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

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 Ea of fructose degradation from 35.99 to 74.04 kJ mol−1 and decreased the Ea 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.