Study on the Solubility of Industrial Lignin in Choline Chloride-Based Deep Eutectic Solvents

Deep eutectic solvents (DESs) have emerged as a promising class of solvents for lignin dissolution, which could significantly expand the application potential of lignin. In this study, over forty ChCl-based DESs of three major types were synthesized and subjected to investigate the solubilisation of two industrial lignins in DES. The results showed that higher HBD content, shorter carbon chain length in the HBD, and fewer functional groups favored lignin solubilization. DESs containing carboxyl groups were found to be more favorable for breaking beta-O-4 bonds and solubilizing lignin. Additionally, high temperature and appropriate water content were observed to promote lignin solubilization. The pretreatment of poplar and maize straw with ChC/FA(1:3), ChC/LA(1:3), ChC/Gly(1:3), ChC/EG(1:3), ChC/Urea(1:3), and ChC/TEOA(1:3) showed good solubilization of lignin, with ChCl/FA(1:3) being particularly effective in solubilizing poplar lignin and maize straw lignin, achieving lignin solubilization of 82% and 57%, respectively. Overall, these findings suggest that DESs have great potential as solvents for lignin dissolution.