Densification pretreatment with a limited deep eutectic solvent triggers high-efficiency fractionation and valorization of lignocellulose

Deep eutectic solvents (DESs) are intriguing for biomass fractionation, but high DES dosage and low biomass loading make them challenging to be used in industry. In this study, a DES was integrated into a novel pretreatment technology, namely "Densifying Lignocellulosic biomass with Chemicals followed by Autoclave (DLCA)", to realize lignocellulose fractionation and valorization with low DES dosage and high fractionation efficiency. Remarkably, a total DES dosage of 2 g g-1 biomass, which is substantially low compared to traditional DES pretreatments, achieved a high delignification ratio of 75% with over 95% cellulose preserved at 50% solid loading. Moreover, DLCA(DES)-CS exhibited superior bioconversion efficiency even at high solid loadings, with the maximum total fermentable sugar concentration exceeding 300 g L-1 and the ethanol concentration as high as 110 g L-1. Comprehensive characterization methods such as SEM, XRD, FTIR and Simons' staining were used to disclose the properties of cellulose-rich solids, and the chemical structure of the extracted lignin was characterized by GPC and 2D-HSQC NMR spectroscopy, revealing that DLCA(DES) pretreatment not only ensured complete exposure of cellulose, but also remarkably protected the native structures of lignin. Without recondensation, lignin extracted by DLCA(DES) preserved more than 98% of native beta-O-4 linkages. In brief, this study demonstrates great potential for lignocellulose valorization initiated by DLCA(DES) pretreatment, including polysaccharide conversion to bioethanol and selective lignin depolymerization to aromatic monomers. A DES was integrated into a novel densification pretreatment technology (DLCA) to realize lignocellulose fractionation and valorization with low DES dosage and high fractionation efficiency.