Heterogeneous enantioselective hydrogenation of an unsaturated carboxylic acid over Pd supported on amine-functionalized silica

The heterogeneous enantioselective hydrogenation of the C--C bond of aliphatic alpha,beta-unsaturated carboxylic acids is a key asymmetric catalytic reaction because of its application in the production of pharmaceuticals. In this study, we investigated the enantioselective hydrogenation of (E)-2-methyl-2-butenoic acid over cinchonidine-modified Pd catalysts. To improve the catalytic performance, a support material, mesocellular silica foam (MCF), was functionalized by grafting with primary amines. The use of the amine-functionalized MCF (NH2-MCF) support resulted in enhanced activity and enantioselectivity. The promoting effects originated from the increased electron density of the Pd nanoparticles and the selective blocking of low-coordinated Pd sites by the strong interaction of the amine groups with Pd nanoparticles. Promisingly, the Pd/NH2-MCF catalyst maintained its enantioselectivity over five successive cycles, thus demonstrating the reusability. Our results provide insights into the origin of the promoting effects of amine-functionalized supports and offer a strategy for improving supported catalysts for structure-sensitive reactions.