Deoxygenation of benzoic acid on metal oxides - 1. The selective pathway to benzaldehyde

The mechanism of the selective deoxygenation of benzoic acid to benzaldehyd e was studied on ZnO and ZrO2. The results show conclusively that the react ion proceeds as a reverse type of Mars and van Krevelen mechanism consistin g of two steps: hydrogen activates the oxide by reduction resulting in the formation of oxygen vacancies. Subsequent re-oxidation of these vacancy sit es by benzoic acid yields benzaldehyde. Inhibition of the deoxygenation rea ction can be achieved by addition of suitable polar compounds with a high a ffinity for the oxygen vacancy sites such as carbon dioxide or water. Diffe rences in the catalytic activity and selectivity of ZnO and ZrO2 can be att ributed to differences in hydrogen activation, redox properties and extent of benzoic acid coverage. (C) 2001 Elsevier Science B.V. All rights reserve d.