Investigation on the hydrated and dehydrated forms of the ion-exchanged microporous stannosilicate EMS-2

Abstract

The ion-exchange properties and characterization of EMS-2, a new stannosilicate isostructural with the natural zirconosilicate lemoynite, are reported. EMS-2 samples were characterized by high resolution synchrotron X-ray diffraction, elemental, thermogravimetric and differential scanning calorimetry analyses. The crystal structure of Mg, Ca, Sr and Ba exchanged samples was refined revealing the positions and occupancies of the extraframework cations and water molecules. After dehydration, the crystal structure of ion-exchanged EMS-2 was investigated by in-situ high temperature X-ray powder diffraction. The experiments indicated that the materials are stable at least up to 400 °C. Mg- and Ca-EMS-2 possess a very rigid structure and only slight variations of the unit cell parameters were noted after dehydration. At a difference, at high temperature Sr- and Ba-exchanged samples undergo a phase transition. The new structures were determined and the phase transition was rationalized as an extensive reorganization of cations after loosing the coordinated water molecules. This process is possibly driven by the need to attain a coordination environment as favourable as possible. This produces a strong framework distortion, associated to a structure contraction in the b direction, which drives to doubling the c cell parameter.