Insights into Structural Changes of Lignin toward Tailored Properties during Deep Eutectic Solvent Pretreatment

Biomass pretreatment enabling extraction of lignin with desirable properties (especially well-preserved beta-O-4 linkages) while ensuring improved cellulose digestibility remains a challenge for sustainable biorefinery. This study aimed to address this challenge using a deep eutectic solvent (DES) comprising choline chloride and ethylene glycol (ChCl/EG) for switchgrass fractionation. Extensive characterization of extracted DES lignin was performed to understand how the lignin properties, including purity, molecular weight, thermal stability, and abundance of the beta-O-4 linkages, were changed in the DESs under varied pretreatment conditions. Finely tuning the solvent system would steer DES-based biomass fractionation toward synchronizing the production of uncondensed lignin and digestible pulp. Appealing lignin properties, such as high-purity (91.4%), well-preserved beta-O-4 linkages, and high volatility similar to that of cellulolytic enzyme lignin (CEL), is obtainable in this designer solvent system. At least 65% lignin can be removed, with the highest delignification reaching near 90% within 30 min. Overall, this versatile and tunable DES system is promising for lignocellulosic biomass processing and would fit into different biorefinery configurations toward value-added valorization of lignocellulosic biomass.