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 TiO₂ and other transition-metal oxides (viz, MnO₂, Fe₂O ₃, CoO_x, NiO, CuO, ZnO, and ZrO₂) or rare-earth oxides (viz, La₂O₃, Sm₂O₃, Eu ₂O₃, and Yb₂O₃) 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 ZrO₂-supported nanogold catalyst (HDP) also showed very good performance. The Au/TiO₂ 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 Au³⁺/ Au⁰ 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 TiO₂, the gold particle size distribution, and also the surface Au³⁺/Au⁰ 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 Au³⁺/Au ⁰ ratio was higher. Both the metallic and ionic gold species (Au ⁰ and Au³⁺) are present in the Au/TiO₂ catalyst. However, the Au³⁺ species are mostly responsible for the high catalytic activity in the oxidation process.