Effect of the K+, Ba2+, and Nd3+ addition to Nb2O5 on intrinsic and effective acidity in relation to biomass reactions

The mitigation of surface acidity and tuning of acid strength of niobic acid were successfully obtained by K+, Ba2+, and Nd3+ surface doping. Three series of doped samples with different amounts of each metal ion on niobic acid (2–15 atoms nm−2, 1–10 atoms nm−2, and 1–6 atoms nm−2 for K, Ba, and Nd, respectively) were prepared and characterized by various surface and bulk techniques (N2-adsorption, TGA, SEM–EDS, and UV–vis-DRS). Particular attention was paid to the acidity determination that was performed with different techniques: by conventional NH3 adsorption in a volumetric–calorimetric apparatus, by temperature-programmed desorption of PEA (2-phenylethyl-amine), and by PEA titrations in water. The acidity results obtained in water have been used to determine the sample effective acidity. The results revealed that Ba and Nd dopants were more effective than K dopant in decreasing the effective acidity of niobic acid. On the three sample series, the reactions of fructose dehydration and sucrose hydrolysis have been taken into account as model reactions to control the nature (Brønsted or Lewis acid sites, BAS, or LAS) of the catalytic acid sites still active in water. Results indicated that K+ ion doped more selectively the BAS while Ba2+ and Nd3+ ions modified more the LAS population of niobic acid.