Phase transitions in natural zeolites and the importance of P_H2O

Abstract

Zeolites are low-density silicates with structures consisting of a negatively charged aluminosilicate framework that creates a system of uniform linked channels and cavities. Variable amounts of extraframework cations and H₂O molecules occupy the channel system, and the H₂O molecules are very responsive to changes in temperature, pressure and partial pressure of water (i.e. P _H2O or relative humidity, RH). As the H₂O molecules occupy much of the volume of the extraframework sites, a gain or loss of H₂O molecules has a direct effect on the extraframework cations and an indirect effect on the framework. Temperature or RH-induced changes can result in both first- and second-order phase transitions, the latter resulting from continuous, minor changes in hydration state and cation position, and the former resulting from discrete changes in hydration state, which can cause similar shifts in cation position. Second-order transitions are typically reversible with no hysteresis, but first-order transitions exhibit considerable hysteresis. As H₂O molecules are crucial in determining zeolite behavior, it is important that any study of thermal behavior involve control of not only temperature but also of relative humidity. Stabilization of a zeolite's hydrated phase to higher temperatures under higher RH conditions can cause some phase transitions to be missed, as is the case with natrolite.

Keywords:

• phase transition
• zeolites
• clinoptilolite
• mordenite
• chabazite
• laumontite
• natrolite
• crystal structure
• X-ray diffraction

Acknowledgements

This manuscript was improved by comments from R. Rinaldi and an anonymous reviewer.