Construction of anhydrous two-step organosolv pretreatment of lignocellulosic biomass for efficient lignin membrane-extraction and solvent recovery

Glycerol organosolv (GO) pretreatment has been revealed to be potent in selectively deconstructing the lignocellulosic biomass and effectively enhancing its enzymatic hydrolysis, but the conventional solid washing and GO lignin extraction processes frequently consume large amounts of water, resulting additionally in difficulty recycling the glycerol. In this study, an anhydrous two-step organosolv pretreatment process was explored, followed by the membrane ultrafiltration of glycerol lignin. The results showed that the solid washing of the residual glycerol after the atmospheric glycerol organosolv (AGO) pretreatment was necessary for the subsequent operation of high-solid enzymatic hydrolysis. Washing with ethanol was desirable as an alternative to water as only a low glycerol content of 5.2% resided in the substrate. Membrane ultrafiltration was helpful in extracting the AGO lignin from the pretreatment liquor, in which a high lignin extraction of 81.5% was made with a regenerated cellulose membrane (cut-off for 1 kDa) under selected ultrafiltration conditions. With the characterization of membrane-extracted lignin, it was observed for the first time that the AGO lignin has a well-preserved structure of G/S type. Moreover, the lignin was enriched with reactive groups, i.e. beta-O-4 ' linkages and aliphatic hydroxyl groups, which was very likely due to the glycerol grafting onto the lignin via alpha-etherification reaction. The two-step organosolv pretreatment process allowed 86% of glycerol and 92% of the ethanol recovery with similar to 78% of distillation energy savings, which was applicable for extraction of organosolv lignin and recycling use of organic solvents.