Characterization of aerogel prepared high-surface-area alumina: In situ FTIR study of dehydroxylation and pyridine adsorption

Mesoporous high-surface-area alumina was prepared by a modified aerogel procedure. Specific surface areas between 530-685 m²g^-1 were obtained after heat treatment at 500°C. Nitrogen adsorption studies have shown that surface areas and pore characteristics change upon decomposition of aluminum hydroxide to oxide as well as upon compaction of oxide powders. The surface area of aluminum hydroxide increased to a maximum, while the pore volume and diameter decreased as the hydroxide was heated to a temperature of 400°C. Heating at higher temperatures resulted in sintering of the particles accompanied by a decline in the surface area. Compaction of activated alumina into pellets was accompanied by a relatively gradual change in the surface area and pore characteristics at pressures below 6.9 × 10⁷ Pa, while severe changes took place at a pressure of 1.4 × 10⁸ Pa. In situ IR studies of the dehydroxylation of the alumina surface, showed v(OH) absorptions for isolated surface hydroxy groups centering at 3670, 3714, and 3765 cm^-1, which are shifted to lower frequencies than common literature values. Pyridine was found to adsorb on Al³⁺ ions as well as through hydrogen bonding to relatively acidic surface OH groups, and IR spectra indicated the presence of strong Lewis acid sites.