Using Lignin Monomer As a Novel Capping Agent for Efficient Acid-Catalyzed Depolymerization of High Molecular Weight Lignin to Improve Its Antioxidant Activity

In this work, a mild and efficient process for acid-catalyzed depolymerization of lignin to improve its antioxidant activity was proposed using lignin monomer, p-hydroxybenzyl alcohol (HBA), as a novel capping agent. High molecular weight lignin (HMWL) with high β-O-4 linkage content was used as the feedstock for depolymerization, and the products were further extracted by ethyl acetate to obtain the depolymerized ethyl acetate soluble lignin (DESL) with high antioxidant capacity. Moreover, the mechanism of the promoted depolymerization through the utilization of the capping agent was proposed and verified using a lignin dimer model with a β-O-4 linkage. The results showed that the HBA substantially suppressed lignin condensation and increased the DESL yield. When the HBA dosage was 3 mmol/g lignin, the DESL yield obtained at 150 °C was 51.88%, which was 2.7 times and 1.9 times those obtained at 150 and 210 °C without HBA addition, respectively. Besides, the DPPH· scavenging capacity (IC₅₀, 65.2 μg/mL) of DESL obtained at 150 °C with HBA addition was significantly higher than that obtained without HBA addition (125.7 μg/mL) and was close to that of commercial antioxidant BHT (57.3 μg/mL). Accordingly, this work demonstrates that the acid catalysis process using lignin monomer as a capping agent is effective to produce depolymerized lignin with high antioxidant activity. Furthermore, this study provides new insights for the efficient acid-catalyzed depolymerization of lignin through condensation suppression using lignin monomer as a capping agent.