Collapse and recrystallization processes in zinc-exchanged zeolite-A: A combined x-ray diffraction, XAFS, and NMR study

The collapse and recrystallization of zinc-exchanged sodium zeolite-A has been observed in situ using combined X-ray spectroscopy and diffraction. The temperature at which this occurs is reduced by the presence of zinc, but the precipitation of NaAlSiO₄ (nepheline) is blocked by the nucleation of ZnAl₂O₄ (gahnite), which persists to at least 1100°C. Within the hydrated zeolite phase zinc occupies disordered octahedral sites that become less disordered with calcination. When zeolite-A collapses, zinc adopts a tetrahedral but strongly disordered geometry, suggesting nonhomogeneity in the amorphous phase from which ZnAl₂O₄ precipitates. Combined X-ray methods have been complemented by solid-state ²⁷Al and ²⁹Si NMR, which have enabled estimates to be made of the extent of disorder in the starting zinc-exchanged zeolite-A structure as well as the degree of crystallinity in the gahnite aluminosilicate glass ceramic that develops after collapse.