We have performed kinetic Monte Carlo (KMC) simulations of benzene mobility in Na-Y with various Na(II) cation occupancies. We evaluate a second-order orientational correlation function with KMC to quantify rates of benzene orientational randomization (BOR). Full Na(II) occupancy gives BOR rates controlled by intracage motion, whereas half Na(II) occupancy gives BOR rates sensitive to both intracage and intercage motion, but insensitive to particular Na(II) spatial patterns. Alternatively, BOR with one quarter Na(II) occupancy demonstrates qualitative sensitivity to different Na(II) spatial patterns.
Several aspects of our model can be improved in order to strengthen our predictions. We plan to calculate dynamically exact hopping rate coefficients to remove assumptions regarding Arrhenius temperature dependence. We also plan to correct these hopping rate coefficients for inhomogeneities introduced by cation vacancies, and will incorporate finite benzene loadings to determine effects from guest--guest interactions. Finally, we plan to extend these calculations to other host--guest systems to determine how widely applicable our conclusions remain. Nevertheless, our results for benzene in cation-containing faujasites are sufficiently remarkable that they deserve attention at this initial level of theory.
This work shows how kinetic Monte Carlo, based on the assumption of random Poissonian statistics, can elucidate orientational randomization in confined geometries such as those in zeolite cavitities.