Langmuir 1998, 14, 4623-4629
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Molecular Sieving Silica Overlayer on γ-Alumina: The Structure and Acidity Controlled by the Template Molecule Nobuaki Kodakari, Kohei Tomita, Koji Iwata, Naonobu Katada, and Miki Niwa* Department of Materials Science, Faculty of Engineering, Tottori University, Koyama-cho, Tottori 680-8552, Japan Received February 27, 1998. In Final Form: May 18, 1998 A silica overlayer was prepared on γ-alumina by a method of chemical vapor deposition (CVD) of silicon alkoxide using preadsorbed benzaldehyde and R-naphthaldehyde as a template. Infrared spectroscopy confirmed that the preadsorbed benzoate anion species remained after the deposition of silica, indicating that silica was deposited on the exposed surface of alumina. On the basis of the adsorption property of modified alumina, it is considered that the silica overlayer had a vacancy whose size was as large as the template molecule. 29Si NMR spectroscopy showed that the Si species deposited using the template was Si(OAl)1(OSi)1(OH)2 or Si(OAl)2(OSi)1(OH)1, while the deposition without the template formed such a species as Si(OAl)1(OSi)3 and Si(OAl)1(OSi)2(OH)1 bonded via a two-dimensional siloxane network. Lack of activity for double-bond isomerization of 1-butene to 2-butene on the silica overlayer prepared using the template was consistent with the previous results that the two-dimensional network of siloxane on the alumina surface induced Brønsted acidity on the Al-O-Si-OH species. Both the adsorption and acidic properties on silica-deposited alumina using R-naphthaldehyde were different from those on the sample prepared using benzaldehyde, showing that the structure of the silica overlayer was controlled by the template molecules.
Introduction In the field of materials science, the design of a solid surface on a molecular scale is one of the most attractive subjects. The “molecular imprinting method” is a technique for creating a cavity on a solid surface by using a molecule as a template. On the basis of the shape of the cavity which is similar to that of the template molecule, the materials prepared by this technique have an affinity for the template analogue molecules.1,2 Lerner et al. have utilized the molecular imprinting method for an immune system to create a catalytic antibody with a specific affinity for a substrate.3 The specificity of these antibodycatalyzed reactions rivals or exceeds that of enzymatic reactions. In parallel to these attempts, many researchers have applied the molecular imprinting method to synthetic polymers to provide the molecular recognition property. For example, Mosbash et al. synthesized the polymers with a stereoselectivity and applied these polymers to adsorbate and catalyst.4,5 While the molecular imprinting method has widely been utilized for antibodies and polymers, few studies have been carried out for the inorganic materials.6-9 The inorganic materials, for example metal oxides, have an advantage for practical use, because of the thermal * Corresponding author. Phone and fax: +81-857-31-5256. E-mail:
[email protected]. (1) Davis, M. E. CATTECH 1997, 1, 19. (2) Davis, M. E.; Katz, A.; Almad, W. R. Chem. Mater. 1996, 8, 1820. (3) Lerner, R. A.; Schultz, P. G. Science 1995, 269, 1835. (4) Sellergren, B.; Lepisto¨, M.; Mosbach, K. J. Am. Chem. Soc. 1988, 110, 5853. (5) Matsui, J.; Nicholls, I. A.; Karube, I.; Mosbash, K. J. Org. Chem. 1996, 61, 5414. (6) Dickey, F. H. J. Phys. Chem. 1955, 59, 695. (7) Wulff, G.; Heide, B.; Helfmeier, G. J. Am. Chem. Soc. 1986, 108, 1089. (8) Morihara, K.; Kurihara, S.; Suzuki, J. Bull. Chem. Soc. Jpn. 1988, 61, 3991. (9) Matsuishi, T.; Shimada, T.; Morihara, K. Bull. Chem. Soc. Jpn. 1994, 67, 748.
stability compared to that of the organic materials. We have proposed a new method of modification of metal oxide in order to control the surface structure on the molecular scale: the silica overlayer was deposited on SnO2 by chemical vapor deposition (CVD) of tetramethoxysilane (TMOS) after benzoate anion had been adsorbed, and then the benzoate anion was removed. In other words, the preadsorbed benzaldehyde was used as the template to prepare the silica overlayer. This modification method is based on the following two findings: the stable adsorption of benzaldehyde and the full coverage of the silica monolayer by CVD of TMOS. Benzaldehyde is adsorbed on weakly basic metal oxides such as Al2O3, TiO2, ZrO2, and SnO2 to form a benzoate anion, and the adsorbed species is desorbed as benzonitrile by the reaction with gaseous ammonia.10,11 On the other hand, a silica overlayer with a monoatomic order thickness is formed by CVD of TMOS on these metal oxides, and it completely covers the metal oxide surface.12,13 The above-mentioned technique to modify the metal oxide surface on the molecular scale was given by combining these two findings. The silica overlayer prepared by the proposed modification method is expected to have a cavity or vacancy whose size is as large as the template molecule and whose depth is of monoatomic order. The silica-deposited tin oxide, called the “molecularsieving silica overlayer”, prepared by this method indeed exhibited the molecular sieving property for the adsorption of aldehydes with various molecular sizes.14-16 Tin oxide was selected as a support metal oxide in our previous (10) Niwa, M.; Inagaki, S.; Murakami, Y. J. Phys. Chem. 1985, 89, 3869. (11) Niwa, M.; Suzuki, K.; Kishida, M.; Murakami, Y. Appl. Catal. 1991, 67, 297. (12) Niwa, M.; Katada, N.; Murakami, Y. J. Phys. Chem. 1990, 94, 6441. (13) Niwa, M.; Katada, N.; Murakami, Y. J. Catal. 1992, 134, 340. (14) Kodakari, N.; Katada, N.; Niwa, M. J. Chem. Soc., Chem. Commun. 1995, 623.
S0743-7463(98)00240-6 CCC: $15.00 © 1998 American Chemical Society Published on Web 07/11/1998
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study, because SnO2 has a semiconductor property which is applied to the gas sensor.17 In this paper, we will examine the preparation of a silica overlayer on alumina using the preadsorption of two aldehyde templates: benzaldehyde and R-naphthaldehyde. Alumina was chosen as the support metal oxide, because the mechanism of CVD and the microstructure of the silica overlayer formed on alumina in the absence of the template have been studied well.18-20 The stoichiometry of the CVD process and the infrared (IR) spectroscopy showed that the deposited Si alkoxide formed a two-dimensional network of Si-O-Si, and the formed monolayer of silica mainly consisted of the Si(OAl)1(OSi)3 and Si(OAl)1(OSi)2(OH)1 species.18 The 29Si nuclear magnetic resonance (NMR) spectroscopy supported the presence of these species.19 Moreover, the catalytic property as a solid-acid catalyst has been shown to be generated by the network structure.20 The structural and chemical properties of the formed silica overlayer will be studied on alumina in this study. For this purpose, the properties of the silica overlayer prepared by CVD using the preadsorption of a template on alumina were evaluated from multiple viewpoints, that is, the chemisorption property, 29Si NMR spectroscopy, and the catalytic activity for the acid-catalyzed reaction (the double-bond isomerization of 1-butene into 2-butene). Experimental Section Preparation of Silica Overlayer Using Template. Alumina was supplied by the Catalysis Society of Japan, as a reference catalyst, JRC-ALO4. The crystal phase was proven to be γ-Al2O3, and the surface area was 151 m2 g-1. The alumina sample (ca. 1 g) was placed on a basket hung by a quartz spring in a vacuum vessel, and the change of the weight was monitored. The sample was evacuated under a reduced pressure less than 0.4 Pa at 673 K until a stable weight was observed, and it was then exposed to vapor of benzaldehyde (227 Pa) or R-naphthaldehyde (13 Pa) at 473 K. The amount of adsorbed aldehyde was calculated from the weight gain. The adsorption amount was adjusted by varying the time for this procedure. Silica was then deposited at 523 K by chemical vapor deposition of tetramethoxysilane (TMOS) after the sample was evacuated. The vapor of TMOS, which was kept at approximately 300 Pa by chilling the reservoir with an ice bath, was admitted onto the sample. Silica was deposited by the repetition of evacuation and introduction of TMOS vapor. This procedure was required for removing the produced gaseous organics such as methanol and dimethyl ether and making the sample exposed to TMOS.13 Finally, the sample was calcined in 26.6 kPa of oxygen at 673 K until a stable weight was observed. The amount of deposited silica was calculated from the weight gain after the calcination by oxygen. Infrared Spectroscopy. The infrared (IR) spectrum was measured in order to show the elementary steps of the preparation. Alumina (10 mg) was pressed into a disk 10 mm in diameter. After the disk was pretreated at 673 K under a reduced pressure less than 0.4 Pa, the sample was exposed to vapor of benzaldehyde (227 Pa) at 323 K. The IR spectrum of adsorbed benzaldehyde on alumina was obtained after the evacuation (
