Carboxylic acid formation by hydroxyl insertion into acyl moieties on late transition metals

Abstract

Aqueous phase reforming of alcohols over Pt has been discussed to operate along two pathways, decarbonylation and decarboxylation. To gain a better understanding of the activity of various catalysts for decarboxylation, we examined computationally a key step of this mechanism on the 12 transition metals of groups 8 to 11, namely the formation of a carboxylic acid intermediate via metal-mediated insertion of OH into an acyl group. The trend of the calculated barriers of OH insertion parallels the oxophilicity of the metals. A separation of the reaction into two formal steps isolates OH activation as a major contribution to the barrier and, not unexpectedly, indicates a strong dependence on the OH adsorption energy. A decomposition analysis of the activation energy reveals that weaker OH adsorption also correlates with the interaction energy between the adsorbed fragments in the transition state, thus indirectly lowering the barrier for OH insertion. Metals in the bottom right-hand corner of the transition metal block studied –Pt, Au, and Ag– bind OH relatively weakly, hence feature a high OH insertion activity. We applied these findings to rationalize various experimental results and suggest catalysts for decarboxylation.