Evidence of three kinds of tetrahedral vanadium (V) species in VSiβ zeolite by diffuse reflectance UV-visible and photoluminescence spectroscopies

VSi beta zeolites prepared by contacting dealuminated Si beta 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 VSi beta in only one kind of tetrahedral coordination. After calcination and then rehydration of VSi beta with low V content, still only one kind of tetrahedral V(V) species is observed. However, in calcined/ rehydrated VSi beta 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 VSi beta 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 beta 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 beta structure are proposed.