Zeolitic encapsulation. Part 2.—Percolation and trapping of inert gases in A-type zeolites

Abstract

Zeolitic encapsulation of small non-polar gas molecules under appropriate p–T conditions (V_r > 1) can be conveniently dealt with in terms of van der Waals' equation of state for real gases. Thus, the amount of gas trapped in the zeolitic cages can be predicted if the cage volumes are known; otherwise, void-volume values may be obtained from encapsulation experiments. These void-volume values, as obtained from hydrogen encapsulation in A-type zeolites, are ca. 20% larger than those reported in the literature, but in agreement with calculated void volumes derived from crystallographic data and from a refined structural model of zeolite A. The ‘jump’ in encapsulation capacity at ca. 2.5 univalent blocking cations per unit cell is explained in terms of the percolation theory as transition from β-cage encapsulation to encapsulation in both the α- and β-cages. Based on the van der Waals equation, calculated available void volumes and the percolation theory, a theoretical encapsulation curve is proposed which agrees fairly well with experimental results reported so far for H₂ in Cs, Na-A zeolites and for Ar, Kr and CH₄ in K, Na-A zeolites.