Glucose–fructose isomerisation promoted by basic hybrid catalysts

As-synthesised mesoporous ordered molecular sieves of the M41S family, containing their organic template and used without any pretreatment, were shown to be efficient weak basic catalysts in glucose–fructose isomerisation in water at 100 °C. Selectivities higher than 80% were obtained for glucose conversion higher than 20%. By comparison, NaOH catalysed glucose–fructose isomerisation operates at lower temperature, but the selectivity is shown to be more strongly dependent on the duration and on the temperature of the reaction. Interestingly, a more conventional inorganic stronger solid base, such as zirconia promoted with Cs, is less active and requires higher reaction temperature, 150 °C, to isomerise glucose into fructose or a pretreatment at high temperature aimed to remove the strongly adsorbed carbonate species. The superiority of the hybrid organic bases compared to the inorganic stronger solid base is correlated to the presence of a high number of weak basic sites intermediate between chemisorption and physisorption, resistant towards irreversible CO2 poisoning. This is demonstrated by calorimetry of CO2 adsorption.