Chemical composition and ion exchange properties of zeolites

Series of exchange selectivity for alkali cations in zeolites change in a regular manner in dependence on the ratio (z) between the number of particles capable of solvating cations in the frameworks of zeolites (molecules of zeolite water, one-fourth of the framework oxygen ions) and the number of zeolite cations. For z=5–6 zeolites have maximum selectivity for large cations, for z<3–4 they selectively absorb sodium, while if z<2 they selectively absorb lithium. An analysis is given of the reasons of this dependence of the ion exchange selectivity of zeolites on the chemical composition of their frameworks. It is shown that the free energy of solvation of cations in zeolites is less than the free energy of their hydration. The effect of transformation of the selectivity series of zeolites can be connected with the contribution, on the part of the effect of redistribution of water between the solid and liquid phases of the ion exchange system, to the free energy of ion exchange reactions, which differs in dependence on the composition and structure of the crystals.Translated from Teoreticheskaya i Éksperimental'naya Khimiya, Vol. 26, No. 3, pp. 378–382, May–June, 1990.