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

Home , Hydrogen spillover

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

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 .... 100

Figure 7.1 Effect of Pd on methanol yield and production rate from CO/CO2/H2

syngases 106

Figure 7.2 Pd promotion of methanol synthesis from CO/CO2/H2 syngases 107

Figure 7.3 Effect of Pd on net reaction and production rates from CO/CO2/H2 syngases 108

Figure 7.4 Effect ofPd on differential methanol production rate under

Figure 7.5 CO/CO,/H, Ill

Pd promotion of differential methanol production under CO/CO2/H2 . Ill

Figure 7.6 Integral/differential methanol production rates under CO/CO2/H2 113

Figure 7.7 Pd promotion of integral/differential methanol production under CO/CO2/H2 113

Figure Al.l Transition from integral to differential conversion as the flow rates through internal recycle and tubular reactors are increased 141

12 List of Tables

Table 2.1 Thermodynamic properties of methanol synthesis reactions at 250°C .. 20

Table 3.1 Overview of Cu/ZnO/Al^O^ catalyst preparation 33

Table 3.2 Bulk chemical analyses of catalysts and physical mixtures 38

Table 3.3 B.E.T. surface area and porosity measurements 39

Table 4.1 Response factors for Gas Chromatography 51

Table 4.2 Effect of impeller speed on methanol production in the Berty reactor ... 53

Table 4.3 Effect of catalyst particle size on methanol production in the Berty reactor 54

Table 4.4 Effect of gas linear velocity on methanol production in the tubular reactor 55

Table 4.5 Effect of catalyst particle size on methanol production in the tubular

reactor 56

Table 4.6 Reproducibility of results from experiments using the Berty reactor .... 57

Table 4.7 Reproducibility of results from experiments using the tubular reactor .. 57

Table 4.8 Carbon and oxygen balances from experiments using the Berty reactor 59

Table 5.1 Yield & selectivity of trace product methane from CO^/H^ at various

flow rates 65

Table 5.2 General selectivity of products under CO^/H^ 65

Table 5.3 Order of activity of catalysts for methanol synthesis from CO2/H2 69

Table 5.4 Comparison of methanol yields from CO2/H2 in the Berty and tubular reactors 76

13 Table 5.5 Effect of Pd on differential methanol production rate under CO2/H2 + CO 83

Table 5.6 Effect of Pd on differential methanol production rate under CO2/H2 + water 84

Table 5.7 Comparison of methanol production rate from CO2/H2 (integral conversion) and CO2/H2/H2O (differential conversion) 87

Table 6.1 Yield & selectivity of trace products firom CO/H2 at various flow

rates 91

Table 6.2 General selectivity of products under CO/H2 91

Table 6.3 Methanol synthesis from CO/H2 over Pd/Al + Cu/Zn/Al and Pd/Al alone 93

Table 6.4 Comparison of methanol yields from CO/H2 in the Berty and tubular reactors 96

Table 7.1 General selectivity of products under CO/CO2/H2 104

14 Nomenclature of catalysts

NOMENCLATURE CATALYST

Cu/Zn/A1 (series prec 1) Cu/ZnO/AlzOg prepared by precipitation of Zn/Al salts and Cu/Zn salts in series in the same vessel

Cu/Zn/AI (series prec 2) Cu/ZnO/Al^Og prepared as above

Cu/Zn/A1 (parallel prec) Cu/Zn0/Al203 prepared by precipitation of Zn/Al salts and Cu/Zn salts in separate vessels before mixing

0.03 Pd/Cu/Zn/Al (wet) Pd/Cu/ZnO/Al^Og prepared by precipitation of Zn/Al salts and Cu/Zn salts in series followed by wet addition of Pd(N03)2 solution (0.03 Pd/Cu atomic ratio)

0.10 Pd/Cu/Zn/Al (wet) Pd/Cu/Zn0/Al203 prepared as above (0.10 Pd/Cu atomic ratio)

0.04 Pd/Cu/Zn/Al (imp) Pd/Cu/Zn0/Al203 prepared by precipitation of Zn/Al salts and Cu/Zn salts in series, then slurrying in Na2C03 solution and finally addition of Pd(N03)2 solution to neutralise (0.04 Pd/Cu atomic ratio)

0.09 Pd/Cu/Zn/Al (imp) Pd/Cu/Zn0/Al203 prepared as above (0.09 Pd/Cu atomic ratio)

0.05 Pd/Cu/Zn/Al (prec) Pd/Cu/Zn0/Al203 prepared by precipitation of Zn/Al salts and Pd/Cu/Zn salts in separate vessels before mixing (0.05 Pd/Cu atomic ratio)

0.12 Pd/Cu/Zn/Al (prec) Pd/Cu/Zn0/Al203 prepared as above (0.12 Pd/Cu atomic ratio)

0.07 Pd/Al + Cu/Zn/Al Physical mixture of Pd/Al203 (1:9 ratio of oxides w/w) and the first Cu/Zn0/Al203 catalyst tabulated above (0.07 Pd/Cu atomic ratio)

0.21 Pd/Al + Cu/Zn/Al Physical mixture of Pd/Al203 (3:7 ratio of oxides w/w) and the first Cu/Zn0/Al203 catalyst tabulated above (0.21 Pd/Cu atomic ratio)

Chapter 1

INTRODUCTION AND AIMS

1.1 SUMMARY OF PRIOR WORK

Methanol is produced industrially from synthesis gas, CO/H2 with a small CO2 fraction, over a Cu/Zn0/Al203 catalyst at ~5 MPa and ~250°C. There is strong evidence from kinetic experiments with CO/CO2/H2 mixtures, using isotope labelled carbon oxides, that methanol is produced by direct CO2 hydrogenation [1.1], with the water-gas shift reaction [1.2] converting CO to CO2 (18,148,149). However, the activity under CO2/H2 is much lower than the activity under synthesis gas (33,36,96,98). Generally, it is thought that the active Cu catalyst sites become oxidised in the presence of excess CO2, or the water produced by [1.1] and reverse [1.2] inhibits Cu by competitive adsorption.

CO2 + 3H2 CH3OH + H2O [1.1] CO + H2O ^ CO2 + H2 [1.2]

Inui et al (1,2) found that methanol production from CO2/H2 over a Cu-based catalyst can be doubled by the addition of Pd in the form of a physical mixture. The promotion was attributed to hydrogen spillover from Pd to Cu which maintains a reductive state of Cu. Fujimoto and Yu (3) proposed in addition that the product water inhibits methanol synthesis through the oxidation of Cu, and hydrogen spillover from Pd counteracts this inhibition. These studies (1-3) were carried out at high conversions, where another possible mechanism for the promotion under CO2/H2 is the secondary reaction of CO to methanol over Pd.

The effect of Pd on methanol production from synthesis gas has not been investigated in great detail. Elliot and Pennella (147) found that Pd impregnation of Cu/ZnO had an adverse effect on the activity, but no explanation was given.

17 1.2 INTRODUCTION TO THE THESIS

This thesis describes a kinetic study of the effect of Pd addition by various methods on the methanol synthesis activity of an industrial Cu/Zn0/Al203 catalyst at industrial conditions. Synthesis under CO2/H2, CO/H2 and mixtures was investigated. An internal recycle reactor operated at integral conversions and a tubular reactor operated at differential conversion were used as kinetic tools. In the former reactor, the catalyst is uniformly exposed to the product gas phase concentration, such that effects of product compounds on the activity of the catalyst are enhanced. In the latter reactor, secondary reactions and other effects of products are eliminated, such that the intrinsic forward rate of reaction for the given gas mixture and catalyst can be measured. A pre-requisite throughout this thesis was the understanding of the reactions taking place on Cu and the promotion/inhibition of these reactions by gas phase components, before analysis of the (often opposing) effect of Pd could be made.

1.3 AIMS OF THE THESIS

It was intended to explain by kinetic means the role of Pd in promoting methanol synthesis from CO2/H2 and other gas mixtures. Specific aims were:

• To determine whether Pd functions as a catalytic site (e.g. under CO2/H2 for the secondary reaction of CO to methanol) or whether Pd enhances the activity of Cu.

• To determine whether methanol is produced by CO and/or CO2 hydrogenation on Cu and which reaction on Cu is promoted in the presence of Pd.

• To determine whether CO2, water or other compounds are necessarily present in the gas phase for Pd promotion to be observed.

• To determine the extent of promotion by Pd in impregnated and co-precipitated catalysts compared to physical mixtures.

18 Chapter 2

BACKGROUND AND LITERATURE

2.1 THE METHANOL AND WATER-GAS SHIFT REACTIONS: THERMODYNAMICS

Some catalysts promote the production of methanol by CO hydrogenation [2.1] and others by CO2 hydrogenation [2.2]. Over Cu/ZnO/Al^O^, it has been claimed that CO; hydrogenation [2.2] is predominant, whilst the water-gas shift reaction [2.3] converts CO to CO2 (18,148,149). Despite this mechanism, a far greater methanol yield is available from CO/H2 synthesis gas compared to CO2/H2 because of the thermodynamics which follows.

CO + 2H2 <=> CH30H [2.1]

C02 + 3H2 <=> CH30H + H20 [2.2] CO + H20 0 C02 + H2 [2.3]

All the above reactions are exothermic in the forward direction (AH°250''c s^e shown in Table 2.1), such that greater equilibrium conversions are achieved at lower temperatures. Table 2.1 also shows the AG°25o°c of reactions, which indicate that the hydrogenation of CO [2.1] is thermodynamically more favourable than the hydrogenation of CO2 [2.2].

The equilibrium carbon yields of methanol as a function of CO2 fraction, C02/(C0+C02), in the syngas have been calculated in Appendix 2 at typical conditions for this thesis (equilibrium constants at 250°C were obtained from literature (108,109); the synthesis gases contained 4 H2:(C0+C02) with 10% inerts; the pressure was 5 MPa). Figure 2.1 shows that the equilibrium methanol yield is 71% under CO/H2, compared to 22% under CO2/H2.

19 Note that with high CO2 fractions (> 0.6), some CO2 is also converted to CO at equilibrium. With a CO2/H2 syngas, the yield of CO at equilibrium is approximately 10% in addition to the 22% yield of methanol.

Table 2.1 Thermodynamic properties of methanol synthesis reactions at 250°C

REACTION AH°250°c* '^G°250°C* (kJ/mol) (kJ/mol)

CO + 2 Hz => CH3OH -90.0 13.6

CO2 + 3H2 => CH3OH + H2O -50.4 33.2

CO + H2O => CO2 + H2 -39.7 -19.6

* H and S data for reactants/products at 250°C obtained from literature (101-104)

0.2 0.4 0.6 0.8 C02/(C0+C02)

Figure 2.1 Effect of COn fraction on equilibrium methanol yield at 250°C and 5 MPa

20 2.2 METHANOL SYNTHESIS FROM CO/COg/Hz OVER Cu/ZnO/AlgOj

2.2.1 Evidence for Cu as the active site and the state of Cu

Cu/ZnO/Al^Oj is normally prepared by precipitation of the metals as hydroxycarbonates, followed by calcination in air to derive the oxides, and finally reduction of the CuO component. The resultant Cu metal, or partially oxidised Cu, is generally considered to be the active species for methanol synthesis. Unsupported polycrystaliine Cu, prepared by the reduction of amorphous CuO powder, is indeed active for methanol synthesis from CO/H2 and CO/CO2/H2 (47,51,52,84). Chinchen et al (46,158) found that the turnover number (moleculescnaoH sitecu"^ s'') under CO/CO2/H2 mixtures was independent of the support used in Cu-based catalysts and the rate of methanol synthesis was directly proportional to the Cu surface area measured after reduction of the catalysts. Burch and co-workers (55,132,159) and others (37,84) found that, although some supports may give higher turnover numbers than others, methanol production was proportional to the Cu surface area for a given support. A similar result was found using various supports under CO2/H2 (139). This suggests that the supports may promote Cu to differing extents, but the rate-determining process for methanol synthesis takes place on Cu. Recently, it has been demonstrated that polycrystaliine Cu prepared by the reduction of CuO (141), clean polycrystalline Cu foil (142) and Cujoo single crystal (155,156) are active for methanol production from CO2/H2. In all cases, the turnover numbers were comparable with those of co-precipitated Cu/ZnO catalysts, and it was concluded that Cu is the active species under CO2/H2.

The state of the active Cu sites is a highly controversial issue. Numerous workers have found spectroscopic evidence for the presence of Cu^^ in Cu-based methanol synthesis catalysts after reduction and under CO/H2 and CO/CO2/H2 mixtures (49,59,163,165,166, 169,171,173,174,178-180), whereas others have found only Cu metal (28,147,162, 175-177,181,189). Chinchen, Waugh and co-workers (31,46,52,149,158,183-188), corroborated by Denise et al (37), found that a fraction of the post-reduction Cu metal

21 surface area could not be measured after exposure to the synthesis gas, and the fraction was controlled by the CO2/CO ratio in the synthesis gas. It was claimed that the Cu metal surface was partially covered by adsorbed oxygen according to the following reactions, and that the subsequent adsorption of CO2 at the oxidised (Cu'^) sites was the first step in the mechanism of CO2 hydrogenation to methanol [2.4].

CO2 + 3H2 ** CH3OH + [2.4] CO + CO2 [2j] H, + Owk [2j]

Others have claimed that active Cu^^ sites are formed by electronic interactions with ZnO. Based on the results of various spectroscopic/characterisation techniques and theoretical considerations, Klier and co-workers (50,59,163-165) and others (160) claimed that Cu^^ dissolved in ZnO was the active centre (and the ZnO was involved in a dual site mechanism; to be discussed in the next Section.) In the case of unsupported Cu (166,167) and Cu/Si02 (168), careful catalyst preparation to prevent the presence of trace alkaline impurities resulted in no activity for methanol synthesis from CO/H2. The activity was promoted by the addition of alkali metals or ZnO, which led to the conclusion that the active site is not Cu metal but Cu^\ stabilised by the promoters. The hypothesis of active Cu^^ sites stabilised by (or in solution with) ZnO does not appear to agree with the correlation of methanol synthesis rate under CO/CO2/H2 with Cu metal surface area (37,84,158,159), as described in the previous paragraph. One explanation is that a fixed proportion of Cu remains as Cu'^ after reduction. Alternatively, the ZnO stabilised Cu'^ sites may only be active for CO hydrogenation to methanol, since in general the supporting work cited (50,59,163-168) was concerned with synthesis under CO/H2, whereas Cu metal or Cu oxidised by exposure to C02-containing syngas may be active for CO2 hydrogenation, which may be the predominant methanol synthesis reaction under CO/CO2/H2 mixtures (18,148,149). Under CO2/H2, strictly pure Cu, in the form of clean polycrystalline Cu foil (142) and Cujoo single crystal (155,156), does promote methanol production, which was not the case under CO/H2. Furthermore, the turnover numbers (moleculescH30H site^u'' s"^) were comparable with those of co-precipitated Cu/ZnO catalysts and no spectroscopic evidence of Cu'^ or adsorbed oxygen was found,

22 suggesting that Cu" is the active site. Conversely, Fujitani and co-workers, who determined turnover numbers under CO^/H^, found that clean polycrystalline Cu foil was promoted by Zn addition (157) and ZnO was superior to many (but not all) other supports of Cu in binary co-precipitated catalysts (137). In both cases (137,157) it was claimed that the formation of Cu'^ in a balance with Cu° is necessary for optimum activity, and there is spectroscopic evidence for the existence of some Cu'^ under CO2/H2 (157,182).

2.2.2 Roles of Zn O and AI2O3

Both ZnO and AI2O3 function as highly effective physical supports, maintaining the dispersion of Cu. Furthermore, the combination of Zn and A1 in a mixed oxide spinel (ZnAlzO^), which is a characteristic of the I.C.I, commercial Cu/ZnO/Al^O^ preparation, greatly enhances the stability of the catalyst (13,19). Both AI2O3 and ZnAl204 are refractory materials, which counteract thermal sintering of Cu (19,50,127,203). Although strongly denied by the I.C.I, group (31,46,52,149,158,183-188,203), in addition to the support role of ZnO there is substantial kinetic evidence of a synergy between Cu and ZnO under CO/H2 and CO/CO2/H2 mixtures (23,170). Many workers have found that pure Cu is in fact inactive for methanol synthesis under CO/H2 (59,166-168,190,196,197) whereas the activity is promoted by the addition of an alkali metal or ZnO (59,166- 168,198). Under CO/CO2/H2 the turnover number (moleculescHsoH site^u'^ s'^) of Cu/ZnO was found to be 10, 8, 6 or 3 times greater than that of Cu (84), Cu/Si02 (55, 132), Cu/MgO (37) or Cu/ZrO^ (132) respectively. Also the addition of ZnO to CU/AI2O3 resulted in up to 5 times greater turnover number under CO/CO2/H2 (57). Under CO2/H2, co-precipitated Cu/ZnO gave a higher turnover number than Cu co-precipitated with Si02, AI2O3, Cr203 or ZrOj (137), although other oxides such as Ga203 (137) and Ti02 (86) appear to be superior to ZnO. The addition of Zn to clean polycrystalline Cu foil under CO2/H2 increased the turnover number by up to 6 times, although the turnover number decreased with higher Zn loadings (157). This is somewhat different firom the behaviour of Cu/ZnO catalysts under CO/CO2/H2, which exhibit a constant turnover number across a wide range of compositions of Cu/ZnO and surface areas of Cu (37,84,158,159) (notwithstanding the relatively high turnover number of Cu/ZnO compared to other Cu- based catalysts as mentioned above).

23 The synergy between Cu and ZnO under all synthesis gases may be the result of electronic interactions, leading to the formation of partially oxidised Cu species, which have been proposed as the active site for methanol synthesis (see previous Section). Furthermore, Klier and co-workers (50,59,163-165) and others (160) claimed that the active centre was a solution of Cu'VZnO, where Cu^^ adsorbs CO and ZnO activates H2. Chinchen and Spencer (150) suggested that ZnO (or alkali metals) was involved in the surface oxidation of Cu, but only to initiate the chemisorption of CO2. This was based on the interpretation of various results in the literature, namely that ZnO or alkali metal addition to pure Cu has been found to promote methanol synthesis, that there is a discrepancy between the rate of dissociative adsorption of CO2 on Cu/Zn0/Al203 and Cu single crystals, that the rate of the dissociative adsorption of CO2 on Cu is promoted by alkali addition, and that the rate of CO2 chemisorption is enhanced on oxidised Cu surfaces. Furthermore, it was claimed that only trace ZnO (or alkali metals) was required due to the autocatalytic nature of CO2 adsorption, i.e. the dissociative adsorption of CO2 provides a partially oxidised Cu surface which promotes the adsorption of molecular CO2 for the production of methanol. The hypothesis by Chinchen and Spencer (150), and indeed all claims of partial oxidation of Cu by ZnO, may be connected to the finding that oxygen (or oxygen containing species) migrates from ZnO to Cu during methanol synthesis (172,195). A highly controversial explanation of the synergy has been given by Frost (190), who predicted theoretically that the presence of a metal (e.g. Cu°) stabilised doubly ionised oxygen vacancies, Vq^, in the so-called support (e.g. Zn^^), where hydrogen is activated and CO or CO2 insertion takes place for methanol synthesis. Ponec (191) criticised this theory claiming that Frost had over-estimated the overall amount of electrons that the metal can receive to form Vq^ vacancies and more importantly that Zn^^ would not be stable in the presence of Cu^^ which, it was claimed, exists in Cu/ZnO catalysts. Nevertheless it has been demonstrated that highly reduced and defective ZnO is active for methanol production from CO2/H2 (32) and it has been claimed that the pivotal intermediate in methanol synthesis from CO2/H2 over Cu/ZnO exists on Zn sites only (199). Finally, Burch and co-workers (55,159,161, 192,193) found that the synergy between Cu and ZnO does not necessarily involve close electronic interactions, since the synergy was observed with physical mixtures of Cu/Si02 and Zn0/Si02. It was proposed that ZnO acts as reservoir for hydrogen, which promotes the hydrogenation of reaction

24 intermediates on Cu by spillover. In support of this theory, there is evidence in the literature for high coverages of ZnO with adsorbed hydrogen under methanol synthesis conditions (31,159,160,185,192,194,200,201); the dissociative adsorption of on Cu is known to be structure sensitive, such that some Cu surfaces may be hydrogen deficient (194); and finally the mobility of hydrogen between Cu and ZnO has been observed (192,194,200).

2.2.2 Effect of CO2 fraction and H2O on methanol production

Optimal methanol production rates are observed with 2-5% CO2 in the synthesis gas over Cu/ZnO (43,49,96,98,117) or Cu/ZnO/AlzOg (33,36,49,60,114-116,118,119) at conditions in the range 0.1-10 MPa and 225-285°C. A typical activity profile (obtained in Chapter 7 of this thesis), is shown in Figure 2.2, where the maximum methanol production occurs at 2% CO2 or 10% 002/(00+002). Note that the methanol production from CO^/H^ is

150 ^ u 60 I

1 I

0.2 0.4 0.6 0.8 C02/(C0+C02)

Figure 2.2 Effect of CO^ fraction on methanol production over Cu/ZnO/AInO] (4 H2:(C0+C02), 5 MPa, 250°C, initial activities)

25 greater than that from CO/H2. Many workers have claimed that methanol is produced predominantly by CO2 hydrogenation rather than CO hydrogenation in CO2 containing synthesis gases, but it is not clear whether this is so at the very low CO2 fractions where the maximum methanol production occurs (see next Section 2.2.4). However, if CO2 is the primary carbon source in all synthesis gases, this would explain the dramatic promotion at low CO2 concentrations. It has been claimed that CO2 containing synthesis gases result in the partial oxidation of Cu (37,46,98). This could also explain the promotion, if methanol production takes place at Cu^^ sites (by CO hydrogenation), which has often been supposed (see previous Section 2.2.1). There are many possible explanations for the loss of methanol production at higher CO2 fractions. Even though the equilibrium methanol yield from CO/H2 compared to CO2/H2 is approximately three times greater, thermodynamic limitation is probably not the cause of the loss since the activity profiles (such as in Figure 2.2) have been obtained at yields far away from equilibrium (Figure 2.1). If Cu° sites, or a balance of Cu°/Cu'^ sites, are necessary for methanol synthesis (see previous Section 2.2.1), the inhibition at higher CO2 fractions may be the result of excessive oxidation of Cu. Alternatively, the presence of excess CO2 may result in disproportionate coverage of various adsorbed species on Cu. One of these may be Ogj; through the dissociative adsorption of CO2 (37,46,98), and this may prevent the dissociative adsorption of H2 at Cu° sites. Alternatively, it has been shown that the adsorption of molecular CO2 is promoted by Oa^s and also that CO2 does not adsorb on fully oxidised (CU2O) surfaces (202), such that C02_ads may become excessive or deficient respectively. The disproportionate coverage of other adsorbed products of CO2 also cannot be ruled out.

Unlike the maxim um at low CO2 fractions (Figure 2.2), Liu et al (154) using Cu/ZnO, corroborated by others (115) using Cu/Zn0/Al203, found that the initial rate of methanol synthesis in a batch reactor increased monotonically as the CO was replaced v^th CO^. Chanchlani et al (49) found a similar trend with both catalysts in a tubular reactor at very high space velocities approximating to differential conversion. Therefore it appears that the maximaum at low CO2 fractions (Figure 2.2), which is observed at integral conversions, must be due to an effect of a product rather than CO2. Methanol is one product and the other major product is water. Given the CO; hydrogenation to methanol

26 and water-gas shift reactions, water production is related to the amount of CO2 in the syngas. Vedage et al (146) found that methanol synthesis from CO/H2 was promoted by up to 2% water addition, and thereafter water had an inhibiting effect. The promotion at low water concentration (160,204) and inhibition at high water concentration (3,154,204) of methanol production from CO/H2 has also been observed by other workers using Cu/ZnO and Cu/Zn0/Al203. It has also been found that the strict removal of water from CO/H2 syngas results in very little (204) or zero (14,18) methanol production. The promotion by water addition observed by Vedage et al (146) was more dramatic than that observed with the addition of CO2 to CO/H2, and by use of isotope-labelled H and O in water, evidence was obtained for the direct reaction of hydroxyl groups with CO in the production of methanol. Therefore the promotion by small CO2 fractions at integral conversions (Figure 2.2) may follow from the small amount of water production by the reverse water-gas shift reaction, and then promotion of the reaction of CO to methanol. Furthermore, it has been found that water inhibits methanol production from CO/CO2/H2 and CO2/H2 (154), and the modelling of various kinetic studies has led to the conclusion that water inhibits the CO2 hydrogenation to methanol (18,148,153) by competitive adsorption at the active sites. Therefore the loss of methanol production at high CO2 fractions (Figure 2.2) may be due to inhibition, following the correspondingly high amount of water production.

2.2.4 Methanol production by CO or CO2 hydrogenation ?

In methanol synthesis from CO/CO2/H2 mixtures, it is difficult to determine the direct carbon source (CO [2.1] or CO2 [2.2]) of methanol because of the possible intermediacy of the water-gas shift reaction [2.3]. To resolve the issue, some researchers have carried out kinetic studies with carbon and oxygen isotopes. Problems with these studies are, firstly, at high conversion the scrambling of isotopes between CO and CO; and, secondly, at low conversion the absence of products, in particular water, which has an effect on the mechanism of methanol synthesis.

CO + 2 H2 <=> CH3OH [2.1] CO2 + 3 H2 <=> CH3OH + H2O [2.2]

27 CO + H2O C* CO2 + Hz [2.3]

Kagan, Rozovskii and co-workers, as far back as 1975 (14-18), were the first to use isotope-labelled reactants to discriminate between methanol synthesis reactions. In the most revealing experiment (15), a ''^CO^/CO/Hz mixture, composition 4%:30%:59% (balance inert), at 50 atm (5.5 MPa) and 240°C was externally recycled over a commercial Cu/ZnO/Al^O^ catalyst. The methanol/water products were removed in each cycle, such that the catalyst was practically only exposed to the synthesis gas. After 0.5 h it was found that the specific radioactivity of all methanol collected was between the initial and final radioactivity of CO2 and substantially exceeded the final radioactivity of CO. Moreover the transient profile of specific radioactivity of methanol, assessed by a mean integral, was similar to that of CO2, which led to the conclusion that virtually all the methanol was produced directly from CO2 and CO was only converted to CO2. The same result was found in experiments with gas mixtures containing higher CO2 contents.

Liu et al (148) measured the initial rate of methanol production over Cu/ZnO in a batch reactor at 220°C and 17 atm (-1.7 MPa) using a lower CO2 content in the synthesis gas, approximately 1.5% C'^02, 27.5% CO and 71% H2. According to the fraction of CHg^^OH in total methanol determined by mass spectrometry, nearly half (37%-47%) the methanol was produced directly from CO2. The effect of water was studied by adding approximately 1.5% H2O to the syngas and measuring the initial rate of methanol production, as before. It was found that CO2 hydrogenation was almost completely inhibited, whereas CO hydrogenation to methanol was virtually unaffected. Whilst water is a product of reaction [2.2] and reverse reaction [2.3], the amount of water used in this experiment was an order greater than the amount which would be present at equilibrium for the 1.5% CO2 synthesis gas. Therefore the effect of water in inhibiting CO2 hydrogenation would be less in normal methanol synthesis. Nevertheless it was still possible to conclude that methanol production from synthesis gas containing a small CO2 fraction took place predominantly by CO hydrogenation, and in the presence of water CO2 hydrogenation is even less significant as a reaction for methanol synthesis.

28 In the above experiments with water addition, there was very fast scrambling of '^0 between C^^02 and H^O, such that was also present in the feed, and given that the product was virtually all it was determined that neither CO2 nor H2O were direct reactants for methanol production. These results are somewhat in disagreement with Vedage et al (146), who studied methanol synthesis from 30:70 CO/H2 in a tubular flow reactor at 225°C and 7.5 MPa over Cu/ZnO with a small amount of H2^^0 or D2O added. They found the major products CHg^^OH or CH2DOH respectively, which showed that hydroxyl groups from dissociative water adsorption are directly incorporated by reaction with CO. However, to the extent that it was demonstrated that CO was the predominant, if not exclusive, carbon source of methanol, Vedage et al (146) are in agreement with Liu e? a/(148).

Finally Chinchen et al (149,158) carried out experiments using the I.C.I, commercial Cu/Zn0/Al203 catalyst in a tubular flow reactor at 5 MPa and 250°C using 20%:80% (C0+''^C02)/H2 mixtures. The fraction of methanol derived directly from CO2 was estimated by a ratio of the specific radioactivity of methanol and the mean (inlet to outlet) specific radioactivity of CO2. Errors were large for small CO2 concentrations or high conversions. At low conversions (< 1.5% methanol in product), with CO2 fractions -0.5%, -7% and 10%, it was found that approximately 70% (±25%), 90% (±10%) and 100% (±5%) of methanol came from CO2 respectively. At moderate conversions (2%- 2.5% methanol in product), with CO2 fractions 4% and 10%, it was found that 80% (±25%) and 90% (±10%) of methanol came from CO2 respectively. Hence Chinchen et al established that for a wide range of CO/CO2/H2 mixtures the majority of methanol was produced directly from CO2, but this did not apply to high conversions.

In summary, kinetic experiments with carbon and oxygen isotopes in CO/CO2/H2 mixtures show that methanol is produced predominantly by CO2 rather than CO hydrogenation. However, in the presence of water (or at high conversions where water is produced) and at small COj fractions (where in fact the maximum methanol productivity is obtained as in Figure 2.2), there is also evidence to suggest that CO is the major carbon source of methanol.

29 2.3 EFFECT OF Pd ON Cu/ZnO CATALYSTS FOR METHANOL SYNTHESIS FROM COg/Hg AND OTHER GAS MIXTURES

2.3.1 Effect of Pd under CO-rich synthesis gases

Elliot and Pennella (147) found that 0.5% Pd impregnation of Cu/ZnO at 250°C and 65 atm resulted in nearly 20% less methanol production from standard synthesis gas (2% CO2, 31.3% CO, 66.7% H2), even though the B.E.T. surface area of the Pd-loaded catalyst was higher. The methanol yields were rather high, being 36% in the case of Cu/ZnO (assuming a catalyst charge density of 1 g/cm^; the yield would be proportionately higher for higher catalyst charge density), but the selectivity of methanol was > 99% in all cases. Elliot and Permella were not able to explain this effect or indeed the stronger inhibition by some other transition metals.

Fujimoto and Yu (3) used a Cu/ZnO/Al^O^ catalyst under 1:2 CO/H2 at 15 atm and 260°C and found that a -12% methanol yield could be increased to ~18% by use of a physical mixture with Pd/Si02. The methanol yields over both Cu/Zn0/Al203 and the physical mixture increased approximately four-fold in the first hour on stream, which was not explained, before reaching the previously stated methanol yields. The selectivity of methanol here was not quoted and, given the low H2:C0 ratio, other carbon products as well as water would be expected. In fact Fujimoto and Yu did carry out an experiment with water addition to the CO/H2 feed and found that the activity of the Pd-promoted catalyst was much less inhibited than the activity of Cu/ZnO/Al^Og. This suggests that Pd counteracts the inhibition of methanol synthesis from CO/H2 by water, but the effect of Pd on synthesis from pure CO/H2 was not established. Fujimoto and Yu proposed that water oxidises the active Cu sites and hydrogen spillover from Pd helps to maintain a more reduced state of the Cu.

Finally, Inui et al (205) found that a 3.4% methanol yield over a Cu/Zn0/Cr203 catalyst was promoted by 50% by the impregnation of 2.8% Pd under 1:2 CO/H2 at 20 atm and 270°C. However, limitations in the experimental procedure make it impossible to make

30 any conclusions about this work, e.g. B.E.T. or metal surface areas were not determined; the preparation of the Pd-promoted catalyst was completely different from that of Cu/ZnO/CrzOg (involving calcination in air at 350°C which was not carried out in the case of Cu/ZnO/CrzOg and two separate reduction/activation procedures instead of one); the methanol yields were determined for a given volume rather than mass of catalyst, without knowing the density of the catalysts (given the different preparation procedures and the fact that the Cu was pre-reduced in the case of the Pd promoted catalyst whereas Cu(0H)2 was probably the Cu phase in the Cu/ZnO/Cr^O^ catalyst when it was loaded into the reactor, the amounts of Cu present in a given volume were probably widely different); and given the reaction temperature and H^zCO ratio, large quantities of water, CO2, dimethyl ether and hydrocarbons were produced.

2.3.2 Effect of Pd under CO/Hs

Inui et al (1,2) studied methanol production from 1:3 CO2/H2 at 250°C and 50 atm over a Cu/Zn0/Al203/Cr203 catalyst prepared by co-precipitation of a uniform gel. At a low space velocity, where the methanol yield was 10.5% over the standard catalyst, it was found that the addition of Pd/Al203 (or Ag/Al203) in the form of a physical mixture promoted methanol production. The promotion was related to the amount of Pd added, but reached a plateau at approximately 0.04 Pd/Cu atomic ratio, where the methanol production was effectively doubled. Since Pd/Al203 alone was found to be inactive under CO2/H2, the promotion was attributed to hydrogen spillover from Pd to Cu which maintains a reductive state of Cu. The source of oxidation of Cu was not discussed, but it is possible that CO2 and the product water (produced as a result of CO; hydrogenation and the reverse water-gas shift reaction) may be involved in redox reactions with Cu. Inui et al proposed in addition that methanol was produced from CO2 by the reverse water-gas shift reaction followed by CO hydrogenation (both reactions on Cu).

The possibility of the secondary reaction of CO to methanol over Pd was not considered as an alternative mechanism for the promotion observed by Inui et al (1,2). Although supported Pd catalysts are inactive for methanol production from CO2/H2 (2,78,136), they are active for methanol production from CO/H2 (19,33,70). Inui et al carried out the

31 kinetic experiments under CO^/H; with physical mixtures at very low space velocity, such that the water-gas shift reaction was equilibrated (with 10% CO2 conversion to CO), and therefore CO was available for reaction over Pd.

Fujimoto and Yu (3) carried out methanol synthesis with an unusual 1:9 CO2/H2 ratio at conditions of 15 atm and 240°C. The methanol yield of 8.9% over Cu/Zn0/Al203 was enhanced by a quarter by use of a physical mixture with Pd/Si02 (Pt/Si02 was also found to be a promoter). The Pd content or Pd/Cu ratio was not specified. Based on experiments with water addition to a CO/H2 synthesis gas (as described in the previous Section 2.3.1), it was proposed that the product water under CO2/H2 inhibits methanol synthesis through the oxidation of Cu, and hydrogen spillover from Pd counteracts this inhibition. Again the secondary reaction of CO to methanol over Pd was not considered as an alternative mechanism for the promotion.

Fujimoto and Yu (3) also tested a Pd-impregnated Cu/ZnO/ AI2O3 catalyst (again of unspecified Pd content) under CO2/H2 at the same conditions as for the physical mixture. The promotion was only 10%, which was much less than that observed using the physical mixture, but no explanation was given and surface areas of the Cu/Zn0/Al203 catalyst compared to the Pd-impregnated catalyst were not determined. Inui et al (205) also tested a Pd-impregnated catalyst, in this case Pd/Cu/ZnO/Cr203 (2.8% Pd), with a 10:27 CO2/H2 mixture at 20 atm and 265°C. Pd was actually found to inhibit methanol synthesis, and it was suggested that the spillover effect of Pd is not easily exerted under the oxidising atmosphere of CO2/H2, which appears to be in contradiction to the previous claims (1,2). However this work (205) is subject to the same criticisms of experimental procedure as were outlined at the end of Section 2.3.1.

In summary, the addition of Pd to Cu/ZnO-based catalysts by use of physical mixtures promotes methanol synthesis from CO2/H2, and it has been claimed that hydrogen spillover from Pd to Cu maintains a reductive state of Cu. The reaction (CO hydrogenation or CO2 hydrogenation) which is promoted has not been determined. It is also unclear whether Pd impregnation can promote methanol synthesis from CO2/H2 and what the effect of Pd addition is in general under CO-rich gas mixtures.

32 Chapter 3

CATALYST PREPARATION AND CHARACTERISATION

3.1 OVERVIEW OF Cu/ZnO/AljOj CATALYST PREPARATION

The catalyst preparations were adapted from the I.C.I, patent for preparation of Cu/Zn0/Al203 (13), and the composition of the catalysts was chosen as 60 CuO; 30 ZnO: 10 AI2O3 (w/w), the industrial standard (19). An overview of the preparation is given in Table 3.1.

Table 3.1 Overview of Cu/ZnO/ALOg catalyst preparation

• Precipitate Zn/Al oxides and Cu/Zn hydroxy carbonates in series/parallel step 1 at pH 6.5 and 65°C

PRECIPITATION • Age precipitated slurry for ~1 hour

• Wash and filter with demin. water

• Dry precipitate overnight at 120°C

step 2 • Calcine in air at 300°C for 6 hours CALCINATION to form the oxides CuO/ZnO/AlgOg

step 3 • Compact catalyst to 2 g/cm^ density

PELLETISATION • Crush / sieve to particle size ranges

step 4 • Reduce CuO to Cu under 5% H2/N2 ACTIVATION at 2°C/min, then 215°C for 12 hours

33 O Precipitation of Zn/Al oxide and Cu/Zn hydroxycarbonates in series or parallel stages

All preparations involved precipitation in two stages. First, Zn and A1 mixed oxides were precipitated as the catalyst support material. Zn(N03)2 solution (85°C) was added by use of a peristaltic pump to NaA102 (sodium aluminate) solution at 65°C in order to effect the precipitation. The ratio Zn:Al was chosen to encourage formation of the Zn Aluminate (ZnAl204) spinel, which leads to enhanced catalyst stability (13,19). The slurry was adjusted to pH 6.5 by pipetting HN03(aq).

In the second stage, Cu and Zn hydroxycarbonates were precipitated. This was carried out in series (i.e. the second stage precipitation was carried out by flowing solutions into the first stage precipitated slurry) or parallel (i.e. the two stages of precipitation were carried out in separate vessels, and then mixed). A mixed solution of Cu(N03)2 and Zn(N03)2 (85°C) was pumped at a fixed rate through the inner chamber of a nozzle to the precipitation vessel (65°C). The flow of Na2C03 solution (85°C) through the outer chamber of the nozzle was adjusted to control the precipitation at pH 6.5.

The slurry was aged for 1 hour. It has been shown that the crystalline hydroxycarbonates vary and grow during the ageing process, causing greater final activity (24-28). The precipitate was successively filtered and reslurried with de-mineralised water to remove sodium salts, which poison the catalyst. The Na content of the precipitate was measured after each wash using Atomic Absorption Spectroscopy (the desired Na content of the catalyst in oxide form was < 500 ppm). Finally, the precipitate was dried at 120°C overnight.

The effect of calcination in air is to decompose the hydroxycarbonates to the normal oxides Cu0/Zn0/Al203. The catalyst is normally stored in this stable form.

34 ® Compaction of catalyst

As a binder and lubricant for the pelletisation process, 2% (w/w) graphite powder was added to the calcined catalyst. The catalyst was compacted to a density of 2 g/cm^ using a 1 inch die and a pellet press. The pellets were crushed and sieved to a particle size range of 106-250 pim for the microreactor and 250-500 p,m for the Berty reactor.

O Activation of catalyst - reduction of CuO to Cu

Prior to reaction, the catalyst was activated in situ by reduction of the CuO in the catalyst to Cu. The reduction being exothermic, there is a danger of sintering (agglomeration of metal, resulting in loss of surface area) the Cu. Therefore a diluted reducing gas (5% H2 in N2) was used, the temperature was ramped very slowly (2°C/min), and the final temperature was chosen for a low reduction rate (215°C).

3.2 PREPARATION OF CATALYSTS AND PHYSICAL MIXTURES

O Cu/ZnO/AloOj prepared by two-stage series precipitation

Two identical Cu/Zn0/Al203 catalysts were prepared by series precipitation, as described in step 1 above. The Zn/Al mixed oxide was precipitated first. Then the Cu/Zn hydroxycarbonates were precipitated into the same slurry, and the slurry aged for 1 hour.

® Cu/ZnO/ALO] prepared by two-stage parallel precipitation

One Cu/ZnO/AlzOg catalyst was prepared by parallel precipitation, as described in step 1 in Section 3.1. The Zn/Al mixed oxide was precipitated in one vessel and the Cu/Zn hydroxycarbonates were precipitated in a separate vessel. Then the two slurries were mixed and aged for 1 hour.

35 ® Pd/Cu/ZnO/AInO^ prepared by series precipitation with Pd wet-added

Two Pd/Cu/ZnO/Al^O^ catalysts were prepared by this method, providing a high and a low Pd loading. The two-stage series precipitation was carried out, as for Cu/Zn0/Al203 (Section 3.2.1). After ageing, Pd(N03)2 solution was added to the slurry in order to give the desired Pd composition. Because Pd(N03)2 solution is acidic, the resultant slurry was pH 5.5 in the case of the low Pd loading and pH 4.75 in the case of the high Pd loading. The slurry was thoroughly mixed before washing/filtering.

G Pd/Cu/ZnO/ALOg prepared by series precipitation with Pd impregnated

Two Pd/Cu/Zn0/Al203 catalysts were prepared by this method, providing a high and a low Pd loading. The two-stage series precipitation was carried out, as for Cu/Zn0/Al203 (Section 3.2.1). After ageing, washing/filtering and drying, the 'impregnation' was carried out. The dried precursor was reslurried with demineralised water. In order to prevent possible dissolution of the Cu/Zn/Al metals in an acidic solution, Na2C03 solution was added first, followed by Pd(N03)2 solution. The amounts of each solution were pre-calculated, Na2C03 to neutralise the Pd(N03)2, and Pd(N03)2 to achieve the desired Pd composition. The addition of the Na2C03 solution followed by the Pd(N03)2 solution gave slurries of pH 11.0 and pH 6.8 respectively in the case of the low Pd loading and pH 11.2 and pH 6.6 respectively in the case of the high Pd loading. The slurries were repeatedly washed and filtered to remove Na salts before drying.

0 Pd/Cu/ZnO/AloOa prepared by parallel precipitation with Pd co-precipitated

Two Pd/Cu/Zn0/Al203 catalysts were prepared by this method, providing a high and a low Pd loading. The two-stage parallel precipitation was carried out, as for Cu/Zn0/Al203 in Section 3.2.2, except that Pd was co-precipitated with Cu and Zn species in the second stage. In other words, a mixed solution of Pd(N03)2, Cu(N03)2 and Zn(N03)2 was precipitated with a Na2C03 solution at a controlled pH 6.5, before mixing with the Zn/Al mixed oxide slurry and ageing.

36 0 Preparation of Pd/ALOg + Cu/ZnO/AinOg physical mixtures

Two Pd0/Al203 were prepared for use in physical mixtures. The preparation involved precipitation from Pd(N03)2 solution and NaA102 solutions. Pd(N03)2 and NaA102 were used in quantities necessary to achieve 10% Pd0/Al203 and 30% Pd0/Al203. The precipitation apparatus consisted of a beaker upon a heater/magnetic stirrer containing temperature and pH probes. NaA102 was dissolved in hot (65 °C) demineralised water. While stirring, the Pd(N03)2 solution was added slowly using a pipette. Finally the slurry was brought to pH 7.0 using aqueous HNO3. The ageing, washing/filtering, drying and calcining processes were as in Table 3.1.

Two Pd/Al203 + Cu/Zn0/Al203 physical mixtures were prepared, using either the 10% Pd0/Al203 or the 30% Pd0/Al203. The first Cu0/Zn0/Al203 catalyst prepared by series precipitation was used for the two physical mixtures. The post-calcined PdO/Al203 and Cu0/Zn0/Al203 powders were thoroughly mixed along with the graphite used for the subsequent compaction. The mass ratio of Pd0/Al203 to Cu0/Zn0/Al203 was 2/3 for both physical mixtures. The only difference between the two physical mixtures was the PdO content of the Pd0/Al203.

3.3 BULK CHEMICAL ANALYSES BY A. A. SPECTROSCOPY

Using the calcined catalyst powders, bulk chemical analyses were performed for Pd, Cu, Zn, A1 and Na metals using an A. A. Spectrophotometer. The compositions, calculated as normal oxides, are shown in Table 3.2. (The catalysts are referred to in Table 3.2 using the nomenclature which was given at the beginning of the Thesis.) The compositions of the three Cu/Zn/Al catalysts were similar. The ratio of Cu0:Zn0:Al203 for all catalysts were close to the nominal 6:3:1. The Pd:Cu atomic ratios, which are used in the catalyst nomenclature throughout this thesis, are also shown in the Table. A high and a low Pd loading were achieved for each of the four methods of Pd addition, including the physical mixtures. The NazO content of all the catalysts was < 500 ppm, which was the desired result of the washing process after precipitation (Step 1 in Section 3.1).

37 Table 3.2 Bulk chemical analyses of catalysts and physical mixtures

Pd/Cu PdO CuO ZnO AI2O3 Total NajO Catalyst (at ratio) (wt%) (wt%) (wt%) (wt%) (wt%) (ppm)

Cu/Zn/Al (series prec 1) 0 0 60.3 2&6 10.8 99.7 420

Cu/Zn/Al (series prec 2) 0 0 60.1 283 11.3 99.7 260

Cu/Zn/Al (parallel prec) 0 0 58J 2&2 13.3 9&8 410

0.03 Pd/Cu/Zn/Al (wet) 0.03 3.2 61.7 24j 10.1 9&8 240

0.10 Pd/Cu/Zn/Al (wet) 0.10 9.5 59.6 20J 10.0 9&8 240

0.04 Pd/Cu/Zn/Al (imp) 0.04 3.7 58.2 27.2 10.7 9&8 300

0.09 Pd/Cu/Zn/Al (imp) 0.09 7.7 55.6 25J 10.6 99.6 340

0.05 Pd/Cu/Zn/Al (prec) 0.05 4.1 57.9 26J 11.0 99.7 220

0.12 Pd/Cu/Zn/Al (prec) 0.12 10.0 54.6 252 10.0 9&8 460

O.OT-Pd/Al + Cu/Zn/Al * 0.07 6.3 5&5 2&8 lO.l'' 99.7 -

0.21 Pd/Al + Cu/Zn/Al * 0.21 16.7 503 218 9.0* 9&8 -

* The compositions of the Pd/Al were not determined. The nominal compositions of Pd/Al (10% or 30% PdO) and the measured composition of Cu/Zn/Al (series prec 1) were used to calculate the compositions of the physical mixtures. ^ The AI2O3 composition does not include the AI2O3 in the Pd/Al component of the physical mixtures.

3.4 B.E.T. SURFACE AREA AND POROSITY MEASUREMENTS

B.E.T. surface area and pore size distribution (by volume and surface) measurements were performed on all the calcined Cu/Zn/Al and Pd/Cu/Zn/Al catalyst powders using a Micromeritics ASAP 2000. The catalysts were degassed in situ under vacuum at 250°C for approximately 10 hours before carrying out all nitrogen adsorptions. The degassed

38 Table 3.3 B.E.T. surface area and porosity measurements

B.E.T. Pore volume (^il/g) Average Catalyst surface (Pore surface area (mVg)) pore area diameter > 500A- 200A- 75A- < 500A 200A 75A 17A 17A (A)

Cu/Zn/Al (series prec 1) 115 6&7 210.1 169.7 44.9 5.3 172 (3.5) (30.4) (54.5) (40.5) (13.0)

Cu/Zn/Al (series prec 2) 112 83 j 209.4 130.1 42.2 5.6 140 (3 6) (30.0) (39.9) (43.0) (13.7)

Cu/Zn/AI (series prec 2) 96 318 107.4 121.7 50.2 2.6 123 after compaction (21) (15.3) (40.6) (49.0) (7.1)

Cu/Zn/Al (parallel prec) 117 54.1 210.9 209.4 47.6 3.9 142 (3 3) (30.8) (69.0) (37.9) (10.5)

0.03 Pd/Cu/Zn/Al (wet) 119 682 204.8 199.0 513 4.0 140 (4.2) (311) (62.9) (48.6) (10.10

0.10 Pd/Cu/Zn/Al (wet) 122 58.1 197.3 209.1 5&8 4.3 145 (14) (28.4) (67.8) (53.9) (11.3)

0.04 Pd/Cu/Zn/Al (imp) 112 73.0 237.3 168.0 27.7 6.5 155 (4j) (33.2) (50.7) (23.8) (15.6)

0.04 Pd/Cu/Zn/Al (imp) 100 17.3 8L7 162.9 47.9 3.9 116 q/?e/- compaction (1.1) (12.3) (55.9) (44.8) (10.1)

0.09 Pd/Cu/Zn/Al (imp) 110 6&9 2273 160.6 40.2 7.1 149 (4.1) (31.6) (47.8) (35.2) (15.90

0.09 Pd/Cu/Zn/Al (imp) 99 19.7 816 153.3 46.0 4.7 114 q/?er compaction (120 (12^0 (52.8) (41.6) (11.8)

0.05 Pd/Cu/Zn/Al (prec) 121 7&3 212.3 188.9 47^ 3.5 174 (5.0) (33.8) (5810 (43.3) (9.6)

0.12 Pd/Cu/Zn/Al (prec) 106 64.7 2173 133.1 52.7 4.3 161 (3.9) (30.1) (44.9) (69.5) (10.90

39 catalyst masses were used in all calculations. The pore volume/surface area distribution data were obtained during the desorption of nitrogen. Table 3.3 shows that the B.E.T. surface areas of all calcined catalysts varied between 106 to 122 mig with no particular trend associated with Pd addition. Average pore diameter (140 to 174 A) showed a similar level of variation, as did pore volume/surface area distributions (e.g. the volumes of 500 A to 200 A pores ranged between 197 and 237 |al/g for all catalysts).

Table 3.3 also shows the surface area and porosity measurements for some compacted catalysts. The pellets of 2 g/cm^ were crushed to powder before making the measurements. The compaction process resulted in the loss of 16, 12 and 11 nxlg of B.E.T. surface area and 17, 39 and 35 A of average pore diameter respectively for those catalysts tested. Compaction also resulted in a decrease of the volume and surface area of micropores <17 A and large pores >75 A, but an increase of the volume and surface area of intermediate size pores (75-17 A).

3.5 TEMPERATURE PROGRAMMED REDUCTION OF THE CATALYSTS

The temperature programmed reduction (T.P.R.) of calcined catalysts was performed at a heating rate of 10°C/min under flowing 5% H; in Argon. Water evolves as PdO and CuO are reduced to the metals. Since Ar and water have a similar thermal conductivity, the consumption of H2 can be monitored using a thermal conductivity detector (T.C.D.). Figure 3.1 shows the T.P.R. profiles of Pd-promoted catalysts and physical mixtures. The reduction of PdO took place at low temperatures, generally below 100°C. The peak reduction temperature of CuO in the calcined Cu/Zn/Al catalyst was 255°C. The addition of Pd in the form of the 0.21 Pd/Al + Cu/Zn/Al physical mixture promoted the reduction of CuO by hydrogen spillover, resulting in a lower peak reduction temperature, 230°C. For the Pd/Cu/Zn/Al catalysts prepared by wet addition and impregnation there were clearly two phases of CuO with different reduction temperatures. One of the peak reduction temperatures was 230°C (similar to the that of the physical mixture); the other was lower at approximately 180°C. The lower reduction temperature may signify

40 .12 Pd/Cu/Za/Al (prec)

0.05 Pd/Cu/Zn/AI (prec)

0.09 Pd/CdZn/Al (imp)

& 0.04 Pd/Cu/Zn/Al (imp) 1'§

0.10 Pd/Cju/Zn/Al (wet!) Q O H

0.03 Pd/Cu/Zn/Al (wfet)

0.21 Pd/Al + Cu/Zn/Al

Cu/kn/Al (series prec 1)

200 250 300 350 400 temperature (°C)

Figure 3.1 T.P.R. of calcined catalysts (10°C/min, 5% H2/Ar, 1 atm)

41 hydrogen spillover from Pd in close proximity to Cu, e.g. the hydrogen may migrate to Cu without involving spillover on the support. The proportion of CuO reduced in the lower temperature phase was greater at the higher Pd loading by wet addition and in the case of the Pd-impregnated catalysts. The reduction of CuO in 0.05 Pd/Cu/Zn/Al (prec) coincided at approximately 180°C, but the reduction of CuO in 0.12 Pd/Cu/Zn/Al (prec) took place with a peak temperature as low as 150°C. This may be due to stronger hydrogen spillover resulting in a shift of the peak otherwise at 180°C. Alternatively the lower reduction temperature may be due to another phase, e.g. Pd-Cu alloy.

3.6 Cu SURFACE AREA MEASUREMENT FOR Cu/Zn/Al

The Cu surface area of the Cu/Zn/Al (series prec 1) catalyst was measured by titration of the N2O reaction with the metallic Cu surface [3.1]. Frontal (105) and pulse (37,106) techniques are commonly used; the latter was used in this case. The catalyst was reduced according to the normal procedure (flowing 5% H2/N2, temperature ramp l°C/min to 215°C and hold for 12 hours). The catalyst was cooled down to 60°C under the reducing gas and then the reactor flow was switched to He, maintaining the temperature at 60°C. 0.5 ml pulses of N^O were added to the He stream using an injection valve. The reactor effluent passed continuously through a Poropak N gas chromatography column at room temperature, where and N2O were separated, to a thermal conductivity detector for analysis. Summing the moles of N2 evolved across all pulses and doubling, according to the reaction stoichiometry [3.1], gave the moles of Cu on the catalyst surface.

2 Cu + N2O CU2O + N2 [3.1]

To evaluate the Cu surface area the Cu atom density was assumed to be 1.46 x 10^^ atoms/m^ (106). The Cu surface area of tlie Cu/Zn/Al (series prec 1) catalyst was measured 5 times and the results were between 23 and 26 m/g. The average Cu surface area was 24 m^/g, corresponding to a Cu dispersion of 6.1%. This compares with a B.E.T. surface area of the catalyst of 115 m^/g before reduction (Table 3.3). Chinchen et al (46) found the Cu surface areas of I.C.I, commercial-type Cu/Zn/Al catalysts of similar

42 composition (60:30:10 w/w as oxides) to be 25-33 m^/g. The Cu surface areas of the other catalysts in the present study were not determined due to the problems outlined in the following Section.

3.7 NOTE: PROBLEMS ASSOCIATED WITH Pd AND Cu SURFACE AREA MEASUREMENTS FOR Pd/Cu/Zn/AI CATALYSTS

The surface oxidation of Cu by N^O was used previously to measure the Cu surface area of Cu/Zn/Al (series prec 1). However N2O reacts readily with the metallic Pd surface also in Pd/Cu/Zn/Al catalysts and partial bulk oxidation of Pd may also occur (107). Since the stoichiometry of the N2O reaction on Pd is unknown, the dispersion of Pd cannot be determined, and more importantly, since the reactions on Cu and Pd take place at room temperature and above, a simple constant temperature procedure, as in Section 3.6, can neither be used to determine the Cu surface area. Similar problems are associated with Cu surface area measurements in Ag/Cu systems (97). With more sophisticated experiments at sub-ambient temperatures it may be possible to distinguish the reactions of NjO on Pd and Cu. Alternatively, it may be possible to back titrate the oxidation of Pd and Cu by N2O by temperature programmed reduction. Other possible methods for the determination of Pd and Cu dispersions could be the adsorption and temperature programmed desorption of H2 or CO on the pre-reduced catalysts.

The lack of Pd and Cu dispersion measurements for Pd/Cu/Zn/Al catalysts is a major limitation of the thesis. It should be noted that higher Pd loadings may not have resulted in higher Pd surface areas and the Cu dispersions may not have been the same in all catalysts. Cu dispersions may have been different, not least because of the effect of different preparations and procedures used for Pd addition, but also because of the effect of different reduction temperatures of CuO in the presence of Pd (Figure 3.1). The most important limitation is that differences in Cu surface area cannot be ruled out as the cause of differences in catalytic activity. This will be discussed in Chapter 8 of the thesis.

Chapter 4

EXPERIMENTAL

4.1 INTERNAL RECYCLE (BERTY) REACTOR SYSTEM

A 'Berty' (34) internal recycle gas phase flow reactor (Figure 4.1), partly designed and constructed by Rockhurst Research Engineering Ltd., was used for kinetic experiments at integral conversions. The reactor contained an impeller which recirculated the reaction gas through the catalyst basket, such that the gas phase was approximately perfect mixed (as confirmed in Section 4.6). In other words the gas phase was 'gradientless' in terms of concentration. Furthermore, three band heaters with associated monitoring and control equipment ensured that the temperature gradient across the catalyst basket was typically only 1 or 2°C. The pressure drop across the reactor was less than 0.1 MPa, despite the gas recirculation.

The complete reactor system included the following features:

• Construction All construction (reactor, piping etc.) was in stainless steel. Maximum design conditions were 10 MPa and 400°C. Pressure sealing of the reactor vessel was by copper gasket and 12-bolt flanged lid. All the reactor internals and heated piping were gold-plated for inertness.

• Reactor and catalyst basket The shape of the reactor vessel (200 ml) and lid aided gas recirculation. The catalyst basket (100 ml) was suitable for catalyst pellets and allowed passage of gas at the top and bottom through meshes.

• Impeller The impeller (up to 2500 r.p.m.) was designed to force the gas up outside the catalyst basket (and the lid was shaped to force the gas down through the catalyst basket). The impeller was magnetically coupled to an external drive unit.

45 r£.vv*'1 A irv\e

CARBONYL TRAP CONTROL

BERTY A REACTOR ^

CaJ:

iwrpelleiT (uivtu a.irms^

WATER COvLp Icci TRAP TUBULAR REACTOR CARBONYL drive TRAP @

P/AG)RP)fVl OF |NTCI\NAL^ Of M—• BeRT-j k^ACToK.

Figure 4.1 Internal recycle reactor and tubular reactor system

46 • Carbonvl trap A heated alumina bead bed upstream of the reactor decomposed Fe and Ni carbonyls formed by reaction of CO with cylinder and piping materials.

• Synthesis gas manifold Gases were supplied pressure regulated from cylinders to a collection of four mass flow controllers (2000, 800, 400 and 100 ml/min s.t.p. - only one shown in Figure 4.1) and various switching valves. This provided flexibility in the choice of syngas composition for any kinetic experiment.

• Condenser Downstream of the reactor was a water-cooled condenser (500 ml), which was operated at the reactor pressure. This allowed collection of liquid products for analysis.

• Reactor effluent The pressure control valve after the condenser allowed the permanent gases to flow through a sampling valve of the gas chromatograph (G.C.) and then to vent. A heated line and bleed valve connected the reactor directly to a second gas sampling valve for the main product analysis (see Section 4.4).

• Process control All process control was supervised by a master control and display unit, which also contained an alarm and trip system for over-conditions. Flow control was by four Brooks high pressure mass flow controllers. Pressure control was by a pressure transducer and an air actuated valve downstream of the condenser. Temperature control of the reactor made use of three band heaters and three each thermocouples in the catalyst basket and on the outside wall of the reactor. Impeller speed control was by tachometer and motor drive unit.

4.2 TUBULAR (MICRO-) REACTOR SYSTEM

The tubular reactor (shown in Figure 4.1 also) was designed for kinetic experiments under differential conversion, hence was also called a micro-reactor. The catalyst bed length was < 1mm. The reactor entrance and exit lengths (approximately 7 and 4 inches respectively) were filled with inert material of the same particle size as the catalyst to ensure the flow over the catalyst was uniform and well distributed. The use of a long entrance length, in which the synthesis gas was heated to the reaction temperature, and a

47 very short catalyst bed length ensured that there was negligible temperature gradient, which was confirmed by the thermocouples above, inside and below the catalyst bed. The pressure drop across the reactor was less than 0.1 MPa.

The complete reactor system included the following features:

• Construction All construction was in stainless steel. Maximum design conditions were 8 MPa and 300°C. Pressure sealing of the reactor tube was by stainless steel gaskets and Swagelok Cajun fittings.

• Reactor and catalyst support The reactor was a 3/8 inch o.d., 1/4 inch i.d., 11 inch length stainless steel tube. The reactor was operated with the gas in downflow. The catalyst charge was placed two-thirds length from the top of the reactor, supported on inert material of the same particle size above a stainless steel mesh at the bottom of the reactor.

• Carbonyl trap A heated alumina bead bed upstream of the reactor decomposed Fe and Ni carbonyls formed by reaction of CO with cylinder and piping materials.

• Synthesis gas manifold Gases were supplied pressure regulated from cylinders to a collection of three mass flow controllers (800, 400 and 100 ml/min s.t.p. - only two shown in Figure 4.1) and various switching valves. This provided flexibility in the choice of syngas composition for any kinetic experiment.

• Reactor effluent The reactor effluent, after passing through the pressure control valve was piped directly to the sampling valve of the gas chromatograph (G.C.) and then to vent. All lines and valves were fully heated to prevent condensation of products.

• Process control Flow control was by three Brooks high pressure mass flow controllers. Pressure control was by a pressure transducer before the reactor and a high temperature, high pressure fine metering valve downstream of the reactor. Temperature control was by use of a Dwyer Thermo speed unit connected to a cylindrical furnace surrounding the reactor. Of the three thermocouple points in the reactor, the centre point inside the catalyst charge was used for the temperature control.

48 4.3 PROCEDURE FOR KINETIC EXPERIMENTS

O Catalyst charging

> Tubular reactor The lower 1/3 reactor length was filled with Inert SiC (160-250|a,m), which just left the centre point of the co-axial three point thermocouple uncovered. The catalyst (106-250|_im) was then added (typically 5 mg for experiments under CO^/H^ at differential conversion) before the reactor was filled with SiC. The reactor was sealed and pressure tested.

> Berty reactor The catalyst (particle size 250-500|xm) was placed in the basket in approximately three layers, supported on quartz wool and separated by glass beads. Between 1 and 8 g of catalyst was charged depending upon the desired space velocity. The three-point thermocouple was pushed into the catalyst basket. The reactor was sealed and pressure tested.

® Reduction of catalyst

> Tubular reactor Reduction was carried out at atmospheric pressure. The flow of 5% H2/N2 (Cryoservice Ltd., 99.995% purity) was set at 200 ml/min. The temperature controller was set to ramp at l°C/min until 215°C and then hold for 12 hours.

> Berty reactor The procedure was as above, except the flow was 250-500 ml/min, depending on the catalyst mass in the reactor. A temperature ramping facility was not available, but the thermal inertia of the reactor limited the heating rate below 2°C/min. The impeller speed was 1500 r.p.m. throughout.

® Switching from reduction to reaction conditions

> Tubular reactor The flow was switched to the syngas. The syngas was either 20% CO2 / 80% H2 or 20 % CO / 80% H; (Cryoservice Ltd., 20% + 0.2% COx in H;, 99.995% purity) or a mixture of the two using two mass flow controllers. The total flow was approximately 800 ml/min s.t.p. for experiments at differential conversion. The reactor

49 was pressurised to 4.5 MPa. Finally the temperature was raised by l°C/min from 215°C to the reaction temperature of 250°C.

> Berty reactor The procedure was as above except that the total syngas flow was 400- 1600 ml/min s.t.p., depending on the desired space velocity. In addition a flow of He (BOC Ltd., 99.995%) was set to achieve a 10% composition (He was used as an internal analytical standard for the mass balances). Given the additional He, the pressure was 5 MPa, instead of 4.5 MPa as in the tubular reactor. Again the temperature ramping facility was not available, but the thermal inertia ensured the heating rate was not above l°C/min.

4.4 GAS CHROMATOGRAPHY

On-line gas analysis was performed by a Perkin Elmer 8500 G.C. A continuous bleed of the effluent from either reactor was provided through a heated line to prevent condensation of products. 0.1 ml (integral conversion) or 0.5 ml (differential conversion) samples were taken by the gas sampling valve. The samples were injected into a Poropak Q column using H2 as a carrier gas. The column was kept isothermally at 120°C and allowed separation of He, CO, CH4, CO2, H^O, CH2O (formaldehyde), CH3OH (methanol), CH3OCH3 (dimethyl ether), CHO2CH3 (methyl formate), CHOOH (formic acid) and CH3CH2OH (ethanol) (compounds in order of retention times). However, the only compounds detected by the G.C. during kinetic experiments were He, CO, CH4, CO2, H2O and CH3OH.

Detection of products was performed by a thermal conductivity detector. Response factors (which calibrate the peak area with the amount of a particular compound) were determined for a number of compounds and are shown in Table 4.1. This was done by sampling pre-calibrated gas mixtures (purchased from Cryoservice Ltd.) or mixed gas/liquid streams where a known liquid flow from a syringe pump was evaporated into a gas stream from a mass flow controller. Given the use of ratios of amounts in the calculation of all kinetic data (see next Section), relative response factors, setting the response factor of CO2 to 1, were acceptable for the calculations.

50 Table 4.1 Response factors for Gas Chromatography

Compound CO2 He CH4 CO H2O CH3OH

Response factor 1 &3507 1.3445 1.1098 1.5118 0.9505

4.5 CALCULATION OF KINETIC DATA

In all calculations the terms such as (CH30H)product refer to the molal analysis of the product gas, i.e. the G.C. peak area corrected by the response factor from Table 4.1:

(CH30H)product = CH3OH peak area x 0.9505 [4.1]

The experiments carried out at integral conversions in the Berty reactor made use of He as an internal analytical standard. In other words by ratioing the amounts of all compounds with He, it was possible to compare feed and products moles quantitatively with ease. The conversion of CO2 for a CO2/H2 synthesis gas was calculated as;

CO2 conversion = (C02/He)product / (C02/He)&ed

All the following equations are generalised for mixed (CO/CO2/H2) synthesis gases. The CH3OH and CO yields were calculated as:

CH3OH yield = (CH30H/He)p,oduct / (CO/He + C02/He)feed [4.2] (CH3OH yield is the carbon conversion to CH3OH)

CO yield = {(CO/He)product - (CO/He)feed} / (CO/He + C02/He)(^ [4.3] (CO yield is the net carbon conversion to CO)

Alternatively, or in the case of experiments in the tubular reactor where He was not used, the yield of methanol (and similarly for the yield of CO) was calculated as:

51 CH3OH yield = (CH30H)product / (CO + CO2 + CH4 + CH30H)product [4.4] (Equations [4.2] and [4.4] gave the same answer.)

; The reaction rate of CO2 (and similarly for the reaction rate of CO) was calculated as:

CO2 reaction rate = {(C02/He)feed - (C02/He)product} x He flow rate [4.5] (the He flow rate was always 10% of the total feed flow rate)

The production rate of methanol (and similarly for the rate of CO, H2O and other products) was calculated as follows:

CH3OH production rate = (CH30H/He)product x He flow rate [4.6]

In the case of experiments in the tubular reactor where He was not used, the CH3OH production rate could also be calculated as:

CH3OH production rate - CH3OH yield x syngas molar flow rate x 0.2 [4.7] (0.2 is the composition of carbon oxides (CO+CO2)/(CO+CO2+H2) in all cases.)

4.6 VERIFICATION OF PERFECT MIXING IN THE BERTY REACTOR

The 'Berty' internal recycle reactor was operated with an impeller speed of 1500 r.p.m. in all kinetic experiments. The mixing characteristics of the reactor used in this study were investigated previously by Boz (99) by monitoring the residence time distribution of a tracer which was added to the feed as a pulse or step input. It was found that the residence time distribution approximated to the perfect mixing model at impeller speeds 500 r.p.m. and above. Boz (99) used other methods, one being the calculation of dimensionless ratios describing mixing characteristics, which also led to the conclusion that the mixing performance was constant at 500 r.p.m. and above.

52 If perfect mixing applies at a given impeller speed (and the reaction productivity is not extra-particle mass transfer limited - see Section 4.7.1) then the reaction productivity should not be effected by greater impeller speeds. This was verified for the reactor used in this study by Boz (99) for mixed alcohol synthesis from CO/H2 over the I.C.I, commercial Cu/Zn/Al catalyst at 290°C. It was found that the catalytic activity was fixed at impeller speeds between 500 and 2500 r.p.m. Moreover, Table 4.2 shows the results of experiments from the present study of methanol synthesis from CO2/H2. A fresh charge of Cu/Zn/Al (series prec 1) catalyst was used for each experiment. The methanol (and CO) production rates at impeller speeds 1000, 1500 and 2000 r.p.m. were well within the range of experimental scatter (± 6% from the mean - see Section 4.8). In other words the catalytic performance was not effected by the gas phase regime, indicating that perfect mixing was approximately achieved in the internal recycle reactor.

Table 4.2 Effect of impeller speed on methanol production in the Bertv reactor (Cu/Zn/Al (series prec 1), initial activities, 4 H2:C02, 13.9 mol h"' g^u'^ 5 MPa, 250°C)

Impeller Speed (r.p.m.) 1000 1500 2000

CH3OH production rate (mmol h"' gcu"^) 98 102 103

CO production rate (mmol h"^ gcu') 71 73 73

4.7 VERIFICATION OF KINETICS NOT LIMITED BY MASS TRANSFER

4.7.7 Extra-particle mass transfer in the Berty reactor

One of the consequences of fast recirculation within the Berty reactor is that there is a high rate of extra-particle mass transfer. This means that external diffusion limited production can generally easily be avoided. The previous kinetic investigation of the effect of different impeller speeds (Table 4.2) was relevant also to the issue of external

53 mass transfer. The experiments were carried out at the highest flow rate relative to catalyst mass (lowest conversion) used in any experiment in the Berty reactor. Therefore the net reaction rate was the greatest and mass transfer limitation was the most likely. However no effect of impeller speed was found (Table 4.2) and therefore the gas recirculation rate resulted in an adequate rate of extra-particle mass transfer. Impeller speeds of 1500 r.p.m. were used in all subsequent kinetic experiments

4.7.2 Intra-particle mass transfer in the Berty reactor

An investigation was also conducted to ensure that methanol production from CO2/H2 was not limited by a low rate of intra-particle diffusion. The investigation involved kinetic experiments with different catalyst particle sizes in order to find the limiting size below which the observed methanol production rate became constant. Again the experiments were carried out at the highest flow rate relative to catalyst mass used in any experiment in the Berty reactor so that mass transfer limitation was the most likely. Catalyst particle size ranges between 106-850 |im were prepared by crushing and sieving Cu/Zn/Al catalyst which had been pre-pelletted to a density of 2 g/cra (as in Section 3.1). 3mm x 3mm cylindrical pellets of the same density were also prepared for testing. Table 4.3 shows that catalyst particle sizes within the range of 106-850 )j.m gave the same methanol (and CO) production rates, indicating that the intra-particle mass transfer rate was adequate. On the other hand the use of the pellets resulted in lower production rates. 250-500 |im catalyst particle sizes were used in subsequent experiments.

Table 4.3 Effect of catalvst particle size on methanol production in the Berty reactor (Cu/Zn/Al (series prec 1), initial activities, 4 H2:C02, 13.9 mol h"' gcu ^ 5 MPa, 250°C) catalyst particle size range (|J.m) 106-250 250-500 500-850 pellets*

CH3OH production rate (mmol h"' gcu^) 102 102 103 86

CO production rate (mmol h"' gcu^) 73 73 73 68

approximately 3mm x 3mm cylindrical pellets

54 4.7.3 Extra-particle mass transfer in the tubular reactor

In order to check that methanol production was not limited by the rate of extra-particle mass transfer, experiments were carried out with different gas linear velocities of the CO2/H2 syngas. The flow through the tubular reactor was varied, whilst maintaining a constant flow rate relative to catalyst mass (see Table 4.4). The flow rate relative to catalyst mass was such that differential kinetics applied (Chapter 5). Therefore the rate of methanol production was the maximum possible and mass transfer limitation was most probable. From Table 4.4, it is clear that there was no effect of linear gas velocity on methanol or CO production from CO2/H2, within the range of experimental error (± 6% from the mean - see Section 4.9). The flow rate used in subsequent experiments was always above 239 ml/min, i.e. the linear gas velocity was never below 125 mm s"' @ s.t.p.

Table 4.4 Effect of gas linear velocity on methanol production in the tubular reactor (Cu/Zn/Al (series prec 1), initial activities, 4 H2:C02, 698 mol h"^ gcu ^ 4.5 MPa, 250°C) catalyst mass (mg) 1.9 2.3 5.3 syngas flow (ml min"^ @ s.t.p.) 239 289 665 syngas linear velocity (mm s'^ @ s.t.p.) 125 152 350

CH3OH production rate (mmol h"' gcu"') 457 438 444

CO production rate (mmol h"^ gcu^) 108 102 105

4.7.4 Intra-particle mass transfer in the tubular reactor

Table 4.5 shows the results of experiments to ensure that methanol production from CO2/H2 was not limited by a low rate of intra-particle diffusion. Again the experiments were carried out at the highest flow rate relative to catalyst mass used in any experiment in the tubular reactor so that reaction rate was the greatest and mass transfer limitation

55 was the most likely. Catalyst particle size ranges between 35-250 |im were tested. Table 4.5 shows that catalyst particle sizes within the range of 35-160 pm gave similar methanol (and CO) production rates, indicating that the intra-particle mass transfer rate was adequate. 106-250 ^m catalyst particle sizes were used in subsequent kinetic experiments with the tubular reactor. Interestingly 250-500 jum particle sizes in the tubular reactor gave a result indicating internal diffusion limited production, which was not the case for the same particles in the Berty reactor. This reflects the fact that reaction rates were far higher in the tubular reactor, which was operated differentially.

Table 4.5 Effect of catalyst particle size on methanol production in the tubular reactor (Cu/Zn/Al (series prec 1), initial activities, 4 H2:C02, 698 mol h"^ gcu '5 4.5 MPa, 250°C) catalyst particle size range (|j,m) 35-106 106-250 250-500

CH3OH production rate (mmol h"' gcu'^) 439 444 407

CO production rate (mmol h"' gcu^) 103 105 97

4.8 REPRODUCIBILITY OF RESULTS

4.8.1 Reproducibility of results using the Berty reactor

Table 4.6 shows the results, in terms of methanol yield, of repeated experiments using Cu/Zn/Al (series prec 1) and 0.09 Pd/Cu/Zn/Al (imp) catalysts under CO^/H^ or CO/H2 at one flow rate in the Berty reactor (further details of these experiments can be found in Chapters 5 and 6). Each data point was obtained as the initial activity from an experiment using a fresh charge of catalyst. It was found that the scatter of results due to experimental error was always within the range ± 6% from the mean result.

56 Table 4.6 Reproducibility of results from experiments using the Bertv reactor (Initial activities, 4 3.47 mol h"' g^u"', 5 MPa, 250°C)

Catalyst Cu/Zn/AI (series prec 1) 0.09 Pd/Cu/Zn/Al (imp)

Syngas CO/H2 CO2/H2 CO/H2 CO2/H2

Runl. CH3OH yield (%) 4.79 6.66 4.49 7.98

Run 2. CH3OH yield (%) 4.66 &33 4.02 8.21

Run 3. CH3OH yield (%) 4.75 &59 4.22 7.91

Run 4. CH3OH yield (%) &47

Run 5. CH3OH yield (%) 6.43

Mean CH3OH yield (%) 4J3 &50 4.24 &03

Variation from the mean ± 1.5% + 2.6% ±5.9% ±:L2

Table 4.7 Reproducibility of results from experiments using the tubular reactor (Initial activities, 4 H^iCOx, 698 (CO^/Hz) / 38.1 (CO/H^) mol h"' gcu\ 4.5 MPa, 250°C)

Catalyst Cu/Zn/Al (series prec 1) 0.09 Pd/Cu/Zn/AI (imp)

Syngas CO/H2 CO2/H2 CO/H2 CO2/H2

Runl. CH3OHrate* 21.5 444 14.4 432

Run 2. CH3OH rate* 225 438 15.1 440

Run 3. CH3OH rate* 21.0 457 455

Run 4. CH3OH rate* 425 463

Run 5. CH3OH rate* 446 449

Run 6. CH3OH rate* 449 437

Run 7. CH3OH rate* 446 448

Run 8. CH3OH rate* 445

Mean CH3OH rate* 21.7 444 14.8 446

Variation from the mean ±3.7% ± 4.3% ±2.7% ±3.8%

* CH3OH production rate (mmol h"' gcu^)

57 4.8.2 Reproducibility of results using the tubular reactor

Table 4.7 shows the results, in terms of methanol production rate, of repeated experiments using Cu/Zn/Al (series prec 1) or 0.09 Pd/Cu/Zn/Al (imp) catalysts under CO^/H^ or CO/H2 at differential conversion in the tubular reactor (further details of these experiments can be found in Chapters 5 and 6). Again each data point was obtained as the initial activity from an experiment using a fresh charge of catalyst. It was found that the scatter of results due to experimental error was always within the range ± 4.5% from the mean result.

4.9 MASS BALANCES

The use of an inert component (10% He) along with the synthesis gas (90% CO+CO2+H2) in the feed to the Berty reactor allowed mass balances in carbon and oxygen to be checked. The reactants/products which were included in the balances were CO, CO2, CH3OH and H2O (the only other product was trace CH4 which made little difference to the mass balances).

The errors EQ and EQ in the carbon and oxygen balances were calculated as:

(CO/He + C02/He)feed - (CO/He + COz/He + CH30H/He)product = [4.8]

(CO/He + 2 C02/He)feed - (CO/He + 2 COj/He + CHgOH/He + H20/He)p„duct = Eo [4.9]

Then the carbon and oxygen balances were defined as the errors in the above equations relative to the amount of carbon or oxygen converted respectively, i.e.:

carbon balance = EQ I {(CO/He + C02/He)feed - (CO/He + C02/He)pj.oduct} [4.10]

58 oxygen balance = EQ / {(CO/He + 2 COz/He)^*,! - (CO/He + 2 C02/He)product} [4.11]

In all kinetic experiments (under CO/H2, CO2/H2 or CO/CO2/H2) in the Berty Reactor, the carbon and oxygen balances were always within ±7.5%. In 95% of experiments the balances were within ±5%. As an example, results of experiments in Chapter 7 along with the carbon and oxygen balances are tabulated in Table 4.8.

Table 4.8 Carbon and oxygen balances from experiments using the Bertv reactor (Cu/Zn/Al (series prec 1), initial activities, 4 H2:C0x, 3.47 mol h-' gcu ^ 5 MPa, 250°C)

COz/CCO+COz) 0 0.1 0.2 0.3 0.5 0.7 0.9 1

CO2 rate* 022 -5.39 -13.10 -17.71 -33.94 -59.69 -91.85 -94.33

CO rate* -33.8 -140.5 -112.7 -82.85 -35.80 -2.04 4422 49.20

CH3OH rate* 32.81 147.3 130.0 99.05 70.31 60.28 51.11 45.01

H2O rate* 0.10 4J9 1L96 17.87 37.63 55.85 93.32 96.45

C balance (%) 2.49 -1.00 -3.37 1.50 -0.82 234 -7.32 0.26

0 balance (%) 1.54 -0.44 -2.23 1.15 -4.11 4J5 -3J5 -1.44

* units of rate (mmol h"' gcu^); negative/positive values indicate reaction/production

4.10 THE VALIDITY OF INITIAL ACTIVITY MEASUREMENTS

Kinetic data tliroughout this thesis were obtained as initial activity measurements (i.e. the initial rate or yield of methanol production) using a fresh charge of catalyst charge each time. In this way it was intended to capture the effect of Pd on the chemical kinetics of methanol synthesis, excluding in particular any changes in catalytic activity caused by deactivation. However, initial rates of deactivation are very high in some catalyst

59 systems, so it is sometimes difficult to ascribe truly initial activities. Also the initial rates of deactivation may be very high for some catalysts in particular, such that these catalysts may appear to have much lower activities than other catalysts if truly initial activity measurements are not taken. The purpose of this section is to show that the initial rates of deactivation were not so high with the present catalysts, thus establishing the validity of initial activity data.

Initial activity was defined as the activity when the catalyst temperature reached 250°C, after the catalyst had been reduced, the reactor pressurised under the synthesis gas, and the temperature ramped from 215°C at l°C/min (as in Section 4.3). Figure 4.2 shows the deactivation, thereafter, of various catalysts under CO/H2 or CO2/H2. (the activity profile of each catalyst has been normalised to the initial activity measurement for that catalyst). The first observation is that the initial rate of deactivation, i.e. the initial slope of the deactivation profile, under either synthesis gas was not significantly effected by the presence of Pd in the catalyst or physical mixture. The second observation is that the initial rate of deactivation in terms of percentage loss of activity, ~4%/hour under CO/H2 and ~1.5%/hour under CO2/H2 was not very high. In fact, differences in the initial activity measurements caused by Pd addition were generally much higher than the deactivation in one hour (e.g. the initial activity of 0.09 Pd/Cu/Zn/Al (imp) compared to Cu/Zn/Al (series prec 1) was 23% greater under CO2/H2 and 10% less under CO/H2 conditions as in Figure 4.2; see Chapters 5 and 6). Therefore it was possible to discriminate between the effect of Pd and the effect of deactivation on the methanol synthesis activity under CO/H2 and CO2/H2, and the rate of deactivation was sufficiently low to validate initial activity measurements.

The rate of deactivation was sufficiently low to validate initial activity measurements with other synthesis gases also. For example, with CO2/CO/H2 (1% CO2) the initial rate of deactivation of 0.09 Pd/Cu/Zn/Al (imp) or Cu/Zn/Al (series prec 1) was ~2%/hour (activity measurements were only taken for a few hours, so deactivation profiles have not been plotted here) whereas the presence of Pd resulted in 30% less initial activity (see Chapter 7). Finally, initial activity measurements were also valid under diffei;ential conversion in the tubular reactor (all the previous data were obtained at integral

60 1.05 COj/Hj synthesis gas 0 Cu/Zn/Al (series prec 1) I'o 1 + 0.04 Pd/Cu/Zn/Al (imp) o 0.09 Pd/Cvi/Zn/Al (imp) 0.95 -L V I \ (D 0.9 initim rate of deactivation (~1.5%/hour) a 0.85 1 H- 10 20 30 40 50

1.05 t3 1 COj/Hj synthesis gas # Cxi/Zn/Al (series prec 1) • 0.07 Pd/Al + Cu/Zn/Al ox O* X 0.21 Pd/Al + Cu/Zn/Al \ aa] X 0.95 - \ I \ X ™ x,'' >f \

0.9 initial rate of deactivation § (~1.5%/hour) (185 0 10 20 30 40 50

1.2

CO/Hj synthesis gas ^ Cu/Zn/Al (series prec 1) T3 13 + 0.04 Pd/Cu/Zn/Al (imp) o 0.09 Pd/Cu/Zn/Al (imp) \ <*+ I 0.8 - \ t3 (U • 0%^$,

0.6 - § initial rate of deactivation (~4%/hour) 0.4 + 10 0 20 30 40 50 time on stream (h)

Figure 4.2 Deactivation of Pd-promoted catalvsts under CO/Ho and COo/Ho (4 H2:C0x, 3.47 mol h"' gcu 5 MPa, 250°C, Berty reactor)

61 conversion in the Berty reactor). For example, under CO/H2 at differential conversion the initial activity of 0.09 Pd/Cu/Zn/Al (imp) was 30% less than that of Cu/Zn/Al (series prec 1) (Chapter 6), whereas the initial rate of deactivation was ~6%/hour in each case.

62 Chapter 5

METHANOL SYNTHESIS FROM COg/Hz

5.1 SUMMARY

• Methanol and CO were produced from COj/Hj with > 99.5% selectivity over all catalysts. The production of trace methane was not effected by the presence of Pd. Water was a stoichiometric co-product of methanol and CO. • Across a range of conversions in the Berty reactor, methanol synthesis over Cu/Zn/Al was promoted by physical mixtures with Pd/Al, whereas Pd/Al alone was inactive. Greater promotion (up to 35%) was obtained over Pd/Cu/Zn/Al catalysts with various methods of Pd addition, but (allowing for likely differences in Cu surface area) the promotions were largely independent of Pd loading and the method of Pd addition. These results support the proposal that hydrogen spillover causes the promotion. • CO production was unaffected by the presence of Pd in catalysts or physical mixtures. • At extremely high flow rates in the tubular reactor, with methanol yields < 0.35%, differential conversion was achieved. There was no promotion by Pd at differential conversion, either with physical mixtures or with Pd/Cu/Zn/Al catalysts, suggesting that the promotion is related to the presence of a product. • No Pd promotion was observed with CO addition to CO2/H2 at differential conversion. • The addition of water at differential conversion caused an order of magnitude loss of methanol production over Cu/Zn/Al, but the inhibition was 30% less with Pd addition. The methanol production rates and Pd promotion were closely comparable to the results at integral conversion in the Berty reactor where the water concentration as a result of water production was similar.

It is proposed that water excessively oxidises the Cu sites or inhibits the catalyst by competitive adsorption. In addition it is proposed that Pd provides hydrogen spillover to

63 Cu, moderating its oxidation state or removing water and dissociated water species. It should be noted that Pd promotion only counteracts a small extent of the inhibition caused by water. Apparently the oxidising potential or the bonding strength of water is much stronger than the effect of hydrogen spillover from Pd, or hydrogen spillover only effects the periphery of Cu crystallites or certain morphologies of Cu.

5.2 TRACE PRODUCTS FROM COg/Hg (BERTY REACTOR)

With all catalysts under CO2/H2 in the Berty reactor, methanol, CO and water were major products, while the only other product detected was trace amount of methane (no trace methane was detected at differential conversion in the tubular reactor^). The amount of methane was always below the linear range of the thermal conductivity detector for the purpose of accurate quantification. So as not to under-estimate its amount, the peak integration of methane was doubled, and the yield was calculated as for methanol (Section 4.5).

Table 5.1 shows the methane yields and selectivities for all the catalysts at two different flow rates of the CO2/H2 synthesis gas. The results were quite scattered due to the measurement of such small quantities, but there appeared to be no relationship between methane production and the presence of Pd in the catalysts or physical mixtures. Furthermore, the two Pd/Al (1:9 and 3:7 as oxides) components of the physical mixtures used alone as catalysts at the same conditions were virtually inactive for methane production (methane was visible in the G.C. chromatogram but was too small a peak to be recognised by the integrator).

Table 5.2 summarises the approximate results of this Section. The selectivity of methane was always <0.5%. No other (carbon) products were detected except for methanol and

^ At differential conversion in the tubular reactor no trace compounds were detected. However, the apparent absence of trace products was probably due to the lower limit of sensitivity of the detector, given the extremely low conversions necessary to achieve differential kinetics.

64 CO, whose sum selectivity therefore was > 99.5%. These results applied for all catalysts, including Pd-promoted catalysts.

Table 5.1 Yield & selectivity of trace product methane from COo/Ht at various flow rates (Initial activities, 4 H2;C02, 5 MPa, 250°C, Berty reactor)

CATALYST CH4 yield (selectivity) (%)

(Jl^mol h"^ gcu'^) * (0.43 mol h"' gcu"^) *

Cu/Zn/Al (series prec 1) 0.047 (0.34) 0.070 (0.25)

Cu/Zn/Al (series prec 2) 0.042 (0.30) 0.074 (0.27)

Cu/Zn/Al (parallel prec) 0.056 (0.39) 0.087 (0.32)

0.03 Pd/Cu/Zn/Al (wet) 0.041 (0.28) 0.081 (0.28)

0.10 Pd/Cu/Zn/Al (wet) 0.051 (0.34) 0.073 (0.25)

0.04 Pd/Cu/Zn/Al (imp) 0.035 (0.23) -

0.09 Pd/Cu/Zn/Al (imp) 0.047 (0.30) -

0.05 Pd/Cu/Zn/Al (prec) 0.053 (0.35) 0.093 (0.33)

0.12 Pd/Cu/Zn/Al (prec) 0.056 (0.38) 0.101 (0.36)

0.07 Pd/Al + Cu/Zn/Al 0.040 (0.27) 0.092 (0.33)

0.21 Pd/Al + Cu/Zn/Al 0.056 (0.37) 0.078 (0.28)

CO2/H2 synthesis gas flow rate

Table 5.2 General selectivity of products under COn/Hn

Carbon products CH3OH CO CH4

Selectivity over all catalysts * > 99.5 % <0.5%

* not including Pd/Al alone as a catalyst

65 6.3 METHANOL SYNTHESIS FROM COg/Hg OVER Cu/Zn/AI, Pd/AI AND PHYSICAL MIXTURES (BERTY REACTOR)

Figure 5.1 shows the methanol yields at 0 and 10 hours over all the Pd promoted catalysts with a moderate flow rate of CO2/H2. Figure 5.2a/b shows the initial methanol yields and production rates across a range of flow rates covering two orders of magnitude. Note that at the lowest flow rate (Figure 5.2b) the equilibrium methanol yield, 22% (from Section 2.1), was almost reached over all catalysts. At the highest flow rate (Figure 5.2a) the methanol yield was lower than 4% over Cu/Zn/Al, but did not correspond to differential conversion, as indicated by the increasing methanol production rate with respect to the flow rate (the methanol production rate should reach an asymptote at differential conversion according to the theory in Appendix 1).

The three Cu/Zn/Al catalysts (Figure 5.2a/b) gave similar methanol yields or rates (± 6%) across the range of flow rates. Figure 5.2a shows that, for any given flow rate, the methanol production was greater over the two physical mixtures in relation to the Pd content. The promotions^ across the range of flow rates can be seen more clearly in Figure 5.3. At the highest flow rate (lowest conversions), where the promotions were greatest, the promotion was 11% or 22% over the 0.07 or 0.21 Pd/Al + Cu/Zn/Al physical mixture respectively. Note that these promotions apply for initial activities, but the promotions were fairly constant with respect to time on stream (for 50 hours), since the deactivation profiles of all catalysts and physical mixtures were similar (see Section 4.10) and the promotion is defined as a relative quantity^.

The two Pd/Al (10:90 and 30:70 ratios of oxides) components of the physical mixtures were each tested alone as a catalyst for methanol synthesis from CO2/H2. Across a wide range of flow rates, there was negligible methanol production (at low flow rates of the

^ Promotion has been calculated relative to the Cu/Zn/AI catalyst which served as a precursor before Pd addition and is defined as the percentage increase in methanol production.

66 synthesis gas, methanol was visible in the G.C. chromatogram but was too small a peak to be recognised by the integrator). The inactivity of Pd/Al for methanol synthesis from CO2/H2 has been found previously (2,78,136). Therefore the promotion observed with physical mixtures is consistent with the proposal (1-3) that hydrogen spillover from Pd to Cu is responsible for the promotion. This proposal will be discussed further in the following Section.

5.4 METHANOL SYNTHESIS FROM COg/Hz OVER Pd/Cu/Zn/AI CATALYSTS (BERTY REACTOR)

Pd addition to Cu/Zn/Al, by whatever method, invariably promoted the production of methanol (Figures 5.1 and 5.2a/b). Greater promotion, up to 35% at the highest flow rate (Figure 5.3), was achieved with the Pd/Cu/Zn/Al catalysts than with the physical mixtures. If it is assumed that the promotion is the result of hydrogen spillover only, the latter result may be due to more effective hydrogen spillover due to the proximity of Pd to Cu. Alternatively, the greater activity of Pd/Cu/Zn/Al catalysts compared to physical mixtures may be due to additional catalytic sites in these catalysts. Firstly, the Cu dispersion in Pd/Cu/Zn/Al catalysts may be greater than that in Cu/Zn/Al, as a result of the different preparation procedures and the different reduction temperature of CuO in the presence of Pd (as discussed in Section 3.7). Secondly, there may be new sites in Pd/Cu/Zn/Al catalysts which may be active for methanol synthesis from CO2/H2 (e.g. Pd supported on ZnO may be active (136) or, as the T.P.R. of the calcined catalysts (Section 3.5) showed that there were a number of phases associated with the reduction of CuO in Pd/Cu/Zn/Al catalysts which implied a high degree of intimacy between Pd and Cu, Pd- Cu alloys or junctions may be active). These issues will be discussed later with the benefit of further kinetic results.

Unlike the physical mixtures, amongst the Pd/Cu/Zn/Al catalysts their was no relationship between Pd loading and the level of promotion (see Table 5.3). Indeed, with the Pd/Cu/Zn/Al catalysts prepared by wet addition or by precipitation, the higher Pd loading

67 resulted in less promotion. In the absence of Pd and Cu dispersion measurements (see Section 3.7), it is difficult to explain this result. If the dispersion of Cu was adversely affected by the presence of Pd (e.g. as a result of the evolution of the Cu metal surface at lower temperatures during the reduction of CuO), and thereby if the Cu dispersion was less in the case of higher Pd loadings, the activity of Pd/Cu/Zn/Al catalysts would appear similar despite greater Pd promotion at the higher Pd loading. However, assuming that the Cu dispersions were similar in all catalysts and that the Pd surface area was related to the Pd loading, the similar activity of Pd/Cu/Zn/Al catalysts containing high and low Pd loadings supports the finding of the previous Section that Pd does not function as an independent catalytic site for methanol production under CO2/H2. Instead, if hydrogen spillover fi-om Pd is responsible for the promotion, the promotion would reach a plateau when the provision of hydrogen becomes sufficient (to achieve the desired catalyst state or as a reactant), and this would explain why higher Pd loadings were not beneficial.

In Figures 5.1-5.3, Pd promotion of methanol synthesis from CO2/H2 has been demonstrated across a much wider range of conversions than previously reported (1-3). The Pd promotion (Figure 5.3) was greater at higher flow rates (lower conversions) and tended to zero at lower flow rates (higher conversions), which is consistent with the methanol synthesis reaction being closer to equilibrium at low flow rates. Since the kinetic experiments were carried out in an internal recycle reactor, where the catalysts were exposed to the product gas phase concentrations, it was considered that experiments at a range of conversions may provide information regarding gas phase concentration dependence of the promotion. However, the promotion was apparently not related to the presence of greater partial pressure of the product CO at higher conversions. This implies that the promotion does not involve the secondary reaction of CO to methanol over Pd (which was a possibility in view of the fact that supported Pd is a catalyst for methanol production fi-om pure CO/H^ (19,33,70)). The promotion was also apparently not related to greater partial pressure of the product water at higher conversions. This is not in accord with the claim of Fujimoto and Yu (3), that water is an oxidising agent of Cu, and the counteraction of this effect by hydrogen spillover is the mechanism of Pd promotion. The dependence of the promotion on the presence of CO or water in the gas phase will be discussed later with the benefit of experiments at differential conversion.

68 • (10 hours) I (initial)

MM Cu/Zn/Al (series prec 1)

Cu/Zn/Al (series prec 2) Cu/Zn/Al (parallel prec) 0.07 Pd/Al + Cu/Zn/Al 0.21 Pd/Al + Cu/Zn/Al 0.04 Pd/Cu/Zn/Al (imp)

0.09 Pd/Cu/Zn/Al (imp)

0.03 Pd/Cu/Zn/Al (wet) 0.10 Pd/Cu/Zn/Al (wet) 0.05 Pd/Cu/Zn/Al (prec) 0.12 Pd/Cu/Zn/Al (prec)

5.5 6 6j: 7 7.5 8.5

methanol yield (%)

Figure 5.1 Effect of Pd on methanol yield from COo/H^ (3.47 mol h"^ 4 H2:C02, 5 MP a, 250°C, Berty reactor)

Table 5.3 Order of activity of catalysts for methanol synthesis from COi/Hn most 0.04 Pd/Cu/Zn/Al (imp) = 0.09 Pd/Cu/Zn/Al (imp) = 0.03 Pd/Cu/Zn/Al (wet) 0.10 Pd/Cu/Zn/Al (wet) s 0.05 Pd/Cu/Zn/Al (prec) 0.12 Pd/Cu/Zn/Al (prec) = 0.21 Pd/Al + Cu/Zn/Al 0.07 Pd/Al + Cu/Zn/Al least Cu/Zn/Al (series prec 1) s Cu/Zn/Al (series prec 2) s Cu/Zn/Al (parallel prec) active

69 u "q

II I

0.09 Pd/Cu/Zn/Al (imp)

0.04 Pd/Cu/Zn/Al (imp)

0.21 PdAl + Cu/Zn/Al

_a_ 0.07 Pd/Al + Cu/Zn/Al € Cu/Zn/Al (series prec 1) I Cu/Zn/Al (series prec 2) I

4 6 8 10

flow rate (mol h"'

Figure 5.2a Effect of Pd on methanol yield and rate from CO^/Hn at various flow rates (Initial activities, 4 HziCO;, 5 MPa, 250°C, Berty reactor)

70 CJ bO 'h "o I

0.03 Pd/Cu/Zn/Al (wet)

20 0.10 Fd/Cu/Zn/Al (wet) 0.05 Pd/Cu/Zn/Al (prec) 18 _s_ 0.12 Pd/Cu/Zn/Al (prec) 16 Cu/Zn/Al (parallel prec) 6 I 14 _4_ Cu/Zn/Al (series prec 2) 'o 12 I 10 8

4 + + 0 1 2 3 4 5 6 7

flow rate (mol h"' gc^"')

Figure 5.2b Effect of Pd on methanol yield and rate from CQo/Ht at various flow rates (Initial activities, 4 H2:C02, 5 MPa, 250°C, Berty reactor)

71 40

35 - 0.04 Pd/Cu/Zn/Al (imp)'

30 0.09 Pd/Cu/Zn/Al (imp)' ^§ 25 20 - I 0.07 Pd/Al + Cu/Zn/Al'

P-(

0.21 PdAl+Cu/Zn/Al'

' relative to Cu/Zn/Al (series prec 1) + 12 14 4 6 8 10

flow rate (mol h"' g^^')

2 3 4 5 7 35 + +

0.03 Pd/Cu/Zn/Al (wet)^ 0.10Pd/Cu/Zn/Al(wet)2

& § I T3 0.05 Pd/Cu/Zn/Al (prec)^ CL, 0.12 Pd/Cu/Zn/Al (prec)

^ relative to Cu/Zn/Al (series prec 2) ^ relative to Cu/Zn/Al (parallel prec)

Figure 5.3 Pd promotion of methanol svnthesis from COi/Hn at various flow rates (Initial activities, 4 H2:C02, 5 MPa, 250°C, Berty reactor)

72 5.5 CO PRODUCTION FROM COg/Hg OVER ALL CATALYSTS (BERTY REACTOR)

Figure 5.4 shows the conversion of CO2 to CO by means of the reverse water-gas shift reaction, which occurred simultaneously with the production of methanol. At the lowest flow rate in Figure 5.4 the equilibrium CO yield, 10% (see Section 2.1), was attained over all catalysts. 10% equilibrium CO yield applies when the methanol yield is also at equilibrium but, given that the reactants for CO and methanol production are the same, the equilibrium CO yield is higher when the methanol yield is not at equilibrium. This explains the higher CO yields at slightly higher flow rates in Figure 5.4. Up to 12% CO yields were obtained in the case of Cu/Zn/Al catalysts, given that the methanol yields were further away from equilibrium than with Pd-promoted catalysts.

Previous studies of Pd-promoted catalysts under CO2/H2 (1-3) have worked at low flow rates, where the water-gas shift reaction was at equilibrium, so the effect of Pd on CO productivity was not established. In the present study, at relatively high flow rates in Figure 5.4, where the water-gas shift reaction was not equilibrated, the presence of Pd did not effect CO yields within 10% of one another. Therefore, hydrogen spillover, if prevalent, does not effect the mechanism of CO production or perhaps the water-gas shift reaction takes place at certain Cu sites which do not accomodate hydrogen spillover. The structure sensitive nature of the water-gas shift reaction on polycrystalline Cu and Cu/Zn/Al, as opposed to the structure insensitive methanol synthesis, supports the contention that the reactions take place at different Cu sites (23,150-152).

The results of this Section have an important bearing on the discussion of the mechanism of promotion in methanol production (Section 5.4). The similar rate of CO production over all catalysts implies that the dispersion of Cu was similar in all catalysts. This suggests that the promotion in methanol production over Pd/Cu/Zn/Al catalysts was not a result of differences in Cu dispersion (as discussed in the previous Section), and therefore indirectly supports the proposal (1-3) that the promotion was caused by hydrogen spillover.

73 Cu/Zn/Al (series prec 1) 12 0.07 Pd/Al + Cu/Zn/Al 0.21 Pd/Al + Cu/Zn/Al - 10

__ 6 -

Cu/Zn/Al (parallel prec) 4 __ 0.05 Pd/Cu/Zn/Al (prec) _g_ 0.12 Pd/Cu/Zn/Al (prec)

+ + + + + + 2 4 6 8 10 12 14 1 2 3 4 5 6 7

flow rate (mol h"^ gc^') flow rate (mol h'^ g^u')

0 2 4 6 8 10 12 14 0 12 3 6 -h + Cu/Zn/Al (series prec 1) 12 -- . 0.04 Pd/Cu/Zn/Al (imp) -x- 0.09 Pd/Cu/Zn/Al (imp) - 10 -

- 8 -

- 6

Cu/Zn/Al (series prec 2) 4 - 0.03 Pd/Cu/Zn/Al (wet) 0.10 Pd/Cu/Zn/Al (wet)

Figure 5.4 Effect of Pd on CO yield from COo/Ht at various flow rates (Initial activities, 4 H2;C02, 5 MPa, 250°C, Berty reactor)

74 5.6 SELECTIVITY BETWEEN METHANOL AND CO PRODUCTION UNDER COg/Hg (BERTY REACTOR)

Given that methanol production, and not CO production, was promoted by Pd addition across the range of flow rates, the selectivity to methanol was also promoted by Pd addition (see Figure 5.5). Over all catalysts the selectivity trends with respect to the flow rate were quite unusual in that they showed that methanol production was faster than CO production at high and low flow rates, whereas CO production was faster at intermediate flow rates. The declining methanol selectivity from high to intermediate flow rates is probably the result of the comparative inhibition of methanol synthesis from CO2/H2 by the product water, since water has been found to inhibit this reaction (148,153,154). The equilibration of the water-gas shift reaction accounts for the increasing methanol selectivity at lower flow rates.

65 - 0.09 Pd/Cu/Zn/Al (imp) ^ 60 --

o 55 - CO

50 Cu/Zn/Al (series prec 1)

I 45 -

40 + 4 6 8 10 12 14 flow rate (mol h"' g^^'^)

Figure 5.5 Effect of Pd on methanol selectivity from COo/Ho various flow rates (Initial activities, 4 H2:C02, 5 MPa, 250°C, Berty reactor)

75 5.7 WATER PRODUCTION FROM COg/Hg OVER ALL CATALYSTS (BERTY REACTOR)

The amount of water produced in all kinetic experiments was always the sum of the methanol and CO produced, or the total CO2 converted, which follows from the stoichiometry of CO2 hydrogenation [5.1] and the reverse water-gas shift reaction [5.2]. (The carbon and oxygen mass balances were always within ±7.5% see Section 4.9.)

CO2 + 3H2 <=> CH3OH + H2O [5.1] CO2 + H2 <=> CO + H2O [5 2]

5.8 COMPARISON OF METHANOL SYNTHESIS FROM COg/Hg IN THE BERTY AND TUBULAR REACTORS

Table 5.4 shows that with the same flow rate of CO2/H2 the methanol yield over Cu/Zn/Al was -30% greater in the tubular reactor than in the Berty reactor. This would be expected of a reversible reaction (and of a reaction inhibited by a product, in this case water (148,153,154)), given that the Berty reactor operates under the product gas phase. The promotions obtained by Pd impregnation of Cu/Zn/Al were similar in the two reactors.

Table 5.4 Comparison of methanol yields from CO2/H2 in the Berty and tubular reactors (Initial activities, 4 H2:C02, 6.93 mol h"' gcu5 MPa*, 250°C)

Reactor Methanol yield (%) Pd promotion (%)

Cu/Zn/Al (series prec 1) 0.09 Pd/Cu/Zn/Al (imp)

BERTY 5^6 &50 28

TUBULAR &56 821 25

* The pressure was only 4.5 MPa in the tubular reactor due to the absence of 10% He.

76 5.9 APPROACH TO DIFFERENTIAL METHANOL PRODUCTION UNDER CO2/H2

Figure 5.6 shows the effect of increasing flow rate of CO2/H2 through the tubular reactor on methanol production over Cu/Zn/Al and Pd-impregnated Cu/Zn/Al. Each data point is the initial activity from a unique experiment using a fresh catalyst charge. At flow rates above 650 mol h"' gcu \ where the methanol yields were < 0.35%, the rate of methanol production reached an asymptote (over both catalysts), signalling differential reaction kinetics (see Appendix 1). This constant methanol production rate, 0.45 mol h"' gcu\ is the infrinsic forward rate of CO2 hydrogenation to methanol at the given conditions for the given catalysts. The extremely high flow rate necessary to achieve differential conversion under CO2/H2 is indicative of the inhibition of CO2 hydrogenation by a product. As has been mentioned previously, water is well known to inhibit methanol synthesis from CO2/H2 (148,153,154).

Figure 5.7 shows the promotion in methanol production, which was calculated at flow rates where repeated experiments were carried out with Cu/Zn/Al and Pd/Cu/Zn/Al. The promotion observed at integral conversions was remarkably absent at differential conversion (above 650 mol h"' gcu^)- The lack of promotion at differential conversion suggests that the promotion which was observed at integral conversions was not a result of additional catalytic sites (e.g. more Cu sites due to higher Cu dispersion or new Pd-Zn sites or Pd-Cu sites as discussed in Section 5.4). Furthermore, the oxidation of Cu by CO2 and the opposite effect of hydrogen spillover from Pd is not an explanation for the Pd promotion. Since at differential conversion the catalyst is only exposed to the pure synthesis gas, CO2/H2, the Pd promotion at integral conversion appears to be related to the presence of a product (i.e. methanol, CO or water) in the gas phase, which will be investigated later.

77 10 100 1000 10000 10 _ (log scale)

flow rate (mol h"' g^u')

200 400 600 800 1000 0.5

0.4 -

0.3

Id —X— Cu/Zn/Al (series prec 1)

I -•m— 0.09 Pd/Cu/Zn/Al (imp)

I"o 0.1

Figure 5.6 Approach to differential methanol production under CO^/Ho (Initial activities, 4 H2:C02, 4.5 MPa, 250°C, tubular reactor)

78 * 0.09 Pd/Cu/Zn/Al (imp) relative to Cu/Zn/Al (series prec 1) &

-1d PL,

+ 0 200 400 600 800 1000

flow rate (mol h"' ')

Figure 5.7 Pd promotion of methanol production from COn/Ho at very high flow rates (Initial activities, 4 H2:C02, 4.5 MPa, 250°C, tubular reactor)

5.10 APPROACH TO DIFFERENTIAL CO PRODUCTION UNDER

CO2/H2

Figure 5.8 shows the effect of increasing flow rate of CO2/H2 on the parallel production of CO by the reverse water-gas shift reaction. At all flow rates the CO production was similar over both catalysts, in agreement with the results in the Berty reactor. The reaction reached differential conversion at relatively low flow rates compared to methanol production. Above a flow rate of 150 mol h"' gcu \ the CO production rate was approximately constant (over both catalysts) at 0.10 mol h"' gcucorresponding to CO yields < 0.33%. The differential methanol production rate was 4.5 times greater than the differential CO production rate. In other words, the intrinsic rate of CO2 hydrogenation to methanol was much faster than the reverse water-gas shift reaction. The finding that CO production was greater than methanol production in some cases in the Berty reactor (even though the equilibrium CO yield is much less than that of methanol) emphasises that the CO2 hydrogenation to methanol is highly inhibited by the product gas phase (in particular

79 water which is well known to inhibit methanol synthesis from CO^/H^ (148,153,154)) under normal synthesis conditions (as discussed previously in Section 5.6).

0.15

'5 0.125 bO

0.1 # 1 X X X X

.2 0.075 I X Cu/Zn/Al (series prec 1) K . 0.09 Pd/Cu/Zn/Al (imp) o 0.05 -- o

0.025 + + 200 400 600 800 1000

flow rate (mol h'^ g^u')

Figure 5.8 Approach to differential CO production under COn/Hn (Initial activities, 4 H2:C02, 4.5 MPa, 250°C, tubular reactor)

5.11 METHANOL PRODUCTION FROM COg/Hz OVER ALL CATALYSTS AT DIFFERENTIAL CONVERSION

Figure 5.9 shows the methanol production rates over all Pd-promoted catalysts under CO2/H2 at flow rates above 650 mol h"' gcu^- For any one catalyst, the rate of methanol production at 698 mol h"' gcu ' was within ± 5% of that at 1046 mol h"^ gcuindicating that differential conversion applied in all cases. The differential methanol production rates were similar over most catalysts, being slightly lower for some Pd/Cu/Zn/Al catalysts, and in particular 0.12 Pd/Cu/Zn/Al (prec). The rates for all experiments varied betwen 0.42 and 0.47 mol h"' gcu ', a relative difference of approximately 12%.

80 il 1046 a 1046 ^ 804 g I ^ ro98 Cu/Zn/Al (series prec 1) 698 o 1 ^'^8 '^698 698 nO Q

Cu/Zn/Al (series prec 2) 1046

Cu/Zn/Al (parallel prec) 1046 '698

0.04 Pd/Cu/Zn/Al (imp) 1046 m 698

-I 698 0.09 Pd/Cu/Zn/Al (imp) Mm ""I,,

BB 698

0.03 Pd/Cu/Zn/Al (wet) 1046 698 0.10 Pd/Cu/Zn/Al (wet) ###w 0.05 Pd/Cu/Zn/Al (prec) 1046 : 0.12 Pd/Cu/Zn/Al (prec) 1016

0.21 Pd/Al + Cu/Zn/Al 10 \e 698

C)J 0.35 0.4 0.45 0.5

methanol production rate (mol h"' gcu"')

Figure 5.9 Effect of Pd on differential methanol production rate under COo/Ho (Initial activities, 4 H2:C02, 4.5 MPa, 250°C, tubular reactor)

The lack of promotion at differential conversion over all the Pd-promoted catalysts proves that the promotion at integral conversions was not due to new (Pd-Cu or Pd-Zn) or additional (Cu) catalytic sites in Pd/Cu/Zn/Al catalysts for methanol synthesis from CO2/H2. The slightly lower production rates of methanol over some Pd/Cu/Zn/Al catalysts (in particular 0.12 Pd/Cu/Zn/Al (prec)) at differential conversion supports the

81 previous supposition that these catalysts in fact contain slightly less Cu dispersion, probably as a result of the lower reduction temperature of CuO in the presence of Pd or the procedures used for Pd incorporation to the catalysts. This would explain, to an extent, why the observed promotions at integral conversions for some of the higher Pd loading catalysts (in particular 0.12 Pd/Cu/Zn/Al (prec)) were lower than expected (Section 5.4). Accounting for the differences in surface area, the promotions at integral conversion appear to be very similar over all the Pd/Cu/Zn/Al catalysts.

5.12 METHANOL PRODUCTION FROM COg/Hg + CO AT DIFFERENTIAL CONVERSION

The lack of promotion at differential conversion under CO2/H2 over all the Pd-promoted catalysts proves that the promotion which was observed at integral conversion is related to the presence of a product. As mentioned previously, a possible mechanism for the promotion is the secondary reaction of CO to methanol over Pd, since supported Pd catalysts are well knovm for methanol synthesis from CO/H2 (19,33,70). Therefore, experiments were carried out at differential conversion under CO2/H2 with a small fraction of the CO2 replaced by CO. A maximum 10% C0/(C0+C02), which represents the equilibrium CO partial pressure that was obtained at the low flow rates of CO2/H2 in the Berty reactor as a result of the reverse water-gas shift reaction, was used in the synthesis gas.

The results of the experiments are shown in Table 5.5. The initial activities of Cu/Zn/Al and Pd/Cu/Zn/Al decreased as the CO fraction was increased, and there was no observed promotion of Pd/Cu/Zn/Al relative to Cu/Zn/Al. This is in agreement with the previous results (Section 5.4), where it was found that the promotion in methanol production was not related to the increasing CO partial pressure in the Berty reactor at higher conversions. As such, Pd does not function as a catalyst for the reaction of CO to methanol in the CO2- rich gas phase, and therefore the secondary reaction of CO to methanol does not account for the promotion of methanol production under CO2/H2 at integral conversions.

82 Table 5.5 Effect of Pd on differential methanol production rate under CO^/Ho + CO (Initial activities, 698 mol h"^ gcu"'» 4 H2:(C02+C0), 4.5 MPa, 250°C, tubular reactor)

SYNTHESIS GAS Methanol production rate (mol h"^ gcu ')

Cu/Zn/Al' Pd/Cu/Zn/Al 2

CO2/H2 0.45 0.45

CO/CO2/H2; 0.05 C0/(C02+C0) 0.43 0.42

CO/CO2/H2; 0.10 C0/(C02+C0) 0.40 0.41

' Cu/Zn/Al (series prec 1) ^ 0.09 Pd/Cu/Zn/Al (imp)

5.13 METHANOL PRODUCTION FROM COg/Hz + HgO AT DIFFERENTIAL CONVERSION

Figure 5.10 and the data in Table 5.4 show that water addition severely inhibits the production of methanol from CO2/H2 over Cu/Zn/Al (in agreement with literature (148,153,154)) and Pd-impregnated Cu/Zn/Al. The inhibition is slowly reversed when the water is switched off (Figure 5.10a), in agreement with literature (154). Figure 5.10a (Table 5.6a) shows an experiment at a high flow rate of CO2/H2 where the differential methanol production rate (0.45 mol h"^ gcu^) was measured over both catalysts during the first hour. The addition of water, which amounted to 2.6% (mol/mol) of the syngas, resulted in an order of magnitude less activity over Cu/Zn/Al. Both catalysts exhibited a slow transient loss of activity during exposure to water, but on average (from 70-120 minutes) there was 33% greater methanol production over the Pd-promoted catalyst.

Figure 5.10b (Table 5.6b) shows an experiment conducted at a lower flow rate of CO2/H2, such that differential kinetics did not apply in the first hour under CO2/H2, and some promotion of methanol production was apparent with Pd addition. Subsequently, water was added to give the same concentration as before, 2.6%. The methanol production rate over either catalyst in the presence of water was approximately the same as the result at

83 the higher flow rate and on average (between 70 and 120 min) 39% more methanol was produced over the Pd/Cu/Zn/Al catalyst. The similar results at the two flow rates (with the same water concentration) shows that differential kinetics applied to the methanol production from CO2/H2/H2O (which is not surprising considering the methanol yields in the presence of water were < 0.2% at the lower flow rate and < 0.05% at the higher flow rate), although differential conversion did not apply at the lower flow rate before water addition. Overall, the results show that water severely inhibits CO2 hydrogenation to methanol, and Pd promotion under CO2/H2 is only prevalent in the presence of water. Hydrogen spillover from Pd may counteract the inhibition of CO2 hydrogenation to methanol caused by water.

Table 5.6 Effect of Pd on differential methanol production rate under COo/Ht + water (4 H2:C02, (a) 698 mol h"^ go/\ (b) 154 mol h"' g^u \ 4.5 MPa, 250°C, tubular reactor)

Cu/Zn/Al ^ Pd/Cu/Zn/Al ^ promotion methanol production rate (mol h'^ gcu"^) (%)

(a) high flow rate

CO2/H2 O.45A 0.46* 1 CO2/H2 + HjO 0.043® 0.057® 33

(a) lower flow rate

CO2/H2 0.26^ 0.28* 11 CO2/H2 + H2O 0.039% 0.054® 39

(a) CO2/H2 (698 mol h"^ gcu"^) + H2O (18.5 mol h"^ gcu"' or 2.6 mol%) (b) CO2/H2 (154 mol h"' gcu ') + H2O (4.1 mol h"' gcu"' or 2.6 mol%) ^ Cu/Zn/Al (series prec 1) ^ 0.09 Pd/Cu/Zn/Al (imp) ^ average from 0-60 min ® average from 70-120 min

The results of this Section, which show that Pd promotion under CO2/H2 depends upon the presence of water, appear to be in contradiction with the previous results at integral

84 + water (18.5 mol J 2.6 mol%)

1 I § 0.1 " I a I

time (min)

30 60 90 120 150 180

0.7 -•m— 0.09 Pd/Cu/Zn/Al (imp) —X— Cu/Zn/AJ (series prec 1) 5 0.6 bO

-X 0.5 + water (4.1 mol h'^ ' I or 2.6 mol%) 1 0.4 0.1 I BO O I 0.3 -X >4 0.2

I 0.1 ^—X—=—X )( 0 0.01

Figure 5.10 Effect of Pd on differential methanol production rate under COi/Ht + water (4 HjiCOj, (a) 698 mol h'^ gcu'% (b) 154 mol h"' gcu"', 4.5 MPa, 250°C, tubular reactor)

85 conversions in the Berty reactor (Section 5.4), where it was found that the promotion in methanol production was not related to the increasing water partial pressure at higher conversions. However, the loss of promotion at higher conversions in the Berty reactor may be due partly to the approach to the equilibrium methanol yield. In addition, it is possible that the promotion (of any particular catalyst containing Pd relative to Cu/Zn/Al) is a maximum with an intermediate concentration of water, since higher water concentrations may restrict hydrogen spillover (e.g. by inhibiting Pd active sites where hydrogen is adsorbed). This would explain why the promotion by Pd increased as the conversion increased from differential conversion in the tubular reactor (Figure 5.7), but decreased at higher conversions in the Berty reactor (Figure 5.3).

5.14 COMPARISON OF METHANOL PRODUCTION FROM COg/Hg/HgO (DIFFERENTIAL CONVERSION) AND COg/Hg (INTEGRAL CONVERSION)

Methanol production at differential conversion with added water can be compared with methanol production at integral conversion where the water concentration as a result of water production was similar (remember that the experiments at integral conversion were carried out in an internal recycle reactor so the catalyst was uniformly exposed to the product gas phase concentration). The results in Table 5.7 show that the given integral methanol production rate over either catalyst was similar to the methanol production rate from CO2/H2/H2O at differential conversion. Therefore the given integral kinetics were determined only by the CO;, and H^O in the gas phase of the Berty reactor, i.e. the integral kinetics can be described by the production of methanol by CO2 hydrogenation, and the inhibition of this reaction by water. Furthermore, the Pd promotion was similar under the two reaction regimes, revealing that Pd promotion at the given integral conversions can be explained solely as a counteracting effect to the inhibition of CO2 hydrogenation by water. As proposed in the previous Section, hydrogen spillover from Pd may be involved in reactions on the Cu surface which prevent the inhibition of Cu by water.

86 Table 5.7 Comparison of methanol production rate from COn/Ho integral conversion^ and COn/Hn/HoO (differential conversion)

Cu/Zn/Al' Pd/Cu/Zn/Al ^

(a) CO2/H2/H2O (differential conversions) water concentration (mol%) 2.6 2.6 methanol production rate (mol h"' gcu^) 0.043 0.057

(b) CO2/H2 (integral conversions) water concentration (mol%) 2.8 3.1 methanol production rate (mol h"' gcu'^) 0.045 0.057

(a) Average activities after water addition (70-120 min), 4 H2:C02 (698 mol h"^ gcu^)> H2O (18.5mol h"' gcu^), 250°C, 4.5 MPa, tubular reactor (data obtained from Table 5.6a) (b) Initial activities, 4 H2:C02 (3.47 mol h'^ gcu^)= H2O production rate (0.096^ or 0.109^ mol h'^ gcu'^), 250°C, 5 MPa, Berty reactor (data obtained from Figure 5.2a) ^ Cu/Zn/Al (series prec 1) ^ 0.09 Pd/Cu/Zn/Al (imp)

5.15 CONCLUDING REMARKS

The addition of Pd to the Cu/Zn/Al catalyst in the form of physical mixtures or, more effectively, by impregnation/coprecipitation promoted methanol synthesis from CO2/H2 by hydrogen spillover. The promotion was observed in an integral, not differential, kinetic regime, where methanol synthesis on Cu by CO2 hydrogenation was severely inhibited by the product water, and hydrogen spillover from Pd counteracted this inhibition to a small extent. It is proposed that the water oxidises or, perhaps through its dissociatively adsorbed species, blocks the active Cu sites for CO2 hydrogenation, whereas hydrogen spillover tends to reduce the Cu or reacts with the adsorbed species of water.

It should be noted that (allowing for possible differences in Cu surface area) the promotions were very similar over all the Pd/Cu/Zn/Ai catalysts in this study, but this

87 only accounted for a small extent of the inhibition caused by water. The similar Pd promotions may have followed from similar Pd surface areas on all the Pd/Cu/Zn/Al catalysts. Alternatively, the accommodation of spillover hydrogen on the catalyst surface may have reached a saturation coverage, where the reducing potential (or the tendency to remove water by reaction) of the hydrogen remained much less than the oxidising potential (or tendency to adsorb at active sites) of water.

88 Chapter 6

METHANOL SYNTHESIS FROM CO/Hg

6.1 SUMMARY

• Methanol was produced from CO/H2 with > 98% selectivity over Cu/Zn/Al and > 97% selectivity over catalysts containing Pd. The trace products, methane, CO2 and water, were produced independently over Pd as well as Cu. • The methanol yields over Pd/Cu/Zn/Al catalysts at moderate/high conversions in the Berty reactor were up to 12% lower than that of Cu/Zn/Al, depending upon the Pd loading and the intimacy of Pd addition. The lower activity was not due to disproportionate deactivation of the Pd/Cu/Zn/Al catalysts under CO/H2. • In the tubular reactor the methanol yields at the same conditions were lower than those in the Berty reactor. Furthermore, with decreasing conversions in the tubular reactor, lower methanol production rates were obtained, suggesting (according to kinetic theory) that methanol production is promoted by the products. The relative inhibition by Pd addition was greater in the tubular reactor than in the Berty reactor. • Differential conversion under CO/H2 was attained with methanol yields < 0.3%. The rate of CO hydrogenation to methanol was up to 40% lower in the case of catalysts containing Pd, depending upon the Pd loading and the intimacy of Pd addition.

It is proposed that the trace by-product water promotes CO hydrogenation to methanol by oxidising the Cu surface or by providing adsorbed species where the secondary adsorption of CO takes place. Assuming the lower activities of catalysts containing Pd was not due to differences in Cu dispersion (based on evidence in the previous Chapter), it is proposed that hydrogen spillover from Pd inhibits the Cu catalyst by excessive reduction of Cu or by reaction with the adsorbed species of water. The observed inhibition by Pd was greatest at differential conversion, because of the opposing (promoting) effect at integral conversions, particularly in the Berty reactor, of the extra trace water produced over Pd.

89 6.2 TRACE PRODUCTS FROM CO/Hg (BERTY REACTOR)

With all catalysts under CO/H2 in the Berty reactor, methanol was the only major product, while there were three trace products, CO2, water and methane (no trace products were detected at differential conversion in the tubular reactor'). The amount of trace products was always below the linear range of the thermal conductivity detector for the purpose of accurate quantification. So as not to under-estimate the amounts of the trace products, their peak integrations were doubled, and the yields were calculated as for methanol (Section 4.5).

Table 6.1 shows the (carbon) yields and selectivities of CO2 and methane at two different flow rates of the CO/H2 syngas. The production of water is expressed simply as a molar ratio with CO2. Although there was some scatter in the results due to the measurement of such small quantities of products, the yields of trace products were significantly greater with Pd-promoted catalysts compared to Cu/Zn/Al and were related to the amount of Pd in the catalyst. Furthermore, the sum of the trace product yields over Pd/Al and Cu/Zn/Al were similar to the yield of trace products over the physical mixture, which shows that Pd and Cu are both catalysts for reactions leading to methane, CO2 and water production. This result is different from the result under CO2/H2 (Section 5.2), where it was found that Pd was not an independent site for any catalytic reaction.

It appears from Table 6.1 that over both Pd and Cu the amounts of trace products were in the order methane > CO2 > water, although this result is not certain since the relative response of the detector to various products in the non-linear range was not known. Table 6.2 summarises the approximate results of this Section. The (carbon) selectivity of trace products detected under CO/H2 was < 2% for Cu/Zn/Al and < 3% for Pd-promoted Cu/Zn/Al catalysts, corresponding to methanol selectivities of > 98% and > 97% respectively.

' At differential conversion in the tubular reactor no trace compounds - above the level of minute impurities in the feed - were detected. However, the apparent absence of trace products was probably due to the lower limit of sensitivity of the detector, given the extremely low conversions necessary for differential kinetics.

90 Table 6.1 Yield & selectivity of trace products from CO/Ht at various flow rates (Initial activities, 4 H2;C0, 5 MPa, 250°C, Berty reactor)

CATALYST CI^yKdd(94)* CO2 yield (%) * H2O/CO2

Syngas flow rate 3.47 mol h"' g^u"'

Cu/Zn/Al (series prec 1) 0.053 (1.09) 0.025 (0.51) 0.68

0.04 Pd/Cu/Zn/Al (imp) 0.062(1.34) 0.035 (0.76) 0.66

0.09 Pd/Cu/Zn/Al (imp) 0.066 (1.53) 0.042 (0.98) 0.61

0.21 Pd/Al + Cu/Zn/Al 0.077(1.56) 0.056 (1.13) 0.75

Pd/Al (3:7 as oxides)^ 0.035 (12.3) 0.030 (10.5) 0.41

Syngas flow rate 0.43 mol h'^ g^u'^

Cu/Zn/Al (series prec 1) 0.18(0.78) 0.12(0.53) 0.82

0.09 Pd/Cu/Zn/Al (imp) 0.18 (0.80) 0.15 (0.67) 0.82

0.12 Pd/Cu/Zn/Al (prec) 0.21 (0.96) 0.15(0.69) 1.04

0.21 Pd/Al + Cu/Zn/Al 0.27(1.13) 0.21 (0.87) 1.06

Pd/Al (3:7 as oxides)^ 0.11 (13.1) 0.086 (10.7) 0.79

* Tabulated values in brackets are the selectivities of CH4 or CO2. The syngas flow rates for Pd/Al were the same as those for the physical mixtures (above in the table) relative to the mass of Pd. The flow rates were 9.64 mol h"^ gp/^ or 1.19 mol h"^ gp^'^ respectively.

Table 6.2 General selectivity of products under CO/Hn

Carbon products CO2 CH4 CH3OH

Selectivity over Cu/Zn/Al catalysts < 0.75% < 1.25% >98 94

Selectivity over Pd-promoted catalysts <1.25% < 1.75% >97%

91 6.3 METHANOL SYNTHESIS FROM CO/Hg OVER Cu/Zn/Al, Pd/Al AND PHYSICAL MIXTURES (BERTY REACTOR)

Methanol production was measured at two flow rates of CO/H2, which were different by about an order of magnitude, as shown in Figure 6.1. The methanol yield over the Cu/Zn/Al (series prec 1) catalyst was very similar to that over the 0.21 Pd/Al + Cu/Zn/Al physical mixture. This result applied for 0 and 10 h time on stream.

[3(10 hours) I (initial)

'' Cu/Zn/Al (series prec 1) r 0.21 Pd/Al + Cu/Zn/Al miMiimii 0.04 Pd/Cu/Zn/Al (imp)

0.09 Pd/Cu/Zn/Al (imp)

2.5' 3.5 4.5

Cu/Zn/Al (series prec 1) "p—'

0.21 Pd/Al + Cu/Zn/Al

0.09 Pd/Cu/Zn/Al (imp)

0.12 Pd/Cu/Zn/Al (prec)

10 12.5 15 17.5 20 225 25

methanol yield (%)

Figure 6.1 Effect of Pd on methanol yield from CO/Hn at various flow rates (4 H2:C0, (a) 3.47 mol h"' gcu (b) 0.43 mol h"^ gcu'5 5 MP a, 250°C, Berty reactor)

92 The Pd/Al component was also used alone as a catalyst under synthesis conditions with CO/H2. The experiments were carried out at the same flow rates relative to the mass of Pd which were used with the physical mixture, as shown in Table 6.3. Pd/Al was active for methanol production from CO/H2, in agreement with many other studies (19,33,70), but compared to the physical mixture, there was much less methanol produced. Nevertheless this result suggests that a small amount of the methanol produced over the physical mixture was produced independently over the Pd/Al component.

Table 6.3 Methanol synthesis from CO/Ho over Pd/Al + Cu/Zn/Al and Pd/Al alone (4 H^iCO, 5 MPa, 250°C, Berty reactor)

CATALYST FLOW RATE METHANOL YIELD (%)

(mol h'^ gcu'*) (mol h"^ gpd"^) (initial) (10 hours)

0.21 Pd/Al + Cu/Zn/Al 0.43 1.19 23 17.3%

Pd/Al (3:7 as oxides) - 1.19 0.61% 0.23%

0.21 Pd/Al + Cu/Zn/Al 3.47 9.64 3.73%

Pd/Al (3:7 as oxides) - 9.64 0.22% 0.08%

6.4 METHANOL SYNTHESIS FROM CO/Hg OVER Pd/Cu/Zn/AI CATALYSTS (BERTY REACTOR)

With repeated experiments for some of the catalysts in Figure 6.1, it was established with a high degree of confidence that incorporation of Pd into Cu/Zn/Al by impregnation or precipitation results in less activity for methanol synthesis. At the higher flow rate (Figure 6.1a), on average 0.09 Pd/Cu/Zn/Al (imp) resulted in 10% less methanol yield initially (or 12% less after 10 hours) than Cu/Zn/Al (series prec 1). At the lower flow rate (Figure 6.1b), the 0.12 Pd/Cu/Zn/Al (prec) catalyst was found to be even less active than 0.09 Pd/Cu/Zn/Al (imp). The effect of Pd addition observed here is in agreement with

93 another study where 0.5% Pd impregnation of Cu/ZnO resulted in approximately 20% less methanol synthesis activity at industrial conditions, although the syngas contained 2% CO2 in this case (147).

The lesser activity of Pd/Cu/Zn/Al catalysts compared to Cu/Zn/Al was not the result of the lower Cu contents of the Pd/Cu/Zn/Al catalysts, since the flow rate of CO/H2 was always scaled according to the mass of Cu. There are three other possible reasons for the lesser activity of Pd/Cu/Zn/Al catalysts. Firstly, the dispersion of Cu may have been less in Pd/Cu/Zn/Al catalysts (e.g. due to the preparation methods necessary for Pd addition, as discussed in Section 3.7). However, the last Chapter provided evidence (namely the similar reverse water-gas shift activity and the similar methanol productivity at differential conversion under CO2/H2) that the Cu dispersions were similar in all catalysts. Secondly, the Pd/Cu/Zn/Al catalysts may have deactivated very quickly under CO/H2, before "initial activity" measurements could be taken. However, the initial rates of deactivation of all catalysts appeared to be the same and, in any case, were not substantial compared to the initial differences in catalytic activity (Section 4.10). The possibility of the dis-proportionate deactivation of Pd/Cu/Zn/Al catalysts under CO/H2 is considered further in the next Section. Thirdly, the Cu catalyst may have been inhibited by a chemical effect of Pd, such as hydrogen "spillover (to be discussed later).

6.5 METHANOL SYNTHESIS FROM COg/Hz AFTER SYNTHESIS FROM CO/H2 (BERTY REACTOR)

Figure 6.2 shows the results of a switching experiment, where the CO2/H2 synthesis gas was introduced after synthesis firom CO/H2 for approximately 10 hours. At the end of the periods under CO/H2 and CO2/H2, Pd impregnation resulted in 11% inhibition compared to 20% promotion respectively. These effects of Pd are very similar to those reported previously in this Chapter and the last. Furthermore, the absolute activity of either catalyst at the end of the experiment under CO2/H2 in Figure 6.2 was as expected from the activity measurements in the last Chapter (Figure 5.1), allowing for the observed deactivation in 10 hours under CO/H2.

94 These results show that there was no particular irreversible deactivation of Pd/Cu/Zn/Al compared Cu/Zn/Al under CO/H2 (before initial activity measurements were taken) which could account for the lower activity of Pd/Cu/Zn/Al catalysts compared to Cu/Zn/Al under CO/H2. Therefore, the lower activity of Pd/Cu/Zn/Al catalysts appears to be a chemical inhibition rather than a deactivation. As mentioned previously, a chemical inhibition of Cu by Pd may be a result of hydrogen spillover.

15.43 10

CO/H. CO2/H2

8 -- e ^ 0.09 Pd/Cu/Zn/Al (imp) t 6 --

Cu/^7M' (s^ies pi^c 1) 1 4 -

10 15 20 time on stream (h)

Figure 6.2 Rffect of Pd on methanol yield from CO/Ht followed by CON/HO (4 H2:C0x, 3.47 mol h'^ gcu ^ 5 MPa, 250°C, Berty reactor)

6.6 COMPARISON OF METHANOL SYNTHESIS FROM CO/Hg IN THE BERTY AND TUBULAR REACTORS

The methanol yield from CO/H2 in the tubular reactor is compared to that in the Berty reactor at the same conditions with the same catalysts in Table 6.4. Less conversion of reactions is normally achieved in recycle reactors compared to tubular reactors, since the

95 former reactor is operated at (or closer to) the product gas phase concentrations. Surprisingly, in this study the methanol yield over either catalyst was found to be greater in the internal recycle reactor. This result is consistent with the methanol synthesis reaction being promoted by one of the products. Other than methanol, the only other products were trace amounts of methane, CO2 and water. A small fraction of CO2 in CO/H2 is well known to promote methanol production from CO/H2 over Cu/ZnO catalysts, although a survey of the literature (Section 2.2.3) favours the primary role of water in the promotion, CO2 being a source of water via the reverse water-gas shift reaction (since a dramatic promotion of methanol synthesis by CO2 addition to CO/H2 has been observed at integral conversions, where there would be a finite pressure of water in the product, but not at very low conversions). Indeed many workers have observed the promotion of methanol synthesis under CO/H2 using Cu/ZnO catalysts with particularly small additions of water (146,160,204). Furthermore, very little (204) or no (18) methanol production has been observed under highly purified CO/H2, and in this case promotion was observed with ppm levels of water (204). Therefore, it is quite possible that the greater methanol production in the Berty reactor was a result of promotion in situ by the trace product water.

Table 6.4 Comparison of methanol yields from CO/Hn in the Berty and tubular reactors (Initial activities, 4 H2:C0, 3.47 mol h ' g^u"', 5 MPa*, 250°C)

Reactor Methanol yield (%) Pd inhibition (%) Cu/Zn/Al (series prec 1) 0.09 Pd/Cu/Zn/AI (imp)

BERTY 4J3 424 10

TUBULAR 422 3.22 24

* The pressure was only 4.5 MPa in the tubular reactor due to the absence of 10% He.

Trace products, including water, were produced over Cu and Pd (Section 6.2). Therefore, if methanol production over Cu is promoted by the trace product water, Pd is involved

96 indirectly in the promotion. Coupled with the proposal in the previous Section, that Pd inhibits methanol production over Cu possibly by hydrogen spillover, Pd may have two opposing roles, although the inhibiting role appears to be the stronger. The opposing roles of Pd are consistent with the greater inhibition of methanol production by Pd in the tubular reactor compared to the Berty reactor, which is shown in Table 6.4. The first role, the promotion of methanol production over Cu by the product water over Pd, would be more effective in the Berty reactor (where the catalyst is uniformly exposed to the product gas phase), whereas the second role, the inhibition of methanol production by hydrogen spillover may be equally effective in both reactors. Hence, the overall inhibition by Pd would be greater in the tubular reactor.

6.7 APPROACH TO DIFFERENTIAL METHANOL PRODUCTION UNDER CO/H2

Figure 6.3 shows the effect of increasing flow rate of CO/H2 on the methanol yield and production rate over Cu/Zn/Al (series prec 1) and 0.09 Pd/Cu/Zn/Al (imp) catalysts. Each data point represents the initial activity from a unique experiment using a fresh catalyst charge each time. The production rate trend over either catalyst was quite unusual in that the rate of methanol production decreased at higher flow rates (the theory of chemical kinetics in flow reactors predicts that the maximum rate of production should occur at high flow rates, i.e. low conversions - see Appendix 1). The profile in Figure 6.3 is consistent with methanol synthesis being promoted by one of the products. This is in agreement with the results of the previous Section, where it was proposed that the trace product water produced over Pd and Cu promotes methanol production from CO/H2 over Cu.

The lesser activity of Pd/Cu/Zn/Al compared to Cu/Zn/Al was observed throughout, as seen in Figure 6.3. In relative terms, the inhibition by Pd appeared to be greater at higher flow rates, i.e. lower conversions (the inhibition was 24% at the lowest flow rate and 30% at the highest flow rate in Figure 6.3). This resuh is again consistent with the proposed

97 Cu/Zn/Al (series prec 1)

— 0.09 Pd/Cu/Zn/Al (imp) I I

20 40 60 80

0.04 & 0.03 I 0 0.02 1 a I 0.01

20 40 60 80

flow rate (mol h' gcu')

Figure 6.3 Approach to differential methanol production under CO/Ho (Initial activities, 4 HziCO, 4.5 MPa, 250°C, tubular reactor)

98 opposing roles of Pd, i.e. the promotion of methanol synthesis by the trace product water - which would be more effective at higher conversions - with an underlying inhibiting tendency (the proposed hydrogen spillover to Cu).

Figure 6.3 shows asymptotes of methanol production rate, 0.021 (Cu/Zn/Al) and 0.015 (Pd/Cu/Zn/Al) mol h'^ g^u^ (at flow rates >35 and > 25 mol h'^ g^u"' respectively), corresponding to methanol yields < 0.3% over both catalysts. The reaction rate asymptotes signify the differential kinetic regime and therefore the asymptotes are the intrinsic forward rates of CO hydrogenation to methanol for the given conditions and catalysts. Note that there were minute amounts of impurities in the feed (CO^ and water were just detectable), but the amounts were at least two orders of magnitude less than the methanol production, i.e. CO and Hj were the only possible direct reactants for the methanol production. Furthermore, the elevated methanol production rates over either catalyst at lower flow rates, which has been ascribed to promotion by trace water production, appears to be the promotion of CO hydrogenation and not CO2 hydrogenation, since the trace amount of CO2 production could not account for the extra methanol production.

6.8 METHANOL PRODUCTION FROM CO/Hg OVER VARIOUS CATALYSTS AT DIFFERENTIAL CONVERSION

Figure 6.4 shows the methanol production rates at differential conversion over various catalysts under CO/H2. As can be seen, the methanol production rates were independent of the flow rate, since the flow rates were above that (35 mol h"' g^u"' from the previous Section) required for differential kinetics. Repeated experiments over Cu/Zn/Al (series prec 1) gave results similar to the Cu/Zn/Al (parallel prec) catalyst, whereas the methanol production rate was lower for all Pd-promoted catalysts, including the 0.21 Pd/Al + Cu/Zn/Al physical mixture. The high Pd loading Pd/Cu/Zn/Al catalysts were the least active, and in particular the 0.12 Pd/Cu/Zn/Al (prec) catalyst was 40% less active than Cu/Zn/Al.

99 The particularly acute inhibition by Pd in all Pd-promoted catalysts at differential conversion compared to the inhibition at integral conversion suggests again that there is a chemical rather than physical consequence of Pd which inhibits CO hydrogenation to methanol in general. It is proposed that hydrogen spillover from Pd to Cu is involved in this inhibition. The greater inhibition with the coprecipitated compared to the impregnated Pd/Cu/Zn/Al catalyst (and with Pd/Cu/Zn/Al catalysts in general compared to the physical mixture) could be due to more effective hydrogen spillover as a result of the proximity of Pd to Cu. As proposed in the previous Section, the inhibiting tendency of Pd (hydrogen spillover) may be opposed by the promotion of Cu by the trace product water (and in particular the greater amounts of trace product water produced over Pd- promoted catalysts). However, since methanol production at differential conversion is not effected by trace (or any) products, this would explain why the maximum inhibition by Pd was realised at differential conversion rather than integral conversion.

TTTT 76.1 Cu/Zn/Al (series prec 1) ^S.l 38.1

Cu/Zn/Al (parallel prec) 76.1 0 0.04 Pd/Cu/Zn/Al (imp) 76.1 1

0.09 Pd/Cu/Zn/Al (imp)

OQ n 0.10 Pd/Cu/Zn/Al (wet) 3&1

0.12 Pd/Cu/Zn/Al (prec) 38.1

0.21 Pd/Al + Cu/Zn/Al 38.1

0.01 0.015 0.02 0.025

methanol production rate (mol h"^ gcu

Figure 6.4 Effect of Pd on differential methanol production rate under CO/H^ (Initial activities, 4 H2:C0, 4.5 MP a, 250°C, tubular reactor)

100 Finally, it is important to discuss the effect of the minute impurities of water in the CO/H2 feed. It has been mentioned (Section 6.7) that there were minute impurities of CO2 and water in the feed and, although the amounts were far too small to account for the methanol production at differential conversion, it is probable that these impurities (primarily water) were involved in the catalysis of CO hydrogenation (e.g. the adsorption of water on Cu may provide sites for the secondary adsorption of CO). This proposal is based on the finding that the rate of methanol production at differential conversion over a given catalyst, although highly reproducible with a particular CO/H2 gas supply cylinder, was rather more variable firom one gas cylinder to the next (all the data in this Section was obtained with two gas cylinders which gave similar results for a given catalyst; results obtained with the use of other gas cylinders were in fact discarded). Others have observed the dependence of methanol production under CO/H2 over Cu/ZnO catalysts on particularly small amounts of water (18,146,160,204), down to even ppm levels (204).

6.9 CONCLUDING REMARKS

Methanol synthesis from CO/H2 over Cu/Zn/Al was inhibited by the addition of Pd, in particular by impregnation/coprecipitation of Pd. The inhibition was particularly severe at differential conversion, which showed that Pd inhibits CO hydrogenation in general. At integral conversion, the inhibition by Pd was partly obscured by the promotion of CO hydrogenation over Cu by the trace water produced over Pd and Cu. It is proposed that the trace product water promotes CO hydrogenation by oxidising the active Cu sites or by providing, through its dissociatively adsorbed species, sites where the secondary adsorption of CO takes place. It is proposed that hydrogen spillover from Pd tends to reduce the Cu or reacts with the adsorbed species of water.

It is important to note that the rates of methanol production at differential conversion in this study were probably affected by the minute water impurity in the feed, since strictly pure CO/H2 has been found to deliver little (204) or no (18) methanol production and water addition down to ppm levels has been found to promote methanol synthesis from CO/H2 (146,160,204). This means that the proposed mechanisms for Pd inhibition.

101 involving the removal of dissociatively adsorbed water species on Cu by hydrogen spillover or the reduction of Cu by hydrogen spillover as opposed to the oxidation by water, is applicable under differential conversion as well as integral conversion.

102 Chapter 7

METHANOL SYNTHESIS FROM CO/COg/Hz

7.1 SUMMARY

• Methanol was produced from CO/CO2/H2 synthesis gases (along with CO at high CO2 fractions) with > 99.5% selectivity over Cu/Zn/Al and Pd-impregnated Cu/Zn/Al. Trace methane production was not affected by the presence of Pd. Water production increased in relation to the CO2 fraction. • In the Berty reactor, a sharp maximum in methanol productivity over Cu/Zn/Al was observed with small CO2 fractions (5-20% of total carbon oxides). Methanol production was inhibited by Pd impregnation by up to 30% at the smallest CO2 fraction, but the inhibition quickly declined to zero with increasing CO2 fractions. At higher CO2 fractions (> 50% of total carbon oxides), Pd promotion of methanol synthesis was observed and the promotion increased to up to 25% under CO2/H2. • At differential conversion in the tubular reactor there was a steep and almost linear increase in methanol production with respect to the CO2 fraction, suggesting that CO2 hydrogenation became the predominant methanol synthesis reaction at quite low CO2 fractions. An effect of Pd was only seen under CO/H2 and at low CO2 fractions: Pd inhibited methanol production by up to 30% under CO/H2, but the inhibition quickly tended to zero with increasing CO2 fraction. • A comparison of the methanol production rates in the integral and differential kinetic regimes showed that the sharp maximum in methanol production at small CO2 fractions and integral conversion was a result of promotion by a product rather than by CO2. This is in line with the proposal of Chapter 6, that small amounts of water production promote CO hydrogenation in situ. The particularly acute inhibition by Pd at the same time suggests again that Pd inhibits the CO hydrogenation to methanol. • There was an increasing departure between the methanol production rates at integral

103 and differential conversion at higher CO^ fractions, which showed that methanol production was inhibited by a product. This is in line with the proposal of Chapter 5, that water production inhibits CO2 hydrogenation in situ. The increasing promotion by Pd at the higher CO; fractions suggests again that hydrogen spillover from Pd counteracts the inhibition of CO2 hydrogenation on Cu by water.

7.2 TRACE PRODUCTS FROM CO/COg/Hz (BERTY REACTOR)

In all experiments under CO/CO2/H2 in the Berty reactor the major products were methanol and water (CO was also a net product at C02/(C0+C02) fractions above 0.7), while the only other product detected was trace amount of methane (no trace methane was detected at differential conversion in the tubular reactor, as explained in Section 5.2 or 6.2). The amount of methane was always below the linear range of the thermal conductivity detector for the purpose of accurate quantification. As in the preceding Chapters, the yield of methane was estimated using double its peak integration.

It was found that, as in the case of CO2/H2, there was no clear relationship between the methane production under CO/CO2/H2 and the presence of Pd in the catalyst, and the methane selectivity was always < 0.5%. The corresponding methanol (and CO) selectivity was always > 99.5% (Table 7.1). These results applied for Cu/Zn/Al (series prec 1) and 0.09 Pd/Cu/Zn/Al (imp) catalysts, which were the only catalysts studied in this Chapter.

Table 7.1 General selecti vity of products under CO/COn/Ho

Carbon products CH3OH CO CH4

Selectivity with 0 < C02/(C0+C02) < 0.7 * > 99.5% - <0.5%

Selectivity with 0.7 < C02/(C0+C02) < 1 * > 99.5% <0.5%

* over Cu/Zn/Al (series prec 1) and 0.09 Pd/Cu/Zn/Al (imp) catalysts

104 7.3 METHANOL SYNTHESIS FROM CO/COg/Hg WITH VARIOUS CO2 FRACTIONS (BERTY REACTOR)

Methanol production in the Berty reactor was measured over Cu/Zn/Al (series prec 1) and 0.09 Pd/Cu/Zn/Al (imp) catalysts across the full range of CO; fractions using the same flow rate and total pressure of carbon oxides in all experiments. The initial activity using a fresh charge of catalyst was obtained from each experiment. The methanol yields and production rates are plotted against the CO2 fractions in the synthesis gas (a) and the product (b) in Figure 7.1. Figure 7.1b is more applicable, since the Berty reactor was operated under the product gas phase, but the broad trends are the same in Figure 7.1a. A small CO2 fraction was found to be most favourable for methanol productivity over Cu/Zn/Al (in agreement with many other kinetic studies in the literature (33,60,96,98,116, 118)) and Pd/Cu/Zn/Al. However, the maximum attainable yield was less in the case of Pd/Cu/Zn/Al, 17% compared to 21%. Also, the maximum occurred at a higher CO2 fraction in the case Pd/Cu/Zn/Al, ~ 0.2 C02/(C0+C02) (~ 4% CO2), compared to ~ 0.1 (2%) in the case of Cu/Zn^Al. In general, Cu/Zn/Al was more active in all experiments with CO-rich feeds, whereas Pd/Cu/Zn/Al was more active with C02-rich feeds.

The same results can be seen in Figure 7.2 where the calculated promotion is plotted as a function of the CO2 fraction. Note that with 0.1 C02(C0+C02) (2% CO2 in the syngas) the Pd inhibition of approximately 20% is in agreement with a previous study (147), where approximately 20% inhibition was caused by 0.5% Pd impregnation of Cu/ZnO under industrial conditions with 2% CO2 in the syngas. The crossover from inhibition to promotion by Pd in Figure 7.2 is seen at approximately 0.4 C02/(C0+C02). Figure 7.2 illustrates further that, whilst the promotion by Pd under C02-rich feeds was related to the CO2 fraction, the inhibition by Pd under CO-rich feeds was not simply related to the CO2 fraction. The inhibition was greater with small CO2 fractions, particularly with 0.05 C02/(C0+C02), than with CO/H2. This trend in the inhibition by Pd (Figure 7.2), as well as the trend in the absolute production rates with respect to the CO2 fraction (Figure 7.1), cannot be explained without reference to the product water, which is considered next.

105 X— Cu/Zn/Al (series prec 1) CM

0.09 Pd/Cu/Zn/Al (imp) "o I i0 1 I& error (^oasea on maxmium absolute error measured; see Table 4.6) I

0.2 0.4 0.6 0.8 inlet C02/(C0+C02)

150 u —M— Cu/Zn/Al (series prec 1) bO Isi — — 0.09 Pd/Cu/Zn/Al (imp) 125 'o

100 f - 75 I of error (based on maximum absolute 50 error measured; see Table 4.6) 25

+ + 0.2 0.4 0.6 0.8 outlet 002/(00+002)

Figure 7.1 Effect of Pd on methanol yield and production rate from OO/OO^/Ho syngases (Initial activities, 4 H2:(00+002), 3.47 mol h"' gcu"\ 5 MPa, 250°0, Berty reactor)

106 0.8 1 inlet COz/fCO+COz)

* 0.09 Pd/Cu/Zn/Al (imp) relative to Cu/Zn/Al (series prec 1)

Figure 7.2 Pd promotion of methanol synthesis from CO/COn/Hn syngases (Initial activities, 4 H2:(C0+C02), 3.47 mol h'^ gcu5 MPa, 250°C, Berty reactor)

7.4 REACTANT/PRODUCT PROFILE FROM CO/COg/Hg WITH VARIOUS CO2 FRACTIONS (BERTY REACTOR)

Figure 7.3 shows the production rates of methanol and water, and the reaction rates of CO and CO2 across the range of CO2 firactions. CO was the net major carbon source for methanol production at small CO2 fractions, but in fact became a net product of CO2, through the reverse water-gas shift, reaction, above 0.7 CO2 fraction. The CO reaction/production profile was similar (+ 5%) over both catalysts above 0.3 CO2 fraction. Note that CO2 was a net reactant under all CO/CO2/H2 feeds, and the Pd promotion in methanol production at CO2 fi^actions > 0.5 was coincident with greater CO2 consumption.

107 — Cu/Zn/Al (series prec 1)

- 0.09 Pd/Cu/Zn/Al (imp)

CH3OH

. u bO

1 &8 1 I inlet 002/(00+00,) a

-150 -L

Figure 7.3 Effect of Pd on net reaction and production rates from OO/OO^/Ho syngases (Initial activities, 4 H2:(C0+002), 3.47 mol h'^ g^u ^ 5 MPa, 250°0, Berty reactor)

Figure 7.3 shows that the water production over both catalysts increased continuously as the OO2 fi"action in the feed was increased. From trace amount under CO/H2 (as in Section 6.2), the water production was equal to the sum of methanol and 00 production under OO2/H2 (as in Section 5.7). In fact, the water production rate was equal, within the tolerances for experimental error, to the OO2 reaction rate across the range of OO2 fractions (which would be expected, given the excess oxygen atom for every molecule of OO2 reacted to methanol or 00). It is clear then that Pd promotion of methanol synthesis

108 occurs in reaction environments rich in CO2 and water, since the increasing Pd promotion above 0.4 C02/(C0+C02) was coincident with increasing CO2 and water concentrations. Therefore the mechanism for the promotion is probably the same as that which was found for methanol synthesis under CO2/H2 in Chapter 5, i.e. methanol synthesis from CO2/H2 is inhibited by the product water, and the inhibition is partially counteracted by Pd through hydrogen spillover.

The discussion of the kinetic behaviour of Cu/Zn/Al for methanol production at small CO2 fractions and the associated inhibition by Pd is rather more involved. First, the particularly high methanol production over Cu/Zn/Al at small CO2 fractions is considered. As it was explained in the literature review (Section 2.2.3), it is not clear whether the promotion in methanol production, which many workers have observed by addition of a small CO2 fraction to CO/H2, is a direct or indirect result of the presence of CO2 in the synthesis gas. The promotion may be caused by the water which is produced from CO2 by the reverse water-gas shift reaction, since promotion by water addition to CO/H2 has been observed with water concentrations as low as 0.25% (146), 0.1% (160), and "a few ppm" (204). The water concentrations in the Berty reactor over either catalyst were -0.06% and -0.13% (mol/mol) at 0.05 and 0.1 C02/(C0+C02) respectively (the water concentrations here were virfually the same over Cu/Zn/Al and Pd/Cu/Zn/Al, since water was produced by the reverse water-gas shift reaction and this reaction was not effected by Pd previously in Chapter 5), so promotion by water was likely. Second, the particularly acute inhibition by Pd at small CO2 fractions (Figure 7.2) is considered. It was proposed in the last Chapter that hydrogen spillover from Pd inhibits CO hydrogenation to methanol under CO/H2, but in Figure 7.2 it can be seen that the inhibition by Pd was greater with small CO; fractions than with CO/H2. This is explained as follows: under CO/H2, methanol production was promoted by the trace product water at integral conversions, and since greater amounts of trace products were produced over catalysts with Pd, the observed inhibition by Pd was relatively low (Chapter 6). However, with small CO2 fractions, although methanol production was again promoted by water (which was no longer a trace product but a product of the reverse water-gas shift reaction), the water production was not effected by the presence of Pd, and therefore the promotion by water did not interfere with the relative inhibition of Cu/Zn/Al by Pd.

109 7.5 METHANOL PRODUCTION FROM CO/COg/Hz AT DIFFERENTIAL CONVERSION

In Figure 7.4 the methanol production rates at differential conversion across the range of CO2 fractions are plotted for Cu/Zn/Al and Pd/Cu/Zn/Al. As before, each data point is the initial activity from a unique experiment using a fresh catalyst charge. In all experiments, the methanol yields were < 0.3%, which was previously found to be the requisite for differential conversion under CO2/H2 and CO/H2 (Sections 5.9 and 6.7). Over both catalysts the differential rate of methanol production was almost linearly related to the CO2 fraction, in agreement with other studies at low conversion using Cu/ZnO catalysts (49,154). The methanol production rate was more than 20 times greater under CO2/H2 compared to CO/H2 (in particular over Pd/Cu/Zn/AI the rate under CO2/H2 was 30 times greater than that under CO/H2). Differences in methanol production over the two catalysts were only apparent under CO/H2 and at small CO2 fractions. This is illustrated more clearly in Figure 7.5; the inhibition by Pd at differential conversion was 30% under CO/H2, but declined to < 10% at 0.3 C02/(C0+C02).

The finite methanol production rate from CO/H2 at differential conversion over both catalysts shows that methanol was produced by the hydrogenation of CO, but the almost linear increase in production with increasing CO2 fraction suggests that CO2 hydrogenation became the predominant methanol synthesis reaction at quite low CO2 fractions, say CO2/ (CO+CO2) > 0.2 or CO2 > 4% in the syngas. This is in agreement with published work using isotope labelled reactants at low conversions (reviewed in Section 2.2.4), where it was found that with 1.5% CO2 more than half of the methanol was produced from CO (148), whereas with 4% CO2 the major carbon source for methanol production was CO2 (15,149). If the effect of Pd at differential conversion was only to inhibit CO hydrogenation, then the trend in Figure 7.5 can easily be explained. Under CO/H2, where methanol is produced by CO hydrogenation, the inhibition was greatest, but the inhibition tended to zero at quite low CO2 fractions as the CO2 hydrogenation became the predominant methanol synthesis reaction. Pd had no effect on CO2 hydrogenation at differential conversion due to the absence of water (see Chapter 5).

110 X— Cu/Zn/Al (series prec 1) I 0.09 Pd/Cu/Zn/Al (imp) I

0.2 -- 1I

0.2 0.4 0.6 0.8

C02/(C0+C02)

Figure 7.4 Effect of Pd on differential methanol production rate under CO/COi/Hn (Initial activities, 4 H2:(C0+C02), 4.5 MPa, 250°C, tubular reactor)

C02/(C0+C02)

1 -20 -- Ph

* 0.09 Pd/Cu/Zn/Al (imp) relative to Cu/Zn/Al (series prec 1)

Figure 7.5 Pd promotion of differential methanol production under CO/CO^/Hn (Initial activities, 4 H2:(C0+C02), 4.5 MPa, 250°C, tubular reactor)

111 In Figure 7.4, it can be seen that at small CO2 fractions and with the Cu/Zn/Al catalyst in particular, there was a slight departure from the linear relationship of methanol production rate with CO2 fraction. The slightly higher methanol production rate at small CO2 fractions (than would be expected from the linear relationship) cannot be explained conclusively, but it was not due to promotion by the product water, since the experiments were carried out at differential conversion. It appears that either CO; hydrogenation is promoted by excess CO (perhaps due to the greater reduced Cu sites where CO2 hydrogenation may take place), or that CO hydrogenation is promoted by CO2 (perhaps due to the partial oxidation of some Cu sites by dissociative reaction of CO2 where CO hydrogenation may take place). The fact that the methanol production rate was more linearly related to the CO2 fraction in the case of Pd/Cu/Zn/Al suggests that the second mechanism may be applicable (hydrogen spillover from Pd would be expected to counteract the oxidising tendency of CO2, thus limiting its promoting effect). However, either of these mechanisms could only account for a small extent of the dramatic promotion in methanol production which was observed at integral conversion (particularly with Cu/Zn/Al) when the synthesis gas contained a small CO2 fraction (Figure 7.1), which will become clear from the following analysis.

7.6 COMPARISON OF METHANOL PRODUCTION FROM CO/CO2/H2 AT DIFFERENTIAL AND INTEGRAL CONVERSIONS

Differential and integral methanol production rates over the Cu/Zn/Al and Pd/Cu/Zn/Al catalysts are compared in Figure 7.6, and the corresponding promotions are compared in Figure 7.7. The most striking observation from Figure 7.6 is that, irrespective of the catalyst, at small CO2 fractions, 0.05 < C02/(C0+C02) < 0.2 (l%-4% CO2 in the syngas), the methanol production rate was greater in the integral kinetic regime. This shows clearly that the promotion of methanol production by small CO2 fractions, which has been observed by many other workers at integral conversions (33,60,96,98,116,118), is primarily a promotion by a product (water) rather than by CO2 in the feed. As has been mentioned before (Section 7.4), the amount of water produced in the present experiments

112 500

Cu/Zn/Al (series prec 1) 400 "o 0.09 Pd/Cu/Zn/Al (imp)

w 300 2 duierential

Ig 200 I

100 -- integral

0.2 0.4 0.6 0.8 C02/(C0+C02)

Figure 7.6 Integral/differential methanol production rates under CO/COo/Hn (curves obtained from Figures 7.1a and 7.4)

20 * 0.09 Pd/Cu/Zn/Al (imp) relative to Cu/Zn/Al (series prec 1) 10 integral ¥ 0.8 1 I C02/(C0+C02) Ph differential

Figure 7.7 Pd promotion of integral/differential methanol production under CO/COn/Hi (curves obtained from Figures 7.2 and 7.5)

113 (at small CO2 fractions and integral conversion) was well within the range of water concentrations found to promote methanol synthesis from CO/H2 in the literature (146,160,204). Furthermore, it has been found, using isotope labelled reactants, that in the presence of water under CO/H2 (146) and 1.5% CO2/CO/H2 (148), CO is by far the major, and perhaps the exclusive, carbon source for methanol. Therefore it appears that the promotion of Cu/Zn/Al by water (at small CO2 fractions and integral conversion) is a promotion of CO hydrogenation to methanol and not CO2 hydrogenation. It is emphasised that the promotion of CO hydrogenation by water does not take place at higher CO2 fractions and integral conversion, where the higher water concentrations would severely or completely inhibit CO hydrogenation (3,146,154,204).

Another striking observation from Figure 7.6 is the increasing departure, irrespective of the catalyst, between the integral and differential methanol production rates at C02/(C0+C02) > 0.4. As it was found in Chapter 5, and as others have found (18,148, 153), water severely inhibits the CO; hydrogenation to methanol (and water, except in the case of low water concentration, also severely inhibits CO hydrogenation to methanol (3,146,154,204)). Therefore, the increasing inhibition of methanol synthesis at integral conversion with C02/(C0+C02) > 0.4 was caused by the increasing water concentration in the Berty reactor, which was shown previously in Figure 7.3.

The trends in the promotions at integral and differential conversions (Figure 7.7) can be explained with reference to the previous proposals (Chapters 5 and 6) that Pd inhibits the CO hydrogenation to methanol in general and Pd promotes the CO2 hydrogenation to methanol in the presence of water. At small CO2 fractions and differential conversion, the contribution of CO hydrogenation to methanol synthesis declined quickly to zero with increasing CO2 fractions, as did the Pd inhibition. At small CO2 fractions and integral conversion, where CO hydrogenation was promoted by the product water and so where CO hydrogenation remained the predominant methanol synthesis reaction, the inhibition by Pd was particularly acute (~ 30%). This extent of inhibition at small CO2 fractions and integral conversion was similar to that found under CO/H2 at differential conversion, where again methanol was produced by CO hydrogenation. (The apparent anomaly under CO/H2 at integral conversion, where the observed inhibition was relatively low, -12%,

114 has been explained at length in Chapter 6 and Section 7.4. The underlying inhibition by Pd remained present, but was obscured by the promotion by trace water produced preferentially over catalysts with Pd.) At higher CO2 fractions and integral conversion, CO2 hydrogenation became the predominant methanol synthesis reaction and, in the absence of water at differential conversion, Pd had no effect, whereas in the presence of water at integral conversion, Pd promoted methanol production.

7.7 CONCLUDING REMARKS

The effect of Pd on methanol synthesis from CO/CO2/H2 was a combination of the individual effects under CO2/H2 and CO/H2 found in the two previous Chapters. Acute Pd inhibition was apparent in situations where CO hydrogenation was the predominant methanol synthesis reaction, e.g. at differential conversion under CO/H2 and, more interestingly, at integral conversion with small CO2 fractions where there was a dramatic promotion of CO hydrogenation due to the small amounts of water produced by the reverse water-gas shift reaction. The much faster CO2 hydrogenation became the predominant methanol synthesis reaction at quite low CO2 fractions. Thereafter, at increasing CO2 fractions, increasing Pd promotion of methanol synthesis was observed, in line with increasing water production and increasing inhibition of CO2 hydrogenation.

115

Chapter 8

SUMMARY OF RESULTS AND DISCUSSION

8.1 METHANOL SYNTHESIS FROM CO^/H;

The promotion of methanol production from CO2/H2 at integral conversions in the Berty reactor was observed with physical mixtures and, since Pd/Al alone was inactive, the results are in agreement with the proposal (1-3) that the promotion involves hydrogen spillover. Greater promotion was observed with Pd/Cu/Zn/Al catalysts (up to 35%). Other kinetic results suggested that the Cu dispersions were no greater in Pd/Cu/Zn/Al catalysts (i.e. CO production by the reverse water-gas shift reaction was similar over all catalysts). Therefore, the greater promotion observed with Pd/Cu/Zn/Al catalysts compared to physical mixtures was probably the result of more effective hydrogen spillover due to the proximity of Pd to Cu (which is consistent v^ith the temperature programmed reduction of the catalysts in Section 3.5).

The direct rate of CO2 hydrogenation to methanol was measured at differential conversion under CO2/H2 over Cu/Zn/Al. There was no significant difference in the CO2 hydrogenation activities of physical mixtures or Pd/Cu/Zn/Al catalysts, suggesting again that the Pd promotion observed previously at integral conversions was not due to differences in Cu dispersion and also that there were no special interfacial catalytic sites in Pd/Cu/Zn/Al catalysts (e.g. Pd-Cu or Pd-Zn) which were responsible for the promotion. The results at differential conversion showed that the promotion at integral conversion was related to the presence of a product (methanol, CO or water). The addition of water to CO2/H2 at differential conversion resulted in an order of magnitude loss of activity over Cu/Zn/Al, giving a similar rate of methanol production as that in the Berty reactor where the water concentration as a result of water production was similar. The inhibition as a result of water addition at differential conversion was less over Pd-impregnated Cu/Zn/Al,

117 resulting in a promotion (~ 35% greater methanol production compared to Cu/Zn/Al) which was similar to that found in the Berty reactor. Therefore, the proposed hydrogen spillover from Pd is only effective as a means to counteract the inhibition of CO2 hydrogenation on Cu by the product water.

Inui et al (1,2) proposed that hydrogen spillover from Pd, causing a more reductive state of Cu, was the basis of the mechanism of Pd promotion under CO2/H2. In addition, Fujimoto and Yu (3) proposed that the product water is an oxidising agent of Cu, which has an adverse effect on the activity, and the opposing reductive effect of hydrogen spillover from Pd leads to the observed promotion. Colboum et al (151) demonstrated that the dissociative reaction of steam evolving H2 and adsorbed oxygen [$.3] takes place readily on a fraction of a polycrystalline Cu surface (possibly at Cujjo sites) at temperatures similar to that of methanol synthesis. As such, the H2/H2O ratio may control the oxygen coverage of Cu in a methanol reaction gas mixture, and, in the presence of Pd, hydrogen spillover may increase the surface availability of hydrogen, thereby decreasing the oxygen coverage at steady state. Chinchen, Waugh and co-workers (31,46,52,149, 158,183-188) claimed that the dissociative reaction of CO2 on Cu [0.4] also, or primarily, controls the oxygen coverage of Cu in Cu/Zn/Al catalysts during methanol synthesis. This conclusion was based on the finding that a fraction of the Cu metal surface area could not be measured after exposure to CO/CO2/H2 synthesis gases (unless the catalyst was re-reduced) and the fraction was confrolled by the CO2/CO ratio in the synthesis gas. However, given that the production of water is also controlled by the CO2/CO ratio in the synthesis gas (this was shown clearly in Chapter 7), it is not possible to conclude whether reaction [8.3] or reaction [5.4] is the primary source of oxidation. Others, who carried out methanol synthesis from CO2/H2 over polycrystalline Cu foil (142) and Cuioo single crystal (155,156), found no evidence of Cu surface oxidation. These studies were carried out at very low conversions (due to low pressure and small overall Cu surface areas), such that the product water partial pressures were very low, which points to the primary role of water [3-3], rather than CO; [SA], in oxidising the Cu surface at integral conversions. Furthermore, thermodynamic calculations show that a typical H2O/H2 ratio in methanol reaction gas mixtures has a much higher oxidising potential than various CO2/CO ratios (23).

118 HjO <» H2 + Oads [g.3] CO2 ^ CO + 0*k [3 4]

Despite all the above arguments, if the CO2 dissociation does control the oxygen coverage of Cu, the theory that adsorbed oxygen inhibits methanol synthesis (and hydrogen spillover counteracts the inhibition by reaction with the adsorbed oxygen) cannot be true, since water is not involved. An alternative mechanism for the inhibition by water involves hydroxyl groups from the dissociative adsorption of water on Cu, reactions [F.5] and [f.6] (U.H.V. studies (206,207) have shown reaction [8.6] to occur on various Cu surfaces and up to eight water molecules interacted with a single oxygen atom at a low oxygen coverage (207)). The hydroxyls may block active Cu sites for methanol synthesis. Hydrogen spillover from Pd may react with the hydroxyls (and with adsorbed oxygen) limiting the inhibition by hydroxyls (and preventing the adsorption of water by reaction [5.6]). In addition, hydrogen spillover may play a more direct role in methanol synthesis, i.e. if adsorbed water species are involved in the inhibition of particular Cu sites where H2 is adsorbed, spillover hydrogen may be an alternative source for CO2 hydrogenation.

H2O ^ OH^ + H,ds [8.5] H2O + Oads ** 2 OHads [8.6]

The promotions at integral conversion were very similar over all the Pd/Cu/Zn/Al catalysts in this study, irrespective of the Pd loading and the method of Pd addition. (There was one possible exception. The promotion observed with the 0.12 Pd/Cu/Zn/Al (prec) catalyst was slightly less than the promotion observed with the other Pd/Cu/Zn/Al catalysts, but considering that its activity for the reverse water-gas shift reaction and for CO2 hydrogenation at differential conversion was slightly less than that of all other catalysts, its Cu dispersion was probably slightly less than that of other catalysts. This would mean that the promotion with 0.12 Pd/Cu/Zn/Al (prec) in terms of the turnover number per Cu site was similar to that of all the other Pd/Cu/Zn/Al catalysts.) The similar promotion observed with all the Pd/Cu/Zn/Al catalysts was ~ 30% (at 3.47 mol h"' gcu' in the Berty reactor), but this only accounted for a small extent of the inhibition caused by water (there was an order of magnitude loss of methanol production over Cu/Zn/Al at

119 '3.47 mol h"' gcu ' due to the product water). The similar Pd promotions may have followed from similar Pd surface areas on all the Pd/Cu/Zn/Al catalysts (e.g. the Pd on the surface of the support may have reached a saturation coverage), resulting in similar rates of spillover hydrogen on all Pd/Cu/Zn/Al catalysts. Alternatively, the accommodation of spillover hydrogen on the catalyst surface may have reached a saturation coverage and therefore may be independent of the Pd dispersion. Given this maximum possible coverage of adsorbed hydrogen, the reducing potential (or the tendency to remove water by reaction) of the hydrogen may be much less than the oxidising potential (or tendency to adsorb at active sites) of water at the partial pressures apparent at integral conversion, which would explain why the maximum Pd promotion was much less than the inhibition by water. Another possible explanation for the deficiency in Pd promotion is that hydrogen spillover only effects the periphery of Cu crystallites or certain morphologies of Cu. The structure sensitive nature of the water-gas shift reaction (as opposed to the structure insensitive methanol synthesis (23,150-152)) and the lack of promotion of this reaction (compared to the promotion of methanol synthesis) suggests that hydrogen spillover does not effect some types of active Cu sites.

The final point of discussion is the impact of the unknown Pd and Cu dispersions in Pd/Cu/Zn/Al catalysts (due to the difficulties outlined in Section 3.7) on the thesis. The lack of Cu dispersion measurements made it difficult to conclude whether the greater activity of Pd/Cu/Zn/Al catalysts was a result of a promotion in the activity of Cu or simply greater dispersion of Cu. Fortunately, as mentioned above, other kinetic results (namely the similar activity of all catalysts for CO2 hydrogenation to methanol at differential conversion and for the reverse water-gas shift reaction in general) suggested that the Cu dispersions were similar in all catalysts. The lack of Pd dispersion measurements made it difficult to conclude whether the similar promotions achieved with all the Pd/Cu/Zn/Al catalysts was a result of similar Pd dispersions in these catalysts or a result of having reached a maximum possible extent of promotion by hydrogen spillover in the present catalyst system. As discussed in the previous paragraph, this issue was not resolved.

120 8.2 METHANOL SYNTHESIS FROM CO/Hg

The direct rate of CO hydrogenation to methanol was measured at differential conversion under CO/H2 over Cu/Zn/Al. The addition of Pd to the catalyst in the form of a physical mixture and Pd/Cu/Zn/Al catalysts resulted in less activity for methanol production at differential conversion. The lowest activities were apparent in the case of the higher Pd loadings amongst Pd/Cu/Zn/Al catalysts and the most intimate method of Pd addition (coprecipitation with Cu resulted in ~ 40% less activity). The lower activities of Pd/Cu/Zn/Al catalysts was not a result of greater deactivation of these catalysts compared to Cu/Zn/Al. The lower activities of Pd/Cu/Zn/Al catalysts could have been due to lower Cu dispersions, although the previous results (Section 8.1) suggested that this was not so. Therefore, it was concluded that there was a chemical effect of Pd which inhibited the CO hydrogenation activity of the Cu catalyst.

The methanol production rate over a particular catalyst was greater at integral conversions, in particular in the internal recycle reactor, compared to differential conversion, which showed that methanol synthesis was promoted by the product gas phase. Based on further evidence in the literature, this was a promotion of CO hydrogenation by the trace product water (rather than the trace product CO2). The underlying inhibiting effect of Pd remained present at integral conversion, but the inhibition (up to 12%) was much less than that found previously at differential conversion. Given that the trace product water promoted methanol production, and that trace products were found to be produced over Pd as well as Cu, the lesser inhibition observed at integral conversion was due to the opposing promotion by the extra trace product water produced over Pd.

It is well known that small amounts of water and CO^ added to CO/H2 promotes methanol synthesis over Cu/ZnO catalysts. However, as it was deduced from an analysis of the literature (Section 2.2.3) and as was found in Chapter 7, water is the primary promoter, since the significant promotion of methanol synthesis by CO2 addition to CO/H2 is observed at integral conversions (where there would be a finite pressure of water in the product due to the reverse water-gas shift reaction) but not at very low conversions.

121 Indeed, many workers have observed the promotion of methanol synthesis under CO/H2 using Cu/ZnO catalysts with particularly small additions of water (146,160,204), even as low as "a few ppm" (204). Furthermore, it has been found, using isotope labelled reactants, that in the presence of water under CO/H2 (146) and 1.5% CO2/CO/H2 (148), CO is by far the major, and perhaps the exclusive, carbon source for methanol production. Hence the promotion of CO hydrogenation by the trace product water is the most likely explanation for the greater activity of catalysts observed at integral conversion, particularly in the Berty reactor, compared to differential conversion.

It should be noted that very little (204) or no (18) methanol production has been observed with highly purified CO/H2. In the present study the CO/H2 synthesis gas did contain minute water impurity and this may have affected the CO hydrogenation rates measured at differential conversion.

The mechanism by which water promotes CO hydrogenation needs to be considered before considering the mechanism of Pd inhibition. Colboum et al {\5\) demonstrated that the dissociative reaction of steam evolving H2 and adsorbed oxygen [5.3] takes place readily on a fraction of a polycrystalline Cu surface at temperatures similar to that of methanol synthesis. As such, water may provide some oxidised Cu sites which many workers have proposed are the active sites for CO hydrogenation to methanol (50,59,160, 163-168). Alternatively, Vedage et al (146) added isotope labelled water to CO/H2 and presented convincing evidence that hydroxyl groups from the dissociative reactions of water on Cu ([F.5] and [0.6]) reacted directly with CO, presumably to produce a formate species, HCO2, which was hydrogenated to methanol. Therefore water may promote CO hydrogenation by providing adsorbed species where the secondary adsorption of CO takes place.

The action of Pd, then, under CO/H2 may be exactly the same as that proposed previously for CO2/H2 (Section 8.1), i.e. hydrogen spillover tends to reduce the Cu or reacts with the adsorbed species of water. But in this case, under CO/H2, Pd causes an inhibition of methanol production, since its action opposes the effect of water which promotes CO hydrogenation. Note that the rates of methanol production by CO hydrogenation at

122 differential conversion may well have been dependent on the minute water impurity in the feed, as mentioned above. This means that the proposed mechanism for Pd inhibition, involving the removal of dissociatively adsorbed water species on Cu by hydrogen spillover or the reduction of Cu by hydrogen spillover as opposed to the oxidation by water, is applicable under differential conversion as well as integral conversion.

8.3 METHANOL SYNTHESIS FROM CO/COg/Hz

The methanol production rate at differential conversion over Cu/Zn/Al was approximately linearly related to the CO2 fraction in the synthesis gas; the rate of CO^ hydrogenation under CO2/H2 was over 20 times greater than the rate of CO hydrogenation under CO/H2. This indicated that CO2 hydrogenation becomes the major methanol synthesis reaction at quite low CO2 fractions, which is in agreement with published work using isotope labelled reactants at low conversions, where it was found that with 1.5% CO2 more than half of the methanol was produced from CO (148), whereas with 4% CO2 the major carbon source for methanol production was CO2 (15,149). Pd impregnation inhibited methanol production at differential conversion under COfRj by ~ 30%, but the inhibition tended to zero at quite low CO2 fractions. There was no effect of Pd at higher CO2 fractions. These results are in line with the previous findings, that Pd inhibits CO hydrogenation to methanol in general (Section 8.2) and that, in the absence of water, CO2 hydrogenation is not effected by Pd (Section 8.1).

At integral conversion in the Berty reactor, a sharp maximum in methanol production rate over Cu/Zn/Al was observed at a small CO2 fraction. In fact, the methanol production rates with small CO2 fractions were higher than those found previously at differential conversion, which showed that the sharp maximum in methanol production rate at integral conversion was a result of promotion by a product rather than by CO2. With further evidence in the literature (i.e. that small amounts of water promote methanol synthesis from CO/H2 (146,160,204) and that, in the presence of small amounts of water, CO is the major carbon source of methanol (146,148), as discussed in Section 8.2), it can be concluded that CO hydrogenation to methanol was promoted by the small amounts of

123 water produced by the reverse water-gas shift reaction. The effect of Pd impregnation with the smallest CO2 fraction at integral conversion was to inhibit methanol production by ~ 30%, the inhibition declining quickly to zero at higher CO2 fractions. These results are again in line with the proposal (Section 8.2) that Pd inhibits CO hydrogenation to methanol in general.

With higher CO2 fractions at integral conversion in the Berty reactor, methanol production over Cu/Zn/Al became increasingly inhibited compared to that attainable at differential conversion (at the extreme, under CO2/H2, the methanol production rate at , integral conversion was an order of magnitude less than that at differential conversion), which showed that methanol production was inhibited by a product. Coincident with the increasing inhibition at higher CO; fractions was the increasing water production. These results are in line with the previous finding (Section 8.1) that the product water severely inhibits CO2 hydrogenation to methanol. Pd impregnation resulted in increasing promotion of methanol production at these higher CO2 fractions (with up to 25% promotion under CO2/H2). These results are in line with the proposal (Section 8.1) that hydrogen spillover from Pd counteracts the inhibition of CO2 hydrogenation on Cu by water.

In summary, the results of the study using CO/CO2/H2 synthesis gases are consistent with the findings of the previous studies using CO2/H2 and CO/H2.

124 Chapter 9

CONCLUSIONS

The addition of Pd to the Cu/Zn/Al catalyst in the form of physical mixtures or, more effectively, by impregnation/coprecipitation promotes methanol synthesis from CO2/H2 by hydrogen spillover. The promotion is observed in an integral, not differential, kinetic regime, where methanol synthesis on Cu by CO2 hydrogenation is severely inhibited by the product water, and hydrogen spillover from Pd counteracts this inhibition to a small extent. It is proposed that the water oxidises or, perhaps through its dissociatively adsorbed species, blocks the active Cu sites for CO; hydrogenation, whereas hydrogen spillover tends to reduce the Cu or reacts with the adsorbed species of water.

Methanol synthesis from CO/H2 over Cu/Zn/Al is inhibited by the addition of Pd, in particular by impregnation/coprecipitation of Pd. The inhibition is particularly severe at differential conversion, which shows that Pd inhibits CO hydrogenation in general. At integral conversion, the inhibition by Pd is partly obscured by the promotion of CO hydrogenation over Cu by the trace water produced over Pd and Cu. It is proposed that the trace product water promotes CO hydrogenation by oxidising the active Cu sites or by providing, through its dissociatively adsorbed species, sites where the secondary adsorption of CO takes place. It is proposed that hydrogen spillover from Pd tends to reduce the Cu or reacts with the adsorbed species of water.

The effect of Pd on methanol synthesis from CO/CO2/H2 is a combination of the individual effects described above. Most interesting, at integral conversion with small CO2 fractions, there is a dramatic promotion of CO hydrogenation due to the small amounts of water produced, and there is also acute Pd inhibition. The much faster CO2 hydrogenation becomes the predominant methanol synthesis reaction at quite low CO2 fractions. At increasing CO2 fractions, with increasing water production, there is increasing inhibition of CO2 hydrogenation, and accordingly increasing Pd promotion.

125

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137

Appendix 1 Kinetic theory

A1.1 Integral and differential conversions

Differential conversion applies when the conversion of reactants to products is so low that, in effect, the catalyst is only exposed to the feed gas mixture and there is no exposure to the products. There is no reverse (or secondary) reaction and the catalyst cannot be inhibited (or promoted as the case may be) by the products at differential conversion, such that the intrinsic forward rate of reaction for the given feed gas mixture, catalyst and conditions is apparent. Integral conversion is a finite conversion of the reactants to products, such that the catalyst is exposed to a lower partial pressure of reactants than was in the feed gas mixture and is also exposed to products. The net reaction rate is subject to reverse (and possibly secondary) reaction and the catalyst may be inhibited (or promoted) by the products.

Assuming a simple first order reversible reaction A ** B, the following equations are the kinetic models for a (perfectly mixed) internal recylce reactor and a (plug flow) tubular reactor. Assuming all parameters equal to unity (arbitrary units). Figure Al.l shows the transition from integral to differential conversion as the flow rate through the reactors is increased. At the left hand side of the Figure, with decreasing flow rates the production rate tends to zero while the yield tends to the equilibrium (50% product yield at equilibrium in this case since the forward and reverse rate constants are of equal magnitude). At the right hand side of the Figure, with increasing flow rate, the product yield tends to zero while the rate tends to the forward rate of reaction for the given gas feed (i.e. r^^ = = 1 in this case). The approach to the forward rate of reaction at very low product yields is the approach to differential conversion.

139 Note that the broad trend in Figure A 1.1 is applicable to reversible and irreversible reactions with any rate orders. There are two kinetic situations in which the trend in Figure A 1.1 does not apply. Firstly, if the product of concern is formed by a secondary reaction, the production rate will be a maximum at intermediate conversions, and will approach zero at equilibrium and differential conversion. Secondly, if the catalytic activity for the product formation is promoted by a by-product or by itself, the production rate will be a maximum at intermedate conversion, will approach a lower production rate at differential conversion, and will approach zero at equilibrium,

Reactions kinetics

Assume a simple reversible gas phase reaction A <=> B at constant pressure P with the feed containing only A. Assuming first order kinetics, the rate of reaction is:

-pf^) [Al.l]

The rate constants and are defined relative to the mass of catalyst m.

Plug flow tubular reactor

In a differential mass of catalyst dm the rate rate of change of A is:

= - {n! V) . {Apf^l dm) [A1.2] where n is the total molar flow rate. Equating [Al.l] and [A1.2] and integrating between the limits pp^-V aXm = 0 (inlet of the reactor) and = PAX at m = M (exit of the reactor), the following equation is obtained:

fAx/P = ( ^A /( ^A + 4 )). ( exp (- M P (^^ + Arg ) /n) - 1 ) + 1 [A1.3]

Then the product yield and the production rate are obtained from:

140 tubular reactor mtemal recycle reactor

0.01 0.1 1 10 100

flow rate (arbitrary units)

0.5

0.4 ^ ^ tubular reactor

internal recycle reactorX

Io 0.2 A

0.1

0 0.01 0.1 1 10 100

flow rate (arbitrary units)

Figure Al l Transition from integral to differential conversion as the flow rates through internal recycle and tubular reactors are increased

141 product yield = 1-/?ax/P [A1.4] production rate = (product yield) . n [A1.5]

Perfectly mixed internal recycle reactor

Since the reaction kinetics are subject to the product gas phase concentration in a perfectly mixed reactor, the reaction rate is simply;

= n(l-/7Ax/P) [A1.6]

Equating [Al.l], with=PAX, and [A1.6] gives;

fAx/P = (n + MP^B)/(n + MP^A + MP^B) [A1.7]

Then the product yield and production rate can be obtained from [A 1.4] and [A 1.5].

A1.2 The internal recycle reactor and tubular reactor

The internal recycle reactor contains a well mixed gas phase as a result of forced recirculation through the catalyst basket. In the limit, the gas phase is perfectly mixed, meaning that there are no concentration gradients in the reactor and the catalyst is only exposed to the final product gas phase. The tubular reactor contains a tubular catalyst bed, which is exposed to a gradient of concentration from the feed gas at the inlet to the final product gas at the outlet of the catalyst bed. In the limit, the reaction gas mixture moves in a plug flow through the reactor, meaning that there is no back-mixing and there are no wall effects such that the entire gas phase cross-section travels at the same linear velocity along the reactor.'

' Note that considerable effort was made to ensure that the gas phase of the internal recycle reactor used in the thesis was perfectly mixed (Section 4.6), but limited attention was paid to the flow characteristics of the tubular reactor. This was because the tubular reactor was used primarily for experiments at differential conversion and, as will be

142 The reactor model equations (assuming a first order reversible reaction A <=> B) were derived in the previous Section and Figure A 1.1 shows that there is no difference between the performances of the two reactors at equilibrium and at differential conversion. However there is a difference at integral conversions not including equilibrium, where it can be seen that there was greater production (yield or rate) in the tubular reactor. This is a result of the greater exposure of the catalyst to reactants (and the lesser exposure to products) in the tubular reactor, and correspondingly the greater forward reaction rate (and the lesser reverse reaction rate).

Note that the greater production in the tubular reactor at integral conversions not including equilibrium is applicable to reversible and irreversible reactions with any rate orders. There are two kinetic situations in which the operation of the the internal recycle reactor at the product gas phase concentration leads to greater production in the internal recycle reactor at integral conversions not including equilibrium. Firstly, if the product of concern is formed by a secondary reaction and, secondly, if the catalytic activity for the product formation is promoted by a by-product or by itself

explained, the mixing/flow characteristics of the gas phase make no difference to the kinetic results at differential conversion.

143

Appendix 2 Methanol synthesis equilibrium calculations

This appendix to Section 2.1 shows the calculation of the equilibrium methanol yields as a function of the CO; fraction, C02/(C0+C02), in the synthesis gas at the conditions which were used for kinetic experiments with the Berty reactor (250°C, 5 MPa, 4 H2:(C0+C02) with 10% inerts). Note that no corrections for fugacity coefficients, which may be necessary at the reaction pressure of 5 MPa, were made in the calculations. Three reactions define the equilibrium composition of methanol synthesis gas mixtures:

CO + 2 Hz ^ CH3OH [A2.1] CO2 + 3H2 o CH3OH + H2O [A2.2] CO2 + H2 o CO + H2O [A2.3]

Only two of these reactions are independent, and for thermodynamics any two can be used. Equilibrium data are commonly available for methanol synthesis jfrom CO hydrogenation [A2.1] and the reverse water-gas shift reaction [A2.3] (19). The established data of Bisset (109) and Kotowski (108) respectively gives the following equilibrium constants at 250°C:

Kcohydrog(250°c) ~ 0.253307 MPa ^ [A2.4] Kfwg shift (ascc) ~ 0.011672 [A2.5]

An iterative calculation procedure is necessary to establish the conversions of a given synthesis gas mixture by reactions [A2.1] and [A2.3] which satisfy the equilibrium constants [A2.4] and [A2.5]. The steps in the calculation are given as follows;

145 O Specify the synthesis gas composition in terms of the number of inlet moles of CO,

CO2, H2 and inerts, Mco,/, »C02./, "m,/ Mi^erts

® Calculate »cH30H,eg by combining [A2.5] and the following equations:

Krwg shift (250°C) ( ^CO,eq • "H20,e? ) / ( ^C02,eq • ^m,eq ) [A2.6]

^CO,eq ~ ^CO,i " "CH30H,e? + ^mO,eq [A2.7] ^m,eq ~ ^m,i - 2 McH30H,eg ' ^mO,eq [A2.8] [A2.9] ^C02,eq = "C02,/ " ^mO,eq

O Calculate the totol number of moles at equilibrium, %otai.e?:

^Total,eq ~ '^CH30H.e? + "H20,e? ^C0,e? "C02,eq'+'^inerts [A2.10]

Kco hydrog (250°C) ~ ( "CH30H,e? ^ ( ^CO,eq • ^m,eq ) ) ^ ( ^Total,eq ^ ^Total,; ) / (^ MPa) [A2.11]

® Compare Kco hydrog (250=c) &om equation [A2.11] with that from [A2.4]. Return to step

2 and iterate until the two values of Kco hydrog (250°c) are the same.

Equilibrium methanol yield = «cH30H,e? / ('^co,/"co2,i) [A2.12]

146