Valorization of native sugarcane bagasse lignin to bio-aromatic esters/monomers via a one pot oxidation–hydrogenation process

Abstract

The development of viable biorefinery routes for the valorization of lignocellulosics into usable products remains a challenge. Despite the significant progress in lignin valorization using the lignin-first approach or lignin-stabilization strategy, a number of issues still remain including poor monomer yield with grasses and the use of toxic chemicals and H₂ with high pressure. Herein, we report a highly effective oxidation–hydrogenation catalytic process for the complete depolymerization of sugarcane bagasse using Pd/C in the presence of atmospheric pressure O₂ and alcohol as a solvent. Under the optimum reaction conditions, 31.7 wt% of ferulic acid (FA) and p-coumaric acid (pCA) derived aromatic esters and 25.8 wt% of other four aromatic monomers were obtained based on the lignin content. In this one step approach, molecular O₂ adsorbed on the catalyst surface enhanced the formation of atomic hydrogen by alcohol oxidation, and hence improved the efficiency of the overall reductive depolymerization process. A linear correlation between the pCA content and its corresponding ester was obtained irrespective of the biomass source or whether the lignin is in the native or technical form. Another feature of this process is the separation of methylated mono-sugar derivatives and a cellulose-rich solid fraction that is readily fermentable. A plausible reaction mechanism was hypothesized which indicates that the carbon defect on the surface and the enhanced oxygen content are strongly correlated with the high activity of the Pd/C catalyst.