A predictive methodology for calculating intracrystalline diffusivitie
s of small molecular gases in 4A zeolite is developed. The intracrysta
lline diffusion path is composed by alternate cavities and windows. An
exchangeable cation located at the eight-membered ring window partial
ly blocks the diffusion path. Only when the diffusing molecule has eno
ugh translational kinetic energy it can overcome the window repulsion.
The probability of a molecule crossing the window depends on its kine
tic energy distribution and on the affinity between the cation and the
window oxygen atoms. Classical and quantum mechanics are used to calc
ulate the kinetic energy distribution and the results are different, s
pecially in the high energy region. This difference directly reflects
on the calculation of the intracrystalline diffusivities. The predicte
d intracrystalline diffusivities and diffusion activation energies for
argon, nitrogen and oxygen in 4A zeolite are in the range of publishe
d experimental data.