Supercritical Isomerization of n-Butane over Sulfated Zirconia. Part I

Similar trends are observed at constant temperature (473 K) and different residence times (MRT), as shown in Figure 4. The highest conversion is achie...
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Ind. Eng. Chem. Res. 2001, 40, 2767-2772

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KINETICS, CATALYSIS, AND REACTION ENGINEERING Supercritical Isomerization of n-Butane over Sulfated Zirconia. Part I: Catalyst Lifetime Bettina Sander, Markus Thelen, and Bettina Kraushaar-Czarnetzki* Institute of Chemical Process Engineering CVT, University of Karlsruhe, Kaiserstrasse 12, D-76128 Karlsruhe, Germany

The isomerization of undiluted supercritical n-butane over a commercial sulfated zirconia catalyst has been studied at temperatures between 443 and 533 K, pressures between 4.6 and 8.1 MPa, and weight hourly space velocities between 17 and 170 h-1 (equivalent to modified residence times between 5000 and 42 000 kg s/m3). The reaction temperature is the decisive parameter with respect to catalyst stability. At temperatures above 500 K, the catalyst is desulfurized and deactivated. An operating area of steady-state processing could be identified below 500 K. The same catalyst suffers from rapid coke deactivation when the isomerization is carried out in the gas phase. Stable conversion levels realized with pure n-butane under supercritical conditions did not exceed 20%. However, because of the high throughputs, production capacities of isobutane are much higher under supercritical conditions than they are at atmospheric pressure using the same catalyst. Introduction Performing heterogeneously catalyzed reactions under supercritical rather than gas-phase conditions could be an interesting option for prolonging catalyst lifetimes and increasing throughputs. For many interesting reactions, however, the lack of physical data, particularly thermodynamic data for multicomponent systems, and the possibility of phase separation during product formation complicate the design of experiments and the interpretation of data. To gain some fundamental knowledge about supercritical processing with respect to the kinetics of mass transfer and reaction at heterogeneous, preferably porous, catalysts, we sought a relatively simple reaction involving compounds about which sufficient data have been collected. The isomerization of n-butane appears to be a suitable system for this purpose. Many physical data and well-known correlations are available to describe the properties of pure supercritical n-butane and of mixtures with isobutane and the byproducts propane and pentanes. The reaction is not only of academic interest. Isobutane is an important feedstock for the manufacturing of reformulated gasoline.1 As the amounts produced in fluid catalytic cracking are not sufficient to meet the requirements, several companies have developed catalytic processes for the hydro-isomerization of n-butane. The catalysts used are bifunctional, and the reaction is typically carried out under gas-phase conditions at elevated pressure in the presence of hydrogen. Chlori* Corresponding author: Professor Dr. Bettina KraushaarCzarnetzki, Institute of Chemical Process Engineering CVT, University of Karlsruhe, Kaiserstrasse 12, D-76128 Karlsruhe, Germany. Phone: +49-721-3947/4133. Fax: +49-721-608 6118. E-mail: [email protected].

nated Pt/γ-alumina is the preferred catalyst.1-3 Organic chlorinating agents and small amounts of water have to be co-fed to the reaction system to produce a highly acidic aluminum chloride surface, thereby continuously releasing hydrogen chloride. Sulfated zirconia, the catalyst used in our present study, could be the alternative for a more environmentally friendly and noncorrosive isomerization process. The material, by itself or promoted with transition metals such as iron and manganese,4-6 exhibits a high activity; however, it deactivates rapidly because of coking under gas-phase conditions.7,8 Coke deactivation can largely be suppressed by using platinum-promoted sulfated zirconia in the presence of hydrogen in the feed.9,10 However, in this hydro-isomerization, the catalyst deactivates through the loss of active sites in the form of hydrogen sulfide.11 Reduction of sulfate to H2S has also been observed in the absence of platinum and hydrogen; however, the temperatures required are much higher than under hydro-isomerization conditions.12 We selected a nonpromoted sulfated zirconia catalyst for our study. In this part, we evaluate the possibility of prolonging the catalyst lifetime under supercritical, as compared to gas-phase, isomerization conditions. Further work on the detailed description of the kinetics is in progress. Experimental Section Catalyst. The catalyst used in this study was a sample of commercially available sulfated zirconium hydroxide [Zr(OH)4] bound to alumina in the form of cylindrical extrudates with a diameter of 2.45 mm and an average length of 3 mm (MEL Chemicals, MEL XZO707/03). To obtain sulfated zirconia, the extrudates were calcined in air for 3 h at 873 K (heating rate of 2.5

10.1021/ie010079w CCC: $20.00 © 2001 American Chemical Society Published on Web 05/26/2001

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Figure 1. Scheme of the experimental unit: P1 ) n-butane pump, P2 ) pressure control valve, R ) reactor, GC ) gas chromatograph.

K/min). After calcination, the amount of alumina binder in the extrudates was 20 wt %, and the sulfur content was 2.6 wt %. X-ray diffraction indicated that the sulfated zirconia exhibited the tetragonal structure. The calcined extrudates exhibited a BET surface area of 129 m2/g (Micromeritics ASAP 2010 instrument). The specific pore volume was 0.25 cm3/g. As determined by means of mercury porosimetry (Micromeritics Autopore III instrument), the pore size distribution was bimodal with an accumulation of pores exhibiting diameters of 3000 and 6 nm. About 60% of the pore volume was related to the large pores. Unit. Catalytic experiments were carried out with a continuous-flow unit. A simplified scheme is depicted in Figure 1. The fixed-bed reactor was made of stainless steel and had an internal diameter of 15 mm and a length of 350 mm. The catalyst bed, consisting of catalyst particles diluted with SiC particles of 0.2-mm diameter in a volumetric ratio of 1:1, was located almost at the end of the reactor tube. The rest of the reactor was filled with SiC of 0.5- and 1.0-mm diameter. This reactor packing ensures that a plug-flow profile is obtained, that the feed has the desired reaction temperature when contacting the catalyst bed, and that the axial temperature gradient within the catalyst bed is negligible during all measurements. Liquified n-butane was first cooled and then pressurized through an airdriven piston pump (Haskel MCPV-71) while the system pressure was kept constant by means of a pressure control valve downflow to the reactor. Feed and product gases were analyzed on-line with a Hewlett-Packard (HP 6890) chromatograph equipped with a STABILWAX capillary column (crossbond carbowax, Restek Corporation) of 0.25-mm diameter and 60-m length and a flame ionization detector. The analysis program of the gas chromatograph was controlled by HP ChemStation software. The process gases used were air (Messer Griesheim, 99.999% purity) for catalyst activation, nitrogen (Messer Griesheim, 99.996% purity) for purging and as a diluent in the gas-phase experiments, and n-butane (Praxair, 99.5% purity). The gas chromatograph was fed with nitrogen as a carrier gas, and the flame ionization detector gases were air and hydrogen (Messer Griesheim, 99.999% purity).

The design of the unit in particular, the small holdup of reactants in the plant, the restrictions in the supply lines, and the installation of all parts of the unit in ventilated cabinets ensured that the potential hazard factor was found to be far below the limit set by the national guidelines for laboratory safety. Reaction Conditions. Because sulfated zirconia is hygroscopic, the catalyst was activated in situ prior to each catalytic measurement by means of a heat treatment at 673 K (heating rate of 2.8 K/min) for 2 h in a flow of dry air [73 cm3(NTP)/min]. Then, the catalyst was cooled to the desired reaction temperature in a flow of nitrogen [73 cm3(NTP)/min], and the plant pressure was adjusted to the desired value. While the flow to the reactor was interrupted and sent through a bypass, the nitrogen purge gas in the unit was replaced by the feed. When the feed flow and reaction conditions were adjusted to the appropriate values, the reaction was started by closing the bypass and sending the fluid stream to the reactor. Blank experiments with a fixed bed consisting of SiC particles only were performed in order to determine the time required to attain a steady state after the feed flow was switched from the bypass to the reactor. Under gasphase conditions, it takes about 15 min until the n-butane concentration at the outlet of the reactor has a constant value. Under supercritical conditions, the steady state is reached after about 25 min. Supercritical Experiments. n-Butane has a critical temperature of 425 K and a critical pressure of 3.8 MPa. Pitzer’s three-parameter correlation for the compressibility factor was used to calculate the fluid density at different temperatures and pressures.13 In Figure 2, the density of pure n-butane is plotted as a function of the pressure for four different temperatures among which the dotted line on top represents the critical temperature. The critical pressure is indicated by a vertical line. The entire area on the right side of this vertical line at 3.8 MPa and below the dotted line (425 K isotherm) represents supercritical conditions. Within this area, the open ellipses indicate the reaction conditions selected for our catalytic experiments. These conditions were chosen for two reasons. First, it was our aim that the reduced densities (Fr), defined as the ratio of installed density (F) and density at the critical point (Fc), should

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Figure 2. Influence of temperature and pressure on the density of n-butane. Open ellipses indicate reaction conditions examined in this work.

Figure 3. Conversion of n-butane versus time on stream at atmospheric pressure and a space velocity (WHSV) of about 6 h-1.

Table 1. Reduced Densities at Reaction Pressures and Temperatures Tr (K) pr (MPa)

443

478

488

498

533

4.6 6.1 8.1

0.77 1.35 1.61

0.46 -

0.41 0.65 0.99

0.39 -

0.33 0.48 0.70

cover a range from Fr < 1 to Fr > 1. Second, we wanted to be able to adjust the same n-butane density at different temperatures and pressures. Table 1 gives an overview on the reaction pressures, temperatures, and resulting reduced densities applied in our work. The data show that the second objective was not met exactly; however, some of the reduced densities, i.e., 0.65 and 0.70, are sufficiently similar to investigate temperature and pressure effects. Supercritical isomerization of n-butane was performed with full extrudates to keep the pressure drop in the catalyst bed low. One experiment at 488 K and 8.1 MPa was carried out with particles obtained from crushed extrudates exhibiting diameters between 0.215 and 0.3 mm. The conversion of n-butane and the product distribution were the same as in the case of the corresponding experiment involving full extrudates. In addition, the external mass transfer coefficient and the Weisz modulus were estimated for all pressures and temperatures applied, indicating that mass transfer resistances can be neglected even when full extrudates are employed. In the supercritical experiments, the catalyst intake varied between 1.5 and 6.0 g, and the weight hourly space velocity was adjusted between 17 and 170 h-1 (modified residence time between 5000 and 42 000 kg s/m3). Gas-Phase Experiments. Some comparative experiments at atmospheric pressure were performed with a mixture of n-butane and nitrogen in a fixed volumetric ratio of 1:4. The extrudates were crushed and sieved to obtain a particle fraction of 0.215-0.3-mm diameter. The mass of the catalyst varied between 1.0 and 6.0 g, and the weight hourly space velocity was adjusted between 0.6 and 6 h-1. The reaction was carried out at temperatures between 423 and 473 K. Results and Discussion Catalyst Deactivation under Gas-Phase Conditions. The catalytic performance of sulfated zirconia in the gas-phase isomerization of n-butane has been the

Figure 4. Conversion of n-butane versus time on stream at atmospheric pressure: 4, WHSV ) 1 h-1; O, WHSV ) 3 h-1; 0, WHSV ) 6 h-1. Supercritical run: b, pr ) 4.6 MPa, WHSV ) 65 h-1.

subject of numerous papers;14-20 however, the catalyst samples used for these studies have a different origin. Our catalyst was tested in gas-phase experiments in order to compare the results with those obtained in supercritical processing. Figure 3 shows typical plots of the n-butane conversion with time on stream at a space velocity of 6 h-1 and atmospheric pressure. The data show that higher temperatures cause an increase in both the isomerization and the deactivation rate. It is also typical that the deactivation rate decreases with time on stream. Similar trends are observed at constant temperature (473 K) and different residence times (MRT), as shown in Figure 4. The highest conversion is achieved at the highest residence time; however, deactivation is also most pronounced in this case. It should be noted that the equilibrium conversion of n-butane at 473 K amounts to 57%. Hence, the system is far from thermodynamic equilibrium. The selectivity to isobutane is higher than 90% in all runs. The byproducts are propane and pentanes. Production Capacities in the Gas Phase and Supercritical Phase. In Figure 4, conversion data of a run under supercritical conditions at 488 K (filled circles) are shown for comparison together with the gasphase data obtained at 473 K. No activity loss can be observed under supercritical conditions, but the conversion level is low despite the much longer residence time and higher temperature. The selectivity to isobutane is almost the same. It should be noted, however, that the throughput in terms of mass feed per unit time is much

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Figure 5. Conversion of n-butane versus time on stream at pr ) 4.6 MPa and MRT ) 10 000 kg s/m3; different reaction temperatures are indicated.

higher during supercritical isomerization. At a constant conversion of 25% in the gas phase at 473 K, which is quite an optimistic assumption, the production capacity of isobutane would only be one-tenth of the capacity in the supercritical reaction at 488 K and 3% conversion. In the latter case, 1.82 kg of isobutane can be produced per kilogram of catalyst per hour. Effect of Temperature. The effect of temperature has been studied at different pressures and residence times. As an example, Figure 5 shows the conversion of n-butane versus time on stream at 4.6 MPa and a residence time of MRT ) 10 000 kg s/m3. The plots indicate that the catalyst deactivates both at high (533 K) and low (478 and 443 K) temperatures, albeit the activity loss at low temperatures is much less pronounced. Between the two extremes exists a temperature range that enables stable operation. The same trend was observed at different pressures and residence times. In all experiments, a steady conversion could be realized at 488 and 498 K. The deactivation at higher temperatures could have two reasons. At temperatures above 508 K, hydrogen sulfide could be continuously detected in the off-gas, and analyses of the spent catalysts confirmed a maximum sulfur loss of 10 wt % within 180 min. In addition, the catalysts could be deactivated by coke deposition. The catalyst used at 533 K, for instance, contained 2 wt % organic matter after 130 min runtime and a subsequent purge in a flow of nitrogen at 573 K. The color was brownish; however, it changed to green after contact with air at ambient temperature for some minutes. This indicates that the “coke” consists of reactive olefinic or aromatic compounds. Adsorbed organic matter was also detected on the spent catalysts used at lower isomerization temperatures. The catalyst employed for 130 min at 498 K without activity loss, for instance, contained 1.5% of coke. The amount of organic material adsorbed on the spent catalysts was found to decrease with decreasing isomerization temperature. Likewise, the intensity of the color ranged from orange after running at 488 K to light yellow after processing at 443 K. The question arises why the catalysts with the lowest coke contents showed a loss of activity during runtime, whereas catalysts with more coke were stable. We assume that the strongest adsorption sites of the fresh catalysts are rapidly covered with organic compounds after the start of a run at medium isomerization

Figure 6. Conversion of n-butane versus time on stream at Tr ) 488 K and MRT ) 16 000 kg s/m3; different pressures are indicated.

Figure 7. Conversion of n-butane versus time on stream at Tr ) 443 K and MRT ) 10 000 kg s/m3; different pressures are indicated.

temperatures of 488 or 498 K. After this initial period, a steady state is attained at which the rate of formation of deposits and the rate of their desorption and transport into the bulk fluid have the same time constant. At lower reaction temperatures, the rate of formation of deposits is lower, and it takes longer before the steady state is reached. Longer runs at 478 or 443 K are required to determine whether the activity finally reaches a stable level. We refrained from these experiments because they are of minor interest for our further studies because of the very low conversion levels. Alternatively, an initial period of deactivation should be observable at medium reaction temperatures 488 or 498 K. Unfortunately, blank experiments without catalyst showed that unit operation is not stationary within the first 25 min on stream. Hence, catalytic data of this period cannot be interpreted and have always been omitted. The selectivity to isobutane is a function of both the temperature and the conversion level. At equal conversion, the selectivity decreases in the following order of isomerization temperatures: 443 ) 478 e 488 ) 498 < 533 K. The byproducts are propane and pentanes, in addition to traces of ethane and hexanes. Effect of Pressure and Density. In Figures 6 and 7, conversion data are shown at varying pressure and density. The data of Figure 6 were obtained at 488 K and a residence time of 16 000 kg s/m3, whereas those of Figure 7 were taken at 443 K and a residence time of 10 000 kg s/m3. Both figures show that the pressure

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Figure 8. Conversion of n-butane versus modified residence time at 488 K and at different pressures.

Figure 9. Selectivity to isobutane versus n-butane conversion at 488 K and at different pressures.

has no effect on the rate of deactivation, i.e., on the slope of the curves. This is surprising because some authors argue that the solvent power of the fluid increases with its density. Subramaniam et al.,21,22 for instance, reported on the existence of an optimum density for the supercritical double-bond isomerization of 1-hexene over a Pt/γ-Al2O3 catalyst. The authors suppose that their catalyst deactivated at low fluid densities because the solvent power was not sufficient to remove all coke precursors, whereas decreased reaction rates at higher fluid densities were explained by pore diffusion limitations. The optimum fluid density was reported to be 0.85Fc (i.e., a reduced density of 0.85). We have verified (see Experimental Section) that external and internal diffusion resistances for n-butane can be neglected at all pressures under investigation. In our experiments (Figures 6 and 7), the fluid density increased by a factor of 2.1-2.4 when the pressure was increased from 4.6 to 8.1 MPa. Diffusion coefficients are proportional to the reciprocal density.13 Correspondingly, they increase by about the same factor in the opposite order. It could be possible that the effects of changing solubility, which influences the driving force for diffusion, and of changing diffusion coefficient compensate for each other. However, this is speculation as we do not know which organic deposits are covering the catalyst and even less is known about how their solubilities in n-butane change with density. Whereas the deactivation rate is not affected, the data in Figures 6 and 7 show that the conversion drops with increasing pressure. This is even more clearly demonstrated in Figure 8, where conversions at different pressures are plotted versus the residence time MRT. These data were obtained at 488 K under steady-state performance of the catalyst. A negative effect of the feed concentration on the conversion level is typical of saturation behavior of the catalyst which can be described by a kinetic expression of the LangmuirHinshelwood type. However, the data obtained with pure n-butane are not sufficient for a kinetic analysis. Recently, we performed experiments with supercritical mixtures of n-butane in propane, which is not reactive but is completely miscible, to cover a wider range of n-butane concentrations and conversion levels. Analysis of the results is in progress, and we will report on this kinetic study soon. The selectivity to isobutane is hardly affected by pressure and density, as can be demonstrated by the data depicted in Figure 9. Also in these experiments,

the temperature was kept constant (488 K), and the conversion was changed by adjusting the residence time. The small differences in selectivity between data measured at different pressures should not be overestimated because they lie within the experimental error. Note that the selectivity axis in Figure 9 is stretched, covering the range between 70 and 100%, only. Conclusions Under supercritical conditions, n-butane can be isomerized over sulfated zirconia in steady-state operation in the absence of hydrogen and without addition of promotors such as platinum, manganese, or iron to the catalyst. In this part of our study, we evaluated the effect of temperature and pressure on the stability of the catalyst. The operational area of steady-state processing is limited by the reaction temperature. It was found that temperatures above about 500 K promote catalyst deactivation because sulfate is removed as H2S. The possibility that blockage of active sites with organic deposits contributes to the activity loss also cannot be excluded. However, deactivation also occurs at lower temperatures, as evidenced by DTA-TGA analyses of spent catalysts. Nevertheless, steady states can be attained below about 500 K, presumably because formation of organic deposits and removal by dissolution and transport into the bulk fluid occur at the same rate. Pressure and density have no effect on the deactivation but do affect the activity level, indicating saturation behavior of the catalyst at high n-butane concentrations. The kinetics of the reaction are currently analyzed through experiments with supercritical mixtures of n-butane in a solvent in order to broaden the concentration range. Results will be reported soon. Despite the low conversion levels realized with pure n-butane under supercritical conditions, the production capacity of isobutane is much higher than with the same catalyst at atmospheric pressure. The maximum production of isobutane achieved at steady-state conditions (Tr ) 488 K, pr ) 8.1 MPa, WHSV ) 170 h-1) amounted to about 3 kg of isobutane per kilogram of catalyst per hour. Regrettably, performance data for commercial processes are scarce. Taking into account that all commercial processes require continuous chlorination, platinum on the acidic catalyst, and hydrogen in the feed, the new processing route presented seems to be less cost-intensive. Possibly, the advantages of supercritical

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isomerization over sulfated zirconia catalysts are still overcompensated with the higher costs for the compression of gases. Nevertheless, our results are encouraging because there is still ample room for improvement of both the catalyst and the process. Definitions and Symbols The weight hourly space velocity is related to the total mass of the catalyst including the binder

WHSV )

m ˘B (h-1) mCat

(1)

The residence time is the quotient of the reactor volume and the volumetric flow into the reactor. In the case of catalytic reactions, it is useful to define a modified reactor residence time

MRT )

mCat (kg s/m3) V˙

(2)

which is related to the catalyst mass rather than to the reactor volume. The volumetric flow through the reactor depends on the reaction temperature and pressure. At constant WHSV, the MRT increases with decreasing temperature or with increasing pressure because the fluid becomes more dense. The conversion of n-butane (X) and the reactor selectivity to isobutane (Siso) are defined as

X)

˘ B)out (m ˘ B)in - (m (m ˘ B)in

(3)

and

Siso )

(m ˘ iso)out (m ˘ B)in - (m ˘ B)out

(4)

respectively. The catalyst performance under gas-phase and supercritical conditions can be compared on the basis of the production capacity of isobutane

C)

(m ˘ iso)out -1 (h ) mCat

(5)

The definitions above contain the following symbols: m ˘ B ) mass flow rate of n-butane (kg/h) m ˘ iso ) mass flow rate of isobutane (kg/h) mCat ) total mass of catalyst including binder (kg) V˙ ) total volumetric flow rate into the reactor at reaction conditions (m3/s) Acknowledgment The authors thank the German Research Foundation (DFG) for financial support and MEL Chemicals for providing the catalyst. Literature Cited (1) Martino, G. Catalysis for oil refining and petrochemistry, recent developments and future trends. Stud. Surf. Sci. Catal. 2000, 130, 83.

(2) Ware, K. J.; Richardson, A. H. New process isomerizes butanes. Hydrocarbon Process. 1972, 11, 161. (3) Sarathy, P. R.; Suffridge, G. S. Etherify field butanes. Hydrocarbon Process. 1993, 2, 43. (4) Arata, K. Solid Superacids. Adv. Catal. 1990, 37, 165. (5) Yadav, G. D.; Nair, J. Sulfated zirconia and its modified versions as promising catalysts for industrial processes. Microporous Mesoporous Mater. 1999, 33, 1. (6) Arata, K. Preparation of superacids by metal oxides for reactions of butanes and pentanes. Appl. Catal. A 1996, 146, 3. (7) Hong, Z.; Fogash, K. B.; Dumesic, J. A. Reaction kinetic behavior of sulfated-zirconia catalysts for butane isomerization. Catal. Today 1999, 51, 269. (8) Vera, C. R.; Pieck, C. L.; Shimizu, K.; Querini, C. A.; Parera, J. M. Coking of SO42--ZrO2 catalysts during isomerization of n-butane and its relation to the reaction mechanism. J. Catal. 1999, 187, 39. (9) Signoretto, M.; Pinna, F.; Strukul, G. Platinum promoted zirconia-sulfate catalysts: One-pot preparation, physical properties and catalytic activity. Catal. Lett. 1996, 36, 129. (10) Liu, H.; Lei, G. D.; Sachtler, W. M. H. Pentane and butane isomerization over platinum promoted sulfated zirconia catalysts. Appl. Catal. A 1996, 146, 165. (11) Xu, B.-Q.; Sachtler, W. M. H. Reduction of SO4) ions in sulfated zirconia. J. Catal. 1997, 167, 224. (12) Ng, F. T. T.; Horva´t, N. Sulfur removal from ZrO2/SO42during n-butane isomerization. Appl. Catal. A 1995, 123, L197. (13) Reid, R. C.; Prausnitz, J. M.; Sherwood, T. K. The Properties of Gases and Liquids, 3rd ed.; McGraw-Hill Book Company: New York, 1977. (14) Morterra, C.; Cerrato, G.; Pinna, F.; Signoretto, M.; Strukul, G. On the acid-catalyzed isomerization of light paraffins over a ZrO2/SO4 system: The effect of hydration. J. Catal. 1994, 149, 181. (15) Ivanov, A. V.; Vasina, T. V.; Masloboishchikova, E. G.; Khelkovskaya-Sergeeva, L. M.; Zeuthen, P. Study of alkane isomerization on superacidic catalysts on the basis of ZrO2/SO4. Kinet. Catal. 1998, 39 (3), 367. (16) Tran, M.-T.; Gnep, N. S.; Szabo, G.; Guisnet, M. Influence of the calcination temperature on the acidic and catalytic properties of sulfated zirconia. Appl. Catal. A 1998, 171, 207. (17) Gonza´lez, M. R.; Kobe, J. M.; Fogash, K. B.; Dumesic, J. A. Promotion of n-butane isomerization activity by hydration of sulfated zirconia. J. Catal. 1996, 160, 290. (18) Song, S. X.; Kydd, R. A. Activation of sulfated zirconia catalysts. Effect of water content on their activity in n-butane isomerization. J. Chem. Soc., Faraday Trans. 1998, 94 (9), 1333. (19) Tabora, J. E.; Davis, R. J. On the superacidity of sulfated zirconia catalysts for low-temperature isomerization of butane. J. Am. Chem. Soc. 1996, 118, 12240. (20) Li, B.; Gonzalez, R. D. The effect of coke deposition on the deactivation of sulfated zirconia catalysts. Appl. Catal. A 1998, 174, 109. (21) Saim, S.; Subramaniam, B. Isomerization of 1-hexene over Pt/γ-Al2O2 catalyst: Reaction mixture density and temperature effects on catalyst effectiveness factor, coke laydown and catalyst micromeritics. J. Catal. 1991, 131, 445. (22) Ginosar, D. M.; Subramaniam, B. Coking and activity of a reforming catalyst in near-critical and dense supercritical reaction mixtures. Stud. Surf. Sci. Catal. 1994, 88, 327.

Received for review January 25, 2001 Revised manuscript received April 16, 2001 Accepted April 17, 2001 IE010079W