A contribution to the physicochemical characterization of nonstoichiometric salts of tungstosilicic acid

Nonstoichiometric cesium and rubidium salts of tungstosilicic acid (TSA) were synthesized by precipitation. These solids are insoluble in water, due to the low solvation energy of large cations, have high specific surface area, and a mainly microporous structure. In turn, nonstoichiometric potassium salts prepared by solution evaporation have a low specific surface area and porosity, high solubility in water, and a negligible porosity. All the synthesized tungstosilicates presented a cubic structure, with a lattice parameter that slightly decreases in the order Cs+ > Rb+ ≅ K+ salts.The salts displayed the characteristic FT-IR bands of TSA, though the band assigned to the stretching of the WO terminal bond presented a slight widening and a shoulder, and the band attributed to a bridged WOW bond was shifted to a lower wavenumber and showed a shoulder. Such effects are indicative of the interaction between the [SiW12O40]4− anion and the alkaline cations.Assuming that an increase in 1H MAS-NMR chemical shift is due to a higher acid strength, the cesium salts would have acid sites with higher acid strength than the others. The potassium salts, however, presented protons in different acid environments, with a low proportion of very strong acid sites.The acidity measurement by potentiometric titration with n-butylamine also showed that the cesium salts have acid sites with higher strength than those of rubidium. The same behavior was observed for the potassium salts, which exhibited an acid strength higher than expected. The number of protons determined by titration decreased in the order Cs+ > Rb+ ≅ K+ salts and also decreased for the more substituted salts.