| Lignocellulosic biomass is the most abundant and renewable biological resource,and its high-value utilization is considered as an important way to achieve sustainable development of society.In lignocellulosic biomass,cellulose and hemicellulose are connected by hydrogen bonds,while lignin is covalently bound to hemicellulose to form lignin carbohydrate complexes(LCC).The presence of lignin makes the structure highly recalcitrant and resistant to solubility;therefore,component separation and delignification by various pretreatment methods are necessary to destroy the recalcitrant and interconnected lignocellulosic structure before high value utilization of biomass.In this thesis,green and efficient separation of broadleaf wood feedstock was carried out by different pretreatment systems in order to deconstruct the lignocellulosic biomass.The isolated cellulose fraction was further applied for enzymatic digestion to obtain excellent enzymatic saccharification efficiency;the lignin fraction was applied to prepare lignin micro/nanospheres(LNSs),which were applied to polyvinyl alcohol(PVA)films to provide a reference for the diversified design and application of lignin-based functional materials to improve the utilization value of lignin.The main contents and results of this thesis are summarized as follows:(1)Using willow as raw material,the acid-catalyzed sulfone/water organic solvent system was used for pretreatment,and the effects of catalyst type,catalyst amount,reaction temperature and time on component dissociation and lignin structure were investigated.The dissociation effect of the system was obvious,and the treatment effects of phosphotungstic acid,phosphomolybdic acid and silicotungstic acid were better than sulfuric acid,among which the pretreatment effect of silicotungstic acid was significantly better than other acid catalysts,and the yield of solid residue after pretreatment was only44.23%,and the lignin removal rate reached 94.67% when the acid catalyst was added at15 m M.The reaction temperature influenced the pretreatment effect,and the lignin removal rate at 150 °C was 2.7 times higher than that at 110 °C.The reaction time was controlled at 1.5 h to achieve excellent lignin removal.The cellulose was characterized by high performance liquid chromatography(HPLC),infrared spectroscopy(FTIR),scanning electron microscopy(SEM)and X-ray diffraction(XRD).The chemical bonding and functional group types of the cellulose fractions isolated under this system did not change significantly,and their enzymatic saccharification rate reached 13.3 times that of the untreated bamboo willow feedstock.The recoveries of lignin extracted by pretreatment separation reached 75.6%,with high purity(>98%),high thermal stability,regular morphology,relatively low molecular weight(～3000 g/mol)and dispersion coefficient(～2.0),rich hydroxyl content,better antioxidant performance than commercial antioxidants,and 85.3% free radical scavenging rate.(2)Using eucalyptus as raw material,the differences in the pretreatment effects of three different DESs,Ch Cl∶OA=1∶1,TBAB∶OA=1∶1,CHCl∶OA∶Sulf=1∶1∶4,were analyzed and compared,and the solid residue obtained under the TBAB-OA-Sulf system The recovery was only 53.24%,and the lignin removal rate reached 92.32%,which was better than the other two DESs.The lignin removal effect was significant,and the lignin removal rate was no longer significantly improved by continuing to increase the temperature,and the high temperature would cause the loss of cellulose.By characterization,the fiber structure was broken and fractured after pretreatment,which increased the accessibility of enzyme and improved the efficiency of enzymatic saccharification,and the rate of enzymatic saccharification reached 97.6%,and the chemical bonds and characteristic functional groups of cellulose were still retained intact.The recovery of lignin under this system was 90.3%,the purity of lignin was higher than94%,the molecular weight was small and uniformly distributed,and the phenolic hydroxyl content was high,thus showing excellent antioxidant potential,which is beneficial for further application in composite materials.(3)Further,the isolated lignin was used as raw material to prepare lignin micro/nanospheres with different morphologies by self-assembly method through different initial concentrations and titration temperatures,and the lignin micro/nanospheres were incorporated into the biodegradable polyvinyl alcohol matrix to prepare polyvinyl alcohol composite films with different lignin micro/nanospheres.The solid lignin micro/nanospheres with the average particle size around 300 nm were analyzed by scanning electron microscopy and transmission electron microscopy,while the hollow lignin micro/nanospheres with the average particle size around 400 nm were produced at low concentration and elevated temperature.The introduction of lignin micro/nanospheres enhanced the mechanical properties of the composite films.The tensile stress of the PVA-SE composite film reached 130.7 MPa,which was about 2.4times that of the pure polyvinyl alcohol film,and the elongation at break of the composite film was also greatly improved.The introduction of lignin micro/nano-spheres endowed the polyvinyl alcohol composite films with UV protection properties,and all the prepared composite films could achieve complete shielding of UV region.The improved thermal stability and hydrophobic properties of the composite films as well as the good water vapor barrier properties increase their potential applications. |
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