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.

Mitigation of the surface acidity of niobic acid was pursued by K, Ba, and Nd doping. The acidity of the doped surfaces decreased with the increasing dopant species added to the surface (1–15 atom nm−2). The samples showed different acid properties when measured in gas/solid phase (intrinsic acidity) and water (effective acidity), which is the reaction solvent. Model reactions of mono- and disaccharides in water indicated that K+ ions doped more selectively the Brønsted acid sites while the Ba2+ and Nd3+ ions doped more selectively the Lewis acid sites of niobic acid.Figure optionsDownload high-quality image (60 K)Download as PowerPoint slideHighlights
► Mitigation and modulation of the surface acidity of niobic acid were realized by ion-doping.
► The intrinsic acidity in gas/solid phase and the effective acidity in water were determined by base adsorption and desorption.
► Model reactions of sucrose hydrolysis and fructose dehydration were performed to study the nature of the acid sites.
► K doped more selectively the Brønsted acid sites, and Ba and Nd doped more selectively the Lewis acid sites of niobic acid.