Observation of a Rhombohedral Phase Change during the Dehydration of Zinc-Exchanged Zeolite A

Temperature dependent powder X-ray diffraction studies of zinc-exchanged zeolite A (LTA structure type; cubic at room temperature) have demonstrated the presence of a non-cubic phase between 150 degrees C and 600 degrees C. Analysis of the data indicated that a rhombohedral phase was formed with refined lattice parameters a = 17.182(3) angstrom and alpha = 59.147(9)degrees (after quenching from 150 degrees C to room temperature under vacuum). Heating above 600 degrees C produced a second cubic phase, which persisted upon cooling, and the rhombohedral phase only appeared for samples that had not been heated above 600 degrees C. The distortion from cubic symmetry may be linked to the formation of species such as Zn-x(H2O)(y)(m+) or Zn-x(H2O)(y)(OH)(z)(n+) during dehydration. (c) 2005 Elsevier Inc. All rights reserved.