Kinetic Monte Carlo Simulations of the loading dependence of diffusion in zeolites

Kinetic Monte Carlo (KMC) simulations are used to model diffusion of molecules in zeolites. A variety of loading dependences of the Maxwell-Stefan diffusivity, D-i, can be realized by allowing the jump frequencies, v, of molecules to be influenced by the presence of neighboring molecules. Neighboring molecules are assumed to reduce or increase the activation energy for diffusion by deltaE and the jump frequencies v are altered by a factor f = exp(deltaE/RT) for each neighboring molecule. By appropriate choice of v and f the loading dependence of D-i can be made to match those obtained from either Molecular Dynamics (MD) simulations or experiment. The major advantage of the KMC simulation strategy is that considerably less computer power is required than for the corresponding MD simulations. Furthermore, KMC simulations allow mixture diffusion to be probed without additional parameter tuning. The KMC simulations also validate the applicability of the quasi-chemical theory of Reed and Ehrlich (Surf Sci. 1981,105, 603-628) for describing the loading dependence of D-i.