Elucidating Traffic Junction Effects in MFI Zeolite Using Kinetic Monte Carlo Simulations

Published experimental studies on the diffusion of n-butane (nC4)/iso-butane (iC4), n-hexane (nC6)/2-methylpentane (2MP), and methane/benzene mixtures in MFI zeolite show that the self-diffusivity of the more mobile (linear) alkanes is diminished by about 1–3 orders of magnitude with increasing proportion of the tardier partners (iC4, 2MP, benzene) in the mixture. The strong reduction can be rationalized on the basis of the preferential location of the tardier partners iC4, 2MP, and benzene at the channel intersections of MFI, serving to slow down the molecular traffic. The primary objective of this article is to investigate, and quantify, such traffic junction effects with the aid of kinetic Monte Carlo (KMC) simulations. The KMC simulations show that the preferential location of branched molecules at the channel intersections can be effectively considered as an intersection blocking with attendant loss of connectivity of the MFI topology. Consequently, both the Maxwell–Stefan (M–S) diffusivity and self-diffusivity for nC4 in the nC4/iC4 mixtures are lowered below that for the unary nC4 diffusion by about 2 orders of magnitude. It is also established that such a lowering of the diffusivity of the more mobile partner is distinctly different from the “correlation” effects that generally manifest in the mixture diffusion in microporous materials.