Oxidation of Benzyl Alcohol to Benzaldehyde by tert-Butyl Hydroperoxide over Nanogold Supported on TiO2 and other Transition and Rare-Earth Metal Oxides

Liquid-phase oxidation of benzyl alcohol to benzaldehyde by tertiary butyl hydroperoxide (at 95 °C) over nanogold supported on TiO 2 and other transition-metal oxides (viz, MnO 2, Fe 2 O 3, CoO x, NiO, CuO, ZnO, and ZrO 2 ) or rare-earth oxides (viz, La 2 O 3, Sm 2 O 3, Eu 2 O 3, and Yb 2 O 3 ) by the homogeneous deposition−precipitation (HDP) method has been investigated. The Au/TiO 2 catalyst, prepared using the HDP method, showed high activity and selectivity in the reaction. The ZrO 2 -supported nanogold catalyst (HDP) also showed very good performance. The Au/TiO 2 catalyst that was prepared using the deposition−precipitation (DP) method, however, showed poor catalytic activity, mostly because of its much-lower gold loading and/or lower Au 3+ /Au 0 ratio. Irrespective of the catalyst preparation method, the catalytic activity increased as the gold loading increased. The gold deposition method strongly influenced the amount of gold that was deposited on TiO 2, the gold particle size distribution, and also the surface Au 3+ /Au 0 ratio; using the HDP method, the gold deposition was much larger, the gold particle size was smaller, the gold particle size distribution was much narrower, and the Au 3+ /Au 0 ratio was higher. Both the metallic and ionic gold species (Au 0 and Au 3+ ) are present in the Au/TiO 2 catalyst. However, the Au 3+ species are mostly responsible for the high catalytic activity in the oxidation process.