CaCl₂molten salt hydrate-promoted conversion of carbohydrates to 5-hydroxymethylfurfural: an experimental and theoretical study

Alkaline earth metal salts are environmentally friendly and economical. They have attracted much attention in the process of biomass conversion, especially for isomerization and dehydration. However, the lack of understanding of the alkaline earth metal-mediated catalytic mechanism prevents the achievement of high HMF yields from glucose or fructose. Herein, we performed experimental and theoretical studies on CaCl₂-catalyzed glucose-to-fructose isomerization and fructose-to-HMF dehydration. In CaCl₂solution, the highest fructose yield (33.6%) from glucose can be achieved at 80 °C. Meanwhile, the presence of CaCl₂in fructose-to-HMF dehydration notably improved fructose conversion. A “one-pot” isomerization-dehydration of glucose to prepare HMF was designed and 100% conversion and 52.1% HMF yield were obtained. The CaCl₂solution can be reused for five runs while maintaining catalytic activity without regeneration. Through¹H NMR,¹³C NMR, DOSY-NMR and DFT calculations, we concluded that the coordination between the C1/C2 sites of the β-glucopyranose tautomer and CaCl₂promoted the 1,2-intramolecular hydride shift of glucose, thus rapidly converting glucose into fructose. Similarly, Ca²⁺can complex with the C2/C6-OH of β-d-fructofuranose and stabilize this tautomer, which easily forms HMF. This is the main reason that CaCl₂can promote the preparation of HMF from fructose.