Efficient Isolation and Structural Characterization of Lignin from <i>Xanthoceras sorbifolium</i> Shell Using Ball-Milling-Assisted Deep Eutectic Solvent

<i>Xanthoceras sorbifolium</i> (a unique woody oil plant) fruit shells (XSS), mostly discarded postcore peeling, are rich in lignocellulose and promising for biomass refining. This work first reports the native lignin structure of XSS (SGH-type, S/G = 1.60, β-O-4 = 51.01/100 Ar) and a ball-milling-assisted deep eutectic solvent (DES) strategy for lignin isolation. At 140 °C, ChCl/LA showed excellent delignification (90.59%) and hemicellulose removal (94.11%); its LAL140 had low Mw (2214 Da), high phenolic OH (1.997 mmol/g), and good antioxidant activity (IC50 = 0.027 g/L). ChCl/EA preserved native lignin backbone (S/G = 1.76, β-O-4 = 44.45/100 Ar) with high thermal stability (char yield = 47.82%), while ChCl/EG increased S/G to 2.47. The purity of all DESL exceeded 92%. This study establishes DES-driven lignin valorization (acidic DES for antioxidants, alkaline for thermally stable polymers) to promote woody biomass waste recycling.