Influence of the preparation procedure on the nature and environment of vanadium in VSiBEA zeolite: XRD, DR UV-vis, NMR, EPR and TPR studies

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• 作者：Rafal Baran ; Yannick Millot ; Thomas Onfroy ; Frederic Averseng ; Jean-Marc Krafft ; Stanislaw Dzwigaj
• 关键词：Vanadium ; BEA ; FTIR ; DR UV&ndash ; vis ; EPR
• 刊名：Microporous and Mesoporous Materials
• 出版年：2012
• 出版时间：1 October, 2012
• 年：2012
• 卷：161
• 期：Complete
• 页码：179-186
• 全文大小：1071 K

文摘

Two series of V_xSiBEA(I) and V_xSiBEA(II) zeolites have been prepared by a two-steps postsynthesis method which consists, in the first step, in the dealumination of TEABEA zeolite by two different procedures to obtain SiBEA(I) and SiBEA(II) zeolites and then, in the second step, in contacting the obtained zeolites with an aqueous NH₄VO₃ solution at pH 2.7. Because the NH₄VO₃ solution at pH 2.7 predominantly contains mononuclear VO₂⁺ ions, vanadium can be incorporated into SiBEA(I) and SiBEA(II) as framework pseudo-tetrahedral non hydroxylated (SiO)₃V(V)O and hydroxylated (SiO)₂(OH)V(V)O species by reaction with silanol groups of vacant T-atom sites. Combined use of FTIR, DR UV-vis, ⁵¹V NMR, EPR and TPR allows to determine the oxidation state, nature and environment of vanadium in as prepared, calcined, rehydrated, activated and reduced V_xSiBEA(I) and V_xSiBEA(II) samples. It is shown that the procedure of dealumination has the effect on the properties of the silanol groups of vacant T-atom sites in SiBEA(I) and SiBEA(II) and on the nature and environment of V species in as prepared V_xSiBEA(I) and V_xSiBEA(II). As evidenced by DR UV-vis and ⁵¹V MAS NMR in V_xSiBEA(II), vanadium is present only as pseudo-tetrahedral V(V) species, and in contrast in V_xSiBEA(I) as pseudo-tetrahedral and octahedral V(V) species. The oxidation state of framework tetrahedral vanadium in V_xSiBEA(I) and V_xSiBEA(II) easily change upon calcination in oxygen, outgassing at 773 K and reducing with hydrogen at high temperature (873 K), as shown by EPR. The presence of Br?nsted and Lewis acidic centres are evidenced in V_xSiBEA(II) by FTIR adsorption investigations of pyridine and CO used as probe molecules.