Deep Eutectic Solvents Recycling for Efficient and Sustainable Lignocellulosic Biomass Processing and the Impact of Recycling on Resultant Lignin and Cellulose Products
Qing Zhang, Enqing Zhu, Chi Zhang, Qingwen Tian, Lili Zhang, Jing Shen, Yonghao Ni, Zhiguo Wang
Abstract
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.