Catalytic oxidation of different lignin over Ni-MOFs based catalyst in oxygen: Effect of catalysts and solvents

The industrial production of monophenols was mainly dependent on fossil resources, due to growing energy demand and escalating environmental issues, more efficient and sustainable strategy, such as utilization of biomass as raw materials to economically produce monophenols, was desired, which could liberate us from the reliance on fossil resource and was of great industrial and social significance. Herein, a novel strategy was reported to produce biomass-derived monophenols from lignin by directly deconstructing C-O bond under mild conditions. A series of NiM-MOFs (M=Co, Mo, and Cu) derived bimetallic catalysts were synthesized via a simple hydrothermal method, which were characterized by XRD, SEM, TEM, XPS and NH3-TPD. The synergistic effect between the Ni and Mo greatly improved the catalytic activity and inhibited the transformation to by-products. In addition, we explored a new binary solvents system of MeOH-THF to effectively enhance the selectivity of monophenols. Under optimal catalytic conditions, Ni4Mo1/C-BN catalyst could break the C-O bonds in lignin dimer under 160 ℃ to afford phenol and benzoic acid, and the possible reaction pathway was proposed. The high yield of lignin oil from the oxidation of various realistic lignin also confirmed the superiority of this catalytic system. The lignin depolymerization products were characterized by element analysis, GPC and GC-MS, confirming the presence of mainly guaiacyl-type (G) products, while the highest yield was assigned to vanillin.