Preparation of Nitrogen-Doped Carbon-Supported Co-V Catalyst for Catalystic Depolymerization of Lignin to Produce Monophenolic Compounds

Lignin, a renewable biomass component, holds promise as a fossil fuel alternative through its depolymerization into monophenolic compounds for liquid fuel production. However, its complex chemical structure presents significant challenges to efficient degradation. This study addresses this limitation by developing a nitrogen-doped carbon-supported Co-V catalyst, synthesized using cobalt nitrate hexahydrate, ammonium metavanadate, and melamine as precursors. The synthesis capitalized on vanadium-nitrogen coordination and the inherent stability of vanadium nitride, eliminating the need for an external carbon source. By regulating the mass ratio of melamine to metal salts, the structural morphology and the types and contents of Co and N in the catalyst were precisely controlled. The optimized Co1-V3/NC (30) catalyst exhibited abundant defect sites, a high specific surface area, large pore volume, and hierarchical porous structure comprising micropores, mesopores, and macropores. The catalyst also featured a high content of pyridinic nitrogen and metallic cobalt (Co0), along with a relatively low content of Co-Nx species. Synergistic interactions between the active components Co0 and V3+, coupled with nitrogen-mediated anchoring, ensured uniform dispersion of active sites, thereby enhancing catalytic activity and stability. Under optimized reaction conditions (180 ℃, 0.5 MPa O2, 8 h), the catalyst achieved 86.3% bio-oil yield and 28.8% monophenolic compounds from organosolv lignin. Notably, Co1-V3/NC (30) outperformed commercial catalysts (Co1-V3/AC and Co1-V3/ZSM-5), demonstrating the critical role of nitrogen-doped carbon in anchoring and dispersing metal active components. 2D-HSQC NMR analysis confirmed the catalyst′s exceptional activity in cleaving both C—O and C—C bonds between lignin structural units, enabling efficient degradation into monophenolic compounds. Mechanistic investigations using a β-1 lignin model compound (diphenylethanone) revealed that the cleavage of Cα—Cβ bonds proceeds via two pathways: (1) O2 is reduced on the catalyst surface to generate reactive oxygen species, which coordinates with the Cβ—H bond to form a peroxide intermediate. This intermediate undergoes dimerization to benzil, followed by Cα—Cβ bond cleavage; (2) alternatively, homolytic cleavage of the peroxide O—O bond generates oxygen-centered radicals, directly breaking the Cα—Cβ bond. Structural characterization via X-ray diffraction (XRD), Brunauer-Emmett-Teller (BET) analysis, and transmission electron microscopy (TEM) confirmed that the catalyst retained its crystalline structure, high surface area, and hierarchical porosity, even after five reuse cycles. Notably, active components (Co0 and V3+) remained uniformly dispersed without aggregation, contributing to sustained catalytic performance. This work provides a scalable synthesis strategy for robust, low-cost catalysts and elucidates a reaction pathway for lignin-to-aromatic conversion, thereby advancing biomass utilization in renewable energy. These findings underscore the potential of Co-V-NC systems to replace fossil-derived catalysts for sustainable chemical production.