Efficient Separation And Modiifcation Of Biomass Based On Ionic Liquids

With the growing shortage of fossil fuels, it is stepping up efforts to find alternatives forfossil fuels. Biomass is the most potential alternative substances, because most of thepetroleum-based chemical raw materials and products can be produced from biomass. Inaddition, biomass is the only source of aromatic groups in the sustainable materials. Plantbiomass composes primarily of carbohydrates, and lignin. Carbohydrates accounted for70%of the total biomass, including cellulose, hemicellulose, and sugar. Lignin accounted for20%,and other substances (such as fat and protein) account for only5%. For biorefining, thevarious components of the biomass must be separated firstly. Lignin and hemicellulose in thebiomass are connected each other by chemical bonds, and these two substances are connectedthrough hydrogen bonds with cellulose. Lignin has a complex structure, and the hydrogenbonding between cellulose molecules are very strong, which has played a very goodprotection for biomass, but now it become a barrier for biomass separation. The ionic liquid isnew type of solvent that can dissolve biomass completely. In the study, pine wood was used asraw materials, and the mixtures of ionic liquid and organic solvents were used as solvents. Aseries of key issues for the separation and conversion of biomass in the mixed solution werestudied, including the design of mixed-solvents, separation conditions, biomass modification,ionic liquids recovery and recycling. The main conclusions are described as follows:1. Different dissolution theory are compared and analyzed, and Hansen'sthree-dimensional dissolution theory was selected as the theoretical basis to design the mixedsolvent of ionic liquids. According to this theory, several mixed solvents with differentfunctions were designed. By experiments, it was found that the design of the mixed solventcan make biomass separation optional.2. Single factor experiments were carried out to determine the optimal reaction condition,and the temperature of170℃and the time of2h were considered to be best.In addition, theexperiments showed that the concentration of the ionic liquid will promote the dissolution anddegradation of cellulose, the presence of water play important role on the separation ofbiomass and can range from10%to30%(relative to the ionic liquid); the presence of organicsolvents can prevent the condensation of lignin. The purity and yield of products afterseparation was calculated, cellulose purity,89.6to94.2%, and yield,64.9to89.9%, ligninpurity,92.1to94.1%, and yield89.1to94.9%.3. By Fourier transform infrared FT-IR,¹H-NMR, thermal analysis (TGA), X-raydiffraction (XRD), gas-mass spectrometry (GC-MS), high performance anion exchange chromatography (HPAEC) and other means the products of the separation was determined ontheir physical properties and chemical structure. It was found that the chemical structure ofcellulose did not change significantly during the separation process, but the degree ofcrystallinity decreased; lignin was hydrolyzed with β-O-4being interrupted, and with littlecondensation; hemicellulose and a small amount of cellulose were hydrolyzed to sugar anddissolved in ionic liquids.4. The conversion and separation of sugars in ionic liquid was studied. Sugars wereconverted into HMF and humin catalyzed by CrCl₃and CrCl₃.6H₂O. After vacuum distillationand filtration, the products were removed easily, and the ionic liquids were recovered withhigh purity.5. Lignin was modified in ionic liquid, catalyzed by CrCl₃and CrCl₃.6H₂O. It was foundthat lignin was occurred to degradation and restructuring. The molecular dispersion of ligninwas improved and large amount of methoxy groups were cleaved, which is very conducive tothe synthesis of the furfural resin.6. The optimal pathway of the recovery of ionic liquid was determined. Firstly, sugar andlignin was one-pot treated in ionic liquids at low temperatures (130℃), and sugars changedinto furfural and separated by decompression; then it was heated to170℃so that theunreacting sugars and lignin polymerized to humin; after filtration,near pure ionic liquid wasrecovered. The recovered ionic liquid was proved to retain the original characteristics andactivity.