Improve Synthesis Of 5-hydroxymethylfurfural From Fructose Over Solid Acid Catalysts By Ultrasound Enhancement And Its Mechanism

5-hydroxymethylfurfural（HMF）is an important platform compound that can be converted into a variety of value-added derivatives to replace fossil fuels.Fructose is the most direct raw material for the preparation of 5-HMF.Conversion from fructose to 5-HMF has high yield and high selectivity.Ultrasound can improve product yield,speed up the reaction rate and even complete the reaction that cannot occur under conventional conditions.In this paper,ultrasound was applied to the synthesis of 5-HMF from fructose,with grafting aluminum SBA-15 molecular sieve as catalyst,dimethylsulfoxide as solvent.The process and kinetics of the synthesis of 5-HMF from fructose by ultrasound enhancement were studied,and the mechanism of ultrasound enhancement was also studied.The specific research contents are as follows:（1）The effects of ultrasonic action mode,catalyst Si/Al molar ratio,ultrasonic power,reaction time and catalyst dosage on the synthesis of 5-HMF were investigated,and the preparation process of 5-HMF was optimized with response surface methodology.The results showed that the suitable type of catalyst is 10-Al/S catalyst,and the suitable mode of ultrasonic action is probe ultrasound.On the basis of single factor experiment,response surface method was used to optimize the synthesis parameters of 5-HMF.A quadratic polynomial regression model was established with 5-HMF yield as the response value.The optimal conditions were obtained as follows:ultrasonic power 846 W,catalyst dosage 0.03 g,reaction time 3.1 h.The yield of 5-HMF synthesized under suitable technological conditions was 49.1%.（2）The kinetics of 5-HMF prepared by fructose dehydration with ultrasonic enhancement was studied.The reaction rate constant and activation energy of four reactions during fructose dehydration to 5-HMF under ultrasonic and without ultrasonic were compared.The results showed that the magnitude relationship of the reaction rate constant of fructose dehydration into 5-HMF（k₁）,the reaction rate constant of fructose condensation into humins（k₂）,the reaction rate constant of 5-HMF rehydration into levulinic acid and formic acid（k₃）and 5-HMF condensation into humins（k₄）was always k₁>k₂>k₃>k₄under both ultrasonic and non-ultrasonic conditions,indicating that the reaction rate of fructose conversion to 5-HMF was higher than all side reactions.The reaction rate constants（k₁,k₂,k₃,k₄）under ultrasound at the same temperature were always larger than those without ultrasound,indicating that ultrasound could accelerate the conversion rate of fructose dehydration into 5-HMF.The activation energy of fructose dehydration to 5-HMF under ultrasound was 12.9kJ·mol^-1,which was 10.2 kJ·mol^-1 and 44.1%lower than that without ultrasound.The activation energy of fructose condensation into humins under ultrasound was 62.6 kJ·mol^-1,which was 13.6 kJ·mol^-1 and 17.8%lower than that without ultrasound.The activation energy of 5-HMF rehydration into levulinic acid and formic acid under ultrasound was 17.5 kJ·mol^-1,which was 9.3 kJ·mol^-1 and34.7%lower than that without ultrasound.The activation energy of 5-HMF condensation into humins under ultrasound was 14.4 kJ·mol^-1,which was 8.2kJ·mol^-1 and 36.3%lower than that without ultrasound.Although ultrasound simultaneously reduced the activation energy of fructose dehydration to 5-HMF,fructose condensation to humins,5-HMF rehydration to levulinic acid and formic acid,and 5-HMF condensation to humins,it still reduced the most for fructose dehydration to 5-HMF.Therefore,it can be concluded that ultrasound is beneficial to fructose dehydration to produce 5-HMF.（3）The thermal,mechanical and cavitation effects of ultrasonic were investigated by calorimetry,catalyst characterization and DPPH dosimetry,respectively.The main conclusions are as follows:In the process of fructose dehydration into 5-HMF under ultrasound,the contribution of ultrasonic thermal effect to 5-HMF yield is about 81%,and the contribution of ultrasonic mechanical effect and cavitation effect is about 19%.XRD,FTIR,N₂ adsorption and desorption,Py-IR and TEM characterizations of the catalysts for unused,once used and for seven times used showed that ultrasound and fructose dehydration reaction did not destroy the two-dimensional hexagonal mesoporous structure of the catalysts,and the functional groups of the catalysts did not change after 7 times of reaction.After 7 times of reaction,the acid content of the catalyst decreased slightly,the total acid content decreased by8.7%,Br（?）nsted acid content decreased by 9.0%,and Lewis acid content decreased by 8.4%compared with that before the reaction,indicating that the catalyst can be recycled.The yield of 5-HMF was 45.3%,44.7%,43.2%,43.6%,42.7%,41.8%and 43.1%after reaction 1～7 times,respectively.The yield of5-HMF almost did not decrease.Studies on the cavitation effect of ultrasound show that ultrasound can enhance the preparation of 5-HMF by fructose dehydration.The possible reason is that ultrasound promotes the solvent to produce free radicals,and the higher the ultrasonic power is,the more free radicals are generated.