| Nowadays,the society is facing the excessive energy consumption and serious environmental pollution problems,so the alternative green and renewable resources are imminently needed.Biomass has been paid more and more attention because of its abundant reserves and renewability,which could be separated and converted into high value-added,diversified products and energy through biorefinery processes.Lignin,as one of the three main components of biomass,is the natural polymer owning the most aromatic rings in the nature,and can be converted into chemicals and materials by chemical catalysis methods to achieve the high-value transformation and utilization.However,the complexity of the lignin structure and the diversity of bonding mechanism are still the critical obstacles of lignin high-value utilization,thus how to efficiently separate lignin component from biomass and enable its transformation and application is one of the important research fields in science.This thesis mainly focuses on the isolation and conversion of lignin based on different biorefinery processes.The effect of autocatalytic organic solvent pretreatment with different straight chain alcohols(methanol,ethanol,n-propyl alcohol and n-butyl alcohol)under subcritical condition 220? on lignin separation was comprehensively compared.It was shown that among all of the solvents,ethanol treatment showed the highest lignin yield(72.1%)on account of its good nucleophilic performance and solubility.Compared with other lignin components,n-butyl alcohol lignin had the highest neutral sugar pollutants(7.53%).Moreover,it was also found that the neutral sugar contents of lignin increased with the increase of the alkyl chain length of alcohols.A large number of S and G units were detected in 2D NMR spectra of all lignin components,as well as the main ?-O-4' and a small amount of ?-?',?-5'linkages.It was noteworthy that the C? positions of ?-O-4' were all substituted by nucleophilic alcohols.The novel expanding pretreatment combined with the subsequent alkali extraction was applied to separate lignin components of agricultural waste rice straw and rice husk.Compared with the lignin of untreated raw materials,expansion obviously improved the yield,and compared with the lignin obtained from steam explosion,that from expansion had the higher purity.Combined with the results of molecular weight and DFRC,it was found that the molecular weight increased with the enhancement of moisture content in the expanding process,which was mainly due to the mild condensation reaction in this process,and the degree of condensation of lignin was proportional to the moisture content in the pretreatment process.From the 2D NMR spectra,the typical G,S and H unit,ferulic acid and coumaric acid in grasses were observed,as well as the common ?-O-4',?-?',?-5' and ?,?-diaryl ether linkages.Moreover,the flavonoid structure was found in the lignin component of rice straw,but not in that of rice husk.Catalytic hydrothermal pretreatment for the separation of lignin component from agricultural waste cotton stalk was explored,which took advantage of the synergy of hot water and catalyst to increase the extraction efficiency of lignin.Combined with the results of molecular weight and NMR,it was found that the obtained lignin from cotton stalk was mainly composed of G and S units,similar to the typical lignin from hardwood,and moreover consisted of ?-O-4' and a small amount of C-C linkage(?-?',?-5').In this process,?-O-4' was significantly disrupted because of the presence of catalyst and the degree of degradation was consistent with the acidity of metal chloride.Simultaneously,the degradation was also accompanied with mild repolymerization,although the degradation rate was greater than condensation.In consideration of the yield and purity,the catalytic hydrothermal pretreatment with AlCl3 was an effective and feasible method to extract lignin component from lignocelluloses.A novel and mild biorefinery method was developed,which applied y-valerolactone(GVL)/water reaction system and the addition of low concentration of inorganic acid to pretreat the agricultural and forestry biomass in the form of one-pot,realizing the separation of three main components by the single step.When GVL/H2O was 80/20,the highest separation efficiency of lignin extracted in the organic-phase reached 64.7%,of which the yield and purity were much higher than MWL.Because the depolymerization and condensation simultaneously occurred in this process,the obtained lignin was condensed component with low molecular weight.Hemicellulose was mainly dissolved in the water-phase,and almost no inhibitors were detected due to the mild reaction conditions.Compared with the untreated feedstock,the yields of enzymatic hydrolysis from pretreated samples increased by two-fold with the mixture of 80/20 GVL/H2O.Combined with the amount of glucose(14.6%)dissolved in the water-phase,over 92.6%of glucose in cotton stalk was released and recovered.The photocatalytic oxidation of lignin model compound vanilla alcohol was preliminarily studied through the preparation of Au-Pd bimetallic catalysts.The support,phosphate-modified Mg-Al hydrotalcite(HT-PO43-)was prepared through a sol-gel and ion exchange method,and then Au-Pd alloy nanocatalysts supported on activated carbon and HT-PO43-were respectively prepared by impregnated-reduction method.The morphology and property of these two catalysts were characterized.Au-Pd@C was used to optimize reaction conditions,then the optimal condition was applied on Au-Pd@HT-PO43-,which could get the conversion rate of 52.8%and selectivity of 49.6%due to the synergy of alloy nanoparticles and alkali support.This catalyst could be used to produce vanillin from lignin model vanilla alcohol at a high conversion and selectivity under mild reaction conditions,including in the air,at room temperature and with no addition of alkali,which was significant for the studies on the photocatalytic oxidation of more complex lignin models,extracted lignin components and biomass raw materials. |
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