HYDROGENATION OF ALPHA,BETA-UNSATURATED ALDEHYDES OVER RU AL2O3 CATALYSTS/

The hydrogenation of citral and cinnamaldehyde has been investigated o ver Ru/Al2O3 catalysts. The effect of metal particle size on the catal ytic activity and selectivity has been studied by using catalysts havi ng a metal dispersion ranging from 0.05 to 0.88. It has been observed that the overall rate of hydrogenation of both the unsaturated aldehyd es is not influenced by the Ru particle size. In the hydrogenation of cinnamaldehyde a higher selectivity to cinnamyl alcohol has been obser ved on catalysts with larger metal particle size. No variations have b een found in the hydrogenation of citral. It is suggested that steric effects do not influence significantly the product selectivity. A repu lsive interaction between the aromatic ring and the catalyst surface w ould explain the higher selectivity to cinnamyl alcohol. On all the in vestigated catalysts the isolated C=C double bond of citral shows a lo w reactivity. It is suggested that the unsaturated aldehyde is adsorbe d through the carbonyl group. This strong adsorption prevents the hydr ogenation of the isolated olefinic bond which is located far away from the adsorption centers.