Metal‐Free Photochemical Degradation of Lignin‐Derived Aryl Ethers and Lignin by Autologous Radicals through Ionic Liquid Induction

Abstract

The degradation of lignin into aromatic products is very important, but harsh conditions and metal-based catalysts are commonly needed to cleave the inert bonds. Herein, an efficient self-initiated radical photochemical degradation for lignin-derived aryl ethers through ionic liquids (ILs) induction is demonstrated. The C-C/C-O bonds can be cleaved efficiently through free-radical-mediated reaction in the binary-ILs system 1-propenyl-3-methylimidazolium bis[(trifluoromethyl)sulfonyl] imide [PMim][NTf₂ ] and the Brønsted acid 1-propylsulfonic-3-methylimidazolium trifluoromethanesulfonate ([PrSO₃ HMim][OTf]) under ambient conditions. [PMim][NTf₂ ] initiates the reaction by promoting the cleavage of the C_β -H bond, and [PrSO₃ HMim][OTf] catalyzes the subsequent C-O-C bond fragmentation. Furthermore, alkyl, hydroxyl, and peroxy radicals are detected, which suggests degradation based on a photochemical free-radical process. Additionally, alkali lignin could also be degraded in the IL system. This work sheds light on sustainable biomass utilization through a self-initiated radical photochemical strategy under metal-free and mild conditions.