High-Solid Lignocellulose Processing Enabled by Natural Deep Eutectic Solvent for Lignin Extraction and Industrially Relevant Production of Renewable Chemicals

This study investigated high-solid loading deep eutectic solvent (DES) pretreatment for lignocellulose fractionation and subsequent conversion into platform chemicals (i.e., furfural, 2,3-butanediol). Switchgrass was pretreated with choline chloride:ethylene glycol (ChCl:EG) under acidic condition at high solid loadings of 20% and 27%. Cellulose was enriched to as high as 72.6% in pretreated switchgrass due to substantial removal of lignin and xylan but minor cellulose loss. Highly concentrated sugar hydrolysate containing up to 241.2 g/L fermentable sugars (206.5 g/L glucose and 34.7 g/L xylose) with 86.2% glucose yield was obtained from pretreated switchgrass via 25% solid loading enzymatic hydrolysis for only 48 h. The high sugar concentration allowed the production of 90.2 g/L 2,3-butanediol upon fermentation, a record high titer produced from lignocellulose hydrolysate without additional sugar concentration. ChCl:EG showed good recyclability, and its reuse facilitated lignin recovery from pretreatment liquor. The pretreatment liquor resulting from four pretreatment cycles was enriched with xylan-based sugars (96.8 g/L xylose equivalents), of which 84.6% was converted to furfural. 2D HSQC NMR revealed that β-O-4 interunit linkage experienced substantial cleavage during the pretreatment. The degree of bond cleavage depended on the solid loading for pretreatment. Lignin derived from a higher solid loading pretreatment preserved the original structure of natural lignin better, thus producing increased amounts of phenolic monomers upon pyrolysis. This study demonstrated that high-solid loading, ChCl:EG pretreatment was not only effective for biomass fractionation but also enabled industrially relevant production of concentrated sugar hydrolysate, furfural, and 2,3-butanediol.