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Methanol Synthesis from CO2/H2 Over Pd-Promoted Cu/Zn0/Al203 Catalysts

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Methanol Synthesis from CO2/H2 Over Pd-Promoted Cu/Zn0/Al203 Catalysts Department of Chemical Engineering and Chemical Technology Imperial College of Science, Technology and Medicine University of London Methanol Synthesis from CO2/H2 over Pd-promoted Cu/Zn0/Al203 catalysts by Mortaza Sahibzada A thesis submitted for the degree of Doctor of Philosophy of the University of London and for the Diploma of Imperial College September 1995 Abstract The kinetics of methanol synthesis ftom CO2/H2 and other gas mixtures was investigated over Pd-promoted Cvi/Zn0/Al203 catalysts using an internal recycle reactor and a differentially operated tubular reactor. Methanol production from CO2/H2 was promoted by use of physical mixtures and, to a greater extent, by impregnation/coprecipitation of Pd. Since Pd/Al203 itself was inactive and since the similar reverse water-gas shift activity of all catalysts indicated that their Cu dispersions were similar, it was concluded that the promotion involved hydrogen spillover. The direct rate of CO2 hydrogenation to methanol, measured at differential conversion under CO2/H2, was much greater than the methanol production rates measured previously at integral conversion, and was not effected by the presence of Pd in the catalysts. The inhibition of CO2 hydrogenation by the product water accounted for the relatively low production rates at integral conversion, and the counteracting effect of hydrogen spillover from Pd explained the observed promotion (water may cause some oxidation of Cu whereas hydrogen spillover maintains the active reduced state). Methanol production from CO/H2 was inhibited by Pd, and again the results pointed to the involvement of hydrogen spillover. The inhibition was particularly acute at differential conversion, which showed that Pd inhibits CO hydrogenation in general. At integral conversion the rate of CO hydrogenation on Cu was promoted by trace amounts of water produced over Cu and also Pd, such that the inhibition by Pd was partly obscured. The effect of Pd on methanol production from CO/CO2/H2 was a combination of the individual effects found previously. Most interesting, at integral conversion with small CO2 fractions there was a dramatic promotion of CO hydrogenation by small amounts of water produced by the reverse water-gas shift reaction, but there was also acute inhibition by Pd. The crossover from inhibition to promotion occurred at CO2/CO £ 1. A cknowledgements I am grateful to my supervisors, Ian Metcalfe and Dave Chadwick, for continuous support and encouragement throughout the last few years, and for highly challenging technical discussions relating to the thesis. I am grateful to past and present friends from the Applied Catalysis Group and from the Department for providing stimulating distraction at dinner times before the trek back to College for the night shift. I am grateful to the E.P.S.R.C. and I.C.I. Katalco for the award of a C.A.S.E. studentship, and to I.C.I. Katalco for accomodating my industrial visits. Finally, thank God it's all over. Contents Title page 1 Abstract 3 Acknowledgements 5 Contents 7 List of figures 11 List of tables 13 Catalyst nomenclature 15 Chapter 1 INTRODUCTION AND AIMS 17 1.1 Summary of prior work 17 1.2 Introduction to the thesis 18 1.3 Aims of the thesis 18 Chapter 2 BACKGROUND AND LITERATURE 19 2.1 The methanol and water-gas shift reactions: thermodynamics 19 2.2 Methanol synthesis from CO/CO2/H2 over Cu/Zn0/Al203 21 2.2.1 Evidence for Cu as the active site and the state of Cu 21 2.2.2 Roles of ZnO and AI2O3 23 2.2.3 Effect of CO2 fraction and H2O on methanol production 25 2.2.4 Methanol production by CO or C02 hydrogenation? 27 2.3 Effect of Pd on Cu/ZnO catalysts for methanol synthesis from CO2/H2 and other gas mixtures 30 2.3.1 Effect ofPd under CO-rich synthesis gases 30 2.3.2 Effect of Pd under CO2/H2 31 Chapter 3 CATALYST PREPARATION AND CHARACTERISATION 33 3.1 Overview of Cu/ZnO/Al^Og catalyst preparation 33 3.2 Preparation of catalysts and physical mixtures 35 3.3 Bulk chemical analyses by A. A. Spectroscopy 37 3.4 B.E.T. surface area and porosity measurements 38 3.5 Temperature programmed reduction of the catalysts 40 3.6 Cu surface area measurement for Cu/Zn/Al 42 3.7 Note: problems associated with Pd and Cu surface area measurements for Pd/Cu/Zn/Al catalysts , 43 Chapter 4 EXPERIMENTAL 45 4.1 Internal recycle (Berty) reactor system 45 4.2 Tubular (micro-) reactor system 47 4.3 Procedure for kinetic experiments 49 4.4 Gas chromatography 50 4.5 Calculation of kinetic data 51 4.6 Verification of perfect mixing in the Berty reactor 52 4.7 Verification of kinetics not limited by mass transfer 53 4.7.1 Extra-particle mass transfer in the Berty reactor 53 4.7.2 Intra-particle mass transfer in the Berty reactor 54 4.7.3 Extra-particle mass transfer in the tubular reactor 55 4.7.4 Intra-particle mass transfer in the tubular reactor 55 4.8 Reproducibility of results 56 4.8.1 Reproducibility of results using the Berty reactor 56 4.8.2 Reproducibility of results using the tubular reactor 58 4.9 Mass balances 58 4.10 The validity of initial activity measurements 59 Chapter 5 METHANOL SYNTHESIS FROM CO^/H^ 63 5.1 Summary 63 5.2 Trace products from CO2/H2 (Berty reactor) 64 5.3 Methanol synthesis from CO2/H2 over Cu/Zn/Al, Pd/Al and physical mixtures (Berty reactor) 66 5.4 Methanol synthesis from CO2/H2 over Pd/Cu/Zn/Al catalysts (Berty reactor) 67 5.5 CO production from CO2/H2 over all catalysts (Berty reactor) 73 5.6 Selectivity between methanol and CO production under CO2/H2 (Berty reactor) 75 5.7 Water production from CO2/H2 over all catalysts (Berty reactor) 76 5.8 Comparison of methanol synthesis from CO2/H2 in the Berty and tubular reactors 76 5.9 Approach to differential methanol production under CO2/H2 77 5.10 Approach to differential CO production under CO2/H2 79 5.11 Methanol production from CO2/H2 over all catalysts at differential conversion 80 5.12 Methanol production from CO2/H2 + CO at differential conversion 82 5.13 Methanol production from CO2/H2 + H2O at differential conversion 83 5.14 Comparison of methanol production from CO2/H2/H2O (differential conversion) and CO2/H2 (integral conversion) 86 5.15 Concluding remarks 87 Chapter 6 METHANOL SYNTHESIS FROM CO/H2 89 6.1 Summary 89 6.2 Trace products from CO/H2 (Berty reactor) 90 6.3 Methanol synthesis from CO/H2 over Cu/Zn/Al, Pd/Al and physical mixtures (Berty reactor) 92 6.4 Methanol synthesis from CO/H2 over Pd/Cu/Zn/Al catalysts (Berty reactor) 93 6.5 Methanol synthesis from CO2/H2 after synthesis from CO/H2 (Berty reactor) 94 6.6 Comparison of methanol synthesis from CO/H2 in the Berty and tubular reactors 95 6.7 Approach to differential methanol production under CO/H2 97 6.8 Methanol production from CO/H2 over various catalysts at differential conversion 99 6.9 Concluding remarks 101 Chapter 7 METHANOL SYNTHESIS FROM CO/COj/Hj 103 7.1 Summary 103 7.2 Trace products from CO/CO2/H2 (Berty reactor) 104 7.3 Methanol synthesis from CO/CO2/H2 with various CO2 fractions (Berty reactor) 105 7.4 Reactant product profile from CO/CO2/H2 with various CO2 fractions (Berty reactor) 107 7.5 Methanol production from CO/CO2/H2 at differential conversion 110 7.6 Comparison of methanol production from CO/CO2/H2 at differential and integral conversions 112 7.7 Concluding remarks 115 Chapter 8 SUMMARY OF RESULTS AND DISCUSSION 117 8.1 Methanol synthesis from CO2/H2 117 8.2 Methanol synthesis from CO/H2 121 8.3 Methanol synthesis from CO/CO2/H2 123 Chapter 9 CONCLUSIONS 125 References 127 Appendix 1. Kinetic theory 139 Appendix 2. Methanol reaction equilibrium calculation 145 10 List of Figures Figure 2.1 Effect of CO2 fraction on equilibrium methanol yield at 250°C and 5 MPa 20 Figure 2.2 Effect of CO2 fraction on methanol production over Cu/Zn0/Al203 25 Figure 3.1 T.P.R. of calcined catalysts 41 Figure 4.1 Internal recycle reactor and tubular reactor system 46 Figure 4.2 Deactivation of Pd-promoted catalysts under CO/H2 and CO2/H2 61 Figure 5.1 Effect of Pd on methanol yield from CO2/H2 69 Figure 5.2a Effect of Pd on methanol yield and rate from CO2/H2 at various flow rates 70 Figure 5.2b Effect of Pd on methanol yield and rate from CO2/H2 at various flow rates 71 Figure 5.3 Pd promotion of methanol synthesis from CO2/H2 at various flow rates 72 Figure 5.4 Effect of Pd on CO yield from CO2/H2 at various flow rates 74 Figure 5.5 Effect of Pd on methanol selectivity from CO2/H2 at various flow rates 75 Figure 5.6 Approach to differential methanol production under CO2/H2 78 Figure 5.7 Pd promotion of methanol production from CO2/H2 at very high flow rates 79 Figure 5.8 Approach to differential CO production under CO2/H2 80 Figure 5.9 Effect of Pd on differential methanol production rate under CO2/H2 81 Figure 5.10 Effect of Pd on differential methanol production rate under CO2/H2 + water 85 Figure 6.1 Effect of Pd on methanol yield from CO/H2 at various flow rates 92 11 Figure 6.2 Effect of Pd on methanol yield from CO/H2 followed by CO2/H2 95 Figure 6.3 Approach to differential methanol production under CO/H2 98 Figure 6.4 Effect of Pd on differential methanol production rate under CO/H2 ...
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