Kinetic Salt Effect on Base-Catalyzed Isomerization of d-Glucose into d-Fructose

Isomerization of d-glucose (Glc) into d-fructose (Fru) is of great importance for food sector as well as for valorization of lignocellulosic biomass. Soluble and solid bases exhibit high catalytic activity for the isomerization. Here, we report a salt effect on the base-catalyzed aqueous-phase Glc-Fru isomerization. Addition of soluble salts (Na₂ SO₄ , NaNO₃ , K₂ SO₄ , and NaCl) results in an increased apparent reaction rate (factors of 1.5 to 6). The salt effect was observed both in the presence of soluble base NaOH at constant pH value and solid bases MgO, Li₃ PO₄ , and Mg-Al hydrotalcite. Apparent activation energy and UV absorption spectra were not significantly influenced by addition of salts. Potentiometric titration showed that the acidity constants of the saccharides increase in the presence of electrolytes. Since the rate of the isomerization depends on the thermodynamic acidity constant of Glc, the isomerization is accelerated by the presence of electrolytes.