Spectral and energetic aspects of water adsorption by Li-, Na-, K- and Cs-X zeolites

Abstract

The infra-red spectra of water adsorbed by X-type zeolites containing different alkali-metal cations have been studied. With small amounts of adsorption in LiNaX, NaX and KNaX zeolites water molecules interact mainly with the zeolite cations. The spectra show that one hydroxyl group of an adsorbed water molecule is bound to the oxygen of the zeolite skeleton and the other is “free”. In the case of CsX zeolite, where adsorption is weaker, both hydroxyl groups of the adsorbed water molecules interact, presumably, with the oxygen atoms of the zeolite. The relative amounts of water molecules interacting with the exchange cations, zeolite oxygen atoms and water molecules forming mutual hydrogen bonds depend on the cation and the number of water molecules in the zeolite cages. As these are filled there is a change in the energy of hydrogen bond formation between water molecules and the zeolite oxygen skeleton and between one another. This change becomes particularly apparent in the case of KX zeolite and is in accordance with the wave-like change of the differential heat of water adsorbtion by KX zeolite and with the maxima and minima of the heat capacity of adsorbed water.