Efficiency of a Bimetallic NiZn/AC Catalyst for Lignin Depolymerization to Phenolic and Ester Chemicals

Abstract Lignin depolymerization into valuable chemicals by non-noble metal catalysts is greatly attractive but remains a big challenge. In this study, a bimetallic NiZn/AC catalyst was developed for the depolymerization of lignin into monomers, with a particular focus on the effect of Zn addition on the electronic structure and morphology of the metallic phase. Zn added into the Ni/AC catalyst promoted the formation of a NiZn alloy, increasing the contents of Ni0 and Niδ+ species, which enhanced hydrogenolysis activity. Additionally, the presence of Zn with a high electronic charge facilitated the adsorption and activation of C−O bonds, thereby improving the bond cleavage efficiency. Zn addition also introduced Lewis acid sites in NiZn/AC catalysts. Compared with the Ni/AC catalyst, the NiZn/AC catalyst exhibited the higher monomer yields, especially NiZn0.45/AC, primarily due to its superior adsorption and hydrogenolysis activity in cleaving C−O bonds within lignin units. Notably, NiZn/AC catalysts also promoted the yield of esters with p-hydroxyphenyl and guaiacyl substructures. The characterization results demonstrated that the synergistic effect of enhanced adsorption and hydrogenolysis activity and increased Lewis acid sites facilitated aldehyde formation as an intermediate, and this process involved the dehydrogenation of hydroxyl-containing degraded monomers, followed by their reaction with dissociated •OCH3 from methanol to generate esters. This work provides new insights into lignin depolymerization pathways, leading to ester formation, and offers a strategy for designing non-noble metal catalysts for the efficient depolymerization of lignin into high-value chemicals.