Efficient and Protective Upgrading of Uncondensed Lignin Extraction with High Yield and High Purity: Mechanistic Insights into Lewis Acid Catalysis Coupled with PEG Stabilization

Maximizing the extraction and valorization of highly reactive lignin from lignocellulosic biomass is crucial for lignin-first biorefining. However, achieving high yield, superior purity, and structural preservation simultaneously is challenging. Herein, we developed a Lewis acid-catalyzed deep eutectic solvent (DES) system employing ferric chloride (FeCl₃) coordinated with polyethylene glycol (PEG) to selectively extract structurally intact lignin. The engineered DES system exhibited outstanding performance, achieving a lignin yield of 32.9% with a purity of 96.2%. Structural analysis indicated that the extracted lignin retained 53.21 β-O-4 linkages per 100 aromatic units, closely approximating the native structure of cellulolytic enzyme lignin (CEL: 58.53 β-O-4/100 Ar). The identification of characteristic β'-O-4 linkages verified that PEG grafting at the C_α position effectively inhibited lignin condensation. Theoretical calculations further confirmed that FeCl₃ enhanced the hydrogen-bonding capacity of the ternary DES with lignin, thereby aiding lignin dissociation and improving separation efficiency. This work provides valuable insights into upgrading lignin fractionation for lignin valorization.