Effect of framework and extra-framework cations substitutions on the zeolite structure: Single-crystal X-ray diffraction analysis of analcime and wairakite solid solutions

Abstract:

There are a large number of natural and synthetic zeolites having a microporous structure, which is generally composed of Si/AlO₄ three-dimensional framework. Analcime, NaAlSi₂O₆·H₂O, is one of the most common naturally occurring zeolites. It forms a variety of solid solutions with wairakite, (Ca,Na₂)Al₂Si₄O₁₂·2H₂O, pollucite (CsAlSi₂O₆·nH₂O), and hsianghualite (Ca₃Li₂Be₃Si₃O₁₂·F₂). This study aimed to clarify structural variations in the solid solution between analcime and wairakite. Single crystals with compositions between analcime and wairakite were synthesized using hydrothermal synthesis method. Aluminium sulfate and sodium metasilicate nonahydrate gelled with distilled water were heated for 24 hours under 200 °C. The hydrothermally synthesized crystalline materials were analyzed by scanning electron microscope (SEM). SEM images showed that the grown single crystals exhibited euhedral trapezohedron shape up to 400μm in size. The effect of the cations substitution on the structural variation was examined by single-crystal X-ray diffraction analysis and electron probe micro-analyser (EPMA). Structure refinements based on the single-crystal X-ray diffraction data exhibited that crystal structures between analcime and wairakite belong to the cubic Ia-3d and kept approximately unchanged. Lattice parameter was monotonously reduced from 13.713 (1) Å to 13.650 (1) Å as a function of Ca concentration. In addition, the lattice parameter was also varied with Si/Al framework cation ratio. The contraction of the 6-membered oxygen rings was closely associated with exchanged cations and substituted framework cations. This study indicates that a geometry of micro-porous zeolite structure is significantly influenced by substitution of not only extra-framework cations but framework cations.