Protective fractionation of highly uncondensed lignin with high purity and high yield: new insights into propanediol-blocked lignin condensation†

Abstract

A lignin-first separation strategy can help improve the economic and environmental benefits of biorefinery. However, the self-condensation of lignin remains an insurmountable bottleneck in its isolation and valorization process. Therefore, the challenge of lignin separation is how to achieve a highly uncondensed lignin with high yield and high purity. In this work, we developed a ternary deep eutectic solvent (DES) based on polyhydric alcohols, which can be used to treat moso bamboo at 80–90 °C to obtain highly uncondensed lignin which is light-colored and highly active. The uncondensed lignin exhibits a high β-O-4 retention rate, up to 52.82/100Ar, accounting for 90.24% of the β-O-4 content in the raw material. At the same time, the maximum yield and purity of uncondensed lignin can reach 33.6% and 95.5%, respectively. Furthermore, through DFT theoretical calculations, we have proved that the lignin protection strategy by 1,2-propanediol is achieved by grafting the hydroxyl functional group to the α position of the benzylic carbocation intermediate, thus revealing the intrinsic mechanism by which polyol effectively blocks lignin condensation. The lignin-first fractionation strategy based on ternary deep eutectic solvents with polyhydric alcohols and Lewis acids overcomes the defects of lignin condensation that is inevitable during lignin separation and depolymerization. It obtains highly uncondensed lignin with high yield and high purity, and provides a new idea for biorefinery under the lignin-first separation strategy.