Insight into the alkaline earth metal salt promotion for alkali-catalyzed glucose isomerization

Abstract

Efficient and selective isomerization of glucose into fructose is the most crucial process of biorefining. Despite its rapid isomerization rate under low-temperature conditions, alkali-catalyzed glucose isomerization is less attractive due to the occurrence of extremely complex side reactions. Herein, we explored an approach for promoting the efficiency of alkali-catalyzed glucose isomerization into fructose by adding alkaline earth metal salts. Kinetic analysis showed that the addition of alkaline earth metal salts increased the E_a of fructose degradation from 35.99 to 74.04 kJ mol⁻¹ and decreased the E_a of glucose conversion from 40.35 to 24.62 kJ mol⁻¹. Adding alkali earth metal salts simultaneously stabilized fructose and promoted glucose isomerization. 70.3% fructose yield with 99% fructose selectivity can be achieved in 0.011 g mL⁻¹ Ca(OH)₂/0.5 g mL⁻¹ CaCl₂ solution at 50 °C for 25 min. From the DFT calculation and DOSY NMR characterization, we inferred that the interaction of CaCl₂ with glucose can reduce the energy barrier of glucose ring opening, and the complexation between CaCl₂ and fructose maintains the stability of fructose. Meanwhile, ¹H NMR titration revealed that the addition of CaCl₂ dramatically promoted the conversion of α-glucopyranose to β-glucopyranose. This alkali-salt-catalyzed glucose isomerization is simple and efficient, providing a new strategy for preparing platform chemicals from glucose.