Electron-transfer at alumina surfaces. Part 5.—Oxidation of aromatic amines

Abstract

The oxidation of primary, secondary and tertiary aromatic amines on oxide surfaces has been studied, using e.s.r. spectroscopy. All the amines give adsorbed radicals whose spectra are not well defined. Addition of a polar molecule displaces the surface species. The nature of the desorbed product depends on the oxide surface involved, e.g., with diphenylamine on alumina the desorbed product is diphenyl nitroxide, whereas from a silica–alumina (∼13 % Al₂O₃) surface NN′-diphenylbenzidine is obtained. These facts and other evidence indicate that the electron-acceptor sites on silica-aluminas of low alumina content differ substantially from those on alumina. On hydrogen Y zeolite two types of site capable of oxidizing aromatic amines exist, depending on the degree of dehydroxylation of the zeolite surface. One of these sites corresponds to an oxidizing site present in silica-alumina; the other is more closely related to the oxidizing centre found on the alumina surface. Possible mechanisms for the surface reactions are considered.