Deep eutectic solvent-driven mild lignocellulose pretreatment: Unlocking lignin valorization and carbohydrate digestibility

High-value utilization of lignocellulose and its components is generally constrained by the pretreatment efficiency and the structural robustness of fractionated components. Harsh pretreatment conditions can promote pretreatment efficiency but also cause serious condensation of lignin structure, thus inhibiting the depolymerization of lignin into valuable chemicals. Herein, we designed a novel deep eutectic solvent (DES) composed of pyridine hydrochloride, ethylene glycol, and AlCl 3 for the pretreatment of lignocellulose. In the developed DES, the lignocellulose could be efficiently fractionated into the natural lignin derivatives with satisfactory hydrogenolysis reactivity and the carbohydrate fraction with high enzymatic digestibility owing to the synthesized DES having adjustable hydrogen-bond acidity, high affinity for lignin, and good protection role on lignin structure. After being pretreated under a mild fractionation condition (70°C, 6 h), a promising delignification ratio of 70.4 % was obtained for poplar, and the extracted lignin exhibited native substructure with well-preserved β-O-4 linkages (42.1 per 100 aromatic units), abundant hydroxyl groups content (4.97 mmol/g), and high molecular weight (5643 g/mol). These structural features contributed to the conversion of the obtained lignin into aromatic monomers with a high yield (27.4 %). Besides, the pretreated residue could be efficiently converted into glucose with a yield of 88.2 % due to the abundant hydroxyl groups of lignin in the cellulose-rich residues. Remarkably, comprehensive studies of lignin structural evolution during pretreatment revealed that the structure-controllable lignin could be effectively isolated by modulating the hydrogen-bond acidity of DESs. This work proposed a mild and efficient lignin-first biorefinery approach that was conducive to the integrated valorization of lignocellulose.