2878
Langmuir 1996, 12, 2878-2880
Catalytic and Crystallization Behavior of a Mullite Precursor Aerogel Prepared from a Double Alkoxide: (Di-sec-butoxyaluminoxy)triethoxysilane (s-OBu)2AlOSi(OEt)3 James B. Miller, Eugene R. Tabone, and Edmond I. Ko* Department of Chemical Engineering, Carnegie Mellon University, Pittsburgh, Pennsylvania 15213-3890 Received January 19, 1996. In Final Form: March 29, 1996X We prepared a mullite precursor aerogel from a (di-sec-butoxyaluminoxy)triethoxysilane double alkoxide. The intimacy of mixing that results from the “built-in” Al-O-Si linkages delivers a mullite precursor displaying higher surface area, less Brønsted acid character, and more facile crystallization to mullite than an aerogel of the same composition prepared by a conventional prehydrolysis technique.
Introduction In preparation of multicomponent oxides as catalytic materials or ceramic precursors, the goodness of atomicscale mixing plays a central role, affecting key textural, structural, and surface chemical properties. The sol-gel preparation method is particularly well suited for controlling the extent of component mixing.1,2 Techniques such as prehydrolysis and complexation have been developed to minimize the effects of reactivity differences among various monomeric alkoxides for improving atomic-scale mixing in multicomponent materials.1 Double alkoxides (also known as heterometallic alkoxides), which contain two different cations (M and M′, for example), have received increasing attention in mixed oxide syntheses.3-5 Because they contain “built-in” M-O-M′ linkages, double alkoxides have the potential to deliver exceptionally wellmixed materials. We recently prepared an aerogel mullite precursor from (di-sec-butoxyaluminoxy)triethoxysilane, a double alkoxide having the nominal molecular formula (s-OBu)2AlOSi(OEt)3. Here, we compare its crystallization behavior and catalytic properties to those of an aerogel made from prehydrolyzed tetraethyl orthosilicate and aluminum secbutoxide, a more traditional approach to preparation of a well-mixed mullite precursor. Methods The sol-gel parameters used to prepare the aluminosilicate alcogels are summarized in Table 1. For the double-alkoxide precursor, we followed a procedure roughly similar to the one reported by Pouxveil et al.4,5 The (di-sec-butoxyaluminoxy)triethoxysilane double alkoxide (Geleste) was first dissolved in sec-butanol. A second solution of water and nitric acid (Fisher, 70 w/w) in sec-butanol was then added. The mixture gelled to a nearly transparent monolith overnight. The prehydrolyzed sample was made using a technique that parallels ones employed by several researchers.4,6,7 In a sec-butanol solvent, tetraethyl orthosilicate (TEOS, Aldrich 99.9+%) was first reacted with * To whom correspondence should be addressed. X Abstract published in Advance ACS Abstracts, May 15, 1996. (1) Miller, J. B.; Ko, E. I. Catal. Today, accepted for publication. (2) Thomas, I. M. In Sol-Gel Technology for Thin Films, Fibers, Preforms, Electronics and Specialty Shapes; Klein, L. C., Ed.; Noyes: Park Ridge, NJ, 1988. (3) Miller, J. B.; Mathers, L. J.; Ko, E. I. J. Mater. Chem. 1995, 5 (10), 1759. (4) Pouxviel, J. C.; Boilot, J. P. In Ultrastructure Processing of Advanced Ceramics; Mackenzie, J. D., Ulrich, D. R., Eds.; Wiley: New York, 1988; p 197. (5) Pouxviel, J. C.; Boilot, J. P.; Dauger, A.; Huber, L. Mater. Res. Soc. Symp. Proc. 1986, 73, 269. (6) Yoldas, B. E.; Paltrow, D. P. J. Mater. Sci. 1988, 23, 1895.
S0743-7463(96)00064-9 CCC: $12.00
Table 1. Sol-Gel Parameters Used in Preparing the Aluminosilicate Alcogels Al:Si (mol/mol) mmol M+/mL s-BuOH H2O:M+ (mol/mol) H2O:Si, PH step (mol/mol) HNO3:M+ (mol/mol) HNO3:Si, PH step (mol/mol) NH4OH:M+ (mol/mol) gel time (h)
double alkoxide
prehydrolyzed
1.0 1.0 10.0 N/A 0.016 N/A 0.0 ∼18
1.0 1.0 3.0 2.0 0.42 0.10 0.09 ∼18
substoichiometric water in the presence of a small amount of nitric acid for a 10 min prehydrolysis period. Additional nitric acid was then added, followed by a solution of aluminum secbutoxide (ASB, Alfa) in sec-butanol. Finally, an ammonium hydroxide solution (EM Science, 29 w/w as NH3) was added to provide the water to complete hydrolysis and to raise the pH for promoting network-building condensation reactions. The prehydrolyzed sample also gelled overnight but was not quite as clear as the double-alkoxide alcogel. We note that both acid ratio and water content differ in the two preparations; this reflects differences in the reactivities of the starting alkoxides. High acid amounts are necessary for the prehydrolyzed recipe to prevent precipitation of ASB upon its addition. Water ratios were chosen to deliver approximately equal gel times for the two preparations. The alcogels were dried by contact with flowing supercritical carbon dioxide in a semicontinuous autoclave (see ref 8 for details). We chose this method to preserve the open, high surface area structure of the gel’s preoxide skeleton that would otherwise collapse under the differential capillary pressures that develop within its pores during evaporative drying. High surface area facilitates measurement of our materials’ catalytic properties. Product aerogels were ground to pass 100 mesh and vacuum dried at 383 K to remove physisorbed water and then at 523 K to remove residual organics. The vacuum dried material was calcined in flowing oxygen at 773 K for 2 h in a tube furnace. Heat treatments at higher temperatures were performed on samples that had been previously heated to 773 K. Aerogels were characterized by nitrogen adsorption, differential thermal analysis (DTA), and X-ray diffraction. Activities of aerogels calcined at 773 K as catalysts for isomerization of 1-butenesa reaction that requires weak Brønsted acid sites9s were measured in a downflow, fixed bed reactor.8 Approximately 50 mg of calcined aerogel, mixed with 100 mg of an inactive silica, was loaded to the reactor. Samples were first pretreated at 573 K for 1 h in 50 sccm helium. The bed temperature was (7) Gerardin, C.; Sundaresan, S.; Benziger, J.; Navrotsky, A. Chem. Mater. 1994, 6, 160. (8) Miller, J. B.; Johnston, S. T.; Ko, E. I. J. Catal. 1994, 150, 311. (9) Goldwasser, J.; Engelhardt, J.; Hall, W. K. J. Catal. 1981, 71, 381.
© 1996 American Chemical Society
Letters
Langmuir, Vol. 12, No. 12, 1996 2879
Figure 1. Differential thermal analysis (DTA) results for double-alkoxide (left) and prehydrolyzed (right) aluminosilicate aerogels. Materials calcined at 773 K for 2 h in oxygen before thermal analysis. then lowered to 383 K and the reactor feed switched to a 100 sccm mixture of 5 mol % 1-butene in helium. Reaction products (cis- and trans-2-butene) were separated and quantified by gas chromatography. Both samples deactivated at low on-stream times but had achieved steady-state activity by 95 min. We therefore report relative catalytic activities as reaction rates at 95 min on-stream time. Relative populations of Lewis (“electron pair acceptors” associated with coordinative unsaturation or regions of unbalanced positive charge) and Brønsted (“proton donors”) acid sites were estimated according to the method of Basila and Kantner10 by analysis of the diffuse-reflectance infrared spectra of calcined (773 K) aerogels that had been dosed with pyridine. These measurements were performed at conditions (383 K in flowing He) chosen to simulate those employed in the isomerization activity tests (see ref 8 for details).
Figure 2. X-ray diffraction (XRD, Cu KR) patterns of doublealkoxide (top panel) and prehydrolyzed (bottom panel) aluminosilicate aerogels. Samples calcined for 2 h at temperatures shown.
Results and Discussion DTA scans of our aluminosilicate aerogels in the vicinity of 1300 K are shown in Figure 1. Note that both samples display an exotherm at approximately 1250 K. This feature has been attributed to crystallization of both mullite (in the “best-mixed” samples) and an aluminarich spinel.7,12-15 A 1250 K feature is usually not observed in “diphasic” mullite precursors such as those prepared from sols and/or powdered boehmite;7,11,14-16 its appearance in the scans of both of our aerogels is evidence that they are, as expected, reasonably well mixed on an atomic scale. Significantly, however, the double-alkoxide sample displays a sharper, more intense feature, which is indicative of better atomic-scale mixing.6,17 This difference suggests that prehydrolysis does not deliver as well mixed a mullite precursor as can be prepared from the double alkoxide with its “preformed” Al-O-Si linkages. X-ray diffraction patterns of the aluminosilicate aerogels at 1173, 1273, and 1373 K appear in Figure 2. Both materials are X-ray amorphous at 1173 K; at 1273 and 1373 K, both display peaks that can be indexed to mullite. Again, the peaks of the double-alkoxide material are sharper and more intense, showing the presence of larger mullite crystallites. This observation is consistent with the DTA results because the better-mixed double-alkoxide sample would be expected to undergo more extensive crystallization under the same heat treatment. The prehydrolyzed aerogel displays an additional weak peak near 2θ ) 22°, perhaps indicative of the start of crystal(10) Basila, M. R.; Kantner, T. R. J. Phys. Chem. 1966, 70, 1681. (11) Aksay, I. A.; Dabbs, D. M.; Sarikaya, M. J. Am. Ceram. Soc. 1991, 74 (10), 2343. (12) Okada, K.; Otsuka, N. J. Am. Ceram. Soc. 1986, 69 (9), 652. (13) Li, D. X.; Thompson, W. J. J. Am. Ceram. Soc. 1991, 74 (3), 574. (14) Hyatt, M. J.; Bansal, N. P. J. Mater. Sci. 1990, 25, 2815. (15) Lee, J. S.; Yu, S. C. J. Mater. Sci. 1992, 27, 5203. (16) Li, D. X.; Thompson, W. J. J. Mater. Res. 1990, 5 (9), 1963. (17) Yoldas, B. E. In Ultrastructure Processing of Advanced Ceramics; Mackenzie, J. D., Ulrich, D. R., Eds.; Wiley: New York, 1988; p 333.
Figure 3. Relative surface areas (area at T/area at 773 K) of the double-alkoxide and prehydrolyzed aluminosilicate aerogels as functions of heat treatment temperature.
lization of a cristobalite phase. The appearance of cristobalite is not unexpected in a silica-rich sample (the Al:Si atomic ratio of mullite is 3:1; recall that the aerogels are 1:1). Cristobalite formation exclusively in the prehydrolyzed sample is an indication of segregation of its alumina and silica components, consistent with its poorer atomic-scale mixing. We also note that in neither sample are there detectable peaks that can be indexed to a spinel or crystalline alumina phase; in both cases crystallization of mullite appears to proceed directly from the amorphous aerogel. This behavior is typical of materials that are richer in silica than stoichiometric mullite.18 At 773 K, the double-alkoxide aerogel has over twice the surface area of the prehydrolyzed material (522 vs 210 m2/g); this result is consistent with Fahrenholz et al.’s association of high surface area and intimate atomicscale mixing in 1:1 Al:Si mullite precursors.19 Figure 3 illustrates that both samples lose area upon heat treat(18) Yoldas, B. E. J. Mater. Sci. 1992, 27, 6667. (19) Fahrenholz, W. G.; Hietala, S. L.; Newcomer, P.; Dando, N. R.; Smith, D. M.; Brinker, C. J. J. Am. Ceram. Soc. 1991, 74 (10), 2393.
2880 Langmuir, Vol. 12, No. 12, 1996
Letters
Table 2. Catalytic/Acidic Properties of the Aluminosilicate Aerogels 1-butene isomerization ratesa area basis (mmol/(m2 h)) mass basis (mmol/(g h)) Lewis/Brønsted ratiob
double alkoxide
prehydrolyzed
0.02 12 2.6
0.44 93 0.9
a Reaction rates reported at 95 min on-stream time; see text for complete reaction conditions. b Ratio estimated at conditions of isomerization trials.
ment at roughly similar rates, with the largest decrease occurring between 1200 and 1300 K, the temperature range in which mullite crystallization occurs. Catalytic and acidic properties of the aerogels are summarized in Table 2. Both samples are active catalysts for isomerization of 1-butene with cis-2-butene/trans-2butene product isomer ratios near unity, confirming reaction at Brønsted sites. On both an area and mass basis, the prehydrolyzed sample displays a significantly higher isomerization activity. For other catalytic mixed oxides, we have linked high activity for 1-butene isomerization with high Brønsted site densities.20,21 We infer, therefore, that of the mullite precursor pair the prehydrolyzed material possesses the higher population of Brønsted acid sites. This result is supported by the results of infrared experiments which illustrate that the prehydrolyzed aerogel has a substantially lower Lewis-toBrønsted ratio. The enhanced Brønsted acidity of the prehydrolyzed aerogel relative to the double-alkoxide material provides additional evidence of atomic-scale mixing differences. Mullite’s crystalline structure allows (20) Miller, J. B.; Ko, E. I. In Advanced Catalyst and Nanostructured Materials: Modern Synthetic Methods; Moser, W., Ed.; American Chemical Society: Washington, DC, accepted for publication. (21) Miller, J. B.; Ko, E. I. Chem. Eng. J., submitted for publication.
formation of surface Lewis acid sites.22 A well-mixed mullite precursor, like our double-alkoxide aerogel, has a more mature pre-mullite structure23 and, therefore, would be expected to display a preference for Lewis acidity. On the other hand, silica-rich aluminosilicates, in which aluminum atoms isomorphically substitute for silicon, possess local charge imbalances that manifest themselves as Brønsted acid sites.22,24,25 A less well-mixed sample, like our prehydrolyzed material, contains silica-rich regions which contribute to its significant Brønsted character. Conclusions Using both crystallization behavior and catalytic data, we have shown how double alkoxides can deliver twocomponent oxides that are better mixed on an atomic scale than materials prepared by the conventional prehydrolysis technique. This work also illustrates that well-mixed samples are not always the goal of a sol-gel preparations our poorly mixed aerogel was a superior Brønsted acid catalyst. The flexibility of sol-gel chemistry to deliver a material of a desired degree of homogeneity for a specific application is, perhaps, its greatest asset as a mixed oxide preparation method. Acknowledgment. This work is supported by the Division of Chemical Sciences, Office of Basic Energy Sciences, Office of Energy Research, U.S. Department of Energy (Grant DE-FG02-93ER14345). LA960064G (22) Leonard, A. J.; Ratnasamy, P.; DeClerk, F. D.; Fripiat, J. J. Discuss. Faraday Soc. 1971, 52, 98. (23) Sundaresen, S.; Aksay, I. A. J. Am. Ceram. Soc. 1991, 74 (10), 2388. (24) Tanabe, K.; Sumiyoshi, T.; Shibata, K.; Kiyoura, T.; Kitagawa, J. Bull. Chem. Soc. Jpn. 1974, 47 (5), 1064. (25) Thomas, C. L. Ind. Eng. Chem. 1949, 41 (11), 2564.
