Structure and texture of some Keggin type heteropolyacids supported on silica and titania

The structure and texture of H3PMo12O40 (HPMo) and H4PVMo11O40 (HPVMo) supported on silica (Aerosil- Degussa and Romsil types) and titania are studied by XRD, FT-IR, low temperature nitrogen adsorption and scanning electron microscopy. All supported heteropolyacids (HPAs) were prepared by impregnation using the incipient wetness techniques with a mixture water: ethanol = 1:1. In order to obtain highly dispersed HPAs species, the samples were prepared with different quantities of active compounds: 5-40 wt. %. The effect of loading on textural and structural properties was examines for supported HPAs catalysts comparatively with parent acids and their mechanical mixtures. It is found that most of HPMo and HPVMo (active phases) in samples are well dispersed on the support and both silica and titania supported HPAs still keeps its Keggin structure. The surface morphology of the silica and titania supported samples is almost similar to that of support, and thereby a relatively uniform distribution of active phase in the support pores are expected. The dispersion of active phase on support was estimated from XRD pattern, and it decreases with acid content. FTIR spectra of mechanical mixtures of HPA with silica are similar to the supported samples, so this method cannot distinguish these materials. Contrasting with supported HPAs on silica, their mechanical mixtures exhibit sharp and narrower diffraction lines, more similar to crystalline pure HPAs.