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 β–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 β–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 β–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.