Rh/ZnO-Al2O3 Catalysts for Selective Hydrogenation of Crotonaldehyde

Gas phase hydrogenation of crotonaldehyde was performed over 1 wt% Rh/ZnO-Al2O3 catalysts with various Zn/Rh atomic ratios. Monometallic Rh/Al2O3 was also prepared for comparison. The samples were prepared by the successive impregnation of Al2O3 with chloride precursors of zinc and rhodium. The solids have been characterized by H2 chemisorption, temperature-programmed reduction, scanning electron microscopy, and cyclohexane dehydrogenation. Their catalytic behaviour in the gas phase crotonaldehyde hydrogenation reaction after reduction treatment in flowing hydrogen at 723 K was investigated. The relationship between catalytic activity, selectivity for crotyl alcohol, and physicochemical properties of the catalysts was examined. Results obtained showed that the presence of Zn clearly promotes the hydrogenation of the carbonyl bond. The catalyst with Zn/Rh atomic ratio of 5 displayed good catalytic stability and the highest selectivity for crotyl alcohol (70%) along with alloy formation.