THE INTERACTION OF CESIUM IONS WITH SILICA-SUPPORTED 12-MOLYBDOPHOSPHORIC ACID

The characterization by X-ray photoelectron spectroscopy (XPS), laser Raman spectroscopy (LRS), 31P NMR spectroscopy, and ion scattering spectroscopy (ISS) of cesium-doped silica-supported 12-molybdophosphoric acid (HPMo) is reported. The addition of cesium to a silica-supported 12-molybdophosphoric acid catalyst has been shown in previous work to poison the catalytic properties for partial oxidation of methane of the original HPMo catalyst. In this work it is shown that the HPMo anions, Keggin Units (KU) are not decomposed upon addition of Cs up to 3 Cs per KU although modification of the surface dispersion of the HPMo seems to occur. Evidence of a direct interaction between the KU and the cesium ions has been obtained from Raman, XPS, and ISS spectroscopies, resulting from an ion exchange between the proton of the acid and the Cs ions. XPS studies show that the Cs ions are well dispersed up to a loading corresponding to the exchange of the three H+ by 3 Cs+. In addition an electronic perturbation of Cs and Mo is detected by XPS as well as a shift of the main Raman band corresponding to a weakening of the MoOt, bond in accordance with the electronic effect of Cs deduced from the catalytic results. © 1991.