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

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 CO 2 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 CO 2 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 NH 2 groups on the surface of lignin-ChCl-PABA played a very important role in the acceleration of the cycloaddition reaction under solvent free conditions.