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 t he temperature range 100-500 K. We apply the kinetic Monte Carlo rando m walk model, with activation energies derived from a new zeolite-hydr ocarbon potential energy surface (PES). An Arrhenius fit yields the ap parent activation energy E(a) = 41 kJ mol(-1), as compared with the pr eviously determined experimental values 14-27 kJ mol(-1). Minimum ener gy paths from the new PES demonstrate ''cartwheel'' and ''skateboard'' hopping mechanisms for benzene in Na-Y. Analysis of the results sugge sts that activation energies from long length scale diffusion measurem ents, e.g. pulsed field gradient NMR, should be interpreted as site-to -window activation energies, whereas those from short length scale exp eriments, e.g. spin-lattice relaxation NMR, correspond to intracage si te-to-site activation energies.