Modeling orientational randomization in zeolites: A new probe of intracage mobility, diffusion and cation disorder

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Abstract
We have performed kinetic Monte Carlo simulations of benzene orientational randomization (BOR) and diffusion in Na-Y zeolite for various Na(II) occupancies and Na(II) spatial patterns. 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. Calculated diffusion coefficients vary weakly with decreasing Na(II) occupancy until ca. one Na(II) per supercage. Diffusion coefficients and mean square displacements reveal no information about intracage motion, and are insensitive to different spatial patterns of Na(II) cations. Our computational results thus suggest that measuring orientational randomization in zeolites can provide important information regarding intracage motion, diffusion and cation disorder.