pubs.acs.org/Langmuir © 2010 American Chemical Society
Tailoring the Porous Hierarchy and the Tetrahedral Aluminum Content by Using Carboxylate Ligands: Hierarchically Structured Macro-Mesoporous Aluminosilicates from a Single Molecular Source Arnaud Lemaire† and Bao-Lian Su*,†,‡ †
Laboratory of Inorganic Materials Chemistry (CMI), University of Namur (FUNDP), 61 rue de Bruxelles, 5000 Namur, Belgium, and ‡State key Laboratory of Advanced Technology for Materials Synthesis and Processing, Wuhan University of Technology, 122 Luoshi Road, 430070, Wuhan, Hubei, China Received August 20, 2010. Revised Manuscript Received September 15, 2010
A novel yet facile synthesis pathway has been developed for the design of hierarchically structured macro-mesoporous aluminosilicates with high aluminum content at tetrahedral sites using a single molecular bifunctional alkoxide (secBuO)2-Al-O-Si(OEt)3 precursor. The use of carboxylate ligands and a highly alkaline media slow down the polymerization rate of the aluminum alkoxide functionality, thus permitting the preservation of the intrinsic Al-O-Si linkage. The hierarchically structured porous aluminosilicate materials present an unprecedented low Si/Al ratio close to 1. Heat treatment applied to the synthesized material seems to favor the incorporation of aluminum into tetrahedral position (intraframework aluminum species). The macro-mesoporosity was spontaneously generated, without the use of any external templating agent, by the hydrodynamic flow of the solvents released during the rapid hydrolysis and condensation processes of this double alkoxide. This method results in materials with an open array of interconnected macrochannels. The synthesized aluminosilicate materials with tailorable macro-mesoporous hierarchy and very high Al content at tetrahedral position hold huge promise in various applications as catalysts, catalysts supports, or adsorbents.
1. Introduction Porous aluminosilicates have attracted great interest in the field of catalysis1-3 and principally in fluidizing catalytic cracking (FCC), owing to their acido-basic properties. The insertion of trivalent aluminum atoms into a tetrahedral silica framework requires additional cations to compensate for the negative framework charges, which yields Lewis and Brønsted acido-basic sites of great interest.4 Optimal selectivity and efficiency should be obtained with the higher aluminum content at tetrahedral positions. Amoros and co-workers tried to synthesize a mesoposorus aluminosilicate material with a Si/Al ratio of 1.0 using two separated precursors, aluminum tributoxide Al(OBus)3 and tetraethoxysilane (TEOS), with a complexing agent, triethanolamine (TEA).5 The material obtained presented very interesting properties, but it was not as ordered as expected and the surface area is low. Further developments led to the synthesis of Al-MCM-41, a mesoporous material of low silica content by Janicke et al. (Si/Al = 1.3).6 In order to improve mass transfer, to decrease poisoning of the active sites and to develop highly efficient catalytic processes, the preparation of advanced catalysts with hierarchical porosity is required.7,8 In many cases, the performance of a classical chemical process can be significantly improved by an integrative coupling of different process *To whom correspondence should be addressed. E-mail: bao-lian.su@ fundp.ac.be or
[email protected]. Tel:þ32-81-724531 or þ 86-2787-85-53-22. Fax: þ32-81-72-54-14. (1) Corma, A. Chem. Rev. 1997, 97, 2373. (2) Corma, A.; Domine, M. E.; Nemeth, L.; Valencia, S. J. Am. Chem. Soc. 2002, 124, 3194. (3) Soler Illia, Galo J. de A. A.; Sanchez, C.; Benedicte, L.; Patarin, J. Chem. Rev. 2002, 102, 4093. (4) Tuel, A. Microporous Mesoporous Mater. 1999, 27, 151. (5) Carbera, S.; Haskouri, J. E.; Guillem, C.; Latorre, J.; Beltran-Porter, A.; Beltran-Porter, D.; Marcos, M. D.; Amoros, P. Solid State Sci. 2000, 2, 405. (6) Janicke, M. T.; Landry, C. C.; Christiansen, S. C.; Birtalan, S.; Stucky, G. D.; Chmelka, B. F. Chem. Mater. 1999, 11, 1342. (7) Leonard, A.; Su, B.-L. Colloids Surf., A 2007, 300, 129. (8) Su, B. L.; Leonard, A.; Yuan, Z. Y. C. R. Chim. 2005, 8, 713.
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units within the catalyst. Recently, there have been reports in the literature outlining the successful synthesis of macroporous materials which possess a secondary mesoporosity and/or microporosity contained into the macroporous walls.9-11 In the near future, such materials with multiple porous hierarchy could allow the diffusion of heavier oil fragments through the macroporous network. These fragments could then be cracked, into smaller-sized particles and admitted to the mesoporous regions where the cracking process would continue, finally ending up with molecules small enough to penetrate the micropores. This is the principle of the “one-pot reactor or hierarchical catalysis” concept.12-15 It is possible to incorporate macropores into a material by using either polymeric beads,16 silica opals,17 soft or hard sacrificial templates,18,19 foams,20 (9) (a) Nakanishi, K.; Kobayashi, Y.; Amatani, T.; Hirao, K.; Kodaira, T. Chem. Mater. 2004, 16, 3652. (b) Amatani, T.; Nakanishi, K.; Hirao, K.; Kodaira, T. Chem. Mater. 2005, 17, 2114. (10) (a) Valtchev, V. P.; Smaihi, M.; Faust, A.-C.; Vidal, L. Chem. Mater. 2004, 16, 1350. (b) Valtchev, V.; Smaihi, M.; Faust, A.-C.; Vidal, L. Angew. Chem,. Int. Ed. 2003, 42, 2782. (11) Huerta, L.; Guillem, C.; Latorre, J.; Beltran, A.; Beltran, D.; Amoros, P. Chem. Commun. 2003, 1448. (12) Leonard, A.; Blin, J. L.; Su, B. L. Chem. Commun. 2003, 2568. (13) Leonard, A.; Su, B. L. Chem. Commun. 2004, 1674. (14) Yang, X. Y.; Li, Y.; Lemaire, A.; Yu, J. G.; Su, B. L. Pure Appl. Chem. 2009, 81, 2265. (15) Yang, X. Y.; Leonard, A.; Ge, T.; Lemaire, A.; Su, B. L. Chem. Commun. 2010, 46, in press. (16) (a) Judith, E. G.; Wijnhoven, J.; Vos, W. L. Science 1997, 281, 802. (b) Holland, B. T.; Blanford, C. F.; Stein, A. Science 1998, 281, 538. (17) Jiang, P.; Bertone, J. F.; Colvin, V. L. Science 2001, 291, 453. (18) (a) Zhang, B.; Davis, S. A.; Mann, S. Chem. Mater. 2002, 14, 1369. (b) Walsh, D.; Arcelli, L.; Ikoma, T.; Tanaka, J.; Mann, S. Nat. Mater. 2003, 2. (c) Kataoka, K.; Nagao, Y.; Nukui, T.; Akiyama, I.; Tsuru, K.; Hayakawa, S.; Osaka, A.; Huh, N. H. Biomaterials 2005, 26, 2509. (d) Dapsens d'Yvoir, P. Y.; Hakim, S.; Shanks, B.; Su, B. L. Chem. Commun. 2010, 46, in press. (19) (a) Su, B. L.; Vantomme, A.; Surahy, L.; Pirard, R.; Pirard, J. P. Chem. Mater. 2007, 19, 3325. (b) Li, Y.; Yang, X. Y.; Tian, G.; Vantomme, A.; Yu, J. G.; Van Tendeloo, G.; Su, B. L. Chem. Mater. 2010, 22, 3251. (20) (a) Wu, M.; Fujiu, T.; Messing, G. L. J. Non-Cryst. Solids 1990, 121, 407. (b) Carn, F.; Colin, A.; Achard, M.-F.; Deleuze, H.; Saadi, Z.; Backov, R. Adv. Mater. 2004, 16, 140. (c) Chandrappa, G. T.; Steunou, N.; Livage, J. Nature 2002, 416, 702.
Published on Web 10/13/2010
DOI: 10.1021/la1033355
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emulsions,21 ice crystals,22 or even bacterial threads.23 However, it has thus far been difficult to successfully synthesize macroporous materials which incorporate meso- and microporosities within the structure as well. Hence, research has focused on targeting novel methods to achieve this aim. From this intense research has arisen a method to incorporate homogeneous macro-mesoporosity/microporosity into various oxide types (such as ZrO2, TiO2, Nb2O5, Y2O3, Al2O3, etc.) as well as aluminosilicates.7,8,12-15,19,24-26 This methodology employs alkoxide precursors and avoids the use of a physical templating agent in the creation of the porous network. This autogeneration of a hierarchy of pores is as a direct result of the generation of an important amount of solvent molecules acting like a porogen. These molecules of solvent are released during fast hydrolysis and polycondensation steps of alkoxides in water. To date, the method used to synthesize these macro-mesoporous/microporous mixed materials required two independent alkoxide sources. This reaction pathway generates heterogeneous binary oxides (two separate phases resulted; a silicate phase and an oxide of the heteroatom) due to the difference in the hydrolysis rates of alkoxide metals.27 Thus, synthesized hierarchically macro-mesoporous aluminosilicate materials with a high tetrahedral Al content in the framework are quite scarce. In order to solve this issue, an elegant and original alternative has been recently developed to use a single molecular precursor di-secbutoxyaluminoxytriethoxysilane (sec-BuO)2-Al-O-Si(OEt)3 that natively possesses the molecular Si-O-Al link.28 This could lead to the synthesis of materials with increased homogeneity which in turn introduces unprecedented acido-basic properties and a totally controlled stoichiometry. Furthermore, this single source contains two alkoxide functionalities that can undergo fast hydrolysis and polycondensation steps, allowing the autogeneration of macro-mesoporosity. The employment of this single molecular precursor has successfully led to ordered mesoporous aluminosilicate with a Si/Al ratio close to 1.28 However, it is well-known that the hydrolysis and condensation rates of aluminum alkoxides are much higher than those of alkoxisilanes even in the case of this single molecular precursor. The polymerization occurs preferably through the Al-O-Al linkage, resulting in the cleavage of the Al-O-Si bonding of di-sec-butoxyaluminoxytriethoxysilane29 and resulting in the formation of extra-framework aluminum. Hence, the main challenge is to preserve this link by fine-tuning the polymerization rate of the two functionalities. It is reported that highly (21) (a) Imhof, A.; Pine, D. J. Nature 1997, 389, 948. (b) Imhof, A.; Pine, D. J. Adv. Mater. 1998, 10, 697. (22) Nishihara, H.; Mukai, S. R.; Yamashita, D.; Tamon, H. Chem. Mater. 2005, 17, 683. (23) Davis, S. A.; Burkett, S. L.; Mendelson, N. H.; Mann, S. Nature 1997, 385, 420. (24) (a) Yuan, Z. Y.; Ren, T. Z.; Azioune, A.; Pireaux, J. J.; Su, B. L. Chem. Mater. 2006, 18, 1753. (b) Ren, T. Z.; Yuan, Z. Y.; Su, B. L. Langmuir 2004, 20, 1531. (c) Blin, J. L.; Leonard, A.; Yuan, Z. Y.; Gigot, L; Vantomme, A.; Cheetam, A. K.; Su, B. L. Angew. Chem., Int. Ed. 2003, 42, 2872. (d) Vantomme, A.; Yuan, Z. Y.; Su, B. L. New J. Chem. 2004, 28, 1083. (e) Yuan, Z. Y.; Vantomme, A.; Leonard, A.; Su, B. L. Chem. Commun. 2003, 13, 61. (f) Yuan, Z. Y.; Ren, T. Z.; Su, B. L. Adv. Mater. 2003, 15, 1462. (g) Ren, T. Z.; Yuan, Z. Y.; Su, B. L. Chem. Phys. Lett. 2004, 388, 46. (h) Yuan, Z. Y.; Ren, T. Z.; Azioune, A.; Pireaux, J. J.; Su, B. L. Catal. Today 2005, 105, 647. (25) (a) Deng, W.; Toepke, M. W.; Shanks, B. H. Adv. Funct. Mater. 2003, 13, 61. (b) Deng, W.; Shanks, B. H. Chem. Mater. 2005, 17, 3092. (c) Hakin, S. H.; Shanks, B. H. Chem. Mater. 2009, 21, 2027. (26) Yang, X. Y.; Li, Y.; Van Tendeloo, G.; Xiao, F.-S.; Su, B. L. Adv. Mater. 2009, 21, 1368. (27) (a) Terry, K. W.; Lugmair, C. G.; Don Tilley, T. J. Am. Chem. Soc. 1997, 119, 9745. (b) Kriesel, J. W.; Sander, M. S.; Don Tilley, T. Chem. Mater. 2001, 13, 3554. (28) Yang, X. Y.; Vantomme, A.; Lemaire, A.; Xiao, F. S.; Su, B. L. Adv. Mater. 2006, 18, 2117. (29) Chaput, F.; Lecompte, A.; Dauger, A.; Boilot, J. P. Chem. Mater. 1989, 1, 199.
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alkaline media could slow down polymerization rates of aluminum species30 and favor the conversion of aluminum precursors into monomeric Al[OH]4- species. This anion is known to copolymerize preferentially with the silicate anions to form an Al-O-Si network. Very recently, hierarchically macro-mesoporous aluminosilicate materials with high tetrahedral Al content have been successfully targeted on the basis of the self-formation mechanism from this single molecular Al-Si ester precursor under high pH condition in the presence of alkoxysilanes such as TMOS,31 TEOS, TPOS, TBOS, or a mixture of TMOS/TEOS32 as silica co-reactant. It was observed that the hierarchical porosity, the textural and structural properties, and the tetrahedral Al content in the framework are sensibly affected by the pH value of the reaction system and the reactivity of alkoxysilanes added as silica co-reactants. Interestingly, the self-generation of the hierarchical porosity is only possible at high pH values (pH = 13.0-13.5). If the mesoporosity is not significantly, but the macrostructure is largely modified by the reactivity, as well as the amount, of the inorganic silica co-reactant. It was reported that some complexing agents, such as acetone, can be used to accommodate the polymerization rate of aluminum alkoxides to prevent the cleavage of the Al-O-Si linkage of the Al-Si ester.28 It is well demonstrated that carboxylate anions are ideal chelating agents for aluminum species, and the use of such anions as controlling agent of the polymerization rate of aluminum in water has been well documented in the literature.33 However, following the self-formation principle of porous hierarchy using metal alkoxide precursor, the rate of hydrolysis and polycondensation is the key factor to generate hierarchical porosities.7,8,12-15,19,24-26,31,32 Thus, the way to balance the reduction of reactivity of aluminum alkoxide functionality of the Al-Si ester precursor to avoid formation of extra-framework aluminum species by using carboxylate anions as chelating agent, and the need of high speed release of alcohol and water molecules as porogen during the hydrolysis and condensation process to produce a hierarchical porosity are of major challenge. In this work, some different carboxylates were investigated, namely, sodium acetate, sodium l-lactate, sodium oxalate, sodium citrate, sodium ethylenediaminetetraacetate, and a long alkyl chained carboxylate molecule (sodium caprylate) (Figure 1). On the basis of our previous studies,31,32 the pH value of the syntheses realized in the present study was maintained at 13.0 in order to favor the formation of Al[OH]4- species and the generation of a hierarchical porosity. The knowledge obtained will contribute to the establishment of nice chemistry in the conception of hierarchically structured porous aluminosilicate materials with high tetrahedral Al content, which are quite desirable for large series of applications as catalysts and adsorbents.2-4
2. Experimental Section 2.1. Preparation of Macro-Mesoporous Materials. The
Al-Si ester di-sec-butoxyaluminoxytriethoxysilane ((BusO)2Al-O-Si-(OEt)3) was purchased from Gelest, and other chemicals from Aldrich. They were used without any further purification. In order to synthesize typical hierarchical macro-mesoporous aluminosilicates with Si/Al ratio close to the unit, 4.3 g of sodium citrate dihydrate (99%) (chelating agent/Al = 1) was dissolved in (30) 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. (b) Irwin, A. D.; Holmgren, J. S.; Jonas, J. J. Mater. Sci. 1988, 23, 2908. (31) Lemaire, A.; Su, B. L. J. Mater. Chem. 2010, 175, 185. (32) Lemaire, A.; Su, B. L. Microporous Mesoporous Mater. 2010, in press. (33) (a) Lu, Q.; Wang, A.; Wang, X.; Zhang, T. Microporous Mesoporous Mater. 2006, 92, 10. (b) van den Brand, J.; Blajiev, O.; Beentjes, P. C. J.; Terryn, H.; de Wit, J. H. W. Langmuir 2004, 20, 6308.
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Figure 1. Schematic formula of the carboxylate chelating ligands employed in this work: (a) sodium acetate, (b) sodium l-lactate, (c) sodium oxalate, (d) sodium citrate, (e) sodium ethylenediaminetetraacetate, and (f) sodium caprylate. 60.0 mL of sodium hydroxide solution (0.24 g NaOH, pH 13.0) under constant stirring. After dissolution, 5.0 g of (BusO)2-AlO-Si-(OEt)3 was slowly added dropwise into the above solution under very slow stirring. After 1 h, the mixture was transferred into a Teflon-lined autoclave and heated at 80 °C for 24 h. The solid product was filtered, washed with water, and dried in an oven at 40 °C. An additional calcination step of the product may be carried out at 650 °C for 10 h under an ambient atmosphere. The reference sample was prepared from the single precursor ((BusO)2-Al-O-Si-(OEt)3) without any chelating agents, and it is named as Al-Si sample. Materials obtained from the single precursor and sodium acetate, sodium l-lactate, sodium oxalate, sodium citrate, sodium ethylenediaminetetraacetate, and sodium caprylate (Figure 1) as chelating agent are labeled as Ac, La, Ox, Ci, EDTA, and C8 respectively. The number following the hyphen (-) represents the ratio of chelating agent/single precursor. The sample synthesized with sodium citrate and a chelating/Al-Si ester ratio of 1 was denoted Ci-1, and after the final calcination step the Ca prefix was added to the sample name, to give Ca-Ci-1. All the information is detailed in Table 1. 2.2. Characterization. Transmission electron microscopy (TEM) experiments were performed on a Philips TECNAI-10 microscope at an acceleration voltage of 80 kV with powder samples embedded in an epoxy resin and sectioned with an ultramicrotome. The N2 adsorption and desorption isotherms were measured at -196 °C with a volumetric adsorption analyzer Micromeretics Tristar 3000. The morphology as well as the macroporous array was studied using a Philips XL-20 scanning electron microscope with conventional sample preparation and imaging techniques. Mercury intrusion-extrusion curves and corresponding pore size distributions were collected with a Micromeritics Autopore IV instrument. Fourier transform infrared (FTIR) spectra were recorded on a Perkin-Elmer Spectrum 2000 spectrometer in the 4000-400 cm-1 frequency range. Products were studied as powders dispersed in a KBr pellet. Finally, the environments of the Al and Si atoms were studied by means of 27 Al and 29Si MAS NMR spectroscopies with a Bruker Avance 500 spectrometer, and the Si/Al ratio was investigated using a Philips PU9200X atomic absorption spectrometer.
Table 1. Names, pH Conditions, and Textural Properties of the Synthesized Aluminosilicate Materials sample
chelating agent/ Al-Si ester
Al-Si-13
SBET (m2/g)
Vp (cm3/g)
318
0.5
3-30
0.5 0.5
2-5 2-5
0.4 0.3
