Yolk-Shell Nickel-Cobalt Phosphides as Bifunctional Catalysts in the Solvent-Free Hydrogenation of Levulinic Acid to Gamma-Valerolactone

Transition metal phosphides deliver promising catalytic potential due to their cheap cost, wide availability, and superior efficiency. However, their potential catalytic application in biomass-related reactions has not been widely investigated. In this work, a series of nickel-cobalt phosphides (NixCoyP) was synthesized by a facile fabrication strategy consisting of hydrothermal and phosphating steps and was applied to the solvent-free hydrogenation of levulinic acid (LA) to gamma-valerolactone (GVL). The effect of Ni:Co molar ratio, the acidity, and the hydrogen activation capability of these phosphides were studied and correlated with their catalytic performances. Among all the catalysts, Ni 2 Co 1 P achieved the complete conversion of LA to GVL (100.0% GVL yield). Characterization results revealed that the outstanding catalytic performance of the phosphides could be attributed to their bifunctional substrate- and hydrogen-activating ability and acidic properties, and the synergistic effect manifested by Ni and Co. The catalysts also demonstrated good recyclability after five recycles without significant loss in their activity, which can be credited to the confinement effect provided by their yolk-shell structure. This successful exploitation of nickel-cobalt phosphides in the hydrogenation of LA to GVL opens new opportunities towards the possible catalytic application of transition metal phosphides in biomass transformation reactions