Evidence of Three Kinds of Tetrahedral Vanadium (V) Species in VSiβ Zeolite by Diffuse Reflectance UV−Visible and Photoluminescence Spectroscopies

VSiβ zeolites prepared by contacting dealuminated Siβ zeolite with aqueous solutions of ammonium metavanadate at room temperature have been studied by diffuse reflectance UV−visible and photoluminescence spectroscopy. UV−visible spectroscopy shows that whatever the vanadium content, vanadium is present in as-prepared VSiβ in only one kind of tetrahedral coordination. After calcination and then rehydration of VSiβ with low V content, still only one kind of tetrahedral V V species is observed. However, in calcined/rehydrated VSiβ with high V content, two kinds of tetrahedral and one kind of octahedral V species are revealed. In contrast, photoluminescence spectroscopy in static or dynamic mode can distinguish in as-prepared VSiβ three kinds of tetrahedral vanadium species. Their relative amounts depend strongly on the vanadium content and on calcination/rehydration treatments. They are present at two different types of framework sites. For samples with low V loading, the tetrahedral V V species are in a site poorly accessible to water, probably in the five-membered rings of the β structure. These V V species are little sensitive to calcination/rehydration treatments. In contrast, for higher V loading, the tetrahedral V V species become highly sensitive to such treatments suggesting that V V ions are in a site more accessible to water, probably in the 12-membered rings, where they can easily change their coordination upon dehydration/rehydration processes. Possible models of tetrahedral and octahedral V species and their probable location in the β structure are proposed.