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Chemical vapor deposition (CVD) of tetramethoxysilane was carried out on alumina after the pre-adsorption of aldehyde as the molecular template, in order to form a molecular-sieving silica overlayer with controlled cavities. The adsorption of a molecule (1-naphthaldehyde) larger than the template was almost completely suppressed when acetic acid was added during CVD, showing a high selectivity based upon the shapes of the controlled cavity and the adsorbed molecule, while the selectivity was low when acetic acid was not used. The infrared (IR) spectrum and nuclear magnetic resonance (NMR) of 29Si showed that the acetic acid enhanced the oligomerization of Si alkoxide via hydrolysis, probably by acid catalysis, resulting in the formation of a dense network of siloxane. Such a dense wall of silica is speculated to determine the shape of the adsorption cavity precisely, and to generate high selectivity.

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