ACTIVITY AND SELECTIVITY IN THE REACTIONS OF SUBSTITUTED ALPHA,BETA-UNSATURATED ALDEHYDES

The effect of substitution of alpha,beta-unsaturated aldehydes on the activity and selectivity of a series of platinum catalysts was investi gated for the hydrogenation of these compounds. The silica-supported p latinum catalysts were promoted by a selected group of additives: comp ounds of Na (alkali metal), V, Ti, Fe (transition metals), Ga, Sn, and Ge (nontransition elements). For all catalysts the selectivity to uns aturated alcohol (UOL) increased with increasing substitution on the t erminal olefinic carbon atom (crotonaldehyde and methyl-crotonaldehyde , respectively). This indicates that the adsorption modes are related to selectivity and the selectivity is influenced by the competition of the C=O and C=C group for the same sites on the promoter. In the hydr ogenation of methacrolein, wherein the methyl substituent is located o n the internal olefinic carbon atom, and in the hydrogenation of methy l vinyl ketone, very pronounced differences in the selectivity to UOL were observed. The trends in activity and selectivity indicate that th e substitutional effects are mainly steric in origin. In general, the activity of a specific group decreases with substitution on that group . (C) 1995 Academic Press, Inc.