Local Structure of Cationic Sites in Dehydrated Zeolites Inferred from 27Al Magic-Angle Spinning NMR and Density Functional Theory Calculations. A Study on Li-, Na-, and K-Chabazite

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• 作者：Petr Klein ; Veronika Pashkova ; Haunani M. Thomas ; Sarah R. Whittleton ; Jiri Brus ; Libor Kobera ; Jiri Dedecek ; Stepan Sklenak
• 刊名：Journal of Physical Chemistry C
• 出版年：2016
• 出版时间：July 7, 2016
• 年：2016
• 卷：120
• 期：26
• 页码：14216-14225
• 全文大小：470K
• 年卷期：0
• ISSN：1932-7455

文摘

High-resolution ²⁷Al magic-angle spinning (MAS) NMR spectroscopy of dehydrated M-forms (M = Li, Na, and K) of chabazite in tandem with density functional theory calculations are employed to study the quadrupolar interaction of ²⁷Al nuclei in dehydrated zeolites and to understand the corresponding high-resolution ²⁷Al MAS NMR spectra. We show that the broadening of the ²⁷Al NMR signal in dehydrated zeolites occurs predominantly because of the deformation of the local structure of AlO₄^– tetrahedra caused by the binding of M⁺ to the zeolite framework. This deformation increases with the decreasing diameter of the cations from K⁺ to Li⁺. The influence of water in hydrated zeolites is limited only to prevent a strong coordination of the M⁺ cation to O atoms of the AlO₄^– tetrahedron, but there is no “averaging” effect concerning the local electrostatic field due to molecular motion of water molecules. Our results show that the ²⁷Al NMR parameters in dehydrated zeolites can be calculated accurately enough to allow the description of the local structure of AlO₄^– tetrahedra in dehydrated zeolites and to infer the local structure of the sites accommodating the extraframework M⁺ cations.