N-Hydroxysuccinimide-catalyzed facile synthesis of high-phenolic-hydroxyl-content lignin for enhanced antioxidant properties

The formation of phenoxy radicals that scavenge free radicals is critical for improving the antioxidant activity of lignin. Novel modified lignin with high phenolic hydroxyl content and enhanced antioxidant properties was prepared from kraft lignin using a two-step method. First, diethylenetriamine-modified amine lignin (DAL) was synthesized via the Mannich reaction. Second, the DAL was reacted with 3,4-dihydroxyhydrocinnamic acid (DHCA) using N-hydroxysuccinimide as the catalyst. The DHCA-modified DAL (DDAL) was characterized and tested for the removal of 1,1-diphenyl-2-picrylhydrazyl (DPPH) from aqueous solutions. The optimum conditions were as follows: a reaction temperature of 80 °C, a 1:1 mass ratio of DHCA to DAL, and a catalytic time of 2 h. The phenolic content of DDAL increased from 0.81 to 1.51 mmol/g. More importantly, the DPPH free radical scavenging capacity of DDAL reached 79.94% at a concentration of 0.50 mg/mL. This study provides a practical strategy for preparing high-performance bio-based antioxidants.