On the correlation between diffusion and self-diffusion processes of adsorbed molecules in a simple microkinetic model

Combining the formalism of transition state theory with an appropriate collisional term, a general relation between diffusion and self-diffusion coefficients (D and D¹ respectively) in zeolites is derived. For low pore filling factors, this relation reduces to Darken's equation, which is shown to coincide in this case with the simple equation $\text{D = (}\text{d}\text{dc}\text{)(D}{}^1\text{c)}$, where c denotes the concentration of adsorbate. Concerning the intracrystalline self-diffusion of ethane in NaCaA zeolites, it is shown, that up to pore filling factors θ ≈ 0.8, Darken's equation should be valid, whereas for higher concentrations considerable deviations from this simple relation may occur and the general formula, as derived in this paper, should be applied.