Promoting the production of 5-hydroxymethylfurfural from high-concentration fructose by creating micro-reactors in a mixed solvent

Abstract

The severe formation of humins during the conversion of high-concentration fructose is a crucial bottleneck restricting the large-scale production of 5-hydroxymethylfurfural (HMF) from fructose. Herein, we report an efficient catalytic system for the conversion of high-concentration (10.0–60.0 wt%) fructose into HMF by creating micro-reactors with cetyltrimethylammonium bromide (CTAB) in a mixed solvent of 1,4-dioxane and water (DIO–H₂O), wherein the formation of humins is remarkably restrained. The micro-reactors were composed of reversed micelles constructed by the assembly of amphiphilic CTAB in the mixed solvent (V_DIO/V_H2O = 95/5). The confinement of fructose and bromine anions within the hydrophilic moiety of the micro-reactors enabled facile interactions between fructose and Br⁻ ions, thereby enlarging the promotional effect of Br⁻ ions on fructose-to-HMF dehydration via accelerating the deprotonation of oxocarbenium ion intermediates. Afterwards, the hydrophobic moiety assisted in the transfer of HMF into the organic solvent outside the micro-reactor protected HMF from further rehydration or condensation to humins. The competitive degradation and condensation of oxocarbenium ion intermediates to humins were significantly mitigated, resulting in high HMF yield (70.3%) and TOF (186.0 h⁻¹) after reacting high-concentration (50.0 wt%) fructose at 140 °C within 15 min. This work highlights the utilization of a multifunctional micro-reactor to fix the reaction zone and facilitate mass transfer, thereby providing a valuable guidance for designing effective catalytic systems for future HMF biorefineries.