Surface chemistry of carbon-templated mesoporous aluminas

Abstract

Using infrared spectroscopy, a comparative investigation was carried out of the surface properties of mesoporous aluminas prepared by the carbon templating route and of a commercial alumina PURAL SB. In the hydroxylated state the mesoporous materials bear surface OH groups terminally bonded to an octahedral Al³⁺ ion, which has basic properties. In contrast, the surface of the commercial oxide additionally exposes OH groups which are terminally bonded to a tetrahedral cation. Bridging OH groups are Brønsted acid sites and are present on both oxides. The sites on the reference sample are stronger protonic acid sites than those on the mesoporous oxide. The strongest Brønsted acid sites are triply bridging OH groups, which are relatively more abundant on the reference oxide. Structural protons in the bulk of the oxides are also present on both oxides; they seem to be somewhat more stable in the reference sample. Condensation of basic OH groups with acidic groups at increasing dehydroxylation temperatures creates Lewis acid sites in the form of coordinatively unsaturated (cus) Al³⁺ ions the acid strength of which is higher on the reference material than on the mesoporous oxide. In conclusion, the surface properties of the investigated oxides are strongly dependent on the preparation route and conditions.