Study On The Synthesis Of 5-HMF From Carbohydrates Catalyzed By Polyacid Catalyst In Deep Eutectic Solvents

In recent years,more and more researchers pay attention to the efficient conversion of biomass resources into effective platform chemicals.In this paper,the dissolution behavior of cassava starch and cellulose in low eutectic solvent and the synthesis of 5-HMF from carbohydrates catalyzed by polyacid catalyst in Deep Eutectic Solvents?DES?were studied.Choline chloride was used as hydrogen bond acceptor and carboxylic acid or alcohol as hydrogen bond donor.With the solubility as the index,when the molar ratio of formic acid was 1:4,the temperature was 100?,the dissolution of cassava starch in CC-FA?1-4?was the best,with the solubility up to 1.1208 g/g DES.After being dissolved by CC-FA?1-4?,the polarized cross characteristic disappeared,the particles broke and agglomerated into blocks,the hydroxyl group of the molecular chain formed a large number of hydrogen bonds with the ions in DES,the layered structure and crystalline structure of the particles basically disappeared,and the weight mean molecular weight decreased.With the mean molecular weight as the index,when the molar ratio of formic acid was 1:4,the temperature was 100?,and the solid-liquid ratio was 1:50,the dissolution of cellulose in CC-FA?1-4?was the best,with the polymerization degree low as DP=94?M= 15,228 g/mol?.After being dissolved by CC-FA?1-4?,the mean molecular weight decreased,the rod-like morphology could not maintain and agglomerated into blocks,the crystallinity increased,and the hydroxyl group of the molecular chain formed a large number of hydrogen bonds with the ions in DES.Therefore,CC-FA?1-4?provides a new idea for the dissolution of cassava starch and cellulose.The CeXH_3-3XPW₁₂O₄₀?X=0.1,0.2,0.3,0.4,0.5,0.6,0.7,0.8,0.9 and 1?polyacid catalyst was prepared by introducing different amounts of metal ion Ce³⁺ to replace the hydrogen proton of H₃PW₁₂O₄₀.The optimal process conditions for the preparation of 5-HMF from carbohydrates catalyzed by polyacid catalysts in DES are as follows: the reaction solvent is CC-FA?1-4?/EAC biphasic system,the amount of Ce_0.1H_2.7PW₁₂O₄₀ is 0.2 mmol,the reaction time is 1 h when the reaction temperature is 160?,the amount of cassava starch is 0.2 g,the yield of 5-HMF reached the highest level at 24.30%.Under the same conditions,the yield of 5-HMF catalyzed by H₃PW₁₂O₄₀ was 19.31%.Although the conversion rate of Ce_0.1H_2.7PW₁₂O₄₀ to cassava starch is lower than that of H₃PW₁₂O₄₀,the selectivity of Ce_0.1H_2.7PW₁₂O₄₀ to 5-HMF is higher than that of H₃PW₁₂O₄₀.This is attributed to the introduction of Ce³⁺ to enhance the Lewis acidity of the polyacid catalyst,thereby improving the selectivity of the product.Therefore,the introduction of Ce³⁺ can improve the yield of 5-HMF.