Hydrogenation of alpha,beta-unsaturated aldehydes over Ru/Al2O3 catalysts

The hydrogenation of citral and cinnamaldehyde has been investigated over Ru/Al2O3 catalysts. The effect of metal particle size on the catalytic activity and selectivity has been studied by using catalysts having a metal dispersion ranging from 0.05 to 0.88. It has been observed that the overall rate of hydrogenation of both the unsaturated aldehydes is not influenced by the Ru particle size. In the hydrogenation of cinnamaldehyde a higher selectivity to cinnamyl alcohol has been observed on catalysts with larger metal particle size. No variations have been found in the hydrogenation of citral. It is suggested that steric effects do not influence significantly the product selectivity. A repulsive interaction between the aromatic ring and the catalyst surface would explain the higher selectivity to cinnamyl alcohol. On all the investigated catalysts the isolated C=C double bond of citral shows a low reactivity. It is suggested that the unsaturated aldehyde is adsorbed through the carbonyl group. This strong adsorption prevents the hydrogenation of the isolated olefinic bond which is located far away from the adsorption centers.