Tuning the acidity of niobia: Characterization and catalytic activity of Nb2O5–MeO2 (Me = Ti, Zr, Ce) mesoporous mixed oxides

• Mesoporous Nb2O5–MeO2 mixed oxides were successfully prepared by EISA method.
• Acidic–basic properties depend on the nature of the oxide that was mixed with niobia.
• Catalytic activity was tested in fructose dehydration in aqueous phase.
• Selectivity and conversion in reaction are correlated to the number of acid sites.

Mesoporous Nb2O5–MeO2 (Me = Ti, Zr, Ce) mixed oxides were successfully prepared using evaporation-induced self-assembly (EISA) method. The structural and textural properties of these materials have been fully characterized using appropriate techniques (low-temperature adsorption–desorption of nitrogen, thermogravimetric analysis, X-ray diffraction analysis (XRD) transmission electron microscopy (TEM), scanning electron microscopy (SEM) and Raman spectroscopy). Acid–base properties were estimated by adsorption microcalorimetry of NH3 and SO2 molecules in order to determine the population, strength and strength distribution of acidic or basic sites. Formation of mesoporous structure was confirmed by the results of XRD, TEM and BET techniques. Results of adsorption microcalorimetry technique showed that the type of transition metal oxide added to niobia has a decisive role for acidic-basic character of investigated mixed oxides. Among the investigated mixed oxide formulations only Nb2O5–CeO2 was amphoteric, while the other samples showed prominent acidic character. All the investigated materials are catalytically active in fructose dehydration; conversion of fructose and selectivity to 5-hydroxymethylfurfural (5-HMF) and levulinic acid (LA) are proved to be dependant on the number of acidic sites on the surface of catalysts. Furthermore, presence of the basic sites on the surface of the catalyst decreases the activity in the fructose dehydration reaction, as in the case of Nb2O5–CeO2 sample.