Comparative Study of Solid versus Liquid Acidic Deep Eutectic Solvents on the Extraction Efficiency and Natural Structural Integrity of Cotton Stalk Lignin

Although deep eutectic solvents (Deep Eutectic Solvents, DES) have shown excellent lignin dissolution capacity in lignocellulose pretreatment, the physical state of DES directly governs the extraction efficiency of lignin and the protection of its structure. Current research has mostly focused on the optimization of chemical composition, while the differences in extraction behavior between the solid and liquid states of different chemical systems, as well as their impact on the natural structural integrity of lignin, have not been systematically reported. In this study, we compared the effects of a liquid deep eutectic solvent (betaine/lactic acid, Bet/Lac) and a solid deep eutectic solvent (choline chloride/tartaric acid, ChCl/Tar) on the extraction efficiency and structural integrity of lignin from cotton stalk. Under optimal conditions, the Bet/Lac system achieved a lignin extraction yield of 17.65% (molar ratio 1:8, 150°C, 6 h), significantly higher than that of the ChCl/Tar system (3.0%, molar ratio 1:2, 140°C, 4 h). The viscosity of the liquid DES ranged from 132.2 to 489.3 mPa·s, which was much lower than that of the solid DES (755.6–974.9 mPa·s), resulting in superior mass transfer performance. The liquid DES well preserves the G, S, and H structural units as well as the side-chain structures of lignin, and the recovered lignin exhibited an amorphous porous morphology. In contrast, the solid DES led to excessive degradation of lignin, giving a denser product morphology. The liquid Bet/Lac system maintained high extraction efficiency while maximally preserving the native structure of lignin, thus offering a green and feasible pretreatment approach for the valorization of agricultural residues such as cotton stalk.