JOWRNAL OF CATALYSiS 63, 346-354 (1980)

Gas Phase Hydrogenation of Benzene on Supported Nickel Catalyst

H. A. FRANCO AND M. J. PHILLIPS Department of Chemical Engineering and Applied Chemistry, University of Toronto, Toronto, Ontario, M5S IA4, Canada

Received March 16, 1979; revised August 23, 1979

The kinetics of benzene hydrogenation over a nickel-kieselguhr catalyst was studied in a differential flow microreactor. Temperatures ranged from 392.2 to 468.2 K, benzene partial pressures from 22.66 to 280 Pa, hydrogen partial pressures from 72.39 to 122.79 kPa, and cyclohexane partial pressures from 5.33 to 40 Pa. Cyclohexane was the only product of the reaction. The results were consistent with a mechanism in which all hydrogen addition steps were assumed to have the same rate constant and to form a set of slow steps. This mechanism could predict the maximum in the hydrogenation rate observed at about 458 K. A temperature- programmed desorption study of hydrogen chemisorbed on the catalyst indicated a number of desorption peaks between 358 and 600 K, one of which was coincident with the rate maximum.

INTRODUCTION effect of cyclohexane with some investiga- tors reporting an inhibiting action In spite of the fact that benzene hydro- (3, 4, 12, 13). genation over transition metals is, to some Reported activation energies for benzene extent, a well-known reaction, its mecha- hydrogenation vary widely below 458 K. nism and kinetic behavior are still far from Although most values fall between 41 and being completely understood. The reaction 58 kJ mol-’ (12, 18, 19, 23) for supported proceeds over most Group VIII metals at catalyst, some extreme values of 67 kJ reasonable rates, giving, in general, cyclo- mol-’ (3, 13) and 94.2 kJ mol-’ (28) have hexane as the only product when moderate been observed. temperatures are employed and cracking A maximum in the hydrogenation rate on products above 623 K. Considerable atten- nickel catalyst around 453 K is mentioned tion has been paid to the kinetics of the by several workers (3, 4, 9, 12, 20, 21). benzene hydrogenation on nickel catalysts Similar results have been reported for other (1-9) since, industrially, standard nickel metals (25, 26). Suggested explanations for catalysts are used for the gas-phase hydro- this phenomenon include: poisoning, de- genation and Raney-nickel for the liquid- creasing reactant adsorption, or decreasing phase hydrogenation (10). concentration of intermediate species. Most of the authors who have studied Widely different mechanisms and kinetic this reaction at temperatures below 373 K equations have been proposed reported reaction orders with respect to (3, 7, 11, 23, 24, 27, 28). Recently van benzene pressure of 0.1 to 0.3 Meerten and Coenen (29) based on the (3, 5, 6, 8, 9, ZZ-Z3), while reaction orders work of Snagovskii (8) have proposed three with respect to hydrogen pressure were possible mechanisms, two of them having found to lie between 0.5 and 0.7 (3- an adjustable rate-determining step and the 5, 8, 9, 1I, 14-17). At higher temperatures third, a set of slow steps. (up to 483 K) the temperature dependence shown by the order becomes more EXPERIMENTALMETHODS significant; for hydrogen the order in- Apparatus (Fig. I). The gas phase hydro- creased from 1 to 3 (3, 5, 8, 9, 28). genation was performed in a differential Less agreement exists concerning the flow reactor. The microreactor, (A), con- 346 0021-9517/80/060346-09$02.00/0 Copyright 0 1980 by Academic Press, Inc. All rights of reproduction in any form reserved. BENZENE HYDROGENATION ON SUPPORTED NICKEL 347

L Ml N

‘uJl FIG. 1. Schematic diagram of the experimental apparatus for kinetic studies.

sisted of a 2.5mm-i.d. Pyrex tube with a molecular sieve trap, (M2), before being fritted glass insert on which the catalyst used. was supported. Reproducible flow rates Dark gray tablets containing 55% nickel were obtained by pressure regulation as nickel oxide on kieselguhr (Harshaw, Ni- valves (Bl) and (B2), connected to line 0102, T l/S”) were used for all experi- metering valves (Cl), (C2), and (C3). Fur- ments. The catalyst was ground and a sam- ther reduction was achieved via capillary ple was taken from the 44 to 37 pm fraction. flowmeters(Dl),(D2), and(D3). The reactor The surface area by BET nitrogen adsorp- gas effluent was measured by a soap-bubble tion was 123.24 m2 g-l. flowmeter (E). A carburettor, (F), com- Analysis of feed and reaction products. posed of two evaporators and one con- Reactor etlIuent samples were injected into denser, provided with fairly good precision a gas chromatograph, (0), via a microvol- the benzene vapor at the pressure corres- ume sampling valve, (P). The gas chro- ponding to the condenser temperature. The matographic column, 2 m in length, 1.5 mm temperature of the condenser was kept at i.d. Teflon, packed with 15% Carbowax 20 289.2 + 0.01 K by means of a constant M on Chromosorb W AW, 250-177 pm, temperature circulating system (G). The gave a good separation of cyclohexane and microreactor was placed inside an electric benzene at 323 K. tubular furnace, (H). The temperature of Procedure. A 1-mg sample of nickel cata- the furnace was controllable to within + lyst diluted with 2 mg of ground Vycor, also 0.01 K by an adjustable microset thermo- from the 44 to 37 pm fraction, was loaded regulator, (I). on the fritted glass of the microreactor. Materials. Cyclohexane and thiophene- After evacuation of the purification train free benzene with a certified purity of 99 for up to 1 hr, a hydrogen flow of 20 cm3 mole% obtained from Fisher Scientific Co. (STP) min-’ was established through the were used without further purification. reactor in order to reduce the catalyst. The Hydrogen from the cylinder, (J), passed reactor temperature was then slowly in- through a Deoxo unit, (K), then through creased to 653 K and kept constant for 12 copper catalyst, (L), at 523 K, a molecular hr. This reduction procedure was carried sieve trap, (M), and Ascarite, (N). Helium out between each series of runs to regener- from the cylinder, (J2), passed through a ate the catalyst. A total gas flow of 995 cm3 348 FRANC0 AND PHILLIPS

(STP) min-’ was sufficient to avoid external mass resistance from the reacting gases to the catalyst surface. A total pressure of 1.55 1 Hydrogen Partial Pres.sure=122.52 KPa 122.92 kPa, indicated in the manometer, 1.40.. (Q), was set for each experiment. s Since it has been observed from previous 5 s 1.25. experiments that a series of runs, in which .- different sets of conditions were estab- ;i (J)s 1.10. e lished, resulted in some loss of activity, the D al. (30) method proposed by Yates et was g 0.95. used to bracket all rate measurements with E measurements at a standard set of condi- 2 0.60. SF tions. The catalyst was discarded when a 1 I third regeneration was necessary and re- 0.65 placed with fresh material. 1 0.501 l RESULTS AND DISCUSSION 2.00 2.10 2.20 2.30 2.40 2.50 2.60 Reciprocal Temperature (x10-S) Variation in the partial pressure of ben- FIG. 3. Effect of the temperature (K) on the rate of zene was carried out at temperatures from hydrogenation (g . mol [g cat.]-’ mix*). x , pB = 392.2 to 468.2 K, keeping the partial pres- 199.9 Pa; 0, pe = 162.6 Pa; 0, pB = 113.3 Pa; A, pe = sure of hydrogen constant. The results are 86.6 Pa. shown in Fig. 2. The order of reaction with respect to benzene pressure ranged from ture, increased the rate of reaction. Of about 0.6 to 0.8, depending upon both tem- special interest is the evidence of a maxi- perature and benzene pressure. mum in the rate of hydrogenation which It can be seen that an increase in benzene occurs around 458 K. This can clearly be partial pressure, at any particular tempera- observed in Fig. 3, where the logarithm of reaction rate is plotted versus reciprocal 1.66 1 temperature for four different partial pres- 1 Hydrogen Partial Pressure=122.52 KPa ^ sures of benzene. 1.49 Unusual catalyst poisoning must be ruled I out as a cause of this maximum since g 1.32.. reaction rates measured for decreasing tem- 2 peratures (468.2 to 438.2 K) were the same, .E 1.15.. ;ii within experimental error, as those ob- 5 0” tained for increasing temperatures. A diffu- 6 0.96.. sion limitation might be considered to cause I” 6 this maximum, but the small particle size of m 0.61 z catalyst used together with the low conver- oz y 0.64 sions obtained (about 1%) make the effec-

I tiveness factor close to unity and diffusion resistance negligible. Nor can this phenom- enon be ascribed to an approach to thermo-

0.344 dynamic equilibrium since the high gas flow 1.12 1.35 1.56 1.61 2.04 2.27 2.50 Log Benzene Partial Pressure rate removed the cyclohexane quickly thus favoring the forward reaction. FIG. 2. Effect of the partial pressure of benzene (Pa) on the rate of hydrogenation ig. mol [g cat.]-’ The temperature-programmed desorp- min-I). 0, 453.2 K; A, 468.2 K; Cl, 438.2 K; x, 423.2 tion (TPD) experiments (Fig. 6) showed K; q , 408.2 K; 0, 393.2 K. that the amount of chemisorbed hydrogen BENZENE HYDROGENATION ON SUPPORTED NICKEL 349 on the surface decreased as temperature Hydrogen Partial Pressure=122.52 KPa increased. This may suggest that an Benzene Partial Pressure=159.9 Pa insufficient amount of adsorbed hydrogen 28.00 Temperature=423.2 K could be the cause of the reaction rate I decrease at higher temperatures. 24.00 s A stream of purified helium was used as H 20.00 diluent to change the partial pressure of s x x x I x- x x hydrogen. Benzene partial pressure and g 16.00 reaction temperature were kept constant in P -xl 1 each run. Figure 4 shows that the order of I” 12.00 reaction with respect to hydrogen pressure 5 2 depends upon both temperature and hydro- d coo-. gen pressure. In this case the order ranged from, approximately, 0.5 to 0.7 Experiments were performed with some standard mixtures containing small concen- :lL---0.00 6.66 13.32 19.99 26.66 33.33 39.99 trations of cyclohexane to determine Cyclohexane Partial Pressure whether the product caused any inhibition FIG. 5. Effect of the partial pressure of cyclohexane in hydrogenation rate. Partial pressures of (Pa) on the rate of hydrogenation (g mol [g cat.]-’ hydrogen and benzene were kept constant min-I). at 422.2 K. No effect of cyclohexane pres- sure was observed as Fig. 5 illustrates. influence on the hydrogenation rate. Since These results are entirely reasonable since in the differential reactor conversions were the reactor behaved differentially in the very low and the partial pressure of cyclo- whole range of cyclohexane pressures used hexane produced has to be calculated from and under these circumstances, it is ex- the difference between the total amount of pected that small changes in the partial cyclohexane in the reactor gas etlluent and pressure of cyclohexane would have no that of the feed, initial partial pressures of cyclohexane beyond 40 Pa were not consid- ered. 1.55 Benzene Partial Pressure=1653 Pa I i‘ 1.40~ Reaction Mechanism 0 x A functional optimization by Rosen- .E 1.25. ;ii brock’s method of hill-climbing (31, 32) 5 was used to treat the extensive data, 164 $ 1.10. 5 independent runs. This computer program L” involves a minimization of the function x 0.95. 2 2 G = 2 log rLf;g-rjlOg rLal 1 $ 0.60.- 1 (1) i=l exP 0.65.. by variation of the values for the constants 0.501 in possible rate equations. In total, 12 equa- 4.72 4.79 4.06 4.93 5.00 5.07 5.14 Log Hydrogen Partial Pressure tions were tried. That of Badilla-Ohlbaum et al. (25) which had successfully corre- FIG. 4. Effect of the partial presure of hydrogen (Pa) onthe rate of hydrogenation (g . mol [g . cat.]-’ min-I). lated data in the same pressure range on a 0, 453.2 K; A, 468.2 K; Cl, 438.2 K; x, 423.2 K; H, singly promoted iron catalyst suggested 408.2 K; 8, 393.2 K). that the rate controlling step is the simulta- 350 FRANC0 AND PHILLIPS neous reaction of three molecules of disso- librium up to a rate-determining step and ciatively chemisorbed hydrogen with one further hydrogen addition steps are faster; molecule of adsorbed benzene. That of (b) all hydrogen addition steps have the Kehoe and Butt (6) developed for a 58 wt% same rate constant; and (c) the hydrogen nickel on kielseguhr catalyst described a addition steps are equilibria up to a rate- Rideal mechanism with the molecular addi- determining step, further addition steps tion of hydrogen to absorbed benzene. having the same rate constant as this step. Van Meerten and Coenen (29), working The first of these mechanisms leads to five on the same basis as Snagovskii (a), sug- equations, the third to four, and the second gested three possible mechanisms, all of to only one. them considering the hydrogenation of ben- Of the 12 equations, that one derived on zene as a sequence of hydrogen atom addi- the basis of van Meeten and co-workers’ tions to adsorbed benzene and partially second, (b), mechanism (29) provided the hydrogenated benzene molecules, where: best fit for the data. That is, (a) the hydrogen addition steps are in equi-

r = [I + (bHzpfi)“*J[bBpe(6A5 + 5A4 + 4A3 + 3AZ + 2A + 1) + (A5 + A4 + A3 + A2 + A + I)] (2) where k+ is the rate constant for the addi- developed from Eq. (2) and are in good tion of an adsorbed H atom to an adsorbed agreement with experimental data (the min- partially hydrogenated benzene molecule imum of the function (1) was G = 0.0155 and is equal to k-, the rate constant for the and the standard deviation, 6.22%). reverse reaction, bH2,and b, are the adsorp- Surface coverages calculated by applying tion equilibrium constants for hydrogen and the stationary state condition to the whole benzene; and A equals (k+/k-)(b,,p,,)1/2. group of slow steps (absorption of benzene, Equation (2) was the only one of the 12 to dissociative adsorption of hydrogen, the six give physically acceptable values for all the constants. Values for the activation en- tropy, hs*, and the activation enthalpy, TABLE 1 AH*, of the forward and reverse hydrogen- Results Obtained from Eq. (2)” ation steps and the adsorption entropies and enthalpies for hydrogen and benzene as kJ mol-’ J mol-’ K-l Benzene sites (g-cat.)-’ defined by van Meerten and Coenen (29) are listed in Table 1. It may be noted that AH,* 55.385 the value of the activation enthalpy of the AS,* -43.04 kinetic rate constant is very close to the AH1 94.408 values reported for the reaction over other AS_* - 12.89 A& -33.328 catalysts (4-8, 25, 29). The adsorption en- As”, -43.16 thalpy of benzene is similarly close to A& -24.129 values reported on nickel (6, 25, 29) and on ASi,, - 144.82 other surfaces (33). The fact that the ad- 43 7.7 x 10’S sorption enthalpy of hydrogen is lower than that of benzene suggests that hydrogen is a Where: k, = nB Texp(AS:/R - AH,*/RT) more weakly adsorbed on the surface than is benzene. This is in agreement with the k- = n, Fexp(AS?/R - AH_*/RT) remarks made by Basset er al. (34). bH = exp(A&/R - AH,/Ri’J The curves in Figs. 2, 3, and 4 were be = exp(AJo,/R - AHJRT). BENZENE HYDROGENATION ON SUPPORTED NICKEL 351

hydrogen addition steps, and the desorp- temperature, and second, the reduction in tion of cyclohexane) are given in Table 2. In the surface coverage of (C6H1Jad, which is particular, since benzene and hydrogen are more rapid at temperatures above 458.2 K. considered noncompetitive, the surface oc- However, it is possible that the decreasing cupied by benzene and hydrogenated spe- surface coverage of hydrogen, shown by TPD experiments and also calculated in cies is given by 8, + 2 &CGHG+ajad= 1 and Table 2 may play a secondary role in ac- n=o counting for this phenomenon. that by hydrogen is given by 8, + &bad = 1. A temperature-programmed desorption The total surface coverage of benzene and (TPD) apparatus, similar to the one devel- hydrogenated species decreases with in- oped by Cvetanovic and Amenomiya (36), creasing temperature from 0.51 to 0.17 im- was used. A 0.9780-g sample of the same plying reversible adsorption of benzene. catalyst size fraction taken for the kinetic The coverage of each of (C,H,J,, through study was used in the hydrogen chemisorp- (GHd,, is effectively equal at the lower tion experiments. temperatures studied although at higher Hydrogen, purity 99.5%, was passed temperatures, the coverage as calculated through copper gauze at 623 K and a char- becomes significantly less for the more coal trap immersed in liquid nitrogen and highly hydrogentated species. was stored in a 5-liter reservoir. This same An interesting observation is that the hydrogen could also be fed to the reactor decrease in the surface coverage by hydro- for catalyst reduction. Prepurified nitrogen, gen does not influence, as much as has been purity 99.998%) used as carried was passed thought, the maximum obtained in the rate through a molecular sieve trap and a liquid of hydrogenation. Careful study of Table 2 nitrogen trap filled with 6-mm-diameter will show that the &sH,,j,d at 468.2 K has glass beads. been reduced about 77% with respect to its Before each run, the catalyst was treated value at 398.2 K, while the 13~“)~~has been as follows: after evacuation of the reduced by only 43% in the same range of purification train and reactor for up to 1 hr, temperature. The surface coverage of ben- a hydrogen flow of 300 cm3 (STP) min-’ was zene itself has been decreased appreciably, established through the reactor in order to which may be caused by the low partial reduce the catalyst. The reactor tempera- pressure of benzene being used. ture was slowly increased to 653 K and kept Therefore, the appearance of a maximum at that temperature for 12 hr. The tempera- in the rate of hydrogenation may be a ture was then increased to 923 K and the consequence of two effects: first, the in- catalyst evacuated for 1 hr. The catalyst creasing values for the rate constant k, with was finally cooled to the actual temperature

TABLE 2

Surface Coverage

Temp. (K) f& (G&L, (GWa, GHtJa, (G&L, (C&dad GHdad (Wad 0~

398.2 0.490 0.086 0.086 0.086 0.086 0.085 0.082 0.007 0.993 408.2 0.553 0.075 0.075 0.075 0.075 0.975 0.071 0.006 0.994 418.2 0.611 0.066 0.066 0.066 0.066 0.065 0.060 0.006 0.994 428.2 0.665 0.057 0.057 0.057 0.057 0.056 0.050 0.005 0.995 438.2 0.713 0.050 0.050 0.049 0.049 0.048 0.041 0.005 0.995 448.2 0.756 0.043 0.043 0.043 0.042 0.041 0.033 0.005 0.995 458.2 0.795 0.037 0.037 0.037 0.036 0.033 0.026 0.004 0.996 468.2 0.829 0.032 0.032 0.031 0.030 0.027 0.019 0.004 0.996 352 FRANC0 AND PHILLIPS

of adsorption while pumping was contin- this type of adsorption has readily occurred ued. After adsorption took place, the cata- at lower temperatures and developed as the lyst was again evacuated before the carrier temperature of adsorption increased. gas was diverted into the reactor. The tem- Curve 2 was obtained by adsorbing hy- perature controller-programmer allowed drogen at 122.52 kPa for 30 min at 635 K, the temperature to increase continuously cooling to 298 K in the presence of hydro- from room temperature to 923 K at a rate of gen (the cooling process took an average of 5 K/min although there was a time lag of 15 min), and then adsorbing at 398 K for about 5 min, at the beginning of the run, another 5 hr at the same pressure. The peak before that rate was reached. The carrier previously obtained at 358 seemed to have gas flow was then set at 40 cm3 (STP) mine1 shifted to 393 K. The remaining part of this and no change in the base line was ob- curve showed a large peak at 458 K instead served over the entire range of scanned of 453 K and smaller peaks at 533, 568, and temperatures. Since the reactor had a vol- 603 K. ume of only 1 cm3, the average residence The conditions for adsorption in curve 3 time in the catalyst bed was about 0.05 sec. were: at 122.52 kPa for 30 min at 775 K and Adsorption of hydrogen was always at 298 K for 1.5 hr. A larger peak at 533 K, rapid since no change of pressure could be instead of two smaller peaks at 533 and 568, observed after a few minutes. In Fig. 6, distinguished this desorption chromato- curve 1, hydrogen was adsorbed at 298 K gram from that represented in curve 2. for 5 hr at 122.52 kPa and then evacuated at Since the first two peaks in curves 2 and 3 the same temperature for 5 min. The first were almost equal, the sites corresponding peak appeared at a temperature of 358 K to these two types of adsorption were al- followed by a smaller peak at about 453 K; ready saturated at 635 K or even below. this may suggest that the peak appearing at However, the peak at 533 K in curve 3 was the higher temperature (453 K) represents a increased appreciably indicating that satu- stronger adsorption and therefore a smaller ration had not been achieved. activation energy of adsorption than that at The total area under these desorption the lower temperature. The peak at 453 K chromatograms coincided within ? 6% indi- did not change position in the desorption cating that the amount of hydrogen de- chromatogram when the adsorption tem- sorbed was the same (within experimental perature was changed to 635 and 775 K, error). As seen in Fig. 6, almost all of the curves 2 and 3, respectively, indicating that hydrogen has been desorbed at 823 K. It

40.

298 373 473 573 673 773 (K)

FIG. 6. Temperature-programmed desorption spectra of hydrogen. BENZENE HYDROGENATION ON SUPPORTED NICKEL 353

should be noted that the base line is re- 6. Kehoe. K. P. G., and Butt, J. B., J. APP/. Chem. gained by about 923 K. Consideration of all Biotechnol. 22, 23 (1972). three curves suggests that the first peak 7. Motard, R. L., Burke, R. F., Canjar, L. N., and Beckman, R. B., J. Appl. Chem. (London) 7, 1 obtained at 358 K, curve 1, shifted to a (1957). higher temperature as adsorption tempera- 8. Snagovskii, Yu.S., Lyvbarskii, G. D., and Os- tures increased. These two facts may be trovskii, G. M., K&et. Katal. 7, 232 (1966). explained if the sites for the adsorption of 9. van Meet-ten, R. Z. C., and Coenen, J. W. E., .I. hydrogen are already saturated at room Catal. 31, 37 (1975). 10. Thomas, C. L., “Catalytic Processes and Proven temperature or higher; the appearance of Catalysts.” Academic Press, New York, 1970. subsequent peaks then indicates distinct II. Hartog, F., Tebben, J. H., and Zwietering, P., types of adsorption taking place on the Actes Congr. Int. Catal. 2nd 1960 1, 1229 (1961). same sites. 12. Nicolai, J., Martin, R., and Jungers, J. C., Bull. It is interesting to note the similarities in Sot. Chim. Belg. 57, 555 (1948). 13. Roethe, K. P., Roethe, A., Rosahl, B., and these TPD spectra and those obtained by Gelbin, D., Chem. Ing. Tech. 42, 805 (1970). Amenomiya and Pleizier (35) for hydrogen 14. Aben, P. C., Platteeuw, J. C., and Stouthamer, on a promoted iron catalyst. Their peaks B., Proc. Int. Congr. Catal. (Moscow) 1968, p. identified as H(II1) and H(IV) which ap- 543 (pap. 3 1). peared at approximately 373 and 473 K IS. Coenen, J. W. E., van Meerten, R. Z. C., and Rijnten, H. T., Proc. Int. Congr. Catal. 5 (Palm after adsorption at 873 K seem to be Beach, Fl) 1972 1, 671 (1973). matched by those observed here at 393 and 16. Dixon, G. M., and Singh, K., Trans. Faraday Sot. 453 K after adsorption at 775 K. 65, 1128 (1969). Badilla-Ohlbaum et al. (25) observed a 17. Lyubarskii, G. D., Acres Congr. Znt. Catal. 1960 maximum in the benzene hydrogenation 1, 1242 (1961). 18. Zlotina, N. E., and Kiperman, S. L., Kinef. Katal. rate over singly promoted iron catalyst at 8, 337, 1129 (1967). about 453 K and suggested that the H(IV) 19. Bond, G. C., “Catalysis by Metals.” Academic of Amenomiya and Pleizier (35) might be Press, New York, 1962. the hydrogen species involved in the sur- 20. Herbo, C., and Hauchard, V., Bull. Sot. Chim. face reaction. It seems similarly significant Belg. 52, 35 (1943). 21. Herbo, C., J. Chim. Phys. 47, 454 (1950). in this work that both a TPD peak and the 22. Herbo, C., and Hauchard, V., BuU. Sot. Chim. rate maximum were observed at 453 K. Belg. 55, 177 (1946). Further work on the TPD spectra is in 23. Polanyi, M., and Greenhalgh, R. K., Trans. Fara- progress. day Sot. 35, 520 (1939). 24. Madden, W. F., and Kemball, C., J. Chem. Sot. 54, 302 (1961). 2.5. Badilla-Ohlbaum, R., Neuberg, H. J., Graydon, W. F., and Phillips, M. J., J. Catal. 47, 273 (1977). ACKNOWLEDGMENTS 26. Kubicka, H., J. Catal. 12, 223 (1968). Financial support from the National Research Coun- 27. Nagata, S., Gasimoto, K., Tanama, I., Nisida, cil of Canada is gratefully acknowledged. Kh., and Ivane, S., Kagaku Kogaku 27, 558 ( 1963). 28. Canjar, L. N., and Manning, F. S., .I. Appl. Chem. 12, 73 (1962). 29. van Meerten, R. Z. C., and Coenen, J. W. E., J. REFERENCES Catal. 46, 13 (1977). I. Candy, J. P.,and Fouilloux, P., J. Catal. 38, 110 30. Yates, D. J. C., Taylor, W. F., and Sinfelt, J. H., (1975). J. Am. Chem. Sot. 86, 2996 (1964). 2. Evzerikhin, R. I., and Lyubarskii, G. D., Kin&. 31. Beveridge, G. S. G., and Schechter, R. S., “Opti- Katal. 7, 934 (1966). mization Theory and Practice,” pp. 396-406. Mc- 3. Germain, J. E., Maurel, R., Bourgeois, Y.,, and Graw-Hill, New York, 1970. Sinn, R., J. Chem. Phys. 60, 1219 (1963). 32. Rosenbrock. H. H., Computer J. 3, 175-184 4. Herbo, C., Bull. Sot. Chim. Belg. 50, 257 (1941). (1960). 5. Jiracek, F., Patek, H., and Horak, J., Collect. 33. Barbenics, L., and Tetenyi, P., J. Catal. 17, 35 Czech. Chem. Commun. 33, 3211 (1968). (1970). 354 FRANC0 AND PHILLIPS

34. Basset, J. M., Dalmai-Imelik, G., Primet, M., and 36. Cvetanovic, R. J., and Amenomiya, Y., in “Ad- Mutin, R., J. Caral. 37, 22 (1975). vances in Catalysis” (D. D. Eley, H. Pines, and P. 35. Amenomiya, Y., and Pleizier, G., J. Catal. 28, 442 B. Weisz, Eds.), Vol. 17, p. 103. Academic Press, (1973). New York, 1967.