Ionic Liquid Pretreatment of Woody Biomass To Facilitate Biorefinery: Structural Elucidation of Alkali-Soluble Hemicelluloses

Woody biomass, eucalyptus, was submitted to pretreatments with five types of ILs followed by alkaline ethanol extraction for isolating hemicelluloses. The structural properties of these hemicelluloses obtained in high yields were compared with the corresponding alkaline fraction extracted from untreated eucalyptus. It was found that the yield of alkaline hemicelluloses increased remarkably after 1-butyl-3-methylimidazolium bromide ([BMIM]Br) pretreatment. Neutral sugars and molecular weight analyses of these hemicelluloses revealed that the molecular weights and distribution of branches along the xylan backbone are different among the hemicelluloses based on different IL pretreatments. The less branched hemicelluloses with lower yield were obtained by 1-ethyl-3-methylimidazolium acetate ([EMIM]Ac) pretreatment, while the more branched and acidic polymers were obtained by 1-allyl-methylimidazolium chloride ([AMIM]Cl) pretreatment. The results showed that these fractions prepared with IL pretreatments exhibited relatively higher molecular weights (50,595-62,090 g/mol) than the hemicelluloses isolated from untreated eucalyptus (49,325 g/mol). No apparent transformation occurred for the main chain of hemicelluloses during these different IL pretreatment processes, which had a backbone of (1 -> 4)-linked beta-D-xylopyranosyl with 4-O-methyl-alpha-D-glucuronic acid attached to O-2. In addition, thermal analysis demonstrated a higher thermal stability of hemicelluloses from IL-pretreated eucalyptus as compared to the fraction obtained from the untreated sample.