Zeolitic encapsulation. Part 1.—Hydrogen diffusion in A-type zeolite encapsulates

Abstract

A simple mathematical treatment provides a convenient temperature-programmed diffusion equation, 2logT_M– logβ=E//_2.3RTM+ logEr²₀//_{2.40 π²RD0} which is analogous to the known equation for first-order temperature-programmed desorption (t.p.d.). With this expression, the energy of activation for diffusion, E, and the pre-exponential factor of the diffusion coefficient, D₀(as D₀/r²₀), were derived for hydrogen diffusion in a series of univalent-ion-exchanged A-zeolite(H₂) encapsulates. Excepting Na-A(H₂) encapsulate, for which very high E and D₀/r²₀ values are obtained, both diffusion parameters increase with the radius of the exchangeable alkali cation, M blocking the O₈-windows (M = K, Rb, Cs). Thus, the diffusion process is attributed to molecular jumps through O₆-windows in the case of Na-A(H₂) which exhibits only β-cage encapsulation, whereas jumps through O₈-windows of varying effective openings are proposed for all other cases in which α-cage encapsulation predominates. Based partially on experiment, the full dependence of E on the degree of exchange, M_z, is predicated and explained in terms of the previously-proposed concept of conditionally-opened windows.