Physicochemical characterization of V-silicalite
- V.le Risorgimento 4, Bologna (Italy); and others
The coordination and nature of V sites in V-silicalite samples prepared by hydrothermal synthesis are characterized. Four vanadium species were detected: (1) a polynuclear vanadium oxide species containing reduced vanadium species, (2) a nearly octahedral vanadyl (VO{sup 2+}) species in the zeolitic channels which interacts with Bronsted sites, (3) a V{sup 5+} species in sites characterized by a nearly symetric tetrahedral environment, and (4) after reduction a V{sup 4+} 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 V{sup 5+} 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. Only weak silanol groups are present on the external surface. The nearly octahedral VO{sup 2+} species interacts with the OH groups associated with tetrahedral V{sup 5+} species. The model of the possible localization of V{sup 5+} sites in the zeolitic structure is also given. 79 refs., 11 figs., 3 tabs.
- Sponsoring Organization:
- USDOE
- OSTI ID:
- 263246
- Journal Information:
- Journal of Physical Chemistry, Journal Name: Journal of Physical Chemistry Journal Issue: 6 Vol. 96; ISSN JPCHAX; ISSN 0022-3654
- Country of Publication:
- United States
- Language:
- English
Similar Records
Tin passivation of vanadium in metal-contaminated fluid-cracking catalysts: Electron paramagnetic resonance studies
Synthesis and characterization of vanadium-substituted mesoporous molecular sieves