Dynamic Monte Carlo simulations of binary self-diffusion in ZSM-5

Dynamic Monte Carlo (DMC) simulations are used to examine binary diffusion in the zeolite ZSM-5. Diffusion in zeolites is strongly influenced by dynamic and static heterogeneity. The former describes the concentration dependent influence of diffusing molecules on each other within the restricted environment of the zeolite pore network; the latter relates to the presence of adsorption sites of different strengths, on which molecules adsorb for different average times, before moving on to a neighboring adsorption site. DMC simulations are an effective tool to study the influence of both forms of heterogeneity on diffusion. The self-diffusivities, as determined from DMC, strongly depend on the pore network topology, the average residence times of both species on each type of site, and the concentrations of those species. The Maxwell–Stefan (MS) approach is known to predict diffusion in silicalite-1, the statically homogeneous version of ZSM-5, rather well. However, it is not rigorous in predicting the self-diffusivity in the presence of any form of strong static heterogeneity. The capability of the MS approach to model the self-diffusion of binary mixtures in ZSM-5 is compared to DMC results, which serve as a benchmark. Alternative theories, which account for the discrete sites of the lattice, are considered for their ability to predict the self-diffusivity in binary mixtures, namely, the effective medium approximation (EMA) and a mean field theory (MFT). The EMA is shown to be a promising method in situations where the MS approach cannot work due to the presence of strong correlation effects.