Electron-transfer at alumina surfaces. Part 3.—Reduction of aromatic nitro-compounds

Abstract

The generation of the anion radicals of various mono-, di- and tri-nitro aromatic compounds by electron transfer at the surface of partially dehydrated catalytic aluminas and silica-aluminas is reported. An increase in the alumina content of the latter results in an increase in reducing power. Maximum reducing activity in alumina and in silica-aluminas occurs with samples dehydrated in the range 600-700°C. A surface defect site containing an excess of oxide ions probably acts as the electron donor on both types of surface; molecular oxygen is not involved in the electron-transfer process. The adsorbed species is stable for long periods and the parent compound can be recovered unchanged from the catalyst surface. The e.s.r. spectrum obtained for all the adsorbed nitro-radicals can be analyzed in terms of a strong anisotropic interaction with one nitrogen atom. Thus, the radical-surface complex behaves essentially as an ion-pair in which the unpaired electron is largely localized on one of the nitro-groups. An increase occurs in the spin density on the nitro-group as the activation temperature is increased up to 700°C. On silica-alumina catalysts of high alumina content, as on pure aluminas, powerful electron-donor and electron-acceptor sites can be stabilized in close proximity and these active centres are to some degree interdependent.