapter The study implemented the paired cell electrosynthesis of guaiacol electrocatalytic hydrogenation (GECH-DO) in the cathode chamber and glycerol electrooxidation (GEOR) in the anode chamber of the SSER, without any product interference on the activity for guaiacol reduction. The paired cell electrocatalytic reactor introduced methanesulfonic acid (MSA) as the electrolyte. The transformation of half-cell to paired cell electrosynthesis contributes to the energy efficiency improvement by increasing the total faradaic efficiency and required energy (𝑀𝑀ℎ 𝑚𝑚𝑟𝑟𝑙𝑙−1). Interestingly, the acid-acid (1.0 M) electrolyte pair is favorable in the paired electrosynthesis of GECH-DO and GEOR. The alkaline electrolyte in GEOR is suitable in the fuel cell application rather than producing specified chemicals of glyceraldehyde due to the rapid oxidation kinetics. In (a) (b) 147 addition, this approach could simultaneously convert by-products from the biomass and biodiesel industries into valuable chemicals. The optimum condition of paired electrosynthesis of GECH-DO and GEOR were determined to be 400 RPM, 5 wt% Pt/C, 100 mM guaiacol and 300 mM glycerol initial concentration, 50 °C, and 66 mA cm-2 (0.4 A). The guaiacol conversion was boosted from 76.85% to 93.72% with the implementation of optimum condition. Furthermore, glyceraldehyde yield doubled compared to the reference conditions. If the total required energy was calculated, the energy demands are less than ~50% compared to the reference condition. In regards to the degree of hydrodeoxygenation in the cathode chamber, the optimized conditions yielded lower O/C ratio and higher H/C ratio compared to the reference condition. In conclusion, paired electrosynthesis is a beneficial process for increasing energy efficiency and product intensification. If a waste or by-product material is implemented as the feed for the paired electrosynthesis, as in this study, it will contribute to the development of green and sustainable processes for valuable chemicals production. The tailored binary (PtNi/C or PtCo/C) and ternary (PtNiCo/C) catalysts were also evaluated in half-cell and paired cell electrosynthesis of guaiacol electrocatalytic hydrodeoxygenation (GECH-DO) and glycerol electrooxidation (GEOR) by cyclic voltammetry and galvanostatic reaction. The catalysts were synthesized using the excess solution co-impregnation method. Several catalyst characterizations were conducted to investigate the intermetallic phase of the binary and ternary metal. Based on X-ray Diffraction (XRD) and high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM), the intermetallic phase was detected in the PtNi/C catalyst. Of the binary and ternary catalysts investigated, only the PtNi/C had comparable particle size with reference to the noble metal catalyst, Pt/C, as determined by TEM and XRD analysis. 148 In terms of the GECH-DO product, alloying Pt with Ni could decrease the amount of Pt loading on carbon support catalyst. The higher cyclohexane selectivity (18%) was observed when PtNi/C catalyst was implemented in the GECH-DO compared to the Pt/C (5%) as the reference at 50 °C. The higher cyclohexane selectivity also observed in higher reaction temperature, 60 °C. Based on the initial reaction rate of GECH-DO, the study effectively tailored the PtNi/C catalyst to reduce the catalyst dependency on noble metals content by as much as 40%. Even though the catalyst had lower activity, the reaction rate of PtNi/C was higher in terms of total metal catalyst (mol s-1 g metal-1). Unfortunately, the other tailored catalysts could not compete with Pt/C catalyst for GECH-DO and GEOR application. The tailored catalysts still could be implemented in hydrogen and oxygen evolution reaction because of decent Tafel slope values. The optimum condition of the paired cell electrosynthesis of GECH-DO and GEOR was determined to be at j = 66 mA cm-2, N = 400 rpm, t = 4 h, and 50 °C using 1.0 M MSA, 0.25 M NaCl, and 25% v/v IPA addition in the cathode chamber electrolyte for 0.1 M CA0 guaiacol and 0.3 M CA0 glycerol on 5 g/L (3.29 wt% metal) PtNi/C catalyst. At this condition, PtNi/C has lower conversion (93.7% vs. 82.3%) compared to Pt/C catalyst in cathode chamber. However, the reduction in Pt amount leads to higher initial kinetics rate (3.96b vs. 3.88c) compared to Pt/C catalyst. On the PtNi/C catalyst, guaiacol was converted through the demethoxylation and ring saturation pathway only, thus, it decreases the amount of intermediate chemicals and reduces the complexity of subsequent separation processes. 149 Chapter 7: Kinetics of guaiacol electrocatalytic hydrodeoxygenation (GECH-DO) and glycerol electrooxidation (GEOR) 7.1 Catalytic reaction kinetics and pathways In Chapter 7, results of a kinetic analysis of GECH-DO and GEOR is reported, based on two related approaches. In the first instance, kinetic models for the rates of reactant consumption (guaiacol and glycerol) were derived from Langmuir–Hinshelwood reaction mechanisms. Utilizing measured reactant concentration versus time data, kinetic parameters of the kinetic models were estimated so as to minimize the error sum of squares between the measured and calculated reactant concentrations.427 Several key assumptions were made in the Langmuir-Hinshelwood kinetic analysis: (i) elementary steps: the elementary steps are first-order, reversible, and involve dual-site reactions, (ii) adsorption and desorption: most of the adsorption and desorption of reactants and products are at equilibrium, involving molecular non-dissociative adsorption without any interaction between adsorbed molecules except through the reaction, (iii) catalyst surface: the catalyst surface is uniform, with all active sites being equivalent. (iv) surface coverage: the catalyst surface is partially covered by adsorbed reactants and intermediates. (v) proton availability: the availability of protons is in excess therefore, the proton concentration does not affect the rate. The Langmuir-Hinshelwood (L-H) reaction mechanism is often more suitable than the Eley-Rideal (E-R) mechanism for reactions involving the adsorption of both reactants onto the catalyst surface. For instance, the L-H mechanism provides a more accurate description of the reaction process in systems where both protons and organic substances adsorb, as in GECH-DO, and for which a dispersed metal on the catalyst surface is required (as demonstrated by control experiments 150 conducted by Wijaya et al.106). Experimental data further support this preference: as shown in Table B.9 (Appendices), the L-H mechanism exhibits a higher R-square value (0.99) compared to the E-R mechanism, indicating a better fit to the observed data. In the case of GEOR, both mechanisms yield comparable R-square values (0.99). However, water dissociation—a crucial step in acidic electrooxidation—requires the surface adsorption of water molecules, which is inherently accounted for in the L-H mechanism. Therefore, the L-H mechanism is deemed more appropriate for describing the reaction kinetics. Consequently, kinetic models for the rates of reactant consumption (guaiacol and glycerol) were derived based on the Langmuir-Hinshelwood reaction mechanism. The rate equation is neccesary information to understand catalyst activity at a molecular level or to design a catalytic reactor.428 The rate-determining step (RDS) is a critical concept in heterogeneous reactions since it identifies the slowest step in the reaction mechanism, which aids in mitigating bottlenecks in catalytic reactions. However, organic synthesis inlvolves many complex reactions and intermediates which hinders the application of Langmuir-Hinshelwood reaction models. Therefore, the second approach to the kinetic analysis was also important to fully investigate the GECH-DO and GEOR reactions. In the second instance, a kinetic analysis of the reaction pathways associated with GECH-DO and GEOR was completed. Experimental data were used to propose and develop suitable reaction pathways to account for the product distributions obtained from GECH-OD and GEOR, under different reaction conditions. By assuming simplified kinetics (1st or 2nd order) for each reaction step of the proposed pathway, component concentration versus time data were generated by numerically integrating the reactor design equations (or mole balance equations), assuming a perfectly mixed batch reactor and based on estimates of the model parameters that include the rate 151 constants and activation energies of each reaction. The optimum model parameters were obtained by minimizing the error sum of squares between the measured and calculated component concentrations. Hence, the kinetically significant reaction pathways for both GECH-DO and GEOR were determined. 7.1.1 Guaiacol electrocatalytic hydrodeoxygenation (GECH-DO) A set of preliminary experiments was conducted with variations in the initial guaiacol concentration to determine its impact on the initial rate of guaiacol consumption. Under the applied conditions, the initial reaction rate at relatively low guaiacol concentration was directly proportional to the initial guaiacol concentration, as shown in Figure 7.1a. Further increasing of guaiacol concentration, as shown in Figure 7.1b, generated a constant initial rate regardless of further increases in reactant concentration (400 – 500 mM). Figure 7.1. The ECH of guaiacol under galvanostatic conditions at (a) lower and (b) higher guaiacol concentration with at 1.0 M MSA and 25% v/v IPA. Reaction condition: I = - 0.4 A (j = -66 mA cm-2), T = 60 °C, t = 4 h. Catalyst: 5 wt%-Pt/C (5 g/L). Initial reaction rates were measured by Equation 3.4. These findings confirm that the surface reaction is the rate determining step (RDS) because the initial rate linearly depends on the guaiacol concentration at low concentration, but it becomes (a) (b) 152 independent at high guaiacol concentrations. Furthermore, if one assumes Langmuir-Hinshelwood kinetics, then all adsorption and desorption steps must be at equilibrium. With these assumptions the Langmuir-Hinshelwood kinetic steps can be written as follows: Adsorption (𝑟𝑟𝑎𝑎 = 0): Guaiacol: 𝐴𝐴 + 𝑆𝑆 𝐴𝐴. 𝑆𝑆 𝑟𝑟𝑎𝑎,𝐴𝐴 = 𝑘𝑘𝑎𝑎,𝐴𝐴𝐶𝐶𝐴𝐴𝐶𝐶𝑣𝑣 − 𝑘𝑘−𝑎𝑎,𝐴𝐴𝐶𝐶𝐴𝐴𝑆𝑆 (7.1) 𝐶𝐶𝐴𝐴𝑆𝑆 = 𝐾𝐾𝑎𝑎,𝐴𝐴𝐶𝐶𝐴𝐴𝐶𝐶𝑣𝑣 (7.2) Isopropanol as proton donor: 𝐼𝐼𝑃𝑃𝐴𝐴 + 𝑆𝑆 𝐼𝐼𝑃𝑃𝐴𝐴. 𝑆𝑆 𝑟𝑟𝑎𝑎,𝐼𝐼𝑃𝑃𝐴𝐴 = 𝑘𝑘𝑎𝑎,𝐼𝐼𝑃𝑃𝐴𝐴𝐶𝐶𝐼𝐼𝑃𝑃𝐴𝐴𝐶𝐶𝑣𝑣 − 𝑘𝑘−𝑎𝑎,𝐴𝐴𝐶𝐶𝐼𝐼𝑃𝑃𝐴𝐴𝑆𝑆 (7.3) 𝐶𝐶𝐼𝐼𝑃𝑃𝐴𝐴𝑆𝑆 = 𝐾𝐾𝑎𝑎,𝐼𝐼𝑃𝑃𝐴𝐴𝐶𝐶𝐼𝐼𝑃𝑃𝐴𝐴𝐶𝐶𝑣𝑣 (7.4) Proton (electrolyte or water splitting): 𝐻𝐻+ + 𝑒𝑒− + 𝑆𝑆 ?̇?𝐻. 𝑆𝑆 𝑟𝑟𝑎𝑎,?̇?𝐻 = 𝑘𝑘𝑎𝑎,?̇?𝐻𝐶𝐶?̇?𝐻𝐶𝐶𝑣𝑣 − 𝑘𝑘−𝑎𝑎,?̇?𝐻𝐶𝐶?̇?𝐻𝑆𝑆 (7.5) 𝐶𝐶?̇?𝐻𝑆𝑆 = 𝐾𝐾𝑎𝑎,?̇?𝐻𝐶𝐶?̇?𝐻𝐶𝐶𝑣𝑣 (7.6) Desorption (𝑟𝑟𝑡𝑡 = 0): Main product: 𝑃𝑃. 𝑆𝑆 𝑃𝑃 + 𝑆𝑆 𝑟𝑟𝑡𝑡𝑤𝑤𝑠𝑠,𝑃𝑃 = 𝑘𝑘𝑡𝑡,𝑃𝑃𝐶𝐶𝑃𝑃𝑆𝑆 − 𝑘𝑘−𝑡𝑡,𝑃𝑃𝐶𝐶𝑃𝑃𝐶𝐶𝑣𝑣 (7.7) 𝐶𝐶𝑃𝑃𝑆𝑆 = 𝐾𝐾𝑡𝑡,𝑃𝑃𝐶𝐶𝑃𝑃𝐶𝐶𝑣𝑣 (7.8) Side product_1: 𝑀𝑀. 𝑆𝑆 𝑀𝑀 + 𝑆𝑆 𝑟𝑟𝑡𝑡𝑤𝑤𝑠𝑠,𝑊𝑊 = 𝑘𝑘𝑡𝑡,𝑊𝑊𝐶𝐶𝑊𝑊𝑆𝑆 − 𝑘𝑘−𝑡𝑡,𝑊𝑊𝐶𝐶𝑊𝑊𝐶𝐶𝑣𝑣 (7.9) 𝐶𝐶𝑊𝑊𝑆𝑆 = 𝐾𝐾𝑡𝑡,𝑊𝑊𝐶𝐶𝑊𝑊𝐶𝐶𝑣𝑣 (7.10) Side product_2: 𝐴𝐴𝐶𝐶𝐸𝐸. 𝑆𝑆 𝐴𝐴𝐶𝐶𝐸𝐸 + 𝑆𝑆 𝑟𝑟𝑡𝑡𝑤𝑤𝑠𝑠,𝐴𝐴𝐶𝐶𝐸𝐸 = 𝑘𝑘𝑡𝑡,𝐴𝐴𝐶𝐶𝐸𝐸𝐶𝐶𝐴𝐴𝐶𝐶𝐸𝐸𝑆𝑆 − 𝑘𝑘−𝑡𝑡,𝐴𝐴𝐶𝐶𝐸𝐸𝐶𝐶𝐴𝐴𝐶𝐶𝐸𝐸𝐶𝐶𝑣𝑣 (7.11) 153 𝐶𝐶𝐴𝐴𝐶𝐶𝐸𝐸𝑆𝑆 = 𝐾𝐾𝑡𝑡,𝐴𝐴𝐶𝐶𝐸𝐸𝐶𝐶𝐴𝐴𝐶𝐶𝐸𝐸𝐶𝐶𝑣𝑣 (7.12) Reaction: 𝐴𝐴. 𝑆𝑆 + 𝐼𝐼𝑃𝑃𝐴𝐴. 𝑆𝑆 + ?̇?𝐻. 𝑆𝑆 𝑃𝑃. 𝑆𝑆 + 𝐴𝐴𝐶𝐶𝐸𝐸. 𝑆𝑆 + 𝑀𝑀. 𝑆𝑆 𝑟𝑟𝑠𝑠 = 𝑘𝑘𝑠𝑠𝐶𝐶𝐴𝐴𝑆𝑆𝐶𝐶𝐼𝐼𝑃𝑃𝐴𝐴𝑆𝑆𝐶𝐶?̇?𝐻𝑆𝑆 − 𝑘𝑘−𝑠𝑠𝐶𝐶𝑃𝑃𝑆𝑆𝐶𝐶𝐴𝐴𝐶𝐶𝐸𝐸𝑆𝑆𝐶𝐶𝑊𝑊𝑆𝑆 𝑟𝑟𝑠𝑠,𝐴𝐴 = 𝑘𝑘𝑠𝑠,𝐴𝐴𝐾𝐾𝑎𝑎,𝐴𝐴𝐶𝐶𝐴𝐴𝐶𝐶𝑣𝑣𝐾𝐾𝑎𝑎,𝐼𝐼𝑃𝑃𝐴𝐴𝐶𝐶𝐼𝐼𝑃𝑃𝐴𝐴𝐶𝐶𝑣𝑣𝐾𝐾𝑎𝑎,?̇?𝐻𝐶𝐶?̇?𝐻𝐶𝐶𝑣𝑣 − 𝑘𝑘−𝑠𝑠,𝐴𝐴𝐾𝐾𝑡𝑡,𝑃𝑃𝐶𝐶𝑃𝑃𝐶𝐶𝑣𝑣𝐾𝐾𝑡𝑡,𝐴𝐴𝐶𝐶𝐸𝐸𝐶𝐶𝐴𝐴𝐶𝐶𝐸𝐸𝐶𝐶𝑣𝑣𝐾𝐾𝑡𝑡,𝑊𝑊𝐶𝐶𝑊𝑊𝐶𝐶𝑣𝑣 𝑟𝑟𝑠𝑠,𝐴𝐴 = �𝑘𝑘𝑠𝑠,𝐴𝐴𝐾𝐾𝑎𝑎,𝐴𝐴𝐾𝐾𝑎𝑎,𝐼𝐼𝑃𝑃𝐴𝐴𝐾𝐾𝑎𝑎,?̇?𝐻𝐶𝐶𝐴𝐴𝐶𝐶?̇?𝐻𝐶𝐶𝐼𝐼𝑃𝑃𝐴𝐴 − 𝑘𝑘−𝑠𝑠,𝐴𝐴𝐾𝐾𝑡𝑡,𝑃𝑃𝐾𝐾𝑡𝑡,𝐴𝐴𝐶𝐶𝐸𝐸𝐾𝐾𝑡𝑡,𝑊𝑊𝐶𝐶𝐴𝐴𝐶𝐶𝐸𝐸𝐶𝐶𝑃𝑃𝐶𝐶𝑊𝑊�𝐶𝐶𝑣𝑣3 (7.13) where: 𝐶𝐶𝜕𝜕 = 𝐶𝐶𝑣𝑣 + 𝐶𝐶𝐴𝐴𝑆𝑆 + 𝐶𝐶𝐼𝐼𝑃𝑃𝐴𝐴𝑆𝑆 + 𝐶𝐶?̇?𝐻𝑆𝑆 + 𝐶𝐶𝑃𝑃𝑆𝑆 + 𝐶𝐶𝑊𝑊𝑆𝑆 + 𝐶𝐶𝐴𝐴𝐸𝐸𝑆𝑆 𝐶𝐶𝜕𝜕 = 𝐶𝐶𝑣𝑣 + 𝐾𝐾𝑎𝑎,𝐴𝐴𝐶𝐶𝐴𝐴𝐶𝐶𝑣𝑣 + 𝐾𝐾𝑎𝑎,𝐼𝐼𝑃𝑃𝐴𝐴𝐶𝐶𝐼𝐼𝑃𝑃𝐴𝐴𝐶𝐶𝑣𝑣 + 𝐾𝐾𝑎𝑎,?̇?𝐻𝐶𝐶?̇?𝐻𝐶𝐶𝑣𝑣 + 𝐾𝐾𝑡𝑡,𝑃𝑃𝐶𝐶𝑃𝑃𝐶𝐶𝑣𝑣 + 𝐾𝐾𝑡𝑡,𝑊𝑊𝐶𝐶𝑊𝑊𝐶𝐶𝑣𝑣 + 𝐾𝐾𝑡𝑡,𝐴𝐴𝐶𝐶𝐸𝐸𝐶𝐶𝐴𝐴𝐶𝐶𝐸𝐸𝐶𝐶𝑣𝑣 𝐶𝐶𝑣𝑣 = 𝐶𝐶𝑇𝑇�1+𝐾𝐾𝑎𝑎,𝐴𝐴𝐶𝐶𝐴𝐴+𝐾𝐾𝑎𝑎,𝐼𝐼𝑃𝑃𝐴𝐴𝐶𝐶𝐼𝐼𝑃𝑃𝐴𝐴+𝐾𝐾𝑎𝑎,?̇?𝐻𝐶𝐶?̇?𝐻+𝐾𝐾𝑅𝑅,𝑃𝑃𝐶𝐶𝑃𝑃+𝐾𝐾𝑅𝑅,𝑀𝑀𝐶𝐶𝑀𝑀+𝐾𝐾𝑅𝑅,𝐴𝐴𝐴𝐴𝐴𝐴𝐶𝐶𝐴𝐴𝐴𝐴𝐴𝐴� (7.14) Reactant, isopropanol, and catalyst surface are represented by (A), (IPA), and (S), resepectively. The product (P) symbol in the above equations represent anisole (E), phenol (B), or 2-Methoxycyclohexanone (G), meanwhile, the side product_1 (W) represents either water (W) or methanol (J) as the side product from the anisole or phenol formation. Meanwhile, 𝐶𝐶𝜕𝜕 and 𝐶𝐶𝑣𝑣 denote the concentrtaion of total catalyst sites and vacant catalyst sites, respectively. Equation 7.14 becomes: 𝑟𝑟𝑠𝑠,𝐴𝐴 = 𝐶𝐶𝑇𝑇�𝑘𝑘𝑠𝑠𝐾𝐾𝑎𝑎,𝐴𝐴𝐾𝐾𝑎𝑎,𝐼𝐼𝑃𝑃𝐴𝐴𝐾𝐾𝑎𝑎,?̇?𝐻𝐶𝐶𝐴𝐴𝐶𝐶?̇?𝐻𝐶𝐶𝐼𝐼𝑃𝑃𝐴𝐴−𝑘𝑘−𝑠𝑠𝐾𝐾𝑅𝑅,𝑃𝑃𝐾𝐾𝑅𝑅,𝐴𝐴𝐴𝐴𝐴𝐴𝐾𝐾𝑅𝑅,𝑀𝑀𝐶𝐶𝐴𝐴𝐴𝐴𝐴𝐴𝐶𝐶𝑃𝑃𝐶𝐶𝑀𝑀��1+𝐾𝐾𝑎𝑎,𝐴𝐴𝐶𝐶𝐴𝐴+𝐾𝐾𝑎𝑎,𝐼𝐼𝑃𝑃𝐴𝐴𝐶𝐶𝐼𝐼𝑃𝑃𝐴𝐴+𝐾𝐾𝑎𝑎,?̇?𝐻𝐶𝐶?̇?𝐻+𝐾𝐾𝑅𝑅,𝑃𝑃𝐶𝐶𝑃𝑃+𝐾𝐾𝑅𝑅,𝑀𝑀𝐶𝐶𝑀𝑀+𝐾𝐾𝑅𝑅,𝐴𝐴𝐴𝐴𝐴𝐴𝐶𝐶𝐴𝐴𝐴𝐴𝐴𝐴�3 (7.15) At initial rate (t = 1 h), Equation 7.15 simplifies: (𝐶𝐶𝑃𝑃 = 𝐶𝐶𝑊𝑊 = 𝐶𝐶𝐴𝐴𝐶𝐶𝐸𝐸 = 0) 𝑟𝑟𝑠𝑠,𝐴𝐴 = �𝑘𝑘𝑠𝑠𝐾𝐾𝑎𝑎,𝐴𝐴𝐾𝐾𝑎𝑎,𝐼𝐼𝑃𝑃𝐴𝐴𝐾𝐾𝑎𝑎,?̇?𝐻𝐶𝐶𝐴𝐴0𝐶𝐶?̇?𝐻0𝐶𝐶𝐼𝐼𝑃𝑃𝐴𝐴0��1+𝐾𝐾𝑎𝑎,𝐴𝐴𝐶𝐶𝐴𝐴0+𝐾𝐾𝑎𝑎,𝐼𝐼𝑃𝑃𝐴𝐴𝐶𝐶𝐼𝐼𝑃𝑃𝐴𝐴0+𝐾𝐾𝑎𝑎,?̇?𝐻𝐶𝐶?̇?𝐻0�3 (7.16) The kinetic equations have many kinetic and equilibrium constants that need to be determined by matching experimental concentration versus time data to the model calculations. In the case of 154 guaiacol consumption, the initial concentration versus time profile was linear so that the initial rate of consumption was calcuated as a finite difference �∆𝐶𝐶𝐴𝐴∆𝑡𝑡�. Then, using OriginPro® 2024b software, the model parameters of Equation 7.16, 𝑘𝑘𝑠𝑠, 𝐾𝐾𝑎𝑎,𝐴𝐴, 𝐾𝐾𝑎𝑎,𝐼𝐼𝑃𝑃𝐴𝐴, and 𝐾𝐾𝑎𝑎,?̇?𝐻 were estimated by minimizing the sum of squares error (SSE) between the measured and model calculated initial guaiacol consumption rates. The lowest sum of squares error (SSE) was selected to obtain the parameter values reported and presented in Figure 7.2. The model (dotted line) was well fitted with the experimental data (square) with an adjusted R-square value of 0.99. Moreover, Equation 7.16 along with the constant parameters (𝑘𝑘𝑠𝑠, 𝐾𝐾𝑎𝑎,𝐴𝐴, 𝐾𝐾𝑎𝑎,𝐼𝐼𝑃𝑃𝐴𝐴, and 𝐾𝐾𝑎𝑎,?̇?𝐻) could be used to predict the initial reaction rate value based on the variation of acid and isopropanol concentrations. The effect of protons (H+) in GECH-DO was evaluated using Equation 7.16 and the kinetic constants from Figure 7.2a. It is clearly seen from Figure 7.2b that higher 𝐶𝐶?̇?𝐻0 provides higher initial reaction rate for GECH-DO until the optimum point at 126 mmol s-1 gPt-1 by implementing 0.9 M MSA. The study also concluded from experimental data that the optimum condition of GECH-DO occurred at 1.0 M MSA as explained in the Chapter 5 and 6 compared to 0.2 and 0.5 M MSA. Therefore, continuous and stable proton supply from the GEOR compared to the OER with the sluggish kinetics would benefit the GECH-DO. Moreover, high acidity complicates the equipment requirements for the process due to the corrosive nature of acidic environments. Therefore, an optimized condition should be justified in the paired electrosynthesis of GECH-DO and GEOR. 155 Figure 7.2. Rate of GECH-DO (a) data and model plot based on Langmuir-Hinshelwood mechanism to obtain kinetic rate constant (see Equation 7.16) where surface reaction is the rate-determining step and the effect of (b) proton and (c) IPA addition in the cathode chamber according to Equation 7.16. With constant concentrations of isopropanol and methanesulfonic acid, Equation 7.16 could be simplified into: �𝐾𝐾𝑎𝑎,𝐼𝐼𝑃𝑃𝐴𝐴𝐶𝐶𝐼𝐼𝑃𝑃𝐴𝐴0 ≫ 1� 𝑟𝑟𝑠𝑠,𝐴𝐴 = �𝑘𝑘𝑠𝑠𝐾𝐾𝑎𝑎,𝐴𝐴𝐾𝐾𝑎𝑎,𝐼𝐼𝑃𝑃𝐴𝐴𝐾𝐾,?̇?𝐻𝐶𝐶𝐴𝐴0𝐶𝐶?̇?𝐻0𝐶𝐶𝐼𝐼𝑃𝑃𝐴𝐴0��1 + 𝐾𝐾𝑎𝑎,𝐴𝐴𝐶𝐶𝐴𝐴0 + 𝐾𝐾𝑎𝑎,𝐼𝐼𝑃𝑃𝐴𝐴𝐶𝐶𝐼𝐼𝑃𝑃𝐴𝐴0 + 𝐾𝐾𝑎𝑎,?̇?𝐻𝐶𝐶?̇?𝐻0�3 (a) (b) (c) 156 𝑟𝑟𝑠𝑠,𝐴𝐴 = �𝑘𝑘𝑠𝑠𝐾𝐾𝑎𝑎,𝐴𝐴𝐾𝐾𝑎𝑎,𝐼𝐼𝑃𝑃𝐴𝐴𝐾𝐾𝑎𝑎,?̇?𝐻𝐶𝐶𝐴𝐴0𝐶𝐶?̇?𝐻0𝐶𝐶𝐼𝐼𝑃𝑃𝐴𝐴0��𝐾𝐾𝑎𝑎,𝐼𝐼𝑃𝑃𝐴𝐴𝐶𝐶𝐼𝐼𝑃𝑃𝐴𝐴0�3 𝑟𝑟𝑠𝑠,𝐴𝐴 = 𝑘𝑘𝑠𝑠𝐾𝐾𝑎𝑎,𝐴𝐴𝐾𝐾𝑎𝑎,?̇?𝐻𝐶𝐶?̇?𝐻0�𝐾𝐾𝑎𝑎,𝐼𝐼𝑃𝑃𝐴𝐴𝐶𝐶𝐼𝐼𝑃𝑃𝐴𝐴0�2 𝐶𝐶𝐴𝐴0 𝑟𝑟𝑠𝑠,𝐴𝐴 = 𝑘𝑘𝐶𝐶𝐴𝐴0 (7.17) 𝑘𝑘 = 𝑘𝑘𝑠𝑠𝐾𝐾𝑎𝑎,𝐴𝐴𝐾𝐾𝑎𝑎,?̇?𝐻𝐶𝐶?̇?𝐻0�𝐾𝐾𝑎𝑎,𝐼𝐼𝑃𝑃𝐴𝐴𝐶𝐶𝐼𝐼𝑃𝑃𝐴𝐴0�2 (7.18) The introduction of isopropanol (IPA) not only triggers the dehydration reaction but also has a positive effect until 20% v/v in cathode chamber, as described by Figure 7.2c. The finding is similar with the experimental data that showed 25% v/v IPA addition in the cathode chamber was the optimum condition in the GECH-DO. Furthermore, at constant concentrations of isopropanol and methanesulfonic acid, Equation 7.17 �𝑟𝑟𝑠𝑠,𝐴𝐴 = 𝑘𝑘𝐶𝐶𝐴𝐴� indicates that 1st-order kinetics for guaiacol consumption is an appropriate approximation of the Langmuir-Hinshelwood kinetics. 7.1.2 Glycerol electrocatalytic oxygenation reaction (GEOR) A similar set of preliminary experiments was also conducted for GEOR, with variations in the initial glycerol concentration to determine the impact on the initial rate of glycerol consumption. The reaction rate is linearly dependent on the initial glycerol concentration, as shown in Figure 7.3a and 7.3b and does not reach a maximum, even as the initial glycerol concentration increases to 400 mM. Hence, GEOR is assumed to follow a Langmuir-Hinshelwood mechanism with adsorption as the RDS. Hence, the Langmuir-Hinshelwood catalytic reaction steps for the GEOR in acidic medium, with adsorption step as the RDS. is written as follows: Adsorption: �𝑟𝑟𝑎𝑎,𝑊𝑊 = 0� Water adsorption: 𝑀𝑀 + 𝑆𝑆 𝑂𝑂𝐻𝐻. 𝑆𝑆 + 𝐻𝐻+ + 𝑒𝑒− 157 𝑟𝑟𝑎𝑎,𝑊𝑊 = 𝑘𝑘𝑎𝑎,𝑊𝑊𝐶𝐶𝑊𝑊𝐶𝐶𝑣𝑣 − 𝑘𝑘−𝑎𝑎,𝑊𝑊𝐶𝐶𝑂𝑂𝐻𝐻𝑆𝑆𝐶𝐶?̇?𝐻 (7.19) 𝐶𝐶𝑂𝑂𝐻𝐻𝑆𝑆 = 𝐾𝐾𝑎𝑎,𝑊𝑊 𝐶𝐶𝑀𝑀𝐶𝐶?̇?𝐻 𝐶𝐶𝑣𝑣 (7.20) Desorption: (𝑟𝑟𝑡𝑡 = 0) Main product: 𝑃𝑃. 𝑆𝑆 𝑃𝑃 + 𝑆𝑆 𝑟𝑟𝑡𝑡𝑤𝑤𝑠𝑠,𝑃𝑃 = 𝑘𝑘𝑡𝑡,𝑃𝑃𝐶𝐶𝑃𝑃𝑆𝑆 − 𝑘𝑘−𝑡𝑡,𝑃𝑃𝐶𝐶𝑃𝑃𝐶𝐶𝑣𝑣 (7.21) 𝐶𝐶𝑃𝑃𝑆𝑆 = 𝐾𝐾𝑡𝑡,𝑃𝑃𝐶𝐶𝑃𝑃𝐶𝐶𝑣𝑣 (7.22) Side product: 𝑀𝑀. 𝑆𝑆 𝑀𝑀 + 𝑆𝑆 𝑟𝑟𝑡𝑡𝑤𝑤𝑠𝑠,𝑊𝑊 = 𝑘𝑘𝑡𝑡,𝑊𝑊𝐶𝐶𝑊𝑊𝑆𝑆 − 𝑘𝑘−𝑡𝑡,𝑊𝑊𝐶𝐶𝑊𝑊𝐶𝐶𝑣𝑣 (7.23) 𝐶𝐶𝑊𝑊𝑆𝑆 = 𝐾𝐾𝑡𝑡,𝑊𝑊𝐶𝐶𝑊𝑊𝐶𝐶𝑣𝑣 (7.24) Reaction: (𝑟𝑟𝑠𝑠 = 0) 𝐴𝐴. 𝑆𝑆 + 𝑂𝑂𝐻𝐻. 𝑆𝑆 𝑃𝑃. 𝑆𝑆 + 𝑀𝑀. 𝑆𝑆 𝑟𝑟𝑠𝑠,𝐴𝐴 = 𝑘𝑘𝑠𝑠,𝐴𝐴𝐶𝐶𝐴𝐴𝑆𝑆𝐶𝐶𝑂𝑂𝐻𝐻𝑆𝑆 − 𝑘𝑘−𝑠𝑠,𝐴𝐴𝐶𝐶𝑃𝑃𝑆𝑆𝐶𝐶𝑊𝑊𝑆𝑆 (7.25) 𝐶𝐶𝐴𝐴𝑆𝑆 = 𝐾𝐾𝑡𝑡,𝑃𝑃𝐶𝐶𝑃𝑃𝐶𝐶𝑣𝑣𝐾𝐾𝑡𝑡,𝑊𝑊𝐶𝐶𝑊𝑊𝐶𝐶𝑣𝑣𝐾𝐾𝑠𝑠,𝐴𝐴𝐾𝐾𝑎𝑎,𝑊𝑊 𝐶𝐶𝑊𝑊𝐶𝐶?̇?𝐻 𝐶𝐶𝑣𝑣 𝐶𝐶𝐴𝐴𝑆𝑆 = 𝐾𝐾𝑅𝑅,𝑃𝑃𝐾𝐾𝑅𝑅,𝑀𝑀𝐶𝐶𝑃𝑃𝐶𝐶?̇?𝐻𝐶𝐶𝑣𝑣𝐾𝐾𝑠𝑠,𝐴𝐴𝐾𝐾𝑎𝑎,𝑀𝑀 (7.26) where, 𝐶𝐶𝜕𝜕 = 𝐶𝐶𝑣𝑣 + 𝐶𝐶𝐴𝐴𝑆𝑆 + 𝐶𝐶𝑂𝑂𝐻𝐻𝑆𝑆 + 𝐶𝐶𝑃𝑃𝑆𝑆 + 𝐶𝐶𝑊𝑊𝑆𝑆 𝐶𝐶𝜕𝜕 = 𝐶𝐶𝑣𝑣 + 𝐾𝐾𝑡𝑡,𝑃𝑃𝐾𝐾𝑡𝑡,𝑊𝑊𝐶𝐶𝑃𝑃𝐶𝐶?̇?𝐻𝐶𝐶𝑣𝑣𝐾𝐾𝑠𝑠,𝐴𝐴𝐾𝐾𝑎𝑎,𝑊𝑊 + 𝐾𝐾𝑎𝑎,𝑊𝑊 𝐶𝐶𝑊𝑊𝐶𝐶?̇?𝐻 𝐶𝐶𝑣𝑣 + 𝐾𝐾𝑡𝑡,𝑃𝑃𝐶𝐶𝑃𝑃𝐶𝐶𝑣𝑣 + 𝐾𝐾𝑡𝑡,𝑊𝑊𝐶𝐶𝑊𝑊𝐶𝐶𝑣𝑣 𝐶𝐶𝑉𝑉 = 𝐶𝐶𝑇𝑇�1+𝐾𝐾𝑅𝑅,𝑃𝑃𝐾𝐾𝑅𝑅,𝑀𝑀𝐾𝐾𝑠𝑠,𝐴𝐴𝐾𝐾𝑎𝑎,𝑀𝑀𝐶𝐶𝑃𝑃𝐶𝐶?̇?𝐻+𝐾𝐾𝑎𝑎,𝑀𝑀𝐴𝐴𝑀𝑀𝐴𝐴?̇?𝐻𝐶𝐶𝑣𝑣+𝐾𝐾𝑅𝑅,𝑃𝑃𝐶𝐶𝑃𝑃𝐶𝐶𝑣𝑣+𝐾𝐾𝑅𝑅,𝑀𝑀𝐶𝐶𝑀𝑀𝐶𝐶𝑣𝑣� (7.27) Adsorption: �𝑟𝑟𝑎𝑎,𝐴𝐴 ≠ 0� 𝑟𝑟𝑎𝑎,𝐴𝐴 = 𝑘𝑘𝑎𝑎,𝐴𝐴𝐶𝐶𝐴𝐴𝐶𝐶𝑣𝑣 − 𝑘𝑘−𝑎𝑎,𝐴𝐴𝐶𝐶𝐴𝐴𝑆𝑆 158 𝑟𝑟𝑎𝑎,𝐴𝐴 = 𝑘𝑘𝑎𝑎,𝐴𝐴𝐶𝐶𝐴𝐴𝐶𝐶𝑣𝑣 − 𝑘𝑘−𝑎𝑎,𝐴𝐴 𝐾𝐾𝑡𝑡,𝑃𝑃𝐾𝐾𝑡𝑡,𝑊𝑊𝐾𝐾𝑠𝑠,𝐴𝐴𝐾𝐾𝑎𝑎,𝑊𝑊 𝐶𝐶?̇?𝐻𝐶𝐶𝑃𝑃𝐶𝐶𝑣𝑣 𝑟𝑟𝑎𝑎,𝐴𝐴 = 𝑘𝑘𝑎𝑎,𝐴𝐴�𝐶𝐶𝐴𝐴− 𝐾𝐾𝑅𝑅,𝑃𝑃𝐾𝐾𝑅𝑅,𝑀𝑀𝐾𝐾𝑎𝑎,𝐴𝐴𝐾𝐾𝑠𝑠,𝐴𝐴𝐾𝐾𝑎𝑎,𝑀𝑀𝐶𝐶?̇?𝐻𝐶𝐶𝑃𝑃��1+𝐾𝐾𝑠𝑠,𝐴𝐴𝐾𝐾𝑅𝑅,𝑃𝑃𝐾𝐾−𝑅𝑅,𝑀𝑀𝐾𝐾𝑎𝑎,𝑀𝑀 𝐶𝐶𝑃𝑃+𝐾𝐾𝑎𝑎,𝑀𝑀𝐶𝐶𝑀𝑀+𝐾𝐾𝑅𝑅,𝑀𝑀𝐶𝐶𝑀𝑀+𝐾𝐾𝑅𝑅,𝑃𝑃𝐶𝐶𝑃𝑃� (7.28) For the initial rate, Equation 7.28 becomes: �𝐶𝐶𝑃𝑃 = 0,𝐾𝐾𝑡𝑡,𝑊𝑊𝐶𝐶𝑊𝑊 ≈ 0,𝑎𝑎𝑛𝑛𝑑𝑑 𝐾𝐾𝑎𝑎,𝑊𝑊𝐶𝐶𝑊𝑊 >>� 𝑟𝑟𝑎𝑎,𝐴𝐴 = 𝑘𝑘𝑎𝑎,𝐴𝐴(𝐶𝐶𝐴𝐴0)�𝐾𝐾𝑎𝑎,𝑊𝑊𝐶𝐶𝑊𝑊� 𝑟𝑟𝑎𝑎,𝐴𝐴 = 𝑘𝑘𝐶𝐶𝐴𝐴0 (7.29) 𝑘𝑘 = 𝑘𝑘𝑎𝑎,𝐴𝐴�𝐾𝐾𝑎𝑎,𝑀𝑀𝐶𝐶𝑀𝑀� (7.30) Reactant, water, and catalyst surface are represented by (A), (W), and (S), resepectively. The product (P) symbol in the above equations represent glyceraldehyde (B) or dihydroxyacetone (F). Meanwhile, 𝐶𝐶𝜕𝜕 and 𝐶𝐶𝑣𝑣 denote total catalyst site and vacant catalyst site. Figure 7.3. The GEOR under galvanostatic conditions at (a) lower and (b) higher glycerol concentration in acidic condition (1.0 M MSA). Reaction condition: I = 0.4 A (j = 66 mA cm-2), T = 60 °C, t = 4 h. Catalyst: 5 wt%-Pt/C (5 g/L). Initial reaction rates were measured by Equation 3.4. (a) (b) 159 To verify the model expression (Equation 7.29), experimental rates and model calculated rates were evaluated using OriginPro 2024b software. The lowest sum of squares error (SSE) between the measured and model calculated rates was selected to obtain the model parameter 𝑘𝑘 as presented in Figure 7.4. The model (dotted line) was well-fitted with the experimental data (circle), with an adjusted R-square value of 0.99. To improve the GEOR initial reaction rate, Equation 7.30 could be optimized by decreasing the water concentration (𝐶𝐶𝑊𝑊) which is strongly related to the anode compartment volume. Therefore, reducing the anode compartment volume could enhance the GEOR initial reaction rate. On the other hand, reducing the anode compartment volume would reduce the inner electrode gap and improve the electrosynthesis reaction by ensuring the ionic conductivity in the reactor.429 Also, note that Equation 7.29 indicates that the initial rate of glycerol consumption can be described by 1st-order kinetics at the chosen reaction conditions. Figure 7.4. Reaction rate of GEOR data and model plot based on Langmuir-Hinshelwood mechanism to obtain the rate constant where glycerol adsorption is the rate-determining step. 160 7.2 Reaction pathways: reaction order and rate constant determination for GECH-DO and GEOR As described earlier, the guaiacol electrocatalytic hydrodeoxygenation and glycerol electrooxidation follow 1st-order kinetics. However, many chemicals are generated during organic reactions. Thus, the Langmuir-Hinshelwood analysis reported above could not predict all the reaction orders and kinetics constants for all intermediates or products. In the cathode chamber, guaiacol demethoxylation and ring-saturation pathways were implemented in this study, according to Wijaya et al.105. However, the introduction of isopropanol (IPA) in this study triggered the dehydration reaction of guaiacol toward anisole, which later produced cyclohexane. Therefore, standalone reactions of phenol, anisole, cyclohexanone, and cyclohexanol were conducted in the SSER under similar conditions to determine the reaction order by graphical analysis for 4 h of reaction. ECH-DO of cyclohexanone produced cyclohexanol with 94% conversion, whereas the ECH of cyclohexanol yielded no other products after 4 h of reaction, as determined by GC-MS analysis. Thus, one concludes that high cyclohexanol concentration inhibits further reaction by hydrogenation. The analysis also showed that anisole reacts to produce cyclohexane (27% selectivity) and methoxycyclohexane (22% selectivity) as the main products after 4 h of reaction. Anisole also yielded cyclohexanone (14%) and cyclohexanol (9%). Meanwhile, phenol only produced cyclohexanol, cyclohexanone, and cyclohexane as the main products. Guaiacol dehydration into anisole was well described by 1st-order kinetics (R2 = 0.98), as presented in Table B7 (Appendices), and the kinetics of GECH-DO are well described by 1st-order reactions, except 161 for the ring saturation pathways. Hence, as shown in Figure 7.5, a reaction pathway can be proposed based on these results. The organic compounds in Figure 7.5 and 7.6 are denoted as follows: guaiacol (A), phenol (B), cyclohexanone (C), cyclohexanol (D), anisole (E), cyclohexane (F), 2-Methoxycyclohexanone (G), 2-Methoxycyclohexanol (H), methoxycyclohexane (I), methanol (J), proton (?̇?𝐻), isopropanol (IPA), acetone (ACE), and water (W). Thermodynamic data are presented in the appendices Table A.3 derived from Chemdraw® 18.1 software to determine the standard Gibbs free energy and enthalpy of formation of the organic chemicals. All ECH-DO reactions are thermodynamically favorable except the reaction of methoxycyclohexane to cyclohexanone (∆G > 0). Figure 7.5. Proposed reaction pathway of GECH-DO with IPA addition in SSER using Pt/C (5 wt%) catalyst with isopropanol oxidation reaction toward acetone as a proton donor in the inserted figure provided with the kinetic rate equations used to estimate the reaction rate constant. 162 As shown in Figure 7.5, the component concentrations are written in terms of the reactor design equation (or mole balance equation), assuming a perfectly mixed batch reactor, for each component, yielding equations of the form 𝑡𝑡𝐶𝐶𝑗𝑗𝑡𝑡𝑡𝑡= 𝑓𝑓(𝑘𝑘,𝐸𝐸,𝐶𝐶) (7.31) where the function f is the net rate of production of component Cj determined from the proposed reaction pathway and k, E and C represent the rate constants, activation energies and component concentrations that are defined by the kinetic rate equations. The kinetic analysis calculations were performed numerically using Matlab® R2017a. Accordingly, the set of ordinary differential equations, representing the component mole balance equations, were integrated over the reaction time using a Runge-Kutta integration algorithm (ODE45). Figure 7.6. The ECH of (a) phenol and (b) anisole kinetic model vs. experiment using SSER with 1.0 M MSA concentration and 25 v/v % IPA content. Reaction condition: I = - 0.4 A (j = -66 mA cm-2), T = 60 oC, t = 4 h, N = 600 rpm. Catalyst: 5 wt%-Pt/C (0.5 g/L). The parameters of the model were estimated using a Levenberg-Marquardt algorithm that minimized the residual sum of squares between the measured component concentrations and the (a) (b) 163 calculated model values. The confidence intervals for the parameter estimates were determined by nlparci code that computes 95% confidence intervals with the Jacobi method to fit a nonlinear regression model and obtain the coefficient estimates. The Matlab script for GECH-DO is presented in Appendices (Section A.7). Figure 7.6 reports experimental data and the model calculations for the case of phenol and anisole as reactants. The agreement between the measured and calculated component concentrations indicates that the reaction pathways proposed for phenol (B) and anisole (E) capture the key kinetic steps of the reactions. Figure 7.7 reports the parity plot for the guaiacol feed at different operating conditions, as discussed below (see Table 7.1), using the kinetic pathways shown in Figure 7.5. Again, the model provides a suitable description of the kinetic pathways for guaiacol ECH-DO. The parity plot for the kinetics rate constants in Table 7.1 has a 0.976 value of R-square which shows a satisfactory relationship between the experimental and predicted values. Therefore, the assumption of 1st-order and 2nd-order could be justified in the study of guaiacol ECH-DO kinetics. Figure 7.7. Parity plot of isopropanol effect 0 – 90 vol.% on the Guaiacol ECH-DO kinetics constants determination in Table 7.1. I = - 0.4 A (j = -66 mA cm-2), N = 600 rpm, T = 60 oC, t = 4 h. Catalyst: 5 wt%-Pt/C (0.5 g/L). 164 A similar kinetic analysis was done for the anode chamber reactions. Accordingly, a set of experiments was conducted to verify the reaction order for glycerol EOR by reacting dihydroxyacetone, glyceraldehyde, glyceric acid, and tartronic acid as the initial substrate in the SSER under similar conditions (1.0 M MSA, 0.25 M NaCl, 5 g/L Pt/C, and 66 mA cm-2) for 2 h of reaction time, as presented in Figure 7.8. The organic compounds in Figure 7.8 and 7.9 are denoted as follows: glycerol (A), glyceraldehyde (B), glyceric acid (C), glycolic acid (D), tartronic acid (E), dihydroxyaacetone (F), lactic acid (G), formic acid (H), acetic acid (I), and water (W). Figure 7.8. The electrooxidation of (a) glyceraldehyde and (b) dihydroxyacetone kinetic model vs. experiment using SSER with 1.0 M MSA concentration. Reaction condition: I = 0.4 A (j = 66 mA cm-2), T = 60 oC, t = 2 h. Catalyst: 5 wt%-Pt/C (0.5 g/L). The glyceraldehyde (B) electrooxidation yielded glyceric acid (5.2%), glycolic acid (7.6%), tartronic acid (7.9%), and formic acid (10.9%). Meanwhile, dihydroxyacetone (F) produced formic acid (29.4%) and acetic acid (60.4%) with 91% conversion. On the other hand, tartronic acid generated glycolic acid and formic acid as much as 21.5% and 15.7%, respectively. The glyceric acid EOR mainly formed formic acid (44.4%), with tartronic acid (29.6%) and glycolic acid (15.6%) as the products. Meanwhile, glycolic acid electrooxidation only produced formic acid. (a) (b) 165 Moreover, the lactic acid electrooxidation generated 32.8% and 67.2% selectivity of formic acid and acetic acid, respectively. Based on these results a proposed reaction pathway for glycerol EOR using SSER is shown in Figure 7.9 along with the kinetic rate equations used to estimate the reaction rate constants. Figure 7.9. Proposed reaction pathway of GEOR in SSER using Pt/C (5 wt%) catalyst with the kinetics rate equation to estimate the reaction rate constant. Thermodynamic data are presented in the appendices (Table A.4) derived from Chemdraw® 18.1 software to determine the standard Gibbs free energy and enthalpy of formation of the organic chemicals. All the glycerol EOR reactions are thermodynamically favorable compared to the OER. The experimental data were well described by 1st order kinetics (R2 > 0.96) in the GEOR reaction, as presented in Table B7 (Appendices). As before, the component concentrations were determined by numerical integration of the kinetic rate equations shown in Figure 7.9 and Figure 7.8 that compare the measured and model calculated component concentrations when glyceraldehyde and dihydroxyacetone were reactants. Together with Figure 7.10 that reports a parity plot for the case of the guaiacol feed (7.10a) and glycerol feed (7.10b) at different operating conditions as discussed 166 below (see Table 7.2), using the kinetic pathways shown in Figure 7.9, one concludes that the model provides a suitable description of the kinetic pathways for glycerol EOR. Figure 7.10. Parity plot of the temperature effect on (a) guaiacol ECH-DO and (b) glycerol EOR kinetics constants determination in Table 7.2. I = - 0.4 A (j = -66 mA cm-2), N = 600 rpm, t = 4 h. Catalyst: 5 wt%-Pt/C (0.5 g/L). Figure 7.10 shows satisfactory results either for guaiacol ECH-DO (R2 = 0.976) in Figure 7.10a or glycerol EOR (R2 = 0.982) in Figure 7.10b. The comparisons of experimental and predicted values have linear (slope ≈ 1) and low intercepts for guaiacol ECH-DO (-0.035) and glycerol EOR (-0.443), respectively. Therefore, the proposed reaction pathways in Figures 7.5 and 7.9 were accordingly justified with 1st and 2nd order kinetics presented in the Figures. 7.3 Isopropanol and MSA concentration variation The rate constant values determined from the ECH-DO kinetic pathway model (Table 7.1), show that IPA affects the desirable route of reaction through anisole formation (route 1, k11 – k14), with optimum conditions at 50% IPA addition, yielding the highest k11 rate constant. As reported previously in Equation 7.18, the apparent rate constant (𝑘𝑘) depends on the combination of acid (𝐶𝐶?̇?𝐻), isopropanol �𝐶𝐶?̇?𝐻𝐼𝐼𝑃𝑃𝐴𝐴� and water (𝐶𝐶𝐾𝐾) concentrations. (a) (b) 167 Table 7.1. Guaiacol ECH-DO rate constant analysis results in SSER at 0.5 g Pt/C, and -66 mA cm-2. Reaction Apparent rate constant (k) NO IPA 10% IPA 25% IPA 50% IPA 75% IPA 90% IPA Wijaya et al.105 Unit Route 1: Dehydration-Ring saturation-Demethoxylation Guaiacol to anisole (k11) - 0.047 ± 0.081 0.066 ± 0.057 0.108 ± 0.041 0.095 ± 0.037 0.064 ± 0.022 - h-1 Anisole to methoxycyclohexane (k12) - 0.103 ± 0.178 0.184 ± 0.228 0.707 ± 0.413 0.232 ± 0.186 0.189 ± 0.077 - h-1 Methoxyclohexane to cyclohexanone (k13) - 0.005 ± 0.008 0.011 ± 0.015 0.021 ± 0.011 0.006 ± 0.001 0.011 ± 0.004 - h-1 Anisole to cyclohexane (k14) - 0.003 ± 0.004 0.006 ± 0.005 0.008 ± 0.012 0.015 ± 0.013 0.023 ± 0.005 - h-1 Route 2: Demethoxylation-Ring saturation-Dehydration Guaiacol to phenol (k21) 0.222 ± 0.041 0.219 ± 0.025 0.181 ± 0.054 0.213 ± 0.034 0.172 ± 0.023 0.114 ± 0.019 0.242 ± 0.009 h-1 Phenol to cyclohexanone (k22) 1.55 ± 1.142 0.822 ± 0.058 1.645 ± 1.61 3.096 ± 2.626 3.351 ± 4.017 1.372 ± 1.091 3.314 ± 0.570 h-1 Cyclohexanone to cyclohexanol (k23) 0.794 ± 0.141 0.697 ± 0.126 0.478 ± 0.255 0.776 ± 0.295 0.686 ± 0.271 0.627 ± 0.088 0.199 ± 0.024 h-1 Cyclohexanol to cyclohexane (k24) - 1.297 ± 1.039 0.688 ± 0.488 0.854 ± 0.76 0.376 ± 0.38 1.029 ± 0.657 - h-1 Route 3: Ring saturation-Demethoxylation-Dehydration Guaiacol to 2-methoxycyclohexanone (k31) 0.099 ± 0.015 0.09 ± 0.005 0.085 ± 0.018 0.048 ± 0.017 0.059 ± 0.029 0.059 ± 0.003 0.093 ± 0.008 h-1 2-methoxycyclohexanone to 2-methoxycyclohexanol (k32) 0.045 ± 0.012 0.151 ± 0.137 0.048 ± 0.024 0.307 ± 0.447 0.049 ± 0.037 0.078 ± 0.036 0.017 ± 0.006 mM-1 h-1 2-methoxycyclohexanol to cyclohexanol (k33) 0.002 ± 0.001 0.004 ± 0.006 0.035 ± 0.04 0.011 ± 0.007 0.009 ± 0.007 0.008 ± 0.008 0.033 ± 0.056 mM-1 h-1 Overall guaiacol conversion 0.303 ± 0.037 0.308 ± 0.035 0.341 ± 0.037 0.312 ± 0.049 0.299 ± 0.081 0.225 ± 0.062 0.336 h-1 168 However, the higher isopropanol concentration drops the conductivity of the electrolyte and negatively affects the whole electrosynthesis process. Therefore, high IPA ratio toward the electrolyte volume (vol. %) could reduce the overall guaiacol rate (0.225 h-1) as shown from Table 7.1 for isopropanol addition > 75% v/v. Moreover, higher isopropanol concentration reduces the possibility of hydrogen donation from the acid protonation due to competition for active surface sites. Therefore, the higher IPA concentration reduces the other two guaiacol reaction routes, demethoxylation (k21) and ring saturation (k31). Furthermore, the presence of IPA enhanced the Meerwein-Ponndorf-Verley (MPV) reduction of 2-methoxycyclohexanone to 2-methoxycyclohexanol (k32), as shown by the higher rate constant compared with the absence of IPA (Table 7.1). The highest apparent 1st order rate constant for total guaiacol consumption of 0.341 h-1, was obtained with 25% IPA (Table 7.1). All three routes dictate the overall initial reaction of guaiacol electrohydodeoxygenation (ECH-DO) process in the cathode chamber. The proposed reaction pathway is also in line with the density functional theory (DFT) study by Verma et al.77 The present study suggests that the formation of anisole occurred due to IPA facilitated proton attack on the C–OH σ bond to form water, which has an activation energy of 336.23 kJ/mol.77 MSA is a potent Brønsted acid catalyst that is solvated in isopropanol (IPA), hence, is more reactive for dehydration.45, 430 7.4 Temperature variation on 5 wt% Pt/C catalyst in 1.0 M acidic electrolyte pairs The kinetic model for GECH-DO and GEOR (see Matlab script in Section A.7 of the Appendices) was applied to obtain the apparent rate constants as the reactor temperature was varied.. The experiment and model plots at 30 °C are presented in Figure B.6 (Appendices) as an example of the outcome of the modelling analysis. 169 Table 7.2. Rate constant of GECH-DO and GEOR on the paired cell electrosynthesis application in SSER using Pt/C (5wt%) catalyst. Code Reactant Product 30 °C 40 °C 50 °C 60 °C Unit k11 Guaiacol Anisole 0.125 ± 0.003 0.075 ± 0.002 0.065 ± 0.002 0.053 ± 0.003 h-1 k12 Anisole methoxycyclohexane 0.128 ± 0.048 0.132 ± 0.026 0.159 ± 0.075 0.186 ± 0.032 h-1 k13 methoxycyclohexane cyclohexanone 0.131 ± 0.065 0.136 ± 0.066 0.182 ± 0.048 0.196 ± 0.031 h-1 k14 Anisole cyclohexane 0.002 ± 0.011 0.004 ± 0.009 0.006 ± 0.01 0.009 ± 0.006 h-1 k21 Guaiacol Phenol 0.117 ± 0.004 0.137 ± 0.004 0.159 ± 0.007 0.191 ± 0.014 h-1 k22 Phenol Cyclohexanone 2.819 ± 0.844 2.463 ± 0.638 2.189 ± 1.401 1.634 ± 0.159 h-1 k23 Cyclohexanone Cyclohexanol 0.104 ± 0.009 0.12 ± 0.011 0.174 ± 0.01 0.261 ± 0.013 h-1 k24 Cyclohexanol cyclohexane 0.406 ± 0.028 0.396 ± 0.022 0.382 ± 0.017 0.359 ± 0.001 h-1 k31 Guaiacol 2-Methoxycyclohexanone 0.061 ± 0.003 0.091 ± 0.012 0.111 ± 0.012 0.126 ± 0.011 h-1 k32 2-Methoxycyclohexanone 2-Methoxycyclohexanol 0.007 ± 0.001 0.012 ± 0.002 0.015 ± 0 0.02 ± 0.002 mM-1 h-1 k33 2-Methoxycyclohexanol Cyclohexanol 0.012 ± 0.001 0.022 ± 0.002 0.031 ± 0.01 0.04 ± 0.013 mM-1 h-1 k1 Glycerol DHA 0.014 ± 0 0.022 ± 0.004 0.032 ± 0.003 0.036 ± 0.001 h-1 k2 Glycerol Glyceraldehyde 0.025 ± 0.002 0.027 ± 0.001 0.032 ± 0.002 0.037 ± 0.001 h-1 k3 Glyceraldehyde Glyceric acid 0.302 ± 0.063 0.398 ± 0.033 0.614 ± 0.058 0.696 ± 0.631 h-1 k4 Glyceric acid Glycolic acid 0.094 ± 0.049 0.24 ± 0.133 0.906 ± 0.363 2.76 ± 0.081 h-1 k5 Glyceric acid Tartronic acid 1.748 ± 0.376 1.758 ± 0.37 2.393 ± 0.004 4.486 ± 1.204 h-1 k6 Tartronic acid Glycolic acid 0.031 ± 0.087 0.099 ± 0.007 0.349 ± 0.164 0.508 ± 0.113 h-1 k7 Glycolic acid Formic acid 0.093 ± 0.388 0.289 ± 0.105 0.493 ± 1.736 0.998 ± 0.26 h-1 k9 Lactic acid Formic acid 0.066 ± 0 0.162 ± 0.157 1.947 ± 2.033 2.962 ± 0.126 h-1 k10 Lactic acid Acetic acid 0.046 ± 0.003 0.28 ± 0.063 1.35 ± 1.299 2.349 ± 0.722 h-1 170 Simultaneously, the kinetic constants obtained at various temperatures are reported in Table 7.2 and the the Arrhenius parameters calculated from these values are shown in Figure 7.11a and Figure 7.11b for the total guaiacol (k11+k21+k31) and glycerol (k1+k2) reaction rate. The complete Arrhenius parameters from the individual reactions are presented in Table B.11 and B.12 for GECH-DO and GEOR, respectively. The Arhenius parameters results showed that guaiacol conversion to phenol has a lower energy barrier (13.7 kJ mol-1) than the conversion to 2-methoxycyclohexanone (20.0 kJ mol-1), and anisole (23.0 kJ mol-1) in Table B.6. Furthermore, the glyceraldehyde formation is favorable at lower temperature compared with the dihydroxyacetone formation in regards to the activation energy for the anode chamber of GEOR, as shown in the Figure B.7 (Appendices). Figure 7.11. Arrhenius plots for the determination of activation energy (Ea) and pre-exponential factor (A) in total kinetics rate of (a) GECH-DO (k11+k21+k31) and (b) GEOR (k1+k2) using 1.0 M MSA electrolyte pairs at different temperatures (30–60 °C). Reaction condition: j = 66 mA cm-2, N = 600 rpm, t = 4 h. Catalyst: 5 wt%-Pt/C (5 g/L). Electrolyte: 0.25 M NaCl with 25% v/v IPA addition in the cathode chamber. As seen in Table 7.2, the magnitude of the kinetic constants of GECH-DO are quite similar to the other findings reported at 60°C from Wijaya et al.105. Unfortunately in the anode chamber, (a) (b) 171 GEOR, there are no available data of kinetic constants using SSER as the reactor configuration. Meanwhile, the total yield of desired product of GECH-DO and GEOR is more favorable at 50 °C (41.9%) than 60 °C (40.7%), eventhough, the higher temperature has a higher conversion rate (Table 6.4). This observation is a consequence of the lower desired glycerol electrooxidation product at higher temperature, due to higher oxidation degree. On the other hand, under lower operating temperature conditions, significant energy savings can be achieved, contributing to more sustainable and cost-effective production. 7.5 Temperature variation on PtNi/C catalyst in 1.0 M acidic electrolyte pairs To evaluate the kinetic parameters of the PtNi/C catalyst, the paired electrosynthesis was conducted at different temperatures to determine kinetic information for scale-up purposes. Figure 7.12 reports the different selectivity, conversion, faradaic efficiency, and the potential of the cathode (a) and anode (b) chambers with the apparent activation energy (Ea) and pre-exponential factor (A) calculation for the guaiacol and glycerol. In general, the conversion, applied voltage and faradaic efficiency improved with the temperature increase. The higher temperature increases the kinetic energy of the molecules in the system, allowing them to overcome the activation barriers more easily. On the other hand, higher temperatures also tend to improve mass transport and diffusion rate of the reactants and products in the electrolyte system. Some products are clearly affected by the temperature by increasing and decreasing amounts for both cathode and anode chamber. For instance, in the cathode chamber, temperature increase had a positive effect on the cyclohexanol selectivity, contrary to the effect on cyclohexanone selectivity. 172 Figure 7.12. The effect of temperature and kinetics rate evaluation in the paired electrosynthesis of PtNi/C-Pt/C catalyst pairs. Reaction condition: j = 66 mA cm-2, N = 400 rpm, t = 4 h. Electrolyte: 1.0 M MSA, 0.25 M NaCl, and 25% v/v IPA addition in the cathode chamber with CA0 guaiacol = 0.1 M, CA0 glycerol = 0.3 M. In the anode chamber, higher temperature had a detrimental effect on lactic acid selectivity and temperatures above 50 °C lead to significantly lower selectivity. The PtNi/C catalyst clearly shows that the GECH-DO had a higher energy barrier (43.5 kJ mol-1) compared to the GEOR (22.0 kJ mol-1). The kinetic parameters in Table 7.3 are comparable to the Pt/C-Pt/C catalyst pair. However, the addition of Ni on Pt on carbon support contributed to a preferable route of demethoxylation 3.0 3.2 3.4−1.5−1.0−0.50.0ln (K)T-1 x103 (K-1)y = -5.234x + 15.722R2 = 0.986EA= 43.5 kJ.mol-1A= 6.73 x 106 h-1(a) (b) 173 and ring saturation with inactive anisole formation which leads to fewer intermediate chemicals. On the other hand, the addition of Ni could decrease the amount of Pt by as much as 40% which might decrease the operational cost of the paired electrosynthesis and reduce noble metal dependency. Table 7.3. The kinetics constant of GECH-DO and GEOR in SSER using PtNi/C-Pt/C catalyst pair. Reaction Index Rate constant (k) Pt/C Unit 30 °C 40 °C 50 °C 60 °C 60 °C GECH-DO Guaiacol to Phenol 21 0.093 0.161 0.217 0.280 0.250 h-1 Phenol to Cyclohexanone 22 5.461 4.547 3.617 2.968 1.492 h-1 Cyclohexanone to Cyclohexanol 23 0.085 0.134 0.165 0.251 0.335 h-1 Cyclohexanol to Cyclohexane 24 0.480 0.394 0.288 0.271 0.279 h-1 Guaiacol to 2-Methoxycyclohexanone 31 0.111 0.223 0.450 0.670 0.257 h-1 2-Methoxycyclohexanone to 2-Methoxycyclohexanol 32 0.182 0.281 0.514 0.894 0.116 mM-1.h-1 2-Methoxycyclohexanol to Cyclohexanol 33 0.253 0.386 0.572 0.718 0.054 mM-1.h-1 GEOR Glycerol to Dihydroxyacetone 1 0.015 0.020 0.027 0.035 0.043 h-1 Glycerol to Glyceraldehyde 2 0.022 0.025 0.032 0.045 0.052 h-1 Glyceraldehyde to Glyceric acid 3 0.424 0.812 1.476 2.482 0.505 h-1 Glyceric acid to Glycolic acid 4 0.103 0.232 1.171 2.504 2.632 h-1 Glyceric acid to Tartronic acid 5 1.736 2.395 4.379 5.261 5.570 h-1 Tartronic acid to Glycolic acid 6 0.037 0.111 0.315 0.466 0.532 h-1 Glycolic acid to Formic acid 7 0.145 0.352 0.620 1.059 1.073 h-1 Dihydroxyacetone to Lactic acid 8 6.873 12.227 24.014 33.526 46.732 h-1 Lactic acid to Formic acid 9 0.069 0.148 1.262 3.009 3.014 h-1 Lactic acid to Acetic acid 10 0.025 0.240 1.632 2.889 2.058 h-1 7.6 Designing an optimum reactor from experimental kinetic data Designing an optimum reactor from experimental kinetic data is a critical and pivotal task in chemical engineering. The obtained kinetic data and model, including the Arrhenius equation parameters, allows one to optimize the reactor volume and reactor design to maximize the process performance (e.g., yield, selectivity, or energy efficiency) while adhering to safety and cost considerations. However, the kinetic data, derived from laboratory studies, can produce fallacious rate expressions that do not account for physical effects.431 Therefore, the study ensures no mass transfer limitation during the reaction. Moreover, the study would like to address the importance and application of kinetics in the further development of the paired electrosynthesis process. 174 Figure 7.13. Modelled (a) glyceraldehyde and (b) cyclohexane yield with (c) van Krevelen diagram of cathode chamber on the paired electrosynthesis of GECH-DO and GEOR using PtNi/C and Pt/C catalyst in stirred slurry electrocatalytic reactor. Reaction condition: j = 66 mA cm-2, N = 400 rpm, t = 20 h. Electrolyte: 1.0 M MSA, 0.25 M NaCl, and 25% v/v IPA addition in the cathode chamber with CA0 guaiacol = 0.1 M, CA0 glycerol = 0.3 M. In this study, the paired electrosynthesis of guaiacol electrocatalytic hydrodeoxygenation (GECH-DO) and glycerol electrooxidation (GEOR) was conducted in a membrane-separated cathode and anode chamber. The reaction time of each chamber was optimized using Polymath software, and a thorough kinetic evaluation. The kinetic rate constants of PtNi/C and Pt/C were 0 5 10 15 20020406080100Cyclohexane yield (%)Time (h) 30 °C 40 °C 50 °C 60 °C0.15 0.20 0.25 0.301.01.52.02.5 Initial and target 30 °C 40 °C 50 °C 60 °CH/C Molar ratioO/C Molar ratioHigher reaction time (0 – 20 h)(a) (b) (c) 175 implemented in the cathode and anode chamber for GECH-DO and GEOR, respectively. The reaction was carried out in an isothermal reaction with jacketed reactor. From the experimental data, a longer reaction time is preferred in the cathode chamber for GECH-DO reaction to improve H/C and O/C ratio, while GEOR requires shorter reaction time to produce higher glyceraldehyde yield as shown in Figure 7.13. Therefore, the batch system still holds the best performance in regards to the cyclohexane and glyceraldehyde yield for the cathode and anode chambers with different reaction times as seen in Figure 7.13. Higher reaction temperature (60 °C) provides higher cyclohexane yield (Figure 7.13a) and better O/C and H/C ratio (Figure 7.13c), however, the higher paired electrosynthesis temperature generated lower glyceraldehyde yield in exchange. Therefore, the optimum condition in terms of cyclohexane yield and van Krevelen diagram was at 50 °C and 16 h of reaction time. On the other hand, in the anode chamber (Figure 7.13b) at 50 °C, the batch reactor has higher glyceraldehyde yield in GEOR for shorter reaction time (4 h) compared to the continuous stirred tank reactor (CSTR). However, the proposed optimum design cuts off 12 h of reaction as the optimum condition considering the length of reaction time in laboratory practice. At this rate, the cyclohexane has high yield (>75%) with comparable value with thermocatalytic process.34-38, 41 Meanwhile, CSTR has a steady state assumption for the calculation, which differs from the batch reactor, however, the different residence time as shown in Figure 7.13b was calculated by the variation of the electrolyte flow using similar reactor volume (100 mL and 50 mL for cathode and anode chamber, respectively). Therefore, a hybrid reactor of different batch operating time reactors is an ideal option in the paired electrosynthesis of GECH-DO and GEOR, respectively, as the stirred slurry electrocatalytic reactor could provide a hybrid system as shown in Figure 7.14. The proposed design is still using 176 laboratory-scale reactor setup (100 mL and 50 mL for catholyte and anolyte volume) to improve the technology readiness level (TRL) of the paired system of GECH-DO and GEOR. The reactor design is equipped with an appropriate filter (200 mesh) to ensure that the catalyst remains confined within the reaction chamber and does not escape with the effluent stream. Keeping the catalyst in the reactor provides more efficient, scalable, and cost-effectiveness, while reducing waste and simplifying the downstream processing. However, there are several challenges on the reactor design application such as (i) the complexity of reactor design and operation and (ii) the charge imbalance during anode electrolyte replacement. Figure 7.14. The ideal reactor design for the paired electrosynthesis of GECH-DO and GEOR with Gantt chart of semi-batch SSER. Time, h 0:00 0:05 1:00 2:00 3:00 3:55 4:00 5:00 6:00 7:00 7:55 8:00 9:00 10:00 11:00 11:55 12:00CathodeAnode 1st cycleAnode 2nd cycleAnode 3rd cycle:::Flow in or flow out of the electrolyteCathode reaction cycleAnode reaction cycle177 7.7 Summary of the chapter A Langmuir-Hinshelwood (L-H) kinetic analysis was conducted to evaluate the reaction mechanism of GECH-DO and GEOR. Accordingly, the consumption of guaiacol was well described by L-H kinetics assuming a surface reaction as the RDS, whereas the consumption of glycerol was described by assuming glycerol adsorption as the rate-determining step. The kinetic constants obtained from this study were comparable to those reported by Wijaya et al.105 for the GECH-DO. Activation energy and pre-exponential factors were also estimated and used to assess the reactor design for scale-up purposes. The study used the estimated kinetic parameters to assess a batch reactor with different reaction times for cathode (12 h) and anode (4h) chambers, with optimum reaction condition: j = 66 mA cm-2, N = 400 rpm, T = 50 °C, t = 20 h, electrolyte: 1.0 M MSA, 0.25 M NaCl, and 25% v/v IPA addition in the cathode chamber with CA0 guaiacol = 0.1 M, CA0 glycerol = 0.3 M. The system would create complexity of reactor design and operation with the possibilities of charge imbalance, which needs to be addressed in future studies. The proposed reactor design would eventually improve the technology readiness level of the paired electrosynthesis on GECH-DO and GEOR. Lastly, the study contributes to improving bio-oil upgrading using electrocatalysis by (1) product intensification through pairing system, (2) reducing the energy demand, (3) providing a better understanding of the reaction pathway of the paired electrosynthesis of GECH-DO and GEOR, (4) decreasing the amount of noble metal (Pt) on the carbon catalyst support, (5) contributing to the waste valorization into valuable products, and (6) improving the technology readiness level to prepare for process scale-up. In general, by integrating the findings from kinetic studies, the reactor design and operation could be systematically optimized, ensuring improved performance and scalability. 178 Chapter 8: Conclusion 8.1 Conclusive summary and key findings The present study has successfully improved the electrocatalytic hydrodeoxygenation (ECH-DO) of the bio-oil model compound, guaiacol, through paired cell electrosynthesis. The full potential of the electrocatalytic reactor was exploited by pairing the system, thereby increasing the competitiveness of electrocatalytic upgrading when compared with thermocatalytic upgrading. A semi-quantitative multi-objective optimization (MOO) analysis was used to screen the anode coupling electrosynthesis (ACE). The consideration included six process variables, namely: economic, environment, energy, safety, technology readiness level (TRL), and separation technology. Thus, glycerol electrooxidation (GEOR) towards glyceraldehyde production had the highest score of the MOO analysis and was implemented in the paired electrosynthesis. Fortunately, the raw materials of paired electrosynthesis of bio-oil ECH-HDO and glycerol electrooxidation could be obtained from one industry: palm oil and the biodiesel industry. For instance, Indonesia, with the highest total production (~59%) of crude palm oil, produced 45.58 million metric tons in 2022. The country had 7 and 4 million tons of biodiesel and glycerol available, respectively, with bio-oil derived from a biomass potential supply of approximately 135 million tons/year. Meanwhile, the biodiesel industry needs around 2 million tons/year of methanol, which could be obtained from the bio-oil ECH-HDO as a by-product. The paired synthesis would primarily generate valuable chemicals such as methanol, cyclohexanone, cyclohexanol, cyclohexane, and glyceraldehyde, as presented in Figure 8.1. 179 Figure 8.1. The plausible industry implementation of the paired electrosynthesis on the bio-oil ECH-HDO and glycerol electrooxidation toward glyceraldehyde in the blue square with the comparison of the CO2RR technology432 in the red square. The paired cell electrosynthesis had comparable kinetics with the half-cell electrosynthesis, with simultaneous total faradaic efficiency improvement and product intensification. The optimum condition of the paired cell electrosynthesis was determined to be 400 RPM, 5 wt% Pt/C, 100 mM guaiacol and 300 mM glycerol initial concentration, j = 66 mA cm-2 (I = 0.4 A), and 50 °C using 1.0 M MSA and 0.25 NaCl as the electrolyte with 25% v/v isopropanol (IPA) addition in the catholyte. This condition boosted the guaiacol conversion from 76.85% (at the reference condition) to 93.72%. Moreover, the addition of IPA triggered the dehydration reaction to produce anisole and cyclohexane in the GECH-DO with guaiacol as reactant. The paired cell electrosynthesis also 180 upgraded the hydrodeoxygenation ratio by improving H/C and decreasing O/C ratio. The implementation of bio-oil and glycerol as the feedstock, contributes to a green and sustainable process toward valuable chemicals production. The study also successfully reduced the noble metal content of the electrocatalyst by as much as 40 wt% by tailoring a non-noble (Ni) metal with the noble (Pt) metal catalyst on a carbon support. The tailored catalyst (PtNi/C) had comparable kinetics toward the GECH. However, the tailored catalyst could not be implemented in the GEOR due to (i) less valuable chemicals such as glyceraldehyde as the main product and (ii) acidic condition of electrooxidation reaction complicated the reaction mechanism. The study demonstrates that the Pt surface exhibits superior performance in glycerol electrooxidation under acidic conditions. However, the tailored catalyst (Pt-Ni-Co) supported on carbon shows promise for use in hydrogen and oxygen evolution reactions, as evidenced by a favorable reduction in the Tafel slope value. To conclude, the study showed that the electrocatalytic reaction has several advantages that include: (i) mild operation condition (50 °C and 1 atm), (ii) in-situ hydrogen supply from the glycerol electrooxidation with better kinetics and thermodynamics compared to water splitting, (iii) enabling the dehydration reaction (cyclohexane and anisole formation) in aqueous environment, (iv) high guaiacol conversion (~94%) with better H/C and O/C ratio in the product, (v) lower required energy, (vi) product intensification from anode electrooxidation, (vii) non-noble metal utilization as the catalyst in cathode chamber, (viii) synergistic integration with renewable electricity sources, and (ix) by-product derived from lignocellulose biomass (bio-oil) and biodiesel industry (glycerol) to valuable chemical production. To reach a higher TRL of the paired electrosynthesis, each reaction cell should accommodate a high faradaic efficiency, low overpotential, high product yield, and minimal separation cost.433 181 The challenge mainly lies in improving the electrosynthesis performance for both chambers as the pairing system creates complicated conditions. For instance, the optimum applied potential windows for bio-oil ECH-HDO and glycerol electrooxidation differ. Furthermore, a comprehensive study will result in the practical realization of the paired system of bio-oil ECH-HDO and glycerol electrooxidation to resolve challenges such as: (i) improve the fundamental understanding of the synergizing effect of the paired electrosynthesis; (ii) a DFT study in collaboration with the experimental study; (iii) tailoring the catalyst for each type of reaction to improve the selectivity, activity, and durability; (iv) optimizing each parameter of the paired electrosynthesis such as the temperature, current density, electrolyte pairs selection, and reaction time; and (v) the scale-up possibility to increase the maturity of the technology. 8.2 Future recommendation The electrocatalytic synthesis remains in the development stage and is not yet ready for widespread commercial deployment. Despite significant advancements in the electrocatalytic process, several works could improve the bio-oil ECH-DO in the cathode chamber specifically and electrosynthesis in general, such as: 1. Mediated electrosynthesis to control the applied potential Some organic mediators like quinones, ferrocene, or TEMPO are used for selective oxidation or reduction of organic molecules. Meditated electrosynthesis involves the use of a redox mediator, a molecule that facilitates the transfer of electrons between the electrode and reactant. Meditated electrosynthesis could be implemented to obtain higher selectivity in complex organic reactions such GECH-DO and GEOR by controlling the applied potential during the reaction. The redox mediator typically has rapid electron transfer rates, accelerating the overall reaction process. It also creates a thermodynamically favorable reaction pathway. In terms of energy efficiency, the redox 182 mediator allows precisely tuned potential, reducing energy consumption. Mediators not only enable the reaction to proceed under mild electrode potential, reducing the energy and increasing the selectivity,434, 435 but also greatly broaden the potential for organic electrosynthesis, enabling the transformation of numerous redox-inactive compounds through strategies such as atom transfer catalysis436 or C–H bond activation437-440. Some studies report the use of vanadium complexes to target C–C and C–O bond cleavage of lignin model compounds441-444 2. Catalyst design and development As the catalyst remains the pivotal part in the reaction process, designing catalysts with high activity for selective hydrogenation and deoxygenation is a crucial part of the electrosynthesis study. From the literature review, transition metals like Pt, Pd, Ni, and Ru are typically used in the electrosynthesis process, however alloying or doping with secondary metals (e.g., Fe, Co, Cu) can improve performance. Designing tailored catalysts directed by DFT studies would reduce the parameter variability in the catalyst design process. A dual electrocatalyst such as silicotungstic acid, SiW12, could also be introduced in the system to improve the reaction kinetics. Electrocatalyst durability and reusability also needs to be addressed, even though many claims state that electrocatalysts are deactivated less easily than thermocatalysts. In SSER configurations, the graphite utilization as electrode needs further consideration to reduce the amount of Pt in the system because of its potential with good electrical conductivity, low cost, and high stability under both reducing and oxidizing conditions. As the current study only considered the geometric area of the electrode, further study could consider the electrochemically active surface area (ECSA) to quantify the active area that participates in the redox reaction with or without the presence of suspended catalysts. Measuring ECSA using a reversible redox system (e.g., 𝑑𝑑𝑒𝑒(𝐶𝐶𝑁𝑁)63− | 𝑑𝑑𝑒𝑒(𝐶𝐶𝑁𝑁)64− or Ru(𝑁𝑁𝐻𝐻3)63+ | 𝑅𝑅𝑔𝑔(𝑁𝑁𝐻𝐻3)62+ 183 is a common and reliable approach and it is does not require double-layer capacitance measurements. The experiment could be conducted in a three-electrode system of cyclic voltammetry (CV) which records the peak current (Ip) for both oxidation and reduction reactions. For a reversible redox system, the peak current (Ip) is related to the ECSA by the Randles-Sevcik equation. Moreover, the implementation of carbon or graphite as electrode should also be considered to increase the economic viability of paired electrosynthesis because of the low cost, high surface area, and chemical stability at moderate potentials. However, the limited catalytic activity, prone to corrosion at high potentials, and lower efficiency of the carbon/graphite must be accounted for. 3. Density functional theory (DFT) study The fundamental reaction mechanism could be evaluated using density functional theory (DFT). DFT analysis predicts the electronic structure and behavior of molecules and materials under specified reaction conditions. The identification of favorable reaction pathways, activation energies, and determining the potential surface for the reaction could be conducted by DFT analysis. Moreover, DFT analysis provides a theoretical prediction on how parameter changes such as the applied potential, catalyst composition, or reaction environment, will influence the reaction performance. For instance, the DFT analysis could study the effect of organic solvents, such as isopropanol, in the guaiacol ECH-DO and combine the data with experimental ones to draw a comprehensive result. 4. Continuous process The current bio-oil ECH-DO occurs mostly in batch reactors because it is still in the early development stage. In the present study, an initial attempt to apply a continuous process in the paired cell electrosynthesis of GECH-DO and GEOR was made. However, there were difficulties 184 associated with controlling catalyst concentration inside the reactor, which required fresh catalyst to be continuously added with the feedstock. Applying a filter on the output line helps prevent the suspended catalyst from leaving the reactor, as shown in Figure 7.12 in stirred slurry electrocatalytic reactor (SSER). Moreover, lower anode reactor volume could improve the kinetics of GEOR according to the kinetic analysis of this study. Lower reactor size on the anode side could decrease the electrode gap between the electrodes, decreasing the cell potential, as the stirred slurry electrocatalytic reactor typically employs a larger inter-electrode gap, contributing in high ohmic resistance for electron transfer.80, 189, 445 On the other hand, the membrane also needs further consideration in the continuous process to minimize the production cost and improve the system by implementing different types of membrane. 5. Larger phenolic compounds and crude glycerol implementation Larger phenolic compounds (e.g., cerulignol and creosol) and crude glycerol as feedstock need to be assessed to further the process development of the paired electrosynthesis. However, the application of the larger phenolic compounds should be considered at the optimum conditions of all process variables such as redox mediator, electrocatalyst and electrolyte selection, reactor configuration, and reaction temperature. Catalyst deactivation is expected to be more rapid with the larger phenolic compounds than the model compound and pure glycerol feedstock. Thus, a durable catalyst also should be another consideration in the catalyst synthesis. Moreover, larger phenolic compounds have poor solubility in water, thus, an emulsification agent and organic solvent should be added to improve solubility. Further studies should focus on using polar protic solvent addition as a component of the catholyte in SSER for processing more complex lignin model compounds and their mixtures with the ultimate goal of converting lignin into valuable products. Meanwhile, crude glycerol also contains impurities such as methanol, soaps, free fatty 185 acids, and salts, which interfere or poison the catalyst surface. Crude glycerol is also very viscous, impeding mass transfer during the GEOR. 6. Life cycle assessment (LCA) analysis and Techno-economic analysis The implementation of the paired system should be evaluated using techno-economic assessment (TEA) and life cycle assessments (LCA). The TEA thoroughly evaluates economic feasibility using all production costs, capital costs and operational expenses. Results from the present study suggest that the paired electrosynthesis of bio-oil ECH-HDO and glycerol electrooxidation toward glyceraldehyde production would be profitable due to (1) by-product/waste to valuable commodities, (2) moderate electrosynthesis operating condition, (3) recyclable electrolyte and long-life of the catalyst due to mild operating condition, and (4) product intensive process accommodating the market demand volatility. Meanwhile, LCA helps to evaluate the cradle-to-gate of the global warming effects for the paired system. The paired system would generate almost net-neutral greenhouse gas emissions with the renewable energy supplied electricity. Life cycle analysis (LCA) along with techno-economic analysis addresses technical, financial, and sustainability aspects, critical for advancing TRL of the developed process. 7. Tandem process of electrocatalytic and thermocatalytic process The tandem process of electrocatalytic and thermocatalytic process might be the solution of high conversion with better H/C and O/C ratio in bio-oil ECH-DO and higher glycerol conversion toward glyceraldehyde production. Currently, there are no studies utilizing electrocatalytic and thermocatalytic processes for bio-oil upgrading. A tandem process has a lot of challenges but there are two possibilities for exploring the processes: • Sequential thermo- and electro- catalytic processes. First, bio-oil is upgraded through the thermocatalytic process, as oligomers in the bio-oil are difficult to break using an 186 electrocatalytic process. Second, the electrocatalytic process is carried out to improve the degree of hydrogenation of the bio-oil and generated secondary product, and the hydrogen gas can be stored and supplied to the thermocatalytic reactor for bio-oil hydrodeoxygenation. 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Upgrading methods advantages and disadvantages Upgrading methods Method type Advantages Disadvantages Ref Solvent addition Physical simple and improve some properties such as viscosity and heating value, also enhances the bio-oil stability it might increase the water content and decrease pH and flash point of bio-oil 446, 447 Emulsification Physical simple and improve some properties such as viscosity and heating value, also enhances the bio-oil stability mainly physical without changing the chemicals, it also has a problem with stability and corrosion 446, 447 Microfiltration Physical Removal of corrosive contaminants (alkali metals) and char particles, improves some properties such as viscosity and stability Increase water content, high-pressure drop on the membrane, reduces HHV, high demand on the membrane 446 Hydrotreatment Chemical Increase some properties of bio-oil such as hydrocarbon content, caloric value, and H/C ratio Need hydrogen, catalyst deactivation, high operational condition (temperature and pressure), lower selectivity 446 Electrocatalytic Chemical High selectivity, increase hydrocarbon content, caloric value, and H/C ratio, produce high-value-added chemicals, mild operational condition (temperature and pressure), use electron instead of Hydrogen gas Energy-intensive process, noble catalyst dependency, increase the water content, and high demand on the membrane 106 233 Table A.2. Glycerol oxidation catalyst based selection. Catalyst/WE Electrolyte Eanode (V vs. SHE) Temp (°C) Time (h) Selectivity (%) Ref Pt/C 0.5 M H2SO4 1.1 60 7 42.1% GLAD, 57.8% GLA, 0.5% GA 309 Pt/C 0.5 M H2SO4 0.4 60 10 35% DHA, 17.1% GLAD, 11.1% GLA, 17.2% GA 118 Pt/C 0.5 M H2SO4 0.6 60 10 2% DHA, 76% GLAD, 8% GLA 118 Pt/C 0.5 M H2SO4 0.8 60 10 2% DHA, 50% GLAD, 20% GLA 118 Pt/C 0.5 M H2SO4 1.0 60 10 4% DHA, 25% GLAD, 22% GLA, 13% GA 118 Pt/C; Bi2O3 addition 0.5 M H2SO4 0.5 – 0.6 - 7 ~100%% DHA 117 Ni/C 0.1 M KOH 0.798 - 3 1.5% DHA, 3.1% GLAD, 29.8% GLA, 17.9% HPA, 28.2% GA, 4.1% OXA 154 Ni/C 0.1 M NaOH 0.833 - 8 32% FA, 11.5% GA, 0.5 mM GLA 155 NiCo/C 0.1 M NaOH 0.833 - 8 7.5% FA, 4.5% GA, 0.25 mM GLA 155 GLAD = glyceraldehyde, GLA = glyceric acid, GA = glycolic acid, DHA = dihydroxyacetone, HPA = hydroxypyruvic, OXA = oxalic acid 234 Table A.3. Thermodynamics data of cathode chamber on the GECH-DO and HER. Reaction Equation ∆𝐺𝐺𝑅𝑅0 (kJ mol-1) ∆𝐻𝐻𝑅𝑅0 (kJ mol-1) ∆𝑆𝑆0 (J mol-1 K-1) E°red (VSHE) 𝜕𝜕𝐸𝐸𝜕𝜕0𝜕𝜕𝑅𝑅 (mV K-1) Route 1: Dehydration – Demethoxylation – Ring saturation Guaiacol to Anisole C7H8O2 + 2H+ + 2 e- → C7H8O + H2O -82.52 -81.87 2.18 0.43 0.01 Anisole to Methoxycyclohexane C7H8O + 6H+ + 6 e- → C7H14O -87.96 -180.25 -309.54 0.15 -0.53 Methoxycyclohexane to Cyclohexanone C7H14O + H2O + H+ + e- → C6H10O + CH4O + 32 H2 39.56 71.00 105.45 -0.41 1.09 Anisole to Cyclohexane C7H8O + 8 H+ + 8 e- → C6H6 + CH4O -162.95 -226.48 -213.08 0.21 -0.28 Route 2: Demethoxylation – Ring saturation – Dehydration Guaiacol to Phenol C7H8O2 + 2 H+ + 2 e- → C6H6O + CH4O -73.07 -70.18 9.69 0.38 0.05 Phenol to Cyclohexanone C6H6O + 4 H+ + 4 e- → C6H10O -57.85 -120.94 -211.60 0.15 -0.55 Cyclohexanone to Cyclohexanol C6H10O + 2 H+ + 2 e- → C6H12O -21.94 -75.90 -180.98 0.11 -0.94 Cyclohexanol to Cyclohexane C6H12O + 2 H+ + 2 e- → C6H12 + H2O -92.61 -41.33 171.99 0.48 0.89 Route 3: Ring saturation – Demethoxylation – Dehydration Guaiacol to 2-Methoxycyclohexanone C7H8O2 + 4 H+ + 4 e- → C7H12O2 -55.93 -118.26 -209.06 0.14 -0.54 2-Methoxycyclohexanone to 2-Methoxycyclohexanol C7H12O2 + 2 H+ + 2 e- → C7H14O2 -21.94 -47.00 -84.05 0.11 -0.44 2-Methoxycyclohexanol to Cyclohexanol C7H14O2 + 2 H+ + 2 e- → C6H12O + CH4O -74.99 -101.76 -89.79 0.39 -0.47 Hydrogen evolution reaction in acidic 2 H+ + 2 e- → H2 0.00 0.00 0.00 0.00 0.00 in neutral or alkaline media 2H2O + 2 e- → H2 + 2 OH- 474.28 571.7 326.77 -0.83 1.69 All the thermodynamic data were obtained based on Joback Method 105, 448, 449 at standard condition (298 K and 1 atm). 235 Table A.4. Thermodynamics data of anode chamber on the GEOR and OER. Reaction Equation ∆𝐺𝐺𝑅𝑅0 (kJ mol-1) ∆𝐻𝐻𝑅𝑅0 (kJ mol-1) ∆𝑆𝑆0 (J mol-1 K-1) E°red (VSHE) 𝜕𝜕𝐸𝐸𝜕𝜕0𝜕𝜕𝑅𝑅 (mV K-1) Glycerol dehydrogenated - Oxygenated Glycerol to DHA C3H8O3 →C3H6O3 + 2 H+ + 2 e- 10.34 83.93 -246.82 0.05 1.28 Glycerol to Glyceraldehyde C3H8O3 →C3H6O3 + 2 H+ + 2 e- 37.30 106.07 -230.66 0.19 1.20 Glyceraldehyde to Glyceric acid C3H6O3 + H2O →C3H6O4 + 2 H+ + 2 e- 70.92 89.86 -63.53 0.37 0.33 Glyceric acid to Glycolic acid C3H6O4 + H2O →C2H4O3 + CH2O2 + 2 H+ + 2 e- 89.18 104.14 -50.18 0.46 0.26 Glyceric acid to Tartronic acid C3H6O4 + H2O →C3H4O5 + 4 H+ + 4 e- 108.22 6.86 339.96 0.28 -0.88 Tartronic acid acid to Glycolic acid C3H4O5 + 2H2O →C2H4O3 + CH2O2 + O2 + 2 H+ + 2 e- 218.10 383.08 -553.35 1.13 2.87 Glycolic acid to Formic acid C2H4O3 + H2O →2CH2O2 + 2 H+ + 2 e- -121.00 -194.86 247.73 -0.63 -1.28 DHA to Lactic acid C3H6O3 + H2O →C3H6O3 + H2O 234.70 280.91 -154.99 1.22 0.80 Lactic acid to Formic acid C3H6O3 + 3 H2O →3CH2O2 + 6 H+ + 6 e- 305.64 311.97 -21.23 0.53 0.04 Lactic acid to Formic acid and Acetic acid C3H6O3 + H2O →CH3COOH + CH2O2 + 2 H+ + 2 e- 89.18 84.90 14.36 0.46 -0.07 Oxygen evolution reaction in acidic 2 H2O → O2 + 4 H+ + 4 e- -474.28 -571.66 -326.77 1.23 -0.85 in neutral or alkaline media 4 OH- → O2 + 2 H2O + 4 e- 474.28 571.66 326.77 0.40 0.85 All the thermodynamic data were obtained based on Joback Method 105, 448, 449 at standard condition (298 K and 1 atm) 236 A.1 Mass-transfer limitation assessment Weisz-Prater (W-P) criterion is typically used to assess the influence of pore diffusion or reaction rates. The equation is described as follow: 𝑁𝑁𝑊𝑊−𝑃𝑃 = −𝑒𝑒𝐴𝐴𝜌𝜌𝑝𝑝𝑅𝑅𝑝𝑝2𝐶𝐶𝑠𝑠𝐷𝐷𝑒𝑒𝑒𝑒𝑒𝑒 ≤ 1𝑒𝑒 (A.1) where −𝑟𝑟𝐴𝐴 denotes the reaction rate, 𝜌𝜌𝑒𝑒 is the catalyst density, 𝑅𝑅𝑒𝑒 is catalyst particle radius, 𝐶𝐶𝑠𝑠 is the reactant at the catalyst surface (equal to 𝐶𝐶𝐴𝐴0 in stirred slurry reactor), 𝐷𝐷𝑤𝑤𝑒𝑒𝑒𝑒 is effective diffusivity, and n is the reaction order (1). The properties data were 𝜌𝜌𝑒𝑒,𝑃𝑃𝑡𝑡 = 21.45 g cm-3, 𝑑𝑑𝑒𝑒,𝑃𝑃𝑡𝑡= 3.91 nm (Table 7.6), and 𝐷𝐷𝑤𝑤𝑒𝑒𝑒𝑒 = 6.04×10-10 m2 s-1 (obtained from literature450). Therefore, the internal mass transfer resistances were calculated for cathode and anode chambers in Table A.5. Table A.5. The internal mass transfer resistance evaluation using Weiz-Prater criteria in paired electrosynthesis of GECH and GEOR Cathode Anode CA0 (mM) -rA (mmol s-1 gPt -1) NW-P NW-P < 1 CA0 (mM) -rA (mmol s-1 gPt -1) NW-P NW-P < 1 200 0.025 5.76×10-5 OK 86 0.009 2.32×10-5 OK 256 0.045 3.99×10-5 OK 171 0.015 9.24×10-5 OK 388 0.078 6.9×10-5 OK 259 0.025 1.72×10-5 OK 559 0.083 5.5×10-5 OK 424 0.038 5.38×10-5 OK Reaction condition: j = 66 mA cm-2, T = 60 °C, t = 4 h. Electrolyte 1.0 M MSA or 1.0 M NaOH with 0.25 M NaCl for both chamber and 25% v/v IPA addition in the cathode chamber using 5 wt%-Pt/C (5 g/L). As dimensionless variable of Sherwood number represents the convective mass transfer and the rate of diffusion. The convective mass transfer coefficient (kS-L) could be calculated accordingly using Sherwood number (𝑆𝑆ℎ), Reynold Number (𝑅𝑅𝑒𝑒), and Schmidt number (𝑆𝑆𝑐𝑐) correlation as follow (Equation A.2 – A.5): 𝑆𝑆ℎ = 2 + 0.4 𝑅𝑅𝑒𝑒1/4𝑆𝑆𝑐𝑐1/3 (A.2) 237 𝑆𝑆ℎ = 𝑘𝑘𝑆𝑆−𝐿𝐿𝑡𝑡𝑝𝑝𝐷𝐷𝑒𝑒𝑒𝑒𝑒𝑒 (A.3) 𝑅𝑅𝑒𝑒 = 𝜌𝜌𝑁𝑁𝑒𝑒2𝜇𝜇 (A.4) 𝑆𝑆𝑐𝑐 = 𝜇𝜇𝜌𝜌𝐷𝐷 (A.5) The properties data were 𝜌𝜌 = 1.01 g cm-3, 𝜇𝜇 = 5.67×10-4 Pa s (at 60 °C), N = 400 rpm, r = 2.54 cm, 𝑑𝑑𝑒𝑒,𝑃𝑃𝑡𝑡/𝐶𝐶 = 75 µm, and 𝑎𝑎𝑒𝑒= 40.37 m-1. Therefore, the external mass transfer could be evaluated using Table A.6 for cathode and anode chambers. Table A.6. The external mass transfer resistance evaluation using Sherwood number in paired electrosynthesis of GECH and GEOR Cathode Anode CA0 (mM) -rA (mol s-1 m -3) kS-L aP CA0 -rA >> kS-L aP CA0 CA0 (mM) -rA (mol s-1 m -3) kS-L aP CA0 -rA >> kS-L aP CA0 112.1 5147.8 5.8 OK 85.6 1498.9 0.2 OK 255.8 7039.6 11.5 OK 170.8 2102.3 11.5 OK 387.6 7153.6 17.4 OK 258.6 3424.2 17.5 OK 558.8 7466.9 25.2 OK 424.5 5032.8 28.5 OK A.2 Experimental reactor cell set-up Figure A.1. The jacketed reactor set-up of (a) cathode half-cell, (b) anode half-cell, and (c) paired cell stirred slurry electrocatalytic reactor (SSER) setup for guaiacol ECH-DO and glycerol EOR with similar size of the chambers. (a) (b) (c) 238 A.3 GC-MS chromatogram results from the ECH-DO of guaiacol Figure A.2. GC-MS chromatograms of (a) standard solution equal to 0.05 M of organic compounds and (b) 0.1 M GECH-DO half-cell at 0.2 M MSA, 0.25 NaCl, and 50% IPA for 2 h of reaction and j = -66 mA cm-2. (a) (b) 239 A.4 HPLC standard solution Figure A.3. HPLC chromatograms of standard solution equal to 0.0125 M of organic compounds in anode chamber (a) RI detection and (b) UV detection. (a) (b) 240 A.5 Cyclic voltammetry analysis of half-cell optimization Figure A.4. Cyclic voltammograms on Pt: (a) the stability test, (b) guaiacol addition, (c) guaiacol and 5 g/L dispersed Pt (5 wt%)/C slurry addition, (d) IPA addition, (e) and (f) IPA, guaiacol, and Pt/C addition at 25 oC and 60 oC respectively in 1.0 M MSA and 0.25 M NaCl. -1 0 1 2-300-200-1000100200300Potential, E(V) vs. Ag/AgClCurrent density (A m-2) MSA MSA + 40 mM Guaiacol MSA + 60 mM Guaiacol MSA + 80 mM Guaiacol MSA + 100 mM Guaiacol-1 0 1 2-300-200-1000100200300400500Potential, E(V) vs. Ag/AgClCurrent density (A m-2) 100/0 MSA/IPA + Guaiacol + Pt/C 90/10 MSA/IPA + Guaiacol + Pt/C 75/25 MSA/IPA + Guaiacol + Pt/C 50/50 MSA/IPA + Guaiacol + Pt/C 25/75 MSA/IPA + Guaiacol + Pt/C-1 0 1 2-200-1000100200Potential, E(V) vs. Ag/AgClCurrent density (A m-2) 1 Cycle 10 CyclesOER ORR HOR HER (a) (b) (c) (d) (e) (f) 241 A.6 RSM and data analysis of half-cell optimization process Figure A.5. The concentration profile of guaiacol ECH-DO with 50% IPA addition using SSER at (a) 0.2 M MSA; (b) 0.5 M MSA; and (c) 1.0 M MSA concentration. Reaction condition: I = - 0.4 A (j = -66 mA cm-2), T = 60 °C, t = 4 h. Catalyst: 5 wt%-Pt/C (5 g/L). (a) (b) (c) 242 Figure A.6. Pareto chart of RSM analysis of XA (guaiacol conversion, %), YB – YJ (product yield, %), Y1 (required power, Wh), and Y2 (H+ concentration, M) for (a) – (l) respectively. (a) (b) (c) (d) (e) (f) (g) (h) (i) (j) (k) (l) 243 Figure A.7. The contour plot of (a) XA (guaiacol conversion, %) and YF (cyclohexane yield, %). Figure A.8. The comparison of experiment and model data obtained from RSM of (a) CA – CJ and (b) Y1. 0 25 50 75 1000255075100Conversion and yield model (%) Conversion and yield experiment (%) R-square : 0.98930 5 10 15 2005101520Y 1 model (Wh) Y1 experiment (Wh) R-square : 0.9400(a) (b) (a) (b) 244 A.7 Energy analysis of half-cell optimization process Figure A.9. The faradaic efficiency profile of guaiacol ECH-DO using SSER at (a) 0.2 M; (b) 0.5 M; and (c) 1.0 M of MSA concentration. Reaction condition: I = - 0.4 A (j = -66 mA cm-2), T = 60 °C, t = 4 h. Catalyst: 5 wt%-Pt/C (5 g/L). (a) (b) (c) 245 Figure A.10. The salt addition effect on half-cell glycerol electrooxidation (0.25 M) using SSER at 1.0 M of MSA concentration in regards of (a) glycerol conversion and (b) glyceraldehyde selectivity. Reaction condition: I = - 0.4 A (j = -66 mA cm-2), N = 600 rpm, T = 60 °C, t = 4 h. Catalyst: 5 wt%-Pt/C (5 g/L). A.8 MATLAB codes for the ECH-DO and GEOR GECH-DO script % Guaiacol kinetics modelling.m clear all clc clf % Tabular data Data = xlsread('ch-data.xlsx'); % Experimental concentration data global CA0 CB0 CC0 CD0 CE0 CF0 CG0 CH0 CI0 CJ0 Cex C Cplot Creport tspan sse SD % A = Guaicol; B = Phenol; C = Cyclohexanone; D = Cyclohexanol % E = Anisole; F = Cyclohexane; G = 2-Methoxycyclohexanone; H = % 2-methoxycyclohexanol; % I = Methoxycylohexane; J = methanol CA0 = Data(1,2);CB0 = 0; CC0 = 0; CD0 = 0; CE0 = 0; CF0 = 0;CG0 = 0;CH0 = 0;CI0 = 0;CJ0 = 0; Cex = Data(:,2:11); % Convert table to homogeneous array tspan = Data(:,1); (a) (b) 246 k0 = [0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01]; % Initial values in a row vector lb = zeros (1,11); % Lower bounds ub = 1000*ones(1,11); % Upper bounds % Optimization options structure using 'lsqnonlin' to solve nonlinear 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 t=tspan; % Independent variables time=0:0.1:4; Cplot; % Plot of the organic chemical concentration (real vs. model) figure(1) plot(time,Cplot(:,1),'b-',time,Cplot(:,10),'r--'); xlabel('Time (h)'); ylabel('Concentration (mM)'); xticks([0:0.5:4]); hold on; plot(t,Cex(:,1),'ob',t,Cex(:,10),'or'); legend({'A model','A real','J model','J real'}); hold off figure(2) plot(time,Cplot(:,2),'r--',time,Cplot(:,3),'b-.',time,Cplot(:,4),'k--',time,Cplot(:,5),'m-.'); xlabel('Time (h)'); xticks([0:0.5:4]); ylabel('Concentration (mM)'); hold on; plot(t,Cex(:,2),'dr',t,Cex(:,3),'ob',t,Cex(:,4),'sk',t,Cex(:,5),'*m'); legend({'B model','C model','D model','E model','B real','C real','Dreal','E real'}); hold off; 247 figure(3) plot(time,Cplot(:,6),'m:',time,Cplot(:,7),'r--',time,Cplot(:,8),'b-.',time,Cplot(:,9),'k--'); xlabel('Time (h)'); xticks([0:0.5:4]); ylabel('Concentration (mM)'); hold on; plot(t,Cex(:,6),'^m',t,Cex(:,7),'dr',t,Cex(:,8),'ob',t,Cex(:,9),'sk'); legend({'F model','G model','H model','I model','F real','G real','H real','I real'}); hold off; % figure end here % Output declaration Sum_of_SE=sse'; standard_deviation=SD'; constants = k' Creport % Calculated dependent variables % Integrate the ODEs using Runge-Kutta 4 method function [sse, SD] =optim(k); global CA0 CB0 CC0 CD0 CE0 CF0 CG0 CH0 CI0 CJ0 Cex C Cplot tspan Creport sse SD time=0:0.1:4; [t,C] = ode45(@balance,time,[CA0 CB0 CC0 CD0 CE0 CF0 CG0 CH0 CI0 CJ0]); Cplot=C; % A = Guaicol; B = Phenol; C = Cyclohexanone; D = Cyclohexanol % E = Anisole; F = Cyclohexane; G = 2-Methoxycyclohexanone; H = % 2-methoxycyclohexanol; % I = Methoxycylohexane; J = methanol y_modelA=interp1(t,C(:,1),tspan); %CA_model y_modelB=interp1(t,C(:,2),tspan); %CB_Model y_modelC=interp1(t,C(:,3),tspan); %CC_Model y_modelD=interp1(t,C(:,4),tspan); %CD_Model y_modelE=interp1(t,C(:,5),tspan); %CE_Model y_modelF=interp1(t,C(:,6),tspan); %CF_Model y_modelG=interp1(t,C(:,7),tspan); %CG_Model y_modelH=interp1(t,C(:,8),tspan); %CH_Model 248 y_modelI=interp1(t,C(:,9),tspan); %CI_Model y_modelJ=interp1(t,C(:,10),tspan); %CI_Model C=[y_modelA y_modelB y_modelC y_modelD y_modelE y_modelF y_modelG y_modelH y_modelI y_modelJ]; Creport=C'; % Function to be integrated function dCdt=balance(t,C) dCdt=zeros(11,1); % Initialization % Material balance expression k11=k(1);k12=k(2);k13=k(3);k14=k(4); k21=k(5);k22=k(6);k23=k(7);k24=k(8); k31=k(9);k32=k(10);k33=k(11); % if there is no Anisole product %k11=0;k12=0;k13=0;k14=0; CT=C(1)+C(2)+C(3)+C(4)+C(5)+C(6)+C(7)+C(8)+C(9)+C(10); 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); CG = CA0*(C(7)/CT); CH = CA0*(C(8)/CT); CI = CA0*(C(9)/CT); CJ = CA0*(C(10)/CT); % ODEs model: multiple equations, multiple parameters\ % A = Guaicol; B = Phenol; C = Cyclohexanone; D = Cyclohexanol % E = Anisole; F = Cyclohexane; G = 2-Methoxycyclohexanone; H = % 2-methoxycyclohexanol; % I = Methoxycylohexane; J = methanol dCA= (-k11*CA-k21*CA-k31*CA); dCB= (k21*CA-k22*CB); dCC= (k22*CB-k23*CC+k13*CI); dCD= (k23*CC+k33*CH^2-k24*CD); dCE= (k11*CA-k12*CE-k14*CE); dCF= (k24*CD+k14*CE); dCG= (k31*CA-k32*CG^2); dCH= (k32*CG^2-k33*CH^2); dCI= (k12*CE-k13*CI); dCJ= (k33*CH^2+k14*CE+k21*CA+k13*CI); 249 dCdt=[dCA,dCB,dCC,dCD,dCE,dCF,dCG,dCH,dCI,dCJ]'; end nt=length(tspan); Cmod = C; sse = (Cmod-Cex).^2; % sum of squares of the residual SD = sqrt(sum(sse)/nt); % standard deviation end GEOR script % Glycerol kinetics modelling.m clear all clc clf % Tabular data Data = xlsread('ch-data.xlsx'); % Experimental concentration data global CB0 CC0 CD0 CE0 CF0 CG0 CH0 CI0 Cex Cplot tspan sse SD Creport % A = Glycerol; B = Glyceraldehyde; C = Glyceric acid; D = Glycolic acid; % E = Tartronic acid; F = DHA; G = Lactic acid; H = Formic acid; I = Acetic acid CB0 = Data(1,1);CB0 = 0; CC0 = 0; CD0 = 0; CE0 = 0; CF0 = 0;CG0 = 0;CH0 = 0;CI0 = 0; Cex = Data(:,12:20); % Convert table to homogeneous array tspan = Data(:,1); k0 = [0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01]; % Initial values in a row vector lb = zeros (1,10); % Lower bounds ub = 10000*ones(1,10); % Upper bounds % Optimization options structure using 'lsqnonlin' to solve nonlinear 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) 250 Cex; % Measured dependent variables t=tspan; % Independent variables time=0:0.1:4; Cplot; % Plot of the organic chemical concentration (real vs. model) figure(1) plot(time,Cplot(:,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 % Plot of product concentration (real vs. model) figure(2) plot(time,Cplot(:,2),'r--',time,Cplot(:,3),'b-.',time,Cplot(:,4),'k--',time,Cplot(:,5),'m-.'); xlabel('Time (h)'); xticks([0:0.5:4]); ylabel('Concentration (mM)'); hold on; plot(t,Cex(:,2),'dr',t,Cex(:,3),'ob',t,Cex(:,4),'sk',t,Cex(:,5),'*m'); legend({'B model','C model','D model','E model','B real','C real','Dreal','E real'}); hold off; figure(3) plot(time,Cplot(:,6),'m:',time,Cplot(:,7),'r--',time,Cplot(:,8),'b-.',time,Cplot(:,9),'k--'); xlabel('Time (h)'); xticks([0:0.5:4]); ylabel('Concentration (mM)'); hold on; plot(t,Cex(:,6),'^m',t,Cex(:,7),'dr',t,Cex(:,8),'ob',t,Cex(:,9),'sk'); legend({'F model','G model','H model','I model','F real','G real','H real','I real'}); hold off; % figure end here % Output declaration 251 Sum_of_SE=sse'; standard_deviation=SD' constants = k' Creport % Calculated dependent variables % Integrate the ODEs using Runge-Kutta 4 method function [sse, SD] =optim(k); global CA0 CB0 CC0 CD0 CE0 CF0 CG0 CH0 CI0 Cex C Cplot tspan Creport SD time=0:0.1:4; [t,C] = ode45(@balance,time,[CA0 CB0 CC0 CD0 CE0 CF0 CG0 CH0 CI0]); Cplot=C; % A = Glycerol; B = Glyceraldehyde; C = Glyceric acid; D = Glycolic acid; % E = Tartronic acid; F = DHA; G = Lactic acid; H = Formic acid; I = Acetic acid y_modelA=interp1(t,C(:,1),tspan); %CA_model y_modelB=interp1(t,C(:,2),tspan); %CB_Model y_modelC=interp1(t,C(:,3),tspan); %CC_Model y_modelD=interp1(t,C(:,4),tspan); %CD_Model y_modelE=interp1(t,C(:,5),tspan); %CE_Model y_modelF=interp1(t,C(:,6),tspan); %CF_Model y_modelG=interp1(t,C(:,7),tspan); %CG_Model y_modelH=interp1(t,C(:,8),tspan); %CH_Model y_modelI=interp1(t,C(:,9),tspan); %CI_Model C=[y_modelA y_modelB y_modelC y_modelD y_modelE y_modelF y_modelG y_modelH y_modelI]; Creport=C'; % Function to be integrated function dCdt=balance(t,C) dCdt=zeros(6,1); % Initialization % Material balance expression k1=k(1);k2=k(2);k3=k(3);k4=k(4); k5=k(5);k6=k(6);k7=k(7);k8=k(8); k9=k(9);k10=k(10); CT=C(1)+C(2)+C(3)+C(4)+C(5)+C(6)+C(7)+C(8)+C(9); CA = CA0*(C(1)/CT); CB = CA0*(C(2)/CT); CC = CA0*(C(3)/CT); CD = CA0*(C(4)/CT); 252 CE = CA0*(C(5)/CT); CF = CA0*(C(6)/CT); CG = CA0*(C(7)/CT); CH = CA0*(C(8)/CT); CI = CA0*(C(9)/CT); % ODEs model: multiple equations, multiple parameters %Kinetics reaction % A = Glycerol; B = Glyceraldehyde; C = Glyceric acid; D = Glycolic acid; % E = Tartronic acid; F = DHA; G = Lactic acid; H = Formic acid; I = Acetic acid dCA=-(k1*CA+k2*CA); dCB=(k2*CA-k3*CB); dCC=(k3*CB-k4*CC-k5*CC); dCD=(k4*CC+k6*CE-k7*CD); dCE=(k5*CC-k6*CE); dCF=(k1*CA-k8*CF); dCG=(k8*CF-k9*CG); dCH=(k9*CG+k10*CG+k4*CC+k7*CD); dCI=k10*CG; dCdt=[dCA,dCB,dCC,dCD,dCE,dCF,dCG,dCH,dCI]'; end nt=length(tspan); Cmod = C; sse = (Cmod-Cex).^2; % sum of squares of the residual SD = sqrt(sum(sse)/nt); % standard deviation end 253 Appendix B : Paired cell electrosynthesis B.1 Catalyst characterization The catalyst used in this study was similar to the one used in Wijaya's105 study; therefore, the catalyst characterization data is derived from that study. Two analysis of N2 physisorption and CO chemisorption were carried out to characterize the catalysts. The resulting data is presented in Table B.1. Table B.1. Characterization of noble metal catalyst with various metal loading content on carbon support.105 Noble metal molecule Metal content (wt%) aSBET (m2/g) bVpore (cm3/g) cDpore (nm) φ (%) Smetal (m2/g metal) dp (nm) Pt 1 1010 0.8 4.4 37.0 91.3 3.1 Pt 3 1448 1.2 3.7 14.6 35.9 7.8 Pt 5 1487 1.4 4.4 28.9 71.5 3.9 Ru 5 777 0.9 6.2 22.0 80.4 6.0 Pd 5 762 0.8 5.2 25.1 111.8 4.5 Pt 10 972 0.9 4.2 8.5 20.9 13.4 aBET surface area; bSingle point adsorption total pore volume (P/P0 = 0.99); cAverage pore diameter by BJH desorption; φ (metal dispersion adapted from CO chemisorption technique), Smetal (metallic surface area), and dp (active particle diameter) determined by CO chemisorption. B.2 Cyclic voltammetry on half-cell to paired cell comparison and optimization The paired electrosynthesis showed identical cathodic and anodic peaks at 60°C (the operating reaction temperature). However, the voltammograms shifted at 25°C, mainly due to the covering of the electrode by organic compounds and Pt/C. The acid-acid pairs indicated the best option for HER and OER activity due to their highest performance in the paired electrosynthesis of guaiacol and glycerol. The paired electrosynthesis did not significantly affect the voltamogram for acid-acid condition at 60 °C (Figure B.1d). Higher Pt loading increases the HER and OER activity as 254 shown in Figure B.2a due to higher surface area, however, higher metal loading (Pt/C) means higher operational cost. The current density in this study is calculated based on the electrode geometric surface. Meanwhile, temperature has significant impact to boost the HER and OER activity as shown in Figure B.1b. Figure B.1. The Cyclic voltammetry of (a-b) 25 °C and (c-d) 60 °C for half-cell and paired electrosynthesis of guaiacol eletrocatalytic hydrogenation (GECH) and GEOR. Electrolyte: 1.0 M Methanesulfonic acid (MSA) or NaOH with 0.25 M NaCl for both chamber and 25% v/v IPA addition in the cathode chamber using 5wt%-Pt/C (5 g/L). -1 0 1 2-2500250500Potential, E(V) vs. Ag/AgClCurrent density (A m-2) Acid-acid Half-cell cathode Acid-acid Paired-cell Acid-base Paired-cell Acid-neutral Paired-celll-1 0 1 2-2500250500Potential, E(V) vs. Ag/AgClCurrent density (A m-2) Acid-acid Half-cell cathode Acid-acid Paired-cell Acid-neutral Paired-cell Acid-base Paired-celll-2 -1 0 1 2-2500250500Potential, E(V) vs. Ag/AgClCurrent density (A m-2) Half-cell Cathode Half-cell Anode Paired electrosynthesis-2 -1 0 1 2-2500250500Potential, E(V) vs. Ag/AgClCurrent density (A m-2) Half-cell Cathode Half-cell Anode Paired electrosynthesis(a) (b) (c) (d) 255 Figure B.2. The effect of (a) Pt loading content and (b) temperature on the cyclic voltammetry of paired electrosynthesis of GECH and GEOR. Electrolyte: 1.0 M MSA with 0.25 M NaCl for both chamber and 25% v/v IPA addition in the cathode chamber using 5wt%-Pt/C (5 g/L). B.3 Tafel slope analysis of binary and ternary metal catalyst on carbon support The experiment was performed using Pt gauze and Pt wire as the working and counter electrode in H-cell jacketed reactor with a potentiostat (CH Instruments Electrochemical Workstation, Model 1100 C). To initiate the test, 20 cycles with 100 mV s-1 scan rate of CV was performed to clean the Pt electrodes surface to acquire stable voltammogram. Reactant and catalyst were then added into the electrolyte solution either for cathode or anode chamber. To investigate the HER and OER performances, the linear voltammetry experiments were conducted and followed with the Tafel slope analysis calculation using Origin 2024b software© as illustrated in Figure B.3. Lower Tafel slope and higher exchange current density are preferable in the HER and OER performances. Pt3Ni/C, PtCo/C, and PtNi0.5Co0.5 seem promising catalysts compared to the Pt/C catalyst in term of HER Tafel slope as seen in Table B.2. Typical value of exchange current density at standard condition is 100 µA.cm-2. -1 0 1 2-2000200400Potential, E(V) vs. Ag/AgClCurrent density (A m-2) 1% Pt/C 3% Pt/C 5% Pt/C 10% Pt/C-2 -1 0 1 2-2500250500Potential, E(V) vs. Ag/AgClCurrent density (A m-2) Paired-cell at 25 οC Paired-cell at 30 οC Paired-cell at 60 οC(a) (b) 256 Figure B.3. Tafel slope analysis plot log current density versus overpotential. Table B.2. The Tafel slope calculation from the linear voltammetry experiments. No Catalyst Catholyte Organic substance Tafel slope (mV/dec) Exchange current density (µA.cm-2) This study Ref 1 - 1.0 M MSA - 60.1 29 - 79 138.6 2 - 1.0 M MSA Guaiacol 60.3 118.2 3 Pt/C* 1.0 M MSA Guaiacol 56.6 138.5 4 - 1.0 M MSA/IPA (75% vol.) Guaiacol 69.5 110.2 5 - 1.0 M MSA/IPA (75% vol.) Guaiacol 64.0 99.5 6 Pt/C 1.0 M MSA/IPA (75% vol.) Guaiacol 65.0 101.4 7 PtNi/C 1.0 M MSA/IPA (75% vol.) Guaiacol 63.0 101.1 8 Pt3Ni/C 1.0 M MSA/IPA (75% vol.) Guaiacol 54.9 98.9 9 PtNi3/C 1.0 M MSA/IPA (75% vol.) Guaiacol 65.4 102.3 10 PtCo/C 1.0 M MSA/IPA (75% vol.) Guaiacol 56.2 152.1 11 Pt3Co/C 1.0 M MSA/IPA (75% vol.) Guaiacol 58.8 139.7 12 PtCo3/C 1.0 M MSA/IPA (75% vol.) Guaiacol 63.9 180.4 13 PtNi0.5Co0.5/C 1.0 M MSA/IPA (75% vol.) Guaiacol 52.1 126.3 14 Pt3Ni0.5Co0.5/C 1.0 M MSA/IPA (75% vol.) Guaiacol 63.2 159.3 15 PtNi2Co/C 1.0 M MSA/IPA (75% vol.) Guaiacol 61.2 150.9 16 PtNiCo2/C 1.0 M MSA/IPA (75% vol.) Guaiacol 62.8 150.4 *Commercial catalyst. Linear voltammetry condition at a scan rate of 50 mV s-1 for -1.00 to 2.00 V of the potential window. 257 Figure B.4. The effect of (a) 0.5 M acid-acid, (b) 0.2 M acid-acid, (c) 1.0 M acid-alkaline, and (d) 1.0 M acid-neutral electrolyte pairs on half-cell GECH (left) and paired cell GECH (right). 0 1 2 3 40255075100Conversion, Selectivity, and FE (%)Time (h) FE Guaiacol Phenol Cyclohexanone cyclohexanol Anisole Cyclohexane 2-Me-cyclo-none 2-Me-cyclo-nol Me-cyclo-xane Methanol0 1 2 3 40255075100Conversion, Selectivity, and FE (%)Time (h) FE Guaiacol Phenol Cyclohexanone cyclohexanol Anisole Cyclohexane 2-Me-cyclo-none 2-Me-cyclo-nol Me-cyclo-xane Methanol0 1 2 3 40255075100Conversion, Selectivity, and FE (%)Time (h) FE Guaiacol Phenol Cyclohexanone cyclohexanol Anisole Cyclohexane 2-Me-cyclo-none 2-Me-cyclo-nol Me-cyclo-xane Methanol0 1 2 3 40255075100Conversion, Selectivity, and FE (%)Time (h) FE Guaiacol Phenol Cyclohexanone cyclohexanol Anisole Cyclohexane 2-Me-cyclo-none 2-Me-cyclo-nol Me-cyclo-xane Methanol0 1 2 3 40255075100Conversion, Selectivity, and FE (%)Time (h) FE Guaiacol Phenol Cyclohexanone cyclohexanol Anisole Cyclohexane 2-Me-cyclo-none 2-Me-cyclo-nol Me-cyclo-xane Methanol0 1 2 3 40255075100Conversion, Selectivity, and FE (%)Time (h) FE Guaiacol Phenol Cyclohexanone cyclohexanol Anisole Cyclohexane 2-Me-cyclo-none 2-Me-cyclo-nol Me-cyclo-xane Methanol0 1 2 3 40255075100Conversion, Selectivity, and FE (%)Time (h) FE Guaiacol Phenol Cyclohexanone cyclohexanol Anisole Cyclohexane 2-Me-cyclo-none 2-Me-cyclo-nol Me-cyclo-xane Methanol0 1 2 3 40255075100Conversion, Selectivity, and FE (%)Time (h) FE Guaiacol Phenol Cyclohexanone cyclohexanol Anisole Cyclohexane 2-Me-cyclo-none 2-Me-cyclo-nol Me-cyclo-xane Methanol(a) (b) (c) (d) 258 Figure B.5. The effect of (a) 0.5 M acid-acid, (b) 0.2 M acid-acid, (c) 1.0 M acid-alkaline, and (d) 1.0 M acid-neutral electrolyte pairs on half-cell GEOR (left) and paired cell GEOR (right). 0 1 2 3 40255075100Conversion, Selectivity, and FE (%)Time (h) FE Glycerol Glyceraldehyde Glyceric acid Glycolic acid Tartronic acid DHA Lactic acid Formic acid Acetic acid0 1 2 3 40255075100Conversion, Selectivity, and FE (%)Time (h) FE Glycerol Glyceraldehyde Glyceric acid Glycolic acid Tartronic acid DHA Lactic acid Formic acid Acetic acid0 1 2 3 40255075100Conversion, Selectivity, and FE (%)Time (h) FE Glycerol Glyceraldehyde Glyceric acid Glycolic acid Tartronic acid DHA Lactic acid Formic acid Acetic acid0 1 2 3 40255075100Conversion, Selectivity, and FE (%)Time (h) FE Glycerol Glyceraldehyde Glyceric acid Glycolic acid Tartronic acid DHA Lactic acid Formic acid Acetic acid0 1 2 3 40255075100Conversion, Selectivity, and FE (%)Time (h) FE Glycerol Glyceraldehyde Glyceric acid Glycolic acid Tartronic acid DHA Lactic acid Formic acid Acetic acid0 1 2 3 40255075100Conversion, Selectivity, and FE (%)Time (h) FE Glycerol Glyceraldehyde Glyceric acid Glycolic acid Tartronic acid DHA Lactic acid Formic acid Acetic acid0 1 2 3 40255075100Conversion, Selectivity, and FE (%)Time (h) FE Glycerol Glyceraldehyde Glyceric acid Glycolic acid Tartronic acid DHA Lactic acid Formic acid Acetic acid(a) (b) (c) (d) 259 B.4 Paired cell optimization Table B.3. Agitation speed and platinum loading effects in paired cell of GECH-DO and GEOR. Parameter Agitation speed with 5 wt% Pt loading content Pt loading content with 600 rpm agitation speed 400 rpm 500 rpm 600 rpm 1 wt% 3 wt% 5 wt% 10 wt% (C) (A) (C) (A) (C) (A) (C) (A) (C) (A) (C) (A) (C) (A) E (V) -0.69 1.71 -0.68 1.70 -0.62 1.78 -1.04 1.65 -0.41 1.65 -0.62 1.78 -0.44 1.62 Ecell (V) 8.6 8.5 8.4 9.4 8.6 8.4 6.0 pH 0.93 -0.05 0.89 -0.04 0.95 -0.03 0.11 0.70 0.04 0.06 0.95 -0.03 -0.11 0.22 XA (%) 77.9 27.3 77.9 28.2 79.1 27.6 23.3 12.8 53.0 16.7 79.1 27.6 45.8 17.5 SB (%) 4.3 10.8 4.5 11.0 4.3 12.5 6.2 46.3 3.7 35.2 4.3 12.5 4.7 5.0 SC (%) 23.3 3.5 23.4 3.0 23.5 4.0 20.4 1.1 14.7 5.6 23.5 4.0 19.3 5.0 SD (%) 11.0 7.4 11.0 9.2 11.5 9.5 9.9 2.5 10.4 4.4 11.5 9.5 10.3 13.8 SE (%) 8.4 7.5 8.0 6.5 8.5 11.2 5.6 9.2 8.6 17.1 8.5 11.2 10.8 15.8 SF (%) 7.0 0.3 6.5 0.6 6.9 0.2 3.7 0.0 2.5 0.0 6.9 0.2 2.8 0.1 SG (%) 20.0 3.6 19.9 1.0 19.3 4.4 33.4 4.2 30.6 1.7 19.3 4.4 26.5 3.8 SH (%) 14.2 38.6 13.9 39.7 14.0 32.8 10.0 19.4 19.5 20.7 14.0 32.8 13.6 20.3 SI (%) 4.2 28.4 5.4 28.9 4.7 25.4 3.2 17.3 4.6 15.2 4.7 25.4 5.3 36.2 SJ (%) 7.5 0.0 7.3 0.0 7.4 0.0 7.6 0.0 5.6 0.0 7.4 0.0 6.6 0.0 C.B. (%) ~99.9 96.2 99.8 95.5 99.7 96.2 99.3 99.0 ~99.9 99.0 99.7 96.2 99.9 ~99.9 F.E. (%) 93.1 34.9 92.5 33.7 97.3 34.7 28.4 12.8 58.3 15.6 97.3 34.7 56.0 22.9 rx,0 × 105* 2.80a 0.76a 2.77a 0.75a 2.84a 0.75a 2.86a 1.41a 2.76a 0.65b 2.84a 0.75b 0.69b 0.22a *Significance difference level between group of Cathode and Anode as formulated in Equation 3.12. Conditions: T = 60 °C, j = 66 mA cm−2, Catalyst loading = 0.50 g (5 g/L), Reaction time = 4 h, all electrolytes also containing 0.25 M NaCl and 25% v/v IPA in the cathode chamber. (C) and (A) denote the cathode and anode chamber. E is either cathode or anode potential (vs. RHE) measured by reference electrode (Ag/AgCl), Ecell was recorded from the potentiostat, pH was measured after the reaction. C.B. carbon balance, F.E. Faradaic efficiency, rx,0 initial reaction rate at 1 h (mol of reactant s−1 gPt−1). Cathode: XA guaiacol conversion, S normalized product selectivity (C mol%) for phenol (B), cyclohexanone (C), cyclohexanol (D), anisole (E), cyclohexane (F), 2-methoxycyclohexanone (G), 2-methoxycyclohexanol (H), methoxycylohexane (I), and methanol (J). Anode: XA glycerol conversion, S normalized product selectivity (C mol%) for glyceraldehyde (B), glyceric acid (C), glycolic acid (D), tartronic acid (E), DHA (F), lactic acid (G), Formic acid (H), and acetic acid (I). 260 Table B.4. Curret density and initial reactant concentration effects in GECH-DO and GEOR paired electrosynthesis with 5wt.% Pt/C cathode and anode catalyst. Parameter Current density effect with 0.1 M initial reactant concentration Initial reactant concentration effect at 66 mA cm-2 66 mA cm-2 99 mA cm-2 132 mA cm-2 0.1 M 0.2 M 0.3 M 0.5 M (C) (A) (C) (A) (C) (A) (C) (A) (C) (A) (C) (A) (C) (A) E (V) -0.62 1.78 -0.94 1.74 -0.83 2.26 -0.62 1.78 -0.84 1.17 -1.02 2.33 -0.93 2.05 Ecell (V) 9.4 12.1 18.9 9.4 8.8 9.1 9.2 pH 0.08 0.03 0.05 0.02 0.00 0.02 0.08 -0.03 0.20 -0.03 0.21 -0.02 0.56 -0.19 XA (%) 79.1 27.6 78.7 40.9 82.5 35.0 79.1 27.6 40.0 17.9 25.3 19.0 19.1 17.1 SB (%) 4.3 12.5 5.0 2.9 5.3 9.7 4.3 12.5 10.0 8.3 7.2 18.6 6.9 11.1 SC (%) 23.5 4.0 24.7 2.8 31.4 4.0 23.5 4.0 36.2 27.7 27.3 22.6 24.2 6.3 SD (%) 11.5 9.5 16.9 18.3 13.4 34.8 11.5 9.5 12.5 2.8 10.2 2.4 10.9 3.7 SE (%) 8.5 11.2 4.3 9.4 4.9 13.0 8.5 11.2 1.0 16.7 1.9 14.7 1.7 30.2 SF (%) 6.9 0.2 3.2 0.8 5.0 0.4 6.9 0.2 6.6 0.6 4.9 0.1 3.8 0.6 SG (%) 19.3 4.4 22.3 3.7 17.1 0.8 19.3 4.4 10.3 35.2 18.7 31.5 20.5 34.0 SH (%) 14.0 32.8 14.3 32.1 10.2 27.3 14.0 32.8 3.6 5.6 11.0 7.3 13.5 7.3 SI (%) 4.7 25.4 1.6 30.0 4.3 10.0 4.7 25.4 9.4 3.1 11.1 2.9 10.8 6.8 SJ (%) 7.4 0.0 7.6 0.0 8.4 0.0 7.4 0.0 10.3 0.0 7.7 0.0 7.8 0.0 C.B. (%) 99.7 96.2 ~99.9 97.3 ~99.9 97.0 99.7 96.2 ~99.9 98.6 ~99.9 97.8 99.9 98.4 F.E. (%) 97.3 34.7 75.8 40.6 49.4 22.2 97.3 34.7 58.0 10.2 33.4 10.5 21.4 11.3 rx,0 × 105 2.84a 0.75b 2.77a 1.19a 2.95a 0.99ab 2.84a 0.75a 1.20a 0.53a 0.82b 0.59a 0.60b 0.52a *Significance difference level between group of Cathode and Anode as formulated in Equation 3.12. Conditions: T = 60 °C, j = 66 mA cm−2, Catalyst loading = 0.50 g (5 g/L), Reaction time = 4 h, all electrolytes also containing 0.25 M NaCl and 25% v/v IPA in the cathode chamber. (C) and (A) denote the cathode and anode chamber. E is either cathode or anode potential (vs. RHE) measured by reference electrode (Ag/AgCl), Ecell was recorded from the potentiostat, pH was measured after the reaction. C.B. carbon balance, F.E. Faradaic efficiency, rx,0 initial reaction rate at 1 h (mol of reactant s−1 gPt−1). Cathode: XA guaiacol conversion, S normalized product selectivity (C mol%) for phenol (B), cyclohexanone (C), cyclohexanol (D), anisole (E), cyclohexane (F), 2-methoxycyclohexanone (G), 2-methoxycyclohexanol (H), methoxycylohexane (I), and methanol (J). Anode: XA glycerol conversion, S normalized product selectivity (C mol%) for glyceraldehyde (B), glyceric acid (C), glycolic acid (D), tartronic acid (E), DHA (F), lactic acid (G), Formic acid (H), and acetic acid (I). 261 Table B.5. Half-cell reaction of GECH-DO and GEOR using binary metal catalysts. Parameter Pt/C (5 wt%) PtNi catalyst PtCo catalyst PtNi/C (3.9 wt% metal) Pt3Ni/C (3.3 wt% metal) PtNi3/C (5.7 wt% metal) PtCo/C (3.9 wt% metal) Pt3Co/C (3.3 wt% metal) PtCo3/C (5.7 wt% metal) C A C A C A C A C A C A C A Ec (V vs RHE) -0.6 -1.2 -0.5 -0.4 -0.5 -0.4 -0.5 -0.4 -0.7 -0.4 -0.6 -0.4 -0.5 -0.4 Ea (V vs RHE) 2.4 1.70 1.9 2.21 4.5 1.03 2.5 1.37 2.5 2.4 2.5 2.0 2.6 1.8 pHc 0.08 0.10 0.03 -0.02 0.17 -0.22 0.10 0.23 0.04 -0.04 0.03 -0.11 0.34 -0.03 pHa 0.01 0.12 -0.03 0.13 -0.26 0.23 0.03 0.25 -0.11 0.08 -0.19 0.16 0.26 0.11 XA (%) 73.1 34.8 70.8 21.3 65.1 27.2 49.7 14.6 70.6 25.4 56.1 14.6 36.3 16.3 SB (%) 7.5 37.2 5.1 10.9 5.5 3.5 7.7 20.3 6.0 6.0 7.6 8.1 9.4 10.8 SC (%) 30.6 0.2 21.9 11.1 21.2 1.2 17.9 2.0 29.3 2.2 31.1 5.2 35.8 4.0 SD (%) 11.0 1.8 27.4 6.8 25.9 5.7 25.4 2.5 25.8 10.8 19.1 8.8 26.6 26.9 SE (%) 16.7 1.7 0.0 9.7 0.0 13.9 0.0 11.7 0.0 12.9 0.0 30.9 0.0 9.9 SF (%) 7.1 0.1 17.4 0.3 9.4 0.2 10.7 0.2 11.1 0.3 11.6 0.2 14.7 0.2 SG (%) 10.3 0.6 8.9 3.4 12.7 66.4 11.8 45.5 8.1 51.2 11.3 20.4 0.0 14.3 SH (%) 2.8 23.7 8.3 47.1 15.6 5.1 16.6 4.4 8.0 10.3 8.4 8.8 0.0 15.3 SI (%) 4.8 34.7 0.0 10.7 0.0 4.1 0.0 13.4 0.0 6.4 0.0 17.6 0.0 18.7 SJ (%) 9.21 11.11 9.61 9.97 11.62 10.85 13.59 C.B. (%) ~99.9 98.2 97.4 97.3 99.1 99.6 99.1 99.8 96.9 99.2 97.1 99.5 97.4 99.7 F.E. (%) 87.2 35.1 78.0 27.5 70.3 14.7 64.1 6.2 77.9 17.3 70.6 11.7 47.2 15.0 rx,0 × 105 (mol s-1 gmetals-1)* 2.54c 1.02bc 4.79a 1.21a 4.39a 1.11b 2.06c 0.64d 4.22a 1.15a 3.74b 0.63d 1.41d 0.72d *The letter shows the significance difference level between group of cathode and anode as formulated in Equation 3.12. Conditions: T = 60 °C, j = 66 mA cm−2, Catalyst loading = 0.50 g (5 g/L), Reaction time = 4 h, all electrolyte also containing 0.25 M NaCl and 25% v/v IPA in the cathode chamber. (C) and (A) denote the cathode and anode chamber. E is the potential (V vs. RHE), pH was measured after the reaction. C.B. carbon balance, F.E. Faradaic efficiency, rx,0 initial reaction rate at 1 h (mol of reactant s−1 gmetals−1). Cathode: XA guaiacol conversion, S normalized product selectivity (C mol%) for phenol (B), cyclohexanone (C), cyclohexanol (D), anisole (E), cyclohexane (F), 2-methoxycyclohexanone (G), 2-methoxycyclohexanol (H), methoxycylohexane (I), and methanol (J). Anode: XA glycerol conversion, S normalized product selectivity (C mol%) for glyceraldehyde (B), glyceric acid (C), glycolic acid (D), tartronic acid (E), DHA (F), lactic acid (G), Formic acid (H), and acetic acid (I). 262 Table B.6. Half-cell reaction of GECH-DO and GEOR using ternary metal catalysts. Parameter Pt/C (5 wt%) PtNiCo catalyst PtNi0.5Co0.5/C (3.9 wt% metal) Pt3Ni0.5Co0.5/C (3.3 wt% metal) PtNi2Co/C (5.7 wt% metal) PtNiCo2/C (5.7 wt% metal) C A C A C A C A C A Ec (V) -0.6 -1.2 -0.5 -0.5 -0.6 -0.5 -0.5 -0.4 -0.5 -0.4 Ea (V) 2.4 1.70 2.6 1.54 2.5 1.42 2.5 1.58 2.5 1.61 pHc 0.08 0.10 0.03 -0.01 0.19 -0.48 0.05 0.09 0.19 0.16 pHa 0.01 0.12 -0.06 0.01 -0.51 0.48 0.02 0.09 -0.13 0.16 XA (%) 73.1 34.8 31.6 20.9 21.4 18.8 17.3 21.3 22.6 19.9 SB (%) 7.5 37.2 2.4 3.7 15.3 6.2 15.5 1.8 35.8 2.0 SC (%) 30.6 0.2 35.7 34.9 24.9 7.8 30.6 68.3 20.7 42.1 SD (%) 11.0 1.8 34.9 4.4 46.2 7.1 27.0 5.3 29.8 1.6 SE (%) 16.7 1.7 0.0 23.9 0.0 50.8 0.0 10.8 0.0 28.0 SF (%) 7.1 0.1 13.5 0.2 0.0 0.1 12.9 0.1 0.0 0.0 SG (%) 10.3 0.6 0.0 25.0 0.0 1.8 0.0 5.1 0.0 1.0 SH (%) 2.8 23.7 0.0 3.5 0.0 6.2 0.0 3.7 0.0 23.6 SI (%) 4.8 34.7 0.0 4.4 0.0 20.1 0.0 4.8 0.0 1.8 SJ (%) 9.21 - 13.49 - 13.59 - 13.97 - 13.74 - C.B. (%) ~99.9 98.2 99.7 98.6 98.7 99.6 98.9 99.8 99.7 89.2 F.E. (%) 87.2 35.1 39.4 12.2 34.6 18.9 20.0 9.1 29.6 11.0 rx,0 × 105 (mol s-1 gmetals-1)* 2.54c 1.02bc 1.76d 0.93c 1.43d 0.83cd 0.70e 0.90c 0.88e 0.87c *The letter shows the significance difference level between group of cathode and anode as formulated in Equation 3.12. Conditions: T = 60 °C, j = 66 mA cm−2, Catalyst loading = 0.50 g (5 g/L), Reaction time = 4 h, all electrolyte also containing 0.25 M NaCl and 25% v/v IPA in the cathode chamber. (C) and (A) denote the cathode and anode chamber. E is the potential (V vs. RHE), pH was measured after the reaction. C.B. carbon balance, F.E. Faradaic efficiency, rx,0 initial reaction rate at 1 h (mol of reactant s−1 gmetals−1). Cathode: XA guaiacol conversion, S normalized product selectivity (C mol%) for phenol (B), cyclohexanone (C), cyclohexanol (D), anisole (E), cyclohexane (F), 2-methoxycyclohexanone (G), 2-methoxycyclohexanol (H), methoxycylohexane (I), and methanol (J). Anode: XA glycerol conversion, S normalized product selectivity (C mol%) for glyceraldehyde (B), glyceric acid (C), glycolic acid (D), tartronic acid (E), DHA (F), lactic acid (G), Formic acid (H), and acetic acid (I). B.5 Kinetics analysis Initially, the reaction order was determined using the graphical analysis using data experiment plotting as presented in Table B.3. The data only compares 1st and 2nd order of reaction from the standalone experiment using similar reaction condition. Along with the reaction order determination, the reaction pathway was also investigated from the standalone experiment. Therefore, reaction rate of GECH-DO and GEOR could be determined and analyzed using Matlab software to predict the kinetics constants. 263 Table B.7. Reaction order determination using graphical analysis Reaction R-square 1st-order �𝒕𝒕 𝒗𝒗𝒗𝒗. 𝒍𝒍𝒍𝒍 � 𝟏𝟏(𝟏𝟏−𝑿𝑿𝑨𝑨)�� 2nd-order �𝒕𝒕 𝒗𝒗𝒗𝒗. 𝒍𝒍𝒍𝒍 � 𝟏𝟏(𝟏𝟏−𝑿𝑿𝑨𝑨)�� Cathode chamber: guaiacol ECH-DO Guaiacol (A) 0.99 0.96 Phenol (B) 0.98 0.87 Anisole (E) 0.98 0.87 Anode chamber: glycerol EOR Glyceraldehyde (B) 0.99 0.96 Glyceric acid (C) 0.99 0.88 Glycolic acid (D) 0.99 0.87 Tartronic acid (E) 0.99 0.89 Dihydroxyaacetone (F) 0.96 0.89 Lactic acid (G) 0.99 0.90 Standard deviation between model and actual data: 𝑆𝑆𝐷𝐷 = �1𝑁𝑁∑ �𝐶𝐶𝑤𝑤𝑒𝑒𝑒𝑒𝑤𝑤𝑒𝑒𝑤𝑤𝑠𝑠𝑤𝑤𝑒𝑒𝑡𝑡 − 𝐶𝐶𝑠𝑠𝑚𝑚𝑡𝑡𝑤𝑤𝑚𝑚�2𝑁𝑁𝑤𝑤=1 (B.1) Table B.8. Standard deviation value of experiment and kinetics model concentration for isopropanol variation experiment. IPA, % v/v MSA, M 0.2 0.5 1.0 0 0.64 0.64 0.53 10 0.97 1.06 0.98 25 0.99 1.14 0.95 50 1.75 1.81 1.59 75 1.61 1.85 1.68 90 1.59 1.76 1.51 264 Table B.9. Langmuir-Hinshelwood and Eley-Rideal mechanism comparison for guaiacol ECH-DO and glycerol EOR Guaiacol ECH-DO Glycerol EOR Langmuir-Hinshelwood Rate determining step (RDS) R2 Rate determining step (RDS) R2 𝑟𝑟𝑎𝑎,𝐴𝐴 = �𝑘𝑘𝑎𝑎,𝐴𝐴𝐶𝐶𝐴𝐴��1 + 𝐾𝐾𝑎𝑎,𝐼𝐼𝑃𝑃𝐴𝐴𝐶𝐶𝐼𝐼𝑃𝑃𝐴𝐴 + 𝐾𝐾𝑎𝑎,?̇?𝐻𝐶𝐶?̇?𝐻� 0.70 𝑟𝑟𝑎𝑎,𝑊𝑊 = 𝑘𝑘𝑎𝑎,𝑊𝑊𝐶𝐶𝑊𝑊�1 + 𝐾𝐾𝑎𝑎,𝐴𝐴𝐶𝐶𝐴𝐴� -1.86 𝑟𝑟𝑎𝑎,𝐴𝐴 = �𝑘𝑘𝑎𝑎,𝐼𝐼𝑃𝑃𝐴𝐴𝐶𝐶𝐼𝐼𝑃𝑃𝐴𝐴��1 + 𝐾𝐾𝑎𝑎,𝐴𝐴𝐶𝐶𝐴𝐴 + 𝐾𝐾𝑎𝑎,?̇?𝐻𝐶𝐶?̇?𝐻� 0.72 𝑟𝑟𝑎𝑎,𝐴𝐴 = 𝑘𝑘𝑎𝑎,𝐴𝐴𝐶𝐶𝐴𝐴�𝐾𝐾𝑎𝑎,𝑊𝑊𝐶𝐶𝑊𝑊� 0.99 𝑟𝑟𝑎𝑎,?̇?𝐻 = �𝑘𝑘𝑎𝑎,?̇?𝐻𝐶𝐶?̇?𝐻��1 + 𝐾𝐾𝑎𝑎,𝐴𝐴𝐶𝐶𝐴𝐴 + 𝐾𝐾𝑎𝑎,𝐼𝐼𝑃𝑃𝐴𝐴𝐶𝐶𝐼𝐼𝑃𝑃𝐴𝐴� -1.88 𝑟𝑟𝑡𝑡𝑤𝑤𝑠𝑠,𝑃𝑃 = 𝑘𝑘𝑡𝑡,𝑃𝑃 3E-16 𝑟𝑟𝑡𝑡𝑤𝑤𝑠𝑠,𝑃𝑃 = 𝑘𝑘𝑡𝑡,𝑃𝑃 -8E-16 𝑟𝑟𝑡𝑡𝑤𝑤𝑠𝑠,𝑊𝑊 = 𝑘𝑘𝑡𝑡,𝑊𝑊 3E-16 𝑟𝑟𝑡𝑡𝑤𝑤𝑠𝑠,𝑊𝑊 = 𝑘𝑘𝑡𝑡,𝑊𝑊 -8E-16 𝑟𝑟𝑠𝑠,𝐴𝐴 = �𝑘𝑘𝑠𝑠,𝐴𝐴𝐾𝐾𝑎𝑎,𝐴𝐴𝐶𝐶𝐴𝐴𝐾𝐾𝑎𝑎,𝑊𝑊 𝐶𝐶𝑊𝑊𝐶𝐶?̇?𝐻��1 + 𝐾𝐾𝑎𝑎,𝐴𝐴𝐶𝐶𝐴𝐴 + 𝐾𝐾𝑎𝑎,𝑊𝑊 𝐶𝐶𝑊𝑊𝐶𝐶?̇?𝐻�2 0.90 𝑟𝑟𝑡𝑡𝑤𝑤𝑠𝑠,𝐴𝐴𝐶𝐶𝐸𝐸 = 𝑘𝑘𝑡𝑡,𝐴𝐴𝐶𝐶𝐸𝐸 -8E-16 𝑟𝑟𝑠𝑠,𝐴𝐴 = �𝑘𝑘𝑠𝑠𝐾𝐾𝑎𝑎,𝐴𝐴𝐾𝐾𝑎𝑎,𝐼𝐼𝑃𝑃𝐴𝐴𝐾𝐾𝑎𝑎,?̇?𝐻𝐶𝐶𝐴𝐴0𝐶𝐶?̇?𝐻0𝐶𝐶𝐼𝐼𝑃𝑃𝐴𝐴0��1 + 𝐾𝐾𝑎𝑎,𝐴𝐴𝐶𝐶𝐴𝐴0 + 𝐾𝐾𝑎𝑎,𝐼𝐼𝑃𝑃𝐴𝐴𝐶𝐶𝐼𝐼𝑃𝑃𝐴𝐴0 + 𝐾𝐾𝑎𝑎,?̇?𝐻𝐶𝐶?̇?𝐻0�3 0.99 Eley-Rideal Rate determining step (RDS) R2 Rate determining step (RDS) R2 𝑟𝑟𝑎𝑎,𝐴𝐴 = 𝑘𝑘𝑎𝑎,𝐴𝐴𝐶𝐶𝐴𝐴 0.87 𝑟𝑟𝑎𝑎,𝐴𝐴 = 𝑘𝑘𝑎𝑎,𝐴𝐴𝐶𝐶𝐴𝐴 0.99 𝑟𝑟𝑠𝑠,𝐴𝐴 = �𝑘𝑘𝑠𝑠,𝐴𝐴𝐾𝐾𝑎𝑎,𝐴𝐴𝐶𝐶𝐴𝐴𝐶𝐶?̇?𝐻𝐶𝐶𝐼𝐼𝑃𝑃𝐴𝐴��1 + 𝐾𝐾𝑎𝑎,𝐴𝐴𝐶𝐶𝐴𝐴� 0.97 𝑟𝑟𝑠𝑠,𝐴𝐴 = �𝑘𝑘𝑠𝑠,𝐴𝐴𝐾𝐾𝑎𝑎,𝐴𝐴𝐶𝐶𝐴𝐴𝐶𝐶𝑂𝑂𝐻𝐻��1 + 𝐾𝐾𝑎𝑎,𝐴𝐴𝐶𝐶𝐴𝐴� 0.19 𝑟𝑟𝑡𝑡𝑤𝑤𝑠𝑠,𝑃𝑃 = 𝑘𝑘𝑡𝑡,𝑃𝑃 -8E-16 𝑟𝑟𝑡𝑡𝑤𝑤𝑠𝑠,𝑃𝑃 = 𝑘𝑘𝑡𝑡,𝑃𝑃 3E-16 *The negative R-square value describes the unconformity of the model toward the experiment data due to high SSres (Sum of square residual) value, indicating worsen data fits compared to a horizontal line. 265 Figure B.6. Concentration profile of the experiment and kinetics model at 30 °C for GECH (left) and GEOR (right). Reaction condition: j = 66 mA cm-2, N = 600 rpm, t = 4 h. Electrolyte 1.0 M MSA with 0.25 M NaCl for both chamber and 25% v/v IPA addition in the cathode chamber using 5 wt%-Pt/C (5 g/L). Cathode: guaiacol (A), phenol (B), cyclohexanone (C), cyclohexanol (D), anisole (E), cyclohexane (F), 2-methoxycyclohexanone (G), 2-methoxycyclohexanol (H), methoxycylohexane (I), and methanol (J). Anode: glycerol (A), glyceraldehyde (B), glyceric acid (C), glycolic acid (D), tartronic acid (E), DHA (F), lactic acid (G), Formic acid (H), and acetic acid (I). 266 Table B.10. Standard deviation value of experiment and kinetics model concentration for paired electrosynthesis on Pt/C catalyst. Chemical symbol 30 °C 40 °C 50 °C 60 °C Cathode Anode Cathode Anode Cathode Anode Cathode Anode A 2.00 0.63 0.97 1.20 0.80 0.37 0.56 0.64 B 0.61 0.59 0.45 0.15 0.26 0.65 0.46 1.62 C 0.81 0.07 0.76 0.12 0.81 0.32 0.80 0.26 D 2.17 0.19 1.60 0.05 0.67 0.28 0.77 1.21 E 1.39 0.27 0.28 0.41 0.40 0.46 0.49 0.94 F 0.25 0.03 1.13 0.03 1.08 0.15 1.53 0.00 G 0.54 0.17 0.69 1.46 1.96 0.27 2.32 0.72 H 0.97 0.19 1.17 2.38 1.60 4.22 1.47 1.52 I 0.76 0.05 0.64 2.39 0.77 4.22 0.17 0.08 J 0.53 - 2.06 - 0.55 - 1.05 - Cathode: guaiacol (A), phenol (B), cyclohexanone (C), cyclohexanol (D), anisole (E), cyclohexane (F), 2-methoxycyclohexanone (G), 2-methoxycyclohexanol (H), methoxycylohexane (I), and methanol (J). Anode: glycerol (A), glyceraldehyde (B), glyceric acid (C), glycolic acid (D), tartronic acid (E), DHA (F), lactic acid (G), Formic acid (H), and acetic acid (I). Table B.11. Arrhenius parameter of GECH reaction in 1.0 M MSA electrolyte pairs at different temperatures (30–60 °C). Reaction condition: j = 66 mA cm-2, N = 600 rpm, t = 4 h. Catalyst: 5 wt%-Pt/C (5 g/L), all electrolyte also containing 0.25 M NaCl with 25% v/v IPA addition in the cathode chamber. No Reactant Product Ea (kJ/mol) A (h-1) k11 Guaiacol Anisole 23.0 1.25E-05 k12 Anisole methoxycyclohexane 11.0 9.59E+00 k13 methoxycyclohexane cyclohexanone 12.6 1.89E+01 k14 Anisole cyclohexane 49.3 5.40E+05 k21 Guaiacol Phenol 13.7 2.67E+01 k22 Phenol Cyclohexanone 14.6 8.72E-03 k23 Cyclohexanone Cyclohexanol 26.0 2.95E+03 k24 Cyclohexanol cyclohexane 3.4 1.08E-01 k31 Guaiacol 2-Methoxycyclohexanone 20.0 1.82E+02 k32 2-Methoxycyclohexanone 2-Methoxycyclohexanol 27.8 4.72E+02 k33 2-Methoxycyclohexanol Cyclohexanol 33.1 6.55E+03 k Total guaiacol reaction (k11 + k21 + k31) 5.82 2.96E+00 267 Table B.12. Arrhenius parameter of GEOR reaction in 1.0 M MSA electrolyte pairs at different temperatures (30–60 °C). Reaction condition: j = 66 mA cm-2, N = 600 rpm, t = 4 h. Catalyst: 5 wt%-Pt/C (5 g/L), all electrolyte also containing 0.25 M NaCl with 25% v/v IPA addition in the cathode chamber. No Reactant Product Ea (kJ mol-1) A (h-1) k1 Glycerol DHA 26.9 6.39E+02 k2 Glycerol Glyceraldehyde 11.4 2.22E+00 k3 Glyceraldehyde Glyceric acid 24.7 5.44E+03 k4 Glyceric acid Glycolic acid 96.1 3.02E+15 k5 Glyceric acid Tartronic acid 26.0 4.47E+04 k6 Tartronic acid Glycolic acid 81.4 3.63E+12 k7 Glycolic acid Formic acid 64.5 1.38E+10 k8 DHA Lactic acid 56.3 3.31E+10 k9 Lactic acid Formic acid 116.9 8.12E+18 k10 Lactic acid Acetic acid 112.9 1.62E+18 k Total glycerol reaction (k1 + k2) 18.07 5.11E+01 Table B.13. Standard deviation value of experiment and kinetics model concentration of paired cell electrosynthesis using PtNi/C and Pt/C in cathode and anode chamber, respectively. Catalyst Temperature (°C) 30 40 50 60 Cathode side PtNi/C – Pt/C 1.07 1.21 2.61 2.50 PtC/C – Pt/C - 1.32 1.43 1.65 Anode side PtNi/C – Pt/C 1.10 1.08 1.39 1.78 PtC/C – Pt/C - 1.31 1.46 1.44 268 Figure B.7. The kinetics rate constant comparison of glyceraldehyde and DHA formation in glycerol electrooxidation. Reaction condition: j = 66 mA cm-2, N = 600 rpm, t = 4 h. Catalyst: 5 wt%-Pt/C (5 g/L), electrolyte also containing 0.25 M NaCl with 25% v/v IPA addition in the cathode chamber.
Thesis/Dissertation
eng
Chemical and Biological Engineering
Vancouver : University of British Columbia Library
Attribution-NonCommercial-NoDerivatives 4.0 International
Graduate
Smith, Kevin J.
Kim, Chang Soo
Paired electrosynthesis in a stirred slurry electrochemical reactor for bio-oil upgrading and glycerol electrooxidation
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