Structural changes of poplar lignin during the ternary deep eutectic solvent (DES) treatment and synergetic alkali-DES treatment

Expenditure isolation of lignin and the understanding of its structural variations during different isolation process is essential for lignin high-value utilization. A green DES consisted with choline chloride (ChCl), ethylene glycol (EG) and 4-chlorobenzene sulfonic acid (4-Cl-BSA) was proposed for rapid lignin separation from poplar in this study. About 97.01% of lignin and 87.41% of hemicellulose were removed while the cellulose preservation achieved 95.02% under 90 °C for 30 min using this DES with the molar ratio of 1:2:0.7 (ChCl: EG: 4-Cl-BSA). The percentage of β-O-4 linkages, molecular weight and associated sugar content of DES lignin declined steadily with the enhancement of treatment severity. Additionally, the pre-extraction of hemicellulose through alkali treatment on subsequent lignin separation and lignin structural variations was also elucidated. The results suggested that alkali pretreatment opened the channels of the cell wall and increased DES permeability to the material, enhancing the DES separation of lignin. It also promoted the cleavage of the β-O-4 and PB linkages during followed DES separation lignin, and reduced the molecular weight and thermal stability of isolated DES lignin. In addition, the integrated DES lignin displayed higher purity and S/G ratio compared to lignin obtained from direct DES treatment. The Py-GC/MS results indicated the obtained DES lignin was suitable for phenolic compounds production and the total phenolic compounds yield ranged from 41.56% to 62.42%. This study provides insight into lignin structural transformation during different pretreatment and guides for dismantling cell walls.