Dissolution And Fractionation Of Lignocellulosic Biomass In Ionic Liquids

Lignocellulosic biomass is the most abundant biorenewable resource in the world, andhas been considered as alternative source to create fuels, energy, chemicals, composites and ahost of other products to replace fossil-based ones. The fundanmental research andcommercial utilization of lignocellulose were, however, seriously limited by the complex cellwall structure and various chemical bonds between different components. Therefore, effectiveand environmental-friendly methods to dissolve and fractionate lignocellulose are in desperateneed. Ionic liquids （ILs） have recently attracted considerable attention and developed manypromising applications due to their unique physicochemical properties in various fieldsincluding lignocellulosic biomass utilization.[C₄mim]Cl,[Amim]Cl and [C₂mim][CH₃COO] were used to dissolve cellulose,holocellulose, bagasse, eucalyptus and pine. Mechanical milling, ultrasonic irradiation andmicrowave heating were applied to assist the dissolution. Research showed that ILs wereeffective solvent for cellulose, but it is difficult to dissolve original material unless it wasmechanical milled. After treating with ILs, the residues were subjected to X-ray diffraction,acid hydrolysis and high performance anion exchange chromatography characterization.Results showed that ILs first dissolved hemicelluloses and amorphous cellulose and thendissolved lignin, swelled and dissolved crystalline cellulose during the dissolution process.This study proposes a novel and feasible fractionation method based on completedissolution of bagasse in [C₄mim]Cl followed by precipitation in90%acetone and extractionwith3%NaOH solution. Bagasse was fractionated into36.78%cellulose containing4.56%lignin,26.04%hemicelluloses containing3.54%lignin, and10.51%lignin, respectively. Thephysicochemical properties of the isolated fractions were characterized by chemical analysis,high-performance anion exchange chromatography （HPAEC）, gel permeationchromatography （GPC）, Fourier transform infrared （FT-IR）, and¹H and2D¹³C-¹Hcorrelation （HSQC） nuclear magnetic resonance spectroscopy. Properties of the fractions weresimilar with microcrystalline cellylose （MCC）, xylan and milled wood lignin （MWL）.[C₄mim]Cl was recycled after concentration and treatment with acetonitrile.¹H NMR analysisconfirmed that there was no obvious difference between the structure of recycled [C₄mim]Cl and fresh material. But the color of recycled [C₄mim]Cl became darker with the increase ofrecycle times, and it was less effective than the fresh one.Bagasse and eucalyptus were dissolved in dimethyl sulfoxide/N-methylimidazole（DMSO/NMI） and fractionated with the abovementioned method. Mechanical milling bagasseand eucalyptus for12h were completely dissolved within5h. Bagasse was fractionated into35.78%cellulose containing10.35%lignin,33.76%hemicelluloses containing8.33%lignin,and10.59%lignin; eucalyptus was seperated into32.42%cellulose containing23.59%lignin,34.30%hemicelluloses containing30.77%lignin, and8.89%lignin, respectively. Productswere characterized by HPAEC, GPC, FT-IR, XRD,¹H NMR,¹³C NMR, and2D¹³C-¹H NMR.[C₄mim]Cl,[Amim]Cl and [C₂mim][CH₃COO] were used as solvent and NaOH solutionwas used as anti-solvent to fractionate components from bagasse, eucalyptus and pine.Mechanical milling duration, concentration of NaOH solution, structure of ILs, and woodspecies all contributed to the results. Bagasse, mechanical milled for12h, could beeffectively fractionated into cellulose, hemicelluloses and lignin after treating with[C₂mim][CH₃COO] at110oC for2.5h following by precipitating in1.0mol/L NaOHsolution. The yields of cellulose, hemicelluloses and lignin were44.86%,28.85%and9.49%,respectively, accounting for83.20%of the original material. HPAEC, GPC, FT-IR,¹H NMR,¹³C NMR, UV were applied to characterize the physicochemical structures of the obtainedfractions. However, the fractionation of eucalyputs and pine with the same process was inpoor performance, which probably due to the different content of lignin and variations inlignin-carbohydrate complexes.