Glucose Aqueous Isomerization Catalyzed by Basic Ionic Liquids

The isomerization of glucose to fructose is a crucial step in the formation of high-value biochemicals from renewable cellulosic biomass. In this work, we have demonstrated a novel and efficient strategy for the aqueous isomerization of glucose catalyzed with basic ionic liquids. At 80 °C and 30 min, a fructose selectivity and yield of 73.8% and 36.8%, respectively, are obtained in the presence of tetrabutyl ammonium proline ([N_4,4,4,4]­Pro). Both of these values are notably larger than those obtained in numerous other technologies, making this technique a significant step toward selective generation of fructose. Further investigation reveals that the isomerization performance of glucose is influenced by the structure and hydrophobicity of the ionic liquids, where the latter enhances glucose isomerization via the formation of a microreactor. Moreover, this process exhibits a low activation energy of 34.4 kJ mol^–1 with the [N_4,4,4,4]­Pro ionic liquid, correlating well with the high activity of this catalytic system. In addition, this IL catalyst shows good reusability as demonstrated by the retention of satisfactory performance after four consecutive runs. Based upon the above results, we have proposed a plausible catalytic mechanism, which can be further confirmed by DFT calculations and isotopic labeling studies.