Supercritical CO2–subcritical H2O system: A green reactive separation medium for selective conversion of glucose to 5-hydroxymethylfurfural

The supercritical CO2 (scCO2)-subcritical H2O (subH2O) system offers a metal- and additive-free reaction solvent that can be used for organic synthesis. The enhanced formation of carbonic acid and ionization of water under elevated pressures and temperatures promote the production of hydronium and hydroxyl ions that can catalyze the conversion of a number of organic compounds. Furthermore, the biphasic nature of the system allows for the simultaneous extraction of less polar products from the aqueous solution by a flowing scCO2 stream. In this study, we investigated the use of scCO2-subH2O as a reactive separation medium for the conversion of glucose to 5-hydroxymethylfurfural (HMF). The highest HMF yield achieved was 32.6 % with a selectivity of 52.7 %. These values corresponded to a 20.25 % and 41.9 % increase in the yield and selectivity that were achieved in the experiments performed under batch operations. The improvement was primarily attributed to the extraction of some of the HMF from the solution. The highest separation efficiency so far was 12.7 %. We also performed semi-empirical quantum calculations to determine a possible mechanism of the production of HMF from glucose. The two-step process, which involved the isomerization of glucose to fructose and the dehydration of fructose to HMF, was shown to proceed with only the OH– and H3O+ that are inherent in the solvent system as catalysts. Insights from both kinetic data and quantum calculations revealed that the reaction is highly favored in the acidic environment of the scCO2-subH2O system.