Improving antioxidant activity of lignin by hydrogenolysis

Alkali lignin (AL) was depolymerized by hydrogenation using Pd/C as the catalyst and formic acid as the hydrogen donor, and the antioxidant activity of hydrogenolysis products of alkali lignin (HPAL) was investigated. It was found that when the degree of depolymerization (DD) was 85.0%, the IC50 of HPAL reached 25.2 ± 0.9 mg L−1 in terms of the DPPH radical scavenging activity, which was better than both AL (63.5 ± 0.6 mg L−1) and commercial antioxidant BHT (38.7 ± 1.0 mg L−1). There was a tight relationship between DD and antioxidant activity, indicating that a lower molecular weight of HPAL was beneficial for improving the antioxidant activity. The hydrogenolysis of lignin contributed to an obvious increase of phenolic hydroxyl content which was the essential for antioxidant activity, and hence hydrogenolysis improved the antioxidant activity of HPAL although the methoxyl content decreased slightly. In addition, phenolic structures with para-alkyl had higher antioxidant activity than those without alkyl substituent. However, the antioxidant activity of the phenolic compounds gradually decreased with an increase of the alkyl chain in para-position due to its lower solubility in a polar solvent. The significant improvement of antioxidant activity of lignin after hydrogenolysis may provide a promising approach to high valued utilization of lignin.