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 Cß -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.