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

Abstract

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 all...