Insights into the reductive catalytic deconstruction of lignin over ultralow-loading palladium–zinc catalysts derived from zinc imidazolate frameworks†

Abstract

The development of high-performance noble metal catalysts at the atomic scale for the selective chemical catalytic conversion of lignin into monophenolic compounds is highly desirable but remains a challenge. Herein, we report a single-atom strategy to fabricate a highly active and stable hydrogenolysis catalyst containing an ultralow Pd content (0.1 wt%) using cobalt and zinc imidazolate frameworks as precursors. The resultant Pd–Zn@NC catalyst exhibits outstanding activity in the reductive catalytic deconstruction of lignin into aromatic compounds. The catalyst affords a phenol monomer yield of up to 49.6%, which surpasses that of commercial Pd/C. Notably, it demonstrates high selectivity towards unsaturated allyl monomers, reaching a maximum of 91% under optimized conditions. Mechanistic studies using β-O-4′ mimics reveal that the high dispersion of Zn contributes to the dissociation of hydroxyl groups, while the atomically dispersed Pd significantly enhances the hydrogenation performance. The synergistic interactions between Pd and Zn active sites activate the C–O bonds, thereby enhancing reductive aryl-ether scission in lignin.