Selective and Tunable Routes for Glucose to Fructose Conversion Using MgCl₂ Catalysis and Comparison to Other Metal Ions

The conversion of glucose to fructose is an important step for the formation of biofuels, fine chemicals and in the food industries. Mg²⁺ is the most abundant divalent cation in living cells and sea water and could be an environmentally friendly biomimetic catalysts for glucose-to-fructose isomerization in water, while holding relevance to prebiotic chemistry. Here, we demonstrate that the catalytic performance of MgCl₂ can be tuned using strategies that limit the presence of basic oxide. Upon calcination and reaction under N₂, glucose isomerization in water at 120 °C approached the thermodynamic equilibrium (~42% fructose) within 30 minutes. Isotope tracking showed that the isomerization proceeds via competing pathways. Compared to Al³⁺ and Cr³⁺, the stereoselectivity is considerably lower for Mg²⁺ than for Al³⁺ and Cr³⁺. Effects of formic acid on the initial rate of glucose-to-fructose isomerization showed a slowing of the reaction catalyzed both by Mg2+, Al³⁺ and Cr³⁺. Inhibition decreased in this order, which resembles decreasing pKa values of the metal ions in aqueous solution. Hydrolysis of aqua ions appears to generate active species for the 1,2-hydride shift in all cases, where the formation of transient and nonspecific interactions between Mg²⁺ and carbohydrate results in a moderate stereoselectivity.