SOLID-STATE NMR AND FT-IR INVESTIGATION OF 12-TUNGSTOPHOSPHORIC ACID ON TIO2

The chemical and dynamic nature of the tungstophosphoric acid (TPA) sp ecies adsorbed on pressed, extruded, and powdered TiO2 support was stu died by P-31 and H-1 solid-state NMR spectroscopy and diffuse reflecta nce FT-LR. From P-31 NMR and FT-IR results the TPA appeared to be pres ent in at least five forms on the titania surface: a bulk salt phase, two weakly bound intact Keggin species, a range of partially fragmente d clusters such as the 11-''defect'' Keggin ion, and a range of specie s formed by high or complete fragmentation of the Keggin ion. The rela tive amount of these species varied depending on the form of the suppo rt. Adsorption was incomplete on extruded titania resulting in bulk sa lt 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 fragmenta tion) without bulk salt formation was attained by impregnation of powd ered TiO2. The acidic nature of the catalysts and the degree of TPA di spersion were investigated by solid-state H-1 high-speed MAS-NMR. Pure TPA salt yielded resonances at about 6.5 ppm. The catalysts yielded a cidic proton resonances at about 7-8 and 10-11.5 ppm. The broad resona nce 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 increa sed with adsorption effectiveness and could, therefore, be related to the formation of hydroxonium ions upon fragmentation of adsorbed Keggi n ions.