Novel dehydration of carbohydrates to 5-hydroxymethylfurfural catalyzed by Ir and Au chlorides in ionic liquids

Catalytic dehydration of carbohydrates into 5-hydroxymethylfurfural (5-HMF) has been studied in the presence of ionic liquids, i.e., 1-butyl-3-methyl imidazolium chloride ([BMIM]Cl) and triethylamine sulphate ([Et3NH][HSO4]) using metal salts as catalysts. Novel metal chlorides IrCl3 and AuCl3·HCl as catalysts in ionic liquid showed remarkable activity and selectivity to 5-HMF for the reaction of fructose dehydration. The reaction conditions of fructose dehydration were optimized as follows: reaction temperature 120 °C, reaction time 30 min, IrCl3 content 7% (mol% based on fructose) in [BMIM]Cl, at which the conversion of fructose and the yield of 5-HMF reached 97.7% and 89.0%, respectively. By simply assuming the fructose simultaneously converts to 5-HMF and other by-products by the first-order reaction kinetics, the energies of activation are estimated to be 165 kJ/mol for the formation of 5-HMF and 124 kJ/mol for the formation of other by-products from 80 to 100 °C, respectively. The existence of water in the media more than 0.5% (v/v) would significantly slow down the dehydration rate of fructose, while it seemed not necessary to further remove water during the reaction when the water was less than 0.5%. Compared with the dehydration of fructose, the dehydration of other carbohydrates such as glucose, sucrose and maltose was rather difficult.