Catalytic oxidative depolymerization of lignin to aromatic compounds using transition metal doped ionic Liquid-based polyoxometalate catalysts

Abstract

Significant attention has been focused on the depolymerization of lignin into monophenolics, as lignin is the most abundant biomass feedstock, and its valorization is considered crucial for a complete biomass refinery. In this study, we investigate catalytic oxidative depolymerization using novel bimetallic transition metal (Ni and Co) doped ionic liquid-polyoxometalate (ILPOM) composites under aerobic conditions. We systematically evaluate various catalysts, including [MIMPS]H₂PW₁₂O₄₀, Ni[MIMPS]PW₁₂O₄₀, Co[MIMPS]PW₁₂O₄₀, [MIMPS]H₂PMo₁₂O₄₀, Ni[MIMPS]PMo₁₂O₄₀, and Co[MIMPS]PMo₁₂O₄₀, assessing their impact on monomer yield and selectivity. Notably, Co[MIMPS]PMo₁₂O₄₀ emerges as a superior catalyst, producing high yields of key aromatic monomers, primarily vanillin and methyl vanillate, under optimized conditions. Additionally, the Co[MIMPS]PMo₁₂O₄₀ catalyst demonstrates effective cleavage of the C-O and/or C_α-C_β bonds within a β-O-4 dimer model compound, indicating potential catalytic cracking capabilities. This investigation elucidates the intricate interplay among transition metals, ionic liquids (ILs), and lignin, providing a novel pathway for lignin transformation.