| Biomass as the most abundant green renewable resource in the world, with its advantages, such as reproducibility and carbon-neutral emission, becomes the best choice to substitute fossil fuel. In this study, eight solvent systems including lithium chloride/dimethylacetamide (LiCl/DMAc), lithium chloride/N-methyl-2-pyrrolidone (LiCl/NMP), tetrabutylammonium fluoride/dimethylsulfoxide (TBAF/DMSO), IL1-allyl-3-methylimidazolium chloride/dimethylsulfoxide ([Amim]Cl/DMSO), NaOH/urea/H2O, NaOH/thiourea/H2O, tetrahydro-1,3-dimethyl-2(1H)-pyrimidine/dimethylsulfoxide (DMPU/DMSO) and4-methyl morpholine N-Oxide/dimethylsulfoxide (NMMO/DMSO) for complete dissolution of ball-milled cotton stalk, and severally separating lignin, hemicelluloses and cellulose gradually under the optimum conditions, finally getting the desired three kinds of products undergoing series of processing were described. After complete dissolution pretreatment, lignin and carbohydrates were separated by the extraction of lignin fraction using ethanol. Then the carbohydrates precipitations were extracted with dimethyl sulfoxide (DMSO) three times to get hemicellulose fractions dissolved in solutions, and then the cellulose residues were also obtained. The aim of this study was to achieve the efficient separation and purification of lignin, hemicelluloses and cellulose from cotton stalk cell walls via gradual separation methods based on complete dissolution. Furthermore, the chemical and physical properties and structures of products were characterized and the separation efficiency of different complete dissolution systems were investigated. The results show as follows:(1) The obtained lignin fractions possessed relatively high purities and lower degrees of degradation as compared with MWL. Structural elucidation of lignin fractions was further investigated and he results indicated a predominance of guaiacyl (G) over syringyl (S) units, and only showed small amounts of p-hydroxyphenyl (H) units. Moreover, the cotton stalk lignins showed a predominance of β-O-4’aryl ether linkages, followed byβ-β resinol-type, and β-5’phenylcoumaran-type structures and with lower amounts of other substructures such as spirodienone and p-Hydroxycinnamyl alcohol. In general, separation of lignin from ball-milled materials by pretreating with complete dissolution in [Amim]Cl/DMSO provided us an effective approach, in consideration of the relatively high yield and purity and less changes in chemical structures.(2) Hemicellulose fractions isolated from cotton stalk consisted of different components and structures due to various pretreatments of complete dissolution. The results showed that glucuronoxylans and glucans were found to be the major hemicellulosic components in the cell walls of cotton stalk, and a-(1â†'4)-glucans which mainly come from starch in cotton stalk were also found.(3) The cotton stalk materials were dissolved in different complete dissolution systems, followed by successive extractions with ethanol and dimethyl sulfoxide (DMSO) to partially remove lignin and hemicellulose fractions and finally obtained the cellulose-rich fractions. The regenerated cellulose fractions mainly composed of glucose. The structural features of the cellulose-rich fractions were comparatively characterized with sugar analysis and a set of spectroscopy including XRD, SEM, FT-IR, CP/MAS13C NMR, etc. The XRD cellulose-rich fractions of indicated the conversion of cellulose I into a less strong cellulose II crystal structure. The results of SEM showed that the cellulose-rich fractions appeared looser and porous and exhibited more internal surfaces as compared with the ball-milled material. The changes of chemical structures and morphology of the treated sample suggested that the complete dissolution pretreatments could effectively disrupt the crystal structure of cellulose, generating highly reactive cellulosic materials for enzymatic saccharification to produce bioethanol.(4) This study provided us a viable method for fractional separation of the three biopolymers via complete dissolution system pretreatment. The isolated lignin fractions with relatively high purity and yield and less structure changes, as compared with that of MWL, could be applied in structural identification and high-value use of lignin in cotton stalks. |
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