Isomerization of Glucose into Fructose over Natural and Synthetic MgO Catalysts

Fructose is one of the most important aldoses and has been gaining attention as the starting material for the synthesis of biobased platform and high-added value chemicals such as 5-hydroxymethylfurfural (5-HMF), levulinic acid and lactic acid. However, due to its low natural occurrence, fructose is produced from glucose, an abundant hexose, via isomerization. Currently, the conventional industrial process utilizes glucose isomerase as a catalyst and is therefore subjected to the limitations of enzymatic reactions. Consequently, an alternative efficient solid catalyst is required that will exhibit high activity, selectivity and stability/reusability. Toward this end, we have demonstrated the effectiveness of using natural MgO, derived from simple calcination of magnesite ores, as a low cost catalyst with increased basicity. A series of industrial and laboratory prepared natural MgO materials with different morphology, porosity and basicity were investigated and the optimum catalyst afforded 44.1 wt % glucose conversion and 75.8 wt % fructose selectivity (33.4 wt % fructose yield), at 90 °C for a 45 min reaction in aqueous solution. The activity of the MgO catalysts was directly correlated with their basicity, which in turn depended on their crystal size, surface area and composition. CaO impurities of the natural MgO materials generated strong basic sites that enhanced glucose conversion but at the expense of fructose selectivity. The stability and reuse of the optimum catalyst was confirmed for at least 4 cycles of reaction–regeneration, whereas the mechanism of glucose isomerization was validated via a first-order kinetic modeling set.