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Chem. Mater. 2008, 20, 1410–1420
Micromesoporous Monolithic Al-MSU with a Widely Variable Content of Aluminum Leading to Tunable Acidity David Ortiz de Zárate,† Frédéric Bouyer,† Heiko Zschiedrich,† Patricia J. Kooyman,‡ Philippe Trens,§ Julien Iapichella,§ Robert Durand,§ Carmen Guillem,4 and Eric Prouzet*,†,⊥ Institut Européen des Membranes, CNRS-UniVersity of Montpellier-ENSCM, 1919 route de Mende F-34293, Montpellier Cedex 5, France, DelftChemTech, Julianalaan 136, 2628 BL Delft, The Netherlands, Institut Charles Gerhardt, CNRS-UniVersity of Montpellier-ENSCM, 8 rue de l’Ecole Normale, F-34296, Montpellier Cedex 5, France, Institut de Ciència dels Materials (ICMUV), UniVersitat de València, P.O. Box 2085, 46071 València, Spain, and Department of Chemistry, UniVersity of Waterloo, 200 UniVersity AVenue West, Waterloo, Ontario N2L 3G1, Canada ReceiVed August 28, 2007. ReVised Manuscript ReceiVed NoVember 20, 2007
We report a new development of the so-called MSU route, the first route to use nonionic surfactants and block-copolymers, for the synthesis of silica with bimodal porosity in the micromesoporous range. Among the different synthesis strategies that have been explored until now, the “two-step” process led to the formation of stable hybrid micelles before silica condensation. This mechanism provided a large freedom in the adjustment of the reaction parameters and consequently of the final material structure. We report here that these hybrid micelles are stable enough to allow them to be concentrated through a new three-step method that involves (i) hybrid micelle assembly, (ii) hybrid micelle concentration (HMC), and (iii) silica condensation that leads to monolithic bodies instead of the usual powder. The silica: surfactant ratio is critical to achieving a well-ordered structure that exhibits a final pore size in between zeolites and mesoporous materials with a bimodal distribution centered at 0.6 and 2.0 nm. This method allowed us to homogeneously insert metal salts that remain trapped between the hybrid micelles after the second step without any disturbance of the silica framework. This is illustrated in this report by the addition of aluminum with different rates. Aluminum was inserted homogeneously up to a Si:Al (respectively, Al:Si %) atomic ratio of 2.6 (respectively, 38%). The Al-MSU obtained by this method exhibits strong acidity with a homogeneous broad range of acidity strength that varies linearly with the aluminum content, as demonstrated by NH3 TPD ranging from 100 to more than 600 °C and preliminary catalytic tests.
Introduction The synthesis of mesostructured materials has opened a new field of research in the domain of porous materials since their (re)discovery in 90s by the groups of Beck and Kuroda.1 Main breakthroughs have been widely reviewed since their initial discovery.2 Among the many possible applications that have been explored until now (including nanocasting, membrane processes and chromatography), the main expectations are in the field of catalysis. Because these materials exhibit a well* Corresponding author. E-mail:
[email protected]. † Institut Européen des Membranes, CNRS-University of MontpellierENSCM. ‡ DelftChemTech. § Institut Charles Gerhardt, CNRS-University of Montpellier-ENSCM. 4 Universitat de València. ⊥ University of Waterloo.
(1) (a) Kresge, C. T.; Leonowicz, M. E.; Roth, W. J.; Vartuli, J. C.; Beck, J. S. Nature 1992, 359, 710. (b) Beck, J. S.; Vartuli, J. C.; Roth, W. J.; Leonowicz, M. E.; Kresge, C. T.; Schmitt, K. D.; Chu, C. T.-W.; Olson, D. H.; Sheppard, E. W.; McCullen, S. B.; Higgins, J. B.; Schlenker, J. L. J. Am. Chem. Soc. 1992, 114, 10834. (c) Inagaki, S.; Fukushima, Y.; Kuroda, K. J. Chem. Soc., Chem. Commun. 1993, 680. (d) Inagaki, S.; Fukushima, Y.; Kuroda, K. In Zeolites and Related Microporous Materials: State of the Art; Weitkamp, J., Karge, H. G., Pfeifer, H., Hölderich, W., Eds.; Elsevier Science: Amsterdam, 1994; Vol. 84, p 125. (e) Di Renzo, F.; Cambon, H.; Dutartre, R. Microporous Mater. 1997, 10, 283. (f) Chiola, V.; Ritsko, J. E.; Vanderpool, C. D. U.S. Patent 3,556,725, 1971.
defined porous framework in a porosity range (2–10 nm) larger than zeolites, they could open the heterogeneous catalytic domain to processing of larger molecules. Many applications have been reviewed but it appears that a main breakthrough still remains to be discovered.3Silica by itself is not active in catalysis and it must be modified as illustrated (2) (a) Behrens, P.; Stucky, G. D. Angew. Chem., Int. Ed. 1993, 32, 696. (b) Behrens, P. Angew. Chem., Int. Ed. 1996, 35, 515. (c) Antonelli, D. M.; Ying, J. Y. Curr. Opin. Colloid Interface Sci. 1996, 1, 523. (d) Beck, J. S.; Vartuli, J. C. Curr. Opin. Solid State Mater. Sci. 1996, 1, 76. (e) Schüth, F. Zeolites and Mesoporous Materials at the Dawn of the 21st Century: Proceedings of the 13th International Zeolite Conference, Montpellier, France, July 8-13, 2001; Elsevier: Amsterdam, 2001; Vol. 135, p 179. (f) Linssen, T.; Cassiers, K.; Cool, P.; Vansant, E. F. AdV. Colloid Interface Sci. 2003, 103, 121. (g) SolerIllia, G. J. d. A. A.; Crepaldi, E. L.; Grosso, D.; Sanchez, C. Curr. Opin. Colloid Interface Sci. 2003, 8, 109. (h) Ford, D. M.; Simanek, E. E.; Shantz, D. F. Nanotechnology 2005, 16, S458. (3) (a) Sayari, A. In Recent AdVances and New Horizons in Zeolite Science and Technology; Chon, H.; Woo, S. I.; Park, S.-E., Eds.; Elsevier: Amsterdam, 1996; Vol. 102, p 1. (b) Sayari, A. Chem. Mater. 1996, 8, 1840. (c) Corma, A. Top. Catal. 1997, 4, 249. (d) Corma, A. Chem. ReV. 1997, 97, 2373. (e) Murugavel, R.; Roesky, H. W. Angew. Chem., Int. Ed. 1997, 36, 477. (f) Brunel, D. Microporous Mesoporous Mater. 1999, 27, 329. (g) Ying, J. Y.; Mehnert, C. P.; Wong, M. S. Angew. Chem., Int. Ed. 1999, 38, 56. (h) Trong On, D.; Desplantier-Giscard, D.; Danumah, C.; Kaliaguine, S. Appl. Catal., A 2001, 222, 299. (i) Weitkamp, J.; Hunger, M.; Rymsa, U. Microporous Mesoporous Mater. 2001, 48, 255. (j) He, X.; Antonelli, D. M. Angew. Chem., Int. Ed. 2002, 41, 214. (k) Xiao, F.-S. Top. Catal. 2005, 35, 9.
10.1021/cm7024558 CCC: $40.75 2008 American Chemical Society Published on Web 01/03/2008
Micromesporous Monolithic Al-MSU
by aluminum addition that provides significant acidity. However, this property is effective only as far as the Al atoms substitute for Si in the silica framework. The final acidity is then controlled by the amount of substituting Al.4 The amount of aluminum actually inserted in the silica framework can be rather low, with Si/Al (respectively, Al/Si %) ranging usually from 20 to 100 (respectively, 1 to 5%)5 and down to 10 or even 5 (respectively, high Al content of 10 or 20%).6,7 Some mesoporous materials have been reported with lower Si/Al ratios but the homogeneity of their structure and the occurrence of a real structural porosity was not always established.8 In parallel, it is worth to mention that more or less mesostructured pure alumina was also synthesized by different methods.9 Along with surface properties, pore size control is one of the other parameters that must be easily tunable for catalytic or separation applications. Unlike zeolites that are rarely synthesized with a high Si/Al ratio, excepted for pure Si zeolite like MFI, mesoporous materials offer a broader range of composition. However, they exhibit a less-well-defined pore size and weaker steaming resistance because of the amorphous structure of their walls. On the other hand, the main drawback of zeolites is that their pore size cannot be tuned easily as for mesostructured materials. Indeed, the (4) Reddy, K. M.; Song, C. Catal. Lett. 1996, 36, 103. (5) (a) Luan, Z.; Cheng, C.-F.; He, H.; Klinowski, J. J. Phys. Chem. 1995, 99, 10590. (b) Luan, Z.; He, H.; Zhou, W.; Cheng, C.-F.; Klinowski, J. J. Chem. Soc., Faraday Trans. 1995, 91, 2955. (c) Schmidt, R.; Junggreen, H.; Stöcker, M. J. Chem. Soc., Chem. Commun. 1996, 875. (d) Climent, M. J.; Corma, A.; Iborra, S.; Miquel, S.; Primo, J.; Rey, F. J. Catal. 1999, 183, 76. (e) Badamali, S. K.; Sakthivel, A.; Selvam, R. Catal. Today 2000, 63, 291. (f) Liu, Y.; Zhang, W.; Pinnavaia, T. J. Angew. Chem., Int. Ed. 2001, 40, 1255. (g) Cesteros, Y.; Haller, G. L. Microporous Mesoporous Mater. 2001, 43, 171. (h) Liu, Y.; Pinnavaia, T. J. Chem. Mater. 2002, 14, 3. (i) Wu, S.; Han, Y.; Zou, Y.-C.; Song, J.-W.; Zhao, L.; Di, Y.; Liu, S.-Z.; Xiao, F.-S. Chem. Mater. 2004, 16, 486. (j) van Grieken, R.; Serrano, D. P.; Melero, J. A.; Garcia, A. J. Catal. 2005, 236, 122. (k) Rabindran Jermy, B.; Pandurangan, A. Appl. Catal., A 2005, 288, 25. (l) Campelo, J. M.; Luna, D.; Luque, R.; Marinas, J. M.; Romero, A. A.; Calvino, J. J.; Rodriguez-Luque, M. P. J. Catal. 2005, 230, 327. (m) Kawabata, T.; Atake, I.; Ohishi, Y.; Shishido, T.; Tian, Y.; Takaki, K.; Takehira, K. Appl. Catal., B 2006, 66, 151. (n) Palani, A.; Gokulakrishnan, N.; Palanichamy, M.; Pandurangan, A. Appl. Catal., A 2006, 304, 152. (o) Palani, A.; Pandurangan, A. J. Mol. Catal. A 2006, 245, 101. (p) San Miguel, G.; Aguado, J.; Serrano, D. P.; Escola, J. M. Appl. Catal., B 2006, 64, 209. (6) (a) Kim, S.-S.; Zhang, W.; Pinnavaia, T. J. Catal. Lett. 1997, 43, 149. (b) Ryoo, R.; Jun, S.; Kim, J. M.; Kim, M. J. J. Chem. Soc., Chem. Commun. 1997, 2225. (c) Gomez, M. V.; Cantin, A.; Corma, A.; de la Hoz, A. J. Mol. Catal., A 2005, 240, 16. (d) Dedecek, J.; Zilkova, N.; Cejka, J. Microporous Mesoporous Mater. 2001, 259, 44–45. (e) Kumaran, G. M.; Garg, S.; Soni, K.; Kumar, M.; Sharma, L. D.; Dhar, G. M.; Rao, K. S. R. Appl. Catal., A 2006, 305, 123–129. (f) Aguado, J.; Serrano, D. P.; Escola, J. M. Microporous Mesoporous Mater. 2000, 34, 43–54. (g) Mokaya, R.; Jones, W. J. Chem. Soc., Chem. Commun. 1997, 2185. (h) Ribeiro Carrott, M. M. L.; Conceiçao, F. L.; Lopes, J. M.; Carrott, P. J. M.; Bernardes, C.; Rocha, J.; Ramoa Ribeiro, F. Microporous Mesoporous Mater. 2006, 92, 270. (i) Selvaraj, M.; Kawi, S. J. Mol. Catal., A 2006, 246, 218. (j) Wu, S.; Huang, J.; Wu, T.; Song, K.; Wang, H.; Xing, L.; Xu, H.; Xu, L.; Guan, J.; Kan, Q. Chin. J. Catal. 2006, 27, 9. (k) Serrano, D. P.; Aguado, J.; Escola, J. M.; Garagorri, E. J. Chem. Soc., Chem. Commun. 2000, 2041. (l) Chiranjeevi, T.; Kumaran, G. M.; Gupta, J. K.; Dhar, G. M. Thermochim. Acta 2006, 443, 87. (7) Wan, Y.; Ma, J.; Wang, Z.; Zhou, W. Microporous Mesoporous Mater. 2004, 76, 35. (8) (a) Yao, N.; Xiong, G.; He, M.; Sheng, S.; Yang, W.; Bao, X. Chem. Mater. 2002, 14, 122. (b) Bonelli, B.; Onida, B.; Chen, J. D.; Galarneau, A.; Di Renzo, F.; Fajula, F.; Garrone, E. Microporous Mesoporous Mater. 2004, 67, 95. (c) Kang, F.; Wang, Q.; Xianga, S. Mater. Lett. 2005, 59, 1426.
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porosity of mesoporous materials can be adapted easily by changing the size of the organic template or by modification of the synthesis parameters, as was especially demonstrated for MSU-type silica prepared with nonionic surfactants.10–13 Moreover, a domain of porosity in the 0.8–2.0 nm range still exists between molecular sieves and mesoporous materials that is only covered by a few compounds that can be either molecular sieves such as AlPO4-8 (0.8 nm)14 VPI-5 (1.3 nm)15 or cloverite (3.0 nm),16 whereas most of the Micelle Templated Structures (MTS) exhibit pores above 2.0 nm. Only some recent reports described the synthesis of supermicroporous (1.0-2.0 nm) materials by using the MTS route with some specific templates as fluorinated surfactants (JLU materials with pores of 1.3 nm)17 or short-chain amines.18 Therefore, it appears that new porous materials with a pore size close to that of zeolites and the versatility of pore adjustment in the micro- to supermicroporous range could be promising candidates for both catalytic applications as well as membranes for gas separation requiring a pore size slightly higher than molecular sieves and smaller than mesoporous materials. These materials could help to fill the gap between zeolites and MTS. We report here, as a new example of the versatility of the two-step synthesis process of MSU-type silica, how we could follow a new pathway named “hybrid micelle concentration” (HMC) without any change in the type of surfactants. This synthesis led to monolithic gels with a homogeneous distribution of inserted species, this latter feature being illustrated by the addition of aluminum with a wide range of composition from Si:Al ) ∞ to 2.6 (Al:Si % from 0 to 38%), and an acidic behavior that evolves in parallel with the Al content. Experimental Section Synthesis Strategy. Our new method is based on the use of a linear nonionic polyoxyethylene-based surfactant (Tergitol T15SN, (9) (a) Vaudry, F.; Khodabandeh, S.; Davis, M. E. Chem. Mater. 1996, 8, 1451. (b) Bagshaw, S. A.; Pinnavaia, T. J. Angew. Chem., Int. Ed. 1996, 35, 1102. (c) Deng, W.; Bodart, P.; Pruski, M.; Shanks, B. H. Microporous Mesoporous Mater. 2002, 52, 169. (d) Gonzalez-Pena, V.; Diaz, I.; Marquez-Alvarez, C.; Sastre, C. E.; Perez-Pariente, J. Microporous Mesoporous Mater. 2001, 203, 44–45. (e) GonzalezPena, V.; Marquez-Alvarez, C.; Dıaz, I.; Grande, M.; Blasco, T.; PerezPariente, J. Microporous Mesoporous Mater. 2005, 80, 173. (f) Cejka, J. Appl. Catal., A 2003, 254, 327–338. (g) Aguado, J.; Escola, J. M.; Castro, M. C.; Paredes, B. Microporous Mesoporous Mater. 2005, 83, 181. (h) Ruihong, Z.; Fen, G.; Yongqi, H.; Huanqi, Z. Microporous Mesoporous Mater. 2006, 93, 212. (i) Xu, B.; Xiao, T.; Yan, Z.; Sun, X.; Sloan, J.; Gonzalez-Cortes, S. L.; Alshahrani, F.; Green, M. L. H. Microporous Mesoporous Mater. 2006, 91, 293. (j) Zilkova, N.; Zukal, ˇ ejka, J. Microporous Mesoporous Mater. 2006, 95, 176. (k) Bore, A.; C M. T.; Marzke, R. F.; Ward, T. L.; Datye, A. K. J. Mater. Chem. 2005, 15, 5022. (10) Bagshaw, S. A.; Prouzet, E.; Pinnavaia, T. J. Science 1995, 269, 1242. (11) Prouzet, E.; Pinnavaia, T. J. Angew. Chem., Int. Ed. 1997, 36, 516. (12) Martines, M. A. U.; Yeong, E.; Larbot, A.; Prouzet, E. Microporous Mesoporous Mater. 2004, 74, 213. (13) Prouzet, E.; Boissière, C. C.R. Chim. 2005, 8, 579. (14) Dessau, R. M.; Schlenker, J. L.; Higgins, J. B. Zeolites 1990, 10, 522. (15) Davis, M. E.; Saldarriaga, C.; Montes, C.; Garces, J.; Crowder, C. Nature 1988, 331, 698. (16) Estermann, M.; McCusker, L. B.; Baerlocher, C.; Merroouche, A.; Kessler, H. Nature 1991, 352, 320. (17) Di, Y.; Meng, X.; Wang, L.; Li, S.; Xiao, F.-S. Langmuir 2006, 22, 3068. (18) Shpeizer, B. G.; Clearfield, A.; Heising, J. M. J. Chem. Soc., Chem. Commun. 2005, 2396.
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CH3(CH2)14(OCH2CH2)≈NOH) as a structure-directing agent.10,19,20 The assembly of both surfactant micelles and silica oligomers (resulting from the hydrolysis of silicon alkoxide (TEOS: Si(OCH2CH3)4 in mild acidity) leads to the formation of stable hybrid micelles consisting of spherical micelles, surrounded by an outer silica shell of low density silica that interacts with the palisade of hydrophilic PEO chains.21 Hybrid micelles are very stable and constitute nanoscopic building blocks that can be used for further reactions. Usually, this first step is followed by a condensation step where condensation of mesostructured silica is catalyzed by the addition of sodium fluoride. Because the assembly step is set apart from the silica condensation, we took advantage of the dynamics of micellar systems to tune the final structure of silica by slightly changing the reaction parameters.12,22 Compared with our previous process that led to monodisperse spherical micrometric powders, several features have been modified to achieve this new process. First, the starting concentration of surfactant was 5 times higher (0.1 mol L-1 instead of 0.02 mol L-1) and the TEOS/T15SN molar ratio was also increased from a previous value of 8–10 up to a value of 20 in the present study, which decreases the relative part of organics that must be burned off afterward. Second, the assembly step is followed by an additional second concentration step that leads to the removal of 70-90% of the solvent (water and ethanol resulting from TEOS hydrolysis). Compared with the two-step synthesis pathway that requires addition of a fluoride catalyst, silica condensation occurred spontaneously in a third step within a couple of hours. This new process cannot be compared with the direct syntheses in preformed liquid crystals that was first illustrated by Attard’s group because the latter was mostly relevant from a confinement mechanism rather than an assembly one.23 Moreover, the final concentration reached by the present process could hardly be achieved by starting directly from the concentrations that are obtained after evaporation, because faster kinetics of silica condensation, which increases with the initial concentration, usually led to the formation of ill-structured gels because of the overlap of the assembly and condensation steps. Our new “three-step” process allowed us to add different doping elements such as metal salts to the concentrated sols of hybrid micelles before reaction. We obtain a homogeneous dispersion at the mesoscopic scale and most of these metallic ions that remain trapped during the further gel condensation do not disturb the final silica structure that is formed from the hybrid micelle condensation. This concept is illustrated in the following with the addition of aluminum salt but it was also verified with other inorganic metal salts (Ag, Cu, Co, Fe, Ni, Zn, . . .). Chemicals. All synthesis reagents are analytically pure and were used as received from Fluka (tetraethyl orthosilicate, TEOS), Union Carbide Chemicals (Tergitol T15SN), Carlo Erba (HCl), and Aldrich (AlCl3 · 6H2O). Synthesis Process. For a typical synthesis of pure MSU silica prepared with a Si:surfactant molar ratio of 20, a 0.1 mol L-1 solution of surfactant was prepared by dissolution of 9.7 g of Tergitol T15S15 (Mw ≈ 860 g) in 100 mL of deionized water. The solution was further acidified to pH 2 with hydrochloric acid. Then, 38.6g (2.0 mol L-1) of TEOS (Mw ) 208 g) was slowly added under magnetic stirring. As was stated elsewhere, these conditions of dilution and acidity favor hydrolysis of TEOS but prevent silica (19) Boissière, C.; van der Lee, A.; El Mansouri, A.; Larbot, A.; Prouzet, E. J. Chem. Soc., Chem. Commun. 1999, 20, 2047. (20) Boissière, C.; Larbot, A.; van der Lee, A.; Kooyman, P. J.; Prouzet, E. Chem. Mater. 2000, 12, 2902. (21) Boissière, C.; Larbot, A.; Bourgaux, C.; Prouzet, E.; Bunton, C. A. Chem. Mater. 2001, 13, 3580. (22) Boissière, C.; Martines, M. A. U.; Tokomuto, M.; Larbot, A.; Prouzet, E. Chem. Mater. 2003, 15, 509. (23) Attard, G. S.; Glyde, J. C.; Göltner, C. G. Nature 1995, 378, 366.
de Zárate et al.
Figure 1. Scheme of the hybrid micelle concentration (HMC) process: (I) a solution of hybrid micelles is prepared and mixed with any heteroelement (• ) salt, particle, . . .); (II) after concentration by evaporation, one obtains a homogeneous transparent gel that has trapped the heteroelement homogeneously dispersed within the gel. Photo: this gel is preserved if one lets the system react at rest.
condensation, leading within minutes to the formation of a clear solution that contains hybrid micelles (HM).21 Even though these concentrations are rather high compared with our previous reports, the next step accounted for an additional HM concentration, by evaporating the solvent (water as well as ethanol resulting from TEOS hydrolysis) with an Ika-Werk Rotavapor until 70% weight loss. The solution was left to gelate for 72 h. When the sample was left to gelate at rest (procedure 1), we obtained a monolithic transparent wet gel as displayed in Figure 1. When gelation was operated under slow rolling (procedure 2), it led to a phase separation between a liquid supernatant (containing surfactant and part of the added metal salt) and a phase that turned progressively from a viscous liquid to a white monolith. Procedure 2 was applied in the present study, for all samples. The material was dried overnight, ground, and calcined in air at 500 °C for 6 h, with a 6 h preliminary step at 220 °C (heating rate of 3 °C/min). Preparation of pure MSU demonstrated that a molar Si/surfactant ratio of 12 is at least required to structure the final material (see below). Since we added an additional reagent (aluminum salt) as precursor of the solid framework, we started with a lower Si/surfactant ratio of 10. Therefore, in a typical preparation of AlMSU silica (molar ratio Si/Al ) 2.5 and TEOS/surf ) 10), a 0.1 M solution of Tergitol T15S15 was prepared by dissolution of 9.7 g of Tergitol in 100 mL of deionized water and adjusted to pH 2 with hydrochloric acid; 23.1 g of TEOS was then slowly dispersed in this solution under magnetic stirring. After evaporation of 70 wt %, 17.9 g of AlCl3 · 6H2O (dissolved in the minimum amount of water) was added to the concentrated sol, which was further left to gelate for 72 h under slow rolling. The supernatant was extracted and the solid that remained in the vial was washed with water, dried overnight, ground, and calcined in air at 500 °C for 6 h, with a 6 h preliminary step at 220 °C (heating rate of 3 °C/min). The Si:Al atomic ratio has been historically used in the zeolite community to describe the composition of aluminosilicates because the trend was to increase this value and achieve pure silica structures. In the domain of mesostructured materials, the trend is the opposite and groups work with the aim of obtaining the highest Al rate. Therefore, we chose to use the Al:(Al+Si) atomic ratio, instead of the Si:Al, as a composition parameter, which seemed to be more meaningful to us. Characterization. Small-angle X-ray scattering experiments were performed in transmission mode using 1 mm Lindemann
Micromesporous Monolithic Al-MSU capillaries on a laboratory SAXS apparatus equipped with a 4 kW copper rotating anode X-ray source (λ ) 1.54 Å) and a multilayer focusing Osmic monochromator giving high flux (1 × 108 photons/ s) and punctual collimation, with a 2D Image Plate as detector. Diffraction patterns are reported as a function of the wave vector q ) 2π/d ) 4π sin θ/λ, where d is the correlation length (also called “d-spacing”) for disordered systems. Nitrogen adsorption isotherms were measured at 77 K on a Micromeritics 2010 porosimeter (Micromeritics, Inc., Norcross, GA) using standard continuous procedures, and samples were first degassed under a dynamic vacuum at 250 °C for 15 h. Surface areas were determined using the BET method in the 0.05–0.2 relative pressure range.24 Because pore distribution measured by N2 adsorption could hardly be determined for the microporous range, we either took the value given by the Horwath-Kawazoe model (HK)25 or calculated the hydrodynamic diameter from the adsorbed volume over surface ratio, which can be applied because the pores are expected to be regular and cylindrical. Both values are only qualitative, but they allowed us to compare the different samples. Argon is a better probe to analyze microporosity, and we were allowed to analyze two samples with a Quantachrome Autosorb-1 MP and argon at 87.4 K as adsorbent (analysis kindly provided by Quantachrome Co.). They were degassed for 10 h and dehydrated at 250 °C, and the isotherms were interpreted using the “silica/cylindrical pore/NLDFT” model applied to the adsorption branch. When mesopores were present, we used a pore correlation curve based on the Broekhoff and de Boer model developed from the Kelvin equation for the mesoporous domain.26,27 SEM micrographs were obtained on a Hitachi S-5400 FEG microscope operating at 5 kV. The Al at % was determined from semiquantitative EDX analysis. These analyses were meaningful because they regarded only two light elements in a matrix that does not contain any heavy atoms. The samples were covered with carbon to increase conductivity. TEM was performed using a Philips CM30 T electron microscope with a LaB6 filament as a source of electrons operated at 300 kV. Samples were mounted on Quantifoil microgrid carbon polymer supported on a copper grid by placing a few droplets of a suspension of ground sample in ethanol on the grid, followed by drying at ambient conditions. The total acidity of the calcined samples was measured by NH3 adsorption and with TPD experiments using a conductivity cell for the detection of the effluent gas. The samples were activated at 500 °C under flowing air for several hours prior to analysis and cooled to 100 °C, at which temperature the samples were contacted with NH3 vapor. They were purged with pure nitrogen in order to remove loosely bound ammonia species and the temperature of the sample cell was ramped up to 600 °C under nitrogen flow (heating rate, 10 °C min-1, nitrogen flow, 40 mL min-1). The desorbed ammonia was trapped in a hydrochloric solution that was titrated by conductivity. As ammonia was released and formed a salt with the hydrochloric acid, the conductivity decreased and the acidic evolution of the sample was given by the first derivative of the conductivity versus temperature, with the surface under the curve giving the total amount of surface acid sites. For catalytic tests, we used a model reaction tested with different catalysts (zeolite 75, MCM-41 (36Å, Si:Al ) 50) and Al-MSU (13Å, Si:Al ) 2.6)), as we thought first that the expected formation of carbocations could be detected by in situ infrared spectroscopy. (24) Brunauer, S.; Deming, L. S.; Deming, W. S.; Teller, E. J. Am. Chem. Soc. 1940, 62, 1723. (25) Horvath, G.; Kawazoe, K. J. Chem. Eng. Jpn. 1983, 16, 470. (26) (a) Broekhoff, J. C. P.; de Boer, J. H. J. Catal. 1968, 10, 368. (b) Broekhoff, J. C. P.; de Boer, J. H. J. Catal. 1968, 10, 377. (27) Prouzet, E.; Cot, F.; Nabias, G.; Larbot, A.; Kooyman, P. J.; Pinnavaia, T. J. Chem. Mater. 1999, 11, 1498.
Chem. Mater., Vol. 20, No. 4, 2008 1413 Scheme 1. Prins Reaction
In fact, it is demonstrated in this paper that, depending on the catalyst employed, this reaction can lead to the formation of phenyldioxane from styrene, that is, the actual Prins reaction, whereas when very strong acid catalysts are used, the reaction path does not lead to the same reaction product (see Scheme 1). As a first clue for the high acidity of Al-MSU, this test could provide a good example of different behaviors of an acid sensitive reaction. The catalytic reaction was performed using a batch reactor with experimental conditions described by Aramendia et al.28 The catalysts were activated for 8 h at 550 °C (ramp under air at 2 °C/min). Two-tenths of a gram of catalyst was put into a flask containing paraformaldehyde (1g), dioxane (5 mL), styrene (0.985 mL), and biphenyl (0.441 g). The catalytic reaction was performed at 100 °C under argon in a 50 mL round flask mounted with a condenser. Dioxane, styrene, and biphenyl were analyzed by chromatography prior to catalysis to ensure the purity of the reagents and biphenyl was used as internal reference, as it does not interfere with the Prins reaction. The chromatographic operating conditions were selected as follows: split ratio ) 20, column oven at 50 °C for 2 min followed by a ramp of 10 °C/min up to 155 °C. The run lasted 12.5 min.
Results Pure MSU Silica. To study the optimum synthesis conditions, we first prepared pure MSU silica with a Si:surfactant molar ratio varying between 8, 12, 16, and 20 and the surfactant concentration being kept constant (see Table 1). Samples were named Si-15-n, where 15 is the number of EO groups in Tergitol T15S15 surfactant and n the Si:surfactant molar ratio. Before thermal treatment, all the X-ray diffraction patterns of the as-synthesized gels exhibit a single peak characteristic of a correlation length of d ) 2π/Q ranging from 5.2 to 4.7 nm for Si-15-8 to Si15-20, respectively (see the Supporting Information, Figure SM.1, and Table 1). After calcination (Figure 2), the X-ray diffraction patterns exhibit two components: one scattering curve characteristic of disordered porous structures and a correlation peak characteristic of ordered porous structures. For Si:surfactant ) 8, the amorphous contribution is very important (intensity scale on the Y axis) and the correlation peak appears as a shoulder at ∼3.9 nm in the scattering curve. As the Si/surfactant ratio increases from 8 to 20, the amorphous contribution decreases in parallel with an increase of the correlation peak: Si-15-12 represents an intermediary stage between mostly amorphous sample (Si-15-8) and well(28) Aramendia, M. A.; Borau, V.; Jiménez, C.; Marinas, J. M.; Romero, F. J.; Urbano, F. J. Catal. Lett. 2001, 73, 203.
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Table 1. Physicochemical Properties of Calcined MSU Silica Prepared with Tergitol 15S15 and Different Amounts of Silica sample
Tergitol
dspacing (nm) as-synthesized
dspacing (nm) calcined
SBET (m2/g)
Vpore (cm3/g)
Dpore (nm)
wall thickness (nm)d
Si-15-8 Si-15-12 Si-15-16 Si-15-20
15S15 15S15 15S15 15S15
5.2 4.7 4.6 4.6
3.9 (sh.)a 3.5 3.6 3.7
762 610 570 500
0.39 0.29 0.27 0.24
3.0b 1.7c 1.5 c 1.4 c
0.9 1.8 2.1 2.3
a shoulder. b From Broekoff and deBoer.26b directly deduced from dspacing - Dpore.
c
From Horvath and Kawazoe.25
Figure 2. SAXS pattern of calcined silica obtained with pure silica and Si/Tergitol 15S15 molar ratios varying from 8 (Si-15-8), 12 (Si-15-12), 16 (Si-15-16), and 20 (Si-15-20). There is a huge discrepancy between the Si-15-8 sample that exhibits a SAXS pattern mostly characteristic of an amorphous porous material and the other samples that display a well-resolved diffraction single peak. Inset: the Si-15-8 SAXS pattern.
ordered samples (Si-15-16 and -20). For this latter sample, the material is well structured. Concomitantly, the associated d-spacing increases slightly from 3.5 to 3.7 nm from Si-1512 to Si-15-20 (Table 1). The structural difference shown by XRD between the Si15-8 sample and the other samples is also observed from the nitrogen adsorption isotherms (Figure 3). All isotherms present a similar shape without textural porosity (no adsorption step in the highest relative pressures) and an adsorption step in the microporous to supermicroporous range (1.0-2.0 nm). The isotherm of the Si-15-8 sample differs from the others (Si-1512 to -20) by its broader pore size distribution and a larger mean value (∼3 nm) than for the other samples (
