The open framework structures of zeolites, containing variable amounts of extra-framework cations and water molecules, are very responsive to changes in temperature and/or water-vapor pressure. Several coupled changes occur in water content and structure whenever a zeolite is subjected to a change in environment. The structural changes include modifications in the unit-cell size and geometry, movement of extraframework cations, and even statistical breakage of the Al-Si framework. The water content in some zeolites is a smoothly varying function of temperature and water-vapor pressure, whereas in others there appear to be distinct transitions between different hydration levels. The response of zeolites...

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