Evidence of three kinds of tetrahedral vanadium (V) species in VSi beta 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 st udied 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 coordin ation. After calcination and then rehydration of VSi beta with low V conten t, 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, photolumin escence spectroscopy in static or dynamic mode can distinguish in as-prepar ed VSi beta three kinds of tetrahedral vanadium species. Their relative amo unts depend strongly on the vanadium content and on calcination/rehydration treatments. They are present at two different types of framework sites. Fo r samples with low V loading, the tetrahedral V-V species are in a site poo rly accessible to water, probably in the five-membered rings of the beta st ructure. 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 t hey can easily change their coordination upon dehydration/rehydration proce sses. Possible models of tetrahedral and octahedral V species and their pro bable location in the beta structure are proposed.