Preparation of activated lignin with high hydroxyl content using lewis acid as demethylation reagent

Activation of lignin by demethylation for improving the reactivity has attracted extensive attentions. However, it still faces many challenges, such as the unsatisfied increase of hydroxyl content and the undesired cracking of linear linkages. Here, the efficient demethylations for significantly increasing the hydroxyl content and protecting the structure of industrial lignin were explored using lewis acid as modification reagent. As BBr₃ was used, the phenolic hydroxyl content (Ar-OH) was increased by 80.65 %, but the lignin structure might be destroyed. About 75 % of the β-O-4 linkages could be fortunately retained by using AlCl₃. This method could also be used for the demethylation of alkaline poplar lignin with up to 171.67 % increase of Ar-OH (from 1.80 to 4.89 mmol/g). After activation, the antioxidant properties were improved 4.64-fold and 2.58-fold for 2,2'-azino-bis (3-ethylbenzothiazoline-6-sulfonic acid) diammonium salt (ABTS) and 2,2-diphenyl-1-picrylhydrazyl (DPPH) radicals, respectively. This work would provide a theory guidance for activation of lignin and facilitate its high-value application.