Selective hydrogenation of crotonaldehyde over amorphous Co-B catalysts: The effect of Zn

The promoting effect of Zn on liquid phase hydrogenation of crotonaldehyde over amorphous Co-B catalysts has been investigated. It was found that when ZnCl2 was introduced to the reactant, zinc ions could selectively block the active sites for C = C bond hydrogenation. Moreover, zinc ions might polarize the carbonyl group as Lewis acid, making the nucleophilic attack of adjacent dissociatively adsorbed hydrogen atom much easier, thus leading to a higher yield of crotyl alcohol. On the other hand, in amorphous Co-Zn-B catalysts, zinc was in the form of Zn(OH)(2) and the amount of CoO and B2O3 increased with more zinc incorporated. These oxides were expected to nonselectively cover the active sites and depress the hydrogenation rates of the C=C bond and the carbonyl group to a similar extent, thus marginally improving the selectivity towards crotyl alcohol.