Heats of adsorption of water vapour on X-zeolites containing Li+, Na+, K+, Rb+, and Cs+ cations

Abstract

Differential heats of adsorption of water vapour on zeolites LiNaX, NaX, KNaX, RbNaX and CsNaX were measured calorimetrically over a wide range of adsorption. The heat of adsorption of water is determined mainly by the energy of specific interaction of the water molecules with the exchange cations, with the negatively charged oxygen ions of the zeolite framework, and with each other. The contributions of these interactions change with change of cation and with degree of filling; hence the dependence of the heat of adsorption of water on the amount adsorbed is wave-shaped in nature. This is due apparently to the presence of some groups of energetically different sites for water adsorption in the cavities of zeolites. Alternation of decrease and increase in the heat of adsorption of water with increasing coverage was particularly evident with KNaX zeolite. At low fillings the heats of adsorption of water decrease from LiNaX to CsNaX zeolites. Removal of Li⁺ by washing, sharply decreases the heat of adsorption of water at small coverages.