Effective conversion of fructose to 5-ethoxymethylfurfural with brønsted acid site (S/Cl)-functional carbon catalysts

Abstract

Brønsted acid site (S/Cl)-functional carbon materials were prepared by ball-milling (bm) L-cysteine and ammonium chloride to form chemical complex solids, by post-heat treatment (Q) of the solids at 500 °C under a nitrogen gas atmosphere to stabilize and fortify amino and functional group solid structures and by post-oxidative treatment with H₂O₂ to add Brønsted acidity. The as-prepared carbon materials (S/Cl@bm_xQ_y; x, y in hours) contained Brønsted acid sites (sulfonates, chlorides), oxygen-containing groups (-COOH, -OH) and amino functional groups. The S/Cl@bm_xQ_y materials were applied as catalysts to promote conversion of fructose in ethanol solvent to 5-hydroxymethylfurfural (5-HMF) and 5-ethoxymethylfurfural (5-EMF). Among the carbon materials, S/Cl@bm₃Q₁ gave 5-EMF and 5-HMF yields of 67.0% and 22.8%, respectively, in pure ethanol at 140 °C for 24 h reaction time, while a reaction time of 18 h gave total 5-EMF and 5-HMF yields of 96.4%. Dimethyl sulfoxide (DMSO) was applied as additive to the ethanol-fructose- S/Cl@bm₃Q₁ reaction system which showed that 5-HMF product selectivity could be controlled with DMSO concentration. The protocol developed allows simple preparation of functional carbon materials that have high catalytic activity and are effective for conversion of fructose to 5-HMF and 5-EMF in ethanol.