Copper and manganese bimetallic catalysts for oxidation of prot lignin: effects of metal oxide on product yield

Lignin is a macro polymer with aromatic structure and is a highly promising renewable carbon source for valuable chemicals and energy. It is available in huge quantities as a by-product from the paper & pulp industry and 2G ethanol industry. Valorization of lignin into useful chemicals can play a vital role in overcoming the demand of diverse chemicals being produced from petroleum feedstock. In the present study, we have examined the oxidative conversion of prot lignin in the presence of Cu/γ-Al2O3, Mn/γ-Al2O3, and Cu-Mn/γ-Al2O3 in water and ethanol-water co-solvent mixture. In water, maximum bio-oil yield of 24.0 wt.% was noticed with Cu/γ-Al2O3. In the case of ethanol-water (50/50) (wt/wt) co-solvent mixture, the maximum bio-oil yield of 74.3 wt.% was obtained with Cu/γ-Al2O3, catalyst. The bio-oil was analysed by GC-MS, 1H-NMR, and FT-IR analyses. The GC-MS compounds were grouped as phenolic (G-, H-, and S-type), heterocyclic, acidic, and other type compounds. Among all the catalysts, Cu/γ-Al2O3 showed the maximum amount of phenolics (84.2%) using 25/75 (wt/wt) ethanol-water solvent. In the case of water as reaction medium, the non-catalytic and catalytic oxidation of lignin showed the maximum selectivity towards acetosyringone with area percentage as 42.0% and 34.4% respectively. In addition, the formation of vanillin (20.7%) was observed in the presence of Cu/γ-Al2O3 using water.