MODELING ORIENTATIONAL RANDOMIZATION IN ZEOLITES - A NEW PROBE OF INTRACAGE MOBILITY, DIFFUSION AND CATION DISORDER

We have performed kinetic Monte Carlo simulations of benzene orientati onal randomization (BOR) and diffusion in Na-Y zeolite for various Na( II) occupancies and Na(II) spatial patterns. Full Na(II) occupancy giv es BOR rates controlled by intracage motion, whereas half Na(II) occup ancy 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 sensit ivity to different Na(II) spatial patterns. Calculated diffusion coeff icients vary weakly with decreasing Na(II) occupancy until ca. one Na( II) per supercage. Diffusion coefficients and mean square displacement s reveal no information about intracage motion, and are insensitive to different spatial patterns of Na(II) cations. Our computational resul ts thus suggest that measuring orientational randomization in zeolites can provide important information regarding intracage motion, diffusi on and cation disorder. (C) 1997 American Institute of Physics.