IR STUDY OF BRONSTED ACIDITY OF AL-PILLARED MONTMORILLONITE

Three different methods have been used to study the hydroxyls of montm orillonite, both as such and pillared with Al13 polycations. The metho ds are (i) formation of H-bonds with benzene, (ii) selective H/D excha nge, and (iii) adsorption of pyridine. Besides non-acidic species, two reactive hydroxyls have been identified, characterized by stretching modes at 3738 and 3660 cm-1. The former is ascribed to a species Al-O- Si-OH, resembling the hydroxyl on silica, and the latter to a species Si-O-Al-OH, similar to extraframework species in zeolites. Both arise from sealing the clay sheet and the Al pillar; their acidity comes fro m a synergic effect between the two constituents, as similar species i n both silica and alumina are definitely less acidic. No bridged speci es Si(OH)Al is formed. Pyridine adsorption shows that (i) only the Si- O-Al-OH species may transfer protons, (ii) at least two distinct Lewis acid sites are present, and (iii) one of the related pyridine species reacts with a nearby hydroxyl to yield alpha-pyridone. (C) 1994 Acade mic Press, Inc.