Insights into the electrochemical degradation of phenolic lignin model compounds in a protic ionic liquid–water system

Abstract

Cleavage of aryl ether (C_aryl–O) bonds is crucial for the conversion and value-added utilization of lignin and its derivatives, but remains extremely challenging under mild conditions due to strong C_aryl–O linkages. In this study, the C_aryl–O bond breaking is achieved through electrocatalytic oxidation of four phenolic lignin model compounds with typical C_aryl–O bonds, i.e., 4-ethoxyphenol (EP), 4-phenoxyphenol (PP), p-benzyloxyphenol (PBP), and 2-(2-phenylethoxy)phenol (2-PEP), in a protic ionic liquid [BSO₃Hmim][OTf]–H₂O electrolyte, and the electrocatalytic oxidation mechanism is also fully explored. The effects of H₂O on the viscosity and conductivity of the [BSO₃Hmim][OTf] ionic liquid system, as well as the solubility and diffusion coefficients of O₂ and the four lignin substrates, are investigated to optimize the optimal ratio of the electrolyte system composed of [BSO₃Hmim][OTf] and H₂O. Electrochemical oxidation–reduction behaviors of the four lignin substrates in the [BSO₃Hmim][OTf]–H₂O system and the effect of O₂ and N₂ atmospheres on degradation are studied in detail by using cyclic voltammetry (CV) curves. Finally, by combining the analysis of degradation products with isotope labeling experiments, the C–O bond cleavage mechanism is obtained, which mainly involves direct and indirect oxidation. Specifically, under a N₂ atmosphere, the substrates are oxidized directly on the RuO₂-IrO₂/Ti mesh electrode through C_aryl–O bond splitting to form quinone and carbonium ions, while under an O₂ atmosphere, apart from the direct oxidation on the electrode, indirect oxidation of the lignin substrates also occurs through in situ generated H₂O₂. This study may provide some insight into developing effective strategies for efficient utilization of lignin under mild conditions.