Abnormal Expansion of a Disordered Zeolite upon Ion Exchange of K+ by Mg2+

Abstract

Here, we present the crystal structures of the Mg2+, Ca2+, Sr2+, and Ba2+ ion exchanged forms of PST-3 and PST-4 in the hydrated state, the two synthetic almuminosilicate natrolites with almost fully disordered and ordered Al and Si atoms, whose frameworks are thus more and less flexible to each other. Unlike the case of the other alkaline earth cations, Mg2+ exchange was found to differ notably according to the framework flexibility of these zeolites: K-PST-3 showed a high level (similar to 85%) of Mg2+ exchange, whereas the opposite was observed for K-PST-4. A combination of the high charge density of Mg2+ ions and the highly flexible nature of the disordered PST-3 framework causes an unexpected expansion in the unit cell volume of this type of zeolites upon ion exchange of K+ by Mg2+.