ACTIVATED DIFFUSION OF PERMANENT GASES IN 4A ZEOLITE AT LOW-COVERAGE

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.