Silica-supported bismuth molybdate catalysts obtained by the sol-gel process via silicon alkoxides

Abstract

The sol–gel route is applied to the preparation, in a one-step process, of bismuth molybdates highly dispersed into silica xerogels produced by hydrolysis/condensation reactions involving silicon alkoxides. The samples correspond to different Bi/Mo and (Bi + Mo)/Si stoichiometric ratios, according to the different bismuth molybdate phases (01-Bi 2 Mo 3 O 12 , β-Βi 2 Μo 2 O 9 , γ-Bi 2 MoO 6 ) and for various metal (Mo + Bi) loadings (2, 5, 10, 25 or 50 mol.% with respect to silica). The samples were characterized by X-ray diffractometry (XRD), X-ray photoelectron spectroscopy (XPS) and energy dispersive X-ray microanalysis coupled with scanning electron microscopy (EDX-SEM). The xerogels containing low molybdate loadings (2, 5, 10 mol %) exhibited high Sbet values (300 to 700 m 2 /g), but the expected phase was never detected as pure in these materials. The XRD spectra of the xerogels characterized by high molybdate loadings (25, 50 mol %) clearly showed the formation of complex mixtures, the a-phase being predominant at 600 °C when the starting Bi/Mo ratio was 0.66, while phase mixtures were observed in all the other cases. Whereas the Bi/Mo ratios determined by XPS were systematically much lower than the expected ones, more satisfactory values were observed using EDX-SEM. This apparent discrepancy reflects the difference in surface depths explored by the two techniques and suggests Mo enrichment of the outer surface layers. This behaviour is thought to be related to the selective migration of ions throughout the incoming silica network, resulting in heterogeneous distributions of Bi-based and molybdate ions, that could actually lead to different phases and anomalous surface Bi/Mo ratios.