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Aluminum incorporation in mesoporous MCM-41 molecular sieves and their catalytic performance in acid-catalyzed reactions
Applied Catalysis A: General 245 (2003) 33–41
Aluminum incorporation in mesoporous MCM-41 molecular sieves and their catalytic performance in acid-catalyzed reactions Suman K. Jana, Hajime Takahashi, Masamitsu Nakamura, Masayuki Kaneko, Reiichi Nishida, Hiroyuki Shimizu, Tsuyoshi Kugita, Seitaro Namba∗ Department of Materials, Teikyo University of Science and Technology, Uenohara-machi, Yamanashi 409-0193, Japan Received 21 September 2002; received in revised form 1 November 2002; accepted 19 November 2002
Abstract Mesoporous aluminosilicate molecular sieves, Al-containing MCM-41, with different Si/Al ratios were synthesized by four different methods: sol–gel, hydrothermal, template cation exchange and grafting. The catalysts prepared by sol–gel, grafting and template cation exchange methods are effective for the incorporation of large amounts of aluminum into the framework of MCM-41. The catalytic activities of the resulting Al-containing MCM-41 samples were tested in the cracking of cumene and the dehydration of 2-propanol, as model acid-catalyzed reactions, and then compared with the results obtained from microporous H-ZSM-5 and HY zeolites. The Al-containing MCM-41 catalysts prepared by different methods behaved differently in acting as acidic catalysts; the catalyst (having almost same Si/Al ratio) prepared by sol–gel method showed higher cracking activity, whereas that prepared by template cation exchange method showed higher dehydration activity. Moreover, these Al-containing MCM-41 catalysts are more catalytically active than microporous HY zeolite for cumene cracking and 2-propanol dehydration reactions. © 2002 Elsevier Science B.V. All rights reserved. Keywords: Al-containing MCM-41; Cracking; Dehydration; Cumene; 2-Propanol
1. Introduction Microporous molecular sieves are widely used acid catalysts. These often suffer from diffusion limitations when applied to petrochemicals and fine chemical synthesis [1]. Hence, the development of aluminosilicate molecular sieves with a pore diameter in the mesopore range have been an increasing demand for use in acid-catalyzed reactions, particularly for the treatment of heavier feeds and the synthesis of large molecules for producing fine chemicals [2]. The dis∗ Corresponding author. Tel.: +81-554-63-6856; fax: +81-554-63-6856. E-mail address: [email protected] (S. Namba).
covery of mesoporous silica, MCM-41, has attracted more attention because of its uniform hexagonal array of cylindrical mesopores [3,4], which provide the potential for its use as catalysts for bulky molecules. There is currently a considerable research interest in the preparation and use of heteroatom-containing mesoporous silicas as heterogeneous catalysts [2,5,6]. The incorporation of Al is particularly important as it gives rise to solid acid catalysts with acid sites associated with the presence of Al in the framework position. A number of reports have appeared on synthesis and characterization of these Al-containing MCM-41 materials [7–14]. The Al-containing mesoporous MCM-41 catalysts can be synthesized by both direct and post-synthesis methods with a wide range
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of framework Si/Al ratios and may exhibit hexagonal arrangement of uniformly-sized mesopores (pore diameter in the range of 1.5 to >10 nm), depending on the template and synthesis conditions used [8]. However, few comparative studies on the synthesis and characterization of Al-containing MCM-41 prepared by different methods are available. The present investigation was a comparative study on the incorporation of Al into the framework of MCM-41 by different methods and their catalytic applications in acid-catalyzed reactions. We report here the syntheses of four series of Al-containing mesoporous MCM-41 with various Si/Al ratios by four different synthesis methods: sol–gel, hydrothermal, template cation exchange and grafting, and their characterization by powder X-ray diffraction, N2 adsorption, 27 Al MAS NMR and NH3 -temperature programmed desorption. The catalytic activities of different Al-containing MCM-41 samples were tested for cumene cracking (as medium to strong Brønsted acid-catalyzed model reaction) and 2-propanol dehydration (as weak Lewis and Brønsted acid-catalyzed model reaction) and their activities were compared with those of microporous H-ZSM-5 and HY zeolites and also with that of amorphous silica-alumina catalyst. The catalytic performance (i.e. cumene cracking or 2-propanol dehydration activity) shown by the Al-containing MCM-41 catalysts is superior to that of microporous HY zeolite. 2. Experimental 2.1. Catalyst preparation Mesoporous aluminosilicate molecular sieves, Alcontaining MCM-41, were prepared as follows. Al-MCM-41 catalysts were prepared by sol–gel method under basic condition similar to that reported earlier [15]. In a typical procedure, calculated amounts of aluminum isopropoxide and tetraethyl orthosilicate (Aldrich, USA) were added to isopropyl alcohol (KANTO Chemical Co., Japan) and then this solution was stirred for 15 min. The resulting solution was then added to a second solution containing measured amounts of tetradecyltrimethylammonium bromide (Aldrich, USA), aqueous ammonia (KANTO Chemical Co., Japan) and water. The gel was vigor-
ously stirred for 30 min and then allowed to react at ambient temperature for 24 h. The solid product was filtered, washed, air dried at room temperature and finally calcined at 540 ◦ C for 6 h. AlMCM-41 catalysts were prepared by a hydrothermal method similar to that described elsewhere [16]. In a typical synthesis, required amounts of sodium silicate (KANTO Chemical Co., Japan) and aluminum sulfate (KANTO Chemical Co., Japan) were separately dissolved in deionized water and then combined together, followed by stirring. Then the aqueous solution of hexadecyltrimethylammonium bromide (Aldrich, USA) was added to the above mixture and this combination was vigorously stirred for 15 min. The pH of the gel was adjusted close to 10 by adding dilute sulfuric acid (KANTO Chemical Co., Japan), and then the resulting gel was transferred into a teflon-lined autoclave and heated statically at 100 ◦ C to achieve its crystallization under autogeneous pressure in an oven for 7 days. After crystallization, the solid product was recovered by filtration, washed with deionized water and dried at 100 ◦ C. The organic template was removed by calcining the samples at 540 ◦ C for 5 h. The ammonium form of MCM-41 was obtained by repeated ion exchanges with 0.5 M ammonium nitrate (KANTO Chemical Co., Japan) solution at room temperature. The protonated form was then obtained by calcining the ammonium form at 540 ◦ C for 6 h. Al/AlMCM-41 catalysts were prepared by a template cation exchange method similar to that reported earlier [17]. The resulting catalysts were obtained by exchanging the hexadecyltrimethylammonium cations of the dried, as-synthesized, hydrothermally prepared AlMCM-41 (Si/Al = 15, 30 or 50) with 0.5 M aqueous aluminum nitrate (KANTO Chemical Co., Japan) solution at 90 ◦ C for a number of times, and then calcined at 600 ◦ C for 6 h to remove the residual template. Al:MCM-41 catalysts were prepared by a grafting method similar to that described earlier [18]. Each calculated amount of aluminum isopropoxide was dissolved in dry hexane (KANTO Chemical Co., Japan), followed by stirring for 30 min, and then an appropriate amount of calcined Si-MCM-41 was added to the above mixture and stirred at room temperature for 24 h. The resulting material was obtained by filtration, washed with dry hexane and dried at room temperature, and finally calcined at 550 ◦ C for 6 h. Si-MCM-41
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was prepared by a conventional hydrothermal process reported elsewhere [16]. Microporous H-ZSM-5 (Si/Al = 50, synthesized by a hydrothermal method) and HY (Si/Al = 2.8, obtained from the Catalysis Society of Japan with reference number JRC-Z-HY 55) zeolites and amorphous silica-alumina (Si/Al = 7.2) were used for comparison. 2.2. Catalyst characterization
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ing were: cumene initial pressure = 0.082 atm, T = 400 ◦ C, W/F (weight of catalyst per mol of the mixture of cumene and He per hour) = 0.42–1.67 g h/mol; the reaction conditions for 2-propanol dehydration were: 2-propanol initial pressure = 0.082 atm, T = 194 ◦ C, W/F (weight of catalyst per mol of the mixture of 2-propanol and He per hour) = 0.73–2.91 g h/mol. The reaction products were analyzed by high performance gas chromatography (Shimadzu GC-8A, fitted with flame ionization detector); the instrument was equipped with a PEG-20M packed column.
Al-containing MCM-41 catalysts were characterized to find: • their mesoporous phase identification and phase purity by XRD, • surface area and pore size distribution by nitrogen sorption, • aluminum environment in the structure by 27 Al MAS NMR, and • acidity by NH3 -TPD. X-ray powder diffraction measurements were obtained on a JEOL JDX-8030 diffractometer using Cu K␣ radiation. The N2 adsorption isotherms were measured in an ASAP-2000 instrument at liquid nitrogen temperature. Specific surface areas of the samples were calculated from the adsorption isotherms by BET method and pore size by BJH method. The 27 Al MAS NMR spectra were recorded using a JEOL JNM-A500 spectrometer. NH3 -TPD of the samples were carried out in a flow-type fixed bed reactor using a Japan BEL TPD-77 instrument. The ammonia adsorption temperature was 150 ◦ C and the temperature was raised to 750 ◦ C at a rate of 10 ◦ C/min. Before ammonia adsorption measurement, each sample was evacuated at 150 ◦ C for 0.5 h. 2.3. Catalytic activity test The catalytic activities of the different Al-containing samples were tested for cracking of cumene and dehydration of 2-propanol. Cumene and 2-propanol were of reagent grade, obtained from KANTO Chemical Co., Japan, and were used as received. A tubular, down-flow quartz reactor was used for both the cracking and dehydration experiments. The reactions were carried out at atmospheric pressure using He as the carrier gas. The reaction conditions for cumene crack-
3. Results and discussion 3.1. Composition and properties of Al-containing MCM-41 catalysts Four series of Al-containing MCM-41 samples were synthesized by four different methods: sol–gel, hydrothermal, template cation exchange and grafting, with a wide range of Si/Al ratios. The powder X-ray diffraction data of mesoporous aluminosilicate catalysts with their highest and lowest Si/Al ratios, prepared by different methods, are shown in Fig. 1. X-ray diffraction patterns show that the catalysts prepared by the sol–gel method give only one broad diffraction peak whether with higher Si/Al ratio (Si/Al = 75) or with lower Si/Al ratio (Si/Al = 3), suggesting the disordered structure of the materials. However, the catalysts prepared by other methods with higher Si/Al ratio exhibit mesoporous phase. Moreover, the catalysts prepared by grafting method even with Si/Al ratio of 3 and also by template cation exchange method with Si/Al ratio of 5.2 exhibit the mesoporous phase well. But the catalyst with Si/Al ratio lower than 9.5 cannot be prepared with a good mesoporous phase obtained by hydrothermal method under the present synthesis conditions. Fig. 1 also shows that, for all synthesis methods, the level of phase purity decreases with increasing the incorporation of Al into MCM-41, however, the decreasing extent of phase purity with increasing Al incorporation depends on the synthesis methods. The above result indicates that the incorporation of large amounts of Al in the framework of MCM-41 was found more effective when Al-containing MCM-41 was prepared by sol–gel, grafting or template cation exchange method
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Fig. 1. Powder XRD patterns of Al-containing MCM-41 catalysts prepared by different methods.
than when it was prepared by the hydrothermal method. Al-containing mesoporous MCM-41 catalysts are also characterized to determine their surface area, pore
size, tetrahedral Al concentration and their acidity. The catalyst characterization data are presented in Table 1. The N2 adsorption isotherms (at liquid nitrogen temperature) of all the calcined Al-containing
Table 1 Composition and physico-chemical properties of Al-containing MCM-4 1 catalysts prepared by different methods Catalyst
Si/Al ratio (mol/mol)
Surface area (m2 /g)
BJH pore size (nm)
Acidity (mmol/g)
Tetrahedral Al concentration for Si/Al ≈ 30 (%)
Al-MCM-41 AlMCM-41 Al/AlMCM-41 Al:MCM-41
3.0–75 9.5–100 5.2–40 3.0–70
700–1100 610–1000 710–970 769–989
1.8–2.5 2.6–2.9 2.6–2.9 2.6–2.8
0.055–0.247 0.112–0.217 0.149–0.231 0.081–0.346
79 86 78 60
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Fig. 2. Data on acidity distribution (strong acidity (- - -) and weak to medium acidity (—)) of different Al-containing MCM-41 catalysts prepared by different methods with different Al contents.
MCM-41 catalysts prepared by different methods showed almost identical types of isotherms characteristic of mesoporous materials with uniform pore size [19]. The average pore diameters are given in Table 1. All the calcined MCM-41 catalysts show a unimodal pore size distribution with a unique peak centered in between 1.5 and 3.0 nm. BET surface areas also confirm the results of mesoporosity comparison. As shown in Table 1, all Al-containing MCM-41 samples prepared by different methods display high BET surface areas from 610 to 1100 m2 /g and pore distribution from 1.8 to 2.9 nm. The results of 27 Al MAS NMR spectral data of Al-containing MCM-41 samples prepared by different methods (having Si/Al ratios close to 30) are given in Table 1. All the samples show similar 27 Al MAS NMR spectra having both tetrahedrally coordinated (framework) Al at around 52 ppm and octahedrally coordinated (non-framework) Al at around 0 ppm; the majority of the Al is in tetrahedral coordination. The tetrahedral Al concentration varied depending upon the synthesis methods (Table 1), it is highest for the
catalyst synthesis by hydrothermal method and lowest for the catalyst synthesis by grafting method. The extra-framework octahedrally coordinated Al species in all the samples are formed during synthesis or calcinations and may be presents in the pores. The strong and weak acidity distributions measured by NH3 -TPD of different Al-containing MCM-41 catalysts are presented in Fig. 2 and the total (weak to medium and strong) acidity values are given in Table 1. The low peak acidity value (i.e. weak to medium acidity) is related to the ammonia desorption at 150–400 ◦ C, whereas the high peak value (i.e. strong acidity) is related to the ammonia desorption at 400–750 ◦ C. Data in Fig. 2 shows that, with increase in the Al concentration in the catalysts, the weak to medium acidity increases, while the strong acidity, which is very small, remains constant. The above result indicates that the acidity of the mesoporous materials is increased with increase in Al incorporation into the framework of the materials and that all the Al-containing MCM-41 catalysts have moderate acidity, with acid sites mostly of medium strength.
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Fig. 3. Variation of apparent reaction rate constant for the cumene cracking (at 400 ◦ C) over different Al-containing MCM-41 samples with the increase in Al concentration in the catalyst.
3.2. Catalytic activities The catalytic activities of different Al-containing MCM-41 samples have been investigated for acid-catalyzed cumene cracking and 2-propanol dehydration reactions. The cracking and dehydration activities are expressed in terms of apparent first-order rate constants by considering that both the reactions followed first-order rate law over Al-containing catalysts. 3.2.1. Comparison of the Al-containing MCM-41 catalysts for cumene cracking The cumene cracking activity versus Al concentration for cracking over different Al-containing MCM-41 catalysts is presented in Fig. 3. The cracking activities of the different Al-containing MCM-41 catalysts (having almost the same Si/Al ratio (Si/Al = 7.5–10)) are compared in Table 2. The conversion proceeds almost exclusively via catalytic cracking to benzene and propene over the Al-containing cata-
lysts, which indicates that the active sites are of the Brønsted acid type [20]. From the comparison of results in Fig. 3 and Table 2, the following important observations can be made: • The cumene cracking activity of different Al-containing MCM-41 catalysts is increased with increase in Al concentration and the related increase of the Brønsted acidity (of medium strength) of the catalysts. The increase in the cracking activity with increase in the Al concentration is almost linear for the Table 2 Results of cumene cracking over different Al-containing MCM-41 catalysts at 400 ◦ C Catalyst
Si/Al ratio (mol/mol)
Apparent first-order rate constant (mol/(g h))
Al-MCM-41 AlMCM-41 Al/AlMCM-41(15) Al:MCM-41
10.0 7.5 10.0 10.0
1.74 0.63 1.08 0.95
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Fig. 4. Variation of apparent reaction rate constant for the 2-propanol dehydration (at 194 ◦ C) over different Al-containing MCM-41 samples with the increase in Al concentration in the catalyst.
catalysts prepared by sol–gel method. In all other cases, the cracking activity is increased linearly up to the Al concentration of 0.08 mol/mol, further increase in the Al concentration has almost no effect on the catalytic activity. • Among the different Al-containing MCM-41 catalysts having almost the same Si/Al ratio and similar total acidity values, the activities of the different catalysts in the cumene cracking are in the following order: Al-MCM-41 > Al/AlMCM-41 (15) ≈ Al:MCM-41 > AlMCM-41. Thus, the catalyst prepared by sol–gel method showed the highest cumene cracking activity, whereas that prepared by hydrothermal method showed the lowest cracking activity. The above observations indicate that the synthesis process is important for the high catalytic activity of Al-containing MCM-41 catalysts in the cracking reaction. 3.2.2. Comparison of the Al-containing MCM-41 catalysts for 2-propanol dehydration The 2-propanol dehydration activity versus Al concentration for dehydration over different Al-containing MCM-41 catalysts is presented in Fig. 4. The dehydra-
tion activity of the different Al-containing MCM-41 catalysts (having almost the same Si/Al ratio (Si/Al = 7–10)) are compared in Table 3. The 2-propanol dehydration produces propene as the main product over the Al-containing catalysts. From a comparison of the results in Fig. 4 and Table 3, the following important observations can be made: • The 2-propanol dehydration activity of different Al-containing MCM-41 catalysts is increased with increase in Al concentration in the catalysts, i.e. with increase in the weak (Lewis and Brønsted) acidity of the catalysts. The increase in the dehydration activity with increase in the Al concentration is almost linear up to the Al Table 3 Results of 2-propanol dehydration over different Al-containing MCM-41 catalysts at 194 ◦ C Catalyst
Si/Al ratio (mol/mol)
Apparent first-order rate constant (mol/(g h))
Al-MCM-41 AlMCM-41 Al/AlMCM-41(15) Al:MCM-41
7.0 7.5 9.0 10.0
0.73 0.46 0.93 0.55
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Table 4 Comparison of Al-containing MCM-41 catalysts with H-ZSM-5, HY and silica-alumina catalysts as regards their activities in the cumene cracking (at 400 ◦ C) and 2-propanol dehydration (at 194 ◦ C) reactions Reaction
Cumene cracking 2-Propanol dehydration
Catalytic activities of different Al-containing catalysts in terms of apparent first-order rate constant (mol/(g h)) Al-containing MCM-41 catalyst
H-ZSM-5
HY
Silica-alumina
3.4 for Al-MCM-41 (Si/Al = 5.0) 0.93 for Al/AlMCM-41(15) (Si/Al = 9.0)
7.99 0.19
2.6 0.64
1.37 0.20
concentration of 0.06 mol/mol for all the catalysts, further increase in the Al concentration has resulted in a very slow increase in the catalytic activity. • Among the different Al-containing MCM-41 catalysts having almost the same Si/Al ratio and similar total acidity values, the activities of the different catalysts in the 2-propanol dehydration are in the following order: Al/AlMCM-41 (15) > AlMCM-41 > Al:MCM-41 > AlMCM-41. Thus, the Al-containing catalyst prepared by template cation exchange method showed the highest 2-propanol dehydration activity and that prepared by hydrothermal method showed the lowest dehydration activity. The above observations indicate that, similar to the cracking activity, the 2-propanol dehydration activity of the Al-containing MCM-41 catalysts largely depends on the catalysis synthesis process. However, the activity orders of Al-containing MCM-41 catalysts prepared by different methods are different for cracking and dehydration reactions. It is also important to note that, although the Al-containing MCM-41 catalyst shows different catalytic activity in cracking and dehydration reactions, still the physico-chemical properties of the catalysts prepared by different methods do not undergo any major change. 3.2.3. Comparison of the Al-containing MCM-41 catalysts with H-ZSM-5, HY and silica-alumina catalysts for cumene cracking and 2-propanol dehydration reactions The results of cumene cracking (typical medium to strong Brønsted acid-catalyzed model reaction) and 2-propanol dehydration (typical weak Lewis and Brønsted acid-catalyzed model reaction) over Al-containing MCM-41 catalysts showing higher activity are compared with microporous H-ZSM-5 and
HY zeolites and also with amorphous silica-alumina catalyst in Table 4. The Al-MCM-41 (Si/Al = 5) prepared by sol–gel method showed lower cumene cracking activity than H-ZSM-5 zeolite, however, it showed higher activity than both HY zeolite and amorphous silica-alumina catalyst. This is expected because H-ZSM-5 zeolite has strong Brønsted acidity, but all other catalysts have Brønsted acidity of medium strength; the high cracking activity of H-ZSM-5 is attributed to its strong Brønsted acidity, since a cracking reaction is accelerated by strong Brønsted acids. The 2-propanol dehydration activity of Al/AlMCM-41(15) (Si/Al = 10) prepared by template cation exchange method is higher than that of H-ZSM-5, this is due to the fact that the dehydration reaction is catalyzed by weak Lewis and Brønsted acids and hence the strong Brønsted acidity of H-ZSM-5 has no significant role on this reaction. Al/AlMCM-41(15) also showed higher dehydration activity than HY zeolite and amorphous silica-alumina catalyst. It is also important to note that, although Al-containing MCM-41 catalysts showed higher cracking or dehydration activity than microporous HY zeolite, however, the acid strengths (weak and strong acid sites) as well as the number of acid sites of Al-containing MCM-41 catalysts are lower than those of HY zeolite [21]. This comparative study indicates that Al-containing MCM-41 catalysts have high potential, particularly in weak to moderate acid-catalyzed reactions.
4. Conclusions The mesoporous Al-containing MCM-41 catalysts prepared by sol–gel, grafting and template cation exchange methods are effective for the incorporation of large amounts of Al into the framework of MCM-41. The acidity of the mesoporous Al-containing MCM-41
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catalysts is increased with increase in Al incorporation into the framework of the materials. The increase in the acidity of Al-containing MCM-41 catalysts up to an optimum level increased the activity of the materials as cracking and dehydration catalysts. However, the activity of the mesoporous Al-containing MCM-41 catalysts can be significantly different depending on the synthesis methods, although the physico-chemical properties of the catalysts prepared by different methods do not undergo any major change along with their acidity. The Al-containing MCM-41 catalyst prepared by sol–gel method showed higher cracking activity, whereas that prepared by template cation exchange method showed higher dehydration activity. These Al-containing MCM-41 catalysts show remarkably high activity in cracking and dehydration reactions, higher than that of microporous HY zeolite. In particular, Al-containing MCM-41 samples prepared by different methods seem to have high potentials in weak to moderate acid-catalyzed reactions. References [1] W.F. Hoelderich, M. Hesse, F. Naumann, Angew. Chem. Int. Ed. Eng. 27 (1988) 226. [2] A. Corma, Chem. Rev. 97 (1997) 2373.
41
[3] C.T. Kresge, M.E. Leonowicz, W.J. Roth, J.C. Vartuli, J.S. Beck, Nature 359 (1992) 710. [4] J.S. Beck, J.C. Vartuli, W.J. Roth, M.E. Leonowicz, C.T. Kresge, K.D. Schmitt, C.T.W. Chu, D.H. Olson, E.W. Shephard, S.B. McCullen, J.B. Higgins, J.C. Schlenker, J. Am. Chem. Soc. 114 (1992) 10834. [5] S. Biz, M.L. Occelli, Catal. Rev.-Sci. Eng. 40 (1998) 329. [6] A. Sayari, Chem. Mater. 8 (1996) 1840. [7] M. Janicke, D. Kumar, G.D. Stucky, B.F. Chmelka, Stud. Surf. Sci. Catal. 84 (1994) 243. [8] A. Corma, V. Fornes, M.T. Navarro, J. Perez-Pariente, J. Catal. 148 (1994) 569. [9] K.M. Reddy, C. Song, Catal. Lett. 36 (1996) 103. [10] K.M. Reddy, C. Song, Catal. Today 31 (1996) 137. [11] R.B. Borade, A. Clearfield, Catal. Lett. 31 (1995) 267. [12] R. Mokaya, W. Jones, J. Catal. 172 (1997) 211. [13] R. Ryoo, S. Jun, J.M. Kim, M.J. Kim, Chem. Commun. (1997) 2225. [14] R. Mokaya, J. Catal. 193 (2000) 103. [15] C. Perego, S. Amarilli, A. Carati, C. Flego, G. Pazzuconi, C. Rizzo, G. Bellussi, Microporous Mesoporous Mater. 27 (1999) 345. [16] T. Kugita, M. Ezawa, T. Owada, Y. Tomita, S. Namba, N. Hashimoto, M. Onaka, Microporous Mesoporous Mater. 44–45 (2001) 531. [17] Y. Yonemitsu, Y. Tanaka, M. Iwamoto, Chem. Mater. 2 (1997) 2679. [18] R. Mokaya, W. Jones, J. Chem. Soc., Chem. Commun. (1997) 2185. [19] S. Namba, unpublished results. [20] J.W. Ward, J. Catal. 9 (1967) 225. [21] S. Namba, unpublished results.
Aluminum incorporation in mesoporous MCM-41 molecular sieves and their catalytic performance in acid-catalyzed reactions Suman K. Jana, Hajime Takahashi, Masamitsu Nakamura, Masayuki Kaneko, Reiichi Nishida, Hiroyuki Shimizu, Tsuyoshi Kugita, Seitaro Namba∗ Department of Materials, Teikyo University of Science and Technology, Uenohara-machi, Yamanashi 409-0193, Japan Received 21 September 2002; received in revised form 1 November 2002; accepted 19 November 2002
Abstract Mesoporous aluminosilicate molecular sieves, Al-containing MCM-41, with different Si/Al ratios were synthesized by four different methods: sol–gel, hydrothermal, template cation exchange and grafting. The catalysts prepared by sol–gel, grafting and template cation exchange methods are effective for the incorporation of large amounts of aluminum into the framework of MCM-41. The catalytic activities of the resulting Al-containing MCM-41 samples were tested in the cracking of cumene and the dehydration of 2-propanol, as model acid-catalyzed reactions, and then compared with the results obtained from microporous H-ZSM-5 and HY zeolites. The Al-containing MCM-41 catalysts prepared by different methods behaved differently in acting as acidic catalysts; the catalyst (having almost same Si/Al ratio) prepared by sol–gel method showed higher cracking activity, whereas that prepared by template cation exchange method showed higher dehydration activity. Moreover, these Al-containing MCM-41 catalysts are more catalytically active than microporous HY zeolite for cumene cracking and 2-propanol dehydration reactions. © 2002 Elsevier Science B.V. All rights reserved. Keywords: Al-containing MCM-41; Cracking; Dehydration; Cumene; 2-Propanol
1. Introduction Microporous molecular sieves are widely used acid catalysts. These often suffer from diffusion limitations when applied to petrochemicals and fine chemical synthesis [1]. Hence, the development of aluminosilicate molecular sieves with a pore diameter in the mesopore range have been an increasing demand for use in acid-catalyzed reactions, particularly for the treatment of heavier feeds and the synthesis of large molecules for producing fine chemicals [2]. The dis∗ Corresponding author. Tel.: +81-554-63-6856; fax: +81-554-63-6856. E-mail address: [email protected] (S. Namba).
covery of mesoporous silica, MCM-41, has attracted more attention because of its uniform hexagonal array of cylindrical mesopores [3,4], which provide the potential for its use as catalysts for bulky molecules. There is currently a considerable research interest in the preparation and use of heteroatom-containing mesoporous silicas as heterogeneous catalysts [2,5,6]. The incorporation of Al is particularly important as it gives rise to solid acid catalysts with acid sites associated with the presence of Al in the framework position. A number of reports have appeared on synthesis and characterization of these Al-containing MCM-41 materials [7–14]. The Al-containing mesoporous MCM-41 catalysts can be synthesized by both direct and post-synthesis methods with a wide range
0926-860X/02/$ – see front matter © 2002 Elsevier Science B.V. All rights reserved. doi:10.1016/S0926-860X(02)00616-6
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of framework Si/Al ratios and may exhibit hexagonal arrangement of uniformly-sized mesopores (pore diameter in the range of 1.5 to >10 nm), depending on the template and synthesis conditions used [8]. However, few comparative studies on the synthesis and characterization of Al-containing MCM-41 prepared by different methods are available. The present investigation was a comparative study on the incorporation of Al into the framework of MCM-41 by different methods and their catalytic applications in acid-catalyzed reactions. We report here the syntheses of four series of Al-containing mesoporous MCM-41 with various Si/Al ratios by four different synthesis methods: sol–gel, hydrothermal, template cation exchange and grafting, and their characterization by powder X-ray diffraction, N2 adsorption, 27 Al MAS NMR and NH3 -temperature programmed desorption. The catalytic activities of different Al-containing MCM-41 samples were tested for cumene cracking (as medium to strong Brønsted acid-catalyzed model reaction) and 2-propanol dehydration (as weak Lewis and Brønsted acid-catalyzed model reaction) and their activities were compared with those of microporous H-ZSM-5 and HY zeolites and also with that of amorphous silica-alumina catalyst. The catalytic performance (i.e. cumene cracking or 2-propanol dehydration activity) shown by the Al-containing MCM-41 catalysts is superior to that of microporous HY zeolite. 2. Experimental 2.1. Catalyst preparation Mesoporous aluminosilicate molecular sieves, Alcontaining MCM-41, were prepared as follows. Al-MCM-41 catalysts were prepared by sol–gel method under basic condition similar to that reported earlier [15]. In a typical procedure, calculated amounts of aluminum isopropoxide and tetraethyl orthosilicate (Aldrich, USA) were added to isopropyl alcohol (KANTO Chemical Co., Japan) and then this solution was stirred for 15 min. The resulting solution was then added to a second solution containing measured amounts of tetradecyltrimethylammonium bromide (Aldrich, USA), aqueous ammonia (KANTO Chemical Co., Japan) and water. The gel was vigor-
ously stirred for 30 min and then allowed to react at ambient temperature for 24 h. The solid product was filtered, washed, air dried at room temperature and finally calcined at 540 ◦ C for 6 h. AlMCM-41 catalysts were prepared by a hydrothermal method similar to that described elsewhere [16]. In a typical synthesis, required amounts of sodium silicate (KANTO Chemical Co., Japan) and aluminum sulfate (KANTO Chemical Co., Japan) were separately dissolved in deionized water and then combined together, followed by stirring. Then the aqueous solution of hexadecyltrimethylammonium bromide (Aldrich, USA) was added to the above mixture and this combination was vigorously stirred for 15 min. The pH of the gel was adjusted close to 10 by adding dilute sulfuric acid (KANTO Chemical Co., Japan), and then the resulting gel was transferred into a teflon-lined autoclave and heated statically at 100 ◦ C to achieve its crystallization under autogeneous pressure in an oven for 7 days. After crystallization, the solid product was recovered by filtration, washed with deionized water and dried at 100 ◦ C. The organic template was removed by calcining the samples at 540 ◦ C for 5 h. The ammonium form of MCM-41 was obtained by repeated ion exchanges with 0.5 M ammonium nitrate (KANTO Chemical Co., Japan) solution at room temperature. The protonated form was then obtained by calcining the ammonium form at 540 ◦ C for 6 h. Al/AlMCM-41 catalysts were prepared by a template cation exchange method similar to that reported earlier [17]. The resulting catalysts were obtained by exchanging the hexadecyltrimethylammonium cations of the dried, as-synthesized, hydrothermally prepared AlMCM-41 (Si/Al = 15, 30 or 50) with 0.5 M aqueous aluminum nitrate (KANTO Chemical Co., Japan) solution at 90 ◦ C for a number of times, and then calcined at 600 ◦ C for 6 h to remove the residual template. Al:MCM-41 catalysts were prepared by a grafting method similar to that described earlier [18]. Each calculated amount of aluminum isopropoxide was dissolved in dry hexane (KANTO Chemical Co., Japan), followed by stirring for 30 min, and then an appropriate amount of calcined Si-MCM-41 was added to the above mixture and stirred at room temperature for 24 h. The resulting material was obtained by filtration, washed with dry hexane and dried at room temperature, and finally calcined at 550 ◦ C for 6 h. Si-MCM-41
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was prepared by a conventional hydrothermal process reported elsewhere [16]. Microporous H-ZSM-5 (Si/Al = 50, synthesized by a hydrothermal method) and HY (Si/Al = 2.8, obtained from the Catalysis Society of Japan with reference number JRC-Z-HY 55) zeolites and amorphous silica-alumina (Si/Al = 7.2) were used for comparison. 2.2. Catalyst characterization
35
ing were: cumene initial pressure = 0.082 atm, T = 400 ◦ C, W/F (weight of catalyst per mol of the mixture of cumene and He per hour) = 0.42–1.67 g h/mol; the reaction conditions for 2-propanol dehydration were: 2-propanol initial pressure = 0.082 atm, T = 194 ◦ C, W/F (weight of catalyst per mol of the mixture of 2-propanol and He per hour) = 0.73–2.91 g h/mol. The reaction products were analyzed by high performance gas chromatography (Shimadzu GC-8A, fitted with flame ionization detector); the instrument was equipped with a PEG-20M packed column.
Al-containing MCM-41 catalysts were characterized to find: • their mesoporous phase identification and phase purity by XRD, • surface area and pore size distribution by nitrogen sorption, • aluminum environment in the structure by 27 Al MAS NMR, and • acidity by NH3 -TPD. X-ray powder diffraction measurements were obtained on a JEOL JDX-8030 diffractometer using Cu K␣ radiation. The N2 adsorption isotherms were measured in an ASAP-2000 instrument at liquid nitrogen temperature. Specific surface areas of the samples were calculated from the adsorption isotherms by BET method and pore size by BJH method. The 27 Al MAS NMR spectra were recorded using a JEOL JNM-A500 spectrometer. NH3 -TPD of the samples were carried out in a flow-type fixed bed reactor using a Japan BEL TPD-77 instrument. The ammonia adsorption temperature was 150 ◦ C and the temperature was raised to 750 ◦ C at a rate of 10 ◦ C/min. Before ammonia adsorption measurement, each sample was evacuated at 150 ◦ C for 0.5 h. 2.3. Catalytic activity test The catalytic activities of the different Al-containing samples were tested for cracking of cumene and dehydration of 2-propanol. Cumene and 2-propanol were of reagent grade, obtained from KANTO Chemical Co., Japan, and were used as received. A tubular, down-flow quartz reactor was used for both the cracking and dehydration experiments. The reactions were carried out at atmospheric pressure using He as the carrier gas. The reaction conditions for cumene crack-
3. Results and discussion 3.1. Composition and properties of Al-containing MCM-41 catalysts Four series of Al-containing MCM-41 samples were synthesized by four different methods: sol–gel, hydrothermal, template cation exchange and grafting, with a wide range of Si/Al ratios. The powder X-ray diffraction data of mesoporous aluminosilicate catalysts with their highest and lowest Si/Al ratios, prepared by different methods, are shown in Fig. 1. X-ray diffraction patterns show that the catalysts prepared by the sol–gel method give only one broad diffraction peak whether with higher Si/Al ratio (Si/Al = 75) or with lower Si/Al ratio (Si/Al = 3), suggesting the disordered structure of the materials. However, the catalysts prepared by other methods with higher Si/Al ratio exhibit mesoporous phase. Moreover, the catalysts prepared by grafting method even with Si/Al ratio of 3 and also by template cation exchange method with Si/Al ratio of 5.2 exhibit the mesoporous phase well. But the catalyst with Si/Al ratio lower than 9.5 cannot be prepared with a good mesoporous phase obtained by hydrothermal method under the present synthesis conditions. Fig. 1 also shows that, for all synthesis methods, the level of phase purity decreases with increasing the incorporation of Al into MCM-41, however, the decreasing extent of phase purity with increasing Al incorporation depends on the synthesis methods. The above result indicates that the incorporation of large amounts of Al in the framework of MCM-41 was found more effective when Al-containing MCM-41 was prepared by sol–gel, grafting or template cation exchange method
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Fig. 1. Powder XRD patterns of Al-containing MCM-41 catalysts prepared by different methods.
than when it was prepared by the hydrothermal method. Al-containing mesoporous MCM-41 catalysts are also characterized to determine their surface area, pore
size, tetrahedral Al concentration and their acidity. The catalyst characterization data are presented in Table 1. The N2 adsorption isotherms (at liquid nitrogen temperature) of all the calcined Al-containing
Table 1 Composition and physico-chemical properties of Al-containing MCM-4 1 catalysts prepared by different methods Catalyst
Si/Al ratio (mol/mol)
Surface area (m2 /g)
BJH pore size (nm)
Acidity (mmol/g)
Tetrahedral Al concentration for Si/Al ≈ 30 (%)
Al-MCM-41 AlMCM-41 Al/AlMCM-41 Al:MCM-41
3.0–75 9.5–100 5.2–40 3.0–70
700–1100 610–1000 710–970 769–989
1.8–2.5 2.6–2.9 2.6–2.9 2.6–2.8
0.055–0.247 0.112–0.217 0.149–0.231 0.081–0.346
79 86 78 60
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Fig. 2. Data on acidity distribution (strong acidity (- - -) and weak to medium acidity (—)) of different Al-containing MCM-41 catalysts prepared by different methods with different Al contents.
MCM-41 catalysts prepared by different methods showed almost identical types of isotherms characteristic of mesoporous materials with uniform pore size [19]. The average pore diameters are given in Table 1. All the calcined MCM-41 catalysts show a unimodal pore size distribution with a unique peak centered in between 1.5 and 3.0 nm. BET surface areas also confirm the results of mesoporosity comparison. As shown in Table 1, all Al-containing MCM-41 samples prepared by different methods display high BET surface areas from 610 to 1100 m2 /g and pore distribution from 1.8 to 2.9 nm. The results of 27 Al MAS NMR spectral data of Al-containing MCM-41 samples prepared by different methods (having Si/Al ratios close to 30) are given in Table 1. All the samples show similar 27 Al MAS NMR spectra having both tetrahedrally coordinated (framework) Al at around 52 ppm and octahedrally coordinated (non-framework) Al at around 0 ppm; the majority of the Al is in tetrahedral coordination. The tetrahedral Al concentration varied depending upon the synthesis methods (Table 1), it is highest for the
catalyst synthesis by hydrothermal method and lowest for the catalyst synthesis by grafting method. The extra-framework octahedrally coordinated Al species in all the samples are formed during synthesis or calcinations and may be presents in the pores. The strong and weak acidity distributions measured by NH3 -TPD of different Al-containing MCM-41 catalysts are presented in Fig. 2 and the total (weak to medium and strong) acidity values are given in Table 1. The low peak acidity value (i.e. weak to medium acidity) is related to the ammonia desorption at 150–400 ◦ C, whereas the high peak value (i.e. strong acidity) is related to the ammonia desorption at 400–750 ◦ C. Data in Fig. 2 shows that, with increase in the Al concentration in the catalysts, the weak to medium acidity increases, while the strong acidity, which is very small, remains constant. The above result indicates that the acidity of the mesoporous materials is increased with increase in Al incorporation into the framework of the materials and that all the Al-containing MCM-41 catalysts have moderate acidity, with acid sites mostly of medium strength.
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Fig. 3. Variation of apparent reaction rate constant for the cumene cracking (at 400 ◦ C) over different Al-containing MCM-41 samples with the increase in Al concentration in the catalyst.
3.2. Catalytic activities The catalytic activities of different Al-containing MCM-41 samples have been investigated for acid-catalyzed cumene cracking and 2-propanol dehydration reactions. The cracking and dehydration activities are expressed in terms of apparent first-order rate constants by considering that both the reactions followed first-order rate law over Al-containing catalysts. 3.2.1. Comparison of the Al-containing MCM-41 catalysts for cumene cracking The cumene cracking activity versus Al concentration for cracking over different Al-containing MCM-41 catalysts is presented in Fig. 3. The cracking activities of the different Al-containing MCM-41 catalysts (having almost the same Si/Al ratio (Si/Al = 7.5–10)) are compared in Table 2. The conversion proceeds almost exclusively via catalytic cracking to benzene and propene over the Al-containing cata-
lysts, which indicates that the active sites are of the Brønsted acid type [20]. From the comparison of results in Fig. 3 and Table 2, the following important observations can be made: • The cumene cracking activity of different Al-containing MCM-41 catalysts is increased with increase in Al concentration and the related increase of the Brønsted acidity (of medium strength) of the catalysts. The increase in the cracking activity with increase in the Al concentration is almost linear for the Table 2 Results of cumene cracking over different Al-containing MCM-41 catalysts at 400 ◦ C Catalyst
Si/Al ratio (mol/mol)
Apparent first-order rate constant (mol/(g h))
Al-MCM-41 AlMCM-41 Al/AlMCM-41(15) Al:MCM-41
10.0 7.5 10.0 10.0
1.74 0.63 1.08 0.95
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Fig. 4. Variation of apparent reaction rate constant for the 2-propanol dehydration (at 194 ◦ C) over different Al-containing MCM-41 samples with the increase in Al concentration in the catalyst.
catalysts prepared by sol–gel method. In all other cases, the cracking activity is increased linearly up to the Al concentration of 0.08 mol/mol, further increase in the Al concentration has almost no effect on the catalytic activity. • Among the different Al-containing MCM-41 catalysts having almost the same Si/Al ratio and similar total acidity values, the activities of the different catalysts in the cumene cracking are in the following order: Al-MCM-41 > Al/AlMCM-41 (15) ≈ Al:MCM-41 > AlMCM-41. Thus, the catalyst prepared by sol–gel method showed the highest cumene cracking activity, whereas that prepared by hydrothermal method showed the lowest cracking activity. The above observations indicate that the synthesis process is important for the high catalytic activity of Al-containing MCM-41 catalysts in the cracking reaction. 3.2.2. Comparison of the Al-containing MCM-41 catalysts for 2-propanol dehydration The 2-propanol dehydration activity versus Al concentration for dehydration over different Al-containing MCM-41 catalysts is presented in Fig. 4. The dehydra-
tion activity of the different Al-containing MCM-41 catalysts (having almost the same Si/Al ratio (Si/Al = 7–10)) are compared in Table 3. The 2-propanol dehydration produces propene as the main product over the Al-containing catalysts. From a comparison of the results in Fig. 4 and Table 3, the following important observations can be made: • The 2-propanol dehydration activity of different Al-containing MCM-41 catalysts is increased with increase in Al concentration in the catalysts, i.e. with increase in the weak (Lewis and Brønsted) acidity of the catalysts. The increase in the dehydration activity with increase in the Al concentration is almost linear up to the Al Table 3 Results of 2-propanol dehydration over different Al-containing MCM-41 catalysts at 194 ◦ C Catalyst
Si/Al ratio (mol/mol)
Apparent first-order rate constant (mol/(g h))
Al-MCM-41 AlMCM-41 Al/AlMCM-41(15) Al:MCM-41
7.0 7.5 9.0 10.0
0.73 0.46 0.93 0.55
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Table 4 Comparison of Al-containing MCM-41 catalysts with H-ZSM-5, HY and silica-alumina catalysts as regards their activities in the cumene cracking (at 400 ◦ C) and 2-propanol dehydration (at 194 ◦ C) reactions Reaction
Cumene cracking 2-Propanol dehydration
Catalytic activities of different Al-containing catalysts in terms of apparent first-order rate constant (mol/(g h)) Al-containing MCM-41 catalyst
H-ZSM-5
HY
Silica-alumina
3.4 for Al-MCM-41 (Si/Al = 5.0) 0.93 for Al/AlMCM-41(15) (Si/Al = 9.0)
7.99 0.19
2.6 0.64
1.37 0.20
concentration of 0.06 mol/mol for all the catalysts, further increase in the Al concentration has resulted in a very slow increase in the catalytic activity. • Among the different Al-containing MCM-41 catalysts having almost the same Si/Al ratio and similar total acidity values, the activities of the different catalysts in the 2-propanol dehydration are in the following order: Al/AlMCM-41 (15) > AlMCM-41 > Al:MCM-41 > AlMCM-41. Thus, the Al-containing catalyst prepared by template cation exchange method showed the highest 2-propanol dehydration activity and that prepared by hydrothermal method showed the lowest dehydration activity. The above observations indicate that, similar to the cracking activity, the 2-propanol dehydration activity of the Al-containing MCM-41 catalysts largely depends on the catalysis synthesis process. However, the activity orders of Al-containing MCM-41 catalysts prepared by different methods are different for cracking and dehydration reactions. It is also important to note that, although the Al-containing MCM-41 catalyst shows different catalytic activity in cracking and dehydration reactions, still the physico-chemical properties of the catalysts prepared by different methods do not undergo any major change. 3.2.3. Comparison of the Al-containing MCM-41 catalysts with H-ZSM-5, HY and silica-alumina catalysts for cumene cracking and 2-propanol dehydration reactions The results of cumene cracking (typical medium to strong Brønsted acid-catalyzed model reaction) and 2-propanol dehydration (typical weak Lewis and Brønsted acid-catalyzed model reaction) over Al-containing MCM-41 catalysts showing higher activity are compared with microporous H-ZSM-5 and
HY zeolites and also with amorphous silica-alumina catalyst in Table 4. The Al-MCM-41 (Si/Al = 5) prepared by sol–gel method showed lower cumene cracking activity than H-ZSM-5 zeolite, however, it showed higher activity than both HY zeolite and amorphous silica-alumina catalyst. This is expected because H-ZSM-5 zeolite has strong Brønsted acidity, but all other catalysts have Brønsted acidity of medium strength; the high cracking activity of H-ZSM-5 is attributed to its strong Brønsted acidity, since a cracking reaction is accelerated by strong Brønsted acids. The 2-propanol dehydration activity of Al/AlMCM-41(15) (Si/Al = 10) prepared by template cation exchange method is higher than that of H-ZSM-5, this is due to the fact that the dehydration reaction is catalyzed by weak Lewis and Brønsted acids and hence the strong Brønsted acidity of H-ZSM-5 has no significant role on this reaction. Al/AlMCM-41(15) also showed higher dehydration activity than HY zeolite and amorphous silica-alumina catalyst. It is also important to note that, although Al-containing MCM-41 catalysts showed higher cracking or dehydration activity than microporous HY zeolite, however, the acid strengths (weak and strong acid sites) as well as the number of acid sites of Al-containing MCM-41 catalysts are lower than those of HY zeolite [21]. This comparative study indicates that Al-containing MCM-41 catalysts have high potential, particularly in weak to moderate acid-catalyzed reactions.
4. Conclusions The mesoporous Al-containing MCM-41 catalysts prepared by sol–gel, grafting and template cation exchange methods are effective for the incorporation of large amounts of Al into the framework of MCM-41. The acidity of the mesoporous Al-containing MCM-41
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catalysts is increased with increase in Al incorporation into the framework of the materials. The increase in the acidity of Al-containing MCM-41 catalysts up to an optimum level increased the activity of the materials as cracking and dehydration catalysts. However, the activity of the mesoporous Al-containing MCM-41 catalysts can be significantly different depending on the synthesis methods, although the physico-chemical properties of the catalysts prepared by different methods do not undergo any major change along with their acidity. The Al-containing MCM-41 catalyst prepared by sol–gel method showed higher cracking activity, whereas that prepared by template cation exchange method showed higher dehydration activity. These Al-containing MCM-41 catalysts show remarkably high activity in cracking and dehydration reactions, higher than that of microporous HY zeolite. In particular, Al-containing MCM-41 samples prepared by different methods seem to have high potentials in weak to moderate acid-catalyzed reactions. References [1] W.F. Hoelderich, M. Hesse, F. Naumann, Angew. Chem. Int. Ed. Eng. 27 (1988) 226. [2] A. Corma, Chem. Rev. 97 (1997) 2373.
41
[3] C.T. Kresge, M.E. Leonowicz, W.J. Roth, J.C. Vartuli, J.S. Beck, Nature 359 (1992) 710. [4] J.S. Beck, J.C. Vartuli, W.J. Roth, M.E. Leonowicz, C.T. Kresge, K.D. Schmitt, C.T.W. Chu, D.H. Olson, E.W. Shephard, S.B. McCullen, J.B. Higgins, J.C. Schlenker, J. Am. Chem. Soc. 114 (1992) 10834. [5] S. Biz, M.L. Occelli, Catal. Rev.-Sci. Eng. 40 (1998) 329. [6] A. Sayari, Chem. Mater. 8 (1996) 1840. [7] M. Janicke, D. Kumar, G.D. Stucky, B.F. Chmelka, Stud. Surf. Sci. Catal. 84 (1994) 243. [8] A. Corma, V. Fornes, M.T. Navarro, J. Perez-Pariente, J. Catal. 148 (1994) 569. [9] K.M. Reddy, C. Song, Catal. Lett. 36 (1996) 103. [10] K.M. Reddy, C. Song, Catal. Today 31 (1996) 137. [11] R.B. Borade, A. Clearfield, Catal. Lett. 31 (1995) 267. [12] R. Mokaya, W. Jones, J. Catal. 172 (1997) 211. [13] R. Ryoo, S. Jun, J.M. Kim, M.J. Kim, Chem. Commun. (1997) 2225. [14] R. Mokaya, J. Catal. 193 (2000) 103. [15] C. Perego, S. Amarilli, A. Carati, C. Flego, G. Pazzuconi, C. Rizzo, G. Bellussi, Microporous Mesoporous Mater. 27 (1999) 345. [16] T. Kugita, M. Ezawa, T. Owada, Y. Tomita, S. Namba, N. Hashimoto, M. Onaka, Microporous Mesoporous Mater. 44–45 (2001) 531. [17] Y. Yonemitsu, Y. Tanaka, M. Iwamoto, Chem. Mater. 2 (1997) 2679. [18] R. Mokaya, W. Jones, J. Chem. Soc., Chem. Commun. (1997) 2185. [19] S. Namba, unpublished results. [20] J.W. Ward, J. Catal. 9 (1967) 225. [21] S. Namba, unpublished results.