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

Abstract

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 PMo₁₂O₄₀^3– depolymerizes to PMo₁₁O₃₉^7– in water, whereas it remains as PMo₁₂O₄₀^3– in solutions acidified with HCl or in ethanol–water. Nevertheless, the species present in the solutions obtained after the adsorption was PMo₁₁O₃₉^7– in the two first solvents, and PMo₁₂O₄₀^3– 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 PMo₁₁O₃₉^7– 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 PMo₁₂O₄₀^3–, 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.