Finely Designing Dicarboxylic Acid-Based Protic Ionic Liquids System for Tailoring Lignin Structure via Demethylation Strategy

As one kind of renewable aromatic polymer, lignin is severely underused due to its chemical recalcitrance. Lignin can endure demethylation modification to improve its activation by releasing more active functional groups. However, the process suffers from expensive, corrosive, and toxic issues by employing halogen-containing reagents, which has become an obstacle to industrial applications. Herein, a series of dicarboxylic acid-based protic ionic liquids (DAPILs) systems composed of ethanolamine and dibasic organic acids (e.g., aspartic acid (Asp), glutamic acid (Glu), succinic acid (SA), and glutaric acid (GA)) with 1~2:1 stoichiometric ratio, have been finely designed for the demethylation of industrial lignin. With [EOA][GA] treatment, the polyphenol content in lignin was favorably increased beyond 1.58 times. The structural tailoring and variation were fully characterized by 2D HSQC and ¹H NMR. The analysis results indicated that, with the increase of phenolic hydroxyl content in lignin, the β-O-4′ bond was broken and the content of structural units (S, G) and the S/G ratio of lignin decreased accordingly. After the treatment, the used IL and tailored lignin can be recovered over 95%. This novel, halogen-free and environmentally friendly lignin-cutting strategy not only opens avenues for high-value utilization of lignin but also expands the field of application of dicarboxylic acid-based protic ionic liquids.