C4= Alkylation under iC4 Supercritical Conditions

Continuous iC4/C4. ) Alkylation under iC4 Supercritical Conditions over K2.5H0.5PW12O40 and H-Beta Solid Acids. Ana Lilia Mota Salinas,‡ Dejin Kong,...
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Ind. Eng. Chem. Res. 2004, 43, 6355-6362

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Continuous iC4/C4) Alkylation under iC4 Supercritical Conditions over K2.5H0.5PW12O40 and H-Beta Solid Acids Ana Lilia Mota Salinas,‡ Dejin Kong,† Younes Ben Taˆ arit,‡ and Nadine Essayem*,‡ Shangaı¨ Research Institute of Petrochemical Technology, 1658 Pudong Bei Lu Pudong, 201208 Shanghaı¨, China, and Institut de Recherche sur la Catalyse, 2 Avenue Albert Einstein, 69626 Villeurbanne, France

The solid-acid-catalyzed iC4/C4) alkylation reaction has been investigated in a continuous fixedbed reactor in supercritical isobutane and compared with that in the liquid phase. The performances of heteropolyacids were compared with those of H-beta zeolites with different Si/Al ratios. Over the two types of catalysts, enhanced catalyst lifetimes were observed when dense supercritical (SC) isobutane was used. It was observed that, when adequate solid acid catalysts are used together with convenient experimental conditions, high alkylate yields (grams of C5+ per gram of C4)) are obtained. This demonstrates that alkylation reactions might be the operative pathways under supercritical iC4 conditions. Otherwise, if the cracking activity under the iC4 SC phase is increased relative to that in the liquid phase, it is seen that, by using a convenient catalyst of lower acid strength, as shown in the present study, one can partly circumvent this drawback. With catalyst aging, the following successive steps are observed: iC4 self-alkylation-cracking/pure alkylation-cracking/multiple alkylation-cracking. Introduction The acid-catalyzed isobutane/butene alkylation reaction presents a considerable economic interest. The trimethylpentanes (TMPs) produced are ideal components for gasoline because of their high research octane numbers (RONs, ∼100) and their low volatility and toxicity. However, the current two commercial processes for this reaction present significant drawbacks, as they use highly corrosive, toxic, and/or hazardous liquid sulfuric or hydrofluoric acid as catalysts. These reasons have prompted extensive research undertaken for more than 2 decades on alternative processes using solid acids. To date, most of the different types of solid acids have been evaluated in this reaction. However, under typical liquid alkylation conditions, solid acid catalysts deactivate rapidly, which prevents a realistic commercial application. To circumvent the fast activity decay, supercritical (SC) fluids have been employed as reaction media. SC fluids are expected to be able to extract heavy oligomers from the catalyst surface, which are responsible for its fast deactivation. However, even though the benefit of SC media in terms of catalyst longevity is often reported, the benefit of SC media in terms of sustainable activity for pure alkylation itself remains controversial. To our knowledge, the use of iC4 supercritical conditions for iC4/C4) alkylation over microporous zeolites was first claimed by Husain et al. in 1994.1,2 A considerable improvement of the catalyst longevity was then reported. The total C4 olefin conversion was maintained for 22 days, together with an instantaneous C5+ yield stabilized at 1.9 g of C5+/g of C4), close to the expected value of 2 for pure alkylation. Note that, in these experiments, a rather low olefin weight hourly space * To whom correspondence should be addressed. E-mail: [email protected]. Tel.: 33(0)472 44 53 15. Fax: 33(0)472 44 53 99. † Shangaı ¨ Research Institute of Petrochemical Technology. ‡ Institut de Recherche sur la Catalyse.

velocity was used (OWHSV ) 0.05 h-1), which also contributes positively to the improved catalyst lifetime. Later, Fan et al.3 reported that the USY zeolite lifetime in isoalkane/olefins alkylation was improved in supercritical isobutane. However, the benefit was not comparable to that reported in the former case. C8 alkylates were the main products only initially, when low amounts of olefins were fed. He et al.4 also reported a considerable improvement in the performance of a HPW (phosphotungstic acid based catalyst) in iC4/C4) alkylation when iC4 SC conditions were applied. Complete olefin conversion was achieved together with a high alkylate selectivity for more than 1400 h. Although the beneficial effect of SC isobutane medium on catalyst activity and stability has been claimed as cited above, other groups consider that performing iC4/ C4) alkylation at a temperature above the critical temperature of the isoparaffin does not allow high alkylate yield to be attained. For that reason, Clark and Subramaniam5,6 studied the use of a cosolvent, such as CO2, to reduce the supercritical temperature of the feed. An improved catalyst stability in iC4/C4) reaction was reported for these conditions. However, one can observe that low alkylate yields were obtained over H-USY and sulfated zirconia. Santana and al.7 investigated SC iC4/C4) on USY zeolite using the experimental conditions used by Subramaniam.6 They showed that C8 olefins are the main products in the presence of CO2. Chelappa et al.8 investigated SC iC4/C4) alkylation over SZ and FeMnSZ under different experimental conditions. In particular, they used ethane as the cosolvent to decrease the critical temperature of the medium. They concluded that TMPs were never formed in high proportion. Olefin dimerization was the dominant reaction. More recently, Ginosar et al.9 investigated iC4/C4) alkylation in SC conditions over different solid acid catalysts including zeolites, sulfated zirconia, and Nafions. They used excess of iC4 and a cosolvent as well to reduce the negative impact

10.1021/ie049792m CCC: $27.50 © 2004 American Chemical Society Published on Web 08/31/2004

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Table 1. Critical Temperatures and Pressures of the Reactant Mixtures (iC4/C4)) iC4/C4) molar ratio

Pc (MPa)

Tc (K)

99 9

3.68 4.15

408.0 410.0

of the high olefin concentration and to reduce the reaction temperature, respectively. The authors reported that the addition of an SC cosolvent to the SC isoalkane decreased the catalyst lifetime as well as the desired product selectivity. They concluded that they “did not demonstrate sustained catalyst activity at supercritical conditions with any of the catalysts or cosolvents examined”. Rather, they reported that, on the best catalyst, the alkylation yield reached 0.8 g of C5+/g of catalyst in the liquid phase versus 0.55 g of C5+/g of catalyst under SC conditions. Our interest in performing iC4/C4) alkylation under supercritical conditions started with experiments under batch conditions on heteropolyacids whose formulation was optimized for the liquid-phase alkylation.10-12 Because the batch conditions did not allow for the direct evaluation of catalyst aging, we undertook similar experiments in a continuous fixed-bed reactor, first on the same HPW catalyst and then on catalysts of lower acid strength: H-beta zeolites of different Si/Al ratios. Moreover, the most important parameters, including the total pressure and the reactant iC4/C4) ratio, were investigated on the most promising catalyst. Our goal was to check the benefit of supercritical conditions on the catalyst lifetime and to obtain better knowledge on the prevailing mechanistic pathway under iC4 supercritical conditions depending on the catalyst employed. Experimental Equipment and Catalyst Preparations 1. Experimental Setup for Supercritical Phase iC4/C4) Alkylation in a Continuous-Flow FixedBed Reactor. A continuous-flow type apparatus with a fixed-bed reactor was specially designed to perform iC4/C4) alkylation reaction under iC4 supercritical conditions. Supercritical alkylation was performed with excess iC4, above the critical temperature (Tc) and critical pressure (Pc) of the reaction medium. The supercritrical constants are Tc ) 407.8, 435.6, and 428.6 K and Pc ) 3.63, 4.20, and 3.99 MPa for isobutane, cisbut-2-ene, and trans-but-2-ene, respectively. The iC4/ C4) alkylation reaction was performed in the presence of a large excess of iC4; therefore, the critical properties of the reaction media were near those of pure iC4. Two limiting feed compositions and their critical constants are reported in Table 1. The critical temperature and pressure of the reactant mixture were calculated with the Kay rule and the Prausnitz and Gunn equation.13 A simplified representation of the setup used for performing continuous isobutane/butene alkylation under supercritical conditions is presented in Figure 1. The micropilot apparatus includes four main parts: the liquid reactant feed, the tubular reactor where the hydrocarbon supercritical state is reached, the product recovery section connected to a fast GC analysis system, and a pressure regulation system. The liquid reactant mixture is kept in a vessel at ambient temperature under a slight He pressure. Its composition is obtained by weighing each of the components and controlled by the on-line GC analysis

system by means of a reactor bypass line. The reactant vessel is placed on a balance to control the feed consumption throughout the progress of the reaction. The reactant vessel is connected to a HPLC pump to deliver a high-pressure liquid (from atmospheric pressure to 200 bar) at a constant low flow rate (0.001-1 cm3‚min-1). The tubular reactor, made of Hastelloy, has an internal diameter of 0.5 cm and a length equal to 22 cm. The catalytic bed is maintained in the upper part of the up-flow reactor by means of packed SiC particles of 0.35-mm diameter placed on the bottom and top parts of the reactor. The catalyst itself is diluted with SiC powder of smaller particle size (i.e., 0.045 mm) to ensure the stability of the catalytic bed and to minimize the creation of preferential voids. A careful loading of the reactor is required to ensure reproducible experiments: The successive SiC beds and catalyst beds are hardly compacted to get fixed bed heights. The catalyst amounts used are as follows: 1 g for K2.5P and 0.34 or 0.64 g for the beta samples. The reactor is connected to an independent pretreatment line, allowing any catalyst treatments to be performed, at atmospheric pressure, before the alkylation reaction. Then, the reaction temperature is stabilized, and the pressure is raised to the chosen value by means of a high-pressure He line equipped with a pressure regulator. Supercritical phases are obtained when the reactor temperature (TR) and total pressure (Ptot) are higher than the corresponding critical constants of the reactant mixture. The supercritical phase is ensured within the tubular reactor volume only. The reaction products are recovered as a high-pressure liquid phase within a glass collector designed to tolerate pressures as high as 150 bar. The high-pressure liquid products can be depressurized and analyzed on-line by means of a fast GC apparatus equipped with a DB1 type column. The fast GC apparatus allows for the separation of the product mixture, from C4 to C16 hydrocarbons, within 30 min. Figure 2 shows chromatograms of two product mixtures and of a standard composed of saturated C8 hydrocarbons. The selectivity to alkylate [Se(C5+), wt %] is defined as the total amount of C5+ alkylate divided by the amount of converted butene (cis + trans)

Se(C5+) (wt %) )

[

C5+ (wt %)

]

(C4))ini (wt %) - (C4))t (wt %)

× 100

For pure isobutane-butene alkylation, the calculated selectivity value is 204%, whereas for butene dimerization Se(C5+) equals 100%. The selectivity to a Cn fraction [Se(Cn, wt %] is defined here as the total amount of Cn divided by the total amount of alkylate C5+ (wt %)

Se(Cn) (wt %) )

[

Cn (wt %)

]

C5+ (wt %)

× 100

2. Catalysts Preparation and Characterization. The Na form of beta zeolite was supplied by PQ. The ammonium form was obtained by exchanging 25 g of Na-beta with 50 mL of 1 M NH4NO3 solution at room temperature for 4 h. The procedure was repeated three times with intermediate H2O washing. The acidic form

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Figure 1. Continuous iC4/C4) fixed-bed micropilot apparatus.

Figure 2. Fast GC analysis using a DB1 column of product mixtures and of a standard mixture within the retention time range of C8 hydrocarbons.

was obtained in situ by using the following treatment under a dry He flow: First, the temperature was increased from room temperature to 110 °C using a temperature ramp of 2 °C‚min-1 and then held at this temperature for 2 h. Second, the temperature was increased to 450 °C at a rate of 2 °C‚min-1 and kept at this temperature for 5 h. Finally, the catalyst temperature was decreased slowly to the reaction temperature under dry He flow. Beta zeolite samples with two different Si/Al ratios were used: Si/Al ) 30 and Si/Al ) 11.5. The acidic potassium salt K2.5H0.5PW12O40 was prepared according to a method described previously14 by adding a KCl aqueous solution (4 M) to a 0.1 M

H3PW12O40 aqueous solution under stirring. Then, the suspension was centrifuged, and the precipitate was washed with deionized water and freeze-dried. The molar ratio of the salt, K/P, was 2.5. For convenience, the potassium salt is abbreviated K2.5P in the following. Prior to the alkylation reaction, the K2.5P sample was pretreated in situ at 473 K for 2 h under a He flow, and then the temperature was reduced to the reaction temperature. The K2.5P textural features are presented in Table 2 together with those of the well-known acidic cesium salt Cs2.5PW12O40 for comparison. As expected, the potassium salt exhibits a BET surface area larger than that

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Table 2. Physicochemical Features of Alkali Salts of H3PW12O4O

sample K2.5H0.5PW12O40 Cs2.5H0.5PW12O40

rate of iC4 external surface micropore δ 1H formationc diameter MAS (10-8 mol‚ SBET area (nm)a NMRb s-1‚m-2) (m2‚g-1) (m2‚g-1)a 150 140

12 28

0.53 0.54

8.1 7.9

2.3 2.1

a Deduced from N adsorption isotherms, using the t-plot 2 method for the external surface and the Horvath-Kawazoe method for the micropore diameter. b Measured after sample treatment for 2 h at 473 K. c n-C4 isomerization: mcata ) 200 mg, TR ) 473 K, total flow ) 0.045 mol‚h-1, 4% n-C4 in N2.

Figure 4. Comparison of the product selectivities in iC4/C4) alkylation over K2.5H0.5PW12O40 in liquid or supercritical phases. Total amount of butene fed ) 0.16 g of C4)/g of catalyst. Conditions: mcata ) 1 g diluted in 0.5 g of SiC (L ) 0.045 mm). iC4/C4) ) 64.3. OWHSV ) 0.27 h-1. Liquid phase TR ) 40 °C, Ptot ) 30 bar. SC phase TR ) 140 °C, Ptot ) 50 bar. Total amount of olefin charged ) 0.16 g of C4) per gram of catalyst.

Figure 3. C4) conversion in iC4/C4) alkylation over K2.5H0.5PW12O40 in liquid and supercritical phases as a function of time on stream expressed in grams of C4) fed per gram of catalyst. Conditions: mcata ) 1 g diluted in 0.5 g of SiC (L ) 0.045 mm). iC4/C4) ) 64.3. OWHSV ) 0.27 h-1. Liquid phase TR ) 40 °C, Ptot ) 30 bar. SC phase TR ) 140 °C, Ptot ) 50 bar.

of the Cs2.5P salt even though the former is almost totally microporous with pore diameters equal to 0.53 nm. Table 2 also summarizes two characteristics of the catalysts’ acidity: their 1H MAS NMR chemical shifts, measured after treatment of the samples for 2 h at 473 K, and their activities in the reaction of n-C4 isomerization at 473 K after the same pretreatment. This reaction is commonly used as a model reaction for strong acidity. It is shown that the two alkali salts exhibit similar activities, in complete agreement with their 1H MAS NMR chemical shifts. However, at this moderate temperature of reaction, the beta zeolites are inactive. This set of experiments confirms the greater acid strength of the acidic salts of H3PW12O40 over H-beta zeolites, in agreement with previous investigations that have already demonstrated the stronger acidity of acidic alkali salts of H3PW12O40 over H-ZSM515or H-mordenite.16 Results and Discussion: HPW Catalyst: K2.5H0.5PW12O40. The experiments performed in the continuous-flow reactor confirmed the enhanced activity and longevity of the microporous acidic salt K2.5H0.5PW12O40 when the reaction is performed under the iC4 supercritical phase compared to the liquid medium (Figure 3). Initially, an almost complete olefin conversion is obtained under SC medium, whereas the olefin conversion does not exceed 40% in the liquid phase. In addition, in SC medium, a high catalyst activity is maintained to an amount of charged olefin equal to 1 g of C4)/g of catalyst. In the

liquid phase, the catalyst rapidly becomes inactive. In other respects, the product selectivity showed that the iC4 supercritical conditions strongly favor the formation of cracked products (Figure 4). TMPs as well as dimethylhexanes (DMHs) are strongly reduced as a result. This enhanced cracking activity is the consequence of two combined phenomena: First, it is clear that the high temperature applied to reach SC conditions favors the cracking activity; the activation energy of cracking reactions is higher than that of alkylation or oligomerization reactions. Second, one would assume that, because of the zero superficial tension of SC fluids, reactants can penetrate into the porosity of the catalyst particles to a higher extent than liquid reactants can. Therefore, the products of secondary reactions such as cracking should be increased compared to the primary products, namely, alkylates and oligomers. H-Beta Zeolites. Because of its lower acid strength, H-beta is expected to be less active for cracking than K2.5P and thus to be a more convenient catalyst for performing alkylation above the iC4 critical temperature. Experiments in liquid and SC phases of different densities were performed over H-beta zeolites with composition Si/Al ) 30. On H-beta zeolite, liquid-phase iC4/C4) alkylation is conducted at 80 °C, the optimal liquid-phase reaction temperature reported in the litterature.17 Note that this temperature is higher than the optimal reaction temperature found for K2.5P, i.e., 40 °C.10 This agrees well with the higher acid strength of heteropolyacids (HPAs). Figure 5 presents the evolution of the olefin conversion versus time on stream in the liquid and supercritical phases. Again, an improved catalyst activity and stability of H-beta in the supercritical phase is obtained. Moreover, significant enhancements of the activity and catalyst lifetime are obtained by applying increasing pressure. When a total pressure of 50 bar or higher is used, the olefin conversion is complete for a long period of time. Furthermore, for the highest pressure, 80 bar, the olefin conversion is maintained at 100% for a total amount of charged olefin of 2 g of C4)/g of

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Figure 5. Variation of the catalyst lifetime over beta zeolite (Si/ Al ) 30), in liquid and supercritical phases: C4) conversion as a function of time on stream expressed in grams of C4) fed per gram of catalyst. Conditions: mcata ) 0.34 g diluted in 0.5 g of SiC (L ) 0.045 mm). iC4/C4) ) 64.3. OWHSV ) 0.8 h-1. Liquid phase TR ) 80 °C, Ptot ) 30 bar. SC phase TR ) 140 °C; Ptot ) 40, 50, or 80 bar.

catalyst, after which the conversion decreases slowly. Once more, this set of experiments confirms our earlier observations made under batch conditions: for the solidacid-catalyzed iC4/C4) reaction, employing a dense SC phase leads to an improvement of the catalyst efficiency and longevity. The lower catalyst deactivation in SC media is generally ascribed to the properties of SC fluids. Such fluids are known to combine the solvation ability of the liquid phase with the transport properties of gas phases. Consequently, it is generally accepted that heavy oligomers, which are responsible of the catalyst decay, can be extracted by dense SC fluids.18 The product distribution obtained in the SC phase is reported in Figure 6, together with that obtained in the liquid medium. Again, one can observe that the use of the SC iC4 medium results in an increased cracking activity of the H-beta zeolite, although to a lower extent compared to that of the HPW catalyst (Figure 4). This lower cracking activity is most likely due to the lower acid strength of beta zeolite acid sites compared to those of HPW, as expected. It is interesting to note that, at higher pressure, 80 bar, the cracking activity is favored. The effect can be ascribed to changes of the SC phase properties: the liquid-type SC phase obtained at 80 bar is characterized by lower diffusion rates, which might favor the cracking of intermediates. These results, obtained over HPW and beta zeolite samples, show that, to perform supercritical iC4/C4) alkylation in excess isoparaffin above iC4 Tc and Pc, it is necessary to design the solid acid catalyst properties to limit the cracking activity and favor the desired reaction, pure alkylation. Influence of the Si/Al Ratio. Because H-beta zeolites with lower Si/Al molar ratios have been reported to be more selective in the liquid phase to alkylates,19 a beta zeolite with a Si/Al ratio of 11.5 was evaluated under SC conditions for comparison. Figure 7 shows that using the H-beta sample of the lower Si/Al ratio, 11.5, led to an improved catalyst stability compared to the H-beta sample of higher Si/ Al ratio (Si/Al ) 30). The total alkylate selectivity (grams of C5+ per gram of C4) converted) is also plotted

in Figure 7 for the two beta samples against the total amount of butene fed per unit weight of catalyst (grams of C4) per gram of catalyst). On the two beta zeolites, the initial alkylate selectivity surpasses a value of 200%, which suggests that iC4 self-alkylation should occur to a large extent on the fresh samples. Then, with time on stream, the alkylate selectivity falls to values lower than 100% for the beta sample of higher Si/Al ratio, indicating carbon retention on the catalyst surface. For the beta zeolite of higher aluminum content, Si/Al ) 11.5, the alkylate selectivity decreases to values lower than 200% when 0.4 g of butene/g of catalyst has been fed and then stabilizes at an intermediate value, 150%, for longer time on stream. The decrease of the alkylate selectivity from 400 to 200% suggests that iC4 selfalkylation does no longer prevail and that pure alkylation becomes the dominant pathway. The stabilization of the alkylate selectivity around 150% might be indicative of different events: (1) C4) oligomerization might become gradually more important while pure alkylation continues to be operative or, simply, (2) double alkylation becomes the dominant mechanism. Double alkylation involves two molecules of butene and one molecule of isobutane to form a C12 aliphatic compounds. In that case, the total alkylate selectivity corresponds to 150%. Regardless, one can clearly observe that the stabilization of the catalyst activity is closely related to high total alkylate selectivity values. When the total alkylate selectivity falls below 100%, a rapid decrease of the olefin conversion occurs. This phenomenon, which is generally observed in the liquid phase,10 still prevails under SC conditions. However, a total alkylate selectivity equal to 200% reflects the fact that each butene molecule reacts with one isobutane molecule. It does not provide any insight into the occurrence of secondary cracking of C8 alkylate or C12 double alkylate. Figure 8 shows the product selectivities at a time on stream equivalent to an amount of fed olefin of 0.4 g of C4)/g of catalyst over the two beta zeolites under the SC conditions that provide the highest catalyst stability with time, i.e., 80 bar and 140 °C. First, let us emphasize the low amount of olefinic compounds (Se < 2%) among the total products for an experiment duration equivalent to 1.2 g of C4)/g of catalyst. This is a strong indication of the high rate of hydride-transfer reactions, which allow for the desorption of intermediate carbenium ions as saturated compounds, according to the possible scheme:20

Regarding the product distributions on the two H-beta samples, it is observed that cracked products are formed in high proportions, the TMP/DMH ratio is close to 2, and significant amounts of heavy compounds, C9+, are formed. Note that cracked products are favored under the present conditions compared to data reported above (Figure 6). The results presented in Figure 8 were obtained with a lower olefin space velocity. This could explain the importance of consecutive reactions, such as branched alkanes cracking. Also, regarding fluids

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Figure 6. Comparison of the product distributions in iC4/C4) alkylation over H-beta zeolite (Si/Al ) 30) in liquid or supercritical phases. Total amount of butene fed ) 0.3 g of C4)/g of catalyst. Conditions: mcata ) 0.34 g diluted in 0.5 g of SiC (L ) 0.045 mm). iC4/C4) ) 64. OWHSV ) 0.8 h-1. Liquid phase TR ) 80 °C, Ptot ) 30 bar. SC phases TR ) 140 °C, Ptot ) 50 bar; TR ) 140 °C, Ptot ) 80 bar. Total amount of olefin charged ) 0.3 g of C4) per gram of catalyst.

Figure 7. Butene conversion and alkylate selectivity during alkylation in the supercritical phase on H-beta samples. Influence of Si/Al ratio. Conditions: mcata ) 0.68 g diluted in 0.5 g of SiC (L ) 0.045 mm). iC4/C4) ) 65. OWHSV ) 0.3 h-1. TR ) 140 °C. Ptot ) 80 bar.

near the SC point, the use of dense SC fluids should lower the mass-transfer rate and thus increase the residence time of the carbenium ion intermediates. Therefore, the larger branched carbon-containing compounds formed can be solvated and extracted slowly by the dense SC fluid (C9+ compounds) or can be cracked (150%, can be stabilized for elapsed times equivalent to at least 1.6 g of C4)/g of catalyst. These results indicate that alkylation reactions between iC4 and C4) might be the operative pathway under these SC conditions. In addition, mainly aliphatic compounds are formed, which demonstrates the high rate of hydride-transfer reactions under SC conditions as well. However, the product distribution evidenced a high proportion of cracked products compared to that obtained in the liquid phase. The proposed explanations are the increased temperature required to reach iC4 SC conditions and the longer intermediate residence time resulting in the less restricted penetration of reactants into the catalyst pores in the SC fluid, although a faster mass transport is expected compared to the liquid phase. Consequently, the solid acids have to be optimized to circumvent the drawbacks of the iC4 SC fluid. As suggested in this study, lower acid strength is preferred to reduce the cracking activity, as well as low Si/Al ratios in the case of beta zeolite, which leads to a improved stability. With catalyst aging, one can observe the following steps that occur more or less successively or might overlap depending of the paraffin-to-olefin ratio:

iC4 self-alkylation-cracking

pure alkylation-cracking

double (multiple) alkylation-cracking

The importance of C4) dimerization in the evolution of the overall reaction mechanism remains questionable under SC conditions. Finally, it is concluded that, if an improved catalyst lifetime in the SC iC4/C4) alkylation reaction is unambiguously demonstrated, the respective contribution of the extraction ability of SC fluids and of the cracking activity of the catalyst to liberate acid sites from

branched carbon-containing deposits requires further investigation. Literature Cited (1) Husain, A. Solid catalyzed supercritical isoparaffin olefin alkylation process. U.S. Patent 5,304,698, 1994. (2) Husain, A. Isoparaffin-olefin alkylation with MCM microporous materials under the supercritical conditions of the isoparaffins. European Patent WO 94/03415, 1994. (3) Fan, L.; Nakumura, I.; Ishida, S.; Fujimoto, K. SupercriticalPhase Alkylation Reaction on Solid Acid Catalysts: Mechanistic Study and Catalyst Development. Ind. Eng. Chem. Res. 1997, 36, 1458. (4) He, Y. Alkylation of isoparaffin with olefin under supercritical fluid state. Chin. J. Catal. 1999, 20, 403. (5) Clark, M. C.; Subramaniam, B. Extended Alkylate Production Activity during Fixed-Bed Supercritical 1-Butene/Isobutane Alkylation on Solid Acid Catalysts Using Carbon Dioxide as a Diluent. Ind. Eng. Chem. Res. 1998, 37, 1243. (6) Subramaniam, B. Enhancing the stability of porous catalysts with supercritical reaction media. Appl. Catal. A: Gen. 2001, 212, 199. (7) Santana, G. M.; Akgerman, A. Alkylation of Isobutane with 1-Butene on a Solid Acid Catalyst in Supercritical Reaction Media. Ind. Eng. Chem. Res 2001, 40, 3879. (8) Chelappa, A. S.; Miller, R. C.; Thomson, W. J. Supercritical alkylation and butene dimerization over sulfated zirconia and iron-manganese promoted sulfated zirconia catalysts. Appl. Catal. A: Gen. 2001, 209, 359. (9) Ginosar, D. M.; Thomson, D. N.; Coates, K.; Zalexsky, D. J. The Effect of Supercritical Fluids on Solid Acid Catalyst Alkylation. Ind. Eng. Chem. Res. 2002, 41, 2864. (10) Gayraud, P. Y. Isobutane-Butene Alkylation in the Presence of Sulfated Zirconia and Heteropolyacids in Liquid and Supercritical Conditions. Ph.D. Thesis, Institut de Recherche sur la Catalyse, Villeurbanne, France, 2000. (11) Gayraud, P. Y.; Stewart, I. H.; Derouane-Abd, H.; Essayem, N.; Derouane, E. G.; Ve´drine, J. C. Performances of potassium 12tungstophosphoric acid salts as catalysts for isobutane/butene alkylation in subcritical and supercritical phases. Catal. Today 2000, 63, 223. (12) Gayraud, P. Y.; Essayem, N.; Ve´drine, J. C. Porous 12tungstophosphoric salts for isobutane/butene alkylation. Influence of the protonic density and surface polarity of the HPA. Effect of the supercritical phase. Stud. Surf. Sci. Catal. 2000, 130, 2554. (13) MacHugh, M. A., Ktukonis V. J. Supercritical Fluid Extraction, Principles and Practice; Butterworth-Heinemann: Boston, 1994. (14) Essayem, N.; Kieger, S.; Coudurier, G.; Ve´drine, J. C. Comparison of the reactivities of H3PW12O40 and H4SiW12O40 and their K+, NH4+ and Cs+ salts in liquid-phase isobutane/butene alkylation. Stud. Surf. Sci. Catal. A 1996, 101, 591. (15) Okuhara, T.; Nishimura, T.; Misono, M. Novel Microporous Solid “Superacids” CsxH3-xPW12O40 (2 < x < 3). Stud. Surf. Sci. Catal. 1996, 101, 581. (16) Essayem, N.; Ben Taˆarit, Y.; Feche, C.; Gayraud, P. Y.; Sapaly, G.; Naccache, C. Comparative study of n-pentane isomerization over solid acid catalysts, heteropolyacid, sulfated zirconia, and mordenite: Dependence on hydrogen and platinum addition. J. Catal. 2003, 219, 97. (17) Nivarthy, G. S.; He, Y.; Seshan, K.; Lercher, J. A. Elementary Mechanistic Steps and the Influence of Process Variables in Isobutane Alkylation over H-BEA. J. Catal. 1998, 176, 192. (18) Subramaniam, B. Enhancing the stability of porous catalysts with supercritical reaction media. Appl. Catal. A: Gen. 2001, 212, 99. (19) Loenders, R.; Jacobs, P. A.; Martens, J. A. Alkylation of Isobutane with 1-Butene on Zeolite Beta. J. Catal. 1998, 176, 545. (20) Pines, H. The Chemistry of Catalytic Hydrocarbon Conversion; Academic Press: New York, 1981.

Received for review March 17, 2004 Revised manuscript received May 15, 2004 Accepted June 14, 2004 IE049792M