Conversion of mono/di/polysaccharides into furan compounds using 1-alkyl-3-methylimidazolium ionic liquids

Abstract

The one-pot hydrolysis/dehydration of mono/di/polysaccharides into furfural (FUR) or 5-hydroxymethyl-2-furaldehyde (HMF) in the presence of an acidic ionic liquid, 1-ethyl-3-methylimidazolium hydrogen sulfate ([EMIM][HSO 4]), at 100 °C, was investigated and the results were compared with those using 1-butyl-3-methylimidazolium chloride ([BMIM][Cl]), with or without CrCl 3 or H 2SO 4. The ionic liquid [EMIM][HSO 4] is effective in converting xylose and fructose or related polysaccharides into FUR (e.g. from xylose: 84% yield at 6 h) or HMF (e.g. from fructose: 88% yield at 30 min), but not glucose and polymers containing these units, which contrasts to that observed for the [BMIM][Cl]/CrCl 3 system. The latter is quite effective in converting glucose and related disaccharides into HMF, but not the polysaccharides cellulose and starch. For the latter feedstocks, the addition of H 2SO 4 or [EMIM][HSO 4] to cellulose/[BMIM][Cl]/CrCl 3 enhances HMF yields, presumably by accelerating the hydrolysis step, although the selectivity of the dehydration of glucose monomers seems rather poor (HMF yield < 13 wt.%). In the case of the xylose to FUR conversion in [EMIM][HSO 4], two systems involving the reaction and simultaneous separation of the target product were investigated, namely reaction/solvent extraction and reaction/evaporation systems, and for both the ionic liquid could be recovered and reused without a significant drop in FUR yield in recycling runs.