Selective Reductive Depolymerization of Lignin to Vanillin over a Ni-NiO-MnO_x/Graphene Oxide Heterojunction Catalyst

Lignin is a renewable aromatic feedstock, but while oxidative depolymerization is well studied, selective reductive strategies remain underexplored due to carbonyl overhydrogenation, necessitating sustainable approaches for efficient valorization. Herein, we report a graphene oxide-supported Ni-Mn heterojunction catalyst for the selective reductive depolymerization of lignin. The catalyst exhibits broad applicability across four different lignin, including dealkaline lignin and sodium lignosulfonate (commercial lignins), as well as lignin isolated from locally available biomass sources such as Prosopis juliflora and Ficus benghalensis. Under mild hydrogenolysis conditions (30 bar H₂, 180°C), the catalyst affords a vanillin yield of 18.4 wt% (11.4 wt% isolated, >97% purity) with 84% selectivity. Solvent optimization enhanced dealkaline lignin solubility and improved depolymerization efficiency. The unique performance arises from synergistic charge redistribution at Ni-NiO-MnO_x heterojunction interfaces, which promote selective CO and CC bond cleavage while fully suppressing vanillin overhydrogenation. Two-dimensional ¹³C-¹H HSQC (Heteronuclear Single Quantum Coherence) NMR and control experiments confirmed efficient cleavage of β-O-4, β-5 and β - β linkages, particularly in guaiacyl (G) and syringyl (S) units, leading to enriched aromatic monomer production.