Evaluation of Acidic Deep Eutectic Solvents Treatment on Enzymatic Hydrolysis Lignins: Structural Analysis and Antioxidant Activity

The high-value utilization of enzymatic hydrolysis lignin (EHL) is essential for promoting the development of the biorefinery industry. This study investigated the enhancement of the antioxidant activity and ultraviolet (UV) absorption capacity of EHL through treatment with choline chloride (ChCl)-based acidic deep eutectic solvents (DESs). The yield, chemical structure, UV absorption properties, antioxidant activity, and thermal stability of the degraded and regenerated enzymatic hydrolysis lignin (DEHL) were analyzed. The results indicated that treatment with DESs effectively preserved the aromatic structure of EHL. Compared to untreated EHL, DEHL exhibited an increased O/C atomic ratio, a decreased UV transmittance, a significant reduction in weight-average molecular weight (<i>M</i>_w), and a notable increase in phenolic hydroxyl (ArOH) content. Notably, DEHL treated with ChCl-<i>p</i>-toluenesulfonic acid had the lowest <i>M</i>_w (1586 g/mol) and the highest ArOH content. Except for the ChCl-malic acid and ChCl-acetic acid systems, all the other five DES treatments enhanced the antioxidant activity of DEHL to varying degrees. Among them, DEHL treated with ChCl-<i>p</i>-toluenesulfonic acid exhibited the highest antioxidant activity, with an <i>IC</i>_{50 DPPH} value of 262.87 μg/mL.