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 terms of hopping lengths and times, yielding the simple-cubic relation D=1/6ka(2), even though the lattice is very different from simple cubic. We have calculated the distribution of cage residence times for benzene in Na-Y, finding single exponential decay controlled by the S-II-->W rate coefficient, even though several processes contribute to intercage motion. Exact agreement between mean square displacement slopes and 1/6ka(2) is obtained only when using kinetic intercage hopping lengths, which are found to be in excess of the static length by as much as 0.54 Angstrom. Constructing diffusion coefficients from intercage lengths and times provides overwhelming computational speedups over calculating mean square displacements. (C) 1996 American Institute of Physics.