Preacetylation of Kraft Lignin Facilitates Acid-Catalyzed Depolymerization in Acidic Deep Eutectic Solvents to Obtain Lignin Oil With High Antioxidant Activity.

Kraft lignin, an abundant renewable aromatic polymer, has attracted increasing interest for acid-catalyzed depolymerization to obtain lignin oil with high antioxidant activity. However, this process is plagued by severe condensation of reactive intermediates, leading to low lignin oil yields. To address this critical issue, we performed preacetylation modification on hardwood kraft lignin (HKL) and investigated its depolymerization behavior in acidic deep eutectic solvents (DESs). Furthermore, the underlying mechanism by which acetylation promotes depolymerization was clarified using lignin monomer model compounds. The results demonstrated that acetylation effectively increased the lignin oil yield. Specifically, at a degree of substitution (DS) of 66.57% (AHKL-20), the lignin oil yield increased markedly from 37.80% to 55.98%. Additionally, lignin oil derived from AHKL-20 (lignin oilAHKL-20) exhibited higher antioxidant capacity (IC50 = 5.18 μg/mL) than HKL (IC50 = 13.02 μg/mL). This study proposes a novel strategy based on the structural premodification of kraft lignin to inhibit condensation, enabling efficient and mild depolymerization in green solvents.