The behavior of natural zeolites is important in any application that utilizes the gas adsorption properties of zeolites. Because of the large variety in the sizes of extraframework cavities and inter-cavity ports, the de/rehydration behavior varies considerably from zeolite to zeolite. Most zeolites have large extraframework cavities and rehydrate rapidly, but those with small ports, such as analcime, rehydrate very slowly. The comparatively slow rates of hydration and rehydration in analcime make it possible to measure the rates experimentally. In order to understand the effects of H2O-vapor pressure and temperature on natural zeolites, we have studied the rehydration kinetics of a natural analcime from Wikieup, Arizona, under controlled conditions.
Rehydration of Wikieup analcime was investigated by placing dehydrated samples into Teflon vessels with deionized water at ~25°, 50°, 75°, and 100 °C and also by analyzing dehydrated samples in an Anton Parr heating/cooling stage on an X-ray powder diffraction instrument in which temperature and humidity were controlled. To monitor rehydration, the samples were periodically examined using X-ray powder diffraction, determining H2O occupancies using Rietveld refinement techniques.
As expected, the analcime rehydration rate was influenced primarily by the temperature of rehydration, with rate increasing with increasing temperature. The amount of H2O available for rehydration and whether it was in the form of liquid or gas also influenced the rehydration rate. Rehydration rates in gaseous H2O were approximately one order of magnitude greater than those measured in liquid water, although activation energies calculated for hydration in liquid and gaseous H2O were identical within errors at 62 ± 1 and 64 ± 5 kJ/mol, respectively.