Insights into bamboo delignification with acidic deep eutectic solvents pretreatment for enhanced lignin fractionation and valorization

Deep eutectic solvent (DES) as novel and green solvents can efficiently promote the fractionation of high-purity lignin from lignocellulosic biomass pretreatment. Revealing the fundamental chemistry of lignin during DES pretreatment will improve methodology to optimize the pretreatment in an integrated biorefinery. Herein, an acidic biomass-derived DES (choline chloride/oxalic acid) pretreatment was developed to deconstruct the recalcitrance of bamboo for enhanced lignin fractionation and valorization. The structure transformation of the regenerated lignin samples obtained at different pretreatment temperatures has been investigated in comparison with enzymatic mild acidolysis lignin (EMAL) by state-of-the-art NMR (2D HSQC, quantitative 13C, 31P), GPC, and TGA analysis. The results revealed that acidic DES assisted bamboo delignification and promoted the cleavage of β-O-4′ linkages at elevated reaction temperature, leading to an increase in the sustained rise of phenolic OH groups and lower molecular weights. Those lignin samples exhibited higher thermal stability and excellent antioxidant activity, suggesting great potential applications in lignin-based chemicals and material. Lignin condensation with unsaturated reactive intermediates which is accompanied by the depolymerization process during this acidic DES system. The results also highlight the utilization of acidic DES as a low-cost, high performance with a long delignification durability and thermal stability. Overall, the insights gained here could shed new light on the design and fabrication of efficient and durable DES-based pretreatment for the production of bio-fuel, bio-based materials and bio-chemicals.