Efficient and Stable Fractionation of Pennisetum × sinese by Three-Stage Countercurrent Recycling of a Fermentation Compatible Deep Eutectic Solvent

Efficient fractionation of lignocellulosic biomass is essential for maximizing carbon utilization and enabling integrated biorefinery processes. We developed a three-stage countercurrent recycling process using a fermentation compatible deep eutectic solvent (DES) for Pennisetum × sinese. In this process, regenerated DES contacted the partially fractionated solids in the final stage, while the lignin-rich DES stream was used for the initial treatment of fresh biomass. This countercurrent arrangement improved the use of the remaining solvent capacity. Lignin and hemicellulose removals remained at approximately 90 and 95%, respectively, over 12 recycling cycles. The glucose yield remained at 34.28–37.26 g per 100 g of dry biomass, with fluctuations between cycles being smaller than those observed in conventional batch cycles. In addition, 10–14 g of low-molecular-weight lignin was recovered per 100 g of dry biomass, with limited structural variation during recycling. The DES recovered remained suitable for microbial fermentation and showed high glycerol utilization, demonstrating its potential for further biological conversion. Overall, the three-stage countercurrent recycling process provides a practical strategy for extending DES reuse and integrating biomass fractionation, fermentable-sugar production, lignin recovery, and further valorization of the solvent within a lignocellulosic biorefinery.