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 1H 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 IC50 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.