Selective furanyl ring hydrogenation of 5-hydroxymethylfurfural at sub-ambient temperature via steric effect on decorated Pd surfaces

It is a great challenge to synthesize the medical intermediate 5-hydroxymethyltetrahydrofurfural (HMTHF) from selective hydrogenation of 5-hydroxymethylfurfural (HMF), as the branched CO group is usually preferentially hydrogenated over active metal, e.g. Pd sites. Herein, we designed a surface-decorated Pd@NSi@Al2O3 catalyst and achieved selective hydrogenation of HMF, yielding 83.5% HMTHF at sub-ambient temperature (5 °C). Combination of experiments and DFT calculations verify the steric effect caused by the decoration of Pd surfaces led to larger bond lengths between O or C in –CHO ligand of HMF and the closest surface Pd sites. The total activation barrier for the hydrogenation reactions on the –CHO group of HMF becomes larger from 0.60 eV on Pd(111) to 0.83 eV on the cramped Pd(111). As a result, CC groups in the furanyl ring was preferentially hydrogenated over the branched CO of HMF. Furthermore, hydrogen bonds between the hydroxymethyl of the adsorbed HMF and the amine groups on Pd surfaces promote the adsorption of HMF and significantly increase the catalytic activity. Notably, this catalyst is robust and readily applicable in the furanyl ring hydrogenation of furfural and 5-methylfurfural. Overall, a controllable surface decoration strategy has been delivered that tailoring an efficient catalyst for selective hydrogenation of aromatic aldehyde.