Enhanced Dehydration of Fructose to 5-Hydroxymethylfurfural in a scCO ₂ /Deep Eutectic Solvent Biphasic System: The Role of CO ₂

This study evaluates the conversion of fructose to 5-hydroxymethylfurfural (HMF) catalyzed by Amberlyst-15 in a supercritical CO2/deep eutectic solvent (scCO2/DES) biphasic system. The work focuses on the operational features of the scCO2/DES system and investigates the role of CO2. The introduction of scCO2 significantly enhanced both fructose conversion and HMF yield in a CO2/choline chloride (ChCl):glycerol system. Kinetic analysis quantitatively revealed that the introduction of CO2 enhances the rate constant of fructose-to-humins formation more strongly than that of fructose-to-HMF. However, the concurrent extraction of HMF by scCO2 decreases its subsequent degradation rate to humins, thus leading to a net increase in the HMF yield. In CO2/DES systems composed of various ChCl-based DESs, the HMF yield is primarily governed by the DES type, while the presence of CO2 universally enhances the yield. Finally, the maximum HMF yield of around 70% was achieved in the scCO2/ChCl:malonic acid DES system compared with around 58% in neat DES.