Deep eutectic solvent for separation and extraction of high purity lignin from Salix babylonica and its characteristics

A choline chloride-based deep eutectic solvent (DES) system utilizing oxalic acid and citric acid as hydrogen bond donors was used for lignin extraction from Salix babylonica biomass under controlled thermal conditions (130 °C). In choline chloride-oxalic acid system, lignin yield increased from 17.2% (1 h) to 66.1% (9 h) and was stabilized after 7 h of extraction. The optimal samples (DES-E1 and DES-E7, 130 °C/1 h and 7 h) showed high purity lignin (> 89%) while maintaining structural integrity. Both DES systems achieved recovery rates (63.3% and 53.6%) of natural lignin content after 7 h, respectively. Spectral characterization identified selective breaking of the β-O-4 bond between the syringyl (S) and guaiacyl (G) units, indicating DES-mediated bond modification. Furthermore, it was found that the carbon-rich macromolecular structure showed a gradual increase in C/O ratio with prolonged reaction time. This investigation provided a thermal regulation strategy for sustainable lignin extraction while establishing a new pathway for the utilization of Salix babylonica biomass through DES driven structural customization.