Dissolution of oil palm biomass by alkyl-imidazolium ionic liquids for efficient enzymatic hydrolysis.

The dissolution of oil palm biomass by ionic liquids (ILs) was comparatively studied. The ionic liquids, 1-ethyl-3-methylimidazolium chloride ([emim]Cl), 1-butyl-3-methylimidazolium chloride ([bmim]Cl) and 1-ethyl-3-methylimidazolium acetate ([emim]OAc) were used to dissolve oil palm biomass and cellulose fibers which are empty fruit bunches (EFB), oil palm frond (OPF) and oil palm trunk (OPT). Dissolution of 5 wt. % (0.1 g) of fibers/ ILs solutions were conducted at 100 °C under inert atmosphere. The heating time for complete dissolution was optimized. It has been shown that [emim]OAc is the best solvent for dissolution of oil palm biomass compared to [emim]Cl and [bmim]Cl. EFB, OPF and OPT fibers dissolved in [emim]OAc shows more than 95 % w/w of dissolution after 16 h of heating, while EFB and OPF fibers dissolved more than 85 % w/w after being heated more than 48 h in [emim]Cl and [bmim]Cl. Cellulose fibers were successfully dissolved after 2 - 3 h of heating in all ILs tested. Fourier-transform infrared spectroscopy (FT-IR)confirmed the absorbance band at 1729 cm-1 and 1512 cm-1 which correspond to hemicellulose and lignin, respectively disappeared after regeneration process indicating that they were diminished after the washing step. Regenerated cellulose-rich solids were obtained in amorphous form (cellulose II), thus decreasing the crystallinity index (CrI) values. The CrI value for regenerated EFB, OPF and OPT fibers decreased after 12 h of dissolution in [emim]OAc which were 79.3, 80.3 and 79.3 % to 39.8, 38.3 and 40.2 %, respectively. The accumulated glucose released was reached to a level approximately 13.8 mg/ml which was at least ten-fold higher than that of untreated fibers samples which only 3.19 mg/ml. From NMR study, the six signals of the unmodified anyhdroglucose unit appear clearly at 102.5 (C-1), 79.67 (C-4), 76.44 (C-5), 75.24 (C-3), 74.19 (C-2) and 60.15 ppm (C-6). Through swelling and dissolution mechanism of fibers, disintegration into rod-like fragments, ballooning followed by dissolution and homogeneous swelling were clearly observed for both oil palm biomass and cellulose fibers. Observation under scanning electron micrograph (SEM) showed that, the loose structures of oil palm biomass fibers and a greater part of the smaller fibrils seemed to be absent in the cellulose-rich solids were observed after regeneration

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