Roles of Cu⁺ and Cu⁰ sites in liquid-phase hydrogenation of esters on core-shell CuZn_x@C catalysts

Liquid-phase hydrogenolytic reduction of esters on Cu-based catalysts is a promising approach for synthesis of alcohols. Here, core-shell CuZn_x@C materials synthesized via pyrolysis of a Zn(NO₃)₂-loaded metal-organic framework (MOF) were proposed, in which well-defined spacing between Cu species in the parent MOF and the homogeneous loading of a Zn salt leads to an ideal distribution and strong interaction of Cu and Zn after pyrolysis of the precursor. The as-synthesized CuZn_x@C showed a much higher catalytic activity than Cu@C. This higher activity is tentatively attributed to the simultaneous presence of Cu⁰ and Cu⁺ sites at ZnO crystal domains. The resulting Cu-O-Zn sites have a high tendency to adsorb the ester in the form of a surface bound species that greatly improves the hydrogenolysis, which is the rate controlling step in the reductive pathway. A theoretical study confirms the ability of this site to bind and catalytically convert fatty acid esters.