Structure–performance correlations of Mg–Al hydrotalcite catalysts for the isomerization of glucose into fructose

Abstract

Mg–Al hydrotalcites with different Mg-to-Al molar ratio, texture, and morphology were studied as catalysts for the isomerization of glucose into fructose. The properties of the hydrotalcites were tuned by varying the preparation procedure including pH during co-precipitation, ageing temperature, and solvent. The catalysts were characterized by ICP-OES, N2 sorption, sorption of acrylic acid, XRD, TG-DSC, and SEM. Tuning the conditions of synthesis enables controlling properties such as crystallite size, dispersion of primary particles, and morphology of the agglomerates. These structural parameters of the hydrotalcites critically influence their basicity, which in turn predominantly determines the catalytic performance. The best fructose yields of up to 30% with 89% selectivity were obtained over hydrotalcites synthesized by co-precipitation in (i) an aqueous medium at pH 10 and (ii) an aqueous-ethanol medium at pH 9.5. These synthesis conditions enabled materials with optimal textural properties and the highest amount of accessible basic centres. Although some leaching of magnesium occurred caused by lactic acid formed as by-product of carbohydrate degradation, the hydrotalcites could be recycled without loss of activity and selectivity.