Solvent Effect And Reaction Mechanism In The Conversion Of Xylose To Furfural

Lignocellulosic biomass is the most abundant renewable resource on earth and can replace petroleum resources to produce fuels and chemicals.Furfural residue,as an industrial lignocellulosic biomass waste,will pollute the environment if it is landfilled randomly.Furfural residue can be used as a precursor of carbon-based catalysts,and this treatment could achieve the efficient utilization of furfural residue,which is environmentally friendly.Carbon materials have the advantages of high surface area,excellent stability?chemical and thermal stability?,good accessibility,easily functionalized performance,and low prices.Therefore,carbon materials are good catalyst supports,which can be loaded with acid groups or metal oxides and exhibit excellent catalytic performance in biomass conversion reactions.Xylose is the main monosaccharide of hemicellulose and can be converted into many valuable chemicals,such as xylitol,furfural and xylulose.Among these xylose converstion,conversion of xylose to furfural is one of the most concerned reactions,and furfural is one of the most important platform compounds,which could be directly or indirectly converted into more than 1600 chemical products.However,due to the complexity of xylose degradation,there are still many hot issues in the catalyst deisgn,solvent effect and reaction mechanism,especially,in structure-performance relationship of catalysts,solvent effect and reaction pathway.In addition,as an isomer of xylose,xylulose is an intermediate product of many reactions such as the xylose-to-furfural reaction.Therefore,improving the yield of xylulose and understanding its generation mechanism are of great significance to the entire biomass conversion process.In this paper,furfural residue was used as a carbon precursor to prepare carbon-based catalysts loaded with sulfonate groups and different metal oxides.The carbon-based catalysts were used for the catalysis of xylose and hemicellulose for the production of furfural,and pore structure and surface properties of catalysts were studied on the effect of the xylse-to-furfural conversion.Molecular dynamics simulation methods were applied in combination of with isotopic labeling method and experimental methods,to understand the catalytic performance of catalysts,solvent effect,and reaction mechanism for the conversion of xylose to furfural or xylulose.The detailed work is listed as follows:1. Carbon-based solid catalysts were prepared using furfural residue as raw materials for the catalysis of xylose and hemicellulose.Using furfural residue as raw material,sulfonic acid-supported carbon-based solid catalysts were prepared by carbonization and impregnation methods.The effects of preparation conditions of carbon-based catalysts?carbonization and activation conditions?on the pore structure and surface oxygen-containing functional groups,and the effects of catalyst properties were investigated on the catalytic performance during the processes of hemicellulose hydrolysis and xylose dehydration.The results showed that chemical activation treatment increased the porosity of the carbonized material and also affected the distribution of surface functional groups.The C-S catalyst prepared by carbonation and sulfonation with the highest-SO₃H density and the lowest surface area had a high catalytic activity in the conversion of xylose to furfural.In the process of hemicellulose hydrolysis,the main products obtained when using the C-S catalyst were xylose and furfural,the highest yield of xylose was 55%?200?-1.5 h?,and the highest yield of furfural is 50%?200?,2.5 h?.The C?K?-S catalyst prepared by KOH-activation carbonation and sulfonation,facilitated the formation of xylooligosaccharides?XOS?during hemicellulose hydrolysis,and the highest yield of XOS was 62%at 180? for 1.5 h.2. The combination of molecular dynamics simulation with experiment was used to study the effect mechanism of the solvent on the reaction of xylose to furfural in monophasic systems.The solubility and configuration of xylose in different solvents,and the effects of different solvents on the degradation reaction of xylose were investigated.Gromacs software was used to simulate the interaction between xylose and furfural in different solvents.The results showed that the rate of xylose conversion and furfural formation was affected by the solubility of xylose,the ratio of?/?configuration,the distribution of solvent around xylose and furfural,the hydrogen bonding between xylose and solvent,and the degradation of solvents.In pure solvents,the hydrogen bonding between xylose and solvents led to different products.Xylose could convert to furfural in water and DMSO,while it mainly degraded to formic acid in other pure solvents.As green solvents,GBL and GVL had weaker affinity for xylose than DMSO,DMF and ISO,but had stronger affinity for furfural,so they could provide a protective shell for furfural and played an active role in furfural production.In the absence of catalyst,the highest yield of furfural was 46.4%in GBL-H₂O at 180?-150 min.3. The combination of molecular dynamics simulation with experiment was used to study the effect mechanism of the solvent on the reaction of xylose to furfural in biphasic systems.The partition coefficients of furfural,xylose conversion and furfural formation in different biphasic systems were investigated.Gromacs software was used to study the distribution and hydrogen bonding of water or organic solvents around xylose or furfural.The results showed that in the biphasic system,the type of organic solvent had a great influence on the partition coefficient of furfural.The highest partition coefficient of furfural existed in DCM-water system,and the higher furfural yield?98.6%?and xylose conversion?99.9%?were obtained in DCM-water system at 180?-90 min under the carbon-based catalyts.The simulation results showed that the different affinity between the organic solvent,xylose,water,and furfural resulted in a different solvent distribution around xylose and furfural.2-Butanol and THF had stronger affinity with xylose.In DCM-water,MIBK-water and MTHF-water systems,there are less water molecules around furfural,indicating that the organic solvents in these three systems prevent more water molecules closing furfural,resulting in higher partition coefficients among these systems.Furfural was stable since hydrogen bonds were hardly formed between the organic solvent and furfural.4. The catalytic performance of the metal oxide-supported carbon-based catalyst and the reaction mechanism of xylose to furfural were studied.The influence of various factors on the reaction process was discussed.The isotope labeling method and nuclear magnetic resonance technology were used to study the evolution path of each carbon in the xylose degradation process.The results showed that C-KOH-Sn,C-KOH-Fe,C-KOH-Zn and C-KOH-Al exhibited higher xylose conversion and furfural yields than the situation of no catalyst.The orders of the activation energy of reactions with different catalysts were as follows:no catalyst>C-KOH-Sn>C-KOH-Al>C-KOH-Mg>C-KOH-Fe>C-KOH-Zn.Xylose degradation mechanism research showed that both C1 and C5 of xylose could be broken to form formic acid.When C1 of xylose was broken to form formic acid,other carbons could generate four to one carbons compounds?such as lactic acid,acetic acid and formic acid?.In this study,the aldehyde carbon of furfural was derived from the C1 of xylose.5. Carbon-based catalysts loaded with magnesium oxide were prepared for the isomerization of xylose.The effects of metal,solvent,temperature,time,catalyst amount,calcination temperature,type and amounts of additives on the xylulose yield were investigated.Isotope labeling method and nuclear magnetic resonance technology were used to systematically study the reaction mechanism of xylose-to-xylulose conversion.The results showed that different calcination temperatures resulted in different types of magnesium metal oxides supported on carbon materials.Among them,C-KOH-Mg?300?was the most effective for the formation of xylulose.The reaction temperature,time,catalyst amounts,solvent,and additives had an important impact on the xylulose yield.Among the ten selected solvents,only water and methanol facilitated the generation of xylulose.Sodium tetraborate could significantly improve the yield of xylulose,but excess sodium tetraborate reduced the conversion rate of xylose and the xylulose yield.The nuclear magnetic resonance results confirmed that sodium tetraborate formed the complex with xylulose,and the experimental results also verified that sodium tetraborate itself had no catalytic effect on xylose.The reaction mechanism of xylose-to-xylulose conversion was studied by isotope calibration method,which showed that the conversion route of xylose to xylulose was mainly intramolecular hydrogen transfer,and a small part of 1,2-enolization.