Production of 5-hydroxymethylfurfural from fructose by a thermo-regulated and recyclable Brønsted acidic ionic liquid catalyst

Abstract

In this work, a thermo-regulated recyclable catalytic system using ionic liquid [HO₂CMMIm]Cl as the catalyst and isopropanol as the solvent has been demonstrated to be effective for the dehydration of fructose to synthesize 5-hydroxymethylfurfural (HMF). The solubility of [HO₂CMMIm]Cl ionic liquid in isopropanol is temperature-dependent, being miscible with isopropanol at a temperature range suitable for fructose dehydration (e.g., above 80 °C) while not soluble at lower temperatures such as room temperature. Temperature-responsive solubilization/precipitation of [HO₂CMMIm]Cl in isopropanol renders the acidic ionic liquid an appealing thermo-regulated phase-switchable catalyst. Furthermore, the effects of various parameters including catalyst loading, fructose concentration, reaction time and temperature on the catalytic performance of fructose dehydration have been studied systematically. Under optimized reaction conditions, up to 91.2 mol% HMF yield could be obtained. Additionally, when the ionic liquid catalyst precipitated out after the reaction, the solvent can be simply decanted and [HO₂CMMIm]Cl could be directly reused for the next run with freshly added solvent and substrate. It was found that [HO₂CMMIm]Cl could be reused at least five times without considerable loss of activity. Furthermore, a kinetic analysis was carried out, indicating the activation energy for the reaction to be 62.1 kJ mol⁻¹. This catalytic system can be envisioned to find applications in a wide range of acid-catalyzed reactions with facile, thermo-regulated catalyst recovery features.