Reactions of ketones on oxide surfaces. II. Nature of the active sites of alumina

Abstract

The nature of the active sites of alumina for the reaction of cyclohexanone was investigated by the introduction of selective poisoning agents both on the surface of the alumina as well as in the reactant itself. Thus additives which are basic in nature such as sodium ions, ammonia, pyridine, as well as acidic additives such as carbon dioxide, carbon monoxide, cyclohexanol, and isoproponal were used. Dual acid-base sites seem to be responsible for the catalytic properties. The sites responsible for the aldol condensation reaction giving a dimer seem to be similar to those sites responsible for the formation of ethers from alcohols, while the sites responsible for the formation of cyclohexene from cyclohexanone seem to be similar to those sites responsible for the formation of a carboxylate species on the adsorption of alcohols on alumina surfaces. A mechanism is proposed for the reaction of cyclohexanone which does involve the intermediacy of cyclohexanol to account for the formation of cyclohexene.