Preparation and characterization of HMS supported 11-molybdo-vanado-phosphoric acid for selective oxidation of propylene

The preparation of Keggin-type phosphovanadomolybdic acid H4PMo11VO40 (PMo11V) supported on a hexagonal mesoporous silicate (HMS) material was performed via three different methods: dry impregnation, wet impregnation and mechanical mixing. The state of the heteropolyacid (HPA) on the siliceous surface and the integrity of the support were characterized by several physico-chemical techniques: elemental analysis, X-ray diffraction, transmission and diffuse reflectance (DR) FT–IR study of the framework vibrations, Raman and X-ray photoelectron spectroscopies, nitrogen physisorption, thermal analysis (TG–DTA) and scanning electron microscopy (SEM). The catalytic performance of supported materials was compared to that of the bulk H4PMo11VO40 in the selective oxidation of propylene by molecular oxygen in the temperature range of 200–400 °C.The physico-chemical characterization of the supported HPA showed mainly that the primary Keggin structure is intact and that a good dispersion of the heteropolyacid on the HMS surface is obtained when the impregnation methods are used. In the case of mechanical mixture, the dispersion of heteropolyacid clusters is not homogeneous. The surface area of HMS decreased when the HPA was added. The impregnation methods lead to mesopores with uniform size of approximatively 3.2 nm in diameter. The TG–DTA showed that the supported HPAs obtained by the impregnation methods have a higher thermal stability than that of bulk HPA. The catalytic results showed that H4PMo11VO40/HMS are more active than the bulk H4PMo11VO40 and lead to oxygenated compounds formation (acrolein, acetaldehyde and acetic acid), whereas the bulk HPA only leads to the formation of COx.