Solvent effect on the preparation of H3PW12O40 supported on alumina

Preparation, structural characterization and solid acidity studies of H3PW12O40 (H3PW) supported (20, 40, 50 and 80 wt.%) on γ-alumina have been performed. Impregnation of H3PW on γ-alumina was performed by the evaporation technique using water (acidified with HCl), ethanol and acetonitrile as solvents. The presence of the Keggin anion on alumina surface is very dependent upon the preparation conditions and pH, as indicated by partial decomposition in water solution. High acid concentration in aqueous solutions is required to obtain intact the Keggin anion. Ethanol showed the formation of distinct species on the alumina surface. In contrast, acetonitrile has not displayed any significant decomposition of the Keggin anion. All these conclusions are based on FTIR, Raman, 31P MAS–NMR, XRD and SEM–EDX measurements. The presence of the Keggin structure on the alumina surface can be followed by those techniques, eliminating any doubt about collapse of the supported anion. Calorimetric and adsorption of pyridine interactions in cyclohexane slurry (Cal-ad method) showed that the catalyst H3PW/Al2O3 is a weaker acid than pure H3PW or supported on silica, but stronger than alumina. The enthalpies indicated partial neutralization of the most basic sites of alumina at the expense of the strongest protons of H3PW.