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 diffra ction. 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 d egrees C. Within the hydrated zeolite phase zinc occupies disordered o ctahedral sites that become less disordered with calcination. When zeo lite-A collapses, zinc adopts a tetrahedral but strongly disordered ge ometry, suggesting nonhomogeneity in the amorphous phase: from which Z nAl2O4 precipitates. Combined X-ray methods have been complemented by solid-state Al-27 and Si-29 NMR, which have enabled estimates to be ma de of the extent of disorder in the starting zinc-exchanged zeolite-A structure as well as the degree of crystallinity in the gahnite alumin osilicate glass ceramic that develops after collapse.