Experimental and theoretical study on the catalytic degradation of lignin by temperature-responsive deep eutectic solvents

Lignin is a renewable biomass resource with excellent application potential; however, the extremely complex structure of lignin renders it difficult to effectively degrade and use. In this context, it is necessary to identify a catalyst that can be easily recovered to increase the lignin degradation rate. In this paper, two temperature-responsive deep eutectic solvents (TRDESs), which can form homogeneous phases with reactants at a high temperature and separate from the reaction system at low temperature, were designed to catalyse lignin degradation. Based on the |HOMO-LUMO| gap, [Bmim]Cl and ChCl, which have a high catalytic activity, were selected from 22 deep eutectic solvents (DESs). [Bmim]Cl-EDTA (mole ratio = 1:1) was obtained by combining [Bmim]Cl and EDTA. ChCl-SA (mole ratio = 1:1) was obtained by combining ChCl and salicylic acid (SA). Results of COSMO-SAC modelling calculations and solubility experiments indicated that TRDESs can respond to temperature. Specifically, these solvents can form a homogeneous phase with the reactants at high temperatures, thereby increasing the number and uniformity of the reaction sites. Moreover, the solvents can be separated from the reaction system at low temperature, which enables efficient recycling of the catalyst. Results of lignin degradation experiments indicated that the catalytic performance of [Bmim]Cl-EDTA is higher than that of the other DESs: the degradation rate of lignin is 83.63%, and the catalyst exhibits a satisfactory catalytic effect even after multiple cycles.