Acid catalysts prepared by impregnation of tungstophosphoric acid solutions on different supports

LUIS RENE PIZZIO; V CACERES, C; N BLANCO, MIRTA

APPLIED CATALYSIS A-GENERAL

ELSEVIER SCIENCE BV

Lugar: Amsterdam; Año: 1998 vol. 167 p. 283 - 294

0926-860X

Tungstophosphoric acid (TPA) in 50% v/v ethanol?water solutions was used to impregnate carbon, TiO2, SiO2 and γ-Al2O3 (Spheralite and Akzo) at 20°C. The solutions were characterized by UV-visible and nuclear magnetic resonance spectroscopies and the results indicated that the main species present was the undegraded anion of the acid both before and after the contact with carbon, TiO2, SiO2 and γ-Al2O3 Akzo. However, the nuclear magnetic resonance spectrum of the solution in contact with γ-Al2O3 Spheralite allowed to observe a tungstodiphosphate anion. X-ray diffraction of the impregnated solids dried at 70°C showed the same diffraction patterns as the supports, possibly owing to a high dispersion of non-crystalline species. The Fourier transform infrared and nuclear magnetic resonance spectroscopies of carbon, TiO2 and SiO2 impregnated with TPA showed that the species present was the undegraded [PW12O40]3− anion. When using carbon and TiO2, the supported acid was thermally stable up to 425°C, similarly to the bulk acid, whereas on SiO2 support, the anion underwent partial degradation from 365°C onwards. In TPA-impregnated γ-Al2O3 samples dried at 70°C, diffuse reflectance and nuclear magnetic resonance spectroscopies allowed us to observe a partial degradation of the anion, the extent of which was not appreciably increased by the subsequent calcination. The specific activity of the catalysts in the isopropanol dehydration reaction showed that TPA supported on carbon, TiO2 and SiO2 present higher acidity than the bulk acid, unlike in alumina where the partially degraded species gave a lower acidity.