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

Graphical UV light on lignin : An efficient self-initiated radical photochemical strategy for degradation of lignin-derived aryl ethers and lignin through ionic liquids induction is developed. In the homogeneous system, the C−C/C−O bonds can be cleaved efficiently through a free-radical-mediated reaction, affording aromatic products in high yields under mild conditions. 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][NTf 2 ] and the Brønsted acid 1-propylsulfonic-3-methylimidazolium trifluoromethanesulfonate ([PrSO 3 HMim][OTf]) under ambient conditions. [PMim][NTf 2 ] initiates the reaction by promoting the cleavage of the C β −H bond, and [PrSO 3 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.