Selective Acrolein Hydrogenation over Ligand-Protected Gold Clusters: A Venus Flytrap Mechanism

Abstract

The catalytic partial hydrogenation of α,β-unsaturated aldehydes is an ideal reaction to understand the selectivity between two different functional groups Here the two functional groups are C═C and C═O, and the hydrogenation of C═O is preferentially desired due to the importance of the issuing products, unsaturated alcohols, in fine-chemical industries. Using density functional theory calculations, we investigate the catalytic competency toward this reaction of a Au nanocluster in the presence of protecting ligands that offer higher stability and the possibility for the uniform distribution of size-selected clusters in the catalytic system. meta-Mercaptobenzoic-acid-protected-protected Au clusters exhibit special (bidentate) ligand-metal interactions: two weak interactions, O═C-OH···Au and Ph(π)···Au, in addition to the strong S-Au covalent bonds. We find that Ph(π)···Au interactions break (or open) to expose unprotected, low-coordinated Au sites on the cluster, which have a high propensity for trapping incoming reactant molecules. We study the partial hydrogenation of acrolein at these sites and find that the unsaturated alcohol, 1-propenol, is selectively favored over possible products. The opening of the π···Au interaction and the trapping of reactant molecules at Au sites are similar to a Venus flytrap mechanism where the flowers in the plant exhibit motion to actively trap its prey.