Adsorption studies and species characterization in catalysts obtained from aqueous solutions or phosphomolybdic acid and gamma-alumina

The equilibrium adsorption on alumina of phosphomolybdic acid (H3PMo12O40), with a Keggin phase structure, has been studied from its solutions in water, 0.1 mol dm(-3) HCl and ethanol-water. The molybdenum adsorption isotherms showed only one plateau, in all cases, indicating the adsorption of only one main species. NMR spectra of the initial solutions showed that the PMo12O403- depolymerizes to PMo11O397- in water, whereas it remains as PMo12O403- in solutions acidified with HCl or in ethanol-water. Nevertheless, the species present in the solutions obtained after the adsorption was PMo11O397- in the two first solvents, and PMo12O403- for ethanol-water. Diffuse reflectance spectra of representative solid samples of the adsorption tests from solutions in water and in 0.1 mol dm(-3) HCl showed that the adsorbed species was PMo11O397- for high-concentration wet samples and that decomposition of the lacunar phase starts at low temperatures. For the samples obtained by adsorption from solutions of H3PMo12O40 in ethanol-water, it can be concluded that the adsorbed species is PMo12O403-, which is not degraded below 230 degrees C. For low-concentration samples, total decomposition of phosphomolybdic anions was observed, regardless of the solvent used. By X-ray diffraction of wet, dried or calcined samples, it was observed that the adsorbed species are highly dispersed on the alumina surface.