Tailored fractionation of lignin with low condensed structure and high yield using acidic ternary deep eutectic solvents based on a diol protection strategy

Deep eutectic solvents (DESs) offer considerable potential for utilization in biorefineries based on the “lignin-first separation” strategy. However, the condensation deactivation of lignin in acidic systems poses challenges to the sustainability of biorefineries relying on DES pretreatments. In this study, choline chloride (ChCl) and lactic acid (LA) were selected as the foundational components for DES, while 1,5-pentanediol (PD) was incorporated as a protective agent, to extract lignin from bamboo cell walls with high yield, well-preserved β- O -4 substructure, and low condensed structure. Results demonstrated that PD, functioning as a small-molecule nucleophile, has a marked preference for capturing and stabilizing lignin reactive intermediates produced during the pretreatments. This stabilization function effectively mitigated the degradation and recondensation of lignin, allowing for the dissolution and recovery of low condensed lignin fragments enriched in β- O -4 substructure. Given the notable yield of 72.64 % and specifically tailored structure (β- O -4 =20.39/100 Ar, S C /S=18.67 %) of the lignin procured, the optimal pretreatment conditions were determined to be a ChCl/LA/PD molar ratio of 1:2:0.6 and a reaction temperature of 130 ℃. Overall, this study introduced an eco-friendly and viable biorefining technique that facilitates the preferential separation of high-quality lignin with tailored structure and high yield. Developing a PD-protected acidic DES system that enables the efficient fractionation of lignin from bamboo cell walls with high yield, well-preserved β- O -4 substructure, and less condensed structure, providing critical theoretical foundations for lignin valorization into platform molecular compounds via catalytic depolymerization. • Lignin was tailored fractionated using acidic deep eutectic solvents (DESs). • Under optimal conditions, the lignin extraction yield was 72.64 %. • Adding 1,5-pentanediol (PD) into acidic DESs protected the structure of lignin. • The mechanism of PD inhibiting lignin condensation was revealed.