Pretreatment of Corn Stover by Levulinic Acid-Based Protic Ionic Liquids for Enhanced Enzymatic Hydrolysis

Economically competitive and sustainable dissolution pretreatment technology is in high demand due to the remarkable achievement in the pretreatment of lignocellulose using ionic liquids (ILs) for enhanced enzymatic hydrolysis. Herein, levulinic acid (Lev), a well-known bio-based platform chemical derived from carbohydrates, in conjunction with a series of organic superbases, was used as a raw chemical to prepare a new class of protic ILs (PILs). The findings indicated that the obtained PILs exhibited a good solubility of up to 10 wt % toward corn stover-based lignocellulose at 140 degrees C in 40 mins, based on which a facile and efficient dissolution pretreatment technology was developed for enhanced enzymatic hydrolysis of corn stover. The facile dissolution and regeneration of corn stover resulted in significant changes in the composition and physical-chemical structures. These changes led to significantly enhanced enzymatic hydrolysis behavior of the pretreated sample with total reducing sugar and glucose yields of 0.8 and 0.49 g/g within 48 h under optimal conditions, respectively. The Kamlet-Taft solvent parameters of the PILs were systematically evaluated, and the interaction between the lignocellulose and PILs was studied by C-13 NMR, indicating that the satisfactory performance of PILs was benefited from the higher beta parameter value and the stronger interactions, particularly the existence of the extra hydrogen-bond interaction from the potential keto-enol tautomerism of the ketone groups in levulinate anions. The compositional and physicochemical changes in the lignocellulose were systematically evaluated to achieve an in-depth understanding of the dissolution activation mechanism using various characterization techniques. The findings also showed that 44.3% of lignin could be fractionated during the facile dissolution and regeneration process, together with a transformation of the crystalline structure of cellulose Ito cellulose II and of the packed morphology to a porous one. The structure of the fractionated lignin was also characterized for a better understanding of the fractionation process. The study demonstrated a new sustainable dissolution pretreatment technology for enhanced enzymatic hydrolysis, which can provide significant insights into the design of new solvents for biomass dissolution and processing.