Solid-state NMR and FT-IR investigation of 12-tungstophosphoric acid on TiO2

The chemical and dynamic nature of the tungstophosphoric acid (TPA) species adsorbed on pressed, extruded, and powdered TiO2 support was studied by P-31 and H-1 solid-state NMR spectroscopy and diffuse reflectance FT-LR. From P-31 NMR and FT-IR results the TPA appeared to be present in at least five forms on the titania surface: a bulk salt phase, two weakly bound intact Keggin species, a range of partially fragmented clusters such as the 11-"defect" Keggin ion, and a range of species formed by high or complete fragmentation of the Keggin ion. The relative amount of these species varied depending on the form of the support. Adsorption was incomplete on extruded titania resulting in bulk salt formation. There was almost complete adsorption on pressed titania pellets, yielding predominantly fragmented Keggin units, along with a small amount of bulk salt. However, complete adsorption (and fragmentation) without bulk salt formation was attained by impregnation of powdered TiO2. The acidic nature of the catalysts and the degree of TPA dispersion were investigated by solid-state H-1 high-speed MAS-NMR. Pure TPA salt yielded resonances at about 6.5 ppm. The catalysts yielded acidic proton resonances at about 7-8 and 10-11.5 ppm. The broad resonance observed at 7-8 ppm is due to a dispersion of acidic sites on the surface. The intensity of the acidic resonance at around 11 ppm increased with adsorption effectiveness and could, therefore, be related to the formation of hydroxonium ions upon fragmentation of adsorbed Keggin ions.