Yolk-shell nickel–cobalt phosphides as bifunctional catalysts in the solvent-free hydrogenation of Levulinic acid to gamma-Valerolactone

The transformation of Levulinic acid (LA) to gamma-Valerolactone (GVL) is deemed an attractive biomass conversion reaction owing to the attainability of LA as a feedstock and the potential applications of GVL as a versatile building block, green solvent, and fuel additive. 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 LA to GVL. These 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. The effect of the 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, Ni2Co1P achieved 100% LA conversion 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.