Supercritical-Phase Alkylation Reaction on Solid Acid Catalysts

Apr 1, 1997 - with an obviously longer lifetime, on a Y-type zeolite catalyst. Lewis acidic sites were found to be the active sites for the target rea...
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Ind. Eng. Chem. Res. 1997, 36, 1458-1463

Supercritical-Phase Alkylation Reaction on Solid Acid Catalysts: Mechanistic Study and Catalyst Development Li Fan,* Ikusei Nakamura, Shintaro Ishida, and Kaoru Fujimoto Department of Applied Chemistry, School of Engineering, University of Tokyo, Hongo, Bunkyo-ku, Tokyo 113, Japan

Alkylation reaction on a zeolite catalyst was conducted in supercritical phase isobutane or isopentane, which was a reactant as well. Compared to the reactions conducted in a liquid phase or gas phase, the supercritical phase alkylation reaction exhibited higher activity, along with an obviously longer lifetime, on a Y-type zeolite catalyst. Lewis acidic sites were found to be the active sites for the target reaction. High-molecular-weight olefins formed through oligomerization reaction readily deactivated the Lewis acidic sites, as in the liquid-phase or gas-phase reaction. Supercritical fluid was able to extract in situ and transport these highmolecular-weight olefins to extend catalyst life successively. La3+-modifed Na-Y zeolite catalyst was found to be active and stable in the supercritical phase alkylation reaction. Introduction Due to the Clean Air Act, increasing attention has been recently paid to the production method of alkylate, which is a very clean burning fuel and has a high MON (motor octane number) with a low octane sensitivity and moderate vapor pressure. As the commercially-operated process of alkylate production uses a liquid acid catalyst such as H2SO4 and HF, a lot of problems in cost, apparatus, and environment should be solved (Cusmano, 1992). As an alternative method, a new synthesis process utilizing solid acid catalysts has been developed for a long time. But no commerical process is realized now, because of the fast catalyst deactivation (Corma and Martinez, 1993). Application of supercritical fluid in heterogeneous catalysis is of great interest recently (Savage et al., 1995). A selected supercritical fluid demonstrated to be able to extract high-molecular-weight hydrocarbons from catalyst micropores, to alleviate catalyst deactivation (Baptist-Nguyen and Subramariam, 1992; Fan et al., 1992), or to reactivate spent catalyst (Madras et al., 1993). In the present paper, we report the effect of the supercritical fluid on alkylation reactions on solid acid catalysts and investigate the catalytic mechanism of this reaction. Experimental Section Catalyst Preparation. Commercially available H-USY (Catalyst & Chemical Ind.; SiO2/Al2O3 ) 8.6, noted as Y-1) catalyst was mainly utilized with a pellet of 20-40 mesh. La-Y zeolites were prepared from H-Y (Catalyst & Chemical Ind.; SiO2/Al2O3 ) 4.5) or Na-Y (LINDE SK-40; SiO2/Al2O3 ) 4.6) zeolites with an aqueous solution of La(NO3)3 through ion-exchanging method. Commercially-available SiO2-Al2O3 (Nikki Chemical; N-631-H, Al2O3 ) 28 wt %) catalyst was also employed in some reactions. Except in a few cases, before reaction the catalysts were calcined in flowing air for 3 h at 450 °C in situ (Nakamura et al., 1995). For investigation on the effect of a trace amount of water in the reactant, pure sodium sulfate was added into the raw reactant to exclude the trace water. * Author to whom correspondence is addressed. Voice/ Fax: (81)-3-5689-0469. Email: [email protected]. S0888-5885(96)00550-7 CCC: $14.00

Apparatus and Analysis. A continuous-flow type fixed-bed reactor equipped with an upstream preheater was utilized. A tailor-made reactant mixture with different compositions was pressurized into the highpressure pump before reaction. Determination of the products was finished on a GC-MASS (Shimadzu GCMS QP1100EX). Quantitative analysis was conducted by on-line gas chromatography (Shimadzu 14-B) with a flame ionization detector where the capillary column (Shimadzu CB-J1) was set. Standard reaction conditions were as follows: reaction temperature, 140 °C; reaction pressure, 6.0 MPa; isobutene/isobutane ) 1/50; W/F ) 40 g‚h/mol. In most figures, reaction time was exhibited in the form of an accumulated feed amount of olefin. Under the standard reaction conditions, 35 mmol of cat‚g-1 was equal to 5.6 h. The critical pressures and critical temperatures of the fluids used are as follows: propane, 96.8 °C and 4.2 MPa; isobutane (2-methylpropane), 135 °C and 3.6 MPa; isopentane (2-methylbutane), 188 °C and 3.3 MPa. Two types of alkylation reactions were studied. One was isobutene with isopentane, and the other was isobutene with isobutane. The paraffins acted here as both reactant and supercritical fluid. Simply, some abbreviations are used in the following text. C3, i-C4, ad i-C5 represent propane, isobutane, and isopentane, respectively. i-C4′ means isobutene (2methylpropene). Results and Discussion 1. Reaction of Isobutene with Isopentane in Various Phases. First, the reaction of i-C4′ in i-C5 was investigated. In Figure 1 the reaction performances of the liquid-phase reaction (50 °C, 3.5 MPa) and supercritical-phase reaction (200 °C, 4.6 MPa) are compared. When the accumulated feed amount of olefin reached 15 mmol of cat‚g-1 (2.4 h), the formation rate of alkylate decreased to zero in the liquid-phase reaction. The formation rate of the olefin oligomer, such as C8 and C12, increased very sharply after the start of the liquid-phase reaction. Concerning the supercriticalphase reaction, even if the initial activity of the alkylate was lower than that of the liquid-phase reaction, the deactivation of the catalyst was not so obvious. Meanwhile, the oligomer formation rate was suppressed to a low level in the supercritical-phase reaction. Detailed © 1997 American Chemical Society

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Figure 1. Effect of supercritical solvent on alkylation of isobuteneisopentane over H-USY catalyst calcined at 450 °C. W/F ) 40 g‚h/mol; i-C4′/i-C5 ) 1/50. Supercritical phase: 200 °C, 4.6 MPa. Liquid phase: 50 °C, 3.5 MPa.

Figure 2. Effect of reaction temperature. Pressure: 4.5 MPa, 3.5 MPa (50 °C). W/F ) 40 g‚h/mol; i-C4′/i-C5 ) 1/50; 450 °C calcined H-USY (Y-1).

analysis of the products showed that the cracking products, such as C1-C3 and C5-C7, formed in a larger amount in the supercritical-phase reaction, which should be attributed to the higher reaction temperature in the supercritical-phase reaction. Also C8 olefin was found in the supercritical-phase reaction products. While the reaction temperature was enhanced from 50 to 200 °C, the i-C4′/i-C5 reaction system changed from the liquid phase to the supercritical phase, as shown in Figure 2. All the product distributions in Figure 2 were obtained when the highest alkylate yield was exhibited at each reaction. It is clear that a lower alkylate yield appeared at high reaction temperature. High reaction temperature favored side reactions or secondary reactions such as olefin oligomerization or cracking, which readily happened on the acidic site of the catalyst. It is clear that, at a reaction of 200 °C, the product selectivity of the cracking reaction was rather higher than that of oligomerization, which indicated that the secondary reaction proceeded more quickly at high reaction temperature. In Figure 3 reaction behaviors with varied reactant compositions (olefin/paraffin ratio) employed are compared. To control the reaction rate, the catalyst was calcined only at 300 °C. From its product distribution, it is clear that a low olefin-to-paraffin ratio was beneficial to alkylation reaction. As i-C5 was utilized as the supercritical fluid as well, its partial pressure was remarkably high. Based on this conclusion, all experiments were conducted at the olefin-to-paraffin ratio of 1/50. 2. Reaction of Isobutene with Isobutane in Various Phases. i-C5 has its critical temperature at 190 °C, but the critical temperature of i-C4 is only 136

Figure 3. Effect of olefin/paraffin ratio of the alkylation activity. 200 °C; 4.5 MPa; 300 °C calcined H-USY (Y-1). Reactant: i-C4′/ i-C5.

Figure 4. Phase effect on the alkylation reaction. i-C4′/i-C4 ) 1/50; W/F ) 40 g‚h/mol; 450 °C calcined H-USY (Y-1).

°C. It is possible to conduct the supercritical-phase alkylation reaction at lower temperature to suppress the influence from oligomerization or other secondary reactions. Low reaction temperature will favor the formation of alkylates, as alkylates are not stable at high reaction temperature. In Figure 4 the durability results of the i-C4′/i-C4 reaction at gas phase, liquid phase and supercritical phase are compared. The catalyst was calcined at 450 °C. Very high initial activity, where the yield of alkylate (2,2,4-trimethylpentane) was as high as 70%, appeared at the liquid-phase reaction, but no activity was observed when the accumulated feed amount of olefin reached 15 mmol/cat‚g. Similarly, obvious catalyst deactivation was observed at the gas-phase reaction. The alkylate yield dropped to zero at the accumulated olefin feed amount of 25 mmol/cat‚g, but for the supercritical-phase reaction, the catalyst deactivation was suppressed by supercritical fluid. While the accumulated olefin feed amount reached 35 mmol/cat‚g (5.6 h), the alkylate yield was still higher than 10%. Although the yield of alkylate decreased with time-on-stream, isobutene conversion was almost 100%. For another liquid-phase reaction whose reaction conditions (125 °C, 5.0 MPa) were slightly different from those of the supercritical-phase reaction, it exhibited deactivation behavior similar to that of the gas-phase reaction. It should be mentioned that supercritical-phase and gas-phase reactions were implemented at 140 °C, but the liquid-phase reaction was conducted at 50 °C. For the gas-phase reaction, high reaction temperature favored side reactions and secondary reactions, deactivating the catalyst easily. Consequently, the initial alkylate yield was only about 30% for the gas-phase reaction, rather lower than that for the supercritical-phase reaction. C5-C7 hydrocarbons formed in relatively high selectivity in gas- or supercritical-phase reaction, which

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Figure 5. Alkylation reaction in different supercritical solvents. i-C4′/i-C4 ) 1/50; W/F ) 40 g‚h/mol; 450 °C calcined H-USY (Y1).

should be attributed to the high reaction temperature in either case. In the liquid-phase reaction, C12 olefin formed in high selectivity, especially after the catalyst deactivation. This can be attributed to the slow diffusion of the reactant and produced olefin oligomer inside the liquid-phase reaction medium. The residence time of the olefins would be prolonged in the catalyst bed at the liquid-phase reaction, compared to that at the supercritical-phase reaction. 3. Extraction Capacity of Supercritical Fluid. Supercritical fluid exhibits a different extraction capacity if their molecular weight or molecular structure is changed. In Figure 5, the durability of the alkylation reaction activity where i-C5, i-C4, and C3 at the supercritical phase were utilized as the reaction media is investigated. It should be mentioned that i-C5 and i-C4 were reactants as well. For the i-C5 case, catalyst deactivation was hardly observed, exhibiting a low initial yield of alkylate of 20%. On the contrary, a high initial alkylate yield of about 60% was reached in the reaction implemented in supercritical phase C3, while catalyst deactivation was most obvious among these. The reaction conducted in i-C4 showed moderate results, compared to the reactions in i-C5 or C3. The reason for the low alkylate yield in i-C5 reaction should be its high reaction temperature. High reaction temperature favored the side reactions, reducing the selectivity of alkylate. Indeed, C5-C7 hydrocarbon products formed in high selectivity in the high-temperature-conducted reaction conducted in supercriticalphase isopentane. It is possible that a hydrocracking reaction, a side reaction, occurred at the same site where the alkylation reaction proceeded. The reaction temperature of the C3-used reaction was low, and the side reactions were suppressed. The deactivation of this reaction should be mainly due to the poor extraction capacity of the propane medium. Low solubility of the catalyst poison into supercritical-phase C3 at the reaction conditions led to catalyst deactivation. In Figure 6, different supercritical fluids mentioned above were used to extract the deactivated catalyst, to prove their regeneration capacity. After i-C5 was used to extract the used catalyst for 1 h, the initial activity of the alkylation reaction was restored to about 70% of the first initial activity. For the extraction where i-C4 was employed, a similar effect was obtained, but if C3 was used to regenerate the deactivated catalyst, no significant result was available. Regenerated catalyst still showed low alkylate yield and high oligomer yield. The results here are in good accordance with those reported in Figure 5. It is clear that catalyst deactivation was closely related to the extraction capacity of the accompanied supercritical fluid. In situ extraction of the formed catalyst poison determined the catalyst

Figure 6. Extraction ability of different supercritical solvents. 450 °C calcined H-USY (Y-1).

durability. Further, failure of the extraction of these catalyst poisons improved the oligomerization reaction of olefin. The extracted hydrocarbons by supercritical-phase i-C5 were analyzed precisely. No hydrocarbons other than C12 olefin (two isomers) were detected. It is clear that C12 olefin derived from the oligomerization reaction deposited onto the catalytic site and deactivated the catalyst. More concretely, high-molecular-weight olefin such as C12, which has a high electron density, could combine strongly with the Lewis acid site inside the catalyst framework to obstruct the participation of Lewis acid in the alkylation reaction. In situ extraction of C12 olefin from these acidic sites was critical to keep catalyst durability. As the extraction ability of the supercritical fluid depends on the molecular structure of the fluid and operation conditions, in situ extraction of the olefins seemed not enough to maintain catalyst activity, especially in the case of supercritical-phase C3. Modification of the catalyst was considered, as introduced later. 4. On the Reaction Mechanism. Corma et al. (1994) proposed that the strongest Bronsted acid sites were responsible for the alkylation reaction, while olefin oligomerization mainly occurred on the weak Bronsted sites. The stronger Bronsted acid sites tend to be deactivated faster than active sites for olefin oligomerization. This theory can explain the decrease in the alkylate yield and the increase in the oligomer yield with time-on-stream. Thus, it can be deduced that a high ratio of stronger to weaker Bronsted acid sites is necessary if one wants to obtain high alkylate activity while avoiding olefin oligomerization. Figure 7 shows the results of the supercritical-phase alkylation reaction

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Figure 7. Supercritical-phase alkylation reaction on SiO2-Al2O3 catalyst. i-C4′/i-C4 ) 1/50; W/F ) 40 g‚h/mol; 140 °C; 6.0 MPa.

Figure 8. Alkylation activity of H-USY catalysts prepared at varied calcination temperatures. i-C4′/i-C4 ) 1/50; W/F ) 40 g‚h/ mol; 140 °C; 6.0 MPa.

with SiO2-Al2O3 catalyst, which has stronger Bronsted acidity than H-USY zeolite. SiO2-Al2O3 catalyst exhibited high activity for oligomerization of isobutene instead of alkylation, from the early stage of the reaction. On the other hand, H-USY zeolite, having not very strong Bronsted acid sites, showed alkylation activity under the same reaction conditions. In order to elucidate the role of different acidic sites in this reaction, H-USY zeolite catalysts calcined at different temperatures were applied in the supercriticalphase i-C4′/i-C4 alkylation reaction. In Figure 8 the yield changes of alkylate and oligomer of these reactions are compared. Enhancement of the calcination temperature from 450 to 600 °C led to the increase of the initial activity of the catalyst for alkylate formation and the decrease of oligomer activity. Meanwhile, the catalyst deactivation was suppressed for the 600 °C calcined zeolite, compared to the case of the 450 °C calcined one. Furthermore, for the catalyst calcined under 650 °C, although the initial alkylate activity was nearly the same as that of the 600 °C calcined one, the catalyst deactivation alleviation was more obvious, but the olefin oligomerization yield was markedly increased for the 650 °C calcined catalyst. Considering the reason for the phenomenon mentioned above, Lewis acidic sites are sure to be formed if the calcination temperature was increased through the dehydration process and inside the zeolite framework. It is well-known that, in this process, a part of the Bronsted acidic site converted to the Lewis acidic site, as reported in many papers (Turkevich and Ono, 1969). It is concluded that alkylation reaction activity is closely related to the Lewis acid on the USY zeolite catalyst. The Lewis acidic site should be involved in the catalytic cycle for the alkylation reaction. As additional evidence to elucidate the role of the Lewis acid in the catalytic cycle of this reaction, the water effect was studied. As trace water (100 ppm) was

Figure 9. Influence of water removal from the reactant. i-C4′/ i-C4 ) 1/50; W/F ) 40 g‚h/mol; 140 °C; 6.0 MPa; 450 °C calcined H-USY (Y-1).

Figure 10. Addition of hydrogen donor in the supercritical-phase alkylation reaction. i-C4′/i-C4 ) 1/50; W/F ) 40 g‚h/mol; 140 °C; 6.0 MPa; 450 °C calcined H-USY (Y-1).

contained in the feedstock, sodium sulfate was employed to exclude water from the feedstock. The water concentration in purified feedstock dropped to 6 ppm. Due to the electron pair of the oxygen atom in the water molecule, the water molecule can readily attack the Lewis acidic site. If water was introduced into the catalyst, the Lewis acidic site would be poisoned by water and the alkylation reaction activity would decrease. The reaction performances where raw feedstocks or purified reactant was used are compared in Figure 9. For the water-removed reaction, the deactivation of the catalyst was suppressed to some extent, compared to the water-involved one. Meanwhile, the selectivity of the oligomerization increased remarkably in the water-removed reaction. It seems that not any kind of Lewis acidic site was favorable for the alkylation reaction. Only a part of Lewis acidic sites could improve the alkylation cycle without enhancing activities of other reactions. To investigate the hydride cycle in this reaction, a hydrogen donor such as adamantane or methylcyclohexane was added into the reaction system. In Figure 10 the reaction performances are compared. In hydrogen-donor-added reactions, the initial activities were lower and the catalysts deactivated more rapidly, compared to those in a conventional reaction. Oligomerization reaction proceeded from the initial stage of the reaction and a high yield of oligomer was still kept, even if alkylation activity disappeared. It is considered that these hydrogen donors could absorb hydride as well after they provide hydride. The hydride cycle related to these additives can compete with carbenium ion and obstruct the hydride cycle of the alkylation reaction. As a result, the carbenium ion transferred the proton to form olefin finally. When a tert-butyl cation is alkylated by a butene molecule, the C8 carbenium ion formed has to be rapidly

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Figure 11. Reaction scheme of the alkylation reaction.

desorbed before it undergoes consecutive oligomerization reaction, leading to the formation of high-molecularweight products, which are difficult to desorb. Thus, one way to decrease the average lifetime of the adsorbed C8 carbenium ion while keeping the reaction chain transfer is to increase the rate of hydride transfer from isobutane, as illustrated in Figure 11. Catalyticallypromoted hydride transfer should exist as the alkylate yield in zeolite-catalyzed alkylation decreased with timeon-stream. Fast hydride transfer in the initial stage of the alkylation reaction might be reasonable if C8 carbenium ion on a stronger Bronsted acid site can abstract hydride from isobutane. However, this hypothesis is not in accordance with the reaction behavior on SiO2-Al2O3, which has a strong Bronsted acidity. SiO2-Al2O3 showed little activity for alkylation as in Figure 7. Hydride can be abstracted from isobutane not only by carbenium ion but also by the Lewis acid site on the zeolite. If a Lewis acid site exists near C8 carbenium ion on a Bronsted acid site, which is quite possible in a zeolite, the hydride may transfer to C8 carbenium. On the other hand, a large number of Bronsted acid sites are not the prerequisites for the occurrence of alkylation reaction, as only a small part of them is necessary to protonate a small amount of isobutene and most of the residual Bronsted acid sites only promote oligomerization of olefins, according to the reaction scheme in Figure 11. The ratio of Lewis to Bronsted acid sites is able to be controlled by pretreatment condition for a given type of zeolite. As discussed in Figure 8, calcination at different temperatures in air could adjust the ratio of Bronsted to Lewis acid of H-USY zeolite. The high ratio of Lewis acid to Bronsted acid, which was realized in high-temperature-calcined zeolite, improved the alkylation to oligomerization ratio to suppress decreasing alkylation activity. Bronsted acid played an important role in the initiation step of the reaction as it provided a proton to olefin to form a carbenium ion. This step is critical for both alkylation reaction and oligomerization reaction. Based on this meaning, Bronsted acid is not related to the catalyst deactivation as oligomerization reaction proceeded remarkably even if the catalyst was deactivated for the alkylation reaction. Close combination between electron-rich heavy olefin such as C12 and Lewis acidic sites is the reason for catalyst deactivation, which can be overcome to some extent by accompanied supercritical fluid, as demonstrated in Figure 12.

Figure 12. Role of the Lewis acid of the catalyst in the alkylation.

Figure 13. Alkylation of isobutane with isobutene on various catalysts. 140 °C; 8.0 MPa; W/F (i-C4′ + i-C4) ) 40 g‚h/mol; i-C4′/ i-C4/N2 ) 1/50/30; catalysts were calcined at 450 °C for 1 h.

5. Catalyst Modification. Catalytically active zeolites are often prepared by exchanging the sodium ions by divalent or trivalent cations. The activity of such ion-exchanged zeolites was first explained by associating an assumed electrostatic field near the cations with the formation of carbon cations by the polarization of the C-H bond of the reactant molecule. This hypothesis has by now largely been abandoned and the polarization may be too weak to form carbenium ion. However, the electrostatic field may promote hydride transfer from isobutane to C8 carbenium ion to weaken the C-H bond of isobutane. In fact, Weitkamp (1980) reported that cerium-exchanged Y zeolite with a high exchanging degree showed excellent activity for liquid-phase isobutane/butene alkylation. The present authors prepared two kinds of La3+ ionexchanged Y zeolites. One of them (La (50%)-H-Y) was prepared by ion-exchanging H-Y with a La(NO3)3 solution. The degree of proton replacement was 50%. La (50%)-H-Y exhibited a catalyst performance almost similar to that of H-USY as compared in Figure 13. The other catalyst (La (76%)-Na-Y) was prepared by

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ion-exchanging Na-Y with a La(NO3)3 solution three times. The degree of Na replacement was 76%. The yield of alkylate was constant from the beginning of the reaction in a La (76%)-Na-Y-catalyzed alkylation reaction, though those of H-USY or La (50%)-H-Y were very low at the reaction start and increased with an increase of the amount of olefin fed up to 40-50 mmol of cat‚g-1. The low hydrocarbon recovery at the early stage of zeolite-catalyzed alkylation was pointed out by Weitkamp, and he proposed that the deficiency of hydrocarbons could be attributed to coke formation (Weitkamp, 1980). From this viewpoint, formation of coke or higher molecule hydrocarbons was suppressed on the La (76%)-Na-Y. Thus, little catalyst deactivation was observed in the case of La (76%)-Na-Y catalyzed alkylation where alkylation activity was at a high level through the reaction. The lower coke formation may be explained by very low proton content on the La (76%)-Na-Y or the existence of Na on zeolite. Conclusions Alkylation reaction conducted in supercritical-phase isobutane or isopentane was performed on a zeolite catalyst while supercritical fluid was a reactant as well. The supercritical-phase reaction exhibited higher activity, along with a remarkably longer lifetime, on Y zeolite catalyst, compared to the reactions conducted in the liquid phase or gas phase. The hydride transfer was promoted by Lewis acid sites on zeolites. Also the hydride transfer was developed by an electrostatic field formed near the trivalent cations such as La3+ exchanged into Na zeolite. High-molecular-weight olefins formed through oligomerization reaction readily deactivated the Lewis acidic sites, as in the liquid-phase or gas-phase reaction. Supercritical fluid was able to extract in situ and transport these high-molecularweight olefins to extend catalyst life successively.

Literature Cited Baptist-Nguyen, S.; Subramaniam, B. Coking and Activity of Porous Catalysts in Supercritical Reaction Media. AIChE J. 1992, 38, 1027. Corma, A.; Martinez, A. Chemistry, Catalysts, and Processes for Isoparaffin-Olefin Alkylation: Actual Situation and Future Trends. Catal. Rev. Sci. Eng. 1993, 35, 483. Corma, A.; Martinez, A.; Martinez, C. Isobutane/2-Butene Alkylation on Ultrastable Y Zeolites: Influence of Zeolite Unit Cell Size. J. Catal. 1994, 146, 185. Cusmano, J. A. New Technology and the Environment. CHEMTECH 1992, 22, 482. Fan, L.; Yokota, K.; Fujimoto, K. Supercritical Phase FischerTropsch Synthesis: Catalyst Pore-Size Effect. AIChE J. 1992, 38, 1639. Madras, G.; Erkey, C.; Akgerman, A. Supercritical Fluid Regeneration of Activated Carbon Loaded with Heavy Molecular Weight Organics. Ind. Eng. Chem. Res. 1993, 32, 1163. Nakamura, I.; Ishida, S.; Fujimoto, K. Supercritical Phase Isobutane/Olefin Alkylation on Solid Acids. International Symposium on Deactivation and Testing of Hydrocarbon Conversion Catalysts. ACS National Meeting, Chicago, 1995. Savage, P. E.; Gopalan, S.; Mizan, T.; Martino, C.; Brock, E. Reactions at Supercritical Conditions: Applications and Fundamentals. AIChE J. 1995, 411, 1723. Turkevich, J.; Ono, Y. Catalytic Research on Zeolites. Adv. Catal. 1969, 20, 135. Weitkamp, J. Isobutane/Butene Alkylation on Cerium Exchanged X and Y Zeolites. Catalysis by Zeolites; Imelik, B., et al., Eds.; Studies in Surface Science and Catalysis; Elsevier: New York, 1980; Vol. 5, p 65.

Received for review September 6, 1996 Revised manuscript received February 3, 1997 Accepted February 10, 1997X IE960550Z

X Abstract published in Advance ACS Abstracts, April 1, 1997.