Intrinsic and effective acidity of K+, Ba2+, and Nd3+ added to Nb2O5 related to stability in biomass reactions

Introduction Niobic acid (Nb2O5•nH2O, here referred as NBO) has been long considered a water-tolerant solid acid, thanks to its ability of maintaining lively acidity in polar and protic solvents, as water [1, 2]. This characteristic prompted its use in various acid-catalyzed reactions of fine chemical and in particular of biomass exploitation, such as the fructose dehydration reaction running in aqueous solvent [3]. In general, dehydration of sugars is considered an important reaction in the biorefinery platform that can form HMF (5-hydroxymethyl-2-furaldehyde) from which wide series of chemicals can be synthesized. A recent work of the authors [4] demonstrated that NBO is valuable in fructose dehydration in water, but it deactivates too fast for a real use. The deactivation of NBO should be associated with the high density and high strength of its surface acid sites due to its polymeric oxide structure (NbO6 octahedral unit repetition). With the aim of decreasing the surface acidity of NBO to obtain surfaces with modulated acid strength, we synthesized three series of doped-NBO samples with different amount (1-15 atoms nm-2) of K+, Ba2+, and Nd3+ ions. Attention was devoted to the acidity determination that was performed by conventional NH3 adsorption in a volumetric-calorimetric apparatus, by TPD (Temperature-Programmed-Desorption) of base probe (2-phenylethyl-amine, PEA), and by PEA titrations in aprotic (cyclohexane) and protic (water) solvents. Results obtained in water have been used to determine the sample effective acidity useful to find relationships with the activity. The doped-NBO samples were tested in long-term reaction (up to 200 h) of fructose dehydration to investigate on the catalyst stability and the activity-selectivity pattern.