DIFFUSION IN ZEOLITES VIA CAGE-TO-CAGE KINETICS - MODELING BENZENE DIFFUSION IN NA-Y

We have performed kinetic Monte Carlo simulations of benzene diffusion in Na-Y (Si:Al=2.0) over the temperature range 200-500 K. For hopping on a tetrahedral lattice, we derive the analytical formula for D in t erms of hopping lengths and times, yielding the simple-cubic relation D=1/6ka(2), even though the lattice is very different from simple cubi c. We have calculated the distribution of cage residence times for ben zene in Na-Y, finding single exponential decay controlled by the S-II- ->W rate coefficient, even though several processes contribute to inte rcage motion. Exact agreement between mean square displacement slopes and 1/6ka(2) is obtained only when using kinetic intercage hopping len gths, which are found to be in excess of the static length by as much as 0.54 Angstrom. Constructing diffusion coefficients from intercage l engths and times provides overwhelming computational speedups over cal culating mean square displacements. (C) 1996 American Institute of Phy sics.