Kinetic Studies of Isobutane Dehydrogenation and Isobutene

The kinetic data for the hydrogenation and dehydrogenation reactions over the Pt/Sn ... The higher rates observed over Pt/Sn/K/SiO2 and Pt/Sn/K−L-ze...
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Ind. Eng. Chem. Res. 1998, 37, 1717-1723

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Kinetic Studies of Isobutane Dehydrogenation and Isobutene Hydrogenation over Pt/Sn-Based Catalysts Randy D. Cortright, Per E. Levin, and James A. Dumesic* Department of Chemical Engineering, University of WisconsinsMadison, Madison, Wisconsin 53706

Rates of isobutane dehydrogenation and isobutene hydrogenation were measured over Pt/Sn/ SiO2, Pt/Sn/K/SiO2, and Pt/Sn/K-L-zeolite catalysts at temperatures from 673 to 773 K and at various partial pressures for a total pressure of 1 atm. Addition of potassium to Pt/Sn/SiO2 enhances the rate of isobutene hydrogenation and isobutane dehydrogenation, and the Pt/Sn/ K-L-zeolite catalyst exhibits considerably higher rates than Pt/Sn/SiO2 and Pt/Sn/K/SiO2. The kinetic data for the hydrogenation and dehydrogenation reactions over the Pt/Sn catalysts could be described by a four-step Horiuti-Polanyi mechanism, with rate-limiting, dissociative adsorption of isobutane and quasi-equilibrated adsorption of hydrogen and isobutene. The higher rates observed over Pt/Sn/K/SiO2 and Pt/Sn/K-L-zeolite may be attributed to stabilization of the activated complex involved in the dissociative adsorption/desorption of isobutane on the Pt/ Sn clusters within the zeolite pore structure and/or stabilization by the presence of potassium. Introduction Catalytic processes for dehydrogenation of light paraffins are of increasing importance because of the growing demand for olefins such as propylene and isobutene (Resasco and Haller, 1994). It has been shown that Pt/Sn-based catalysts exhibit high selectivity and stability for dehydrogenation of light paraffins at elevated temperatures (e.g., Brinkmeyer and D. F. Rohr, 1987; Cortright and Dumesic, 1994, 1995a,b, Imai and Hung, 1983; Miller, 1986)). Recently, it was shown that tin interacts with platinum on silica to form Pt/Sn alloy particles, which decreases the size of surface Pt ensembles and inhibits the formation of highly dehydrogenated surface species required for the competing isomerization, hydrogenolysis, and coking reactions (Cortright and Dumesic, 1994). In a related investigation (Cortright and Dumesic, 1995a), it was shown that the addition of potassium to Pt/Sn/SiO2 catalysts further increases the selectivity for isobutane dehydrogenation and enhances the rate of isobutane dehydrogenation (Cortright and Dumesic, 1995a). Finally, it was found that supporting tin and platinum on the potassium form of L-zeolite produces a unique material which exhibits high dehydrogenation activity and selectivity (Cortright et al., 1996; Hill et al., 1997). We have reported elsewhere the development of a kinetic model that describes the essential surface chemistry involved in isobutane dehydrogenation and isobutene hydrogenation over Pt/Sn-based materials (Cortright et al., 1996). This model, based on a four-step Horiuti-Polanyi mechanism, incorporated results of microcalorimetric measurements for hydrogen and isobutene adsorption, steady-state kinetics investigations for isobutane dehydrogenation and isobutene hydrogenation, and deuterium-tracing investigations for the dehydrogenation and hydrogenation. In the present paper, we focus on that portion of the analysis dealing with data from the steady-state kinetic studies of isobutane dehydrogenation and isobutene hydrogena* To whom correspondence should be addressed. E-mail: [email protected]. Fax: (608) 262-5434.

tion over three Pt/Sn-based materials. These hydrogenation and dehydrogenation reactions over the silica and L-zeolite-supported Pt/Sn catalysts proceed with little deactivation (Cortright and Dumesic, 1994, 1995a,b), making it possible to collect reaction kinetic data for these reactions over relatively clean surfaces. Experimental Section Platinum was added to Cab-O-Sil using the ionexchange method of Benesi et al. (1968). After impregnation, the resulting 1.3 wt % Pt/SiO2 catalyst was filtered and dried overnight at 390 K. Tin was added to this Pt/SiO2 catalyst by evaporative impregnation of a solution of tributyltin acetate in methanol. This method was used to produce a catalyst with a Pt:Sn atomic ratio of 1:1. After impregnation with tin, the resulting Pt/Sn catalyst was dried overnight in air at 390 K. Potassium was added to a portion of the 1:1 Pt/ Sn/SiO2 by incipient wetness impregnation with an aqueous solution of KOH to produce a catalyst with atomic ratios of Pt:Sn:K of 1:1:3. The resulting Pt/Sn/ K/SiO2 catalyst was dried in air at 390 K overnight. The Pt/Sn/SiO2 and Pt/Sn/K/SiO2 catalysts were treated with flowing oxygen at 673 K for 2 h, followed by reduction for 2 h in flowing hydrogen at 773 K. An L-zeolite-supported Pt/Sn catalyst was prepared by sequential impregnation using methods described elsewhere in detail (Cortright and Dumesic, 1995b). In this method, K-L-zeolite (Tosoh) was calcined in dry air at 873 K for 18 h, followed by incipient wetness impregnation with a solution of tributyltin acetate in methanol under a N2 atmosphere. Following impregnation, the zeolite was dried in air at 393 K for 2 h and treated in a flowing mixture of 25 mol % oxygen in helium at 573 K. The dried Sn/K-L-zeolite was impregnated with an aqueous solution of Pt(NH3)4(NO3)2 (Aldrich) under a N2 atmosphere, treated with a flowing mixture of 25 mol % oxygen in helium at 533 K for 1 h, and reduced at 873 K in flowing hydrogen. Reduction of the Pt/Sn/K-L-zeolite catalyst at 873 K was found necessary to achieve the desired selectivity and stability for the L-zeolite-supported catalyst (Cortright and

S0888-5885(97)00917-2 CCC: $15.00 © 1998 American Chemical Society Published on Web 04/11/1998

1718 Ind. Eng. Chem. Res., Vol. 37, No. 5, 1998

Figure 1. Turnover frequency for (a) isobutane dehydrogenation at 0.016 atm of isobutane and 0.098 atm of hydrogen and (b) isobutene hydrogenation at 0.013 atm of isobutene and 0.13 atm of hydrogen [O, Pt/Sn/K-L-zeolite; 0, Pt/Sn/K/SiO2; and ], Pt/ Sn/SiO2].

Dumesic, 1995b). The resulting Pt/Sn/K-L-zeolite catalyst contained 0.5 wt % Pt and had a Pt:Sn atomic ratio of 1:2.5. Kinetic studies of isobutane conversion were conducted using a quartz, downflow reactor. Helium (Liquid Carbonic) was employed as a carrier gas, and it was purified by passage through copper turnings at 473 K, followed by activated molecular sieves (13X) at 77 K. Isobutane (Liquid Carbonic, 99.5%) was treated by passage over beds of reduced Oxytrap (Alltech) and reduced Ni on alumina at 373 K to remove oxygen and sulfur impurities, respectively. Isobutene (Liquid Carbonic, 99.5%) was treated by passage over a bed of reduced Oxytrap (Alltech) at 298 K. Hydrogen (Liquid Carbonic) was treated by passage through a Deoxo unit (Engelhard) and a bed of molecular sieves (13X) at 77 K. The reactor inlet and outlet gases were analyzed by a HP-5890 gas chromatograph with a FID detector and a 10-ft 15% Squalane Chromsorb PAW column held at 318 K. Reaction rates and kinetic orders for isobutane dehydrogenation and isobutene hydrogenation were determined over the catalysts at 673, 723, and 773 K. Sieved fractions (80-120 mesh) of the various catalysts were mixed with Cab-O-Sil at a dilution ratio higher than 19:1. These conditions were chosen to ensure that the rates were not influenced by transport limitations (Mears, 1971a,b). All data were collected at a total flowrate of 304 sccm, and catalyst amounts were adjusted to maintain conversions less than 15% of the equilibrium conversion values. Turnover frequencies were calculated from the kinetic data on the basis of the number of surface platinum atoms determined by the saturation uptakes of hydrogen at 403 K (22 ( 1, 38 ( 1, and 4.0 ( 0.5 µmol of surface Pt/g for the Pt/ Sn/SiO2, Pt/Sn/K/SiO2, and Pt/Sn/K-L catalysts, respectively).

Figure 2. Turnover frequency for isobutene hydrogenation versus isobutene pressure for 0.13 atm hydrogen pressure at (a) 773 K, (b) 723 K, and (c) 673 K [experimental results: O, Pt/Sn/K-Lzeolite; 0, Pt/Sn/K/SiO2; and ], Pt/Sn/SiO2]. Predicted rates given by solid line.

Results Experimental. Figure 1a shows the effect of temperature on the rates of isobutane dehydrogenation at 0.016 atm of isobutane and 0.098 atm of hydrogen over the three investigated catalysts. This figure shows that the addition of potassium to Pt/Sn/SiO2 nearly doubles the dehydrogenation rate, and the dehydrogenation rates are over an order of magnitude higher over Pt/ Sn/K-L compared to Pt/Sn/SiO2 or Pt/Sn/K/SiO2. The apparent activation energies for isobutane dehydrogenation at these conditions were 74, 71, and 95 kJ/mol for the Pt/Sn/K-L, Pt/Sn/K/SiO2, and Pt/Sn/SiO2 catalysts, respectively. Figure 1b shows the effect of temperature on the rates of isobutene hydrogenation at 0.013 atm of isobutene and 0.13 atm of hydrogen over the three catalysts. As with the dehydrogenation reaction, the addition of potassium to Pt/Sn/SiO2 significantly increases the hydrogenation rate, and the hydrogenation reaction rates are over an order of magnitude higher over Pt/Sn/K-L compared to those of the silicasupported catalysts. The apparent activation energies for isobutene hydrogenation were -50, -33, and -19 kJ/mol for the Pt/Sn/K-L, Pt/Sn/K/SiO2, and Pt/Sn/SiO2 catalysts, respectively. Figure 2 shows the dependence of the hydrogenation rate on the isobutene pressure over the three catalysts

Ind. Eng. Chem. Res., Vol. 37, No. 5, 1998 1719 Table 1. Reaction Orders with Respect to Hydrogen and Hydrocarbon Pressures for Isobutane Dehydrogenation and Isobutene Hydrogenation over Silica- and L-Zeolite-Supported Pt/Sn Catalysts temp. (K)

hydrogen pressure (atm)

C4 pressure (atm)

Pt/Sn/L-zeolite

Pt/Sn/K/silica

Pt/Sn/silica

hydrogenation

reaction

673

hydrogenation

723

hydrogenation

773

hydrogenation hydrogenation hydrogenation dehydrogenation dehydrogenation dehydrogenation dehydrogenation

673 723 773 673 723 773 673

dehydrogenation

723

dehydrogenation

773

0.13 0.13 0.13 0.13 0.13 0.13 0.026-0.33 0.026-0.33 0.026-0.33 0.099 0.099 0.099 0.033-0.099 0.099-0.59 0.033-0.099 0.099-0.59 0.033-0.099 0.099-0.59

0.0013-0.013 0.013-0.066 0.0013-0.013 0.013-0.066 0.0013-0.013 0.013-0.066 0.013 0.013 0.013 0.0066-0.13 0.0066-0.13 0.0066-0.13 0.016 0.016 0.016 0.016 0.016 0.016

0.7 0.6 0.8 0.7 1.0 0.7 0.5 0.6 0.6 0.8 0.8 0.9 -0.5 -0.6 -0.6 -0.5 -0.5 -0.6

0.9 0.6 0.9 0.7 1.2 0.7 0.4 0.4 1.0 0.8 0.8 0.9 -0.5 -0.6 -0.4 -0.7 -0.3 -0.3

0.6 0.6 0.9 0.8 1.2 0.8 0.7 0.9 1.1 1.1 1.0 0.9 -0.2 -0.6 -0.2 -0.3 -0.1 -0.2

at 773, 723, and 673 K. These data were collected at a hydrogen pressure of 0.132 atm over an isobutene pressure range of 0.00066-0.066 atm. Table 1 lists the reaction orders with respect to the isobutene pressure determined from the data presented in Figure 2. The isobutene reaction orders orders increase from values of near 0.6 at lower temperatures and higher isobutene pressures to values near 1.0 at higher temperatures and lower isobutene pressures. Figure 3 shows the dependence of the hydrogenation rate on the hydrogen pressure for the three catalysts at various temperatures. These data were collected at an isobutene pressure of 0.013 Torr and over a hydrogen pressure range from 0.026 to 0.33 atm. Table 1 lists the reaction orders with respect to hydrogen pressure determined from the data presented in Figure 3. The hydrogen reaction order increased from half-order at 673 K to first-order at 773 K over the silica-supported catalysts, while the hydrogenation reaction was halforder in hydrogen over all conditions for the Pt/Sn/K-L catalyst. Figure 4 shows the rate of isobutane dehydrogenation over the three catalysts versus the isobutane pressure at various temperatures. These data were collected at a hydrogen pressure of 0.099 atm at isobutane pressures between 0.0066 and 0.13 atm. The reaction orders with respect to isobutane pressure determined from these data are tabulated in Table 1. The dehydrogenation reaction is nearly first-order in the isobutane pressure over the three investigated catalysts. The rates of isobutane dehydrogenation over the three catalysts versus the hydrogen pressure at various temperatures are shown in Figure 5. These data were collected for a hydrogen pressure range of 0.033-0.59 atm and at an isobutane pressure of 0.016 atm. The reaction orders with respect to hydrogen are shown in Table 1. The reaction order in hydrogen pressure remained nearly constant at -0.5 over the Pt/Sn/Lzeolite catalyst. Negative reaction orders were also observed over the silica-supported catalysts, but these hydrogen orders became less negative at higher temperatures and lower hydrogen pressures. The observed reaction orders and activation energies for isobutane dehydrogenation over Pt/Sn/SiO2 and Pt/Sn/K-L are similar to those values reported earlier over similarly prepared catalysts (Cortright and Dumesic, 1995b). Kinetic Analysis. As noted above, the combined results from reaction kinetics studies, microcalorimetric

Figure 3. Turnover frequency for isobutene hydrogenation versus hydrogen pressure for 0.013 atm isobutene pressure at (a) 773 K, (b) 723 K, and (c) 673 K [experimental results: O, Pt/Sn/K-Lzeolite; 0, Pt/Sn/K/SiO2; and ], Pt/Sn/SiO2]. Predicted rates given by solid line.

measurements of hydrogen and isobutene adsorption, and deuterium tracing during isobutane dehydrogenation and isobutene hydrogenation can explained by a Horiuti-Polanyi mechanism shown below (Cortright et al., 1996):

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Figure 4. Turnover frequency for isobutane dehydrogenation versus isobutane pressure for 0.099 atm hydrogen pressure at (a) 773 K, (b) 723 K, and (c) 673 K [experimental results: O, Pt/Sn/ K-L-zeolite; 0, Pt/Sn/K/SiO2; and ], Pt/Sn/SiO2]. Predicted rates given by solid line.

C4H10 + 2* h C4H9* + H*

(step 1)

C4H9* + 2* h C4H8** + H*

(step 2)

C4H8** h C4H8 + 2*

(step 3)

H2 + 2* h 2H*

(step 4)

On the basis of the deuterium tracing results, it appears that the dissociative adsorption of isobutane is the slow step in this process (Cortright et al., 1996). A similar conclusion was reached by Lok et al. (1986) in their investigation of isobutane dehydrogenation over aluminasupported Pt/Sn catalysts. Therefore, we may assume that steps 2-4 are quasi-equilibrated for the experimental conditions of the present study, leading to the following rate expression for the hydrogenation-dehydrogenation reactions:

[

rDH ) k1θ*2 PiC4H10 -

]

PiC4H8PH2 Keq

(1)

where rDH is the net dehydrogenation rate, k1 is the rate constant for the dissociative adsorption of isobutane, Keq is the overall equilibrium constant for isobutane dehydrogenation, and θ* is the fraction of sites that is free

Figure 5. Turnover frequency for isobutane dehydrogenation versus hydrogen pressure for 0.016 atm isobutane pressure at (a) 773 K, (b) 723 K, and (c) 673 K [experimental results: O, Pt/Sn/ K-L-zeolite; 0, Pt/Sn/K/SiO2; and ], Pt/Sn/SiO2]. Predicted rates given by solid line.

of adsorbed species. Previous kinetic analyses indicated that the fractional coverage of isobutyl species is less than 10-5 at the reaction conditions of this kinetic study (Cortright et al., 1996). Accordingly, the value of θ* is dependent on the adsorption/desorption equilibria for hydrogen and isobutene and can be determined from the following expression:

θ* )

[

-K3 1 + xK4PH2 4PiC4H8

x

1 + 2xK4PH2 + K4PH2 +

]

8PiC4H8 K3

(2)

where K3 is the equilibrium constant for isobutene desorption (step 3) and K4 is equilibrium constant for hydrogen adsorption (step 4). The combination of eqs 1 and 2 provides a rate expression for the dehydrogenation/hydrogenation reactions which is dependent on the values of k1, K3, and K4 (as well as the overall equilibrium constant, Keq). Estimates for these kinetic parameters can be made in terms of physically meaningful quantities such as entropies and enthalpy changes. Transition state theory gives the following expression for k1:

Ind. Eng. Chem. Res., Vol. 37, No. 5, 1998 1721 Table 2. Fitted Parameter Table step 1 forward activation

step 3 quasi-equilibrium

step 4 quasi-equilibrium

catalyst

S1q (J/mol/K)

∆H1q (kJ/mol)

SC4H8** (J/mol/K)

∆H3 (kJ/mol)

SH* (J/mol/K)

∆H4 (kJ/mol)

relative error (%)

sticking coefficient

Pt/Sn/silica Pt/Sn/K/silica Pt/Sn/L-zeolite

273.7 ( 9.3 256.9 ( 8.4 265.4 ( 7.7

60.9 ( 7.6 41.9 ( 7.1 24.3 ( 7.1

253.6 ( 3.9 256.5 ( 3.5 262.2 ( 4.9

118 fixed 118 fixed 118 fixed

22.9 ( 7.0 26.4 ( 4.2 35.1 ( 5.5

-78 fixed -78 fixed -78 fixed

12.9 16.0 14.4

4.0 × 10-3 5.0 × 10-4 1.5 × 10-3

k1 )

kBT (∆Sq1/R) (∆H1q/RT) e e h

Table 3. Entropies at 723 K, 1 atm

(4)

where kB is the Boltzmann constant, h is Planck’s constant, R is the gas constant, and ∆S1q and ∆H1q are the standard entropy change and the standard enthalpy change, respectively, for formation of the transition state from gaseous isobutane. Similarly, the equilibrium constants may be described in terms of standard entropy changes (∆S3 and ∆S4) and enthalpy changes (∆H3 and ∆H4) for adsorption of isobutene and hydrogen. These standard entropy changes can be expressed in terms of known gaseous entropies and entropies of the three surface species: adsorbed H atoms, adsorbed isobutene, and the activated complex for step 1. Accordingly, a quantitative description of the hydrogenation/dehydrogenation reactions may be determined for a given catalyst in terms of six parameters: S1q, SH*, SiC4H8**, ∆H1q, ∆H3, and ∆H4. The values for this set of parameters were determined for each of the Pt/Sn catalysts using a general regression analysis of the apparent rate expression combined with CSTR reactor design equations to fit the combined results of the isobutane dehydrogenation and isobutene hydrogenation kinetic studies. The number of adjustable parameters was reduced to four by fixing the heats of isobutene and hydrogen adsorption (∆H3 and ∆H4) using the results of microcalorimetric experiments (Cortright et al., 1996; Natal-Santiago et al., 1997). Over the range of reaction conditions investigated, the average fraction of free sites was found to be 0.65 (Cortright et al., 1996). This average fraction of free sites was used to estimate average heats of isobutene and hydrogen adsorption of 118 and 78 kJ/mol, respectively. The solid lines shown in Figures 2-5 represent the turnover frequencies predicted using the above rate expression and the fitted parameters listed in Table 2. This table lists the values of the fixed and fitted parameters, the 95% confidence limits for the fitted parameters, and the overall relative error of the fitted kinetic data. It can be seen in Figures 2-5 that the kinetic model provides a good fit of experimental data for the three Pt/Sn catalysts, with average relative errors between 13% and 16% (see Table 2). Table 2 shows the best fit for parameters that describe the forward rate constant of step 1. Similar fits could be obtained using different sets of values for S1q and ∆H1q because of the compensation effect caused by the limited temperature range of this investigation. However, the fitted values for the entropies for the transition state correspond to sticking coefficients between 5.0 × 10-4 and 4.0 × 10-3. These sticking coeffients are within the range of values reported for isobutane dissociation on Pt (110)-(1 × 2) (Weinberg and Sun, 1992). Table 3 lists the values of entropies (Stull et al., 1969) for gaseous atomic hydrogen, molecular hydrogen, isobutane, and isobutene at 723 K and 1 atm. This table

compound

Stotal (J/mol/K)

Sloc (J/mol/K)

Sloc+1D (J/mol/K)

Sloc+2D (J/mol/K)

H H2 i-C4H10 i-C4H8

133.0 156.6 420.6 404.7

0.0 20.8 242.8 227.4

25.6 49.3 285.3 269.8

43.0 69.5 319.5 303.8

also lists the entropies when these species possess 0 degrees of translational freedom (Sloc), 1 degree of translational freedom (Sloc+1D), and 2 degrees of translational freedom (Sloc+2D). The values of Sloc+1D and Sloc+2D were calculated assuming surface densities of (1015)0.5 cm-1 and 1015 cm-2, respectively. Comparison of the fitted entropies in Table 2 with the calculated entropies of Table 3 provides information about the mobility of the various surface species. The fitted entropies for the transition state of step 1 are between Sloc and Sloc+1D, indicating a fairly immobile transition state. Tables 2 and 3 show that the entropies of adsorbed isobutene are also between Sloc and Sloc+1D for the three catalysts, and adsorbed hydrogen atoms are more mobile than adsorbed isobutene. In addition, the entropies for adsorbed isobutene and atomic hydrogen on the various catalysts appear to increase in the following order:

Pt/Sn/K-L > Pt/Sn/K/SiO2 > Pt/Sn/SiO2 These results suggest that adsorbed isobutene and hydrogen are least mobile on Pt/Sn/SiO2 and most mobile on Pt/Sn/K-L. Discussion The apparent activation energies for the dehydrogenation reaction are lower over the three investigated catalysts than the overall heat of the dehydrogenation reaction (122 kJ/mol at 723 K). Accordingly, negative apparent activation energies were observed for the hydrogenation reaction. It is well-known that there is an optimum temperature for the rate of olefin hydrogenation over metals, and the reaction rate decreases with increasing temperature above the optimum temperature ((Horiuti and Miyahara, 1968) and references within). The average hydrogenation/dehydrogenation reactions rates were 2.5 times faster over Pt/Sn/K/SiO2 and 50 times faster over Pt/Sn/K-L compared to those of Pt/Sn/SiO2, as shown in Figures 1-5. These turnover frequencies are based on the number of surface platinum atoms determined from the saturation uptakes of hydrogen at 403 K. If the turnover frequencies are based on the total number of platinum atoms present on these catalysts, then the factor for Pt/Sn/K/SiO2 would increase from 2.5 to 4.3 and the factor would decrease from 50 to 24 for Pt/Sn/K-L. Importantly, these turnover frequencies are still considerably higher

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over the Pt/Sn/K-L catalyst when the rates are based on total platinum. The parameters of Table 2 suggest that the higher reaction rates over Pt/Sn/K-L and Pt/Sn/K/SiO2 may be attributed to the stabilization of adsorbed species on the small clusters of Pt/Sn within the zeolite micropores. For example, equilibrium constants for hydrogen adsorption at the average temperature of 723 K appear to be 2.3 and 19 times higher over Pt/Sn/K/SiO2 and Pt/ Sn/K-L, respectively, compared to Pt/Sn/SiO2. Furthermore, the equilibrium constants for isobutene adsorption at 723 K are 1.4 and 2.8 times higher over Pt/ Sn/K/SiO2 and Pt/Sn/K-L, respectively, compared to Pt/ Sn/SiO2. Moreover, the free energy change to form the activation complex for step 1 at 723 K decreases in the following order:

Pt/Sn/SiO2 > Pt/Sn/K/SiO2 > Pt/Sn/K-L (167 kJ/mol) (160 kJ/mol) (136 kJ/mol) This trend suggests that the activated complex of step 1 is also stabilized over Pt/Sn/K/SiO2 and Pt/Sn/K-L, compared to that of Pt/Sn/SiO2. The negative reaction orders with respect to the hydrogen pressure for isobutane dehydrogenation at low conversions can be attributed to competitive adsorption of hydrogen, which blocks adsorption sites for the dissociation of isobutane. Accordingly, the less-negative reaction orders observed over Pt/Sn/SiO2 and Pt/Sn/K/ SiO2 are caused by lower coverages of hydrogen on these materials, which is reflected in the lower values of the equilibrium constants for hydrogen adsorption on these catalysts. Furthermore, the observed change in hydrogen order with hydrogen pressure is attributed to decreasing hydrogen coverage with decreasing hydrogen pressure and increasing temperature. The rate of isobutane dehydrogenation is nearly firstorder in the isobutane pressure, indicating that the surface coverage by adsorbed isobutyl species is low under the conditions of this study. Furthermore, these kinetic studies were conducted at low conversions, such that the pressure of isobutene (and thus the coverage by adsorbed isobutene) remained low. In contrast, reaction orders with respect to isobutene pressure for the hydrogenation reaction were found to be dependent on temperature and isobutene pressure, as shown in Table 1. This table shows that the isobutene order increased from 0.6 at 673 K and higher isobutene pressures to values near unity at 773 K and lower isobutene pressure. These observations can be explained in terms of a decrease in the surface coverage by adsorbed isobutene at higher temperatures and lower isobutene pressures. In previous investigations of ethene hydrogenation over silica-supported Pt at temperatures below 333 K (Cortright et al., 1991; Rekoske et al., 1992), the reaction is zero-order or negative-order with respect to the olefin pressure, and this behavior was attributed to high coverages of the Pt surface by adsorbed hydrocarbon species. The zeolite pore structure and/or the presence of potassium may enhance the reaction rates over Pt/Sn/ K-L through promotion of the adsorption rates of isobutane and isobutylene. For example, molecularly adsorbed isobutane and isobutene precursor states may be stabilized by the zeolite pore structure or by excess potassium, thereby increasing the rates of adsorption of these hydrocarbons. However, it was previously shown that scaling the rates of isobutane and isobutene

adsorption over Pt/Sn/SiO2 did not adequately describe the observed steady-state kinetics over Pt/Sn/K/SiO2 and Pt/Sn/K-L (Cortright et al., 1996). Accordingly, the enhanced reaction rates over these catalysts cannot be attributed to faster adsorption of the hydrocarbons due to stabilization of the molecularly adsorbed precursor states. Finally, we note that Langmuirian kinetics on a uniform surface were used to analyze the results of this kinetic investigation. The successful application of the proposed rate expression based on the four-step HoriutiPolanyi mechanism is attributed to the relatively clean surface at the reaction conditions investigated (0.45 < θ* < 0.75). Furthermore, the fitted entropy values shown in Table 2 are appropriate for the indicated heats of adsorption, and similar fits could be achieved using different entropy values with different heats of adsorption according to the compensation effect. However, the heats of adsorption are relatively constant over the coverage regime of this investigation (Cortright et al., 1996; Natal-Santiago et al., 1997) (i.e., ∆H4 increases from 70 to 85 kJ/mol as θ* increases from 0.45 to 0.75). Accordingly, the heats of adsorption used for this analysis (see Table 2) are representative for the conditions of this investigation. The results of these kinetic analyses suggest that the mobility of adsorbed isobutene is limited to less than 1 degree of translational freedom and adsorbed atomic hydrogen has approximately 1 degree of translational freedom on Pt/Sn catalysts under the conditions of this study. Importantly, these values of the fitted entropies were determined using reasonable heats of adsorption determined from independent microcalorimetric measurements. Accordingly, these values of surface entropies that describe the observed kinetic results may be useful “rules of thumb” in future analyses of the reactions of hydrogen and hydrocarbons on platinum surfaces at similar reaction conditions. Conclusions Addition of potassium to silica-supported Pt/Sn enhances the rates of isobutane dehydrogenation and isobutene hydrogenation at temperatures from 673 to 773 K. Furthermore, supporting platinum and tin in the potassium form of L-zeolite produces a material that exhibits significantly higher rates for both reactions, compared to silica-supported catalysts. Reaction kinetics data over these Pt/Sn catalysts can be described using a rate expression derived from a four-step HoriutiPolanyi mechanism, assuming quasi-equilibrated adsorption of hydrogen and isobutene adsorption, and ratelimiting, dissociative adsorption of isobutane. This kinetic model contains six physically meaningful parameters, and this set can be reduced to four parameters using heats of adsorption for hydrogen and isobutene determined from microcalorimetric measurements. The results of kinetic analyses suggest that mobility of adsorbed isobutene is limited to less than 1 degree of translational freedom on the Pt/Sn catalysts under the conditions of this study. Adsorbed atomic hydrogen has approximately 1 degree of translational freedom on the surface. Adsorption of isobutene and hydrogen is favored with the addition of potassium to silica-supported Pt/Sn and is further enhanced on the Pt/Sn/K-L catalyst. The higher reaction rates over Pt/Sn/K-L are caused by stabilization of the activated complex for isobutane dissociation on the small clusters of Pt/Sn in

Ind. Eng. Chem. Res., Vol. 37, No. 5, 1998 1723

the zeolitic micropores and/or stabilization by the presence of potassium. Acknowledgment We acknowledge financial support for the experimental work of this research by the National Science Foundation and financial support for the modeling work of this research by the National Center for Clean Industrial and Treatment Technologies. We thank Professor W. E. Stewart for providing us with his general regression analysis software (GREG) and Rod Bain for providing us with his nonlinear equation solver (NNES) Literature Cited Benesi, H. A.; Curtis, R. M.; Studer, H. P. Preparation of Highly Dispersed Catalytic Metals. J. Catal. 1968, 10, 328. Brinkmeyer, F. M.; Rohr, D. F., Jr. Hydrocarbon Conversion Processes. U.S. Patent 4,866,211, 1987. Cortright, R. D.; Dumesic, J. A. Microcalorimetric, Spectroscopic, and Kinetic Studies of Silica-Supported Pt and Pt/Sn Catalysts for Isobutane Dehydrogenation. J. Catal. 1994, 148, 771. Cortright, R. D.; Dumesic, J. A. Effects of Potassium on SilicaSupported Pt and Pt/Sn Catalysts for Isobutane Dehydrogenation. J. Catal. 1995a, 157, 576. Cortright, R. D.; Dumesic, J. A. L-Zeolite-Supported Platinum and Platinum/Tin Catalysts for Isobutane Dehydrogenation. Appl. Catal. A: General 1995b, 129, 101. Cortright, R. D.; Goddard, S. A.; Rekoske, J. E.; Dumesic, J. A. Kinetic Study of Ethylene Hydrogenation. J. Catal. 1991, 127, 342. Cortright, R. D.; Bergene, E.; Levin, P.; Natal-Santiago, M.; Dumesic, J. A. Reactions of Isobutane and Isobutene over Silicaand L-Zeolite-Supported Pt/Sn and Pt/Sn/K Catalysts. In 11th International Congress on Catalysis, Baltimore, MD; Elsevier: New York, 1996; Vol. 101, p 1185. Hill, J. M.; Cortright, R. D.; Dumesic, J. A. Silica- and L-zeolitesupported Pt, Pt/Sn and Pt/Sn/K Catalysts for Isobutane Dehydrogenation. Accepted for publication in Appl. Catal.: A, 1997.

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Received for review December 15, 1997 Revised manuscript received February 19, 1998 Accepted February 22, 1998 IE970917F