Transition state theory of zeolitic diffusion. Diffusion of CH4 and CF4 in 5A zeolite

Abstract

A simple theory of zeolitic diffusion is developed from the principles of transition state theory. General expressions for the diffusivity are derived in terms of the lattice parameter and the partition function for the transition state which, for the type A zeolites, is identified as a molecule in passage through the 8-membered oxygen window. Theoretical diffusivities calculated for CH₄ and CF₄ in 5A zeolite agree well with experimental data and it is shown that, in the transition state, the CF₄ molecule has a high degree of rotational freedom whereas the rotation of the CH₄ molecule is severely restricted. This difference is attributed to the difference in the moments of inertia.