TRANSPORT-THEORY FOR CATIONIC ZEOLITES - DIFFUSION OF BENZENE IN NA-Y

We have modeled benzene diffusion in Na-Y zeolite (Si:Al = 2.0) over the temperature range 100-500 K. We apply the kinetic Monte Carlo random walk model, with activation energies derived from a new zeolite-hydrocarbon potential energy surface (PES). An Arrhenius fit yields the apparent activation energy E(a) = 41 kJ mol(-1), as compared with the previously determined experimental values 14-27 kJ mol(-1). Minimum energy paths from the new PES demonstrate ''cartwheel'' and ''skateboard'' hopping mechanisms for benzene in Na-Y. Analysis of the results suggests that activation energies from long length scale diffusion measurements, e.g. pulsed field gradient NMR, should be interpreted as site-to-window activation energies, whereas those from short length scale experiments, e.g. spin-lattice relaxation NMR, correspond to intracage site-to-site activation energies.