The behaviour of encapsulated non-polar gases in Cs,Na-A zeolites

Abstract

Zeolitic encapsulation of non-polar permanent gases in Cs,Na-A is compared and discussed. H₂, Ar, O₂, N₂ and CH₄ are shown to be trapped by α cages whose 8R windows are blocked by at least 2.3 Cs⁺ ions per pseudo-cell (z_Cs 2.3). Below this value, trapping is restricted to the β(sodalite) cages. As shown for the large gases, encapsulation in α and β cages can be distinguished during temperature-programmed diffusion (t.p.di.) which releases the trapped gas and a transition region of combined α- and β-cage encapsulation is clearly observed. The activation parameters for diffusion, E and D₀, for these two types of cages were derived both by the graphical method (g.m.) and the peak-shape method (p.s.m.)(see preceding paper). The simpler and more convenient p.s.m. is shown to be generally applicable and not less reliable than the g.m. for pure t.p.di. peaks; in fact, it should be the method of choice at high-E-value cases. β-cage decapsulation and α-cage decapsulation above a degree of exchange, z_Cs of 3.5 exhibit constant E while at the region 2.3 < z_Cs < 3.5 E for the α cages increases sharply with z_Cs. The pre-exponential factor, D₀, depends predominantly on the type of window (i.e. size of ring, blocking cation) and decreases as the window becomes more widely open.