Preparation and characterization of H3PW12O40 supported on niobia

Catalysts of H3PW12O40 (H3PW) supported on Nb2O5 were prepared with a wide range of loadings (20–60 wt%) and characterized by FTIR, FT-Raman, XRD, 31P MAS NMR, the BET method and pyridine adsorption in the gas phase. In addition, the acidity was measured by the Cal-Ad method, which combines microcalorimetry and adsorption of pyridine on the solid samples in cyclohexane slurry. Preparation of the catalysts involved two procedures: impregnation in aqueous acid or acetonitrile solutions with further calcination at 200 °C for 8 h. No decomposition of H3PW was observed under these conditions. The results indicated the formation of well-dispersed H3PW species on materials with loadings up to 30 wt%, prepared preferentially in aqueous acid solution. A strong interaction between the heteropolyacid and niobia surface was observed forming species such as [NbOH2]n+[H3-nPW12O40]n-3. The H3PW/Nb2O5 materials containing 20–40 wt% presented type II isotherms and showed that surface area, pore volume and pore size decreased as the amount of H3PW increased, whereas the micropore area increased with H3PW loading. According to gaseous pyridine adsorption results, the catalysts presented only Brønsted sites. Microcalorimetric titration demonstrated that the 25 wt% material was the most acidic. The Cal-Ad method provided the enthalpy and number of sites for the strongest interaction with pyridine as: ΔH1 = −119.7 kJ mol−1 and n1 = 0.086 mmol g−1. About 33% of the protons were available for 25% H3PW/Nb2O5, in comparison to only 8% for pure H3PW. A considerable increase in the surface protons was achieved by H3PW on niobia surface.