Synthesis Of Novel Solid Acid Catalysts For Carbohydrates Conversion To 5-Hydroxymethylfurfural And Its Catalytic Mechanism

5-Hydroxymethylfurfural(5-HMF)is regarded as one of the top 10 important and value-added platform chemicals from the conversion of carbohydrates,which can be widely used in the industrial chemical manufactures.In recent years,methods for the synthesis of 5-HMF from carbohydrates have attracted much attention.However,strong acid condition was early employed in these methods,and there were serious drawbacks with respect to difficult separation of catalyst,corrosivity and by-products.In this study,novel solid acid catalysts were prepared for the conversion of carbohydrates to 5-HMF.Niobium phosphotungstate catalysts were synthesized for the conversion of fructose and sucrose to 5-HMF,which significantly reduced the reaction temperature.Based on the conversion of glucose to 5-HMF,tin(?)modified organic polymer,niobium-loaded montmorillonite and hafnium-tin composite oxide catalysts were further developed and exhibited excellent catalytic activity.Finally,the catalytic mechanism of 5-HMF synthesis was further studied.Phosphotungstate catalysts were prepared by simple alcohol-mediated thermal process,and niobium phosphotungstates(NbPW)catalyst with strong Bronsted acidity exhibited high activity for fructose dehydration under mild conditions.The highest 5-HMF yield of 96.7%was achieved with complete fructose conversion at 80? in DMSO after 1.5 h,because the introduction of niobium ion enriched P-OH groups on the surface of the catalyst,which offered Br(?)nsted acid sites to activate the surface hydrogen species and facilitate 5-HMF synthesis.Moreover,NbPW-06 could be recycled four times without significant loss in catalytic activity.Meanwhile,the reaction kinetics of fructose conversion was established,and the apparent activation energy and pre-exponential factor were 54.09 kJ·mol-1 and 2.3×106 min-1,respectively.The 5-HMF yield was 62.54%when sucrose conversion was catalyzed by NbPW-06.The reaction mechanism of converting sucrose to 5-HMF was proposed:(1)sucrose was firstly hydrolyzed to fructose and glucose under the action of H+;(2)Nb5+accelerated the transformation of glucose to fructose;(3)fructose was dehydrated to form 5-HMF by removing three water molecules.In order to promote glucose dehydration,Sn modified polyimidazole epichlorohydrin(PIL-Sn)was prepared through polymerization and ion exchange reaction,and used as solid acid catalyst for converting carbohydrates to 5-HMF.PIL-Sn and PIL-Fe possessed Br(?)nsted and Lewis acidity,and the catalytic activity for glucose and fructose dehydration were in the order of PIL-Sn>PIL-Fe>PIL-pre and PIL-Fe>PIL-Sn>PIL-pre,respectively.51.1%yield of 5-HMF was achieved from glucose catalyzed by PIL-Sn in DMSO at 130? after 1 h.In addition,the yield of 5-HMF from fructose catalyzed by PIL-Fe was 87.1%.With sucrose,maltose,cellobiose,inulin and starch as reactants,the yields of 5-HMF were 35.6%,26.2%,24.2%,25.7%and 10%,respectively.The DFT calculation verified the reaction pathway for glucose isomerization to fructose in PIL-Sn catalytic system.Penta-coordinated SnCl5-as the dominant species possessed low Gibbs free energy,and SnCl5--PIL played an important role in glucose isomerization.Moreover,the hydrogen transfer from C2 to C1 in glucose would form a five-membered chelate intermediate,which was a rate-controlling step.Adding a small amount of water to DMSO could effectively reduce the reaction energy barrier.In order to reduce the catalyst cost,niobium-loaded montmorillonite(Nb-MMT)was synthesized by cation exchange reaction,and Br(?)nsted and Lewis acid sites on the surface played an important role in glucose dehydration.The results showed that 1 wt%Nb was the optimal loading of MMT,and the highest 5-HMF yield of 70.52%with almost complete glucose conversion was obtained in MIBK/water biphasic system at 170? for 3 h.The catalyst retained high catalytic activity after four cycles.Moreover,MIBK-H2O biphasic solvent was better than single solvent for Nb-MMT catalytic system,and the volume ratio of 7:3 for MIBK and water phase was optimal.Saturated sodium chloride solution instead of pure water could effectively change intermolecular interactions in liquids,improving the immiscibility of water phase and organic phase,and increasing 5-HMF distribution coefficient in the organic phase.The reaction mechanism was verified by DFT calculation,indicating the mononuclear[NbO(OH)]species of Nb-MMT could directly catalyze the glucose conversion to 5-HMF with low activation energy.The formation of enol intermediates is a rate controlling step for glucose isomerization.On account of hafnium-based catalyst favorable for the sugar dehydration,hafnium-tin composite oxide(Sn-HfO2)catalyst was prepared through hydrothermal reaction at room temperature.The results showed that tin was successfully introduced into HfO2,and the catalytic performance of the synthesized Sn-HfO2 was better than that of monometallic HfO2.Sn-Hf02 was a kind of bifunctional catalyst with strong Lewis acidity and very weak Bronsted acidity.The introduction of tin played a positive role in the catalytic activity.When the molar ratio of Sn to Hf was 1,Sn-Hf02 possessed the optimal L/B acid ratio and the maximum specific surface area,and promoted glucose isomerization and dehydration.The yield of 5-HMF was 75.5%in MIBK-H2O biphasic solvent at 170? for 2 h.0.5Sn-HfO2 possessed excellent catalytic activity in the dehydration of other carbohydrates.With fructose and starch as reactants,the optimal yields of 5-HMF were 65.2%and 54.1%,respectively.The reaction mechanism was proposed that Lewis acid sites were derived from hafnium species and tin species in the catalyst skeleton,while Br(?)nsted acid sites were derived from a small amount of Hf-OH and Sn-OH groups on the catalyst surface.The synergistic effect of hafnium and tin species facilitated the isomerization and dehydration of glucose into 5-HMF.To sum up,the active components of solid acid catalysts have significant influence on the catalytic conversion of carbohydrate to 5-HMF.The modification of metal ions enhanced the acidity on the surface of catalyst.Lewis and Bronsted acid sites were critical factors for catalytic activity of solid acid.The structure of solid acids and the reaction conditions were optimized for efficient production of 5-HMF.