methanol synthesis estudio cinetico

Applied Catalysis, 50 (1989) 265-285

Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands 265

Methanol Synthesis from Hydrogen, Carbon Monoxide and Carbon Dioxide over a CuO/ZnO/ A1203 Catalyst

II. Development of a Phenomenological Rate Expression

MELANIE A. MCNEIL, CARL J. SCHACK and ROBERT G. RINKER* Department of Chemical and Nuclear Engineering, University of California, Santa Barbara, CA (U.S.A.)

(Received 1 August 1988, revised manuscript received 10 January 1989)

ABSTRACT

New steady-state kinetics data for the synthesis of methanol from H,/CO/C02 over a CuO/ ZnO/Al,O,, catalyst were correlated on the basis of reported catalytic mechanisms to formulate a rate expression for methanol formation. In developing the mechanistic model, it was assumed that hydrogen adsorption occurs on ZnO in contrast to carbon monoxide adsorption on Cu’+ and carbon dioxide adsorption on Cu’. Also, the concentration of sites containing adsorbed hydrogen was assumed constant during reaction. These assumptions led to the formulation of a rate expression containing the sum of contributions from carbon monoxide and carbon dioxide hydrogenation.

INTRODUCTION

Generally it is agreed that, for a reactant mixture of CO, CO, and Hz, the main reaction for methanol synthesis is

CO + 2Hz %CH,OH AH,,,, K = - 21.7 kcal/mol (1)

Carbon dioxide is almost always added to the synthesis mixture and, as pointed out by Denny and Whan [ 11, it needs to be accounted for since it can form methanol according to

COP +3Hz *CH,OH+H,O AH,,, K = - 11.9 kcal/mol (2)

The hydrogenation of carbon dioxide proceeds much slower than the hydrogenation of carbon monoxide at industrial conditions of 5-10 MPa and 500-

0166-9834/89/$03.50 0 1989 Elsevier Science Publishers B.V.

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575 K [ 21 although the opposite has been found to be true at lower temperatures [ 3-61. The reversible water-gas shift reaction also occurs. It, of course, is obtained by subtracting reaction ( 1) from (2 ) , giving

COz + H, %CO + H, 0 AH,,,, K = + 9.8 kcal/mol (3) and, consequently, is not an independent reaction in this system.

REVIEW OF PREVIOUSLY PROPOSED MECHANISMS

The following review is presented in order to give the reader an overall ap- preciation of this complex reacting system. A major emphasis of the present study has been to develop a mechanistic model from the apparently contradictory results presented in previous studies as well as from our own experimental kinetics data. The key ideas which provide the basis for our model development are summarized at the end of the review section. Table 1 lists available information on the catalyst types and operating conditions for most of the studies referred to in this review.

Phenomenological models and rate laws have been proposed in the literature for methanol synthesis for over 30 years. However, disagreement still exists, especially concerning the contribution of carbon dioxide during methanol synthesis as well as the identity of the active site or sites on the CuO/ZnO catalyst.

Prior to 1970, no attempt was made to account for the effect of the carbon dioxide added to the synthesis gas mixture. In fact Natta’s model [ 71, which was the basis of many models developed after 1953 for both high and low pressure methanol synthesis catalysts, did not include any terms to account for the effect of carbon dioxide. Natta’s rate expression is shown below.

YCOPCO Y EI,PzI, - YCH30HPCH30HIKeq

r= (A+%opco +CYap~z +~YCH~OH~CH~OH)~ (41

where yi is the fugacity coefficient of species i, pi is the partial pressure of species i, Keg is the thermodynamic equilibrium constant of reaction (11, and A, B, C, and D are empirical constants which are functions of temperature and which are different for each catalyst. Even though this rate expression did not include any carbon dioxide terms, it fits Natta’s data well.

Bakemeier et al. [S] were the first to consider the contribution of carbon dioxide in their rate equation. They realized that carbon dioxide could partic- ipate in the water-gas shift reaction as well as form methanol. The influence of carbon dioxide was accounted for by using a Langmuir adsorption isotherm; and methanol desorption was assumed to be the rate determining step. The resulting rate expression is shown below.

r=

l- PCH~OH

~co~&Kes >I I+ DeF ‘RTpCdpH2

(5)

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TABLE 1

Catalyst and operating conditions of selected literature studies

Ref. Catalyst composition Reaction conditions’

Bakemeier et al. ref. 8, 1970

Kagan et al. ref. lo,1975

Kieffer et al. ref. 4, 1981

Klier et al. ref. 14,1982

Okamoto et al. ref. 22, 1984

Liu et al. ref. 18,1985

Edwards and Schrader ref. 40,1985

Bridgewater et al. ref. 37,1986

Chinchen et al. ref. 13,1987

Schack et al. ref. 20, 1989

ZnO 78 wt.-% Cr,O, 22 wt.-% BASF S 5-10 red.

CuO/ZnO/AlaO, SNM-1 catalyst Unspecified percentages

CuO 60 wt.-% zno 30 wt.-% A&O, 10 wt.-% Lab preparation

Cu 30 metal-atom-% Zn 70 metal-atom-% Lab preparation

Cu 30-80 wt.-% ZnO balance Lab preparation

cue 30 wt.-% ZnO 70 wt.-% Lab preparation

Cu/ZnO 5/95 mol-% lo/90 mol-% CuO/ZnO/CrzO, 5/90/5 mob% 10/80/10 mol-% Lab preparation

cu 72.8 wt.-%” Zn 23.1 wt.-%” Al 4.1 wt.-%” UC C79-4 catalyst

CuO 60 wt.-% ZnO 30 wt.-% Al,O, 10 wt.-% Unspecified vendor

CuO/ZnO/Al,O, BASF S 3-85 proprietary percentages

Nonisothermal T=562-612 K, initially P=20-36 MPa 0.875 kg cat. Z*lO“-6*104 Nm3/m3cat. h

T=443-513 K P=4.9 MPa Unspecified cat. wt. Unspecified flow-rate

T=523-573 K P=5.2 MPa 3*10-4 kg cat. 6700 I/kg cat. h

T=498-523 K P= 7.6 MPa 2.45*10-3 kg cat. 6100 l/kg cat. h

T=413-453 K P=l.Ol*lO-’ MPa unspecified cat. wt. 3.4 and 3.9 slh

T= 468-501 K P= 1.7 MPa 2.81.10-* kg cat. Batch reactor

T= 473-523 K P=5 MPa GHSV= 1800 h-l T= same P=same GHSV=3600 h-’ Unspecified cat. wt.

T= 493 K P=5 MPa Unspecified cat. wt. GHSV= 10000 h-l

T=523 K P=5 MPa Unspecified cat. wt. GHSV=1*104-4.105 h-l

T=483-513 K P=2.9-4.3 MPa 7.9.10-3 kg cat. 75 slh

*Metal percentages were normalized to exclude oxygen. *Nm” is measured at 1 atm and 273 K; GHSV is gas hourly space velocity; slh is standard liters per hour (1 atm, 294 K).

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where A and D are pre-exponential constants, E and F are activation energies, and m and n are orders of reaction. This expression predicts that the methanol production rate decreases as the partial pressure of carbon dioxide increases but does not consider the contribution of carbon dioxide as a reactant.

In an interesting series of papers, Rozovskii and co-workers [9-121 proposed that carbon monoxide is converted to carbon dioxide through the water-gas shift reaction and that most of the methanol is consequently formed from carbon dioxide via reaction (2). As stated above, carbon dioxide hydrogenation occurs faster than carbon monoxide hydrogenation at lower temperatures and pressures. This might explain why Rozovskii et al. [ 91, who operated at 495 K, found such a large degree of production from carbon dioxide. While some investigators [ 131 have accepted reaction (2) as the principal path to methanol formation, the majority feel that reaction (1) is the principal path, at least under typical synthesis conditions of 5-10 MPa and 500-575 K.

Klier et al. [ 141 were the first to account for the effect of carbon dioxide as a catalyst promoter and as a reactant to form methanol. Three models were proposed based on different assumptions concerning how CO, CO, and Hz compete for the active site. The rate-determining step was postulated to be the termolecular surface reaction of carbon monoxide with two hydrogen molecules to form methanol. All three rate expressions fit the data adequately. The one which most satisfactorily fit the data is shown below, and it represents the general form of the other two models.

r=kAi W’PC02/PCO I3 &oK2,, (PCO ~$2 -PCH~OHKJ

[l+K’ (~con/~coH~‘(l+Kco~co+Kcoz~c~oz+K~~~~~)~ (6)

( 1 PCH~OHPI~O +A?' I-%02--

=4 PL >

where Keq and & are the thermodynamic equilibrium constants of reactions (1) and (2)) respectively; A, is the sum of the concentrations of oxidized and reduced active centers; and K’ is the equilibrium constant of the following reaction:

Ared+COz,,, %A,, +co,,, (7)

where Ared and A,, are reduced and oxidized active centers of catalyst, respectively. Klier et al. also found that, under the conditions of their work, the term multiplied by l/K& was negligible. The contribution of carbon dioxide as a catalyst promoter was taken into account through the term which represents the active-site concentration (i.e. the term containing the pcoz/pco ratio). Klier et al. concluded that the main contribution of carbon dioxide was that of a catalyst promoter. In fact, the models of Klier et al. show that the methanol

production rate decreases to zero when no carbon dioxide is present in the feed, because PCOJPCO and therefore the active site concentration are zero. One main limitation of the models of Klier et al. is that other researchers [X5-17] have shown that synthesis gas containing no carbon dioxide can still produce methanol. In fact, Edwards and Schrader [40] ran a synthesis gas mixture of H2/C!O/C02 =67/33/O over a CuO/ZnO/C!ri03 catalyst and measured rates four times higher than those obtained by running a 66/28.6/6.6 mixture over the same catalyst. On the other hand, carbon dioxide or another oxidizing agent such as water seems to be necessary for optimum synthesis since investigators [ 2,6,18] have reported that running synthesis gas mixtures without an oxidizing agent over the CuO/ZnO catalyst results in partial deactivation of the catalyst. Herman et al. [ 21 have suggested that the deactivation can be explained in the following manner: they have reported that the active site for carbon monoxide hydrogenation consists of Cu’+ sites dispersed in ZnO. In a strongly reducing environment, large copper crystallites form from this Cd+/ZnO phase, thus reducing the number of active sites, and resulting in irreversible deactivation.

Another interesting observation regarding carbon dioxide participation was made by Klier et al. [ 141 who found that increasing the CO,/CO molar ratio produced methane while no methane was detected for feed gases of CO/H,. In a seemingly conflicting report, Nappi et al. [ 161 reported that methane production occurs with no carbon dioxide in the feed and increases with increasing carbon monoxide concentration. Amenomiya [ 191 studied methanol synthesis from carbon dioxide and hydrogen and reported that no methane is formed unless the reaction is conducted at temperatures above 570 K. Although no explanations have been proposed for these varying results, it is worth noting them.

Liu et al. [ 6,181 have recently published interesting results which raise fur- ther questions regarding the contribution of carbon dioxide during methanol synthesis. Their initial rate studies showed that the methanol production rate increases as the COJCO ratio increases and does not go through a maximum. This result is in contrast to studies by Klier et al. [ 141 and separately by Schack et al. [ 201 who have found that an optimum CO,/CO ratio exists, beyond which the production rate of methanol is inhibited. However, in Schack’s and Klier’s studies, the production rate of methanol was influenced by the reverse synthesis reaction. Liu et al. also found that water inhibits carbon dioxide hydrogenation but not carbon monoxide hydrogenation to methanol. Coupled with that are the experimental results obtained by Schack et al. [ 201 which suggest that carbon dioxide inhibits carbon monoxide hydrogenation but not its own hydrogenation to methanol. These results indicate that carbon dioxide and carbon monoxide hydrogenation occur on different sites, The experiments of Liu et al. using I80 labelled carbon dioxide gave particularly clear evidence sup-

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porting separate sites for carbon monoxide and carbon dioxide adsorption, They showed that water inhibited only the carbon dioxide hydrogenation rate while the carbon monoxide hydrogenation rate was not changed.

Identification of the active site (s ) for methanol synthesis on Cu/ZnO catalysts has also generated considerable controversy. Proposals have included Cue (X-241, Cul+ [2,25-311 andZn0 [32].

Herman et al. [ 21, Mehta et al. [ 251 and Klier [ 331 have extensively studied binary and ternary Cu/ZnO catalyst samples (oxidized, reduced and used) in order to elicit the activation characteristics, electronic structure and surface composition of these catalysts. Their optical spectroscopy studies of the catalyst led to the proposal that Cul+ dissolved in ZnO is the active site for methanol synthesis.

Klier [ 331 also investigated the activity of pure copper metal and pure ZnO and found that neither of the two, used individually, can synthesize methanol in appreciable amounts. Hence, Klier concluded that neither Cue nor ZnO is active for methanol synthesis. However, this conclusion is only valid for Cue and ZnO used in isolation. The situation could be quite different when they are present together, particularly if each site chemisorbs one of the reactants such as hydrogen on ZnO and carbon monoxide on Cu’. Consequently, the experiments of Klier, on the individual sites, do not unequivocally eliminate Cue and ZnO as active sites for methanol synthesis.

Numerous groups [ 25,28,34-361 have provided evidence for the existence of oxidation states of copper in methanol synthesis catalysts during synthesis. However, Bridgewater and co-workers [24,37] dispute the claim that copper can exist in an oxidized form under methanol synthesis conditions. Thus, they also disagree with the proposal of Mehta et al. [25] that Cu’+ is the active site for methanol synthesis. Bridgewater’s study compared methanol production over Raney copper catalysts with production over typical methanol synthesis catalysts using a synthesis gas which contained carbon dioxide, and found that catalysts with more copper surface area had more activity. Bridgewater and co-workers also felt that Klier’s work shows the same trend of increasing copper surface area providing increasing activity. It would be interesting to see what level of methanol production rates would be obtained on Bridgewater’s catalysts without the presence of an oxidizing agent such as carbon dioxide.

Other investigators have also found that increased copper surface area pro- duces increased activity. For example, Chinchen et al. [ 381 measured the copper surface area of Cu/ZnO/Alz03 catalysts by in situ frontal chromatography. They found a linear relationship between the methanol synthesis activity and the total copper surface area of the catalyst.

Okamoto et al. [22] seem to provide support for the observations of Brid- gewater and Chinchen. Their studies on methanol decomposition indicated that, as the concentration of Cue dispersed in ZnO decreased, the concentration of Cu’+ dispersed in ZnO increased and the activity of their methanol

271

synthesis catalysts decreased. This conclusion was in contrast to their earlier proposal [ 291 that Cul+ in ZnO was the active site.

On the other hand, support for the proposal of Mehta et al. [ 25 1 that Cult is the active site for methanol synthesis was provided by Apai et al. 1261 who surface-stabilized Cul+ ions by vacuum heating Cu/CrzOB evaporated films. The concentration of Cu’ + could be varied’with the length of time for vacuum heating. When these surface-stabilized films were compared to a Cu/Cr,O, powder which had been reduced in the typical manner, it was found that the activity of the films was approximately four times greater. Thus, the results of Apai et al. indicate that Cu’+ is the active site for methanol catalysis.

Generally, most of the investigations have been concerned with the adsorption sites for carbon monoxide and carbon dioxide since hydrogen is believed to adsorb on ZnO and is not believed to be a rate-limiting influence [ 391. Thus, the main controversy is whether Cue or Cu’+ is the active site for methanol synthesis, although some investigators feel that all or most of the methanol synthesis pathway occurs on ZnO [ 39,401. This is not an unreasonable conjec- ture since the high-pressure methanol synthesis catalysts consist of ZnO/Cr,O, and no copper. However, Klier [33] has pointed out that ZnO is active for methanol synthesis at 620 K and pressures of 20.4 MPa but not under typical low-pressure methan synthesis conditions. In any case, the contribution of ZnO has received some attention in the literature, scant as it may be.

Cheng et al. [32] studied the decomposition of methanol on the different Crystal faces of ZnO. No decomposition occurred below 580 K which is above typical methanol synthesis conditions. The main question brought out by Cheng et al.‘s work is whether methanol synthesis proceeds through a pathway which occurs in part on the ZnO surface. In other words, is copper, in whatever form, only active for the adsorption of carbon monoxide while the rest of the steps occur on the ZnO surface? If the answer were affirmative, this would agree with a reaction pathway proposed by Edwards and Schrader [ 401.

There are three factors which seem to exclude ZnO as the principal site for methanol synthesis: one is the result from the investigation by Denise et al. [ 411 who found that ZnO is not a unique support. Other supports such as ZrO, also permit the synthesis of methanol. The second factor is the study by Brid- gewater et al. [ 371 in which a Raney Cu/ZnO catalyst was compared to a typical industrial methanol synthesis catalyst. Bridgewater et al. concluded that the active site in both types of catalyst was the same since the activity per unit copper surface area was the same. Furthermore, experiments showed that Ra- ney copper, completely free from zinc, was comparatively active for methanol synthesis. The third factor is that several reports have indicated that ZnO is not modified during the course of methanol synthesis. Okamoto et al. [29] have shown, by XPS analysis, that ZnO does not reduce to zinc until temperatures exceed 770 K. Also, Himelfarb et al. [42] performed in situ static X-ray diffraction studies on a Cu/ZnO/AlzO, catalyst and showed that the reflection

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of ZnO did not change throughout the reaction, thereby showing that ZnO is not reduced.

The published reports discussed above present conflicting viewpoints on the identity of the active site(s) on the Cu/ZnO methanol synthesis catalysts. Most of the above studies have considered carbon monoxide hydrogenation to be the only significant reaction for methanol production. A possible reason for the seemingly contradictory results in the literature is that carbon dioxide hydrogenation also occurs to a significant extent under certain reaction conditions (i.e. in the lower temperature and pressure regimes of methanol synthesis) . Perhaps, as mentioned by Liu et al. [ 181, two main sites are necessary for methanol synthesis, one for activation of carbon monoxide and one for activation of carbon dioxide. If that is the case, then the apparent disagreement over the identity of the active site could be resolved.

The next point of interest is the identity of the most abundant reaction intermediate (MARI) during methanol synthesis. In earlier studies, rate laws were proposed by investigators in which the MARI as well as the rate determining step (RDS) were arbitrarily chosen and many models differed from each other solely on that basis. Even if such models adequately predicted experimental kinetics trends, they often were not based on physical reality. However, in the recent literature, a number of investigators have published results on intermediates actually identified on the catalyst surface.

For example, Edwards and Schrader [40,43,44] conducted an in situ infrared spectroscopy study with Cu/ZnO and Cu/ZnO/Cr,03 catalysts at 5.0 MPa and 473 and 523 K. Although these catalysts were limited to about 10 mol percent copper because of an infrared adsorption constraint, the results are still considered applicable to industrial catalysts (which usually contain at least 30% copper), because the catalysts with 10 mol percent copper showed good selectivity for methanol. Edwards and Schrader found spectroscopic evidence for formate, formaldehyde and methoxy intermediates. Based on their findings, Edwards and Schrader developed a model (but not a rate expression) in which the hydrogenation of the methoxy intermediate was proposed as the RDS. The work of Edwards and Schrader represents some of the first good spectroscopic evidence for formaldehyde as an important intermediate. Their study indicates that the reaction involving formaldehyde occurs rapidly, which may explain why formaldehyde has been difficult to measure in the past.

Surface intermediates were also studied for the reaction of carbon dioxide and hydrogen over a methanol synthesis catalyst. Kieffer et al. [4] identified formates and acetates as intermediate surface species on a Cu/ZnO/Al,O, catalyst by chemical trapping. Tagawa et al. [21] also studied carbon dioxide hydrogenation, this time by infrared spectroscopy and temperature-pro- grammed desorption. In both studies, copper formate was proposed as the MARI for methanol synthesis. The hydrogenation of copper formate was proposed as the RDS. It is interesting to note that it is copper formate which is the species

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identified in the above studies while Edwards and Schrader [40] identified formate on ZnO sites.

In the present study, attempts were made to derive a phenomenological model and associated rate law that would not only include the contribution of carbon dioxide to methanol formation, but also be based on the reported chemical and catalytic mechanisms discussed above. While Klier et al. [ 141 did propose a rate expression which accounts for the effect of carbon dioxide as a catalyst promoter as well as a methanol producer, they contended that the main contribution of carbon dioxide is that of a promoter. Moreover, their carbon dioxide hydrogenation rate term is not based on mechanistic factors. While we do not dispute the fact that carbon dioxide can perform as a catalyst promoter, we feel that the contribution of carbon dioxide as a reactant must be more rigorously taken into account. Thus, we attempted to develop a phenomenological model and associated rate law that includes carbon dioxide hydrogenation to methanol and is based on the following features: (i) the surface reaction of the methoxy intermediate with a dissociatively adsorbed hydrogen atom is the RDS for carbon monoxide hydrogenation; (ii) the methoxy intermediate on Cu’+ sites is the MARI for carbon monoxide hydrogenation; (iii) hydrogen is dissociatively adsorbed on ZnO sites; (iv) the surface reaction of the formate intermediate with a dissociatively adsorbed hydrogen atom is the RDS for carbon dioxide hydrogenation; and (v) the formate intermediate on Cue sites is the MARI for carbon dioxide hydrogenation. It is worth noting that no unequi- vocal evidence has been reported in the literature which proves that the RDS and MARI described above are the true RDS and MARI in the methanol synthesis system. However, the recent spectroscopic evidence in the literature seems to suggest that the proposed RDS and MARI have at least a measure of credibility. It must be remembered that in many earlier studies, the RDS and MARI were chosen without the benefit of such evidence.

EXPERIMENTAL

The reactor system used to perform the kinetics study has been described in detail, along with the results of the experiments used in this modelling study, in a separate paper [ 201. Basically, a 0.400 1 continuous stirred reactor (gradientless) was used at temperatures between 483 and 513 K, pressures of 2.89 MPa and 4.38 MPa and varying CO,/CO/H, ratios. A limited number of CO,/ He/H2 experiments were conducted to study the conversion of carbon dioxide to methanol. BASF methanol synthesis catalyst, type S 3-85, was used in all experiments.

Before the gradientless reactor was used for experiments, step tests were performed to confirm that there was essentially perfect mixing in the fluid phase. Calculations also indicated that heat and mass transfer resistances out- side and inside the catalyst particles were negligible.

274

MODEL DEVELOPMENT

The group at Lehigh University [ 2,14,25,33,36,45-481 have conducted one of the most extensive studies on methanol synthesis and the synthesis catalyst to date. That body of work provides a good starting place for the formulation of a rate equation which includes reported catalytic mechanisms. Thus, for example, as proposed by Mehta et al. [25], our carbon monoxide hydrogenation path is based on carbon monoxide adsorbing on Cul+ sites.

As discussed before, catalytic studies by other groups also provided evidence of the MARI for carbon monoxide hydrogenation. Infrared studies by Edwards and Schrader [40,43,44] over binary and ternary catalysts suggested that the methoxy intermediate was a plausible candidate as the MARI for carbon monoxide hydrogenation. Thus, our model was developed with the methoxy intermediate as the MARL In addition, based on Edwards and Schrader’s study, the hydrogenation of the methoxy intermediate was chosen as the RDS for the carbon monoxide hydrogenation rate term in our model.

Regarding the site for hydrogen adsorption, we chose ZnO sites based on Edwards and Schrader’s infrared analysis [ 40,43,44]. Since Kung [ 391 has indicated that hydrogen adsorption is rapid, we assumed that the concentrations of ZnO sites and adsorbed hydrogen on ZnO sites remain constant during synthesis. Consequently, each hydrogenation path could be represented by a single-site model (e.g. the Cul+ site for carbon monoxide adsorption and the Cue site for carbon dioxide adsorption ) .

Based on the preceding points, the rate equation for carbon monoxide hydrogenation was formulated as follows:

H, +ZnO%H*ZnO*H (8)

H*ZnO*H+ZnO%2 ZnO*H (9)

co + cul+ %co*cul+ (10)

CO*Cul+ +H*ZnO%CHO*Cu’+ + ZnO (11)

CHO*Cul+ + H*ZnO%CH, O*Cul+ + ZnO (12)

CH, O*Cul+ + H*ZnO %CH, O*Cul+ + ZnO (13)

RDS:

CHBO*Cul++H*ZnO%CH,OH*Cul’+ZnO (14)

CH,OH*Cul+%CH,OH+Cul+ (15)

Reactions (8) and (9) are based on Edwards and Schrader’s infrared analysis [ 401. Reactions (11 )- ( 13 ) were combined to give

CO*Cu’++3 H*ZnO%CH,O*Cul++3 ZnO (16)

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A site balance gives

L=Cu’+ +CO*Cu’+ +CHBO*Cul+ + CH,OH*Cul+ (17)

Rearranging reactions (8), (9), (lo), (15) and (16) to solve for [H2*ZnO], [H*ZnO], [CO*Cul+ 1, [ CH30*Cu1+ ] and [ CH,OH*Cul+ 1, respectively, leads to

[H*ZnO*H] =KH2pH2 [ZnO] (18)

[H*ZnO] =KgzKg’p$ [ZnO] (19)

[CO*Cul+] =Kcopco[Cu’+ ] (20)

[CH,O*Cu’+] = Ko,[H*Zn0]3[CO*Cu1+]

[Zn013 (21)

[CH30*Cu1+] =KCHKCOK~~K~2pCOp3H/22[C~1+] (22)

[CH,OH*Cul+] =KMpCH30H[C~1+] (23)

wherep; is the partial pressure of species i in the gas phase and the concentration of each surface species is given in brackets. The rate of reaction for the RDS is given by

~.=$K~~‘K~‘P~~O~~~~*~~~+ [ZnO] -k,KmpCHzoH 0 cuI+ [ZnO] (24)

where 0; is the fractional surface coverage by species i. Converting eqns. (18)- (23) in terms of fractional surface coverage and substituting into eqn. (24) leads to

kiKwKL,K&Kco (PCOP~H~ -PCH~OH/&)

r=1+K~HKf(~K~2K p p312 co co Hz + Kc, PCO + KCH~OH PCHZOH (25)

for the carbon monoxide hydrogenation rate term, where k; is the forward rate constant lumped with [ ZnO ] (since [ ZnO ] is considered constant), and Keg is the thermodynamic equilibrium constant of the overall reaction (1). Based on the kinetics experiments, the Kcopco and the KCH30HpCH30H terms were found to contribute negligibly to the denominator term. Klier et al. [ 141, Kuc- zynski [ 171, and Villa et al. [49] also claimed that the methanol adsorption term is negligible. However, it was found necessary to include adsorption terms in the denominator accounting for hydrogen and carbon dioxide adsorption on Cul+ sites. The final version of the rate expression for carbon monoxide hydrogenation becomes

(26)

The Ku, in the denominator of eqn. (26) is primed as a reminder that, in this case, hydrogen is adsorbing on a Cu’ + site and not a ZnO site.

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Before developing the carbon dioxide hydrogenation rate term, it was necessary to determine a possible site for carbon dioxide activation since both Liu et al.‘s [6,18] and our own kinetics experiments have indicated that carbon dioxide adsorbs on a different site than carbon monoxide. A possible site for carbon dioxide activation was suggested by the investigation of Okamoto et al. [22]. In this study, both the reverse carbon dioxide and reverse carbon monoxide hydrogenation reactions were investigated in order to determine the na- ture of the active site(s) of the methanol synthesis catalyst. Okamoto et al. found that, as the Cul+ concentration increased, the catalyst activity decreased. Metallic copper was considered the major species active for the de- hydrogenation reactions. Okamoto proposed that electron transfer was occurring between Cue and ZnO which apparently contributes to the activity of the Cu’. Herman et al. [ 21 and Mehta et al. [ 251 also noticed fine particles of copper metal dispersed in ZnO along with Cult ions. The optical d-hump tran- sition of the copper particles in the experiments of Herman et al. was slightly shifted from that of free metallic copper, indicating that a small amount of electron transfer between the copper and zinc oxide was occurring [ 2). How- ever, in that study, it was not felt that this effect contributed to the activity of the catalyst.

Based on the above studies as well as on Okamoto et al.‘s [22] and Fleisch and Mieville’s [23] XPS analysis showing that Cu2+ sites are non-existent in the reduced catalyst, carbon dioxide hydrogenation is assumed to occur on Cue sites. Based on Okamoto et al.‘s [22,29], Klier’s [33] and Denise et al’s [41] investigations, we also feel that it is microcrystalline copper particles and not large amorphous copper agglomerates that are the active sites for carbon dioxide hydrogenation. It is noteworthy that Denise et al. [41] suggested the pos- sibility of an oxidized form of copper as active for carbon monoxide hydrogenation and copper metal as active for carbon dioxide hydrogenation.

Again, as in the case for carbon monoxide hydrogenation, the RDS and MARI for carbon dioxide hydrogenation were chosen based on reported spectroscopic evidence. Copper formate has been detected as a surface intermediate in carbon dioxide hydrogenation to methanol by Kieffer et al. [ 41 and Tagawa et al. [ 211. Edwards and Schrader [ 401 also noted a surface formate species during reaction over a Cu/ZnO catalyst for a synthesis gas containing CO/CO,/H,, although they attributed it to a formate absorbed on a ZnO site. Based on these studies, our carbon dioxide hydrogenation rate term includes copper formate as the MARI and the hydrogenation of a copper formate species as the RDS. The rate equation for carbon dioxide hydrogenation is formulated from the following steps:

Hz + ZnO %H*ZnO*H (27)

H*ZnO*H+ZnO+s2 H*ZnO (28)

co, + CuO% co, *cue (29)

CO2 *Cue + H*ZnO% CHOz *Cue+ ZnO (36)

RDS:

CHOz*Cuo+H*ZnOfsCH,O,*Cuo+ZnO

CH, O2 *Cue + H*ZnO%CH3 0, *Cue +ZnO

CHB 02*Cuo + H*ZnO%C!H, OHO*CuO + ZnO

CH3 OHO*Cu”GCH, OH + O*Cu”

O*Cu” + H*ZnO%HO*CuO + ZnO

HO*CuO+ H*ZnO %H, O*Cu” + ZnO

H,0*Cu”%H20+Cuo

L= Cue + HCO, *Cue + CO, *Cue + H, O*Cu”

(31)

(32)

(33)

(34)

(35)

(36)

(37)

(33)

Reactions (32 )- (37 ) were combined and the following rate equation was developed for carbon dioxide hydrogenation.

(39)

where the double primes denote the fact that these are carbon dioxide hydrogenation rate terms. It is noteworthy that the Kco2 in the carbon monoxide hydrogenation rate term [ eqn. (26) ] is not equal to Kho2 since different sites are involved. As in the case of carbon monoxide hydrogenation, the k$’ contains the [ZnO] term which is taken as constant and the K& is the equilibrium constant for reaction (2 ) .

The contribution of unity in the denominator of eqns. (26) and (39) was found to be negligible and was dropped before adding the two equations to form the overall rate expression shown below.

~;K~HK~,K~K~~(P~oP~~-P~H~oH/K~~)

r=Kc~K?i:K?i!2&o~co~~; +&o~Pco~+K;I~PH~

+~FKH%KHKc:~~K~H~~[Pco~PH~ -PcH~oHPH~o/(~cIP~z) 1 K~~22K~12KC02K CHOYPCOZPHZ 1’2 + Go2 P go:! + K+o P&O

RESULTS

(49)

If the constants in the numerators of each hydrogenation rate term in eqn. (40) are divided into their respective denominators, a six-parameter model is formed, i.e.,

278

r=(PcoP&z -PCH~OH/&)+ [Pco~PH~-PCH~OHPH~O/(~~P~~) 1 u&O&2 +@H, +'%O, ~‘Pco,PX~ +b’p:o, +C’P&O

(41)

The parameters in this model were fit to the experimental data with the aid of a multiple nonlinear regression subroutine, LMDIF, obtained from the San- dia Laboratories in New Mexico. First, the parameters for the carbon dioxide hydrogenation rate term, a’, b’, c’, were fit by using data which was obtained by replacing carbon monoxide with helium in the feed [ 201. An average of 0.6 mol percent carbon monoxide was formed during these runs, most likely through the reverse water-gas shift reaction. Although the resulting carbon monoxide could react to form methanol, contribution from this source was assumed negligible. Only the carbon dioxide hydrogenation rate term in eqn. (41) was used to fit a’, b’, and c’. Then, with these parameters fixed, the remaining parameters, a, b, and c, were fit to the data from the CO/H,/CO, experiments with the complete eqn. (41) . Table 2 lists the values of these parameters under the conditions of our experiments.

The overall sum of the squares of the residuals for a, b, and c was of the order of 10W1’ for each temperature while the overall residuals for a’, b’, and c’ varied between lo-l3 for the 513 K data to 10-l” for the 483 K data. The lumped parameter containing the Koo2 term, c or c’ , corresponding to carbon monoxide or carbon dioxide hydrogenation, respectively, was always the largest parameter. This result agrees with the finding of Klier et al. [ 141 that the adsorption strength of carbon dioxide is-higher than for hydrogen or carbon monoxide. Kuczynski [ 171 reported that the pcopH2 term in his model was negligible, at least at pressures of 6 MPa. However, as shown in the values of parameter a in Table 2, that term was significant in our model for each temperature and for both pressures, 2.89 and 4.38 MPa. Experimental data for 513 K at 2.89 and 4.38 MPa and for varying H,/CO/CO, inlet compositions are presented in Table 3.

DISCUSSION

As shown in eqn. (40), inhibition terms involving carbon dioxide and hydrogen adsorption on Cul+ appear in the denominator of the carbon monoxide

TABLE 2

Parameter values for the methanol synthesis rate law, eqn. (41)

Temp [K] a b c a’ b’ C’

483 3.167.lo6 4.595.105 1.493*109 4.687.lo6 1.133.lo6 2.423.lo8 488 2.145.lo6 8.902*107 6.086*108 1.237.lo6 2.864.lo6 1.072.lOa 498 1.000-10~ 6.302*106 1.031~108 3.512-10' 7.877*105 2.140.lo7 513 3.762.105 1.536*107 2.993*107 8.939-10" 2.368*106 7.702*106

279

hydrogenation rate term. Based on the data from our kinetic study, it was found necessary to include these two inhibition terms. A plausible reason for the necessity of adding the hydrogen adsorption term is that the reduction of Cu’ + to Cu* by hydrogen, which eliminates sites for carbon monoxide activation, occurs to a significant extent during methanol synthesis. The necessity of adding an inhibition term involving carbon dioxide adsorption on Cul + sites indicates that carbon dioxide adsorbs strongly on these sites, which in turn inhibits carbon monoxide from adsorbing. Recall that oxidation of Cu’+ to Cu2+ has already been ruled out [ 22,231.

The carbon dioxide hydrogenation rate term in eqn. (40) contains a term in the denominator for carbon dioxide adsorption on Cue sites, As we show later, the carbon dioxide adsorption term does not appear to diminish the carbon dioxide hydrogenation rate under the conditions of our experimental kinetics study [ 201. However, the modelling studies presented here indicate that the carbon dioxide adsorption term must be present in order to correctly predict the experimental plots of overall methanol production rate versus carbon dioxide percentage in feed.

The predicted overall methanol production rate versus mol percent carbon dioxide in the feed is shown in Figs. 1 and 2 and is compared to the experimental rate at 513 K and at pressures of 2.89 MPa and 4.89 MPa. The accumulated error in these experiments was calculat.ed to be i 5%. Duplicate runs at 2 mol percent carbon dioxide in the feed exhibited a 3.4% difference in methanol production rate at 2.89 MPa and a 0.8% difference at 4.38 MPa.

An explanation for the maximum in the overall methanol production rate versus mol percent carbon dioxide in the feed is of particular interest. Two possibilities are: (i) Klier et al.‘s proposal [ 141 that carbon dioxide adsorbs strongly on the Cul+ sites necessary for CO activation, thus causing inhibition of the carbon monoxide hydrogenation rate and/or (ii) Liu et al.‘s proposal [6,18] that the water formed in the methanol synthesis reaction and/or the water-gas shift reaction via reaction (2) or (3)) respectively, adsorbs strongly on sites necessary for carbon dioxide activation, thus causing inhibition of the carbon dioxide hydrogenation rate. However, as shown in Fig. 3, which is a plot of methanol production rate versus mol percent carbon dioxide in a carbon monoxide free feed, the carbon dioxide hydrogenation rate directly increases with increasing carbon dioxide, at least up to 8 mol percent carbon dioxide in the feed. Although this result could be misleading due to the limited amount of data available, it indicates that carbon monoxide and not carbon dioxide hydrogenation is being inhibited, thus leading to the maximum in Fig. 2. These results suggest that, as proposed by Klier et al. [ 141, carbon dioxide is adsorbing on &I” strongly enough to inhibit the methanol production rate. While water might inhibit the carbon dioxide hydrogenation rate, not enough was formed to be an inhibitor under our synthesis conditions, perhaps due to participation in the water-gas shift reaction.

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77

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7.09

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0.25

17.11 2.05 75.34 4.70

2.89

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3 8.63

25.98

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7.25

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0.20 23.98 2.38 68.11 4.88

2.89

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32.59

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2.89

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29.99

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0.19 28.59 2.43 63.43 4.91

2.89

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9.98

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7.78 2.02 86.20 3.38

2.89

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7.78

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8.07

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4.38

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8.25

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32.66 2.03 65.30 7.69

7.89

0.25 27.95 2.67 61.24 7.66

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0.38 14.35 2.58 75.03 7.40

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19.35 5.01 75.64 7.70

6.99

0.55 14.20 5.23 73.03 6.80

4.38

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18.77 7.98 73.26 7.89

6.10

0.77 14.13 8.16 70.84 6.07

281

2I”“l”“fI I”I’I/‘I 0 2 4 6 6

Mel Percent CO, in the Feed

Fig. 1. Predicted (-_) compared to experimental (m) methanol production rate versus mol percent carbon dioxide in the feed at 513 K and 2.89 MPa. The CO:HP mol ratio was held constant at 1.0:3.9.

0 2 4 6 a

Mol Percent CO, in the Feed

Fig. 2. Predicted (-_) compared to experimental (m) methanol production rate versus mol percent carbon dioxide in the feed at 513 K and 4.38 MPa. The CO:H, mol ratio was held constant at 1.0:3.9.

00 “‘i”J”l”J 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14

Mol Percent CO2 I” the Feed

Fig. 3. Experimental methanol production rate versus mol percent carbon dioxide in the feed over a range of temperatures at two pressures. Carbon monoxide was replaced with helium in the feed. The He:H, mol ratio was held constant at 1.0:3.9. ( V ) 483 K, 2.89 MPa; (V ) 483 K, 4.38 MPa; (W) 488 K, 2.89 MPa; (A ) 498 K, 2.89 MPa; (A ) 498 K, 4.38 MPa; (0) 513 K, 2.89 MPa; (0) 513 K, 4.38 MPa.

282

7/ 1

‘a 1 I I, I ,I, I

10 20 30

Mol Percent CO in the Feed

Fig. 4. Predicted (-) compared to experimental ( n ) methanol production rate versus mol percent carbon monoxide in the feed at 513 K and 2.89 MPa. The carbon dioxide mol percent in the feed was held constant at 2% and the balance was hydrogen.

4 I’I”““‘l”l”I’ 0 10 20 30

Mel Percent CO in the Feed

Fig. 5. Predicted (-) compared to experimental (m) methanol production rate versus mol percent carbon monoxide in the feed at 513 K and 4.38 MPa. The carbon dioxide mol percent in the feed was held constant at 2% and the balance was hydrogen.

Figs. 4 and 5 compare the experimental and predicted rate versus mol percent carbon monoxide in the feed under the same conditions as above. The accumulated experimental error was calculated to be k 5%. In both cases, our model is able to predict the experimental trends adequately. The maxima in Figs. 4 and 5 are relatively flat indicating that carbon monoxide negligibly inhibits its own hydrogenation rate. This could be a possible explanation for our finding that the Kcopco term in the denominator of the carbon monoxide hydrogenation rate term was negligible.

Eqn. (41) fit all of the experimental data (66 points) to within an absolute error of ? 18%, with the exception of 4 points which were about 25% off. The quality of the fit is illustrated in Fig. 6, which shows the predicted reaction rate versus the experimental value. The average deviation between the two is 8.4%.

Our overall rate expression includes the effect of carbon dioxide as a methanol producer via the carbon dioxide hydrogenation rate term as well as a methanol production rate inhibitor through the inhibition term in the denom-

283

1 2 3 4 5 6 7 a

Measured Methanol Production Rate (kmol/kg set) x10”

Fig. 6. Scatter plot of predicted versus measured methanol production rate over a range of temperatures at two pressures. (0 ) 483 K, 2.89 MPa; ( 0 ) 483 K, 4.38 MPa; (a ) 488 K, 2.89 MPa; ( 0 ) 488 K, 4.38 MPa; (A ) 498 K, 2.89 MPa; ( A ) 498 K, 4.38 MPa; (7 ) 513 K, 2.89 MPa; (V ) 513 K, 4.38 MPa.

inator of the carbon monoxide hydrogenation rate term. These dual qualities distinguish our model from that of Natta [ 71, since the effect of carbon dioxide was not considered in his model, and from that of Bakemeier et al. [S] whose model only took carbon dioxide inhibition into account. The model of Klier et al. [ 141 does take into account both of the above contributions of carbon dioxide. However, our model differs from that of Klier et al. in several significant respects. In the absence of carbon dioxide our model predicts that methanol production will occur while the model of Klier et al. predicts that the methanol production rate will decrease to zero. Several studies reported in the literature have confirmed methanol production in the absence of carbon dioxide, although to a lesser degree than in the presence of carbon dioxide [ 15-171.

Carbon dioxide hydrogenation in our model is also treated differently from that of Klier et al. In their models, carbon dioxide hydrogenation is accounted for by an empirical term. In our case, we developed a carbon dioxide hydrogenation expression based on mechanistic information reported in literature. To the best of our knowledge, our rate expression is the first which develops a carbon dioxide hydrogenation rate term based on reported mechanistic information.

Additionally, in our model, Cul+ only activates carbon monoxide methanol synthesis whereas, in Klier et al.‘s model [ 141, Cul+ can activate carbon monoxide, carbon dioxide and hydrogen. However, in another report, Klier [33] had suggested that hydrogen may also be activated on ZnO with one of the two forms being the significant kinetic form. In fact, Herman et al. [2] originally proposed that the adsorption of hydrogen occurs on the ZnO surface.

Finally, Klier et al. [ 141 propose the termolecular surface reaction of an adsorbed carbon monoxide with two adsorbed hydrogen molecules as the RDS.

284

However, one would expect termolecular surface reactions to be relatively rare. None of the elementary steps proposed in our model contains reactions which are higher than bimolecular. In the development of our reaction rate model some elementary steps were lumped together for convenience. Although the lumped steps have orders higher than two, they obviously do not represent what is occurring at the molecular level.

CONCLUSIONS

A model and resultant rate expression were developed which includes both carbon monoxide and carbon dioxide hydrogenation rate terms. In our judge- ment, the rate expression given by eqn. (40) adequately models our kinetics data and also retains most of the salient features of mechanistic studies presented in the literature. Reiterating, the important reported mechanistic features included in our proposed phenomenological model are: (i) the surface reaction of the methoxy intermediate with a dissociatively adsorbed hydrogen atom is the RDS for carbon monoxide hydrogenation; (ii) the methoxy intermediate on Cult sites is the MARI for carbon monoxide hydrogenation; (iii) hydrogen dissociatively adsorbs on ZnO sites; (iv) the surface reaction of the formate intermediate with a dissociatively adsorbed hydrogen atom is the RDS for carbon dioxide hydrogenation; and (v) the formate intermediate on Cue sites is the MARI for carbon dioxide hydrogenation.

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