Structural study of Mg-exchanged NaX and CaX zeolites in hydrated (25°C) and dehydrated (400°C) states

Abstract

The crystal structures of partially Mg-exchanged zeolites Mg 31Na 30X (h-MgNaX) and Ca 23Mg 23X (h-CaMgX) with a Si Al ratio of 1.1 and their dehydrated forms (d-MgNaX and d-CaMgX) were determined using single-crystal X-ray diffraction data. For the dehydrated forms, X-ray intensities were measured at 400°C. The structures were refined by the least-squares method in the cubic space group Fd3 to final R-factors of 0.056 and 0.062 for hydrated and 0.046 and 0.057 for dehydrated forms, respectively. In h-MgNaX, Mg ions are located in three sites, I′, II, and III; in h-CaMgX, they occupy sites I′ and II. In the dehydrated forms, the sites of Mg location are I, I′, and II in d-MgNaX and II′ and II in d-CaMgX. In site II, Mg ions are situated in the center of the six-membered ring. Large distortions of the zeolite framework on dehydration are observed. The incomplete ion exchange of Na + for Mg 2+ is explained by the blockade of 12-membered rings with Mg ions in site III. The influence of Mg ions on the positions of other cations is discussed.