Oxidation of lignin to value-added aromatics through selective C-C bond cleavage via metal-free and mild strategies is promising but challenging. It was discovered that the cations of ionic liquids (ILs) could effectively catalyze this kind of strong bond cleavage by forming multiple weak hydrogen bonds, enabling the reaction conducted in air at temperature lower than 373 K without metal-containing catalysts. The cation [CPMim]+ (1-propylronitrile-3-methylimidazolium) afforded the highest efficiency in C-C bond cleavage, in which high yields (>90 %) of oxidative products were achieved. [CPMim]+ could form three ipsilateral hydrogen bonds with the oxygen atom of C=O and ether bonds at both sides of the C-C bond. The weak bonds joint effects could promote adjacent C-H bond cleave to form free radicals and thereby catalyze the fragmentation of the strong C-C. This work opens up an eco-friendly and energy-efficient route for direct valorization of lignin by enhancing IL properties via tuning the cation.
Keywords: hydrogen bond; ionic liquids; lignin; oxidation; weak bonds joint effects.
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