Ind. Eng. Chem. Res. 1989,28,693-697 Klinzing, G. E. Vertical Pneumatic Transport of Solids in the Minimum Pressure Drop Region. Znd. Eng. Chem. Process Des. Dev. 1979,18(3),404. Kunni, D.; Levenspiel, 0. Fluidization Engineering; Krieger Publishing Co.: Huntington, NY, 1977. Leung, L. S.;Jones, P. J. Coexistence of Fluidized Solids and Packed Bed Flow in Standpipes. In Fluidization; Davidson, J. F., Keairns, D. C., Eds.; Cambridge University Press: New York, 1978a; p 116. Leung, L. S.; Jones, P. J.; Knowlton, T. M. Analysis of Moving-Bed Flow of Solids Down Standpipes and Slide Values. Powder Technol. 1978b,7, 1. Reddy, K. V. S.; Pei, D. C. T. Particle Dynamics in Solids-Gas Flow in a Vertical Pipe. Ind. Eng. Chem. Fundam. 1969,8(3),490. Rose, H. E.; Barnacle, H. E. Flow of Suspensions of Non-Cohesive Spherical Particles in Pipes. Engineer 1957,203(5290),898,939.
693
Singh, B. Lean Phase Vertical Pneumatic Conveying of Particulate Material-An Analysis of the Pressure drop in the Non-Accelerating Zone. Chemeca 1977, 77, 315. Stemerding, S. The Pneumatic Transport of Cracking Catalyst in Vertical Risers. Chem. Eng. Sci. 1962,17, 599. Van Swaaij, W. P. M.; Buurman, C.; von Breusel, J. W. Shear Stresses on the Wall of a Dense Gas-Solid Riser. Chem. Eng. Sci. 1973,25,31. Yang, W. C. A Correlation for Solid Friction Factor in Vertical Pneumatic Conveying Lines. AZChE J. 1978,24(3),548. Yoon, S.M.; Kunii, D. Gas Flow and Pressure Drop Through Moving Bed. Znd. Eng. Chem. Process Des. Dev. 1970,9(4),559.
Received for review J u n e 13, 1988 Revised manuscript received February 10, 1989 Accepted February 25, 1989
Influence of Reaction Parameters on the Stereoselectivity of the Nickel-Catalyzed Gas-Phase Hydrogenation of o -Cresol. 1. Kinetics and Reaction Pathway Werner K. Schumann, Oemer M. Kut, and Alfons Baiker* Department of Industrial and Engineering Chemistry, Swiss Federal Institute of Technology (ETH), 8092 Zurich, Switzerland
T h e thermodynamics and kinetics of the nickel-catalyzed gas-phase hydrogenation of o-cresol t o the two stereoisomers, cis- and trans-2-methylcyclohexanol,were studied. T h e kinetics were investigated in a continuous fixed-bed reactor over a commercial nickel on silica catalyst in the temperature range 150-240 "C and a t atmospheric pressure. T h e formation of the stereoisomer products was found t o occur via 2-methylcyclohexanone as intermediate. The rate of formation of this intermediate is much smaller than the rate of the consecutive hydrogenation t o the stereoisomer products. Consequently, the sorption equilibrium of the intermediate 2-methylcyclohexanone is not reached. The conversion of the o-cresol shows a maximum a t about 200 "C, and the hydrogenation to the intermediate is first order in hydrogen. T h e isomer ratio of the 2-methylcyclohexanol production is only weakly influenced by the reaction conditions. T h e kinetic results of the o-cresol hydrogenation are compared t o those obtained for the hydrogenation of o-tert-butylphenol. T h e cis selectivities reached with both reactants are higher than those predicted by equilibrium calculations. Catalytic hydrogenation of alkylphenols represents an economic way for the synthetic of alkylated cyclohexanols, some of which are important intermediates in the fragrance and perfume industry. In several applications, only one of the two stereoisomers of the alkylated cyclohexanols is desired. The effects of different catalysts and operating parameters on the stereoselectivity of the hydrogenation of substituted phenols have been studied by various groups, and the present state of the art in liquid-phase hydrogenation has been reviewed recently by Bartok (1985). To our knowledge, kinetic studies of the stereoselective hydrogenation of alkylphenols are scarce and furthermore confined to the liquid phase only. Recently, Kut et al. (1988) reported the kinetics of the liquid-phase hydrogenation of o-tert-butylphenol over nickel, cobalt, and noble metal catalysts. Gas-phase hydrogenations of aromatics over metal catalysts have been studied extensively using benzene and phenol as reactants (Kiperman, 1986) but not with more complex 2-alkylphenols. With this in mind, we have investigated the kinetics of the nickel-catalyzed gas-phase hydrogenation of o-cresol to the two stereoisomers, cis- and trans-2-methylcyclohexanol. First, a thorough study of the thermodynamics of this reaction is presented, including a discussion of the 0888-5885/89/2628-0693$01.50/0
influence of the various reaction parameters onto the equilibrium concentrations of all reaction components. Subsequently, we shall discuss the influence of the different reaction parameters on the kinetics and the reaction pathway. Finally, in the concluding section, the kinetic results of the o-cresol hydrogenation are compared with corresponding results obtained for the hydrogenation of o-tert-butylphenol.
Experimental Section Materials. The kinetic experiments were performed with o-cresol (STIA, Pratteln, BL, Switzerland, >99% 1, 2-methylcyclohexanone (>99 % 1, 2-methylcyclohexanol (Fluka, Buchs, SG, Switzerland, purum, >98%, x , , - ~=~ 0.27), and o-tert-butylphenol (STIA, Pratteln, BL, Switzerland, >99%). The intermediate product, 2methylcyclohexanone, was produced by hydrogenating o-cresol over palladium (Engelhard, Iselin, NJ, Type 99812). o-Cresol, 2-methylcyclohexanone, and o-tert-butylphenol were purified by distillation. A commercial granular catalyst consisting of 8 wt % nickel supported on silica (Katalysator Werke Huls, Marl, W. Germany, Type H1207) was used for the kinetic measurements. A sieve fraction corresponding to a mean particle size of 2.4 mm was used. The specific surface area of the reduced 0 1989 American Chemical Society
694 Ind. Eng. Chem. Res., Vol. 28, No. 6, 1989 catalyst as determined by krypton adsorption was sp = 0.4 m2.g-', and the total pore volume was up = 0.17 mL-g-' (mercury porosimetry). Apparatus. The experimental setup consisted essentially of a tubular fixed-bed reactor, a metering system for the reactants, and a condenser/gas-liquid separator. The stainless steel reactor tube (i.d. = 25 mm) was 445 mm long. The reactor temperature was controlled by a molten salt bath. The axial temperature profile within the catalyst bed was measured with a thermocouple (0.d. = 5 mm), installed along the center of the reactor. The auxillary equipment prior to the reactor consisted of a metering system for hydrogen, nitrogen, and organic reactant. A condenser, gas-liquid separator (including cold-trap), and a burner for the waste gas were installed behind the reactor. Kinetic Experiments. The kinetic experiments were carried out in the fixed-bed tubular reactor described above. Before use, the catalyst was reduced by treatment with hydrogen (for 15 h at 200 "C and 0.3 mL of H2.min-l followed by 3 h a t 300 "C and 1.0 L of H2.min-'). The catalyst showed a slight deactivation with time on stream (13 mol % over 6.7 h a t 170 "C, p M p = 0.023 bar, and p~ = 0.7 bar) but could be regenerated to its original activity by treatment with hydrogen (12 h at 150 " C and 0.3 L of H2.min-'). Special care was taken that the kinetic measurements were not affected by this deactivation. The hydrogen pressure was varied between 0.36 and 1.08 bar, the reactant pressure between 0.006 and 0.06 bar, and the temperature between 150 and 240 "C. Higher reactant partial pressures were avoided to prevent the condensation of o-cresol. In addition to the experiments with pure ocresol, various other feed mixtures were used (o-cresol:2methylcyclohexanone = 75:25; o-cresol:2-methylcyclohexanol = 60:40; pure 2-methylcyclohexanol (x,,.oL = 0.27)). Analysis. Organic components were analyzed on a FID gas chromatograph. o-Cresol, 2-methylcyclohexanone, and 2-methylcyclohexanol were separated on a 50 % phenylmethylsilicone capillary (HP 19095L) and the two stereoisomers of 2-methylcyclohexanol on a methylsilicone capillary (HP 19091-60250) by on-column injection. The configuration of the 2-methylcyclohexanol isomers was identified by NMR spectroscopy (NMR-Spectra, 1975). Mass-Transfer Limitations. To estimate possible external diffusion effects, experiments were carried out under similar reaction conditions but at different space times. These measurements indicated that film diffusion becomes significant a t NRe = G d p / p < 8 (G = 0.9 kg m-2 e s-1 ). Theoretical calculations of the Damkohler number (ratio of chemical reaction rate to the maximum diffusion rate) and effectiveness factors (ratio of measured reaction rate to chemical reaction rate) confirmed these observations. Reaction conditions were chosen such that NRe > 10 were obtained (Carberry, 1970). To determine the influence of intraparticle diffusion effects, the Weisz-Prater model was determined (Froment and Bischoff, 1979). This indicated clearly that no pore diffusion limitations exist in the macropore range. Mercury porosimetry measurements showed that the catalyst used mainly contained macropores (Schumann, 1988). Calculations of the temperature gradients across the film ( A T < 0.4 "C) surrounding the catalyst particle and within the catalyst particle indicated that such gradients were negligible in our system (Froment and Bischoff, 1979).
Results and Discussion Thermodynamics. Based on the analysis of the product mixtures, the reaction steps presented in Figure 1 were
A
ON
MP
>HZ
CH,
U tr-OL
Figure 1. Scheme of the reaction system.
I
0
I
100
200
300
400
TEMPERATURE
Iocl Figure 2. Thermodynamic equilibrium concentration profile for the hydrogenation of o-cresol: ptot= 1.1 bar; PH = 0.7 bar.
found to be relevant for the thermodynamic consideration of the overall reaction. The thermodynamic equilibrium constants for this reaction scheme were calculated from thermodynamic data available in the literature (Kabo and Frenkel, 1983; Fedoseenko et al., 1983) and from data estimated by an incremental method (Van Krevelen and Chermin, 1961). The temperature dependence of these constants was as follows (Schumann, 1988):
Kpl = exp(15.486
X
103/T - 30.98)
(1)
Kp2= exp(7.529
X
103/T - 15.53)
(2)
Kp3= exp(8.032
X
103/T - 15.69)
(3)
The resulting equilibrium concentration profile, calculated with these constants, is shown in Figure 2. As the temperature increases, the reverse reaction is favored (exothermic reaction). trans-2-Methylcyclohexanol is the more stable isomer, but the fraction of this isomer is reduced with increasing temperature. Since the amount of organic species is unchanged over the whole reaction path, the reactant partial pressure has no influence on the equilibrium concentration. On the other hand, hydrogen is consumed by the reaction. Therefore, a shift to the right is observed with increasing hydrogen partial pressure. Since both isomers consume an equal amount of hydrogen, the equilibrium ratio of cis- to trans-2-methylcyclohexanolis not influenced by the hydrogen partial pressure. To check the calculated equilibrium concentrations, experiments were carried out in an integral reactor using pure 2-methylcyclohexanol as feed. A comparison between the reactor effluent concentrations of these experiments and the thermodynamic equilibrium concentrations (Figure 3) confirms the thermodynamic data described above. Figure 3 also indicates that below approximately 200 "C
Ind. Eng. Chem. Res., Vol. 28, No. 6, 1989 695
0.8
q
~
0 150
170
160
TEMPERATURE [OC
160
190
180
(TR)
I
0
.
I
.
i
I
0.2
.
.
I 1 I
0.4
I
/ I
,
,
/
l
,
,
/ 1
0.6
1
1
0.8
1
I
,
#
'
I
,
I
1 .O
V
I
,
I
1.2
HYDROGEN PARTIAL PRESSURE (PHI [bar]
Figure 4. Rate of o-cresol hydrogenation as a function of hydrogen partial pressure: ym = 1.00, 7'~= 170.3 "C; p m = 0.023 bar; (0) rMp, (A) ON, and !+) rob Table I. Rate of o-Cresol Hydrogenation as a Function of Substrate Partial Pressure (ym0= 1.00, T R= 170.3 "C, p a = 0.7 bar) reaction rate, mmol.min-'.g-' PMP, mbar MP ON OL 5.2 12.3 22.1 33.3 43.9 54.4
-0.0255 -0.0298 -0.0324 -0.0332 -0.0335 -0.0317
240
(TR)
Figure 5. Rate of o-cresol hydrogenation as a function of reaction rm, temperature: ymo = 1.00; pH = 0.7 bar; pMp = 0.023 bar; (0) ( A ) TON, and (+) TOL. Table 11. Isomer Ratio of the 2-Methylcyclohexanol Produced at Various Temperatures ( p m = 0.024 bar; ynrp0 = 1.00; p a = 0.7 bar) cs-OL/(cscs-OL/ (csTR,"C OL + tr-OL) TR,"C OL + tr-OL) 156 165 180 192
i
220
[OCI
0 . 0 4 -I
. '
200
TEMPERATURE
1
Figure 3. Comparison between the reverse reaction and the thermodynamic equilibrium concentration at varying temperatures: you = 1.00;ptot= 1.1bar; pH = 0.7 bar. Full line, calculated thermodyMP, (A) namic equilibrium ratio. Symbols, experimental points: (0) ON, (+) cs-OL, and ( X ) tr-OL.
I i
180
0.0066 0.0110 0.0140 0.0159 0.0173 0.0163
0.0188 0.0188 0.0185 0.0174 0.0162 0.0155
the reverse reaction is only significant for the conversion of 2-methylcyclohexanone to 2-methylcyclohexanol, and not for the conversion of o-cresol to 2-methylcyclohexanone. Influence of Reaction Parameters. In a differential flow reactor, the influence of the various reaction parameters was investigated using pure o-cresol as the feed. Even at low conversions most of the o-cresol was converted to 2-methylcyclohexanol. Table I shows the rate of o-cresol hydrogenation a t various substrate partial pressures. We note that the reaction order in substrate partial pressure rapidly tends to zero, suggesting a strong adsorption of the organic substrate. Figure 4 depicts the rate of o-cresol hydrogenation as a function of the hydrogen partial pressure. Although 2 mol of hydrogen is consumed in the first reaction step, the reaction only exhibits a first-order dependence in hydrogen. The slight curvature of the o-
0.484 0.472 0.440 0.414
206 220 234
0.395 0.388 0.412
cresol consumption line at higher hydrogen partial pressures indicates that hydrogen is only relatively weakly adsorbed under the conditions given. As to the temperature dependence of the hydrogenation, the conversion of o-cresol shows, similar to the hydrogenation of other aromatics (Kiperman, 19861, a maximum at apparoximately 200 O C (Figure 5). It should be emphasized that this maximum is not due to an attained thermodynamic equilibrium. This emerges from a comparison with the thermodynamic data. With regard to the isomer distribution, we note that, within the experimental range, it was only weakly influenced by the reaction conditions. A tendency to slightly higher cis concentrations is observed with decreasing reaction temperature. This emerges from Table 11, where the ratio of cis isomer to the total produced 2-methylcyclohexanol is given at various temperatures. Experiments with a feed mixture consisting of o-cresol and 2-methylcyclohexanone indicated that the conversion of 2-methylcyclohexanone to 2-methylcyclohexanol takes place much more rapidly than the conversion of o-cresol to 2-methylcyclohexanone. However, it should be noted that this feed mixture caused an enhanced deactivation of the catalyst and a shift of the conversion maximum to lower temperatures. Mixing 2-methylcyclohexanol to o-cresol caused a reduction in the conversion relative to the reaction with pure o-cresol (Figure 6), while the amount of produced 2methylcyclohexanone was almost unchanged. A comparison of the ratio of 2-methylcyclohexanone, cis-2methylcyclohexanol, and trans-2-methylcyclohexanolof the reactor effluent from the feed mixture with that obtained from thermodynamic data shows that these overlap. This confirmed that for the conversion of 2-methylcyclohexanone to 2-methylcyclohexanol the thermodynamic equilibrium is reached when the reaction mixture is used as the feed. In addition, a reduced activity is observed with the reaction mixture. This suggests that a fraction of the catalyst surface is occupied with the competing equilibrium reaction. Hence, less active sites are available for the
696 Ind. Eng. Chem. Res., Vol. 28, No. 6, 1989
0'04
5:
I Table 111. Comparison between the Hydrogenation of o -Cresol and o-tert -Butylphenol under Similar Reaction Conditions (N,,, = 299 mmolomin-'; ppe = 0.023 bar; p a = 0.703 bar; T R= 175 "C) substrate mK, g XPH,mol 70 YON/YOL MP 19.6 10.9 0.73 BP 19.6 47.9 11.0 MP 43.5 25.0 0.32 BP 43.5 69.9 5.3
.E
1.0 -1
I
I
- 0 06 150
160
170
TEMPERATURE
180
190
(TR)
roc1 Figure 6. Comparison between the hydrogenation of pure o-cresol, b ~ p= p 100; ( X ) rMp, (0) TON, and (e)roL)and of a feed mixture consisting of o-cresol and 2-methylcyclohexanol CyMpo = 0.60; yoLo = 0.40; (0) F M ~ (A) , PON, and (+I FOL; PH 0.7 bar; Porg,subak = 0.023 bar).
o-cresol adsorption. It also indicates that the adsorption of the intermediate and end product is significant. The free 2-methylcyclohexanolis linked via the thermodynamic equilibrium ratio to the 2-methylcyclohexanone and the desorption of the latter becomes limiting. Reaction Pathway. In the hydrogenation of o-cresol, most of the substrate is converted to 2-methylcyclohexanol and very little intermediate product is found in the product mixture. As mentioned above, the organic substances are relatively strongly adsorbed onto the catalyst surface. These adsorptions are of comparable strength. In addition, the intermediate product is rapidly converted to 2methylcyclohexanol. These factors suggest that the intermediate product formed has not enough time to desorb before it is converted to the end product. Consequently, the sorption equilibrium of the intermediate is not reached. As to the adsorption of the reactants, our experiments indicate that the organic species are adsorbed significantly stronger than hydrogen (see Table I and Figure 4). Catalyst Deactivation. Long-term experiments showed a significant catalyst deactivation. This deactivation was found to be reversible, and the catalyst could be regenerated by treatment with hydrogen. The deactivation behavior was characterized by a significant initial drop in activity, which was followed by a weak progressive deactivation. The rate of deactivation increased with increasing temperature. The latter observation is in agreement with a dehydrogenation reaction of the organic substances to hydrogen-deficient surface species, which is favored by higher temperatures. Hydrogenation of o-tert-Butylphenol. To gain more general information about the hydrogenation of 2-alkylphenols, experiments were carried out using o-tert-butylphenol. The thermodynamic equilibrium concentration profile for this reaction has the same shape as that for the o-cresol hydrogenation. However, the curve is shifted to slightly lower temperatures, the ketone maximum is much higher, and the cis isomer is now more stable than the trans isomer. The rate of hydrogenation of o-tert-butylphenol is significantly higher than the hydrogenation rate of ocresol. In addition, the intermediate product Z-tert-butylcyclohexanone becomes the prevalent product. These
1
0.2 W R
s
0
~
I / , l , , ,
,,, ,;,,,,,,,
TEMPERATURE
,
,
,
(TR)
[OCI
Figure 7. Comparison of the isomer ratio between the hydrogenap ~1.0 (A))and the hydrogenation of 2-terttion of o-cresol C y ~ = ~ (0)); broken lines, thermodynamic equilibbutylphenol ( y ~ =p 1.0 rium ratio; p~ = 0.7 bar; p ~ =p p ~ =p 0.023 bar.
differences in the hydrogenation of o-cresol and o-tertbutylphenol are illustrated by the experimental results listed in Table 111. Similar to the hydrogenation of ocresol, the reaction with o-tert-butylphenol also showed a maximum in the conversion at about 195 "C. Finally, it is interesting to note that the ratio of cis isomer to trans isomer was for both substances higher as predicted by the thermodynamic equilibrium ratio. Calculated and experimental isomer ratios are compared in Figure 7.
Conclusions The nickel-catalyzed hydrogenation of 2-alkylphenols proceeds via the ketone derivative to the two stereoisomers of the 2-alkylcyclohexanol. This has been evidenced for the hydrogenation of o-cresol and o-tert-butylphenol. While most of the o-cresol is directly converted to the alcohol, o-tert-butylphenol shows a high intermediate product concentration (2-tert-butylcyclohexanone).Both substrates exhibit a maximum in conversion at about 200 "C. The hydrogenation rate of o-tert-butylphenol is considerably higher than the rate of o-cresol hydrogenation. The selectivities to the two stereoisomer forms of the alcohol were only weakly influenced by the reaction conditions. In either case, more cis isomer than predicted by thermodynamic equilibrium calculations was produced. Acknowledgment W.K.S. thanks the 'Eidgenossische Stipendienkommission fur auslandische Studierende-and the ETH-Zurich for financial support. This work was initiated by the late Prof. G. Gut, deceased on Oct 4, 1986. This paper is a tribute to him.
Nomenclature d, = equivalent particle diameter, m G = mass velocity, kg.m%-' Kpj = thermodynamic equilibrium constant, bar-Au NRe= Reynolds number
697
Ind. Eng. Chem. Res. 1989,28, 697-702
N,, = molar velocity, mmol-min-l m K = mass of catalyst, g pi = partial pressure, bar s = specific surface area, m2.g-l .P = temperature, K TR = reaction temperature, *C up = catalyst pore volume, m3.g-l xi = liquid mole fraction Xi = conversion, mol % yi = gas mole fraction Greek Symbols v = stoichiometric coefficient g =
viscosity, Pa-s
Subscripts BP = o-tert-butylphenol cis-OL = cis-2-methylcyclohexanol H = hydrogen i = component i j = reaction step j MP = o-cresol (2-methylphenol) OL = 2-methylcyclohexanol ON = 2-methylcyclohexanone PH = phenol derivative tr-OL = trans-2-methylcyclohexanol 0 = initial condition Registry No. MP, 95-48-7; ON, 583-60-8; CS-OL, 7443-70-1; Tr-OL, 7443-52-9; BP, 88-18-6; Ni, 7440-02-0.
Literature Cited Bartok, M. Stereochemistry of heterogeneous metal catalysts; Wiley: New York, 1985. Carberry, J. Chemical and catalytic reaction engineering; McGrawHill: Cambridge, 1970. Fedoseenko, V. I.; Yursha, I. A.; Kabo, G. J. Dokl. Akad. Nauk BSSR 1983, 27, 926. Froment, G. F.; Bischoff, K. B. Chemical reactor analysis and design; Wiley: New York, 1979. Kabo, G. J.; Frenkel, M. L. Thermodynamics of diastereomeric transformations of alcohols with different carbon-skeleton structures. J. Chem. Thermodyn. 1983,15, 377. Kiperman, S. L. Some problems of chemical kinetics in heterogeneous hydrogenation catalysis. In Catalytic hydrogenation; Cerveny, L., Ed.; Elsevier: Amsterdam, 1986. Kut, 0. M.; Datwyler, U. R.; Gut, G. Stereoselective Hydrogenation of 2-tert-Butylphenol to cis-2-tert-Butylphenol. 2. Kinetics of the Liquid-phase Hydrogenation of 2-tert-Butylphenol over Nickel, Cobalt, and Noble Metal Catalysts. Ind. Eng. Chem. Res. 1988, 27, 219. NMR-Spectra, Sadler Research Laboratories, Philadelphia, 17117 M and 21229 M, 1975. Schumann, W. K. Stereoselektive Gasphasenhydrierung von 2-A1kylphenolen. Ph.D. Thesis 8524, ETH-Zurich, 1988. Van Krevelen, D. W.; Chermin, H. A. G. In Landolt-Bdrnstein Zahlenwerte und Funktionen aus Physik, Chemie, Astronomie, Geophysik, Technik, 6th ed.; Springer Verlag: Berlin, 1961; Vol. 11/4, p 18. Received for review J u n e 6, 1988 Accepted February 13, 1989
Influence of Reaction Parameters on the Stereoselectivity of the Nickel-Catalyzed Gas-Phase Hydrogenation of o -Cresol. 2. Kinetic Modeling W e r n e r K. S c h u m a n n , Oemer M. K u t , and Alfons B a i k e r * Department of Industrial a n d Engineering Chemistry, Swiss Federal Institute of Technology (ETH), 8092 Zurich. Switzerland
A modified Langmuir-Hinshelwood model was used to describe the gas-phase hydrogenation of o-cresol over a nickel on silica catalyst. To optimize the various estimated model parameters, a stepwise procedure, which includes differential and integral measurements in a plug-flow reactor, was used. The model is based on nonequilibrium adsorption of the intermediate product, a reaction order in hydrogen and substrate of one for each reaction step, competitive adsorption between hydrogen and organic substances, and thermodynamic restrictions for the reaction system. T h e estimated kinetic parameters were found to be consistent with imposed physical criteria. The kinetic model was also successfully applied to describe the kinetic data of the hydrogenation of o-tertbutylphenol. A critical analysis of the method used for the development of the kinetic model for these complex reaction systems is given. The effort which has been expended to search for kinetic models to describe the gas-phase hydrogenation of simple aromatics (such as benzene) has been reviewed by Kiperman (1986). Van Meerten and Coenen (1977) have shown that the kinetic data of the benzene hydrogenation can be fitted to various models based on different mechanisms. All of these models explained the observed kinetic behavior, including the conversion maximum at increasing temperatures. Chou and Vannice (1987) report that the observed kinetic behavior as well as results obtained by isotopic exchange and deactivation reactions can be described by including an auxillary reaction-the btnzene hydrogenation-to various carbonaceous surface species. To our knowledge, there has been no report in the literature dealing with the kinetic modeling of more complex 0888-5885/89/2628-0697$01.50/0
stereoselective hydrogenations of aromatics, such as alkylphenols, in the gas phase. Kut et al. (1988) reported on the influence of reaction conditions as well as the influence of various metal catalysts on the stereoselective liquid-phase hydrogenation of o-tert-butylphenol. The observed kinetic behavior was described by a modified Langmuir-Hinshelwood-typemodel based on nonequilibrium sorption of the intermediate product. In the preceding paper in this issue, we studied the influence of the reaction parameters on the stereoselectivity of the hydrogenation of o-cresol and o-tert-butylphenol in the gas phase (Schumann et al., 1989). A reaction pathway, which includes nonequilibrium sorption of the intermediate product and thermodynamic restrictions for the reaction system, was postulated. The aim of
0 1989 American Chemical Society
