Efficient Separation and Targeted Activation of Lignin by Ethanolamine Pyruvate Protic Ionic Liquid

To address the challenges of inefficient depolymerization and undesirable condensation side reactions of lignin in lignocellulosic biomass, this study employed an ethanolamine pyruvate protic ionic liquid (EAP) pretreatment system to achieve selective separation of lignin from eucalyptus while simultaneously enabling its in situ structural activation. Under optimized conditions (pyruvate-to-ethanolamine mole ratio of 1:3, 120 °C, 40 min), the EAP system afforded a lignin separation yield of 79.0 ± 0.6%, with dissolution yields of cellulose and hemicellulose of 9.6 ± 0.3% and 11.2 ± 0.4%, respectively. According to 2D-HSQC NMR and 31P NMR analyses, the relative content of β-O-4 ether linkages in the isolated lignin decreased from 18.4 ± 0.4% to 14.2 ± 0.3% after EAP treatment. The total phenolic hydroxyl content reached 2.26 ± 0.08 mmol/g, and the syringyl-to-guaiacyl (S/G) ratio declined from 1.72 ± 0.04 to 0.71 ± 0.03. Based on these observations, it is proposed that the ethanolamine component facilitates the dissociation of the lignin network through hydrogen bonding and stabilizes reactive intermediates, while the pyruvate component participates in the cleavage of β-O-4 ether linkages and the removal of methoxy groups via proton catalysis and nucleophilic attack. Compared with the ethanolamine and ethanolamine acetate systems, EAP pretreatment yielded lignin of higher purity (98.4 ± 0.3%) under milder conditions, and the isolated lignin exhibited stronger antioxidant activity (IC50 = 0.17 ± 0.02 mg/mL). This work offers insights into the development of pretreatment systems that combine efficient separation with structural preservation of lignin.