Study On Catalytic Conversion Of Hemicellulose Into Platform Chemicals Over Metal Chlorides And Their Reaction Mechanisms

Hemicellulose,as an abundant,biodegradable and renewable biomass polysaccharide resource,can be transformed into important platform chemicals.However,the research on conversion biomass to chemicals by chemocatalytic is mainly focused on cellulose and lignin,and the research on hemicellulose is rarely.Therefore,transformation hemicellulose into high value-added chemicals has important theoretical value to realize the high-value utilization of hemicellulose,and will promote the innovation and development of biobased platform compounds.This thesis starts with the valorization of hemicellulose,the green and efficient catalytic systems for conversion hemicellulose into important chemicals（lactic acid and furfural）were built through adoption of highly activity metal chloride catalysts,and selection of appropriate solvent systems.Furthermore,the conversion mechanism of hemicellulose to lactic acid was also studied.Aimed at valorization of industrial hemicelluloses,water-soluble hemicelluloses were fabricated with mild acetylation in 1-ethyl-3-methylimidazolium acetate ionic liquid and dichloroacetyl chloride system by a facile and novel method.The resultant modified products could fully dissolve in water with the degree of substitution valued between 0.17 and 0.37.The modified hemicelluloses were chiefly composed of the（1→4）-linkedβ-D-Xylp backbone with hydroxyl or-COCH₃ linked to O-2 and O-3 of the Xylp units.Rheological property measurements revealed that acetylated hemicelluloses solutions showed shear-thinning behavior and indicated lower viscosity compared with those of the referenced hemicelluloses.The excellent water-solubility of industrial hemicelluloses would widen its application field.The catalytic conversion of water-soluble hemicellulose was also studied by using Al Cl₃-Sn Cl₂catalyst.The results showed that the products were a mixed acid composed of lactic acid,formic acid and acetic acid,indicating that the selectivity of lactic acid preparation still needs to be improved.In order to improve the selectivity of synthesis lactic acid,the research on efficient preparation of lactic acid from hemicellulose catalyzed by Er Cl₃ in aqueous system was carried out.The yields of lactic acid derived from xylose and industrial hemicellulose were 78.7%and 68.1%under the optimum reaction conditions of 180°C,1 h,0.1 g substrate,and 0.07 g Er Cl₃.The conversion efficiency of three kinds of hemicellulose from industrial（hardwood）,Miscanthus（annual plants）,and spruce（softwood）was also studied.The results showed that the difference in lactic acid yield was caused by the diversity of hemicellulose composition and chemical structure.The conversion efficiency of hemicelluloses was higher when the substrates possessed higher content of xylose lower content of galactose.To explore the reason of the result,the catalytic mechanism of xylose and galactose was studied by combining experimental research and theoretical calculation with key intermediates as substrates.The mechanism indicated that the reaction pathways of galactose and xylose were similar,but the highest energy barrier for the conversion of galactose to dihydroxyacetone was higher than that of xylose,impeding the conversion of galactose into lactic acid.In order to tailor product distribution by varying the solvent system and reaction conditions,the preparation of furfural from hemicellulose catalyzed by Er Cl₃ in a biphasic system was studied.Biphasic systems（dichloromethane/water,methyl isobutylketone/water,toluene/water and2-methyltetrahydrofuran/water）and monophase miscible systems（γ-valerolactone/water,γ-butyrolactone/water,tetrahydrofuran/water and dimethyl sulphoxide/water）were compared for the transformation of hemicellulose.The results indicated that the solvent system had a significant effect on the conversion of hemicellulose to furfural catalyzed by Er Cl₃,and the furfural yield was the highest in the biphasic system of dichloromethane/water.The higher furfural yield using dichloromethane/water was becase it had the best extraction capacity,the least degradation of furfural,and the role of controlled release of monosaccharides generated by hemicellulose.When the reaction time was less than 4 h,the furfural yield was obviously higher than other solvent systems.Under the optimal reaction conditions（0.05 g hemicellulose,0.2 mmol Er Cl₃,15 mmol Na Cl,160°C,1 h,10 m L H₂O,and 5 m L dichloromethane）,the furfural yield from hemicellulose reached up to 63.1%.Furthermore,the applicability of Er Cl₃ was studied,indicating that the Er Cl₃ with high catalytic activity was more suitable for high polymerization degree substrates such as xylan and biomass,and the Er Cl₃ might be used for monosaccharides after adjusting the reaction conditions.In order to reduce the catalyst cost,the cheap and readily available Cu Cl₂ was used to catalyze hemicellulose conversion to furfural with high selectivity in a green and efficient solvothermal conversion system.The solvent system had influence on the distribution and content of product,and the THF solvent system with 20%H₂O had the best furfural yield.The furfural yield could reach at 63.8%under the optimum reaction conditions of 180°C,2 h,0.25 mmol Cu Cl₂,and 0.05 g hemicellulose.The solid residue characterization showed that as the residues increased,the furfural molecules and intermediates reacted to form soluble polymers,humins and carbon materials,resulting in the decrease of the furfural yield.Mass spectrometry showed that there might be two kinds of catalytic active species Cu²⁺and Cu（OH）⁺.Under the role of catalytic active species,xylose was isomerized into xylose and then dehydrated to generate furfural.Moreover,xylose was converted into furfural in deuterium water catalyzed by Cu Cl₂,and the reaction solution was detected by H¹ NMR.The results showed that the deuterium atoms in the solvent were not introduced into furfural.In order to improve the yield of furfural and the reusability of Cu Cl₂,a biphasic system composed of Na Cl aqueous solution and methyl isobutylketone was used to convert hemicellulose to furfural by Cu Cl₂.The catalytic efficiency of Cu Cl₂ in the biphasic system composed of methyl isobutylketone and water was better than in toluene/H₂O,2-methyltetrahydrofuran/H₂O and guaiacol/H₂O.The effects of reaction conditions were investigated,the yield of furfural could reach at 74.8%under the optimal conditions.When used monosaccharides,hemicelluloses and biomass feedstock as substrates,furfural or 5-hydroxymethylfurfural could be obtained,which indicating Cu Cl₂ has good catalytic applicability.In addition,the furfural yield was 77.6%after 7 cycles when Cu Cl₂ catalyzed hemicellulose to furfural in the biphasic system,showing that the Cu Cl₂ had good recyclability.