INTERACTIONS AT AN ALUMINA-AQUEOUS GLYCINE INTERFACE - CHARACTERIZATION OF GLYCINE-MODIFIED BOEHMITE

Glycine is adsorbed on the surface of boehmite at room temperature by impregnation from an aqueous solution at different pH values (2.5, 6.0, 12.2). The boehmite sample impregnated at pH 6.0 was subjected to heating at 120, 250, 450, and 550 degrees C in air. DTA, TGA, IR, XRD, and N-2 adsorption at 77 K were used to examine the modified boehmite samples. At pH 6.0 the IR and XRD results proved the presence of glycine in both the alpha and beta forms; the presence of the beta form on the surface, in spite of its ignored presence in the aqueous phase, is attributed to the removal of hydration water through the drying step, which caused the adsorbed glycine to adopt the noncharged form to account for the constraints of electroneutrality. Above 120 degrees C only the beta form is found, which resists heating up to 250 degrees C, and complete desorption of glycine is effected at 450 degrees C. The adsorption appears to proceed via hydrogen bonding in which the surface hydroxyls are the proton accepters. The beta form is also identified on the surface of boehmite impregnated in the highly acidic medium and adsorbed through hydrogen bonding. In highly alkaline medium coordination with the coordinatively unsaturated surface A1 ions is believed to take place through the lone pair of electrons on the nitrogen of NH2. N-2 adsorption results showed that the parent boehmite possesses micropores, mesopores of limited sizes, and wider interstitial voids. The decrease in area and pore volume caused by adsorbed glycine was confined to micropores in the case of interaction through hydrogen bonding, while wider interstitial voids are the most affected parts when coordination takes place. (C) 1995 Academic Press, Inc.