Adsorption studies and species characterization in catalysts obtained from aqueous solutions of phosphomolybdic acid and alumina"

M. CASTILLO; P. VÁZQUEZ; M. BLANCO; C. CÁCERES

Journal of Chemical Society Faraday Transactions

Royal Society Chemistry

Lugar: Cambrigde; Año: 1996 vol. 92 p. 3239 - 3246

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 °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.