Physicochemical characterization of V-silicalite

The coordination and nature of V sites in V-silicalite samples prepd. by hydrothermal synthesis are characterized by wide-line solid-state 51V-NMR, ESR, UV-visible diffuse reflectance (DR) and FT-IR spectroscopies, x-ray diffraction, XPS, NH3 temp.-programmed desorption (NH3-TPD) and H temp.-programmed redn. Four V species were detected: (i) a polynuclear V oxide species contg. reduced V species, (ii) a nearly octahedral VO2+ species in the zeolitic channels which interacts with Broensted sites, (iii) a V5+ species in sites characterized by a nearly sym. tetrahedral environment, and (iv) after redn. a V4+ species in a nearly tetrahedral environment. The polynuclear vanadium oxide species can be removed from the zeolite by an ammonium acetate extn. and is derived from excess V present in the prepn. of V-silicalite. The excess V remains as an amorphous oxide in pores or in external positions, as indicated by XPS. The tetrahedral V5+ species can be attributed to atomically dispersed V species anchored to the silicalite framework, probably as a framework satellite, and localized inside the pore structure, as indicated by FT-IR and XPS. UV-visible DR spectrum of this species differs from that of other tetrahedral compds., suggesting the presence of a short V-O bond. The FT-IR spectroscopy data obtained in the characterization of surface acidity show the presence of strong Lewis acid sites assocd. with V only, inside the zeolitic channels, indicating the internal localization of these V5+ sites. Only weak silanol groups are present on the external surface of the zeolite, and these groups generate very weak Lewis sites upon evacuation. Silanol groups are also formed inside the zeolite channel in the V-silicalite, as indicated by FT-IR and NH3-TPD. The nearly octahedral VO2+ species interacts with the OH groups assocd. with tetrahedral V5+ species. The model of the possible localization of V5+ sites in the zeolitic structure is also given. [on SciFinder(R)]