The Conversion Of Hexose To Pentose And Furfural

As an effective substitute for fossil resources,lignocellulosic biomass can be transformed into platform chemicals and biofuels.Pentose（xylose and arabinose）hydrolyzed from xylan-rich hemicelluloses and furfural obtained by the dehydration of pentose are used to produce functional sugar,biofuel,medicine and plastics because of their multifunctionality.The ever-increasing market demand for pentose and furfural is continuing due to their wide application.Hexose（glucose and fructose）derived from cellulose are usually converted into5-hydroxymethylfurfural（HMF）by dehydration,while could also be selectively converted to furfural with the cooperation of specific catalysts and solvents.Direct production from hexose could maximize current yield of pentose and furfural,and reduce the accumulation of furfural residue（cellulose and lignin）,thus achieving efficient utilization of carbohydrate（cellulose and hemicellulose）which accounts for 50～80%of total plant biomass.In this thesis,a strategy of high selective conversion of hexose to pentose and furfural was proposed by adjusting the acidity of catalysts,polarity and basicity of solvents.The effects of catalysts acidity and solvents properties were studied on the product distribution of hexose.Isotopic labeling,molecular dynamics simulation and density functional theory（DFT）calculation methods were applied to understand the reaction mechanism,solvent effect and difficulty of product formation for the conversion of hexose to pentose and furfural.The detailed work was listed as follows:1.The selective conversion of glucose to pentose（xylose and arabinose）and formic acid was studied using artificial zeolite and GBL-H₂O catalytic system.Catalysts and solvents with different properties were used to explore the influencing factors and rules of the selective conversion of glucose to pentose,furfural and HMF.Isotope labeling and apparent activation energy were performed to study the reaction mechanism and solvent effect for the conversion of glucose to xylose,arabinose and formic acid.The effects of reaction conditions on the yield of xylose,arabinose,formic acid were also investigated as well as the cyclic stability of catalyst.The results showed that the selective conversion of glucose to HMF,pentose and furfural could be regulated by controlling Br?nsted acid strength.The weak Br?nsted acid could promote the formation of pentose,while strong Br?nsted acid lead to the formation of HMF and further degradation of pentose to furfural.Glucose was isomerized to fructose by Lewis acid,followed by the selective C1-C2 bond cleavage of fructose by weak Br?nsted acid to form pentose and formic acid.GBL-H₂O reduced the apparent activation energy of the reaction and promoted the formation of pentose and formic acid.The favorable yield reached to 54.3%pentose and 54.7%formic acid from glucose at 453 K for 1.5 h.2.The efficient conversion of glucose to pentose and formic acid was investigated using sodium lignosulfonate（by-product of the paper industry）as catalyst in the GBL-H₂O system combined with microwave.The effects of the acidity of different catalysts and the basicity of solvents were discussed on the product distribution of glucose.The mechanism and the interaction of solvents,catalysts,reactants and products of glucose to pentose were verified by isotope labeling experiments,DFT calculation and molecular dynamics simulation.Finally,the influence of reaction conditions on the yields of pentose and formic acid,the recycling performance of sodium lignosulfonate and the universality of the catalytic system for other hexoses（fructose,mannose and galactose）were also analyzed.The results showed that the selective production of pentose,HMF and furfural from glucose could be achieved by controlling Br?nsted acid strength of catalysts and solvent basicity.Glucose was isomerized to fructose by Lewis acid of sodium lignosulfonate,and then fructose selectively broke C1-C2bond to form pentose by weak Br?nsted acid of sodium lignosulfonate.The selective C1-C2bond cleavage of fructose was rate-determining step,and xylose was easier to be produced than arabinose.The GBL-H₂O promoted the production of pentose and formic acid,and inhibited the further degradation of pentose.The favorable yields reached to 86.5%pentose（60.2%xylose,26.3%arabinose）and 93.1%formic acid at the microwave of 400 W,413 K for 1.5 h.3.The conversion of fructose to furfural and formic acid was studied using dealuminized Hβzeolite as the catalyst.Hβzeolite was dealuminized by monoic acid,binary acid and tribasic acid to regulate the acidity and porosity of Hβzeolite.The catalytic effects of dealuminized Hβzeolite were also analyzed on the production of furfural from fructose,the optimal condition and the mechanism of dealuminization process.Isotope labeling and DFT calculation were applied to understand the mechanism of fructose conversion to furfural and formic acid.Finally,the influence of reaction conditions on the yield of furfural and formic acid was investigated.The cyclic stability of the catalyst and the applicability of the catalytic system to other hexoses were also discussed.The results showed that the optimum p H value of all the acid solutions was 1.6 for the dealuminization of Hβzeolite,among which citric acid dealuminized Hβzeolite had the best catalytic effect on the preparation of furfural from fructose.Hβzeolite channels were expanded and the ratio of strong Br?nsted acid to weak Br?nsted acid was increased due to complexation of citric acid in the dealuminization process,thus increasing the yield of furfural and formic acid.The conversion mechanism of fructose to furfural and formic acid included two pathways:the C1-C2 bond of fructose was selectively broken to form pentose,and then pentose was dehydrated to furfural;fructose was converted to 1,2-enediol,followed by the selective C5-C6 bond cleavage and subsequent dehydration of1,2-enediol to produce furfural and formic acid.DFT calculation showed that selective C1-C2or C5-C6 bond cleavage was the rate-determining step of the two reaction pathways,and C1-C2 bond was easier to be broken than C5-C6 bond.The highest yields of furfural and formic acid reached to 76.2%and 83%at 433 K for 1 h.4.Solvent effect was studied in the conversion of fructose to furfural.Molecular dynamics simulation combined with DFT calculation and experiments were used to study the mechanism of the selective conversion of fructose to HMF and furfural in different solvent systems.The effects of polarity and basicity of solvents were investigated on the product distribution of fructose.The effects of dealuminated Hβzeolite catalyst and 5 wt%H₂O on the formation of HMF and furfural were explored.The impact of fructose tautomers in different solvents and the distribution of solvents around fructose on the conversion of fructose to HMF and furfural were analyzed by nuclear magnetic resonance technology and molecular dynamics simulations.Combined with the mechanism of fructose conversion to HMF and furfural and the data in previous chapters,DFT calculation was conducted to analyze the difficulty of fructose conversion to pentose,furfural and HMF.The results showed that high polarity and high basicity solvents were favorable for the conversion of fructose to HMF,while furfural was formed in high polarity and low basicity solvents more easily.The HMF was formed without catalyst and water,while furfural needed both of them to achieve high yield.DMSO and GBL could inhibit the side reaction of fructopyranose to humins and promote the conversion of fructofuranose to HMF and acyclic fructose to furfural.DFT calculation showed that HMF was easier to be formed than furfural,because HMF was formed by the direct dehydration of fructofuranose,while furfural was produced by the cleavage of C1-C2 or C5-C6 bond and subsequent dehydration of acyclic fructose.Combined with the results of previous chapters,glucose/fructose was the most likely to produce HMF,followed by pentose and furfural.The yield of HMF,pentose and furfural converted from fructose were higher than glucose because there was no need to overcome the energy barrier required for glucose isomerization into fructose.In this work,the new strategy and methods were developed for high selective conversion of hexose to pentose and furfural.The theoretical system and reaction network for the conversion of hexose to pentose and furfural were also established.This research could provide theoretical support for the new utilization of hexose and the production of platform chemicals,and has great significance for sustainable development and efficient utilization of biomass resources.