Deep Eutectic Solvents Recycling for Efficient and Sustainable Lignocellulosic Biomass Processing and the Impact of Recycling on Resultant Lignin and Cellulose Products

Deep eutectic solvents (DESs) present a promising pathway for sustainable lignocellulosic biomass fractionation, yet challenges such as solvent recyclability and maintaining product quality over repeated cycles persist, which is largely caused by hemicelluloses. A lactic acid/choline chloride (LA/ChCl) DES was employed to fractionate acid-hydrolyzed poplar chip biomass. After removing ∼96% of hemicellulose via acid hydrolysis pretreatment, the fractionation using fresh DES achieved a delignification degree of 90%, yielding cellulose-rich pulp over 90%. The DES was recycled up to five times, maintaining over 90% recovery in each cycle. However, the delignification degree decreased to 76% after five cycles. Supplementing fresh DES restored the delignification degree to 78%, while activated carbon adsorption further improved it to 88%. Importantly, structural analysis confirmed minimal degradation of DES, and the recycling process maintained the quality of the fractionated products. The lignin fraction exhibited high purity (>85%), stable molecular weight (∼7000 g/mol), low polydispersity index (∼1.70), and the cellulose fraction displayed efficient fiber separation while maintaining its natural crystalline structure (crystallinity index ∼80%). This work provides new process concepts for DES fractionation of biomass, offering a scalable and environmentally sustainable solution for lignocellulosic biomass processing within biorefinery frameworks.