Reduction of benzaldehyde on metal oxides

The study of benzaldehyde reduction over simple metal oxides was carried out at 300 degrees C in a helium or dihydrogen atmosphere. The results obtained show that benzyl alcohol is produced by the Cannitzaro reaction under helium and by direct hydrogenation under dihydrogen on reducible oxides such as ZnO and Cr2O3. Over slightly reducible oxides such as ZrO2, both reactions occur under Hz: the Cannizzaro reaction involves the most basic OH groups and is followed by the reduction of benzoate species by H-2 to benzaldehyde, regenerating the active hydroxyl groups. Toluene is formed under He on the most acidic oxides (Al2O3, TiO2) from a reaction involving benzyl alcoholate species, and under Hz on reducible oxides (Fe2O3) by means of consecutive reduction reactions. The present study shows that the benzaldehyde transformation via the Cannizzaro reaction is a good test for determining the mobility of surface hydroxyl groups on metal oxides, whereas its direct reduction under H-2 characterizes the metal oxide redox properties. (C) 1997 Academic Press.