Intrinsic and effective acidity study of niobic acid and niobium phosphate by a multitechnique approach

The acidity properties of niobic acid (Nb2O5•nH2O) and niobium phosphate (NbOPO4) surfaces have been studied by different methods and under various conditions in order to obtain comprehensive information about the number, strength, and strength distribution of their acid sites. Four base probe molecules of different basicities (ammonia, pyridine, 2-phenylethylamine, and aniline) have been used. Gas-solid and liquid-solid titrations by these basic probes have been performed, using static or dynamic adsorption (calorimetry linked to volumetry, reaction calorimetry, pulse flow volumetry using a HPLC apparatus) and desorption (thermogravimetric analysis) techniques. The intrinsic surface acidities of the two solids, determined under vacuum or inert atmospheres, were compared with the effective acidities measured in liquids of different polarities and proticities (decane, cycloexane, toluene, methanol and isopropanol). The effective acidities of the two surfaces could be related with their catalytic activities measured in the fructose dehydration reaction in aqueous phase. The superior catalytic activity of niobium phosphate compared to niobic acid is related to its higher number of surface sites maintaining acid characteristics even in highly polar and protic solvents.