Industrial lignin activation with enhanced antioxidant capability in mild halogen-free protic ionic liquids

Lignin commonly has a low antioxidant activity despite acting as a natural antioxidant . Most of the reported chemical reagents for enhancing lignin phenolic hydroxyl content contain halogen or metal elements, which is expensive, toxic environment and non-recyclable. Here, a cost-effective and eco-friendly technique for increasing the phenolic group content and antioxidant capability of lignin was described. Protic ionic liquids , an effective demethylating reagent with advantages of zero halogen, low cost and recyclability was applied for lignin demethylation . Quantitative 1 H NMR and semi-quantitative 2D HSQC NMR analyses revealed the conversion of methoxy groups to phenolic groups. Typically, ethanolamine acetate was able to efficiently demethylate the methoxy of raw lignin at a rate as high as 44.73 %, which increased the amount of phenolic group in RL to 215.69 %. Besides, the recovery rate of [HOAc]-based protic ionic liquids and lignin was both over 90 %. The IC 50 value of organic lignin (OL) on scavenging of 2,2-Diphenyl-1-picrylhydrazyl was decreased to 0.15 mg/mL, which was better than natural or synthesized antioxidative chemicals.