Selective Hydrogenation of Cinnamaldehyde to Cinnamyl Alcohol over Au Catalysts: Influence of the Oxide-Supports

Uniform Au particles with a mean size of 1.9 nm were initially synthesized with polyvinyl alcohol (PVA) as protecting agent, and then deposited on MgO, Al2O3, TiO2, Fe2O3, Co3O4, ZnO and In2O3. After calcinations at 300 A degrees C in air to remove PVA, Au particles were highly dispersed and immobilized on all the supports, displaying narrow diameter distributions and almost identical mean sizes of 2.5-3.2 nm. The Au-oxides catalysts were further used to catalyze selective hydrogenation of cinnamaldehyde (CAL) to cinnamyl alcohol (COL). It was found that the selectivity of COL was tightly related to the acidity/basicity of the Au/oxides catalysts. The catalyst with only acidic sites such as the Au/Al2O3 catalyst gave very low selectivity of COL of 15%. The catalysts with only basic sites such as Au/MgO catalyst displayed medium selectivity of COL of 52%. Interesting, the Au catalysts with both acidic and basic sites such as the Au/In2O3 and Au/ZnO catalyts exhibited the highest selectivity of COL of around 86%. The synergistic cooperation of acidic sites and basic sites should be responsible for preferential adsorption and activation of the C=O group than C=C group in CAL, and then preferential hydrogenation of C=O to COL