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Methane homologation by the two-step cycle on Co Catalysts Zadeh, Jaafar Sadegh Soltan Mohammad 1998

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Methane Homologation by the Two-Step Cycle on Co Catalysts by Jaafar SadeghjSoltan Mohammad Zadeh B.Sc., Abadan Institute of Technology, 1987 M.Sc, The University of Shiraz, 1993 A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY in THE FACULTY OF GRADUATE STUDIES (Department of Chemical Engineering) We accept this thesis as confirming to the required standard THE UNIVERSITY OF BRITISH COLUMBIA January, 1998 © J.S. Soltan Mohammad Zadeh, 1998 In presenting this thesis in partial fulfilment of the requirements for an advanced degree at the University of British Columbia, I agree that the Library shall make it freely available for reference and study. I further agree that permission for extensive copying of this thesis for scholarly purposes may be granted by the head of my department or by his or her representatives. It is understood that copying or publication of this thesis for financial gain shall not be allowed without my written permission. Department of The University of British Columbia Vancouver, Canada Date Tfth. 1\ 1W DE-6 (2/88) Abstract Conversion of natural gas to liquid hydrocarbons upgrades a low density fuel to a valuable source of chemicals and liquid fuel. To eliminate the expensive intermediate step of methane-steam refonning in the commercial Fischer-Tropsch, methanol to gasoline and Shell middle distillate synthesis processes, a direct method of CFLt conversion to higher hydrocarbons is very desirable. In direct conversion of CH4 to higher hydrocarbons in the presence of O2 (e.g. oxidative coupling and partial oxidation), deep oxidation of CH4 to CO and C02 is a major drawback. In the two-step homologation of CH4 in the absence of O2, CFL, is first activated on a reduced transition metal catalyst at high temperature (e.g. 450 °C) to produce H2 and carbon species on the catalyst. The carbon species are then hydrogenated in the second step at a lower temperature (e.g. 100 °C) to produce CFLt and higher hydrocarbons. In the present study of the two-step homologation of CH4, Si02 supported Co catalysts were prepared by incipient impregnation. The catalysts were characterized by BET surface area and pore volume measurement, powder X-ray diffraction, temperature programmed reduction, H2 chemisorption and Co re-oxidation. Carbon species deposited in the activation step were recovered by isothermal hydrogenation at 100 °C, temperature programmed surface reaction and temperature programmed oxidation to account for the reactivity of different carbon species. ii The effect of catalyst loading, activation time, activation temperature, carbon aging, reaction cycle and isothermal medium on both the CtL, activation step and the isothermal hydrogenation to C2+ hydrocarbons, were studied. Based on the findings from deposition of more than a nominal monolayer carbon coverage on the supported metal a semi-empirical kinetic model for the activation of CH4 on Co-SiC»2 catalysts was developed. In the kinetic model, gas phase CH4 is first activated on Co to produce adsorbed H and CH3 species. Migration of some of the CH3 species from the metal to the support liberates Co sites for further reaction. H2 is generated by further dehydrogenation of CH3 species on the metal and support, and desorption of adsorbed H. The kinetic model and rate constants of different steps were used to interpret the effect of changes in operating conditions on the rate of different steps of the CH4 activation reaction. Metal-support interactions in the Co-Si02 system play an important role in CH4 activation and in detennining the activity of carbon species. With more than a nominal monolayer coverage of metal by carbon, a considerable amount of inactive carbon, which can only be removed by high temperature oxidation, is produced on the support. Hydrogen content and age of the carbon species were among the important factors affecting C2+ production in isothermal hydrogenation. It was shown that C-C bond formation occurs to a great extent before the isothermal hydrogenation step. iii Table of Contents Abstract ii Table of Contents v List of Tables ix List of Figures xi Nomenclature xiv Acknowledgments xviii Chapter 1 Introduction 1 1.1 Conversion of Methane 1 1.2 Two-step Homologation of Methane 4 1.3 Motivation 7 1.4 Objectives of the Research 8 Chapter 2 Literature Review 10 2.1 Methane Upgrading2.2 Homologation of Methane Using a Two-Step Cycle 12 2.2.1 Methane Homologation Using a Temperature Cycle 15 2.2.2 Methane Homologation Using a Pressure Cycle 20 2.3 Carbonaceous Deposits on Group VUI Metal Catalysts 24 2.4 Modifications of the Catalyst and Reactor for the two-step Cycle 29 2.4.1 Effect of the Catalyst Support on the Two-Step Cycle 29 2.4.2 Effect of a Catalyst Promoter on the Two-Step Cycle 30 iv 2.4.3 Bimetallic Catalysts for the Two-Step Cycle 31 2.4.4 Molecular Sieve Catalysts for the Two-Step Cycle 32 2.4.5 Membrane Reactors for the Two-Step Cycle 33 2.5 Summary 3Chapter 3 Experimental Methods 34 3.1 Overview 33.2 Catalyst Preparation 35 3.3 Experimental Set-Up 6 3.3.1 Kinetic Apparatus 33.3.2 Diffuse Reflectance JR. Spectroscopy 39 3.4 Catalyst Characterization 42 3.4.1 Temperature Programmed Reduction 42 3.4.2 H2Desorption 43 3.4.3 Co Re-Oxidation3.4.4 Powder X-Ray Diffraction 44 3.4.5 BET Surface Area 5 3.5 Two-Step Reaction Cycles 47 3.5.1 Methane Activation3.5.2 Isothermal Hydrogenation 48 3.5.3 Temperature Programmed Surface Reaction 49 3.5.4 Temperature Programmed Oxidation 49 3.6 DRIFTS Experiments 50 v Chapter 4 Catalyst Characterization 52 4.1 Overview 52 4.2 Surface Area and Pore Volume 53 4.3 Bulk Phase Analysis 57 4.4 Reduction Properties of Calcined Co-Si02 Catalysts 61 4.5 H2 Chemisorption on Co-Si02 Catalysts 64 4.6 Extent of Catalyst Reduction 67 4.7 DRIFTS Study 70 4.8 Discussion 74 Chapter 5 Activation of CH4 78 5.1 Overview 78 5.2 Activation of CH4 79 5.3 Kinetics of CH4 Activation 82 5.3.1 Coverage of Surface Species 89 5.4 Parametric Study of the Kinetic Model 93 5.4.1 Initial Activation of CH4 94 5.4.2 Carbon Migration 96 5.4.3 Hydrogen Desorption 98 5.4.4 Dehydrogenation of Surface CH3 100 5.5 Effect of Operating Variables 102 5.5.1 Effect of Reaction Time 102 5.5.2 Effect of Metal Loading 105 5.5.3 Effect of Activation Temperature 106 5.5.4 Effect of Support and K Promoter on the Activation of CH» 108 5.6 Conclusions 110 Chapter 6 Hydrogenation 1 6.1 Overview 116.2 Carbon Recovery 113 6.2.1 Isothermal Hydrogenation 116.2.2 Temp erature Programmed Surface Reaction 115 6.2.3 Temperature Programmed Oxidation 117 6.2.4 Carbon Balance 119 6.3 Effect of Operating Variables 121 6.3.1 Effect of Loading 126.3.2 Activation Time 4 6.3.3 Activation Temperature 126 6.3.4 Carbon Aging 128 6.3.5 Reaction Cycle 131 6.3.6 Isothermal Medium 133 6.4 Conclusion 137 Chapter 7 Conclusions and Recommendations for Future Work 139 7.1 Conclusions 137.2 Recommendations for Future Work 143 References 145 vii Appendices 1.1 Calibration of Mass Flow Controllers 1.2 Conditions of BET Analysis 1.3 Analysis with Gas Chromatograph 1.4 Calibration of Quadrupole Mass Spectrometer 1.5 Analysis Conditions of Inffa-Red Spectroscopy 2.1 Temperature Programmed Reduction Profiles 2.2 Experimental Data 2.3 Equations for Calculations 2.4 Study of the Effect of Activation Temperature by the Kinetic Model 3.1 Computer Program Listing viii List of Tables Table 1.1 Operating conditions and yield of some direct CH4 upgrading processes. 3 Table 2.1 Selectivity of different catalysts in two step homologation of CH4. 18 Table 2.2 Effect of K and Cu promoter on Co catalysts. 30 Table 2.3 Kinetic parameters of hydrogenation step on [Cox]red/NaY and Co/Si02. 32 Table 4.1 Average pore diameter and pore size distribution for the support and A series Co- Si02 calcined catalysts. 57 Table 4.2 Peak 20 location and the relative peak intensity of different phases in C0-S1O2 system in the 20 range of 30-50°. 58 Table 4.3 Co dispersion based on the PXRD of the calcined catalysts. 61 Table 4.4 Extent of reduction of Co catalysts based on TPR. 64 Table 4.5 Moles of chemisorbed H2 and surface Co in Co-Si02 catalysts. 67 Table 4.6 Extent of reduction of Co catalyst based on re-oxidation of the reduced Co. 70 Table 4.7 Summary of the Co-Si02 catalyst characteristics. 75 Table 5.1 Effect of metal loading on the reaction rate constants. 106 Table 5.2 Effect of temperature on CH4 activation over 12% Co-Si02 catalyst. 107 Table 5.3 Effect of support and K promoter on CH4 activation. 109 Table 6.1 Summary of carbon balance calculations for A-12% Co-Si02 catalyst. 120 Table 6.2 Effect of the catalyst metal loading on its hydrogenation performance. 122 Table 6.3 Effect of activation time on the hydrogenation step. 125 Table 6.4 Effect of activation temperature on the hydrogenation step. 127 ix Table 6.5 Effect of carbon aging on the hydrogenation step. 130 Table 6.6 Effect of reaction cycle on the hydrogenation step. 132 Table 6.7 Effect of the isothermal medium on the product distribution. 135 x List of Figures Figure 1.1 Schematic representation of the two-step cycle for the homologation ofCHL,. 5 Figure 2.1 Gibbs free energy as a function of temperature for activation of CFLt on Co and hydrogenation of cobalt carbide to C2FL3. 14 Figure 2.2 CFLt production profile as a function of temperature in a TPSR. 17 Figure 2.3 A probable reaction sequence for activation of CH4 (step 1) and hydrogenation of surface species (step 2). 26 Figure 3.1 Block diagram of the experimental set-up. 37 Figure 3.2 DRIFTS reaction cell. 40 Figure 3.3 Block diagram of the DRIFTS reaction cell connections. 41 Figure 4.1 N2 adsorption and desorption on Si02 as a function of relative pressure (P/Po). 54 Figure 4.2 BET plot for N2 adsorption on Si02. 56 Figure 4.3 PXRD diffractogram of the calcined A series 12%Co-Si02 catalyst. 59 Figure 4.4 Profile of the temperature programmed reduction (TPR) of the A series 12%Co-Si02 catalyst with 54 rnl/inin of 20%H2/Ar gas mixture. 62 Figure 4.5 Profile of temperature and H2 desorption from A series 12%Co-Si02 catalyst in 15 ml/min of Ar flow. 66 Figure 4.6 02 uptake profile from pulses of 1.0 ml 02 in 15 ml/min of He flow by the reduced A series 12%Co-Si02 catalyst during the Co oxidation step. 69 Figure 4.7 Diffuse reflectance infra-red Fourier transform spectra of the dilute passivated A series 12%Co-Si02 catalyst with respect to pure KBr background. 71 Figure 4.8 Diffuse reflectance infra-red Fourier transform spectra of the diluted A series 12%Co-Si02 reduced catalyst after activation, subsequent evacuation and hydrogenation with respect to the reduced, diluted catalyst background. 73 Figure 4.9 Schematic diagram of the two types of metal-support interaction (MSI) on Co-Si02 catalyst. 77 Figure 5.1 Rate of CFLt consumption and H2 generation per mole of Co during 2 min flow of 5%CIV95%Ar gas naixture at 54 ml/min and 450 °C over A series 12%Co-Si02 catalyst. 80 Figure 5.2 Cumulative moles of CH4 consumed per mole of surface Co and the value of x in the CHX surface species during 2 min flow of 5%CIV95%Ar gas mixture at 54 ml/min and 450 °C over A series 12%Co-Si02 catalyst. 81 Figure 5.3 Cumulative moles of CH4 consumed per mole of surface Co measured during 2 min flow of 5%CHV95%Ar gas mixture and 54 ml/min at 450 °C over A series 12%Co-Si02 catalyst, compared to model fit. 88 Results of simulation of reaction rates for 2 min activation of 5%CU4/95%Ar Figure 5.4 90 gas mixture at 450 °C over A series 12%Co-Si02 catalyst. Figure 5.5 Results of simulation of surface coverage for 2 min activation of 5%CIV95%Ar gas mixture at 450 °C over A series 12%Co-Si02 catalyst. 91 Figure 5.6 Effect of ki on nominal metal coverage, 6C Figure 5.7 Effect of k2 on nominal metal coverage, 6C 95 97 xii Figure 5.8 Effect of k3 on nominal metal coverage, 0C. 99 Figure 5.9 Effect of tj on nominal metal coverage, Gc. 101 Figure 5.10 Cumulative moles of CH4 consumed per mole of surface Co measured during 7 min flow of 5%CH4/95%Ar gas mixture at 54 ml/min and 450 °C over the A series 12%Co-Si02 catalyst, compared to model fit. 103 Figure 5.11 Value of x in the CHX surface species during 2 min and 7 min flow of 5%CH4/95%Ar gas mixture at 54 ml/min and 450 °C over the A series 12%Co-Si02 catalyst. 104 Figure 6.1 Profile of CH4, C2H4 and C2H6 production as a function of isothermal hydrogenation at 100 °C in 11 ml/min H2 flow over the A series 12%Co-Si02 catalyst after 2 min activation with a 5% CEVAr mixture at 450 °C. 114 Figure 6.2 Profile of CH4 production and temperature as a function of time during temperature programmed surface reaction (TPSR) in 11 ml/min of H2 flow on A series 12%Co-SiC>2 catalyst. 116 Figure 6.3 Profile of C02 production and temperature as a function of time in temperature programmed oxidation (TPO) in 11 ml/min of 02 flow on A series 12%Co-Si02 catalyst. 118 xiii Nomenclature a absorptivity AES Auger electron spectroscopy a.u. arbitrary unit BET Brunauer-Emmett-Teller C concentration (mol/1) C constant (Section 3.4.5) C2+ hydrocarbons with carbon number of 2 and higher CHX carbon species on the surface with the H/C ratio of x Ca active carbon on the surface that hydrogenates at a temperature below 100 °C Cp moderately active carbon on the surface that hydrogenates at a temperature of about 200 °C Cy least active carbon on the surface that hydrogenates at a temperature of about 500 °C d average pore diameter (A) DRIFTS diffuse reflectance infra-red Fourier transform spectroscopy FID flame ionization detector FT Fischer-Tropsch GC gas chromatograph HREELS high resolution electron energy loss spectroscopy IR infra-red TVS injection valve system k reaction rate constant xiv K Scherrer constant (=0.9) LEED low energy electron diffraction LNG liquefied natural gas M reduced metal MCT mercury-cadmium-telluride MSI metal-support interaction MTG methanol to gasoline N0 Avogadro's number (=6.02 x 1023 molecules/mol) OC oxidative coupling P pressure (Pa) P0 saturation pressure of the adsorbate at the adsorption temperature (Pa) PO partial oxidation PXRD powder X-ray diffraction QMS quadrupole mass spectrometer R gas constant (8.314 J.mol'.K'1) R' relative reflectance R2 correlation parameter S scattering coefficient (Section 3.6) S surface site 51 active site on the metal surface 52 active site on the support surface SBET specific surface area measured by BET method (m2/g) SMDS Shell middle distillate synthesis t crystal size (A) T temperature (°C or K) TCD thermal conductivity detector TCM trillion cubic meters (1012 m3) TOF turn over frequency (min"1) TPD temperature programmed desorption TPO temperature programmed oxidation TPR temperature programmed reduction TPSR temperature programmed surface reaction UUP ultra high purity UHV ultra high vacuum VA volume of gas adsorbed per gram of adsorbent at STP (ml/g) VBET specific pore volume measured by BET method (m3/g) vm monolayer capacity of the adsorbent (ml/g) w instrumental peak broadening in Scherrer's equation (radians) = 0.11 degrees W peak width at half height (radians) Wt% weight percent XPS X-ray photoelectron spectroscopy xvi Greek Letters a molecular cross-section of adsorbate (~0.162 nm2/molecule of N2) a chain growth probability in Schulz-Flory model (Section 2.2.1) P peak broadening in PXRD (radians) AG0 standard Gibbs free energy change (J/mol) O catalyst metal dispersion (mol metal on surface/mol metal in the catalyst) X radiation wavelength in Scherrer's equation (1.5406 A) 9 coverage 6 peak position in PXRD (radians) (Section 3.4.4) 6C nominal metal coverage by carbon (mol CFL/mol surface Co) a standard deviation xvii Acknowledgments I am deeply indebted to my supervisor Dr. Kevin J. Smith for his support, guidance and enthusiasm throughout my graduate career at UBC. My thanks are also due to my supervisory committee members Dr. John Grace, Dr. Keith Mitchell and Dr. A.P. Watkinson for their help and contribution to this work. Special thanks must go to the staff of the Mechanical Shop, Electronic Shop and Stores, and Office of the Chemical Engineering Department and UBC Libraries for all the skill, patience and kindness that made an experimental graduate research work possible. Finally, I would like to dedicate this thesis to my family and to my first grade teacher, Mr. Hasan Mahdad. xviii Chapter 1 Introduction 1.1. Conversion of Methane CHU is the primary constituent of both natural gas and the gas resulting from crude oil production and processing. Currently, natural gas is used mostly as a fuel in industrial and domestic applications. However, the low energy density of natural gas makes its transport from production site to the consumer difficult and expensive. Fox (1993) reported that the cost of transporting natural gas as liquefied natural gas (LNG) over a 1000 km distance can be about six times the value of the natural gas at the well. Conversion of CH4 to higher hydrocarbons and transportable liquid fuels would make natural gas a more attractive fuel and source of chemicals. Currently, two approaches for the conversion of CH4 to liquid hydrocarbons are practiced commercially (Anderson, 1989; Fox et al., 1990; Fox, 1993). Both begin with the production of synthesis gas from CH4 steam reforming (reaction 1.1) which is a high temperature, endothermic and costly operation. CH4 + H20 -> CO + 3H2 (1.1) In the Fischer-Tropsch (FT) process (Anderson, 1984), the synthesis gas is converted to a range of higher hydrocarbons, including gasoline, by polymerization reactions on Fe based catalysts. In the methanol to gasoline (MTG) process (Anderson, 1989), CH3OH is first produced from CO and H2. The CH3OH is subsequently dehydrated to (CH3)20 which is 1 passed over a ZSM-5 zeolite catalyst. The gasoline product is the result of a complex series of oligomerization, cracking, cyclization and aromatization reactions. The gasoline produced by either the FT or the MTG process is more expensive than gasoline obtained from crude oil. This higher production cost arises mainly from the reforrriing of the natural gas which accounts for more than 60% of the total cost of these processes (Poirier et al., 1991; Mc Carty, 1992; Sundset et al., 1994; Ross et al., 1996; Lange and Tijm, 1996; Rostrup-Nielsen, 1994; Lange et al., 1997; Rostrup-Nielsen et al., 1997; Vora et al, 1997). As a result, direct conversion of CEL. to higher hydrocarbons without reforming to synthesis gas is a very attractive alternative (Fox et al., 1990). Direct conversion of CFL, to higher hydrocarbons is not practical under normal industrial operating conditions because of the positive Gibbs free energy change associated with reaction 1.2. 2CH4 C2H6 + H2 AG°298 = 74 kJ/mol (1.2) One approach to overcoming the Gibbs free energy change barrier is to add 02 to the reaction system, producing H20 or oxygenates instead of H2. In oxidative coupling (OC) reactions (Amenomiya et al., 1990), CH4 and 02 react on an alkali metal supported on BeO, MgO and other alkaline earth oxides to give a higher hydrocarbon (e.g. C2H<;) and H20: 2C//4 +102 -> C2H6 + H20 AG°298 =-217 kJ/mol (1.3) Deep oxidation of hydrocarbons to carbon oxides (CO and C02) are important undesirable reactions in this route. In the partial oxidation of CFL, (PO) (Pitchai and Kher, 1986), CH3OH and CH20 are produced from 02 plus CFL, but this process also suffers from CO and C02 producing side 2 reactions. The product yield from both of the above routes is too low for a viable commercial operation (see Table 1.1). In other processes, CFLt /halogen or CH4 / halogen/ O2 mixtures are used (Oxyhydrohalogenation) to produce CH3X (X is a halogen) which is then converted to higher hydrocarbons and HX or hydrolyzed to CH3OH (Anderson, 1989). In other direct CH4 conversion reactions, CfL, is coupled with C3H5 or C<>H5CH3 to produce higher hydrocarbons (Fox, 1993). The yield from all of these processes is too low for commercial application, as shown in Table 1.1, in which the operating temperature and catalyst for these reactions are compared. Table 1.1. Operating conditions and yield of some direct CH4 upgrading processes. process Temperature Yield Catalyst Reference °C % OC <800 15-25 Metal oxide Amenomiya et al, 1990 PO 350-500 <8 M0O3 Pitchai and Kher, 1986 Oxyhydrohalog. 100-250 <30 (based on CI) Pt Fox et al., 1990 C1-C3 Coupling 350 4 Ni Ovalles et al., 1991 Another approach to overcome the Gibbs free energy change barrier of direct homologation of CH4 (equation 1.2) in the absence of 02, is to perform the reaction in two steps. The temperature and pressure of each step of the cycle is chosen to overcome the positive Gibbs free energy change of the overall reaction (Koerts and van Santen, 1991; Koerts et al. 1992; Belgued et al., 1992). 1.2. Two-Step Homologation of Methane In the two-step homologation of CFL to higher hydrocarbons, CFL is first activated on a transition metal catalyst at high temperature and low H2 partial pressure, generating H2 and surface carbonaceous species. The carbon species are subsequently hydrogenated in the second step at low temperature and high H2 partial pressure to produce CFI4 and higher hydrocarbons such as C2FL; and C3H8, herein referred to collectively as C2+ products. The different temperatures and H2 partial pressures of the two steps are necessary to overcome the positive Gibbs free energy change associated with the single-step CFL homologation reaction (1.2). Figure 1.1 shows a schematic illustration of the two-step CFL homologation cycle. In previous studies of the two-step homologation reaction, CFL activation was performed over supported Group VIII metal catalysts either at high temperature (>300°C) in a dilute stream of CFL (Koerts et al., 1992; Koranne et al., 1995), or with short pulses of CFL introduced to the feed gas (Carstens and Bell, 1996), or at temperatures below 300° C (Belgued et al., 1992). In each case, the number of moles of CFL converted was less than the number of moles of surface metal atoms available on the catalyst, suggesting less than a nominal monolayer coverage of the metal by the resulting CFL. species (i.e. the moles of CFL reacted per mole of surface metal atoms was less than one). 1. Activation Step+ CH4+M^H2+M.CHX 2. Hydrogenation Step+ H2 + M.CHX -*M+CH4,C2H6,. High Temperature (450 °C) Low Temperature (100 °C) Remove H2 (low P^) Supply H2 (high Pm) Figure 1.1. Schematic representation of the two-step cycle for the homologation of CFL. M designates the reduced metal. + Equations are not stoichiometrically balanced. 5 A consequence of the low metal coverage was a low C2+ mole yield in the subsequent hydrogenation step since the moles of carbonaceous species available for hydrogenation were limited. In terms of moles of C2+ product per mole of surface metal of the catalyst, the yield reported by van Santen and coworkers (Koerts et al., 1992), Amariglio and coworkers (1994) and Koranne et al. (1995) was less than 1%. One possible approach to increasing the yield of the two-step cycle is to increase the concentration of the carbonaceous species on the surface by reacting more CFLt during the activation step. This may be achieved by, for example, operating with a higher concentration of CH4 in the feed. Despite the fact that in recent years different research groups in the Netherlands (Koerts and van Santen, 1990, 1991a, 1991b; Koerts et al., 1992a, 1992b), France (Amarigho et al., 1994; Belgued et al., 1992, 1996a, 1996b, 1996c) the United States (Koranne et al, 1995; Carstens and BelL 1996) and Hungary (Solymosi et al., 1992; Gucczi et al., 1996b, 1997) have worked on various aspects of the two-step cycle for the homologation of CHt, this is a new approach in the early stages of development. Consequently, despite the contributions made in previous studies, many unanswered questions remain. In particular, the fate of the carbon deposited on the catalyst surface during the activation of CHt and the sequence of carbon-carbon bond formation and product generation steps, is not well understood. Other points relevant to the two-step CHt homologation reaction that require further attention include the following: 1. The kinetics of CHt activation on supported metal catalysts are not known. 6 2. The properties of the carbonaceous deposits resulting from CH4 activation and how these affect C-C bond formation need to be established. 3. The role of H2, as opposed to other reactants, in the second step of the two-step homologation reaction needs clarification. The present research is motivated by the need to address these issues, as described in detail in the following section. 1.3. Motivation With a higher concentration of CH4 in the feed of the activation step, the rate of CEL. activation and the fate of the generated carbon species are unknown. Previous studies of the two-step cycle, in which the coverage of the metal by carbonaceous species was nominally less than a monolayer, identified three different types of carbon species on the catalyst surface. These species were identified according to their reactivity in H2 during temperature programmed surface reaction (TPSR). Only the most active carbon species are capable of producing C2+ in the second step of the two-step cycle. By exposure of the carbon species to high temperature, an "aging" phenomenon occurs in which active carbon species transform to less active species (Koerts and van Santen, 1991b; Koerts et al., 1992b). With higher concentrations of CFLtinthe feed, higher activation temperatures and with the deposition of more than a nominal monolayer of carbon species on the catalyst, the relative amount and activity of the surface carbon species needs to be determined. Also, the significance and effect of aging of the carbonaceous deposit on reactivity need to be evaluated. It is also important to determine the operating conditions that maximize the amount of active carbon species and minimize the loss of carbon reactivity due to aging. After CU, activation in the first step of the homologation cycle, an isothermal hydrogenation follows to produce C2+ and CH4. The significance of the isothermal medium and the importance of its chemical reactivity has not been addressed in previous studies (Koerts and van Santen, 1991b; Koerts et al., 1992a; Koranne et al., 1995; Belgued et al., 1996a, 1996b, 1996c; Carstens and BelL 1996). Clarification of the role of the isothermal medium in the two-step cycle would help to identify the best isothermal gas for higher C2+ production. From a mechanistic point of view, it is not known at what stage of the two-step cycle C-C bond formation occurs and reaction products desorb. Clarification of this point is needed to rnaximize the production of C2+ hydrocarbons and to minimize the production of CFL, and inactive carbon in the complete cycle. 1.4. Objectives of the Research The present study focuses on the effect of using a higher concentration of CFL, in the feed of the two-step cycle. The specific objectives are: 1. To prepare and characterize Si02 supported Co catalysts. 2. To study the kinetics of the CFL activation step, to develop a kinetic model for CFL activation and to obtain the reaction rate equations. 3. To use the kinetic model and rate equations as a quantitative tool to interpret some of the experimental observations and to quantify the effect of operating conditions on the activation step. 8 4. To evaluate the reactivity of the carbonaceous surface species after the activation step. 5. To detennine whether the carbon combination, which generates the desired C2+ products, occurs in the hydrogenation step or the CH4 activation step. 6. To clarify the significance of the chemical reactivity of the isothermal gas in the second step of the two-step cycle. 7. To determine the effect of operating variables such as activation temperature, catalyst metal loading and carbon aging on the overall performance of the two-step cycle. 9 Chapter 2 Literature Review 2.1. Methane Upgrading The world supply of natural gas continues to increase as a result of the discoveries of new fields and the conservation methods being employed during gas and oil production (Vora et al., 1997). In 1992, the proven gas reserves of the world stood at 145 trillion (1012) cubic meters (TCM), and annual production was 2.5 TCM, an increase of more than 60% from the 1973 production of about 1.6 TCM. The composition of natural gas varies widely but it consists predominantly of CH4. The heavier hydrocarbon components of natural gas are usually separated for use as fuel or as feedstocks for chemical processes. Transportation of natural gas from production site to the remote consumer is difficult and costly. An attractive solution is to convert the natural gas to higher hydrocarbons at the production site. The product liquid hydrocarbon could then be more easily transported and it would be a valuable source of chemicals and fuel in the consumer market. Activation of CFL, is difficult because it is a thermodynamically stable compound with very strong tetrahedral C-H bonds (bond energy of 435 kJ/mole), stronger than any C-H or C-C bond present in higher hydrocarbons. Current practice for converting natural gas to higher hydrocarbons proceeds by an indirect route in which natural gas is converted at high temperature (450-550 °C) to synthesis gas (Fox et al., 1990; lb and Hansen, 1997). Hydrocarbons are produced at lower temperature from exothermic synthesis gas reactions, 10 either by the Fischer-Tropsch (FT) process (Anderson, 1984), the methanol to gasoline (MTG) process (Anderson, 1989) or by the Shell middle distillate synthesis (SMDS) process (Sie et al, 1991). The gasoline produced by either of these indirect processes is more expensive than that obtained from crude oil refining. The higher production costs arise mainly from the reforming of natural gas which accounts for more than 60% of the cost of production (Pokier et al., 1991; Ross et al., 1996; Lange and Tijm, 1996; Rostrup-Nielsen, 1994) Direct CFL conversions such as pyrolysis to acetylene or benzene operate at temperatures above 1000 °C and generate coke as a side product. At moderate reaction conditions, direct conversion of CFL, to higher hydrocarbons is thermodynamically unfavorable because of the positive Gibbs free energy change of the reaction 2.1. 2CH4 -*C2H6 + H2 AG°298 = 74 kJ/mol (2.1) One approach to overcome the Gibbs free energy change barrier is to add oxygen to the reaction system and produce oxygenates or water instead of hydrogen. In oxidative coupling (OC) reactions (Lunsford, 1995) CFL and 02 react at a temperature of about 800 °C on an alkali metal supported on BeO, MgO and other alkaline earth oxides to produce C2FL5 and H20. 2CH4 +^02 -> C2H6 + H20 AG°298= -217 kJ/mol (2.2) In OC, the role of the catalyst is to activate CFL, by abstraction of H from CFL to generate a methyl radical. Subsequent coupling of the radicals yields C2FL3. Abstracted hydrogen is removed by water formation using both oxygen from the gas phase and oxygen adsorbed on the catalyst surface. 11 Deep oxidation of hydrocarbons to carbon oxides is an undesirable reaction in OC which limits the C2+ yield to about 25%. In partial oxidation (PO), CFL, is partially oxidized to methanol and formaldehyde at a temperature range of350 to 500 °C in an oxygen deficient atmosphere. M0O3 based catalysts have been widely used for partial oxidation reactions. CH4 +02-> CH20 + H20 AG°298 = -300 kJ/mol (2.3) Deep oxidation of CFL to CO and C02 is also the most important drawback of this process limiting the reaction yield to less than 8%. Reacting 02 with CFL is beneficial in overcoming the Gibbs free energy change barrier of the direct homologation of CFL, but it leads to the non-selective conversion of CFL to CO and C02. Another approach is needed to overcome the Gibbs free energy change barrier of the direct homologation of CFL in the absence of 02 and to avoid deep oxidation to CO and CO2 that occurs with OC and PO. 2.2. Homologation of Methane Using a Two-Step Cycle In another approach to overcome the Gibbs free energy change barrier of direct homologation of CFL to higher hydrocarbons, the effect of temperature and pressure on the Gibbs free energy change is utilized in a two-step cycle (Guczi et al., 1996). In the first step, CFL is activated at high temperature on a transition metal catalyst to produce hydrogen and carbonaceous deposits on the catalyst. In the second step, the carbonaceous species are hydrogenated at a lower temperature to produce C2+ products. 12 For illustration purposes, assume that the reaction of CFLt with a Co catalyst produces H2 and Co3C (eq. 2.4a) in the first step of the cycle (Koerts et al., 1992a). The Co3C species are hydrogenated (eq. 2.4b) in the second step to produce C2FL. For the CFL activation on Co to produce Co3C in the first step and hydrogenation of Co3C to produce C2H<> in the second step, the AG0 vs. temperature diagram is shown in Figure 2.1. 2CH4 + 6C0 -> 2Co3C + 4H2 (2.4a) 2Co3C + 3H2 -+C2H6 + 6C0 (2.4b2CH4 -» C2H6 + H2 AG° (727 °C) = 71 kJ/mole (2.4) The activation of CFL on Co forming cobalt carbide (eq. 2.4a) is endothermic with a positive change in entropy. This reaction is only possible at temperatures above 360 °C at standard conditions. The hydrogenation of cobalt carbide to form ethane (eq. 2.4b) is exothermic with a negative change in entropy. This reaction is favorable at low temperature and can occur below 77 °C. There is no common temperature at which both reactions 2.4a and 2.4b can occur, so that the temperature gap of 283 °C is a thermodynamic stipulation to meet the condition of negative Gibbs free energy change for a favorable reaction. This temperature gap can be decreased by the partial pressure effect. The effect of pressure on the Gibbs free energy of the two steps is expressed by equations 2.5a and 2.5b. 13 Figure 2.1. Gibbs free energy as a function of temperature for activation of CH4 on Co (2.4a) and hydrogenation of cobalt carbide to C2H3 (2.4b). 2CH4 + 6C0 -> 2Co3C + 4H2 (2.4a) 2Co3C + 3H2 -*C2H6+ 6C0 (2.4b14 —AG stepl RT (2.5a) —AG step! (2.5b) = exp RT Instead of using the effect of temperature, it is possible to use the effect of partial pressure to overcome the Gibbs free energy change barrier of reaction 2.4. Lower H2 partial pressure and higher CFL partial pressure in the first step, by continuous removal of the H2 produced, makes reaction 2.4a more favorable at low temperature. Low partial pressure of C2H6 (by removal of C2FL from the system) and high partial pressure of H2 in the second step (eq. 2.4b) utilizes the effect of pressure on Gibbs free energy change and makes an isothermal two-step cycle possible. Although reactions 2.4a, 2.4b and 2.4 are used as an example to illustrate the concept of the two-step cycle, Co3C species are reportedly involved in the two-step cycle reactions over Co catalysts (Koerts and van Santen, 1991a; Koerts et al., 1992a). 2.2.1. Methane Homologation Using a Temperature Cycle Van Santen and co-workers (Koerts and van Santen, 1990, 1991a, 1991b; Koerts et al., 1992a, 1992b) reported a two-step CFL activation and hydrogenation sequence for C2+ hydrocarbon formation. At a temperature between 170 and 520 °C, CFL was activated on reduced Group VTJI metal catalysts to produce H2 and adsorbed surface carbonaceous species. In the second step, hydrogenation of the surface species at 30-130 °C produced hydrocarbons. Among the Group Vm metal catalysts studied, they reported a maximum C2+ yield of 13% for a silica-supported Ru catalyst. 15 Van Santen and co-workers (Koerts and van Santen, 1990, 1991a, 1991b; Koerts et al., 1992a, 1992b) also studied the CFL; activation activity of silica-supported Ru, Rh, and Co catalysts as a function of temperature. From these studies they reported the activation energy of the CFL activation reaction on Ru, Rh and Co as 26, 31 and 42 kJ/mole, respectively. By subsequent temperature-programmed hydrogenation of the carbonaceous deposits, three types of surface carbon species were identified (Figure 2.2). A very reactive surface carbon species Ca could be hydrogenated at <50 °C to CFL and C2+ hydrocarbons. Single crystal studies have identified the Ca carbon to be carbidic (Koerts and van Santen, 1992). A less reactive surface carbon species designated Cp could be hydrogenated between 100 and 300 °C. During the hydrogenation of Cp only traces of higher hydrocarbons up to hexanes were detected. The Cp species from studies of CO disproportion on Ru was identified as an amorphous carbon (Koerts et al., 1992a). At temperatures above 400 °C, the poorly reactive Cy reacted to produce only CFL. The Cr type surface carbon was reportedly a graphitic phase which was probably located partially on the support (Nakamura et al., 1987; Koerts et al., 1992a). By analyzing hydrocarbons produced by hydrogenation of surface carbon species, van Santen and co-workers (Koerts et al., 1992a) found that hydrocarbon selectivity followed a Schulz-Flory type distribution, indicating that adsorbed surface CxHy species can either be hydrogenated to CxH2x+2 or increase in carbon number to Cx+iHz species. The chain growth probability a, was reported as 0.25 for Co and Ru catalysts. 16 Figure 2.2. CFL, production profile as a function of temperature in a TPSR (Koerts and van Santen, 1992). a.u. is an arbitrary unit. 17 Table 2.1 shows the product selectivity of different transition metal catalysts supported on silica reported by van Santen and co-workers (Koerts et al., 1992a). Table 2.1. Selectivity of different catalysts in two-step homologation of CFL, (Activation at 460 °C, Hydrogenation at 95°C). selectivity for hydrocarbons (%) catalyst CH4 C2H6 CsHg C4H10 10% Co 79.6 11.3 6.11 2.92 5% Ru 80.5 15.8 2.69 1.00 10% Ni 89.3 7.3 1.89 1.40 3%Rh 95.8 4.12 0.09 0 4%Pt 91.3 8.4 0.23 0 5% Re 98.4 1.3 0 0.29 Since only the reactive Ca type carbon has reasonable selectivities for C2+ hydrocarbon formation upon hydrogenation, it is desirable to maximize the amount of Ca carbon generated during CH, activation. Parameters that affect the amount of Ca are CH, deposition temperature, time, CH, concentration, gas velocity and aging time. It has been found that a high selectivity for reactive Ca type carbon is only obtained at low surface coverage by carbon. The selectivity of more reactive carbidic type carbon is related to the ability of the catalyst to form multiple bonded surface carbon species. It has been proposed that CH, is initially activated into carbidic Ca carbon and that there is an interconversion between Ca and Cp 18 carbon with a dynamic equihbrium of conversion of Ca to Cp(Koerts et al., 1992a) and transformation of Cp to Ca(Carstens and BelL 1996). Carstens and Bell (1996) studied the two-step CFL, homologation reaction on a 2.75% Ru-Si02 catalyst. In this work, pulses of 10.4% CFL/He rnixture were used in the activation of CFL on the reduced catalyst in the temperature range of 200 to 400 °C with exposure times of less than 40 s, producing a nominal metal coverage (0C) of less than 80%. The results from this study showed that CFL underwent dissociative adsorption on Ru at temperatures above 100 °C. The pre-exponential factor for dissociative adsorption was 2xl0"7 to 5.7xl0"7 s"1 and the activation energy was 24.8 to 29.4 kJ/mole. Based on TPSR in H2, three types of carbon species were distinguished, namely Ca, Cp and Cy carbons. The distribution among these carbon species was dependent on the total carbon coverage and the length of time that the carbon had been aged at a given temperature after deposition. For the first time in this study, the conversion of Cp to Ca and its activation energy of 92.4 kJ/mole, was reported. In the two-step cycle using high temperature CFL activation in the first step and lower temperature hydrogenation in the second step, the effect of both temperature and H2 partial pressure on the Gibbs free energy change is utilized. From a practical point of view, the temperature cycle has an undesirable energy flow. The exothermic low temperature hydrogenation step cannot provide heat for the endothermic high temperature activation step. Using a pressure cycle in which both the activation and hydrogenation steps are carried out at the same temperature does not have an unfavorable energy flow. In the pressure cycle two-step homologation of CFL, removal of the H2 produced in the activation step and provision of 19 high partial pressure of H2 in the hydrogenation step, provides the driving force to overcome the Gibbs free energy change barrier of direct homologation of CFL,. 2.2.2. Methane Homologation Using a Pressure Cycle Instead of working in a temperature cycle, Belgued and co-workers (Belgued et al., 1992; Amariglio et al., 1994; Belgued et al., 1996a, 1996b, 1996c) performed the two-step reaction isothermaffy in a pressure cycle. CFL was activated on Ru and Pt supported catalysts producing adsorbed CEL. fragments and H2. Subsequently, the CEL. fragments were hydrogenated in pure H2 flow to produce C2+ hydrocarbons. Belgued and co-workers (Belgued et al., 1992; Amariglio et al.; 1994) reported the production of higher hydrocarbons from CFL over EUROPT-1 (a standard Pt-Al203 catalyst in European catalysis laboratories) by using a two-step reaction sequence. In the first step, 100 mg of EUROPT-1 was exposed to pure CEL at a flow of400 ml/rnin and 250 °C. They observed C2EL evolution with a selectivity of 63.5%, which was equivalent to 40% of the amount of C2EL formed in the reaction of 2CH4 -> C2H6 + H2 at equihbrium. The total CEL conversion was 19.3% and this increased to 29% when the process was conducted at 200 °C, but the corresponding amount of higher hydrocarbons produced was lower since less CEL was adsorbed during the activation. The key factor of success in their experiments was the continuous removal of H2 by the flow of CEL during activation. These results were compared with those obtained on Ru and Co at low temperature (250 °C) (Belgued et al., 1992). On the basis of adsorbed CEL, the yields of C2+ hydrocarbons amounted to 19.3% on Pt at 250 °C, to 36.9% on Ru at 120 °C, but only 7.5% on Co at 270 20 °C. The distribution shifted toward products of higher molecular weight hydrocarbons at higher temperatures on Pt, but Co and Ru displayed an opposite trend. Strikingly, on Ru the pentanes were the most abundant products at 120 °C. The conversion of the adsorbed CFL, and the distribution of the products were strongly influenced by the flow rate of CFL during the activation step. The product distribution was significantly affected by the flow of H2, that is, at 300 °C with a H2 flow rate of 50 ml/min, the C2 selectivity on the Co catalyst was 85%, but at a H2 flow rate of 300 ml/min, the C2 selectivity decreased to 66%, but selectivity to C3 and C4 products increased. Considerable improvement in selectivity was achieved in the work of Pareja et al. (1994). They established that during CFL adsorption on EUROPT-1, to increase the conversion of CFL, the produced H2 had to be removed. Therefore a hydrogen trap containing 5 wt% Pd on A1203 operating at room temperature was attached to the adsorption loop and during CFL activation H2 was removed by the Pd trap. Using this device the conversion of CFL to higher hydrocarbons increased to about 40% at 250 °C , and the distribution of C2+ products shifted toward higher hydrocarbons when the exposure time increased from 3 to 16 min. In a more recent work, Belgued and co-workers (1996a, 1996b) showed that at atmospheric pressure, CFL was activated on EUROPT-1 catalyst (6.3 wt% Pt-Si02) with parallel evolution of H2 at temperatures above 150 °C. C2FL was also produced at an order of magnitude lower rate, so that H deficient hydrocarbon fragments built up on the metal surface. Temperature programmed desorption (TPD) after CFL activation showed that CFL was desorbed in two peaks, one at about 60 °C and the other between 180 and 250 °C. TPD was 21 interrupted at 300 °C. During subsequent temperature programmed surface reaction with H2, formation of CH, was accompanied by small amounts of C2Hs and traces of C3H8. They reported that the amount of chemisorbed CFL, increased and the H content (i.e. the value of x) of surface CHX species decreased when the temperature or the duration of exposure was increased. When a CIL/Ar mixture was used instead of pure CFL,, the increase of the CFL, content of the feed gas had an effect similar to that of an increase of either the temperature or the duration of exposure. In this study, out of 16 mmol of CFL, fed to the reactor, only 1.8 to 12.6 pmol was reacted on the 21 pimol available Pt atoms. Hydrogenation of the adspecies formed from CH, chemisorption on EUROPT-1 produced a mixture of alkanes ranging from Ci to Cg. They reported that the C2+ selectivity reached a maximum at a temperature of 250 to 260 °C, with a maximum C2+ yield occurring at about 210 °C. Increasing the duration of exposure to CEL, in the activation step at 250 °C, increased the C2+ production for exposure times of less than 1 min. Exposure to static CEL, at 250 °C resulted in negligible CHU activation, while increasing the flow rate to <400 ml/min increased both the CFL, consumption and C2+ production. The effect of increasing CH, content in a CHL/He feed in the activation step at 250 °C had a similar effect as the CH, flow rate. The effect of partial pressure of H2 in the H2/Ar mixture, which was used in the TPSR step, was also similar to the effect of flow rate in the activation step. They showed that with all other factors fixed, there existed an optimum temperature for the two-step isothermal homologation of CH, which resulted in maximum C2+ production. All the results were interpreted by assuming that C-C bond formation takes place between H-deficient CHX fragments during the CH, chemisorption step. In the second step, H2 saturates the alkane precursors and removes them from the surface. 22 From a conceptual point of view, the driving force for this isothermal homologation cycle in the absence of 02 is the energy which would be required to compress the necessary quantity of H2 from low partial pressure, at which it is produced in the first step, up to the operating pressure of the second step in order to make it able to remove alkanes from the Pt surface. The two-step cycles used by Belgued et al. (pressure cycle) and van Santen and co workers (temperature cycle), are similar since in both cases CFL forms an intermediate carbonaceous product on the catalyst which is subsequently hydrogenated to produce hydrocarbons. The main difference in the two approaches is in the hydrogen content of the carbonaceous species generated by the isothermal two-step cycle due to the lower temperature of the CFL activation step. Belgued argued that since C-C bond formation occurred during the activation step, a lower activation temperature decreased the possibility of hydrogenolysis reactions, leading to higher selectivity in the isothermal process. In a similar study Belgued et al. (1996c) used a 4.7% Ru-Si02 catalyst instead of EUROPT-1 Pt-Si02 for the two-step reactions. The main contribution from this study constituted extension of the results obtained with Pt to Ru. The possibility of CFL adsorption at lower temperatures, no release of C2H6 during CFL adsorption, lower x value of CHX surface fragments and stronger contribution of hydrogenolysis during the hydrogenation step were the main differences displayed by Ru. In the pressure cycle two-step homologation of CFL, both the first step activation and the second step hydrogenation are performed at the same temperature of 200-300 °C. At this lower temperature of activation (compared to about 450 °C in the temperature cycle) the CFL activation is slow and a very small amount of carbon species is deposited on the catalyst. Low 23 temperature activation generates more active carbon species and more C-C bond formation occurs. The temperature of the hydrogenation step in the pressure cycle (200-300 °C) is higher than that of the temperature cycle (about 100 °C). Consequently the possibility of hydrogenolysis reactions that break the C-C bonds is higher in the pressure cycle case. In both the temperature and pressure cycles for two-step homologation of CEL,, active carbon species are generated on the catalyst. Determination of the chemical nature of these species can enhance our understanding of the controlling mechanisms of the process. 2.3. Carbonaceous Deposits on Group VTH Metal Catalysts Ln the first step of the two-step cycle, H2 and surface carbonaceous species are produced by catalytic activation of CEL,. A temperature programmed surface reaction (TPSR) in which carbon species are removed by reaction with H2 can differentiate the carbon species based on their reactivity with H2. TPSR is a kinetic method of characterizing the reactivity of the deposited carbon species. The chemical nature of the deposited carbon species is often determined by surface science techniques. Goodman and co-workers (Lenz-Solomun et al., 1994; Wu et al. 1994) used an ultra high vacuum (UEIV) system to study the surface intermediates and to elucidate the reaction mechanisms of the two-step CEL, activation and hydrogenation reactions on single crystals of Ru. They studied C2EL; and C3EL3 formation from CEL, using combined elevated pressure kinetic measurements and surface analytical techniques. They acquired kinetic data in a combined elevated pressure reactor and UETV surface analysis chamber equipped with Auger electron spectroscopy (AES) and temperature programmed desorption (TPD). They employed 24 a separate UHV system containing high resolution electron energy loss spectroscopy (HREELS) and elevated pressure reactors to characterize various forms of surface hydrocarbon species from CEL. activation reactions. Based on HREELS results of UHV experiments they identified two species, methylidyne (CH) and vinylidene (CCH2 ) at temperatures between 130 and 430 °C. They identified another species, ethylidyne (CCH3) below 130 °C. In the elevated pressure kinetic studies, they measured turnover frequency (moles of hydrocarbon produced per mole of catalyst surface atoms per unit time) for C2IL; and C3H-8 products on single crystals as a function of activation temperature. By subsequent hydrogenation at 102 °C, they detected, CH4, C2H<;, C3H8 and C4H10. The turnover frequencies for C2FL and C3Hg products increased with increase in activation temperature and then decreased at temperatures exceeding 230 °C. At an activation temperature of 520 °C, C2H6 and C3H8 production was negligible. It was observed that the surface concentration of methylidyne species first increased as activation temperature increased from 130 to 230 °C and then reached a plateau between 230 and 330 °C. At temperatures exceeding 330 °C the CH concentration decreased. The vinylidene species was found to be present in a narrow temperature range between 192 and 277 °C. From the comparison of the product turnover frequencies and the HREELS results at different temperatures, they concluded that the CCH2 species was likely the key precursor to C2H6 and C3H8. The possibility of methyhdyne polymerization to vinyUdyne intermediates and then hydrogenation to form C2H;, was not rejected. Based on these findings they proposed the reaction sequence shown in Figure 2.3. 25 Step 1 activation (130-330 °C) Step 2 hydrogenation (100°C) Figure 2.3. A probable reaction sequence for activation of CEL (step 1) and hydrogenation surface species (step 2) proposed by Lenz-Solomun et al. (1994) and Wu et al. (1994). 26 In the first step of the reaction, CH4 is activated on the surface to produce the methylidyne and vinylidene species at temperatures between 130 and 330 °C. Subsequent polymerization of the CH species to the vinylidene intermediates competes with direct hydrogenation to CH4. C2H5 is formed from the hydrogenation of vinylidene at about 100 °C. Wu et al. (1994) compared the results of single crystal studies with the supported metal catalyst experiments of van Santen and co-workers (Koerts et al., 1992a). There was no strong one-to-one correlation between the detected surface intermediates and Ca, Cp and Cr species that were considered to be carbidic, amorphous and graphitic carbons, respectively. However, the results from the supported catalyst experiments revealed that the Ca and Cp were associated with H. The average number of hydrogens contained in the surface carbon species following CH4 activation was near one in the 280-530 °C activation temperature range. This result agreed with the single crystal studies in which methylidyne species were found to be dominant at activation temperatures above 270 °C. In addition, the results from the supported Ru catalysts indicated that higher hydrocarbon production was favored when the surface species were dehydrogenated. The optimum average number of hydrogens contained in the surface species was found to be near one, in agreement with single crystal studies that represented the surface species as CH and CCH2 (Wu et al., 1994). In an extension of the studies of CHt homologation on single crystal Ru, Goodman and co-workers (Koranne et al., 1995) examined the performance of supported Ru-Si02 catalysts, hi these studies, CHt was activated on 3 wt% Ru-Si02 catalyst at temperatures between 130 and 530 °C to produce surface carbon species, followed by hydrogenation of these species to higher hydrocarbons at 95 °C. It was found that the Ru-Si02 supported 27 catalyst exhibited a trend similar to that of single crystal Ru. However, the temperature at which a maximum in C2H5 selectivity occurred shifted toward higher temperature. Shen and Ichikawa (1997) used transmission IR to compare a Co4(Co)i2 cluster catalyst with supported 10% Co-Si02 catalysts in the two-step homologation of CEL. In this study, an IR wafer of [Cox]red-NaY (reduced form of the Co4(Co)i2-NaY catalyst) was briefly exposed to a flow of 20 ml/min of 5% CFL/He at high temperature, quickly cooled to 25 °C and then in situ IR spectra were recorded. The weak and medium bands observed at 2960, 2880, 1520 and 1393 cm"1 were attributed to CH» dissociation products. Comparison of these IR bands with vibrational bands of hydrocarbon species suggested that the 2960 and 2880 cm"1 bands were due to C-H stretching of CH and CH2 adspecies and the 1520 and 1393 cm'1 bands arose from the deformation vibrations of CH2 adspecies. From a comparison of the band intensity due to activation in the temperature range of 200 to 400 °C, it was concluded that the dissociated fragments increased with increasing activation temperature. By admission of H2 into the IR cell with heating, the bands of CHX adspecies disappeared and new bands at 3024 and 1310 cm"1 appeared, an indication of physisorbed CEL. The dissociation of CD4 studied in the same manner as CFL generated new bands due to CDX adspecies. The trends of changes in band intensity observed with CD4 were the same as with CFL. However, with the same procedure of activation on a Co-Si02 catalyst, no carbon species was detected by DC After activation of CH4 on [Cox]red-NaY catalyst, the carbon species were treated with D2 at 200 °C and the product distribution was analyzed by mass spectroscopy. The distribution of reaction products showed that the carbon species in the form of CH2, CH and carbidic carbon were present on [Cox]refj-NaY. The same experimental method with Co-Si02 28 catalyst showed that only CH and carbidic carbon were present. The presence of CHX on the [Cox]red-NaY catalyst was essential for production of high molecular weight products of hydrogenation of [Cox]red-NaY compared to Co-Si02. 2.4. Modifications of the Catalyst and Reactor for the Two-Step Cycle The effect of various parameters of the catalyst, process and reactor configuration on the performance of the two-step cycle for CH, homologation, has also been studied by different research groups. 2.4.1. Effect of the Catalyst Support on the Two-Step Cycle Solymosi and Cserenyi (1994) activated CFLt on Ir supported on Ti02, A1203, Si02, MgO and ZSM-5 and subsequently hydrogenated the produced carbon species. Ir-MgO proved to be the most effective catalyst for CH, activation. Ir-Al203 catalyst produced the highest C2+ selectivity upon hydrogenation. This study proved the significance of the support in the performance of the catalyst metal for both CH, activation and hydrogenation. Infra-red (LR) study of the activation of CH, identified CH3 as a reaction intermediate. La another study Solymosi et al. (1994b) used 5 wt% Pd supported on Ti02, A1203, Si02 and MgO for CH, activation and subsequent hydrogenation of the carbon species. This study identified Pd-Ti02 as the most effective catalyst for the CH, activation reaction, whereas a large amount of C2H§ was produced by Pd-Si02. 29 2.4.2. Effect of a Catalyst Promoter on the Two-Step Cycle Smith and co-workers (Boskovic et al., 1996, 1997; Boskovic and Smith, 1997) studied the activation of CHU at 450 °C and hydrogenation of the resulting carbon species at 100 °C with subsequent TPSR characterization of the unreacted carbon species. This study reported that 10 wt% Co-Al203 catalyst was more active than Co-Si02 catalyst for the CHU activation reaction, while Co-Si02 proved to be more effective in the hydrogenation of carbon species to higher hydrocarbons. Addition of 1% K promoter to the Co supported on both A1203 and Si02 catalysts increased the CH» activation on both the catalysts, while Cu promoter increased the activity of Co-Si02 and decreased the activity of Co-Al203. K promoter increased the C2+ selectivity of the Co-Si02 catalyst from 14% to 36%, while it had a very small effect on the selectivity of Co-Al203 catalyst. Cu promoter decreased the C2+ selectivity of both the catalysts. Table 2.2 simrmarizes the effect of K and Cu promoters on the Co-Al203 and Co-Si02 catalysts (Boskovic et al., 1997). Table 2.2. Effect of K and Cu promoter on Co catalysts (Data from Boskovic et al., 1997). Catalyst a CH4 c2+ c2+ mol CHd/mol Co fimol/g cat jjmol/g cat Catom% Co-Si02 1.15 7.57 0.63 14.2 K-Co-Si02 1.33 1.55 0.44 36.0 Cu-Co-Si02 1.31 3.79 0.23 10.8 Co-Al203 1.92 4.11 0.25 10.8 K-Co-Al203 2.88 3.16 0.20 11.3 Cu-Co-Al203 1.84 3.28 0.09 5.0 30 2.4.3. Bimetallic Catalysts for the Two-Step Cycle Guczi et al.( 1996b) reported the results of a study of CH4 activation and coupling of CHX species formed from CFL, into higher hydrocarbons over NaY, Pt-NaY, Co-NaY, Co-Pt-NaY and Co-Pt-Al203 catalysts. Co-Pt-NaY and Co-Pt-Al203 showed high yields, referred to the adsorbed CHX species, and high selectivity for the formation of C2+ hydrocarbons (84 and 92% respectively) at 250 °C activation and hydrogenation conditions. However, the amount of CHX after activation was four times higher on Co-Pt-NaY than Co-Pt-Al203. The synergistic effect was interpreted as insertion of Co into Pt inside the zeolite cages which caused a preferred coupling of CHX versus its hydrogenation to CEL. Experiments carried out for the removal of CHX species with D2 showed that deep dissociation of CFL did not occur on bimetallic catalysts and weakly bound CHX species easily participated in a chain building reaction on the surface. Dissociative chemisorption of CEL over Ru, Co and Ru-Co bimetallic catalysts supported on A1203, Si02 and NaY was investigated in the temperature range of 150 to 450 °C (Guczi et al., 1997). Based on this study it was reported that the amount of chemisorbed CEL and the evolution of H2 during CEL activation increased with temperature and followed the sequence of reducibility of the supported metals and the particle size, which in turn depends on the support and the alloy formed. CH species prevailed on A1203 and Si02 supported catalysts while on NaY supported metals, CH2 species were dominant when the metal particles were stabilized inside the super cage. 31 2.4.4. Molecular Sieve Catalysts for the Two-Step Cycle Shen and Ichikawa (1997) compared the reduced Co4(CO).2-NaY and 10% Co-Si02 catalysts in the two-step cycle. Pulses of CFLt/He were used to activate CFL, at 450 °C and the generated carbon species were hydrogenated in TPSR in the temperature range of 27 to 400 °C. The result of hydrogenation after 20 pmol CFL, activation on the two catalysts in terms of product selectivity and the temperature for optimum selectivity of each product, is shown in Table 2.3 (Shen and Ichikawa, 1997). Table 2.3. Kinetic parameters of hydrogenation step on [Cox]red-NaY and Co-Si02. [Cox]red-NaY Co-Si02 hydrocarbon optimum temp, °C selectivity, % optimum temp, °C selectivity, % c2 60 2.4 270 3.7 c3 210 17.7 270 6.6 c4 150 11.7 -c5 180 46.9 -[Co]x-NaY showed a high activity in the CFL, activation step. TSPR experiments proved that [Co]x-NaY produced higher hydrocarbons at lower temperatures compared to the Co-Si02 catalyst. Based on the LR studies, CH2 species were detected on the encapsulated Co catalysts which were believed to be responsible for propagation of C-C bond formation in the [Co]x-NaY catalysts. 32 2.4.5. Membrane Reactors for the Two-Step Cycle Gamier et al. (1997) used a 5% Ru-Al203 catalyst in a Pd-Ag membrane reactor for activation of CH4 at 450 °C for a 2.4% mixture of CH4 in a 3 min pulse. Upon hydrogenation of the carbon species to form higher hydrocarbons it was reported that the use of the Pd-Ag membrane reactor significantly enhanced the CFL, conversion into H2 and carbon species in the first step and the yield of hydrocarbons in the second step. 2.5. Summary In the two-step CFL, homologation, H2 and carbon species are generated in the activation step. Subsequent hydrogenation of the carbon species produces CFL, and C2+ hydrocarbons. Although no clear understanding of the effect of metal, support and promoter has been attained, transition metal catalysts have proved effective in the two-step cycle. In a temperature cycle two-step sequence, Co-Si02 catalyst has produced a high C2+ selectivity. Because of the small amount of carbon deposited in the activation step, a maximum C2+ yield per site of about 1% has been reported. Because of better C2+ selectivity, Co-Si02 catalysts will be used in the present work. Since a temperature cycle in a flow system uses the effect of both the temperature and pressure on the Gibbs free energy, a flow system based on temperature cycle is used in this work. To increase the C2+ yield per site, more than a nominal monolayer of carbon will be deposited on the catalyst by a 2 min activation of 5% CLL/Ar at 450 °C. 33 Chapter 3 Experimental Methods 3.1. Overview In this chapter the details of the experimental methods used in the present study for catalyst preparation, catalyst characterization and CH4 homologation by the two-step cycle are presented. In section 3.2 the preparation method of two different SKV supported Co catalysts is discussed. The two catalysts, designated as A series Co-Si02 and B series Co-Si02, are referred to as A-X%Co-Si02 and B-X%Co-Si02, where X% is the nominal weight percent of Co in the catalyst. In section 3.3 the experimental set-ups are discussed. Section 3.3.1 gives the details of the kinetic apparatus used for some of the catalyst characterization and all of the two-step homologation reactions. Section 3.3.2 gives the details of the diffuse reflectance infra-red Fourier transform spectrometer (DRIFTS) that was used to study the catalyst and deposited carbon species during the two-step cycle reactions. In section 3.4 the details of the catalyst characterization methods are given. Catalysts were characterized by a sequence of temperature programmed reduction (TPR), H2 desorption and Co re-oxidation. TPR in H2 was used to study the reduction characteristics of the calcined Co-Si02 catalysts and to estimate the extent of reduction of the cobalt oxide to Co. H2 desorption experiments were used to determine the number of surface Co atoms available for H2 chemisorption, and this was taken as the number of active metal sites of the 34 catalyst. In the Co re-oxidation experiments, the amount of reduced Co atoms was deterinined by re-oxidation of the reduced Co catalyst. Catalyst characterization experiments by TPR, H2 desorption and Co re-oxidation were performed using the kinetic apparatus. For further characterization of the catalysts (BET surface area, BET pore volume and powder X-ray diffraction) standard procedures were followed using commercial equipment. The experimental methods for the two-step homologation of CFL, are discussed in section 3.5. In a complete sequence of experiments, the reduced Co-Si02 catalysts were used for activation of CFL (section 3.5.1), isothermal hydrogenation of the deposited carbon species (section 3.5.2), temperature programmed surface reaction (section 3.5.3) and temperature programmed oxidation (section 3.5.4). The sequence of the four experiments was performed using the kinetic apparatus. Diffuse reflectance infra-red Fourier transform spectroscopy (DRIFTS) was used to study the catalyst during the reaction cycle and to characterize the carbon species generated by activation of CFL on the Co-Si02 catalysts. The DRIFTS study experimental method is presented in section 3.6. 3.2. Catalyst Preparation The cobalt catalysts were prepared by incipient wetness impregnation of the silica support (Silica geL grade 62, 60-200 mesh, 15A, Aldrich 24398-1) using an aqueous solution (de-ionized water) of Co(N03)2.6H20 (98+%, Aldrich 23926-7). For catalyst series A, silica gel was used as the support following calcination in air at 500°C for 25 hours. After impregnation of the support, the catalysts were dried under 98.6 kPa vacuum for 37 hours at 35 110 °C and then calcined in air for 10 minutes at 450 °C. Series B catalysts were prepared similarly except that there was no high temperature calcination step and the drying time was extended to at least 10 days in vacuum. The catalysts were reduced using the temperature-programmed reduction procedure. Similar preparation procedures using extended drying times and short calcination times have been shown to provide improved Co dispersions on Si02 supports (Ho et al., 1990; Coulter and Sault, 1995). 3.3. Experimental Set-Up An experimental set up (referred to as the kinetic apparatus) was used for the two-step cycle kinetic studies and some of the catalyst characterization experiments. In addition a diffuse reflectance Fourier transform infra-red spectrometer (DRIFTS) was used to study the catalyst during the two-step cycle and to characterize the carbon species generated after activation of CH, on the Co-Si02 catalyst. 3.3.1. Kinetic Apparatus A flow diagram of the reactor and the on-line analytical equipment used in the present study is shown in Figure 3.1. The reactor feed gas flow rate was controlled by calibrated Brooks 5878 mass flow controllers and two Whitey flow selector valves were used to introduce different gas mixtures to the reactor feed. The calibration data of the mass flow controllers are given in Appendix 1.1. 1/8" (3.2 mm) gas flow lines with Swagelok connections were used in the experimental system 36 to DRIFTS from DRIFTS DRIFTS: diffuse reflectance Fourier transform infra-red spectrometer GC: gas chromatograph rVS: injection valve system QMS: quadrupole mass spectrometer TCD: thermal conductivity detector Figure 3.1. Block diagram of the experimental set-up. 37 Air actuated valves with known-volume sample loops (0.1 and 1 ml) installed in a 5710A Hewlett Packard Gas Chromatograph were used for injection of doses of gas into the reactor feed stream (injection valve section in Figure 3.1). A connection was provided to direct the feed gas stream to the reaction cell of the DRIFTS set up. The 20 cm long fixed-bed differential micro-reactor was made from quartz glass (id. = 4 mm) and included an in-bed thermowell and fused silica catalyst bed support. The catalyst particles (0.074 to 0.27 mm with average size 0.17 mm) were placed on the support and held in place with glass wool. The total volume of the reactor and the tubing system between the two by-pass valves was about 5 ml. The gas flowrate and catalyst particle size were chosen to eliminate any external or internal mass transfer effects. The reactor was placed in a Hoskins cylindrical furnace and the reactor temperature was controlled by an Omega linear temperature-programmable controller. Rapid cooling of the reactor was achieved by lowering the furnace from the reactor. Product analysis was achieved in a variety of ways, depending on the type of experiment being performed. Hydrogen consumption during catalyst reduction was determined using the thermal conductivity detector (TCD) of the 5710A Hewlett Packard Gas Chromatograph. In this case the reactor feed gas was passed through the reference side of the TCD prior to entering the reactor, while the reactor effluent was passed through the sample side of the same TCD. A Spectramass DAQ100/DXM quadrupole mass spectrometer (QMS) was used to continuously monitor the reaction products and a Varian 3400CX gas chromatograh (GC), equipped with a 10 loop Valco sampling valve, was used to calibrate the QMS and store samples during the short reaction time for subsequent analysis. 38 The Labtech Notebook® and Model PCL-718 Pc-Lab Card (Advantech) computer interface were used for data logging and recording. Reactor temperature, TCD response and QMS mass number recordings were logged by the computer. The gas analysis section could also be used to analyze the products from the FTTH reaction cell. 3.3.2. Diffuse Reflectance IR Spectroscopy FT-IR diffuse reflectance spectroscopy (DRIFTS) was performed with a Bio-Rad FTS 175 spectrometer. The spectrometer is equipped with a tungsten-halogen infra-red source, an extended range KBr beamsplitter and a liquid nitrogen-cooled mercury-cadmium-telluride (MCT) detector. A Harrick "praying mantis" diffuse reflectance accessory and an FTVC-DR3 controlled atmosphere high temperature, high pressure reaction cell were used for studying the catalyst under treatment and reaction cycle experimental conditions. A schematic diagram of the diffuse reflectance attachment and the reaction cell is shown in Figure 3.2. The reaction cell was equipped with a thermocouple and a heater which was controlled by a Watlow Series 999 control unit. Figure 3.3 shows a schematic diagram of the DRIFTS set-up. The gas niixture from the kinetic apparatus was diverted to the DRIFTS cell with a Nupro Bellows sealed open-close valve and a Nupro control valve. A sensitive pressure gauge which was connected to the reaction cell was used to indicate the pressure and vacuum level in the reaction cell. The reaction products then passed through a 15p:m Swagelok filter. The reaction cell could be evacuated with an Edwards 1.5 vacuum pump via a Nupro Bellows sealed open-close valve and a Nupro control valve. 39 The High Pressure Reaction Chamber (HVC-DR3). Figure 3.2. DRIFTS reaction cell. 40 Vacuum Pump QMS Gas flow system QMS: quadrupole mass spectrometer DRIFTS: diffuse reflectance Fourier transform infra-red spectrometer P: pressure gauge Figure 3.3. Block diagram of the DRLFTS reaction cell connections. 41 The effluent from the DRIFTS cell was connected to the GC and QMS in the analysis section of the kinetic apparatus with a Nupro Bellows sealed open-close valve. 3.4. Catalyst Characterization To determine the turnover frequency (TOF) of a reaction on a catalyst it is essential to quantify the amount of active catalyst metal available for the reaction in a given amount of catalyst. Catalyst characterization is also important to quantify the effects of various treatments on the catalyst. 3.4.1. Temperature Programmed Reduction Temperature programmed reduction (TPR) is used to monitor the reduction characteristics of the calcined catalysts. The shape of a TPR profile, the number of peaks, their corresponding temperature and area are useful information which can be extracted from a TPR experiment. TPR profiles were recorded by placing 0.5 g of the calcined catalyst in the reactor and increasing the temperature from 30 °C to 450 °C at a ramp rate of 10 °C/min while flowing 60 ml/min of a 20% H2/80% Ar feed gas mixture. UHP grade (Praxair) gases were used for the TPR experiments (without further treatment). During the TPR measurement, the effluent gas was passed through a dry ice trap prior to entering the sample side of the TCD. The TCD response, which was proportional to the H2 consumption, was recorded by data acquisition software. The H2 consumption profile was integrated and calibrated against the H2 consumption of a CuO (99.9%) standard during TPR. Using the stoichiometry of reduction 42 of C03O4 to Co (equation 3.1), the extent of reduction of the calcined catalysts could be calculated (Rosynek and Polansky, 1991). CO3OA + 4H2 -> 3Co + 4H20 (3.1) 3.4.2. H2 Desorption H2 chemisorbs dissociath/ely on the reduced Co with a Co/H stoichiometric ratio of 1/1 (Reuel and Bartholomew, 1984). The chemisorbed H desorbs as H2 at high temperature. Since only surface Co is available for H2 chemisorption, the number of moles of surface Co is determined by quantifying the amount of the desorbed H2. Catalyst dispersion (O) is the ratio of the number of moles of surface Co to the total number of moles of reduced Co. Co dispersions were measured by a H2 desorption method. Following TPR, the catalyst was cooled to 30 °C in 20 ml/min of H2, and then purged with 15 ml/min of Ar at 30 °C for 30 min. The catalyst was then heated to 450 °C within 3 minutes in 15 ml/min of Ar, and the H2 desorption was monitored with the TCD. The H2 desorption profile was calibrated against 0.1 ml pulses of H2 under the same flow conditions. Assuming a Co/H stoichiometric ratio of 1/1, the number of moles of surface Co atoms, and hence the catalyst dispersion, were calculated. 3.4.3. Co Re-Oxidation The reduced Co can be oxidized with 02 at 400 °C (Reuel and Bartholomew, 1984). By measuring the number of moles of 02 consumed for Co oxidation, the number of moles of the reduced Co is calculated. Comparing the number of moles of the reduced Co to that of the 43 nominal Co loading of the catalyst, the degree of reduction of the Co catalyst after reduction in H2 was determined. After fast desorption of H2, the catalyst was purged for 15 minutes at 400 °C in 15 ml/min of He. Subsequently, the extent of catalyst reduction was measured by injecting pulses of UHP grade 02 (1 ml) into the He (UHP grade, Praxair ) stream and monitoring the effluent with the TCD until no further 02 uptake was observed. Assuming that during this process all the reduced Co was re-oxidized (equation 3.2) to Co304 at 400 °C, the degree of reduction of the catalyst was calculated. 3.4.4. Powder X-Ray Diffraction Powder X-ray diffraction (PXRD) is a non-destructive, bulk sensitive characterization method to identify the type of ciystalline phases in a catalyst and to estimate the size of the crystals. For nominal Co loadings above 5 wt%, PXRD patterns of both the calcined and reduced catalysts were recorded with a Siemens D5000 powder (iiffractometer. With power settings of 40 kV and 30 mA a CuKa (X=1.5406 A) radiation source was used for PXRD. The powder samples were loaded into the powder sample holder without any treatment. The size of both the Co304 and Co crystallites were estimated from Scherrer's peak broadening equation: 3Co + 202 ->Co304 (3.2) (3.3) t = ficosd 44 where: j3 = ylw2 -w2 t = Crystal size (A) X = Radiation wavelength (1.5406 A) K = Scherrer constant (=0.9) 6 = Peak position (radians) W = Peak width at half height (radians) w = Instrumental peak broadening (radians) = 0.004 radians For the case where XRD of the reduced catalysts was not performed, the C03O4 particle size was converted to the corresponding Co metal particle size using the equation dco = 0.75da,3O4, which is based on a comparison of the molar volumes of Co and Co304 (Schanke et al., 1995). Assuming cubic particles of Co on the support, catalyst dispersion (O) was found from the particle size (t) using the approximate equation <!>(%) = —-—. t(nm) 3.4.5. BET Surface Area Based on physisorption of gases on porous surfaces, the BET (Brunauer-Emmett-Teller) method (LoweL 1979; Uer, 1979) is used to determine the surface area and pore volume of the catalysts. Brunauer-Emmett-Teller (Brunauer, et al., 1938) developed an adsorption isotherm which has been used to determine specific surface area from physisorption of gases on solid surfaces. The BET equation is of the form: 1 1 C-l P (3.4) + vmc VmC P0 45 in which: P = Adsorption pressure (Pa) Po= Saturation pressure of the adsorbate at the adsorption temperature (Pa) Va= Volume of adsorbate gas adsorbed per gram of adsorbent solid at STP (ml/g) Vm = Monolayer capacity of the adsorbent (ml/g) C = Constant, depending on the temperature, adsorbate and adsorbent surface By plotting [V^Po/P-l)]'1 versus P/P0 a straight line is obtained with the slope of (C-l)/(VraC) and intercept of l/(VmC) from which the value of Vm can be obtained. The specific surface V N a area is obtained from SRPT = ——— in which: BET 22414 SBET = BET specific surface area of the adsorbent solid (m2/g) No = Avogadro's number (=6.02 x 1023 molecule/mole) a = Molecular cross section of the adsorbate gas (=0.162 mn2/molecule of N2) Assuming that at P/P0=0.95 the pores of the porous solid are filled with the liquid adsorbate, the pore volume of the porous solid (VBET) can be calculated by determining the liquid volume of the corresponding adsorbed gas volume at P/P0=0.95. Assuming a cylindrical pore structure allows the average pore diameter to be estimated V by d = 4 —^ in which: "BET VBET = Pore volume (cm3/g) SBET = BET surface area (cm2/g) d = Cylindrical pore diameter (cm) 46 BET surface area and pore volume of the support before impregnation and catalysts with different metal loadings were determined using a Micromeritics ASAP 2010 system For each analysis about 0.2 g sample was used for N2 adsorption at liquid nitrogen saturation temperature. The details of the experimental conditions are reported in Appendix 1.2. 3.5. Two-Step Reaction Cycles Following TPR the reduced catalyst was used for two-step reaction cycles. In the first step of a complete cycle, CEL, was activated on the reduced catalyst to deposit carbonaceous surface species on the catalyst. In the second step of the cycle, the carbon species were hydrogenated isothermally to produce higher hydrocarbons. Subsequently, a temperature programmed surface reaction and a temperature programmed oxidation were used to remove the unreacted carbon species from the catalyst. In all of the four steps of the complete run experiments, transport resistances and diffusion effects (mass transfer or heat transfer) were insignificant. Residence time distribution experiments showed that the flow pattern in the reactor was nearly ideal with an average residence time of about 0.1 min in the activation step and about 0.5 min in the isothermal hydrogenation, TPSR and TPO steps. 3.5.1. Methane Decomposition Heterogeneous reaction of CFLt on Si02- supported Co catalysts produces H2 and generates carbon species on the catalyst. Activation of CHt on the catalyst to produce surface carbon species is the first step of the two-step CHt homologation cycle. 47 Decomposition of CFL on the supported Co catalysts was performed in the same quartz, fixed-bed micro-reactor, with the same amount of catalyst as was used for the TPR and H2 desorption measurements. Following TPR, the reactor and catalyst were purged with 48 ml/min of Ar at 450 °C for 15 minutes. Once the temperature and flow had reached steady-state, the reduced catalyst was exposed to a 5% CFL/95% Ar gas mixture (premixed, analyzed Linde specialty gas) at a flow rate of 54 ml/min for periods of 2 to 7 minutes at 450 °C. During CFL activation, the reactor effluent was monitored continuously using the calibrated QMS. The catalyst was subsequently cooled to 100 °C in less than 30 s. The catalyst was then purged with 54 ml/min of Ar at 100 °C for 5 min. 3.5.2. Isothermal Hydrogenation The second step of the two-step CFL homologation cycle is isothermal hydrogenation of the surface carbon species to generate C2+ and CFL. After stabilizing the H2 flow and QMS, the catalyst was exposed to 11 ml/min of H2 at 100 °C for 10 min. The reaction products were monitored with the QMS and nine samples were taken with the GC multi-loop automatic sampling valve for subsequent analysis. A 5' (1.5 m) x 1/8" (3.2 mm) 60/80 Carbosive G packed column (Supelco Canada Ltd) was used for separation of CFL, C2FL, C2FL, C3IL; and C3Hg with 30 ml/min of UHP grade He carrier gas. A flame ionization detector (FID) was used for sample analysis. The GC was regularly calibrated with an analyzed calibration gas mixture (Praxair). The list of the GC control program for sampling and analysis is given in Appendix 1.3. 48 3.5.3. Temperature Programmed Surface Reaction Not all the surface carbon species are active enough to react with H2 at 100 °C during the isothermal hydrogenation. Using a temperature programmed surface reaction (TPSR) the amount and the activity of the less active carbon species are quantified. In TPSR the temperature of the reactor is increased linearly while H2 flows through the reactor. A TPSR in 11 ml/min of H2 followed the isothermal hydrogenation step. For TPSR, the temperature was increased from 30 °C to 700 °C at a ramp rate of 15 °C/min and the reactor effluent was monitored with the calibrated QMS. The QMS calibration and calculation procedures are reported in Appendix 1.4. 3.5.4. Temperature Programmed Oxidation Temperature programmed oxidation (TPO) was used to quantify the amount of any residual carbon species not removed in the TPSR. In TPO, the reactor temperature is increased linearly while 02 flows through the reactor. In TPO, (the last step of a complete reaction cycle) the catalyst was exposed to 11 ml/min of 02 (UHP, Praxair) flow while the reactor temperature was increase from 40 °C to 700 °C at a ramp rate of 15 °C/min. To determine the amount of C02 production the reactor product was monitored with QMS. 49 3.6. DRIFTS Experiments Infra-red techniques are very powerful to study the catalyst under actual reaction conditions. Unlike techniques such as X-ray photoelectron spectroscopy (XPS) and low-energy electron diffraction (LEED) spectroscopy, infra-red (IR) methods do not need an ultrahigh vacuum environment, which is an unrealistic operating condition for most catalytic reactions. Diffuse reflectance infra-red Fourier transform spectroscopy (DRIFTS) is based on the diffusely reflected radiation from the sample. Even though many strongly absorbing catalysts must be diluted with a nonabsorbing material such as KBr, the fact that pellet pressing is unnecessary for diffuse reflectance work has advantages. With DRIFTS, the catalysts are studied under more realistic reaction conditions. There is no need for difficult self-supporting disk preparation. In transmission infra-red , the KBr binder can inhibit diffusion of gases into the pellet and adsorption on the catalyst sites, whereas diffuse reflectance uses a physical mixture of catalyst and dispersant (usually KBr) in the form of a loose powder. To relate band intensity linearly to concentration in DRIFTS, the relative reflectance spectrum is converted using the Kubelca-Munk (K-M) equation (Blitz and Augustine, 1994). JK ' 2R S where: R' = Relative reflectance a = Absorptivity C = Concentration S = Scattering coefficient 50 The mid-LR range (400-4000 cm"1 wave number) DRIFTS is a useful method to monitor the changes in the reaction system during the reaction conditions close to the two-step cycle. Passivated Co-Si02 catalysts were used in DRIFTS experiments. To prepare the passivated catalyst, the reduced Co-Si02 catalyst (reduction by TPR) was slowly oxidized in a 50 ml/min of 1.5% 02/He flow at 30 °C for 30 min. For DRIFTS studies about 0.08 g of a mixture of passivated 12% Co-Si02 catalyst and KBr was loaded into the reaction cell. Spectra were collected with respect to a KBr background. The catalyst was reduced with 10 ml/min of H2 flow at 450 °C for 60 minutes. The reaction cell was purged with 10 ml/min of Ar and bypassed. Background spectra were recorded. 10 ml/min of a 5%CIL;/Ar was then admitted into the reaction cell at 450 °C for 7 min and spectra were recorded with respect to the dilute reduced catalyst background. The reaction cell was then evacuated for 30 min while cooling to 100 °C. 10 ml/min H2 was admitted to the reaction cell at 100 °C and spectra were again collected. While flowing 10 ml/min H2 through the reaction cell the temperature was increased stepwise from 100 °C to 400 °C with 100 °C increments and a temperature ramp of 100 °C/min holding at each temperature for 5 min and collecting spectra. DRIFTS data collection conditions are given in Appendix 1.5. 51 Chapter 4 Catalyst Characterization 4.1. Overview Catalyst characterization is an important step in any study of catalytic reactions (Delannay, 1984; Imelik and Vedrine, 1994). By characterizing the catalyst, the physico-chemical state of the catalyst active species may be determined and changes in the state of the catalyst during various treatments can be observed. Perhaps one of the most important results from a catalyst characterization study is determination of the number of active sites exposed to the reaction medium on the surface of the catalyst. The actual nature of the active sites responsible for any particular reaction may be different for different reactions (Somorjai, 1994). In practice, for supported metal catalysts, the number of moles of the active metal on the surface of the catalyst metal crystallite is usually taken as a measure of the number of active sites. By expressing the rate of reaction in terms of the number of active sites, instead of the catalyst weight or reactor volume, a reaction frequency is obtained. The reaction frequency (moles of reactant consumed per mole of catalyst surface atoms per unit time) with the dimension of (time1) is referred to as the turnover frequency (TOF). Expressing the reaction rate in terms of TOF facilitates comparison of reaction rates reported by different research groups with different catalysts. To express the reaction rate in terms of TOF it is essential to determine the number of catalyst metal atoms on the surface of metal crystallites and hence catalyst dispersion. 52 In the present work different techniques were used for catalyst characterization. Surface areas and pore volumes of catalysts with different metal loadings were measured using the BET method. Powder X-ray diffraction (PXRD) was used to identify the presence of different bulk cobalt phases in the catalysts after calcination and reduction. PXRD was also used to estimate the size of the cobalt oxide particles in the calcined catalyst and Co particles in the reduced catalyst. To study the reduction behavior of the calcined catalysts and to estimate the extent of reduction of the catalyst, a temperature programmed reduction (TPR) technique was used. The number of moles of surface Co atoms and hence the catalyst metal dispersion were quantified by H2 desorption. By re-oxidizing the reduced catalyst, the extent of reduction of catalysts was also determined. The catalyst was also studied under reaction conditions similar to the kinetic studies using diffuse reflectance Fourier transform infra-red spectroscopy (DRIFTS) experiments. DRIFTS was also used to study the carbon species after catalyst activation. In this chapter the findings from each characterization technique are reported and discussed. Since each characterization method provided a partial view of the catalyst properties, a conclusion section summarizes the overall catalyst characteristics. 4.2. Surface Area and Pore Volume BET surface area, pore volume and average pore diameter of the calcined Si02 catalyst support and the calcined catalysts with different metal loadings were measured by the BET method using N2 adsorption and desorption at the normal boiling point of N2 (-196 °C). 53 Figure 4.1. N2 adsorption on (0) and desorption from (A) Si02 as a function of relative pressure (P/Po). 54 In Figure 4.1, the amount of adsorbed and desorbed N2 is plotted as a function of the relative pressure (P/Po) at N2 normal saturation temperature of -196 °C measured on the Si02 support. N2 adsorption and desorption curves showed that the support had a type IV isotherm with meso-pore structure (Lowell, 1979). Figure 4.2 shows the BET surface area plot for the Si02 support. According to the BET method in the P/P0<0.2 range, a plot of rVa(Po/P-l)]"' versus (P/P0) is a straight line with slope of (C-l)/(C.Vm) and intercept of l/(C.Vm). The data of Figure 4.2 show a straight line. Using the slope and intercept of the best fit straight line, Vm and hence the specific surface area of the porous material were calculated. From repeat experiments, the BET surface area, pore volume and average pore diameter of the Si02 support were found to be 279.2 ± 0.9 m2/g, 1.05 ±0.01 ml/g and 15.1 ±0.1 nm respectively. These values compare well with the technical specifications of specific surface area, pore volume and average pore diameter of 300 m2/g, 1.15 ml/g and 14 nm provided by the silica gel supplier (Aldrich). Similar measurements were made on the series A Co-Si02 catalysts, and Table 4.1 reports their average pore diameters and pore size distributions. 55 0.0035 0 0.05 0.1 0.15 0.2 0.25 Relative Pressure (P/P0) ure 4.2. BET plot for N2 adsorption on Si02. 56 Table 4.1. Average pore diameter and pore size distribution for the support and A series Co-Si02 calcined catalysts. Catalyst Average Pore Dia. % Area of pores in (nm) Micro1 Meso2 Macro3 Si02 15.1 9.0 90.9 0.1 0.6% Co-Si02 14.5 11.8 88.0 0.2 2% Co-Si02 14.4 12.1 87.6 0.3 5% Co-Si02 14.3 12.3 87.5 0.2 12% Co-Si02 14.1 12.2 87.7 0.1 1 pores with diameter smaller than 20 A. 2 Pores with diameter in the range of 20 to 500 A. 3 Pores with diameter larger than 500 A. The data of Table 4.1 showed that by increasing the metal loading of the catalyst the average pore diameter decreased. This is possibly due to partial coverage of meso-pore walls with cobalt oxide since the pore size distribution of the support and catalysts were similar except for a small decrease in the percentage of meso-pores of the catalyst. 4.3. Bulk Phase Analysis Powder X-Ray Diffraction (PXRD) was used to determine the bulk phases and particle size of cobalt oxides in the calcined catalyst. The reduced catalysts were also examined by 57 PXRD to determine the presence of any cobalt oxide or cobalt silicate phase, and to estimate the reduced cobalt crystallite size. Figure 4.3 shows the PXRD diffractogram of the calcined A series 12% Co-Si02 catalyst. In Table 4.2 the major peak location and relative intensity of peaks of different compounds in the Co-Si02 system in the 20 range of 30° to 50° are reported (Data from ICDD, 1994). If one compares the peak location in Figure 4.3 with Table 4.2 it can be concluded that C03O4 was the only major cobalt oxide phase present in the calcined catalyst. This is in accordance with previous characterizations of Co-Si02 catalysts (Khodakov et al., 1997). Table 4.2. Peak 20 location and the relative peak intensity of different phases in the Co-Si02 system in the 20 range of 30-50° (ICDD, 1994). Crystalline phase Peak 2© location, degrees (Peak Intensity) Degree, (%) Co 47.6 (100), 44.8 (60), 41.7 (20) CoO 42.4 (100), 36.5 (67) C03O4 36.9 (100), 31.3 (34), 44.8(19) Co203 31.2(100), 38,6(100) Y- Co2Si04 36.6 (100), 44.5 (43), 48.8 (10) p- Co2Si04 36.5 (100), 44.5 (79), 33.9 (68), 33.1 (57) Co2Si04 36.9(100), 44.5 (44), 38.3 (13) Co(N03)2 40.5 (45), 42.2 (40), 29.5 (35) 58 eoo 30 31 32 33 34 35 36 37 38 39 40 41 42 43444546 47 434950 2Theta(Deg) Figure 4.3. PXRD diffractogram of the calcined A series 12%Co-Si02 catalyst. 59 PXRD patterns of the reduced catalysts showed only metallic Co, indicating reduction of C03O4 to Co. The absence of bulk cobalt oxide or cobalt silicate compounds does not completely exclude the possibility of their presence. As discussed in later sections of this chapter (Section 4.3, 4.4 and 4.5), other characterization techniques such as TPR, H2 desorption and Co re-oxidation showed that not all the cobalt oxide of the calcined catalyst was reduced by TPR. Reports in the literature (Ming and Baker, 1995; van Steen et al., 1996) indicate that for Co-Si02 catalysts, unreduced cobalt silicate compounds existed after reduction under the same conditions as used in this work. Since PXRD is a bulk sensitive technique for catalyst characterization, it cannot detect amorphous phases and its detection limit of crystalline phases is about 3 wt% (Wachs, 1992). Because of the limitations of PXRD, the presence of cobalt silicates or other metal-support interaction products cannot be ruled out. Using the line broadening equation (see Section 3.3.4), the crystallite particle size of C03O4 and Co particles in the calcined and reduced catalysts, and hence percent dispersion of the Co, was estimated. Comparison of the Co dispersion calculated by PXRD line broadening of the C03O4 in the calcined catalyst and Co in the reduced catalyst, showed that there was very good agreement between the two methods of measurement. For example, for B series 8% Co-Si02 catalyst, Co dispersion based on PXRD of the reduced catalyst was 7.3±1.0%, while Co dispersion based on the PXRD of the calcined catalyst was 8.8+1.0%. In Table 4.3 the catalyst Co dispersion, <f> (the ratio of the number of moles of surface Co atoms to the total number of moles of reduced Co in the catalyst) based on PXRD line broadening of C03O4 is reported for the A and B series Co-Si02 catalysts. 60 Because of the detection limitation of the PXRD the 2% and 0.6% catalysts were not characterized by PXRD. Table 4.3. Co dispersion based on the PXRD of the calcined catalysts. Catalyst Co DispersionfO) (%) A-12%Co-Si02 10.0 A-5% Co-Si02 11.0 B-8% Co-Si02 8.8 4.4. Reduction Properties of Calcined Co-SiC>2 Catalysts In temperature programmed reduction (TPR), the temperature of the catalyst is increased in a dilute H2 flow and the H2 consumption is recorded by a thermal conductivity detector (TCD) of a gas chromatograph (GC). TPR is a very useful technique in determining the operating conditions for catalyst reduction, the reduction behavior and the extent of reduction of a catalyst. In the present work, TPR was used to study the reduction characteristics of the calcined Co-Si02 catalyst precursor. Figure 4.4 shows the TPR profile of the calcined A series 12% Co-Si02 catalyst. TPR profiles of the rest of A series catalysts are provided in Appendix 2.1. 61 0 1000 2000 3000 4000 Time (sec) Figure 4.4. Profile of the temperature programmed reduction (TPR) of the A series 12%Co-Si02 catalyst with 54 ml/min of 20%H2/Ar gas mixture. The two H2 consumption peaks of the TPR profile were due to the two-step reduction of C03O4 according to the stoichiometry Co304 + H2 —> 3CoO + H20 occurring at about 250 °C and CoO + H2 —> Co + H20 which occurs at about 400 °C. The higher temperature reduction peak is an indication of a more difficult to reduce cobalt oxide (Lapidus et al., 1991). Given the overall stoichiometry of reduction of C03O4 to metallic Co Co304 + 4H2 —» 3Co + 4//2Othe total peak area of the TPR profile was used to quantify the extent of reduction of the calcined catalysts. The extent of reduction of the catalysts determined from TPR are reported in Table 4.4. Data in Table 4.4 showed that the reduction of C0-S1O2 catalysts was not complete in the TPR. Incomplete reduction of the Co304 to Co is attributed to a metal-support interaction (MSI) and formation of cobalt silicates that are only reducible at temperatures higher than the maximum TPR temperature. (Rathousky et al., 1991; Jabionski et al., 1995; Rosynek and Polansky, 1991) Although TPR is a powerful technique to determine the reduction behavior of the catalyst precursor, it provides only an approximate value for the degree of reduction. The fact that the TCD response is used for peak area calculations imposes some limitations on the accuracy of the calculation of H2 consumption. The TCD detects only the difference between the thermal conductivity of the reactor feed and product streams. As a result, any change in gas composition due to liberation of non-condensable gases from the catalyst (e.g. due to the incomplete decomposition of nitrates in the calcination step), or H2 uptake due to hydrogen spillover, or possible reduction of some support material, can have positive or negative effect on the peak area. In addition, the accuracy of the TPR is limited by the nonlinear response of 63 the TCD. The above errors become more significant at lower metal loadings. Because of these limitations the extent of reduction of the 0.6% Co-Si02 catalyst is clearly in error. Table 4.4. Extent of reduction of Co catalysts based on TPR. Catalyst Extent of Reduction (%) A-12%Co-Si02 91 A-5% Co-Si02 84 A-2% Co-Si02 90 A-0.6% Co-Si02 >100+ B-8% Co-Si02 79 + The calculated value was 105. Nonetheless TPR is a valuable method for the preliminary study of catalyst reduction behavior and optimization of the reduction conditions. In the present work, TPR profiles were used primarily to quantitatively monitor the catalyst reduction behavior. Re-oxidation of the reduced catalysts at high temperature proved to be a more reliable method to quantify the extent of reduction of the catalysts. 4.5. H2 Chemisorption on Co-Si02 Catalysts In the temperature range 25 °C to 100 °C, hydrogen chemisorbs dissociatively on reduced Co with a H/Co stoichiometry of 1/1 (Reuel and Bartholomew, 1984). At high temperature, the chemisorbed H atoms combine and desorb as H2. In the present study, the H2 64 desorption technique was used to quantify the number of moles of surface Co atoms present in the reduced Co-SiC*2 catalysts. Based on the amount of H2 desorbed from a known amount of catalyst, catalyst dispersion O was calculated. Catalyst dispersion (O) is the ratio of the number of moles of surface Co atoms to the total number of moles of reduced Co in the catalyst. The latter quantity was obtained from the re-oxidation of the reduced catalyst to be described in Section 4.5. Figure 4.5 shows a typical H2 desorption profile for the A series 12% Co-Si02 catalyst. The profile of temperature rise and the TCD response due to H2 desorption are plotted as a function of time. The amount of desorbed H2 was determined by comparing the H2 desorption peak area with that of H2 calibration peaks. In Table 4.5 the amount of desorbed H2 and number of moles of surface Co for the catalysts of the present study are reported. From repeat experiments, the experimental error was established at less than ±10% of the reported values. Previous experiments (not reported here) showed that the Si02 support did not chemisorb H2. Hence the amount of desorbed H2 reported in Table 4.5 was assumed to be chemisorbed on the surface Co atoms alone. Lower metal loading catalyst have less reduced Co content for H2 chemisorption. Using the amount of desorbed H2, the amount of surface Co atoms in the catalyst was detennined. 65 66 Table 4.5. Moles of chemisorbed H2 and surface Co in Co-Si02 catalysts. Catalyst Chemisorbed H2 Surface Co /wiol/g ptmol/g A-12% Co-Si02 49 98 A-5% Co-Si02 17 33 A-2%Co-Si02 4 8 A-0.6% Co-Si02 2 4 B-8% Co-SiQ2 24 49 4.6. Extent of Catalyst Reduction PXRD showed that oxidation of the reduced catalyst at high temperature caused Co to be oxidized to C03O4. To quantify the extent of reduction of the Co-Si02 catalysts, the re-oxidation of Co to C03O4 ( 3Co + 202 —> Co304) was used. In the present work, the reduced catalyst was oxidized to Co304 at 400 °C with pulses of 02 in He flow. Using calibration peaks of 02 at the same flow conditions, the total 02 consumption was calculated. Given the oxidation stoichiometry, the amount of the reduced Co and hence the extent of reduction of the initial C03O4, was quantified. Figure 4.6 shows the re-oxidation profile of the reduced A series 12%Co-Si02 catalyst at 400 °C. In Figure 4.6 each arrow corresponds to one pulse of 02 injection. 02 injected by the first five pulses was completely consumed by the reduced Co catalyst. A portion of the 67 sixth pulse completed re-oxidation of the reduced Co. Due to completion of the Co re-oxidation reaction, O2 consumption from the last two pulses was negligible. Extent of reduction calculated from Co oxidation does not have the limitations of the TPR technique discussed in Section 4.3. High temperature oxidation ensures a fast and complete oxidation of all the reduced Co to Co304. PXRD characterization of the catalysts after Co oxidation confirmed that C03O4 was the only detectable phase in the catalyst. In Table 4.6 the extent of reduction of the catalysts, based on Co oxidation, is reported. The trend of extent of reduction of the cobalt oxides with change in catalyst metal loading shows that by decreasing the metal loading, the extent of reduction decreased. This result is an indication of the presence of strong metal support interactions on low metal loading catalysts. Most probably with low metal loading catalysts, the size of catalyst precursor particles before calcination is smaller and the relative particle-support interface is larger. Under the high temperature calcination conditions the low loading catalysts generate more cobalt silicate compounds (Rosynek and Polansky, 1991; Coulter and Sault, 1995). Cobalt silicates are non-reducible under the reduction conditions of this work. Consequently the low loading catalysts have lower extents of reduction. 68 Figure 4.6. 02 uptake profile from pulses of 1.0 ml 02 in 15 ml/min of He flow by the reduced A series 12%Co-Si02 catalyst during the Co oxidation step. (Arrows indicate injection of an 02 pulse.) 69 Table 4.6. Extent of reduction of Co catalyst based on re-oxidation of the reduced Co. Catalyst Extent of Reduction (%) A-12%Co-Si02 85 A-5%Co-Si02 76 A-2% Co-Si02 42 A-0.6% Co-Si02 26 4.7. DRIFTS Study Diffuse reflectance infra-red Fourier transform spectroscopy (DRIFTS) was used to characterize the catalyst in a cycle of two-step reaction under very similar operating conditions to the actual kinetic studies. Figure 4.7 shows the DRIFTS spectrum of a 16 wt% mixture of passivated A series 12% Co-Si02 catalyst and KBr with respect to pure KBr powder background at 50 °C. Absorption bands due to Si-O-Si bonds below 1100 cm"1 and OH vibrations above 3500 cm"1 due to isolated, paired and hydrogen-bonded hydroxyl groups, were observed (Unger, 1979; fler, 1979). To detect very small changes in the catalyst structure, the reduced catalyst/KBr mixture was used as a background to study the effect of activation and hydrogenation conditions on the catalyst. CH, was activated on the reduced catalyst at 450 °C for 7 min. The reaction cell was then closed and evacuated while cooling to 100 °C. H2 was then aclmitted 70 3700 3200 2700 2200 cm'1 1700 1200 700 Figure 4.7. Diffuse reflectance infra-red Fourier transform spectrum of the dilute passivated A series 12%Co-Si02 catalyst, with respect to pure KBr background. (K-M is the Kubelca-Munk unit.) 71 into the evacuated cell at 100 °C until the pressure reached one atmosphere. In Figure 4.8 the three spectra of the catalyst after activation, evacuation and hydrogenation are shown. Because of the incomplete purging of the sample compartment, the atmospheric C02 band at about 2350 cm"1 is present in all the spectra (Socrates, 1994). The activation spectrum showed two absorption bands at 1400 cm"1 and 3016 cm"1 due to gas phase CFL,. The spectra of catalyst after evacuation and hydrogenation at 100 °C did not show any band in the 3000 cm"1 range which could be an indication of C-H bonds. The bands in the 900-1000 cm"1 range were due to the effect of high temperature on Si-O-Si bonds which are caused by dehydrosilation of the Si02 support (Her, 1979; Unger, 1979) according to the reaction: OH OH 72 Hydrogenation at 100 C (+0.1 KM) A. Evacuation (+0.05KM) Activation 0.12 0.10 0.08 K-M 0.06 0.04 0.02 0.00 3700 3200 2700 2200 1700 1200 700 cm" Figure 4.8. Diffuse reflectance infra-red Fourier transform spectra of the diluted A series 12%Co-Si02 reduced catalyst after activation, subsequent evacuation and hydrogenation with respect to the reduced diluted catalyst background. Activation: 7 min activation of 5% CFL/Ar at 450 °C Evacuation: 30 min evacuation to 99.97 kPa. Hydrogenation: 10 ml/min H2 at 100 °C. (Evacuation and Hydrogenation spectra have been shifted by 0.05 and 0.1 K-M units, respectively.) 73 4.8. Discussion Table 4.7 summarizes the catalyst characterization data for the series A and B catalysts in which the nominal metal loading was varied from 0.6 wt% to 12 wt% Co on Si02. By decreasing the nominal metal loading, the degree of reduction based on the 02 uptake measurement decreased from 85% to 26%. Previous studies (Coulter and Sault, 1995; Rosynek and Polansky, 1991; Reuel and Bartholomew, 1984) have reported somewhat lower levels of reduction for Co catalysts than those reported here. The higher values obtained in the present study are most likely due to the longer drying time and shorter calcination times used in the preparation of the catalysts. Generally, a low degree of reduction has been interpreted as evidence for the presence of cobalt silicates or a strong metal support interaction (MSI) that result in cobalt species that are difficult to reduce (Coulter and Sault, 1995; Rosynek and Polansky, 1991; Reuel and Bartholomew, 1984). The Co dispersions, reported as a percentage of reduced Co and measured by H2 desorption, did not show any strong dependence on metal loading. Of more significance, however, was the observation that the Co dispersions estimated by PXRD line broadening were approximately twice the value measured by H2 desorption. Previous studies (Rosynek and Polansky, 1991; Reuel and Bartholomew, 1984; Bartholomew and Pannell, 1980) have shown that metal dispersions measured by PXRD line broadening and H2 chemisorption agree within approximately 30%. Since the repeat analyses showed the experimental error for each method to be less than ±10%, it was concluded that the differences in dispersions estimated by PXRD and chemisorption as shown in Table 4.7 were significant. 74 The data of Table 4.7 suggest that not all of the Co detected by PXRD was exposed to H2 in the chemisorption measurement. This observation was interpreted as evidence for an additional MSI effect, namely migration of the Si02 support onto the reduced Co (Raupp, et al., 1987). A comparison of the difference between the PXRD and chemisorption measurements for the 12 wt% and 5 wt% Co catalysts (Table 4.7), suggests that this decoration of Co by Si02 became more significant as the metal loading decreased. Although Si02 is generally regarded as an inert support with weak MSI (Yoshitake and Iwasawa, 1992), other studies have shown the importance of MSI with Co, including the effect of incomplete metal reduction (Coulter and Sault, 1995; Rosynek and Polansky, 1991; Reuel and Bartholomew, 1984) and alloy formation (Potoczna-Petru and Kepinski, 1993). Table 4.7. Summary of the Co-Si02 catalyst characteristics. Extent of Reduction by Co Dispersion Measured by H2 desorption PXRD of Co304 Series Co loading TPR in H2 02 uptake reduced catalyst calcined catalyst wt% mol % mol % % % A 12 91 85 5.6 10.0 A 5 84 76 5.1 11.0 A 2 (90)+ 42 5.3 -B 8 78 _ 4.6 8.8 Degree of reduction based on the TPR is not reliable 75 Note that the presence of cobalt silicates or other unreduced Co species, as deduced from the incomplete cobalt reduction, cannot explain the difference in metal dispersions obtained by PXRD and H2. The difference between Co dispersion measured by PXRD and H2 desorption suggests that almost 50% of the reduced Co surface is covered by Si02. Possible penetration of Co below the Si02 support surface cannot explain the difference between the Co dispersion measured by PXRD and H2 desorption. Such a phenomenon can happen at the edges of Co particles where thin Co would be neither detectable by PXRD nor available for H2 chemisorption. On the relatively large Co particles (about 20 nm) the contribution of such an effect would be small. Figure 4.9 shows a schematic visualization of the state of Co-Si02 catalyst after reduction. In Figure 4.9 regions ® and © are the Si02 and metallic Co phases, respectively. Regions ® and © illustrate the two forms of metal-support interaction (MSI) in the Co-Si02 system. In region © cobalt silicate formation gives rise to incomplete reduction of the cobalt oxide phase during the TPR. Region © is not detected by PXRD, and H2 adsorption does not occur in this region. Cobalt under the Si02 decoration in region © is reducible. Cobalt in region © can be observed by the PXRD, but not by H2 chemisorption. As a result due to the effect of MSI in region ©, the PXRD measures a higher Co dispersion than the H2 desorption method. 76 SiCb decoration on Co reduced Co ure 4.9. Schematic diagram of the two types of metal support interaction (MSI) on Co-Si02 catalyst. 77 Chapter 5 Activation of CH4 5.1. Overview In the two-step cycle, CH4 reacts on the catalyst in the activation step to generate H2 and deposit carbon species on the catalyst surface. The deposited carbon species are converted in the subsequent hydrogenation step to produce CEL, and C2+ products. The amount and properties of the carbonaceous species deposited in the activation step have a major effect on the two-step cycle performance. In this chapter, results from the present study of the activation step are presented. For the first time, both the H2 generation and CEL consumption profiles as a function of exposure time have been quantified. Based on the experimental observations from this work, and published computational and experimental results, a semi-empirical model of CEL activation kinetics has also been developed. In the proposed modeL gas phase CEL reacts with the active metal of the catalyst to produce adsorbed H and CH3 species. Then, the adsorbed CEL. species either dehydrogenate or migrate from the metal site to the support site to liberate the occupied metal site for further reaction of CEL. Dehydrogenation of CH3 on the support sites, combination of surface H species and desorption generate H2. The kinetic model has been used to quantify the effect of various parameters on the CEL activation reaction. The kinetic model, the parametric studies and the results of the 78 application of the kinetic model to the analysis of the activation step, are presented in this chapter. 5.2. Activation of CH4 The QMS was used to monitor the reactor effluent stream during CH4 activation on the C0-S1O2 catalyst. Figure 5.1 shows a typical profile of the CFL, consumption and H2 generation per minute, during a 2 minute period of flow of a 5% CEL/95% Ar gas mixture at 450 °C using the A series 12% Co-Si02 catalyst. The CFL consumption profile showed a short period of high reaction rate followed by a longer period of lower reaction rate. However, the rate of CFL consumption did not decrease to zero during the 2 minute reaction period. By numerical integration of the CFL consumption and H2 production molar rate profiles, the cumulative CH4 consumption and H2 generation were calculated as functions of the reaction time. Hence, the cumulative CFL consumption per mole of surface Co on the reduced catalyst (the CEL/Co ratio taken as a measure of the metal coverage) and the average value of x for the CHX surface species, could be calculated and are presented in Figure 5.2. A decrease in the slope of the cumulative CFL consumption curve is indicative of a decrease in the CEL consumption rate. As shown in Figure 5.2, the slope decreased over a period of time and then remained constant after approximately 1 min, corresponding to coverage of the Co by about a monolayer of CHX (Boskovic and Smith, 1997). 79 2.5 c E o" o o E 1.5 £ 1 A A A A 0.5 o o 0 o o o o A ° A 06— 0.00 0.50 1.00 1.50 Time (min) 2.00 2.50 3.00 Figure 5.1. Rate of CFL, consumption (O) and H2 generation (A) per mole of Co during 2 min flow of 5%CFL/95%Ar gas mixture at 54 ml/min and 450 °C over A series 12%Co-Si02 catalyst. 80 4* 3.5 2.5 O I * 3 (A C X 1.5 0.5 O O 2 A A A 06-0.5 1 1.5 Time (min) 2.5 Figure 5.2. Cumulative moles of CHU consumed per mole of surface Co (O) and the value of x in the CHX surface species (A) during 2 min flow of 5%CHU/95%Ar gas mixture at 54 mVrnin and 450 °C over A series 12%Co-Si02 catalyst. 81 The data of Figure 5.2 also show that the H content of the carbonaceous deposit decreased as the reaction time increased. Similar trends of decreasing x in CHX surface species with increasing reaction time have been reported previously (Koerts et al., 1992). 5.3. Kinetics of CBU Activation The data of Figure 5.2 clearly show that CEL consumption continued well beyond a nominal monolayer coverage of the metal sites. In addition, the data show that the species on the surface continued to lose hydrogen as time proceeded. To quantify these observations, a kinetic model of CEL activation was developed, based on the experimental observations from this work and published experimental and computational studies relevant to CEL activation kinetics. Previous studies, reviewed recently by Guczi et al. (1996), have shown that CEL interacts with metal surfaces to produce EL and some form of carbon species on the surface (Koerts and van Santen, 1991; Koerts et al., 1992; Solymosi et al. 1992; Belgued et al. 1996; Boskovic et al. 1996; Ferriera-Aparicio, 1997). Since the activation energy for the activation of gas phase CEL ( CH4(g) + IS —> CH3S + HS where S represents an active catalyst site) is less than that of adsorbed CEL (CH4S + S-> CH^S + HS ) over Group Vffl metal catalysts (Shustorovich and BelL 1991), it is reasonable to assume that the first step of the CEL activation reaction can be written according to reaction (5.1). Furthermore, estimates of the activation energies for the subsequent dehydrogenation of CELS to CELS (x=2,l,0) are 40 -60 kJ/mol larger than those for the initial adsorption step (Shustorovich and BelL 1991). Hence it is reasonable to assume that subsequent dehydrogenation of surface CH3 species 82 generates H2 and various carbon fragments CHX (x=2,l,0). Few studies on the structure of the carbonaceous fragments are available, although, vinylidene and methylidyne species have been identified on Ru (Koranne and Goodman, 1995; Lenz-Solomun, et al., 1994). Detection of small amounts of C2Hs and H2 during the activation of CFL, on Ru supported catalysts has been taken as evidence for the presence of surface CH3 species (Erdohelyi et al., 1993), the combination of which would yield C2FL. Based on these observations, but neglecting the combination reaction of surface carbon species since in the present work production of higher hydrocarbons during the activation step was negligible, one can approximate the surface reactions by the following sequence of steps: CHUg) + IS H> CH,S + HS (5.1) CH,S + S-^> CH2S + HS (5.2CH2S + S^> CHS + HS (5.3) CHS + S^ CS + HS (5.42HS^H2 + 2S (5.5) where XS refers to an adsorbed surface species and S is the active metal site. However, these steps imply that the rate of CFL activation will decrease with time due to the occupation of vacant sites and finally cease upon complete coverage of the metal sites by the carbonaceous species. Since experimental observations have shown that the supported metal catalysts activate more CEL than that corresponding to a monolayer coverage (Lenz Solomun et al., 1994; Boskovic et al., 1996; Boskovic and Smith, 1997; Tsipouriariet et al., 1996), possible migration or spillover of the carbon species from the metal site to the support (Driessen and Grassian; 1996) was included in the kinetic model. Incorporating migration of 83 all CHX (x=l, 2, 3) surface species together with the dehydrogenation reactions, requires an additional eight equations to describe the reaction mechanism. CH3S1 + S2-*CH3S2+Sl CH2Sl+S2->CH2S2+Sl CH S1 + S2-+CH S2+Sl CSt+S2 ->CS2 +S1 CH3S2+S2-*CH2S2+HS2 CH2S2 + S2 —> CH S2 + HS2 CH S2+S2-+ CS2 + HS2 2HS2->H2+2S2 Here Si represents a metal active site while S2 represents a support site. Since the present work is concerned primarily with CHt activation kinetics, the reaction mechanism was simplified further to reduce the number of rate constants in the kinetic model. Semi-empirical calculations (Koerts and van Santen, 1991) indicate that highly hydrogenated CHX fragments such as CH3 are more mobile than for example, CH2 or CH, due to the weaker metal-adsorbate interaction of the CH3 species. Consequently migration from the metal site to the support site was assumed to be dominated by adsorbed CH3 species. Furthermore, it was assumed that CH3 adsorbed on both the metal and support is dehydrogenated to produce H2 and a less hydrogenated carbon species CHX (x<3). Since only carbon species were considered in the modeling, the possibility of migration of H from the metal site to the support site has been neglected. The simplified reaction mechanism is then as follows: 84 CH4 + 2Sl —CH& + HS, (5.6) CrYj^, + S2 —CH3S2 + 5", (5.72HSl—^H2 + 2Sl (5.8) CffjS", —CH^, + H2 (5.9O/JS'J —CHXS2 + H2 (5.10) PXRD characterization of the catalysts before and after reaction (not reported here) has shown that under the reaction conditions of the two-step cycle, catalyst metal particle size did not change. Since the surface area of the support is much larger than the metal and since in the absence of sintering the metal-support interface does not change, it is safe to assume that the coverage of support sites is relatively constant during CEL activation. Given the small amount of CFL consumption, the change in CFL concentration in the gas phase is neglected. Sin^lifying the rate constants k'i and k'2: k2 — k2ds (5.11) the rate of change of coverage of different surface species (0) follows directly from equations (5.6)-(5.10): dd = -2k,6l + k£rH , + kJdL (5.12) 85 ^- = ^-2*3^ (5.14) ^L = WCHA (5-15-f^-^c** (5.16) The set of first order differential equations is subject to the initial condition: att = 0 8Si = 1, 6CH Si = 6HSi = 6CHA = 0CH^ = 0 (5.17) Using eq. (5.6) and (5.8)-(5.10) the cumulative CHU consumption and H2 production as a function of exposure time are given by: "CH, =-i(mc«)k,e2sdt (5.18) = J0'(mco)(^k + ^L\kficH& + Wai&Wt (5-19) The simplified reaction mechanism proposed herein represents the carbon surface species as CHX. Experimental data have shown that the value of x changes with exposure to high temperature, and Figure 5.2 shows that x decreased from about 4 to approximately 1 after 2 min reaction. To avoid the complicated problem of describing the transformation kinetics for CHX species and the change in x (Lenz-Solomun, et al., 1994), only the CHU consumption equation (5.18) together with equations (5.12)-(5.15) were used to estimate the rate constants ki, k2, k3 and kj. The rate constants were estimated using a combined pattern search and steepest descent optimization algorithm, with an initial guess of the rate constants. By means of a 4-th order Runge-Kutta method, the set of initial value first order differential equations was solved 86 and the CFL consumption was calculated as a function of time. Using the sum of squares of the difference between the actual and calculated CFL, consumption profiles as an objective function, the optimization routine improved the estimate of the rate constants until the rninimum of the objective function corresponding to the optimum rate constants ki, k2, k3 and kt, was obtained. The computer code used to perform these numerical calculations is listed in Appendix 3.1. Figure 5.3 is a typical comparison of the actual and calculated cumulative CFL consumption to surface Co molar ratio as a function of time, during CFL activation on the series A 12% Co-Si02 catalyst at 450 °C for a 2 min reaction period. The estimated rate constants and their corresponding standard deviations were ki = 3.9 ± 0.2 min"1, k2 = 40.0 ± 12.0 min"1, k3 = 0.5 ± 0.1 min"1 and k, = 1.1 ± 0.3 min"1, and the agreement between the measured and calculated values was excellent (R2 = 99.9%). Note that without the migration step (k2 = 0) the cumulative CFL/Co molar ratio would approach on asymptote of 1, while the activation rate would approach on asymptote of zero (Kristyan, 1997). Clearly, the kinetic data are in accord with the proposed migration or spillover of CH3 species from the Co to the support. 87 1.4 0.00 0.50 1.00 1.50 2.00 2.50 Time (min) Figure 5.3. Cumulative moles of CHU consumed per mole of surface Co (O) measured during 2 min flow of 5%CHU/95%Ar gas mixture and 54 ml/min at 450 °C over A series 12%Co-Si02 catalyst, compared to model fit (—). Model parameter estimates are given in the text. 88 5.3.1. Coverage of Surface Species By applying the kinetic model to the experimental data, the optimum rate constants for reactions (5.6) to (5.9) were determined. Using these optimum reaction rate constants, the surface coverage 0 of different species, the rate of CFL, activation on Co (eq. 5.6) and CH3 migration from the metal to the support site (eq. 5.7) were calculated. In Figure 5.4, the rate of CFL activation on Co ( CH4 + 2S, —CH& + HS, , eq. 5.6) and CH3 migration from Co to Si02 sites (C7/3S, + S2 )C//352 + S, , eq. 5.7) are presented. The nominal metal coverage by carbon (mol CFL reacted per mol Co, 0C) as a function of reaction time is also presented in Figure 5.4. Figure 5.5 shows the profiles of surface coverage for catalyst metal vacant site (Si), adsorbed CH3 species (CH3Si), adsorbed hydrogen (HSi) and the adsorbed lumped carbon species CHX (CHxSi) as functions of time. In addition, the nominal metal coverage by carbon (0C) as a function of reaction time is presented in Figure 5.5. The relative rate of CFL activation on the metal site (eq. 5.6) and CH3 migration (eq. 5.7) from metal (Si) to support site (S2) has a major effect on the rate of activation of CFL on the Co-Si02 catalyst. Initially, the number of vacant metal sites (Si) is high and there are no carbon species on the catalyst. As a result, the initial rate of CH3 migration is zero whereas the CFL activation reaction is fast. As reaction time proceeds, CFL reacts with more metal sites (Si) to produce CFLSi surface species. 89 0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2 Tim e (m in) Figure 5.4. Results of simulation of reaction rates for 2 min activation of 5%CH4/95%Ar gas mixture at 450 °C over A series 12%Co-SiC>2 catalyst. Left y axis: Rate of Activation of CH4 on metal (CH4 + 2SX —^—> CH3S1 + HS^ , eq. 5.6) and migration of CH3 from metal to support site ( O/jS, + S2 *'2 ) CH2S2 + eq. 5.7) Right y axis: Nominal metal coverage (9c) by carbon Parameters: lc, = 3.9 min"1, k2 = 40.0 min"1, k3 = 0.5 min"1 and = 1.1 min"1 90 Time (min) Figure 5.5. Results of simulation of surface coverage for 2 min activation of 5%CH4/95%Ar gas mixture at 450 °C over A series 12%Co-Si02 catalyst. Left y axis: Coverage (6) of catalyst vacant site Si, HSi, CH3Si and CHxSi surface species Right y axis : Nominal metal coverage (0C) by carbon Parameters: ki = 3.9 min"1, k2 = 40.0 min"1, k3 = 0.5 min"1 and Id = 1.1 min"1 91 With deposition of these carbon species, the number of vacant sites Si, decreases, the rate of CEU activation decreases and the rate of CH3 migration increases. At this initial stage, the rate of carbon deposition on the metal site is higher than the rate of carbon removal. This trend continues until the concentration of CH3S1 on the metal site reaches a maximum which corresponds to the maximum CH3S1 coverage (see Figure 5.5). Beyond this point, the rate of both the CEL, activation on the metal site and the rate of CH3 migration continue to decrease and approach asymptotic levels. The rate of CHxSi build-up on the catalyst is much smaller than the rate of CEL, activation on the catalyst. Figure 5.5 shows that by increasing the exposure time and hence the nominal metal coverage (0C) by carbon species, coverage of vacant sites Si decreases sharply at first then levels off as the rate of CEL, activation and CH3 migration reach a nearly constant value. With increasing activation time, the coverage of HS, species increases sharply at first and then levels off as the rate of initial CEL, activation on the metal decreases and the rate of CH3 migration from the metal to the support site increases. The coverage of CH3S1 species starts from zero at first, reaches a maximum and then decreases to a small nearly steady value. This is an indication of the initial build-up of CH3S1 species during the initial activation of CEL, on the metal site with negligible carbon migration. By increasing the coverage of CH3S1 species, the rate of migration of CH3S1 becomes significant and the coverage of CH3S1 reaches an asymptotic level. By increasing the exposure time, the coverage of CHxSi species increases slowly. 92 5.4. Parametric Study of the Kinetic Model To examine the significance of different reactions and their rate constants on the performance of the reaction model a parametric study was performed. The 2 min activation of 5%CH4/Ar on 0.5 g A series 12%Co-Si02 catalyst at 450 °C was analyzed by the kinetic model. The optimum rate constants (kj) and the corresponding standard deviations (at) were established above as ki = 3.9 ±0.2 min'1, k2 = 40.0 ± 12.0 min"1, k3 = 0.5 ± 0.1 min"1 and = 1.1 ± 0.3 min"1. In the parametric study, the value of each reaction rate constant k; was set as ki+2Cj, ki-2o"i (i=l to 4 with CT; being the standard deviation of the corresponding rate constant) and zero, while the kj (j=l to 4, j;4) were set at their optimum values. The predicted nominal metal coverages 0C (mol CFL, converted /mol surface Co atoms) were compared with the calculated 0C using the optimum rate constants and the experimental data. Although the performance of the kinetic model depends on the complete set of the reactions (5.6 to 5.10), a study in which the reaction rate constants are varied one at a time provides qualitative insight into the physical significance of different steps of the kinetic modeL Reaction (5.6) represents the initial activation of CFL, on the catalyst metal. A high value of k, indicates a more active catalyst for CFL, activation and generation of surface CH3Si species. Reaction (5.7) is responsible for transport of the adsorbed CH3 species from the metal site to the support site and regeneration of the metal site S, for further activation of gas phase CFL,. A high k2 increases the rate of regeneration of S, sites which leads to more CFL, consumption. Reaction (5.8) generates H2 by combination of two HSi species. This reaction 93 also liberates Si active sites. Reaction (5.9) does not directly affect the rate of CFL activation, but it transforms CH3S1 species to CFLSi species and generates H2. 5.4.1. Initial Activation of CBLj Initial activation of CFL is expressed by reaction (5.6) and the corresponding reaction rate constant ki. Figure 5.6 shows the results of the kinetic model prediction when ki is set at 4.3 min"1 (ki+2o~i) and 3.5 min"1 (ki-2ci) with optimum k2, k3 and Lt values ( 40, 0.5 and 1.1 min"1, respectively). For comparison the optimum nominal metal coverage and the experimental values are also included in Figure 5.6. By increasing the initial CFL activation activity of the catalyst (increasing ki), more CFL dissociates per unit of time on the catalyst to produce surface carbon species on the catalyst metal This leads to higher initial activity of the catalyst. A more active catalyst metal with a high ki is more active for CFL activation only at low nominal metal coverage (0C). With increasing exposure time and higher 0C the rate of CFL activation becomes nearly independent of the metal activity. This is due to the fact that at higher nominal metal coverage (0C) the overall CFL consumption rate is governed by the carbon migration from the metal to the support site. Consequently, a more active metal is not as effective at high 0C as at low 0C. 94 1.6 0.00 0.50 1.00 1.50 2.00 Time (min) Figure 5.6. Effect of ki on nominal metal coverage, 6C. (O) Experimental 9C, (—) and the best fit (ki=3.9 min"1) compared to: ki= 3.5 (kr2ai) and ki= 4.3 (ki+2a() k2, k3 and kt were the optimum values. 95 5.4.2. Carbon Migration After initial activation of CH4 on the catalyst metal site (reaction 5.6), the surface carbon species migrate from the active metal site to the support site (reaction 5.7). A high k2 leads to a faster migration of the carbon species from the metal to the support site and hence faster regeneration of the active metal site for reaction with more CH4. The effect of migration of carbon species is initially low when 0C is small, whereas the effect of the migration step becomes significant at higher 0C. In Figure 5.7 the kinetic model predictions for different k2 values of 64 min"1 (k2+2c>2), 16 min"1 (k2-2a2) and zero with optimum ki, k3 and L, values (3.9, 0.5 and 1.1 min"1, respectively) are compared with the optimum 0C profile (with k2=40 cm"1) and the experimental values. Figure 5.7 shows that 0C increases with increasing migration rate constant k2. The effect of varying k2 is more pronounced at higher 0C. The k2 = 0 profile corresponds to the case where the effect of migration of carbon species from the metal site to the support is neglected (Kristyan, 1997). In this case, the slope of 0C vs. time of reaction decreases with increasing exposure time. The slope of 0C versus exposure time approaches zero when 0C approaches unity. 96 Figure 5.7. Effect of k2 on nominal metal coverage, 0C. (O) Experimental 9C, and (—) the best fit (k2= 40 min'1) compared to: k2= 0, k2= 16 (k2-2a2) and k2= 64 (k2+2c2) ki, k3 and Lt were the optimum values. 97 5.4.3. Hydrogen Desorption Initial activation of CH4 on the active metal site generates CH3S1 and HSi (reaction 5.6). Reaction (5.8) represents the combination of two HSi surface species to generate H2 and liberate two active metal sites. A higher k3 indicates a higher rate of H2 evolution and faster regeneration of active metal sites which were occupied by adsorbed H. Hence, increasing k3 leads to faster CH4 activation. Figure 5.8 shows the predicted Gc with rate constant k3 set at 0.7 rnin"1 (k3+2a3), 0.3 min1 (k3-2a3) and zero, while retaining optimum ki, k2 and k, values of 3.9, 40 and 1.1 min"1, respectively. For comparison, the experimental 0C values and the optimum profiles are included in Figure 5.8. A higher k3 gives rise to a higher nominal metal coverage (0C) because the rate of H2 desorption increases and the rate of regeneration of active metal sites increases making them available for further CH4 activation. The 0C profile corresponding to k3=0 represents the case where the adsorbed H does not desorb from the active metal site. In this case the rate of change of 0C with time decreases which corresponds to a very small rate of CH, activation. With k3=0 (i.e. no desorption of the adsorbed H2) as the exposure time increases, the metal active sites become occupied and the rate of CFL, activation approaches zero. This is in accordance with the physical expectation that with k3=0, once the active sites Si become occupied by CH3, CHX and H species, the rate of CH, activation decreases to zero. 98 1.6 1.4 0.00 0.50 1.00 1.50 2.00 Time (min) Figure 5.8. Effect of k3 on nominal metal coverage, 6C. (O) Experimental 6C, and (—) the best fit (k3=0.5 min'1) compared to: k3=0, k3= 0.3(k3-2a3) and k3= 0.7(k3+2c3) ki, k2 and Li were the optimum values. 99 5.4.4. Dehydrogenation of Surface CH3 Equation 5.9 is a representation of the lumped dehydrogenation of surface CH3S1, CH2Si and CHSi to the less hydrogenated species. Figure 5.9 compares the effect of changing kt to k4+2o"4 (=1.7 min'1), kr2cj4 (=0.5 min"1) and zero while optimum ki, k2 and k3 values of 3.9, 40 and 0.5 min"1 respectively, are retained. Equation 5.9 has an important effect on the rate of H2 generation in the CFL, activation reaction, but its effect on CFL consumption kinetics is small. Hence kt has little effect on the performance of the present kinetic model. 100 1.6 0.00 0.50 1.00 1.50 200 Time (min) Figure 5.9. Effect of Lj on nominal metal coverage, Gc. (O) Experimental 0C, and (—) the best fit (k4=l. 1 min"1) compared to: k4=0, kt= 0.5(k4-2a4) and k4= 1.7(k4+2rj4) ki, k2 and k3 were the optimum values. 101 5.5. Effect of Operating Variables The kinetic model was used to assist in the interpretation of experimental data obtained to determine the effect of various operating conditions on the CHU activation step. The experimental data and calculation methods are provided in Appendix 2.2 and 2.3, respectively. 5.5.1. Effect of Reaction Time In Figure 5.10, the cumulative CHU consumption per surface Co molar ratio as a function of time for the series A 12% Co-SiC>2 catalyst is shown for a total reaction time of 7 min. These data were also well described by the kinetic model (R2 = 99.4%) with the estimated parameter values of ki = 4.3 ± 0.8 min"1, k2 = 54.3 ± 12.3 min"1, k3 = 0.6 ± 0.1 min"1 and kt = 0.8 + 0.2 min'1. Taking account of the standard deviation of the estimates, these values are in good agreement with the estimates obtained from the 2 min reaction time experiment (Figure 5.2). In Figure 5.10, the deviation in the model fit in the first 2 min activation time was greater than in the subsequent 5 min. This was due mostly to the fact that each data point carried equal weight in the fitting procedure. Assuming that the carbon species on the surface can be represented as CHX, the calculated value of x as a function of time is shown in Figure 5.11. Increasing exposure time decreased the hydrogen content of the CHX surface species. In previous reports with less than a monolayer coverage (Koerts et al., 1992), x was reported to be close to 1.0. 102 0.00 1.00 2.00 3.00 4.00 5.00 6.00 7.00 8.00 Time (min) Figure 5.10. Cumulative moles of CFL, consumed per mole of surface Co (O) measured during 7 min flow of 5%CFL/95%Ar gas mixture at 54 ml/min and 450 °C over the A series 12%Co-Si02 catalyst, compared to model fit (—). Model parameter estimates given in the text. 103 4n 3.5 2.5 01 a 2 3 in c X 1.5 0.5 O A 0 0.00 o A 2 6 O O 0 o o 1.00 2.00 3.00 4.00 5.00 Time (min) 6.00 7.00 8.00 Figure 5.11. Value of x in the CHX surface species during 2 min (A) and 7 min (O) flow of 5%CH4/95%Ar gas mixture at 54 ml/min and 450 °C over the A series 12%Co-SiC>2 catalyst. 104 In addition, the presence of CH and C2H4 species on Ru catalysts has been detected spectroscopicalfy after activation (Lenz-Solotnun, et al, 1994) and computational techniques (Koerts and van Santen, 1991) have shown that CH carbon species on a Pt catalyst can combine to form C2+ products more readily than CHX species with x > 1. The data of Figure 5.11 suggest that to obtain x = 1, a reaction time of about 2.5 minutes is required at the conditions of the present study. The continuous dehydrogenation of carbon species with reaction time is an indication of production of less active carbon species. This is in accordance with previous reports of the transformation of more active carbon species to less active forms with increased reaction time (Koerts et al., 1992b). 5.5.2 Effect of Metal Loading The A series Co-Si02 catalysts with 2, 5 and 12 wt% nominal Co were used to study the effect of catalyst loading on the CH4 activation kinetics. Table 5.1 reflects the nominal metal coverage, reported as the moles of CH4 reacted per mole of surface Co, and the estimated rate constants for CH4 activation on the Co catalysts. At lower metal loadings the amount of activated CH4 decreased and hence the relative error of measurement with QMS increased, leading to an increasing trend of R2 with decreasing metal loading. The catalyst characterization data of Table 4.7 showed that by decreasing the Co loading, the extent of metal support interaction increased (i.e. there was a lower extent of reduction and increased silica decoration of the reduced Co). Table 5.1 shows that an increased MSI not only increased the CHj activation activity (increased ki), but also facilitated the migration of carbonaceous species from the metal to the 105 support (increased k2). Expectedly, the rate constant of the hydrogen combination reaction was not influenced by the extent of the MSI. The electronic effect of supports on metal catalysts is mostly limited to a few atomic layers and is considered a short range effect (Raupp et al, 1987). The increased MSI at lower loadings observed in the present study suggests that the overall electronic effect of the support on the metal becomes significant and results in an increase in ki. In addition, as metal loading decreases, the metal-support contact surface area is expected to increase, resulting in an increases in the rate of migration (increased k2). Table 5.1. Effect of metal loading on reaction rate constants estimated at 450°C for a 2 minute reaction time with 5% CEL, in Ar at 54 (STP)ml/min. Regression Catalyst Model Parameters Std. Error Coefficient Series A CR) Reacted ki k2 k3 k4 oe R2 mol CHVmol Co min'1 min"1 min"1 min"1 mol CRt/mol Co % 12%Co-Si02 1.81 5 23 0.7 1.8 0.02 99.7 5%Co-Si02 2.01 117 153 0.5 12.0 0.06 98.5 2% Co-SiQ. 2.3 121 346 0.6 10.9 0.37 93.1 5.5.3. Effect of Activation Temperature The series A 12% Co-Si02 catalyst was also used to study the effect of temperature on the CFL activation kinetics. The metal coverage, the value of x of the CHX carbon species after a reaction time of 2 minutes, and the rate constants are reported in Table 5.2 for 106 temperatures in the range 360-450 °C. In Appendix 2.4, the experimental kinetic data and kinetic model fit are compared. Increasing temperature increased the CH4 conversion and the metal coverage (total moles of CH4 reacted per mole of surface Co), but decreased the H content of the resulting surface species. The activation energies for initial activation of CH4 on Co (ki), migration of the CH3 species to the support (k2) and H2 evolution (k3) were estimated from the data of Table 5.2 as 56, 48 and 171 kJ/mole, respectively. Table 5.2. Effect of temperature on CFL, activation over 12% Co-Si02 catalyst (Series A) measured with a 2 minute reaction time and 5% CFL, in Ar at 54 (STP)ml/min. Temperature Total CH4 Reacted x of Model Parameters R2 Ratio Conv. CH, k. k2 k3 k, °C mol CH4A110I Co % min"1 min"1 min"1 min"1 ol CHVmol Co % 360 0.5 7.2 2.1 1.4 5.9 0.01 0.87 0.01 90.6 390 0.6 9.8 1.4 2.0 5.1 0.24 1.13 0.03 99.2 450 1.3 20.8 0.9 5 16.8 0.56 1.24 0.01 99.7 The activation energy for k, was in reasonable agreement with the value of 42 kJ/mole reported for the activation of CFL, on Co-Si02 catalysts at less than a monolayer coverage (Koerts et al., 1992a). Koerts et al. (1992a) reported the activation energy of CFL, activation on Co based on cumulative H2 production resulting from CFL, activation. Extent of dehydrogenation of CFL, on the catalyst and hence stoichiometry of the H2 evolution during 107 CHU activation is a function of temperature (Koerts et al., 1992a). As a result the activation energy of 42 kJ/mole is considered an approximate value. The apparent activation energy for the migration of CHX species from Co to Si02 has not been reported. However, the activation energies for similar surface diffusion phenomena (e.g. surface diffusion of CO on Ni and H spillover onto AI2O3) have activation energies of similar magnitude (Conner and Pajonk, 1986; Roop et al., 1987 ). The activation energy of hydrogen desorption from silica-supported Co catalysts has not been reported either. However, the activation energy distribution for hydrogen desorption from silica-supported Ni catalysts has been reported to be in the range of 35 to 135 kJ/mole, depending on the preparation method, metal loading and surface coverage (Arai et al., 1995). 5.5.4. Effect of Support and K Promoter on the Activation of CBU Results of CH, activation on the 10% Co catalysts supported on Si02 and A1203, with and without 1 wt% K promoter (Boskovic et al, 1996) were also analyzed by the kinetic model. Catalyst characterization and details of experimental results for this section are reported by Boskovic et al. (1996). The experimental results and estimated rate constants are summarized in Table 5.3. A1203-supported Co catalysts showed a higher activity than Si02-supported catalysts (Boskovic and Smith, 1997; Boskovic et al, 1996, 1997) and this observation is reflected in the higher value of ki estimated for the 10%Co-Al2O3 catalyst compared to the 10%Co-SiO2 catalyst. Previously it was suggested that the differences in total CHU activation obtained with the different supports may be due to differences in the rate of migration of the carbonaceous deposit from the metal to the support (Boskovic and Smith, 108 1997). The kinetic analysis shows, however, that the rate of CFL, activation (ki) was more important, being much greater on the A1203 supported catalyst than on the Si02 supported catalyst. Although differences in the estimated values of k2 are shown in Table 5.3 for the two supports, the parameter estimates were based on a 1 minute activation time so the effect of migration did not dominate the kinetic analysis. Hence, the estimated values of the parameters k2, k3 and kt have significant errors associated with them and definitive conclusions regarding changes in these parameters between the two supports were not possible. The rate of migration (k2) on the A1203 and Si02 supported catalysts increased significantly with addition of K promoter. The increase in k2 may be due to the electron donor capability of the K promoter. The semi-empirical calculations referred to previously (Koerts and van Santen, 1991) show that increased d orbital filling decreases the adsorption energy of the CH3 species to the metal. Hence, addition of K would be expected to weaken the Co-CH3 bond and thereby increase the rate of migration or spillover to the support site, in agreement with the kinetic model parameters. Table 5.3. Effect of support and K promoter on CFL activation at 450°C for a reaction time of 1 min. Catalyst Total CFL Reacted Model Parameters Series C Ratio Conversion k, k2 k3 k4 ae R2 mol CrL/mol Co % min"1 min'1 min'1 min"1 mol CHVmol Co) % 10% Co-Si02 1.15 37 5.8 18.6 74.1 11.0 0.05 98.5 K-10%Co-SiO2 1.33 27 18.5 54.5 30.2 21.5 0.17 98.2 Co-Al203 1.92 32 17.7 11.5 57.4 5.6 0.08 99.1 K-10%Co-Al2O3 2.40 31 19.3 41.9 121. 8.9 0.21 99.1 109 5.6. Conclusions The initial high rate of CFL, activation on supported Co catalysts at 450°C and 101 kPa, decreased rapidly but continued despite a nominal coverage of the surface Co by CHX that was > 1. The kinetic model developed to describe this observation assumed that activation of gas phase CFL on a Co site, was followed by migration of the resulting CHX surface species from the Co to the support. Hence, the effects of reaction temperature, catalyst metal loading, support and promoter were interpreted in terms of the changes in the magnitudes of the activation and surface migration rate constants. The results indicate that metal-support interaction plays a key role in activation of CFL on the catalyst. Since the dispersion of the Co-Si02 catalysts were all about 5%, the possibility of strong effect of catalyst metal dispersion and particle size in this study can be ruled out. 110 Chapter 6 Hydrogenation 6.1. Overview After deposition of the carbon species in the activation step, a low temperature (100 °C) isothermal hydrogenation was used to produce CH4 and C2+ hydrocarbons. In the isothermal hydrogenation step, the most active surface species are hydrogenated. A temperature programmed surface reaction (TPSR) followed the isothermal hydrogenation step. In TPSR the temperature of the reactor was increased linearly, while pure H2 flowed through the reactor. The reaction products were monitored by the calibrated quadrupole mass spectrometer (QMS). During TPSR the less active carbon species not hydrogenated in the isothermal hydrogenation step were hydrogenated at higher temperatures. Following TPSR a temperature programmed oxidation (TPO) was used to quantify the amount of inactive carbon species not removed in the TPSR step. In TPO, the temperature of the reactor was increased while pure 02 flowed through the reactor. Any inactive carbon that had not been removed in TPSR reacted with 02 to produce C02 at high temperature. The amount of C02 was quantified by the calibrated QMS. Isothermal hydrogenation, TPSR and TPO comprised a complete set of carbon recovery experiments that could be compared to the initial carbon deposited in the activation step of the two-step CFL, homologation cycle. A complete set of carbon recovery experiments was beneficial to establish a reasonable carbon balance in the experiments. In previous studies 111 of the two-step cycle, the quantitative analyses and results were based on either the CFL, consumption in the activation step or hydrocarbon generation in the isothermal hydrogenation and TPSR. It was assumed that all the deposited carbon in the first step was recovered as hydrocarbons in the subsequent isothermal hydrogenation and TPSR (Koerts et al, 1992a; Pareja et al., 1994; Koranne et al., 1995; GuczL et al., 1997; Shen and Ichikawa, 1997). The complete carbon balance experiments of the present work showed that not all the carbon deposited in the first step was recovered by hydrogenation. In this chapter the results of carbon recovery after CFL, activation are presented in two sections. In the first section, the three carbon recovery steps of isothermal hydrogenation, temperature programmed surface reaction (TPSR) and temperature programmed oxidation (TPO) are discussed. The carbon recovered in the isothermal hydrogenation step, TPSR and TPO is compared to the carbon deposited in the activation step. The second section of this chapter is devoted to a discussion of the results of the effects of different operating variables on the two-step cycle. 112 6.2. Carbon Recovery In this section, a typical set of isothermal hydrogenation, temperature programmed surface reaction and temperature programmed oxidation results are presented. 6.2.1 Isothermal Hydrogenation A ten minute isothermal hydrogenation followed the activation step. During the isothermal hydrogenation, nine 0.25 |il samples of the reactor effluent were taken in 1 minute intervals, stored in the multi-loop valve of the GC and analyzed at the completion of the experiment. Figure 6.1 shows the profile of CFL,, C2FL and C2H<; production as a function of time in the isothermal hydrogenation of A series 12% Co-Si02 catalyst after 2 min activation of 5% CFL/Ar at 450 °C. By numerical integration of the profiles, the number of moles of each hydrocarbon and carbon selectivity of C2+ products were determined. 113 0.3 Time (min) Figure 6.1. Profile of CHU (•), C2H4 (O) and C2IL5 (A) production as a function of isothermal hydrogenation at 100 °C in 11 mVniin H2 flow over the A series 12%Co-Si02 catalyst after 2 min activation with 5% CHU/Ar mixture at 450 °C. 114 6.2.2. Temperature Programmed Surface Reaction Temperature programmed surface reaction (TPSR) followed the isothermal hydrogenation step. The calibrated QMS was used to monitor the products in the TPSR. Figure 6.2 shows the TPSR profile of the A series 12% Co-SiC>2 catalyst after 2 ruin activation of 5%CFL/Ar at 450 °C and 10 min isothermal hydrogenation. CFL, was the only significant product in the TPSR. Figure 6.2 shows two CFL production peaks at about 200 °C and 540 °C due to hydrogenation of Cp and C? carbon species. The 10 min isothermal hydrogenation at 100 °C removed the Ca active carbon form. The amount of inactive carbon in the TPSR was determined by numerical integration of the peak areas of Cp and Cy carbon species. Activation of CFL with transition metal catalysts generates H2 and surface carbon species (see also section 2.2.1). In order of decreasing reactivity with H2 in a TPSR, the carbon species have been classified as Ca, Cp and Cy. Ca is the most active type of carbon that produces CFL and C2+ at low temperature (ca. 100 °C). Cp is less active and Cy the least active carbon. These generate only CFL in the temperature range of 200 and 500 °C, respectively. 115 0.006 0.005 0.004 0.003 -° o E 0.002 O 0.001 0.001 10 20 30 40 Time (min) Figure 6.2. Profile of CH4 production (—) and temperature (O) as a function of time in temperature programmed surface reaction (TPSR) in 11 ml/min of H2 flow on A series 12%Co-Si02 catalyst. Activation of CJT, (2 min flow of 5% CHVAr mixture at 450 °C) followed by 10 min isothermal hydrogenation at 100 °C. 116 6.2.3. Temperature Programmed Oxidation Temperature programmed oxidation (TPO) was used to quantify the amount of carbon that was not removed by TPSR in H2. In TPO, the reactor temperature was increased in an 02 flow and the reactor product was analyzed by the calibrated QMS. Figure 6.3 shows the C02 production profile and the reactor temperature as a function of time. By numerical integration of the C02 production profile, the total amount of inactive carbon was determined. In the TPO step, in addition to C02, the mass numbers 18, 31 and 28 corresponding to H20, CH3OH (or C2H5OH), and CO were also monitored. H20 mass number (m/e=18) showed a distinct peak due to H20 production from oxidation of the adsorbed hydrogen in the catalyst. The trends of mass numbers 31 and 28 did not show any CH3OH, C2H5OH or CO oxygenates production in the TPO step of the two-step cycle experiments. 117 803 703 603 g 503 *_ 3 S 400 a. E ai 303 200 A 100 A o ql 10 20 30 Time (min) 40 0.014 0.012 0.01 0.008 0.006 0.004 % o E 0.002 O O [0 -0.002 -0.004 -0.006 -0.008 50 Figure 6.3. Profile of CO2 production (—) and temperature (O) as a function of time during temperature programmed oxidation (TPO) in 11 ml/min of 02 flow on A series 12%Co-Si02 catalyst. (Activation: 2 min flow of 5% CHVAr mixture at 450 °C) 118 6.2.4. Carbon Balance To examine the rehabihty of the two-step cycle experiments, and to establish the fate of carbon generated from activation of CFL,, the amount of carbon in the activation step was compared with that from hydrocarbons produced in the isothermal hydrogenation step, subsequent temperature programmed surface reaction (TPSR) in H2 and finally temperature programmed oxidation (TPO). Since the activation, TPSR and TPO steps were monitored by the calibrated quadrupole mass spectrometer (QMS) and the isothermal step was monitored by both the gas chromatograph (GC) and QMS, a reasonable carbon balance was an indication of an accurate analysis of the complete set of reaction cycles. Table 6.1 presents results from a series of experiments aimed at quantifying the carbon balance that could be achieved from each of the reaction steps. The first column of the table corresponds to an experiment in which the isothermal hydrogenation step was omitted. After activation the catalyst was exposed to TPSR and TPO only. In the second column, the results of an experiment without TPSR are shown. In this experiment, the catalyst was exposed to TPO after activation and isothermal hydrogenation. The third column summarizes the results of an experiment that had an activation and TPO step without exposure to hydrogen in isothermal hydrogenation or TPSR. The fourth column corresponds to a complete experiment with isothermal hydrogenation, TPSR and TPO after activation of CFL, on the reduced catalyst. Comparison of the relative magnitude of the deposited and recovered carbon in different steps of isothermal hydrogenation, TPSR and TPO showed that less than 10% of the deposited carbon was recovered in the isothermal hydrogenation step. Depending on the 119 experimental sequence 30-40% of the deposited carbon was recovered in the TPSR step and approximately 50% of the deposited carbon was recovered in the TPO step. The last row in Table 6.1 represents the error in carbon balance which was calculated as: % error=100 X [(mol carbon deposited in activation step)/(mol total carbon recovered)-1] The carbon balance error in a complete two-step cycle reaction was less than ±20%. Table 6.1. Summary of carbon balance calculations for A series 12% Co-Si02 catalyst. Steps Ac, TPSR, TPO Ac, Is, TPO Ac, TPO Ac, Is, TPSR, TPO Total pmol On Ac 61 60 63 57 pmol Cout Is - 2 - 5 umol C0ut TPSR 27 - - 19 pmol Cout TPO 33 62 65 25 Total pmol Cout 60 64 65 49 % error +2 -7 -3 +14 Catalyst: 0.5 g reduced A series 12%Co-Si02 Activation (Ac): 2 minutes exposure to 5% CEL/Ar at 450 °C Isothermal hydrogenation (Is): 10 minutes exposure to 11 ml/min of H2 at 100 °C Temperature programmed surface reaction (TPSR): Exposure to 11 ml/min of H2 Temperature programmed oxidation (TPO): Exposure to 11 ml/min of 02 The results in Table 6.1 prove that a TPSR alone does not remove all of the inactive carbon from the catalyst. In previous studies (Koerts et al., 1992a; Pajero et al, 1994; 120 Koranne et al., 1995; Guczi, et al., 1997; Shen and Ichikawa, 1997) the amount of carbon recovered from the isothermal hydrogenation and TPSR steps was assumed equal to the deposited carbon in the activation step. Based on the results of the present work in these studies, the presence of inactive carbon which can only be removed by TPO, has been overlooked. The data in Table 6.1 showed that inactive carbon (which is only removed by TPO) may account for as much as 50% of the deposited carbon. Product yield data reported based on the amount of carbon deposition in the activation step or the carbon recovered in the isothermal reaction and TPSR in H2, can be in error by a factor of 2. 6.3. Effect of Operating Variables In this section the effect of different operating variables on the performance of the two-step cycle is discussed. 6.3.1. Effect of Loading Using Si02-supported catalysts with 2, 5 and 12% Co loading the effect of catalyst metal loading on the two-step cycle was studied. Table 6.2 shows a summary of the results from the hydrogenation step on these catalysts. The data of Table 6.2 show that decreasing the catalyst metal loading from 12% to 2% increased the catalyst effectiveness for CFL activation. As discussed above, decreasing the metal loading increased the metal-support interaction, resulting in faster migration of carbon species from the primary activation site on the metal to the support (see section 5.4.2). 121 Table 6.2 also presents the hydrogen content (x) of the surface species CHX generated in the activation step. Computational work based on semi-empirical quantum chemistry calculations of the reaction path for C-C bond formation (Koerts and van Santen, 1991) has shown that the activation energy for the combination of three-fold bonded carbon with a CH2 species to form a vinylidene surface species was the lowest of all possible combinations. Table 6.2. Effect of the catalyst metal loading on its hydrogenation performance. Catalyst 12% Co-Si02 5% Co-Si02 2% Co-Si02 pmol CHVpmol Co (Ac) 1.8 2.0 2.3 xofCHx 1.1 1.3 1.6 C2+ selectivity (Is) 17 5 2 C2+ yield/site, mol C2+/mol Co(Is) 0.01 0.004 0.001 %C recovery (Is+TPSR) 43 21 12 Catalyst: 0.5 g reduced A series 12%Co-Si02, 5%Co-Si02 or 2%Co-Si02 Activation (Ac): 2 ruinutes exposure to 5% CEL/Ar at 450 °C Isothermal hydrogenation (Is): 10 minutes exposure to 11 ml/min of H2 at 100 °C Temperature programmed surface reaction (TPSR): Exposure to 11 ml/min of H2 Experimental results (Koerts et al., 1992a) have also confirmed that on silica supported Rh, Co and Ru catalysts, the C-C bond formation was favorable for CHX species in which the average value of x was about one. The trend of increasing hydrogen content of CHX surface 122 species with decreasing metal loading of the catalysts, indicate that the C-C bond formation tendency of the surface species decreases with decreasing metal loading. Hence the C2+ carbon selectivity would be expected to decrease with decreasing metal loading and the experimental data of Table 6.2 confirm this observation. C2+ hydrocarbons are generated in the isothermal hydrogenation of species adsorbed on the metal surface only in the isothermal hydrogenation step. The decreasing trends of C2+ selectivity and C2+ yield per site (number of moles of C2+ generated/ number of moles of surface Co) with metal loading indicate that although lower loading catalysts have higher CH4 activation capability, they have lower C-C bond formation capability. The final set of data presented in Table 6.2 shows the total percent recovery of initially deposited carbon by the isothermal and TPSR hydrogenation steps. The carbon recovery also decreases with decreased metal loading of the catalyst. With reference to the previous discussion (section 5.4.2), by decreasing the metal loading of the catalyst, the metal-support interactions increased. In the activation step, increased metal-support interactions give rise to a higher rate of migration of carbon species from the initial activation metal site to the support site. Carbonaceous species residing on the support are not only inactive for C-C bond formation in isothermal hydrogenation, but they are not easily hydrogenated in TPSR. Hence, although a lower loading metal catalyst performs better in the activation step, it generates more inactive carbon on the support and less C2+ product in the isothermal hydrogenation step. Based on these results, to obtain a higher yield of C2+ products, a higher metal loading is recommended. 123 6.3.2. Activation Time In the activation step of the two-step cycle, the carbonaceous species resulting from CFL activation on the metal not only migrate to the support site but also dehydrogenate and accumulate on the catalyst. As the duration of the activation step is increased, more carbon is deposited on the catalyst. In addition, a longer exposure to the high activation temperature can lead to a decrease in the amount of active carbon due to migration and aging of the carbon species. To exaniine these phenomena, 5% CFL/Ar was activated for 1, 2 and 7 minutes on A series 12% Co-Si02 catalyst. The deposited carbon species were hydrogenated in the isothermal and TPSR steps, and then oxidized in TPO. Table 6.3 summarizes the results from these experiments. The data of Table 6.3 show that longer reaction times of CPU at 450 °C generate more carbon species on the catalyst and these species dehydrogenate on the surface resulting in decreased values of x in CHX. In the subsequent isothermal hydrogenation step, selectivity to C2+ hydrocarbons and the total amount of carbon recovered, decreased in the isothermal hydrogenation step as the activation time increased. Finally the last row in the table corresponds to the total carbon recovery in the form of CH4 in the TPSR step. A longer CFL, activation time generates more carbon deposits on the catalyst. Longer exposure to high temperature has two effects on the deposited carbon species. First, more carbon species migrate from the metal site to the support site. Second, the surface carbon species undergo more dehydrogenation. This effect is reflected in the x value of the CHX shown in Table 6.3. The combined effect of temperature and time is loss of activity of surface carbon species and lower selectivity and carbon recovery in the isothermal hydrogenation step. 124 Table 6.3. Effect of activation time on the hydrogenation step. activation time (min) 7 2 / Umol CEL, consumed (Ac) 177 71 45 pimol H2 generated (Ac) 352 110 64 x of surface CHX 0.0 0.9 1.1 pmol C2+ generated (Is) 0.1 0.16 0.24 % C2+ selectivity (Is) 5 6 7 pimol C produced (Is) 4 6 7 pimol C produced (TPSR) 163 38 1 Catalyst: 0.5 g reduced A series 12%Co-Si02 Activation (Ac): 1, 2 or 7 minutes exposure to 5% CFL/Ar at 450 °C Isothermal hydrogenation (Is): 10 minutes exposure to 11 ml/min of H2 at 100 °C Temperature programmed surface reaction (TPSR): Exposure to 11 ml/min of H2 Using the results of the kinetic model calculation of coverage of different species on the catalyst as a function of activation time (Figure 5.4 and Figure 5.5), one can safely assume that after approximately one minute of reaction, the coverages of different species reach their asymptotic values, and the difference between one and two minutes exposure on the coverage of different species on catalyst metal becomes negligible. As a result, the effect of migration of carbon species on the catalyst after one and two minutes reaction is the same. Consequently the only major difference between the carbon species in the three experiments of Table 6.3 is the activity of the surface carbon species which is reflected in the hydrogen content of the 125 surface species. Longer exposure of surface species to a high activation temperature of 450 °C generates dehydrogenated surface carbon species that are less active in C-C bond formation, resulting in less C2+ generation and lower C2+ selectivity. The low carbon reactivity in the isothermal hydrogenation step indicates that the carbon species are less active not only for C-C bond formation but also for hydrogenation to produce CFL. TPSR of the carbon species after the isothermal step, generated a larger amount of CFL on the catalyst as the exposure time to CFL increased. This is due to the larger amount of initial carbon deposited on the catalyst. It is also worth noting that the 163 pimol CFL, produced in the experiment with 7 minutes exposure, was more than the 48 pmol surface Co atoms available on the catalyst. This suggests that in TPSR, a large amount of carbon residing on the support can also be hydrogenated. The mechanism of this phenomenon is not clearly known, but may be related to a H spillover mechanism 6.3.3. Activation Temperature The effect of activation temperature on the two-step cycle was studied by activation of CFL in the temperature range of 360 to 450 °C. Table 6.4 summarizes the results of this study. Data in Table 6.4 show that decreasing the activation temperature decreased the amount of both activated CFL and generated H2. The x-value of CHX carbon species also increased as a result of decreasing temperature of activation. Results of the kinetic model showed that in the temperature range of 360 to 450 °C, the coverage of adsorbed H atoms on 126 the surface was about 60% and it did not change appreciably with activation temperature. Consequently, the higher x value of CHX species at low temperature indicates higher hydrogen content of the surface species. Data of C2+ generation and selectivity in the isothermal hydrogenation step showed that C2+ production dropped from 0.28 Ltmol for an activation temperature of 450 °C to 0.09 pimol for an activation temperature of 360 °C. C2+ selectivity also dropped by a factor of 3 by decreasing the temperature of activation from 450 °C to 360 °C. Table 6.4. Effect of activation temperature on the hydrogenation step. Activation Temperature • 450 390 360 (°C) pimol CFLj consumed (Ac) 57 37 27 Ltmol H2 generated (Ac) 90 47 26 x of surface CHX 0.9 1.4 2.1 Ltmol C2+ generated (Is) 0.28 0.21 0.09 % C2+ selectivity (Is) 12 7 4 Ltmol C produced (TPSR) 18.7 0.6 0.1 Catalyst: 0.5 g reduced A series 12%Co-Si02 Activation (Ac): 2 minutes exposure to 5% CEVAr at 360, 390 or 450 °C Isothermal hydrogenation (Is): 10 minutes exposure to 11 ml/min of H2 at 100 °C Temperature programmed surface reaction (TPSR): Exposure to 11 ml/min of H2 127 The last row in Table 6.4 shows the total amount of CH4 recovered in the TPSR step after isothermal hydrogenation. By decreasing the activation temperature from 450 °C to 360 °C a significant drop in carbon recovery in the TPSR step was observed. Decreasing the activation temperature had a number of effects on the deposited carbon species. At lower activation temperature, the total amount of carbonaceous species deposited on the catalyst was lower. As a result, less carbon was available on the surface for combination. Activation temperature also affects the nature of the carbon species. A low activation temperature generated carbon species with higher hydrogen content. As has been demonstrated by computational methods (Koerts and van Santen, 1991), carbon species with high hydrogen content have lower tendency for C-C bond formation. This result has also been demonstrated by experimental observations (Koerts and van Santen, 1991; Koerts et al., 1992a) that a CHX with x close to 1 is the most active type of carbon species for C-C bond formation. Higher mobility of carbon species at high temperature contributes to the migration of more carbon from the metal site to the support site, resulting in production of more inactive carbon on the support at high temperature. 6.3.4. Carbon Aging Carbon species generated by CEL, activation transform to less active forms by exposure to high temperature. The extent of this transformation depends on the exposure period (Koerts et al., 1992a; Carstens and Bell, 1996). In the present study, experiments were performed with different time delays between the activation and the isothermal hydrogenation steps in an attempt to quantify the carbon aging effect. 128 In these experiments, the A series 12% Co-Si02 catalyst was exposed to a 5% CFL/Ar mixture at 450 °C for 2 minutes. The deposited carbon species were then aged in Ar atmosphere for 5, 10 and 20 minutes at 100 °C and hydrogenated isothermally at 100 °C for 10 minutes. A TPSR and TPO followed the isothermal hydrogenation step. A summary of the results from this set of experiments is given in Table 6.5. The operating conditions for the activation step of this set of experiments were approximately the same. In the activation step, more than a nominal monolayer of carbon was deposited on the catalyst. Increasing the age of the carbonaceous deposits (by increasing the time delay between the activation and the isothermal step) decreased the production of both CH4 and C2+ hydrocarbons in the isothermal hydrogenation step. A decreasing trend of C2+ selectivity with increased deposit age indicates that the effect of aging was more important for C-C bond formation than hydrogenation of carbon species to CFL,. Interestingly, there was some C2FL production for the shorter aging times which decreased to zero at longer aging times. A small amount of C3+ hydrocarbons generated in the five minutes aging experiment and its absence in the longer aged experiments, indicates that the most active carbon which is capable of multiple C-C bond formation either undergoes C-C bond breakage to generate C2+ and eventually CFL or further C-C bond formation to produce inactive polycarbon species. The total amount of carbon recovered in the isothermal hydrogenation is reported as Ca in Table 6.5. 129 Table 6.5. Effect of carbon aging on the hydrogenation step. Carbon Aging (min) 5 10 20 pmol CFL, consumed (Ac) 52 51 55 pmol H2 generated (Ac) 79 78 81 x of surface CHX 0.95 0.95 1.04 CFL, production (pimol) 4.8 3.9 2.8 C2+ production (pmol) 0.84 0.74 0.21 % C2+ selectivity (Is) 29* 27 13 ^IVCzFL; product 0.28 0.29 0 Ca production (pimol) 6.6 5.4 3.2 Cp production (pmol) 6.2 4.3 0.1 Cy production (pimol) 18.6 19.5 N/A+ Umol C produced (TPSR) 24.8 23.8 N/A+ C02 production in TPO (pmol) 11 15.3 26.9 * Selectivity includes both the C2 and C3 products. + Because of mass spectrometer failure this data point was not available. Catalyst: 0.5 g reduced A series 12%Co-Si02 Activation (Ac): 2 minutes exposure to 5% CEVAr at 450 °C Isothermal hydrogenation (Is): 10 minutes exposure to 11 rnl/min of H2 at 100 °C Temperature programmed surface reaction (TPSR): Exposure to 11 ml/min of H2 Temperature programmed oxidation (TPO): Exposure to 11 ml/rnin of 02 130 TPSR after isothermal hydrogenation showed that the amount of Cp decreased with aging time while the least active form carbon Cy increased with aging time (for the first ten minutes). This indicates that aging decreased not only the amount of the most active carbon Ca, but also the amount of less active carbon Cp. The increasing trend of CO2 production (in the TPO step) with aging indicated that the least active Cy deactivated to a less active carbon that could only be recovered by TPO. The effect of aging on the carbon species with more than a monolayer nominal coverage proved to be somewhat similar to the case with less than a monolayer coverage in the sense that the activity of carbon species decreases with aging. The fact that even at 100 °C aging can have significant effects on the activity and distribution of the carbon species and can generate inactive carbon has not been reported in the literature. In previous work (Koerts et al, 1992a; Koranne et al., 1995; Carstens and BelL 1996), it was assumed that fast cooling of the carbon species after activation, to about 100 °C (the temperature of the isothermal hydrogenation step) prevented deactivation of the carbon species. The results of the present study proved that at a temperature of 100 °C there were significant deactivation effects due to aging of the carbon species. 6.3.5. Reaction Cycle To study the effect of the number of two-step cycles on the catalyst, the A series 12% C0-S1O2 catalyst was exposed to a series of three CH4 activation, isothermal hydrogenation 131 and TPSR cycles without the TPO step. Only at the end of the third cycle was TPO performed . The results of this study are summarized in Table 6.6. Table 6.6. Effect of reaction cycle on the hydrogenation step. Cycles of Operation 1st 2nd 3rd pimol CH4 consumed (Ac) 66 53 48 p:mol H2 generated (Ac) 95 51 42 x of surface CHX 1.1 2.1 2.2 CH4 production (pmol) 7.7 2.1 1.83 C2+ production (pmol) 0.30 0.04 0.01 % C2+ selectivity (Is) 7.2 3.7 1.0 C recovered in TPSR (umol) 10 38 36 C recovered in TPO (umol) - - 6 Catalyst: 0.5 g reduced A series 12%Co-Si02 Activation (Ac): 2 minutes exposure to 5% CFLVAr at 450 °C Isothermal hydrogenation (Is): 10 minutes exposure to 11 ml/min of H2 at 100 °C Temperature programmed surface reaction (TPSR): Exposure to 11 ml/min of H2 Temperature programmed oxidation (TPO): Exposure to 11 ml/min of 02 In the activation step, increasing the number of cycles decreased the amount of generated H2 and activated CH4, indicative of a gradual loss of activity of the catalyst as the 132 number of operating cycles increased. However, by increasing the number of cycles, the extent of dehydrogenation of surface carbon species decreased. As discussed in section 6.2.1, a higher H content of the surface carbon species decreased the activity of the carbon species in the C-C bond formation reaction. As has been discussed previously, activation of CH4 with more than a monolayer metal coverage generates some inactive carbon species which cannot be removed by TPSR. The build-up of inactive carbon on the catalyst not only reduced the activity of the catalyst, but also affected the activity of the generated surface species. Surface CHX species with x close to one are most active for C-C bond formation. Carbon species with x>l are less active in the isothermal hydrogenation step. By increasing the number of cycles, not only did the amount of CH4 and C2H5 products decrease, but the C2+ selectivity also decreased by a factor of seven. TPSR after the isothermal hydrogenation showed that except for the first cycle, about 40 |imol of carbon was recovered in TPSR. This is due to stronger interaction of inactive carbon species with the catalyst in the first cycle. Inactive carbon species in the second and the third cycles have a weaker interaction with the support and are more easily removed by the TPSR. 6.3.6. Isothermal Medium In the two-step CH4 homologation on supported metal catalysts, the question of whether the C-C bond formation occurs during isothermal hydrogenation or if the carbon 133 species combine before hydrogenation, needs to be answered. In an attempt to address this question, different gases were used in place of H2 in the isothermal step. B series 8% Co-Si02 catalysts were exposed to 5% CEL/Ar gas at 450 °C for 2 min to activate CEL and generate carbon deposits on the catalyst. After purging the reactor while cooling, the catalyst was exposed to either H2, Ar, C02 or 02 at 100 °C for ten niinutes. In each experiment, nine samples were taken and stored for later analysis by GC. After purging the isothermal medium, TPSR in H2 and TPO were performed to determine the amount of less active carbon species in each case. The results of these experiments are summarized in Table 6.7. Table 6.7 shows that the activation conditions were practically the same for all experiments. The isothermal hydrogenation data show that small amounts of C2+ and CEL with a C2+ carbon selectivity of 13% were generated on the catalyst. A large part of the deposited carbon species were removed as less active and inactive carbon in the TPSR and TPO steps. The isothermal reaction with Ar generated some CEL and almost the same amount of C2H6 as the isothermal hydrogenation. The carbon selectivity of C2+ in the isothermal Ar experiment was about 39%. The carbon recovery in the isothermal Ar experiment was almost half that of the isothermal hydrogen experiment. Comparing with the isothermal hydrogenation, in Ar medium more carbon was recovered in TPSR and less inactive carbon was recovered in TPO. The presence of chemisorbed H in the isothermal step proves that either the Ar purge after CEL activation did not remove the chemisorbed EL or surface carbon species underwent 134 further dehydrogenation to generate chemisorbed H on the Co catalyst. The first possibility can be ruled out because H chemisorption at temperatures above 100 °C is negligible (Reuel and Bartholomew, 1984). Table 6.7. Effect of the isothermal medium on the product distribution. Isothermal Medium H2 Ar C02 o2 pmol CFL, consumed (Ac) 55 51 60 55 p:mol H2 generated (Ac) 81 76 86 78 x of surface CHX 1.0 1.0 1.1 1.1 CFL, production (pmol) 2.76 0.87 0.90 0.63 C2+ production (pmol) 0.21 0.28 0.04 0.04 % C2+ selectivity (Is) 13 39 8 11 C recovered in isothermal (pmol) 3.20 1.43 0.98 0.71 C recovered in TPSR (pimol) 14.3 24.7 58.5 -C recovered in TPO (pimol) 26.9 9.9 7.2 8.6 Catalyst: 0.5 g reduced B series 8%Co-Si02 Activation (Ac): 2 minutes exposure to 5% CHVAr at 450 °C Isothermal hydrogenation (Is): 10 min exposure to 11 ml/min of H2, Ar, C02 or 02 at 100 °C Temperature programmed surface reaction (TPSR): Exposure to 11 ml/min of H2 Temperature programmed oxidation (TPO): Exposure to 11 ml/min of 02 135 It is possible that after CFL, activation and Ar purge, surface carbon species undergo further dehydrogenation to produce chemisorbed H on the surface. Surface H then reacts with some surface carbon species and saturates some of the H deficient CFL, and C2+ precursors which then desorb and are removed by the gas flow in isothermal step. Isothermal reaction in C02 also generated some CFL, and a small amount of C2H6. The large amount of carbon recovered in TPSR was due to adsorption of C02. Isothermal 02 generated the smallest amounts of CFL, and almost the same quantity of C2FL5 as was obtained for the isothermal CO2 case. There was a 6.5 °C temperature rise during the isothermal O2 flow. Product analysis showed some H20 production in the isothermal step. The amount of CO, CH3OH or C2H5OH was negligible. Comparison of the results of the four (lifferent reaction media under isothermal conditions showed that H2 generated the highest amount of CFL,, while the amounts of CFL, produced in the Ar, CO2 and O2 reaction media were approximately the same. Surprisingly, H2 and Ar generated almost the same amount of C2FL5. This indicates not only that C-C bond formation occurs to some extent before the isothermal step but also that the homologation products can be released from the catalyst by an inert medium In the activation step, carbon species with an overall H/C ratio of 1 to 1.1 were deposited on the catalyst. The fact that C2+ products with H/C ratio of 4 to 3 could be produced by Ar proves that there is C-C bond formation, together with breakage and hydrogen exchange on the catalyst after activation. The temperature rise of 6.5 °C and H2O production in the isothermal O2 medium proves that there was chemisorbed H on the catalyst surface before the isothermal step. Since after activation the catalyst was purged with Ar the possibility of adsorbed hydrogen remaining on the surface 136 is ruled out. The adsorbed hydrogen must have come from the adsorbed CHX species after activation. These results show that as a result of C-C bond formation and breakage and hydrogen exchange between the surface species as well as the metal surface, some C2+ products are produced on the catalyst surface before the isothermal step. Some of the C2+ products are saturated and are readily desorbed, whereas some of the species need hydrogen to generate CFL, and C2FL;. In isothermal hydrogenation, hydrogen acts partially as a physical desorbing medium and it fiirnishes H for production of CFL and C2H6 from adsorbed species. H can also facilitate hydrogenolysis of some of the C-C bonds which are already formed on the surface. Possibly, C02 dissociated on the catalyst, generating some carbon species and adsorbed oxygen. The adsorbed oxygen reacts with active carbon species on the catalyst and reduces the production of CFL and C2FL products dramatically. 6.4. Conclusion Based on the results discussed in this chapter, the following sequence of steps in the surface reaction can be proposed. After the initial activation of CFL on the catalyst that produces surface carbon species, some of the carbon species migrate from the active metal site to the support. C-C bond formation and C-C bond breakage reactions start immediately after carbon deposition in the activation step. Longer exposure to high temperature leads to deactivation of the carbon species partially because of more migration of carbon species from the metal to the support site. Decreasing temperature from the high activation temperature to the lower isothermal hydrogenation temperature reduces the rate of the deactivation reactions. 137 Exposure to any gas medium at low temperature in the isothermal step liberates some of the CH4 and C2+ products that result from reactions on the catalyst before the reaction with the isothermal medium In addition, the nature of the isothermal gas medium and its reactions with the surface species affect the isothermal step product distribution. Exposure to hydrogen at high temperature during the TPSR removes most of the carbon species from the metal site and some of the carbon species residing on the support site. Only high temperature oxidation in the TPO step ensures complete removal of the inactive carbon species from the catalyst. To increase the C2+ yield of CH4 homologation with a two-step cycle it is better to use a high metal loading catalyst. A short activation time at about 450 °C followed immediately by the isothermal hydrogenation at about 100 °C gives a high yield of C2+ products and reduces the production of inactive carbon. In the CH4 activation step, deposition of more than a nominal monolayer carbon on the catalyst leads to more carbon recovery in the isothermal hydrogenation and TPSR. In this work, it was shown that by deposition of more carbon in the activation step a C2+ yield per site of about 2% can be obtained in the isothermal hydrogenation step. Although this is considered an improvement in comparison to the previous reports of 1% C2+ yield per site, one should note that more inactive carbon is also produced in this case. From a practical point of view, inactive carbons that are recovered only by high temperature hydrogenation or oxidation as CFL, or CO2 are considered a serious drawback. Based on the present operating conditions, one cannot devise an economically feasible process based on the two-step cycle for CFL homologation. Further improvements need to be made before a viable process based on this concept can be proposed. 138 Chapter 7 Conclusions and Recommendations for Future Work 7.1. Conclusions Two-step homologation of CFL, with Si02 supported Co catalysts was investigated in this work. Si02 supported Co catalysts were prepared by the wetness impregnation method. The catalysts were characterized by BET surface area and pore volume measurements, powder X-ray diffraction (PXRD), temperature programmed reduction (TPR), H2 desorption and Co re-oxidation. The characterized catalysts were used in the two-step homologation of CFL,. In the two-step homologation cycle, high temperature (ca 450 °C) activation of CFL, on the reduced catalyst produced H2 and surface carbon species. Lower temperature (ca 100 °C) hydrogenation of the active carbon species generated CFL and higher hydrocarbons. Less active carbon species were hydrogenated at high temperature in a temperature programmed surface reaction (TPSR) which followed the isothermal hydrogenation. After TPSR, the remaining inactive carbon species were removed by oxidation in a temperature programmed oxidation (TPO). By complete analysis of the reactor effluent, comparison of the carbon deposited in the activation step and carbon recovered in the isothermal hydrogenation, TPSR and TPO steps, became possible. Based on the results of this study, a kinetic model for CFL, activation on Si02-supported Co catalysts was developed. The kinetic model and its rate equations were used to interpret the effects of activation time, catalyst loading, activation temperature and catalyst support and promoter on CFL activation. In the isothermal hydrogenation step the effects of catalyst loading, activation time, activation temperature, carbon aging, reaction cycles and isothermal medium were studied. The principal observations and conclusions from the study are listed below: 1. Catalyst characterization results showed that there were two possible types of metal-support interaction (MSI) on Si02 supported Co catalysts. In the first case, cobalt-silicate compounds, which were not reducible under the reduction conditions of this work, were formed. In the second case, Si02 patches decorated the Co metal particles. The Co located below the Si02 decorations was detected by PXRD. It could be reduced and re-oxidized. However, it was not available for H2 chemisorption or CFL activation. 2. Activation of CFL on reduced Co catalysts generated H2 and surface carbon species. Experimental observations showed that the supported catalysts could activate more CEL than that corresponding to monolayer coverage of the surface Co by carbon species. It was suggested that migration of carbon species from the metal active site to the support site was responsible for liberation of metal active sites for further activation of gas phase CEL- Based on the initial activation of gas phase CEL on reduced Co and subsequent migration of carbon species from a metal site to a support site, a kinetic model was developed. By fitting the experimentally determined CEL consumption profile to the kinetic model, the rate constants of 140 the different steps of the CEL, activation kinetic model were obtained. The rate constants were used to interpret the effect of different operating parameters on the CEL, activation reaction. 3. Carbon deposited in the activation step was recovered by isothermal hydrogenation, temperature programmed surface reaction and temperature programmed oxidation. It was shown that not all the deposited carbon was recoverable by high temperature hydrogenation; only about 50% of the deposited carbon was removed by high temperature oxidation. The presence of this inactive carbon has not been reported by previous researchers. 4. Co-Si02 catalysts with nominal metal loadings of 2-12% were used for the two-step CEL, homologation. 2% Co-Si02 catalyst proved to be about 30% more active in the CEL, activation reaction, but its C2+ yield and selectivity were an order of magnitude lower than the 12% Co-Si02 catalyst. It was shown that lower loading catalysts had stronger metal-support interactions and produced carbon species with higher H content which led to lower tendency for C-C bond formation. 5. The effect of activation time on the two-step cycle was studied by activation of CEL, for 1-7 minutes and hydrogenation of the resulting carbon species. Longer activation times produced carbon species with lower hydrogen content. As a result, C2+ selectivity and yield corresponding to 7 min activation time was about half that obtained after one minute activation, while about four times as much carbon was deposited in the 7 min activation time. It was shown that longer activation time produced mainly inactive carbon deposits. 6. CEL was activated in the temperature range of 360 to 450 °C, and the deposited carbon species were hydrogenated at 100 °C. The activation energies of CEL activation on Co, adsorbed CH3 migration from metal to the support and H desorption were estimated as 141 56, 48 and 171 kJ/mole, respectively. It was shown that lower activation temperatures generated carbon species with higher H content leading to C2+ selectivities and yields which were about one third of those corresponding to a CH4 activation temperature of450 °C. 7. After CH4 activation at 450 °C, carbon species were aged at 100 °C for different periods of time. Hydrogenation of aged carbon species showed that, by increasing the aging time from 5 min to 20 min, C2+ selectivity and yield decreased by a factor of 2 and 4, respectively. Despite the accepted notion that only aging at high temperature deactivates the active carbon, present study showed that aging at a low temperature (100 °C) could also have a significant effect on the amount of active carbon. 8. It was shown that after CH4 activation on the Co-Si02 catalyst, some C-C bond formation occurs on the catalyst before the isothermal hydrogenation step. As a result of this C-C bond formation, some CH4 and C2H5 could be released by Ar instead of H2 at 100 °C. 142 7.2. Recommendations for Future Work As a result of this study, answers to some of the questions in the two-step cycle for CH4 homologation were obtained. In addition, the results of this work led to some new questions. For further expansion of this work, a list of recommended tasks is given below. 1. Preparation and characterization of Co-Si02 catalysts and understanding the nature and mechanism of metal-support interactions proved to be an important problem in the present study. A better understanding of the chemical transformations during impregnation, drying, calcination and reduction could make a significant contribution to the control of catalyst properties. 2. The chemical identity of carbon species on the metal and support site and the effect of time and temperature on these carbon species would be a significant extension of the present work. Such a study should include the kinetics of dehydrogenation of each carbon species on the catalyst. The results of such a study should be linked to the kinetic model of this work. 3. In the present kinetic model, only the rate of CEL, consumption was considered in the rate equations. The kinetics of hydrogen combination, desorption and spillover has not been treated. By considering these steps with the kinetics of dehydrogenation of carbon species (see above item) a complete kinetic model could be developed. 4. 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Wachs, I.E., Characterization of Catalytic Materials; Butterworth-Heineman: London, 1992. Wu, M.; Lenz-Solomun; Goodman, D.W., 'Two-Step, Oxygen-Free Route to Higher Hydrocarbons from Methane over Ruthenium Catalysts", J. Vac. Sci. Technol. A- Vac, Surf, Films 1994, 12,4, 2205-2209. Yoshitake, H.; Iwasawa, Y., 'Electronic Metal-Support Interaction in Pt Catalysts under Deuterium-Ethane Reaction Conditions and the Microscopic Nature of Active Sites", J. Phys. Chem. 1992, 96, 1329-1334. 151 Appendices Appendix 1 1.1. Calibration of Mass Flow Controllers 1.2. Conditions of BET Analysis 1.3. Analysis with Gas Chromatograph 1.4. Calibration of Quadrupole Mass Spectrometer 1.5. Analysis Conditions of Infra-Red Spectroscopy 152 Appendix 1.1. Calibration of Mass Flow Controllers Two 4 way-flow selection valves and two Brooks 5850E mass flow control sensors (see section 3.2) were used to prepare different gas mixtures for flow experiments. Mass flow control sensor "A" and "B" was factory calibrated for 20 mVmin (STP) CTL and 200 ml/min (STP) of N2, respectively. To use these flow sensors for other gases, calibration charts and calibration factors were developed. Using a soap bubble flow meter, calibration factors were obtained for different gases in the two control cells. The calibration factors agreed very well with the flow correction procedure outlined in the instrument manual (1). Figure Al. 1(a) is a typical calibration plot which was obtained for 02 flow with control cell "A". Based on the calibration plots, flow correlations in the form of Y = m. X were developed. Ln the flow correlation: Y: Volumetric gas flow, ml/min STP X: Percent reading of the controller (0 < X < 100) m: Flow correlation factor, rnl/min In Table Al. 1 flow correlation factors are given. (1) Installation and Operating Instructions, Brooks Mass Flow Controller, Model 5850E Brooks Instrument Division, Emerson Electric Co, Hatfiled, Pennsylvania 19440 153 30 Brooks Reading (%) Figure Al.l. Measured (O) vs. best fit for calibration plot of 02 with Brooks mass flow sensor (A). 154 Table Al. 1. Flow correlation factor (m). Gas Flow Cell m, ml/min CO2 A 0.19 at, A 0.20 A 0;24 H2 A 0.25 Ar B 2.79 5.16%CH4/Ar A 0.33 5.16%CH4/Ar B 2.69 155 Appendix 1.2. Conditions of BET Analysis Catalyst surface area and pore volume were determined using a Micromeritics (1) ASAP 2010 Accelerated Surface Area and Porosimetry System ASAP 2010 is a computer controlled system which performs the analyses and calculations based on the user programmed parameters. The summary of the analysis options are given below. Analysis Adsorptive: Nitrogen Maximum manifold pressure: 925.00 mmHg Non-ideality factor: 0.000066 Density conversion factor: 0.0015468 Therm, tran. hard-sphere diameter: 3.860 A Molecular cross-sectional area: 0.162 nm2 Fast evacuation: Yes Crossover pressure: 5.0 rnrnHg Leak test: No Leak test duration: 120 sees Evacuation time: 0.1 hours Backfill Gas: Analysis Equilibration interval: 5 sees Maximum volume increment: No Target tolerance. 5.0 % or 5.0 mmHg Mn. equil. delay at P/Po >= 0.995: 600 sees (1) Micromeritics, One Micromeritics Drive, Norcross, GZ, 30093-1877 156 Free space group: Measured Warm free space: 16.0000 cm3 Cold free space: 45.0000 cm3 Post-free space evacuation: No Evacuation time: 0.1 hours Leak test: Yes Leak test duration: 180 sees Low pressure dosing: No Dose amount: O.OOcmVgSTP Minimum equilibration delay: 0.00 hours Maximum equilibration delay: 999.00 hours Po type: Measured Po. 740.000 rnmHg Measurement interval: 120 minutes Temperature type: Entered Temperature: 77.35 K Measurement gas: N2 Inside diameter of sample tube: 9.530 mm Appendix 1.3. Analysis with Gas Chromatograph A calibrated gas chromatograph (GC) with a flame ionization detector (FID) was used for sampling and analysis of the reaction products in the isothermal hydrogenation step. For calibration, an analyzed gas mixture (Praxair) of 4.89% H2, 10.60% CFL, 2.01% C2H4, 3.95% C2H6 and 78.62% Ar was used. By repeat analyses, calibration factors (ratio of peak area of component '1" to mole% of component 'Y' )were determined. Typical calibration factors and corresponding retention times are reported in Table A3.1. Table A1.3. Calibration factors and retention time of different components. Component Retention time calibration factor (peak area/mole%) min mol°/o' CFL, 0.71 8.005 X 107 C2H4 3.16 1.511 X 108 C2FI6 4.15 1.598 X 108 During the sampling step, the sampling valve and the flow direction valve were programmed to direct the reactor effluent through the sampling valve and to take nine samples with an interval one minute. The GC operating program for sampling is provided in Figure A3.1(a) After sampling, the GC column had to be purged completely before analyzing the samples. In Figure A. 1.3(b), the GC program for pre-analysis purging is given. Sequential analysis of the collected samples were performed according to the program provided in Figure A. 1.3(c) 158 INITIAL AUK HOLD 71Mb 8.89 RELAYS PRGM TIME ST i 1,00 1 c. um -i 3 d. . 1 " 4 £. 18 -i ir ._• •i.00 1 6 3.18 -1 — 4.90 1 o 4.18 -•; q 5.88 1 5,16 _-; ' li 6.00 1 " Ih' 6.18 -1 13 7.80 1 14 7.16 -1 15 3.00 1 16 8.18 -1 17 4,fiH 1 Is 9.18 -1 19 9.50 c Figure A1.3. (a). List of program for sampling and storage of the reactor effluent in the sampling loops. 159 METHOD £ TIME ii:12 15 AUG 37 REV 9S05i£17£4 INITIAL COLUMN TEMP 145= INITIAL COL HOLD TIME 0.0W FINAL HOLD TOTAL FROM TEMP RATE TIME TIME 1 195 6.9 5.68 13.33 INJECTOR TEMP 145" RUAILIARY TEMP £26= INITIAL AUX HOLD TIME C=B0 DETECTOR TEMP ££©= FID B ATTEN RANGE A/2 8 9 YES PLOT SPEED 0.6 CM/MIN ZERO OFFSET 15 PLOT SIGNAL E TIME TICKS NO IHSTR EVENT CODES NO USER NUMBER 6-0 PRINT USER NUMBER HO PRINT REPORT NO PRINT RUN LOG NO INITIAL RELAYS 1-2 Figure A1.3. (b). List of program for purging the GC column before sample analyses. 160 METHOD 3 TIME 11; 12 5 AUG 97 REV 9609061711 INITIAL COLUMN TEMP 145* INITIAL COL HOLD TIME 0.08 FINAL HOLD TOTAL PRGM TEMP RATE TIME TIME I 195 6.0 5.80 13.33 INJECTOR TEMP 145E AUXILIARY TEMP £EOc INITIAL AUM HOLD TIME 8.68 DETECTOR TEMP £28= TCD A ATTEN RANGE A/Z SIG 16 .5 YES HEG FILAMENT TEMP OFF= FID B ATTEN RANGE A/2 128 1£ YES PLOT SPEED 8,5 CM/MIN ZERO OFFSET 15 •< PLOT SIGNAL B TIME TICKS NO INSTR EVENT CODES YES USER NUMBER 8-8 PRINT USER NUMBER HO PRINT REPORT YES PRINT RUN LOG NO INITIAL RELAYS l-£ RELAYS PRGM TIME STATE 1 8,81 1 £ 8,03 -1 Figure A1.3. (c). List of program for analysis if the stored samples. 161 Appendix 1.4. Calibration of Quadrupole Mass Spectrometer A mass spectrometer operates based on ionization of components and determination of relative amounts of ionized species based on differing "mass/charge" ratio. If each molecule produced only one characteristic ion, it would be a relatively easy task to find the composition of the analyte stream by deterrrrining the peak intensity of the characteristic mass numbers. In practice, because of different effects of isotope abundance and fragmentation, most of the molecules produce more than one charged species. Each molecule has a characteristic cracking pattern and a mixture of gases produces a complicated spectrum of charged species with overlapping mass numbers. Analysis of partial pressure versus mass number of a mixture of gases to find the gas composition is difficult when more than one molecule produces charged species at the same mass number (overlapping spectrum). For a system containing "n" gases and "m" mass number peaks, one can relate the measured mass number partial pressures to the actual partial pressures: M = Ax(SxP) Where M is the peak intensity vector, A is the cracking pattern matrix, S is the sensitivity vector and P is the actual partial pressure vector (1-5). Calculation of partial pressure i.e. chemical composition from mass spectrometer mass numbers measurement involves a tedious experimental procedure and mathematical solution algorithm. In this work the quadrupole mass spectrometer (QMS) was used to monitor the three steps of activation of CFLt, temperature programmed surface reaction (TPSR) and temperature programmed oxidation (TPO). One common feature of these three steps was the simplicity of 162 the reaction mixture. In CEL. activation step, a mixture of 5.16% CFL, and 94.84% Ar flowed through the reactor. Because of the CFL activation on the catalysts, the reactor effluent contained the unreacted CFL (less than the inlet stream), generated H2 and Ar (the same amount as inlet stream). In TPSR, H2 was admitted into the reactor and the effluent stream contained H2 (approximately the same amount as inlet stream) and CFL which was produced by reaction. In TPO, 02 was introduced into the reactor and the outlet stream contained 02 (approximately the same amount as the inlet stream), C02 and H20 which were produced by the oxidation reaction. In all these systems the chemical components of the analysis stream were known and there was a main component with high concentration and negligible change in composition i.e. Ar in CFL activation, H2 in TPSR and 02 in TPO. Simplicity of the reaction mixture was a great advantage to avoid the difficult experimental and computation task and to devise a simple method of converting the mass number recordings to chemical composition. In this simple method, the QMS was calibrated with mixtures of gases with a similar composition to the analysis gas. Calibration gas flows were established using the calibrated mass flow meters and UFD? grade pure gas or analyzed premixed gas mixtures. With (iifferent concentrations of the calibration gas stream, the mass number and total pressure from the QMS were recorded. By plotting the known mole fraction of the calibration stream versus (atomic mass unit)/(total pressure) readings from the QMS a calibration plot was prepared. Figure A1.4(a) shows a typical calibration plot of CFL/Ar niixture. From the least square best fit of the experimental points a calibration factor was extracted. 163 0.5 AMU 15/ P, Figure A1.4(a). Calibration plot of CHL, mole fraction (O) vs. AMU 15/Pt (O) is the experimental data and ( ) is the best fit. 164 In fact the calibration factor contained the effects of the cracking pattern, relative peak intensity and relative sensitivity of QMS for each component. Table A1.4 typical calibration factors for various gas mixtures are given. Table A1.4. Calibration factors for different components. Component Major Component Mass Number Calibration Factor H2 Ar 2 1.0 CH4 Ar 15 10.2 Ar Ar 40 15.2 H2 H2 2 0.9 CFL, H2 15 3.5 C2H4 H2 25 37.4 C2H6 H2 30 13.1 co2 o2 44 21.8 H20 o2 18 -o2 o2 32 9.0 In the simple reaction mixture of this work the accuracy of the above method was better than 95% in most cases. 165 1. O'Hanlon J.F., "A User's Guide to Vacuum Technology", second ed., John Wiley & Sons, 1989. 2. Harris, N., "Modern Vacuum Technology", McGraw Hill Books Co., 1989 3. Dobrozemsky R., "Experience with a computer program for residual gas analyzers", The Journal of Vacuum Science and Technology, vol. 9, No. 1, 220. 4. Easton D.S., Clausing R.E., "Outguessing of nuclear rocket fuel elements ", The Journal of Vacuum Science and Technology, Vol. 7, No. 6, SI 16. 5. Raimondi D.L., et al., "Automation of a residual gas analyzer on a time-shared computer", IBM J. Res. Dev., 1971, 307. 166 Appendix 1.5. Analysis Conditions of Infra-Red Spectroscopy Diffuse reflectance Infra-red Fourier transform spectroscopy (DRIFTS) was used to study the catalyst and deposited carbon species during the two-step reactions. The analysis conditions and data manipulation parameters of the DRIFTS studies are listed in this appendix. Requested_Scans_per_Scanset= 100 Number_of_Scansets= 60 Scan_type= K-Munk Background_File= c:\win_ir\data\soltan\try2\kbr50. spc Resolution= 4 Igram_Symmetry= Single Sided Sampling= UDR-2 Gain Range_Radius= 40 Scanning= Unidirectional Aperture= Open Detector= Back Beam_Path= Internal Source= MIR Garn_Amplirier= 1 Velocity= 20 KHz Low Pass_Filter= 5 KHz Apodization= Triangular Zero_Filling_Factor= 1 Laser_Wavenumber= 15800.82 Starting_Wavenumber=4000 Ending_Wavenumber= 700 Experiment_type= Normal Scans_per scanset= 100 167 Appendix 2 2.1. Temperature Programmed Reduction Profiles 2.2. Experimental Data 2.3. Equations for Calculations 2.4. Study of the Effect of Activation Temperature by the Kinetic Model 168 Appendix 2.1. Temperature Programmed Reduction Profiles The calcined Co-Si02 catalysts were reduced by a temperature programmed reduction (TPR) procedure in H2 (Section 3.4.1). The TPR profile for the A series 12% Co-Si02 catalyst is presented in Section 4.4. In this section, the TPR profiles of A series 5, 2 and 0.6% Co-Si02 catalysts are presented in Figure A.2.l.(a), A.2. l.(b) and A.2. l.(c), respectively. 169 0 500 1000 1500 2000 2500 3000 3600 4000 4500 5000 Time (sec) Figure A.2.1.(a). Profile of the temperature programmed reduction (TPR) of the A series 5%Co-Si02 catalyst with 54 ml/min of 20%H2/Ar gas mixture. 170 171 172 Appendix 2.2. Experimental Data Experimental reaction data of CEL, in CEL activation step, production of hydrocarbons in the isothermal hydrogenation step, CEL formation in the TPSR step and C02 formation in the TPO step were used to evaluate the effect of operating variables on the two-step cycle. In this section the experimental data for each set of experiments are reported in Table A2.2.(a) through A2.2.(g). 173 Table A2.2(a). Effect of activation time. Activation Time (min) 7 2 1 experiment series P52 P48 P53 ltmol CH4 consumed (Ac) 176.9 71.3 44.5 Ltmol H2 generated (Ac) 352.3 110.3 64 x of surface CHX (Ac) 0 0.91 1.12 ltmol CH4 production (Is) 3.70 5.27 6.30 Ltmol C2+production (Is) 0.10 0.16 0.24 Ltmol C production (TPSR) 163.0 38.0 1.0 pmol C02 production (TPO) 37.0 N/A N/A Catalyst: 0.5 g reduced A series 12%Co-Si02 Activation (Ac): exposure to 5% CFL/Ar at 450 °C and atmospheric pressure Isothermal hydrogenation (Is): 10 minutes exposure to 11 nWmin of H2 at 100 °C Temperature programmed surface reaction (TPSR): Exposure to 11 ml/min of H2 Temperature programmed oxidation (TPO): Exposure to 11 ml/min of 02 174 Table A2.2(b). Effect of catalyst metal loading. Catalyst Metal Loading (%) 12 5 2 experiment series P37 P38 P39 pimol CFL consumed (Ac) 85.1 43.6 36.7 pmol H2 generated (Ac) 102.0 59.4 44.3 x of surface CHX (Ac) 1.6 1.3 1.6 pmol CFL production (Is) 4.3 3.2 1.1 Umol C2+production (Is) 0.44 0.08 0.013 pimol C production (TPSR) 31.1 5.8 1.4 pmol CO2 production (TPO) N/A N/A N/A Catalyst: 0.5 g reduced A series 12%Co-Si02, 5%Co-Si02 and 2%Co-Si02 Activation (Ac): 2 minutes exposure to 5% CFL/Ar at 450 °C and atmospheric pressure Isothermal hydrogenation (Is): 10 minutes exposure to 11 ml/min of H2 at 100 °C Temperature programmed surface reaction (TPSR): Exposure to 11 ml/min of H2 Temperature programmed oxidation (TPO): Exposure to 11 ml/min of 02 175 Table A2.2(c). Effect of activation temperature. Activation Temperature (°C) 450 390 360 experiment series P60 P55 P56 pmol CFL, consumed (Ac) 56.8 36.7 27.0 ltmol FL generated (Ac) 89.5 47.2 26.3 x of surface CHX (Ac) 0.90 1.43 2.06 pmol CFL production (Is) 4.1 5.70 3.90 ltmol C2+ production (Is) 0.28 0.21 0.09 Ltmol C produced (TPSR) 18.7 0.2 0.1 Ltmol CO2 production (TPO) 25.0 N/A 29.4 Catalyst: 0.5 g reduced A series 12%Co-Si02 Activation (Ac): 2 minutes exposure to 5% CFL/Ar at atmospheric pressure Isothermal hydrogenation (Is): 10 minutes exposure to 11 ml/min of H2 at 100 °C Temperature programmed surface reaction (TPSR): Exposure to 11 ml/min of H2 Temperature programmed oxidation (TPO): Exposure to 11 rmVmin of 02 176 Table A2.2(d). Effect of various steps on carbon balance. Steps Ac Ac Ac Ac TPSR Is TPO Is TPO TPO TPSR TPO experiment series P57 P58 P59 P60 Ltmol CFL, consumed (Ac) 61.0 60.0 63.0 56.8 umol Ff2 generated (Ac) 91.4 89.6 88.8 89.5 x of surface CFfx (Ac) 1.0 1.0 1.2 0.9 Ltmol CFL production (Is) - 2.03 4.1 Ltmol C2+ production (Is) - N/A 0.28 Ltmol C produced (TPSR) 27.3 - - 18.7 |imol C02 production (TPO) 33.0 61.5 64.9 25 Catalyst: 0.5 g reduced A series 12%Co-Si02 Activation (Ac): 2 minutes exposure to 5% CFL/Ar at 450 °C and atmospheric pressure Isothermal hydrogenation (Is): 10 minutes exposure to 11 ml/min of H2 at 100 °C Temperature programmed surface reaction (TPSR): Exposure to 11 ml/min of H2 Temperature programmed oxidation (TPO): Exposure to 11 mVmin of 02 177 Table A2.2(e). Effect of carbon aging. Carbon Aging (min) 5 10 20 experiment series P67 P68 P63 umol CHU consumed (Ac) 52 51 55 Umol H2 generated (Ac) 79 78 81 x of surface CHX (Ac) 0.95 0.95 1.04 pmol CEL, production (Is) 4.8 3.9 2.8 Umol C2+ production (Is) 0.84 0.74 0.21 Umol Cp production (TPSR) 6.2 4.3 0.1 pmol Cy production (TPSR) 18.6 19.5 N/A umol C produced (TPSR) 18.7 0.6 0.1 umol C02 production (TPO) 11 15.3 26.9 Catalyst: 0.5 g reduced A series 12%Co-Si02 Activation (Ac): 2 minutes exposure to 5% CHLj/Ar at 450 °C and atmospheric pressure Isothermal hydrogenation (Is): 10 minutes exposure to 11 ml/min of H2 at 100 °C Temperature programmed surface reaction (TPSR): Exposure to 11 ml/min of H2 Temperature programmed oxidation (TPO): Exposure to 11 ml/min of 02 178 Table A2.2(f). Effect of cycles of operation. Cycle 1 2 3 experiment series P51a P51b P51c umol CFL, consumed (Ac) 66 53.3 47.8 Umol H2 generated (Ac) 95.0 50.5 41.6 x of surface CHX (Ac) 1.1 2.1 2.2 umol CFL, production (Is) 7.7 2.1 1.8 umol C2+production (Is) 0.3 0.04 0.01 p:mol C produced (TPSR) 10 38 36 umol C02 production (TPO) N/A N/A 6.0 Catalyst: 0.5 g reduced A series 12%Co-Si02 Activation (Ac): 2 minutes exposure to 5% CEVAr at 450 °C and atmospheric pressure Isothermal hydrogenation (Is): 10 minutes exposure to 11 rnl/niin of H2 at 100 °C Temperature programmed surface reaction (TPSR): Exposure to 11 ml/min of H2 Temperature programmed oxidation (TPO): Exposure to 11 mymin of 02 179 Table A2.2(g). Effect of isothermal medium. Isothermal Medium H2 Ar C02 o2 experiment series P63 P64 P65 P66 umol CFL, consumed (Ac) 54.6 51.3 59.8 54.8 itmol H2 generated (Ac) 80.6 75.9 85.6 78.1 x of surface CHX (Ac) 1.0 1.0 1.1 1.1 umol CFL production (Is) 2.76 0.87 0.90 0.63 Ltmol C2+production (Is) 0.21 0.28 0.04 0.04 umol C produced (TPSR) 14.3 24.7 58.5 N/A pmol C02 production (TPO) 26.9 9.9 7.2 8.6 Catalyst: 0.5 g reduced B series 8%Co-Si02 Activation (Ac): 2 minutes exposure to 5% CFL/Ar at 450 °C and atmospheric pressure Isothermal hydrogenation (Is): 10 minutes exposure to 11 ml/min of H2 at 100 °C Temperature programmed surface reaction (TPSR): Exposure to 11 rnl/min of H2 Temperature programmed oxidation (TPO): Exposure to 11 rnl/min of 02 180 Appendix 2.3. Equations for Calculations In this section the definitions and equations for various parameters are presented. CH4 conversion: (number of moles of CFL, activated)/(number of moles of CFL hi the feed) x of CHX surface species: Hydrogen content (H/C) in the surface species. x = 4 - 2*(number of moles of H2 generated)/(number of moles of activated CH4) Qc, nominal metal coverage by carbon = (number of moles of CH, actfvated)/(number of moles of surface Co in the catalyst) C2+ yield per site: (number of moles of C2+ generated in isothermal hydrogenation)/(number of moles of surface Co in the catalyst) C2+ carbon selectivity in isothermal hydrogenation step: (number of moles of surface carbon converted to C2+ products in isothermal hydrogenation step)/(number of moles of recovered surface carbon in isothermal hydrogenation step) C2+ carbon selectivity = [(2*number of moles of C2H<; and C2H4)+(3*number of moles of C3 products)]/[(number of moles of CH,)+ (2*number of moles of C2H5 and C2H4)+(3*number of moles of C3 products)] 181 Appendix 2.4. Study of the Effect of Activation Temperature by the Kinetic Model In this section, experimental results of the effect of CH4 activation temperature are compared with the kinetic model profiles. Figure A2.4(a), A2.4(b) and A2.4(c) show this comparison for the activation temperature of450 °C, 390 °C and 360 °C, respectively. 182 1.4 0.00 0.50 1.00 1.50 2.00 2.50 Tim e (m in) Figure A2.4(a). Cumulative moles of CFJL* consumed per mole of surface Co (O) measured during 2 min flow of 5%CH4/95%Ar gas mixture and 54 mi/min at 450 °C over A series 12%Co-Si02 catalyst, compared to model fit (—). Model parameter estimates: ki=5 min"1, k2=16.8 min"1, kr=0.56 min"1 and k4=1.24 min'1 (R2=.997) 183 0 0.5 1 1.5 2 2.5 Time (min) Figure A2.4(b). Cumulative moles of CFL, consumed per mole of surface Co (O) measured during 2 min flow of 5%CFL/95%Ar gas mixture and 54 ml/min at 390 °C over A series 12%Co-Si02 catalyst, compared to model fit (—). Model parameter estimates: ki=2 min"1, k2=5.1 min"1, k3=0.24 min"1 and k4=1.13 min"1 (R2=992) 184 0 0.5 1 1.5 2 25 Tim e (m in) Figure A2.4(c). Cumulative moles of CFL, consumed per mole of surface Co (O) measured during 2 min flow of 5%CHJ95%Ai gas mixture and 54 ml/min at 360 °C over A series 12%Co-SiC>2 catalyst, compared to model fit (—). Model parameter estimates: ki=1.4 min"1, k2=5.9 min'1, k3=0.01 min"1 and kr=0.87 min"1 (R2=.906) 185 Appendix 3 3.1. Computer Program Listing 186 Appendix 3.1. Computer Program Listing REM COMBINED MODEL AND OPTIMIZATION CLS REM Dimensions DIM Yl(100), FHOLD(IOO) DMTIME(IOO), CH2EXP(100), CH2CAL(100), CCH4EXP(100), CCH4CAL(100) DIMRK1(5), RK2(5), RK3(5), RK4(5) DIM RH2CAL(100), RCH4CAL(100), CUMXCAL(100), CUMXEXP(IOO) DIM S(100), CH3S(100), HS(IOO), CHXS(IOO), S2(100), CH3S2(100), CHXS2(100) REM Coverage Functions DEF FNS (TS, TCH3S, THS, TCH3S2) = -2 * Kl * TS A 2 + K2 * TCH3S + 2 * K3 * THS DEF FNCH3S (TS, TCH3S, THS, TCH3S2) = Kl * TS A 2 - (K2 + K4) * TCH3S DEF FNHS (TS, TCH3S, THS, TCH3S2) = Kl * TS A 2 - 2 * K3 * THS A 2 DEF FNCH3S2 (TS, TCH3S, THS, TCH3S2) = K2 * TCH3S1 - K5 * TCH3S2 DEF FNCHXS2 (TS, TCH3S, THS, TCH3S2) = K5 * TCH3S2 REM Read Data H=.001 REM READ DATA FOR TTME(20), CH2EXP(20), CCH4EXP(20) OPEN"CiNsoltanVKTNETICSXD^URSEVEN.PRN" FOR INPUT AS #1 INPUT #1, NDAT FOR 1=1 TO NDAT INPUT #1, TIME(I), CH2EXP(I), CCH4EXP(I) NEXT I CLOSE #1 REM Optimization conditions CONS = 2410.7 187 MCO = 40.75 TTMEMX = TTMEfNDAT) WH2 = 0 WCH4 = 1 NP = 4 TH(1) = 2.782 TH(2) = 12.67 TH(3)=1 TH(4) = .228 TH(5) = 1 NOB = NDAT FORI= 1 TO NOB Y1(I) = 0! NEXT I INPUT "Output data saved as"; A$ b$ = "C:\soltan\kuietics\STAT\" + A$ + ".pm" OPEN "o", #1, b$ REM Optimization program REM REM MAIN ROUTINE FOR NON-LINEAR REGRESSION ANALYSIS REM 60 REM REM calculate sel and conv DIM signs(NP), DIFZ(NP) EPS1 = .000001 EPS2 = .000001 FLAM = .5: MIT = 6: XFNU = 5! FORks=lTONP 188 DIFZ(ks) = .5:signs(ks)=l! PRINT #1, signs(ks) NEXTks signs(l) = 0 IH = NOB *NP DEVI Q(NP), P(NP), E(NP), Pffl(NP), TBfNP) DIM F(1700), r(1700), A(8000), d(1700), DELZ(9000) DIM XA(NP, NP), XB(NP, NP) GOSUB 340 CLOSE #1 CLOSE #2 CLOSE#3 END PRINT #1 "*********FINISE{ED***********" 340 DEF FNACOS (X) = ATN(SQR(1 / X / X - 1)) ERS2 = EPS2 NPSQ = NP *NP LX$ =" TEST ONE" PRINT #1, "NON-LINEAR REGRESSION ANALYSIS"; LX$: PRINT #1, "NUM OBS = "; NOB PRINT #1, "NUM PARAMS = "; NP PRINT "NON-LINEAR REGRESSION ANALYSIS"; LX$: PRINT #1, "NUM OBS = "; NOB PRINT "NUM PARAMS = "; NP GA = FLAM nit= 1 LAOS = 0 IF EPSK 0 THEN EPS1 = 0 SSQ = 0 FORI=lTONP XTH(1) = TH(1).NEXTI GOSUB 7000 FORI=lTONOB 189 REM PRINT F(I) r(D = Yl(T)-F(D SSQ = SSQ + r(I) * r(T): NEXT I PRINT #1, "INITIAL SUM OF SQUARES "; SSQ PRINT "IMTIAL SUM OF SQUARES "; SSQ REM REM BEGIN ITERATION REM 550GA = GA/XFNU INCNT = 0: PRINT #1,: PRINT #1,: PRINT #1, PRINT #1, "ITERATION NUMBER"; nit PRINT "ITERATION NUMBER"; nit 580 JS = 1 - NOB FORJ= 1 TONP TEMP = TH( J) P(J) = DIFZ(J)*TH(J) TH(J) = TH(J) + P(J) Q(J) = 0: JS = JS + NOB FOR ks = 1 TO NOB: FHOLD(ks) = F(ks): NEXT ks FORKX= 1 TONP XTH(KX) = TH(KX): NEXT KX GOSUB 7000 ij = JS -1 FORI=lTONOB ij=ij + l DELZ(ij) = F(I): F(I) = FHOLDtT) DELZ(ij) = DELZ(ij) - F(I): Q(J) = Q(J) + DELZ(ij) * rfl) NEXT I Q(J) = Q(J) / P(J): REM STEEPEST ASCENT TH(J) = TEMP NEXT J IF LAOS > 0 GOTO 2410 190 F0RI=1T0NP F0RJ=1T0I SUM = 0! KJ = NOB*(J- 1) KI = NOB *(I- 1) FOR K = 1 TO NOB KI = KI+ 1 KJ = KJ+1 SUM = SUM + DELZCTO) * DELZ(KJ) NEXT K TEMP = SUM / (P(I) * P(J)) JI = J + NP*(I- 1) d(JT) = TEMP ij = I + NP*(J- 1) d(ij) = TEMP NEXT J E(T)==SQR(d(Jl)) NEXT I REM 920 REM FORI=lTONP ij = I - NP FORJ=lTOI ij = ij + NP A(ij) = d(ij)/(E(T)*E(J)) JI = J + NP*(I- 1) A(JI) = A(ij): NEXT J NEXT I REM REM A SCALED MOMENT MATRIX REM H = -NP 191 F0RI=1T0NP P(T) = Q(n/E(D:Pffl(T) = P(T) H=NP+ i + n A(n) = A(ii) + GA NEXT I 1=1 FORKX= 1 TONP FORks=lTONP KJS = ks + W*(KX- 1) XA(ks, KX) = A(KJS): XB(ks, 1) = P(ks): NEXT ks: NEXT KX NB = I: NNVAR = NP GOSUB 2770 DET = XDET FORks = lTONP: FORKX = lTONP KJS = KX + NP*(ks- 1) P(ks) = XB(ks, 1): A(KJS) = XA(KX, ks): NEXT KX. NEXT ks REM REM P/E CORRECTION FACTOR REM XSTEP = 1: SUM1 = 0: SUM2 = 0: SUM3 = 0 1250 FORI= 1 TONP SUM1 = P(T) * PFfl(T) + SUM1 SUM2 = P(T) * P(I) + SUM2 SUM3 = Pffl(I) * Pffl(I) + SUM3 Pffl(T) = P(i):NEXTI TEMP = SUM1 / SQR(SUM2 * SUM3) IF TEMP >= 1! THEN TEMP = 1! TEMP = 57.29 * FNACOS(TEMP) PRINT #1, "DETERMINANT "; DET;" ANGLE DSf SCALED COORDS "; TEMP FORI=lTONP P(I) = Pffl(I) * XSTEP / E(T) TB(I) = TH(T) + P(T) 192 NEXT I PRINT #1, "TEST POINT PARAMS " FORI=lTONP PRINT #1, TB(I); NEXT I FORI=lTONP IF signs(I) < 0 GOTO 1470 IF TH(I) < 0 THEN TH(I) = 1 IF TB(I) < 0 THEN TB(I) = 1 IF TH(I) * TB(I) < 0 GOTO 1560 1470 NEXT I SUMB = 0 FOR ks = 1 TO NP: XTH(ks) = TB(ks): NEXT ks GOSUB 7000 FORI=l TO NOB r(I) = Yl(I)-F(D SUMB = SUMB + r(I) * r(I): NEXT I PRINT #1,: PRINT #1, "TEST POINT SUM OF SQUARES "; SUMB IF SUMB - (1 + EPS1) * SSQ < 0 GOTO 1650 1560 LF TEMP - 30 < GA GOTO 1580 ELSE LF GA > 0 GOTO 1620 GOTO 1590 1580 IF TEMP - 30 > 0 GOTO 1620 1590 XSTEP = XSTEP/2 INTCNT = ENTCNT + 1 IF INTCNT - 36 < 0 GOTO 1250 ELSE GOTO 1870 1620GA = GA*XFNU INTCNT = INTCNT + 1 IF INTCNT - 36 < 0 GOTO 920 ELSE GOTO 1870 1650 PRINT #1, "PARAMETER VALUES VIA REGRESSION" FORI=lTONP TH(I) = TB(I) NEXT I 193 ITYPE = 1: GNQ = NP: FOR ks = 1 TO NP GAS(ks) = TH(ks): NEXT ks GOSUB 3160: PRINT #1, PRINT #1, "LAMDA ="; GA;" SUM OF SQUARES AFTER REGRESSION "; SUMB PRINT " SUM OF SQUARES AFTER REGRESSION "; SUMB IF EPS2 > 0 GOTO 1750 IFEPS1 >0GOTO 1810 1750FORI=l TONP IF ABS(P(I)) / (9.999999E-21 + ABS(TH(I))) - EPS2 < 0 THEN GOTO 1780 IF EPS1 <= 0 GOTO 1840 ELSE GOTO 1810 1780 NEXT I PRINT #1, "ITERATION STOPS.REL CHANGE IN EACH PARAM < "; EPS2 GOTO 1890 1810 LF ABS(SUMB - SSQ) - EPS1 * SSQ > 0 GOTO 1840 PRINT #1, "ITERATION STOPS. REL CHANGE IN SUM OF SQRS < "; EPSl GOTO 1890 1840 SSQ = SUMB nit = nit + 1 IF nit - MIT <= 0 GOTO 550 ELSE GOTO 1890 1870 PRINT #1, "THE SUM OF SQRS CANNOT BE REDUCED TO THE SUM OF " PRINT #1, "SQRS AT THE END OF THE LAST ITERATION ..ITERATING STOPS" 1890 GOSUB 8000 PRINT #1, PRINT #1," RESPONSES PRINT #1," MEASURED PREDICTED RESIDUAL" FOR mags = 1 TO NOB PRINT #1, USING "####.###^A... Yl(mags); F(mags); r(mags) NEXT mags PRINT #1, PRINT #1," X TRANSFORM X MATRIX" ITYPE = 4: FOR ks = 1 TO NP: FOR KX = 1 TO NP. KJS = KX + NP * (ks -1) GC(KX, ks) = d(KJS): NEXT KX: NEXT ks 194 GOSUB3160 SSQ = SUMB IDF = NOB-NP PRINT #1,: PRINT #1," CORRELATION MATRIX" NB = 0: NNVAR = NP FORks=lTONP: FORKX = lTONP KJS = KX + NP*(ks- 1) XA(KX, ks) = d(KJS): XB(ks, 1) = P(ks): NEXT KX: NEXT ks GOSUB 2770 FORks = lTONP: FORKX=1 TONP KJS = KX + NP*(ks- 1) d(KJS) = XA(KX, ks): P(ks) = XB(ks, 1): NEXT KX: NEXT ks DET = XDET FORI=lTONP n = i + NP*a-1) E(I) = SQR(d(TI)): NEXT I FORI=lTONP JI = I + NP *(I- 1)- 1 ij = I + NP * (I - 2) FORJ = ITONP JI = JI+ 1 A(JI) = d(JI)/E(i)/E(J) ij = ij + NP A(ij) = A(JI): NEXT J NEXT I rTYPE = 3.FORks = lTONP: FOR KX = 1 TONP: KJS = KX + NP * (ks -1) GC(KX, ks) = A(KJS): NEXT KX: NEXT ks GOSUB 3160 IF DDF = 0 GOTO 2740 SDEV = SSQ/1DF PRINT #1,: PRINT #1, "VARIANCE OF RESIDUALS"; SDEV;" DEGREES OF FREEDOM" IDF 195 SDEV = SQR(SDEV) F0RI=1T0NP P(B = TH(l) + 2! *E(I)*SDEV TB(I) = TH(I) - 2! * E(T) * SDEV NEXT I PRINT #1, PRINT #1, "INDIVIDUAL CONFID LIMITS ON EACH PARAM (UNDER LIN HYPOTH)" ITYPE = 2: FOR ks = 1 TO NP: GAS(ks) = TB(ks): GB(ks) = P(ks): NEXT ks GOSUB 3160 LAOS = 1 GOTO 580 2410 FOR K= 1 TO NOB TEMP = 0! FORI=lTONP FORJ= 1 TONP ISUB = K + NOB *(I- 1) DEBUG1 = DELZTISUB) REM DEBUGl=DELZ+NOB*(J-l) ISUB = K + NOB *(J- 1) DEBUG2 = DELZ(ISUB) REM DEBUG2=DELZ+NOB*(J-l) ij = I + NP*(J- 1) DEBUG3 = d(ij) / (DIFZ(D * TH(l) * DIFZ(J) * TH(J)) TEMP = TEMP + DEBUG 1 * DEBUG2 * DEBUG3 NEXT J NEXT I TEMP = 2! * SQR(TEMP) * SDEV r(K) = F(K) + TEMP F(K) = F(K)-TEMP NEXT K PRINT #1,: PRINT #1, "APPROX CONFID LIMITS FOR EACH FUNCTION VALUE" IE = 0 196 F0RI=1T0N0B STEP 7 PRINT #1, IE = IE + 7 IF NOB - IE >= 0 GOTO 2670 IE = NOB 2670 FORks = ITOIE PRINT #1, USING "###.###/WVA"; r(ks); NEXT ks: PRINT #1, FORKX = ITOIE PRINT #1, USING "###.###AAAA"; F(KX); NEXT KX: PRINT #1, NEXT I 2740 RETURN PRINT #1, "PARAMETER ERROR" GOTO 2740 2770 REM THIS SUBROUTINE DETERMINES THE INVERSE OF ANY SQUARE MATRIX REM REM XB IS THE INVERSE OF XA PIVOTM = XA(l, 1) XDET=1 FOR ICOL = 1 TO NNVAR PIVOT = XA(ICOL, ICOL) IF PIVOT < PIYOTM THEN PIVOTM = PIVOT XDET = PIVOT * XDET REM REM DIVIDE PIVOT ROW BY PIVOT ELEMENT REM XA(ICOL, ICOL) = 1 IF PIVOT < 9.999999E-21 THEN PIVOT = 9.999999E-21 PIVOT = XA(ICOL, ICOL) / PIVOT FORL=l TO NNVAR XAflCOL, L) = XA(ICOL, L) * PIVOT 197 NEXTL IF NB<=0 GOTO 3020 FOR L = 1 TO NB XB(ICOL, L) = XB(ICOL, L) * PIVOT NEXTL REM REM REDUCE NON PIVOT ROWS REM 3020 FOR LI = 1 TO NNVAR IF (LI - ICOL) = 0 GOTO 3130 T = XA(Ll,ICOL) XA(L1,ICOL) = 0 FORL=lTONNVAR XA(L1, L) = XA(L1, L) - XAflCOL, L) * T NEXTL IF NB = 0 GOTO 3130 FOR L = 1 TO NB XB(L1, L) = XB(L1, L) - XB(lCOL, L) * T NEXTL 3130 NEXT LI NEXT ICOL RETURN 3160GNQ = NP REM NR = F1X(NP/ 10) LOW=l LUP=10 3210 IF NR < 0 THEN RETURN IF NR > 0 GOTO 3260 LUP = GNQ IF LOW - LUP > 0 THEN RETURN PRINT #1,SPC(7); 198 3260 FOR J = LOW TO LUP PRINT #1, J; SPC(8); NEXT J: PRINT #1, IF ITYPE = 1 GOTO 3410 LF ITYPE = 2 GOTO 3450 IF ITYPE = 4 GOTO 3380 FOR I = LOW TO LUP PRINT #1,1; SPC(2); FOR ks = LOW TO I PRINT #1, USING "##.##AAAA"; GC(ks, I); NEXT ks: PRINT #1, NEXT I: GOTO 3490 3380 FOR I = LOW TO LUP PRINT #1,1; SPC(2); FOR ks = LOW TO I PRINT #1, USING "##.###AAAA"; GC(I, ks); NEXT ks. PRINT #1, NEXT I: GOTO 3490 3410 FOR ks = LOW TO LUP PRINT #1, USING "##.###AAAA"; GAS(ks); NEXT ks: PRINT #1, GOTO 3660 3450 FOR ks = LOW TO LUP PRINT #1, USING "##.###AAAA"; GB(ks); NEXT ks: PRINT #1, GOTO 3410 3490 REM LOW2 = LUP + 1 IF LOW2 - GNQ > 0 GOTO 3660 LF ITYPE = 4 GOTO 3600 FORI = LOW2TOGNQ PRINT #1,1; SPC(2); 199 FOR ks = LOW TO LUP PRINT #1, USING "##.###AAAA"; GC(ks, I); NEXT ks: PRINT #1, NEXT I GOTO 3660 3600 FOR I = LOW2 TO GNQ PRINT #1,1; SPC(2); FOR ks = LOW TO LUP PRINT #1, USING "U.#m"^"; GC(I, ks); NEXT ks: PRINT #1, NEXT I 3660 LOW = LOW + 10 LUP = LUP + 10 NR = NR- 1 GOTO 3210 RETURN 7000 REM REM Initialization for RK optimization COMPDAT = 2 K1=XTH(1) K2 = XTH(2) K3=XTH(3) K4 = XTH(4) K5 = XTH(5) CH2CAL0) = 0 CCH4CAL0) = 0 TIME0 = 0 TS0 = 1 TCH3S0 = 0 THS0 = 0 TCH3S20 = 0 200 TCHXS20 = 0 CMETHO = 0 CHYDRO = 0 7500 IF TIMEO > TTMEMX THEN 10000 REM RK Parameters REM Kl REM PRINT TIMEO, TSO, TCH3S0, THSO REM PRINT TCH3S20, TCH3 SXO RK1(1) = H * FNSCTSO, TCH3S0, THSO, TCH3S20) RK1(2) = H * FNCH3S(TS0, TCH3S0, THSO, TCH3S20) RK1(3) = H * FNHS(TS0, TCH3S0, THSO, TCH3S20) RK1(4) = H * FNCH3S2(TS0, TCH3S0, THSO, TCH3S20) RK1(5) = H * FNCHXS2(TS0, TCH3S0, THSO, TCH3S20) REM K2 RK2(1) = H * FNS(TSO + RK1(1) / 2, TCH3S0 + RK1(2) / 2, THSO + RK1(3) / 2, TCH3S20 + RK1(4) / 2) RK2(2) = H * FNCH3SCTS0 + RK1(1) / 2, TCH3S0 + RK1(2) / 2, THSO + RK1(3) / 2, TCH3S20 + RK1(4) / 2) RK2(3) = H * FNHS(TSO + RK1(1) / 2, TCH3S0 + RK1(2) / 2, THSO + RK1(3) / 2, TCH3S20 + RK1(4) / 2) RK2(4) = H * FNCH3S2(TS0 + RK1(1) / 2, TCH3S0 + RK1(2) / 2, THSO + RK1(3) / 2, TCH3S20 + RK1(4) / 2) RK2(5) = H * FNCHXS2(TS0 + RK1(1) / 2, TCH3S0 + RK1(2) / 2, THSO + RK1(3) / 2, TCH3S20 + RK1(4) / 2) REM K3 RK3(1) = H * FNSfTSO + RK2(1)7 2, TCH3S0 + RK2(2) / 2, THSO + RK2(3) / 2, TCH3S20 + RK2(4) / 2) RK3(2) = H * FNCH3S(TS0 + RK2(1) / 2, TCH3S0 + RK2(2) / 2, THSO + RK2(3) / 2, TCH3S20 + RK2(4)/2) RK3(3) = H * FNHS(TS0 + RK2(1) / 2, TCH3S0 + RK2(2) / 2, THSO + RK2(3) / 2, TCH3S20 + RK2(4) / 2) 201 RK3(4) = H * FNCH3S2(TS0 + RK2(1) / 2, TCH3S0 + RK2(2) / 2, THSO + RK2(3) / 2, TCH3S20 + RK2(4) / 2) RK3(5) = H * FNCHXS2(TS0 + RK2(1) / 2, TCH3SO + RK2(2) / 2, THSO + RK2(3) / 2, TCH3S20 + RK2(4) / 2) REM K4 RK4(1) = H * FNS(TSO + RK3(1), TCH3SO + RK3(2), THSO + RK3(3), TCH3S20 + RK3(4)) RK4(2) = H * FNCH3S(TSO + RK3(1), TCH3S0 + RK3(2), THSO + RK3(3), TCH3S20 + RK3(4)) RK4(3) = H * FNHSCTSO + RK3(1), TCH3SO + RK3(2), THSO + RK3(3), TCH3S20 + RK3(4)) RK4(4) = H * FNCH3S2(TSO + RK3(1), TCH3SO + RK3(2), THSO + RK3(3), TCH3S20 + RK3(4)) RK4(5) = H * FNCHXS2(TS0 + RK3(1), TCH3SO + RK3(2), THSO + RK3(3), TCH3S20 + RK3(4)) REM NEXT STEP REM Coverage of species TS 1 = TSO + RK1(1) / 6 + RK2(1) / 3 + RK3(1) / 3 + RK4(1) / 6 TCH3S1 = TCH3SO + RK1(2) / 6 + RK2(2) / 3 + RK3(2) / 3 + RK4(2) / 6 THS1 = THSO + RX1(3) / 6 + RK2(3) / 3 + RK3(3) / 3 + RK4(3) / 6 TCH3S21 = TCH3S20 + RK1(4) / 6 + RK2(4) / 3 + RK3(4) / 3 + RK4(4) / 6 TCHXS21 = TCHXS20 + RK1(5) / 6 + RK2(5) / 3 + RK3(5) / 3 + RK4(5) / 6 CMETH1=H*MC0*K1 * TS 1 A 2 + CMETHO CHYDR1 = H * MCO * (K3 * THS1 A 2 + K4 * TCH?SI + K5 * TCH3S21) + CHYDRO TTME1 =TTMEO + H REM MATCH RATES IF ((TTME(COMPDAT) < TTMEl) AND (TTME(COMPDAT) > TTMEO)) THEN CH2CAL(COMPDAT) = CHYDRO + (CHYDR1 - CHYDRO) * (TTME(COMPDAT) - TTMEO) /H CCH4CAL(COMPDAT) = CMETHO + (CMETH1 - CMETHO) * (TTME(COMPDAT) -T1ME0)/H 202 REM PRINT "RATE-1; COMPDAT, CONS * CH2EXP(COMPDAT); CH2CAL(COMPDAT); CONS * CCH4EXP(COMPDAT); CCH4CAL(COMPDAT) COMPDAT = COMPDAT + 1 END IF TIMEO = TTME1 TS0 = TS1 TCH3S0 = TCH3S1 THS0 = THS1 TCH3S20 = TCH3S21 TCHXS20 = TCHXS21 CMETHO = CMETH1 CHYDRO = CHYDR1 GOTO 7500 10000 REM SUM = 0 FOR 11 = 1 TONDAT F(I1) = (WH2 * (CH2CAL(I1) - CONS * CH2EXP(H)) A 2 + WCH4 * (CCH4CAL(H) - CONS * CCH4EXP(H)) A 2) A .5 SUM = SUM + F(H)*Fai) NEXT II PRINT KI, K2, K3, K4, K5, "SUM="; SUM RETURN REM Use optimum parameters to find the coverages and rates 8000 REM CALCULATE FINAL FORM REM Initialize S(l)=l CH3S(1) = 0 HS(1) = 0 CHXS(1) = 0 S2(l)= 1 203 CH3S2(1) = 0 CHXS2(1) = 0 K1=XTH(1) K2 = XTH(2) K3=XTH(3) K4 = XTH(4) K5=XTH(5) CH2CAL(1) = 0 CCH4CAL(1) = 0 COMPDAT = 2 CONS = 2410.7 MCO = 40.75 TIMEO = 0 TSO = 1 TCH3S0 = 0 THS0 = 0 TCH3S20 = 0 TCHXS20 = 0 CMETH0 = 0 CHYDRO = 0 RMETHO = 0 RHYDRO = 0 RH2CAL(1) = 0 RCH4CAL(1) = 0 CH2CAL(1) = 0 CCH4CAL(1) = 0 CUMXCAL(l) = 4 CUMXEXP(l) = 4 TLMEMX = TLME(NDAT) 8500 IF TIMEO > 1TMEMX THEN 11000 204 REM KI PRINT TTMEO, TSO, TCH3S0, THSO REM PRINT TCH3S20, TCH3SXO RK1(1) = H * FNS(TS0, TCH3S0, THSO, TCH3S20) RK1(2) = H * FNCH3S(TS0, TCH3SO, THSO, TCH3S20) RK1(3) = H * FNHS(TSO, TCH3SO, THSO, TCH3S20) RK1(4) = H * FNCH3S2(TSO, TCH3SO, THSO, TCH3S20) RK1(5) = H * FNCHXS2(TS0, TCH3SO, THSO, TCH3S20) REM K2 RK2(1) = H * FNSOTSO + RK1(1) / 2, TCH3SO + RK1(2) / 2, THSO + RK1(3) / 2, TCH3S20 + RK1(4) / 2) RK2(2) = H * FNCH3S(TSO + RK1(1) / 2, TCH3SO + RK1(2) / 2, THSO + RK1(3) / 2, TCH3S20 + RK1(4) / 2) RK2(3) = H * FNHS(TSO + RK1(1) / 2, TCH3S0 + RK1(2) / 2, THSO + RK1(3) / 2, TCH3S20 + RKl(4)/2) RK2(4) = H * FNCH3S2(TS0 + RK1(1) / 2, TCH3SO + RK1(2) / 2, THSO + RK1(3) / 2, TCH3S20 + RK1(4) / 2) RK2(5) = H * FNCHXS2(TS0 + RK1(1) / 2, TCH?SO + RK1(2) / 2, THSO + RK1(3) / 2, TCH3S20 + RK1(4) / 2) REM K3 RK3(1) = H * FNS(TSO + RK2(1) / 2, TCH?SO + RK2(2) / 2, THSO + RK2(3) / 2, TCH3S20 + RK2(4) / 2) RK3(2) = H * FNCH3S(TSO + RK2(1) / 2, TCH3SO + RK2(2) / 2, THSO + RK2(3) / 2, TCH3S20 + RK2(4)/2) RK3(3) = H * FNHSCTSO + RK2(1) / 2, TCH3S0 + RK2(2) / 2, THSO + RK2(3) / 2, TCH3S20 + RK2(4) / 2) RK3(4) = H * FNCH3S2(TS0 + RK2(1) / 2, TCH?SO + RK2(2) / 2, THSO + RK2(3) / 2, TCH3S20 + RK2(4) / 2) RK3(5) = H * FNCHXS2(TS0 + RK2(1) / 2, TCH? SO + RK2(2) / 2, THSO + RK2(3) / 2, TCH3S20 + RK2(4) / 2) REM K4 RK4(1) = H * FNS(TSO + RK3(1), TCH?SO + RK3(2), THSO + RK3(3), TCH3S20 + RK3(4)) 205 RK4(2) = H * FNCH3S(TS0 + RK3(1), TCH3S0 + RK3(2), THSO + RK3(3), TCH3S20 + RK3(4)) RK4(3) = H * FNHS(TSO + RK3(1), TCH3SO + RK3(2), THSO + RK3(3), TCH3S20 + RK3(4)) RK4(4) = H * FNCH3S2(TSO + RK3(1), TCH3SO + RK3(2), THSO + RK3(3), TCH3S20 + RK3(4)) RK4(5) = H * FNCHXS2(TSO + RK3(1), TCH3S0 + RK3(2), THSO + RK3(3), TCH3S20 + RK3(4)) REM NEXT STEP TS1 = TSO + RK1(1) 16 + RK2(1) / 3 + RK3(1) / 3 + RK4(1) / 6 TCH3S1 = TCH3SO + RK1(2) / 6 + RK2(2) / 3 + RK3(2) / 3 + RK4(2) / 6 THS1 = THSO + RK1(3) / 6 + RK2(3) / 3 + RK3(3) / 3 + RK4(3) / 6 TCH3S21 = TCH3S20 + RK1(4) / 6 + RK2(4) / 3 + RK3(4) / 3 + RK4(4) / 6 TCHXS21 = TCHXS20 + RK1(5) / 6 + RK2(5) / 3 + RK3(5) / 3 + RK4(5) / 6 CMETH1 =H * MCO * Kl * TS1 A2 + CMETHO CHYDR.1 = H * MCO * (K3 * THS1 A 2 + K4 * TCH3S1 + K5 * TCH3S21) + CHYDRO RMETHl = MCO * Kl * TS1 A 2 RHYDR1 = MCO * (K3 * THS1 A 2 + K4 * TCH3S1 + K5 * TCH3S21) TIME1 = TIMEO + H REM MATCH RATES IF ((TIME(COMPDAT) < TTME1) AND (TTME(COMPDAT) > TIMEO)) THEN CH2CAL(COMPDAT) = CHYDRO + (CHYDR1 - CHYDRO) * (TTME(COMPDAT) - TIMEO) /H CCH4CAL(COMPDAT) = CMETHO + (CMETH1 - CMETHO) * (TLME(COMPDAT) -TIMEO)/H RH2CAL(COMPDAT) = RHYDRO + (RHYDR1 - RHYDRO) * (TIME(COMPDAT) - TIMEO) /H RCH4CAL(COMPDAT) = RMETHO + (RMETHl - RMETHO) * (TIME(COMPDAT) -TIMEO)/H CUMXCAL(COMPDAT) = 4 - 2 * CH2CAL(COMPDAT) / CCH4CAL(COMPDAT) 206 CUMXEXP(COMPDAT) = 4 - 2 * CH2EXP(COMPDAT) / CCH4EXP(COMPDAT) S(COMPDAT) = TSO + (TS1 - TSO) * (TIME(COMPDAT) - TTMEO) / H HS(COMPDAT) = THSO + (THS1 - THSO) * (TTME(COMPDAT) - TTMEO) / H CH3S(COMPDAT) = TCH3S0 + (TCH3S1 - TCH3S0) * (TTME(COMPDAT) - TTMEO) / H CH3S2(COMPDAT) = TCH3S20 + (TCH3S21 - TCH3S20) * (TTME(COMPDAT) - TTMEO) / H CHXS2(COMPDAT) = TCHXS20 + (TCHXS21 -TCHXS20) * (TTME(COMPDAT) -TTMEO)/H S2(COMPDAT) = 1 - (CH3S2(COMPDAT) + CHXS2(COMPDAT)) CHXS(COMPDAT) = 1 - (S(COMPDAT) + HS(COMPDAT) + CH3S(COMPDAT)) PRINT "RATE-'; COMPDAT, CONS * CH2EXP(COMPDAT); CH2CAL(COMPDAT); CONS * CCH4EXP(COMPDAT); CCH4CAL(COMPDAT) COMPDAT = COMPDAT + 1 END IF TTMEO = TTME1 TS0 = TS1 TCH3S0 = TCH3S1 THS0 = THS1 TCH3S20 = TCH3S21 TCHXS20 = TCHXS21 CMETHO = CMETH1 CHYDRO = CHYDR1 RMETHO = RMETH1 RHYDRO = RHYDR1 GOTO 8500 11000 REM REM REcord the results in the file REM WRITE DATA FOR TTME(20), RH2EXP(20), RCH4EXP(20) SUMH = 0 SUMC=0 EXC = 0 207 EXH = 0 FOR 11 = 1 TO NDAT SUMH = SUMH + (CH2CAL<I1) - CONS * CH2EXP(I1)) A 2 SUMC = SUMC + (CCH4CAL(I1) - CONS * CCH4EXP(I1)) A 2 EXH = EXH + (CONS * CH2EXP(I1)) A 2 EXC = EXC + (CONS * CCH4EXP(I1)) A 2 NEXT II OUTPT$ = "C:\soltan\KJJSIETICS\OUTPUT\,, + A$ + ".PRN" OPEN OUTPT$ FOR OUTPUT AS #2 WRITE #2, "H=", H, "SUM H=", SUMH, "SUM C=", SUMC PRINT "SUM H="; SUMH, "SUM C="; SUMC WRITE #2, "DT# PT=", NDAT, "EXS H=", EXH, "EXS C=", EXC PRINT "EXS H="; EXH "EXS C="; EXC WRITE #2, "Kl=", Kl, "K2=", K2, "K3=", K3, "K4=", K4, "K5=", K5 WRITE #2, "t (MIN)", "sigH2(EX)", "sigH2(CL)M, "sigCH4(EX)", "sigCH4(CL)H, "X(EX)", "X(CL)", "rtH2(CL)", "rtCH4(CL)", "Tet(Sl)M, "Tet(CH3Sl)", "Tet(HSl)", "Tet(CHxSl)", "Tet(S2)", "Tet(CH3S2)", "Tet(CHxS2)" FOR I = 1 TO NDAT WRITE #2, TLME(l), CONS * CH2EXP(I), CH2CAL(D, CONS * CCH4EXP(I), CCH4CAL(I), CUMXEXP(I), CUMXCAL(I), RH2CAL(l), RCH4CAL(I), S(I), CH3S(I), HS(I), CHXS(I), S2(I), CH3S2(I), CHXS2(I) NEXT I CLOSE #2 RETURN 208 

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