Lignin modified by deep eutectic solvents as green, reusable, and bio-based catalysts for efficient chemical fixation of CO2

Abstract

Lignin is a biodegradable, abundant and renewable natural polymer on earth. In this study, deep eutectic solvents (DESs)-modified lignin heterogeneous catalysts were prepared for the first time via a facile acid-base interaction between choline chloride (ChCl) and p-aminobenzoic acid (PABA) or its derivatives, which were used to efficiently catalyze the cycloaddition of CO2 and terminal epoxides under green and mild conditions. Among them, lignin-ChCl-PABA was found to exhibit the highest catalytic activity for the cycloaddition of epoxides in the presence of 1.0 MPa of CO2 while retaining high selectivity, which far outperformed those achieved on the homogeneous EDSs catalytic system. A series of cyclic carbonates could be obtained in high yields (90–99%), and lignin-ChCl-PABA was stable and reusable for the cycloaddition reaction (84% yield after 5 recycles). The attaching DES of ChCl/PABA onto the functional lignin increased the catalytic activity and also made the catalyst easier to recycle from the reaction mixture. The OH and NH2 groups on the surface of lignin-ChCl-PABA played a very important role in the acceleration of the cycloaddition reaction under solvent free conditions.