PHYSICOCHEMICAL CHARACTERIZATION OF V-SILICALITE

The coordination and nature of V sites in V-silicalite samples prepared by hydrothermal synthesis are characterized by wide-line solid-state V-51-NMR, electron spin resonance (ESR), UV-visible diffuse reflectance (DR) and infrared (FT-IR) spectroscopies, X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), ammonia temperature-programmed desorption (NH3-TPD) and hydrogen temperature-programmed reduction (H-2-TPR). Four vanadium species were detected: (i) a polynuclear vanadium oxide species containing reduced vanadium species, (ii) a nearly octahedral vanadyl (VO2+) species in the zeolitic channels which interacts with Bronsted sites, (iii) a V5+ species in sites characterized by a nearly symmetric tetrahedral environment, and (iv) after reduction a V4+ species in a nearly tetrahedral environment. The polynuclear vanadium oxide species can be removed from the zeolite by an ammonium acetate extraction and is derived from excess vanadium present in the preparation of V-silicalite. The excess vanadium 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. The UV-visible DR spectrum of this species differs from that of other tetrahedral compounds, 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 associated 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 associated with tetrahedral V5+ species. The model of the possible localization of V5+ sites in the zeolitic structure is also given.