 
Summary: Modeling the concentration dependence of diffusion in zeolites. I.
Analytical theory for benzene in NaY
Chandra Saravanan
Department of Chemistry, University of Massachusetts, Amherst, Massachusetts 01003
Scott M. Auerbacha)
Departments of Chemistry and Chemical Engineering, University of Massachusetts, Amherst,
Massachusetts 01003
Received 12 June 1997; accepted 14 August 1997
We have developed an analytical expression for the diffusion coefficient of benzene in NaY at finite
loadings in terms of fundamental rate coefficients. Our theory assumes that benzene molecules jump
among SII and W sites, located near Na ions in 6rings and in 12ring windows, respectively. We
assume that instantaneous occupancies in different supercages are identical, a mean field
approximation yielding D 1
6k a2
where a 11 Å is the mean intercage jump length and 1/k is
the mean supercage residence time. We show that k ·k1·P1 , where P1 is the probability of
occupying a W site, k1 is the total rate of leaving a W site, and is the transmission coefficient for
cagetocage motion. We assume 1
2 for all loadings, and derive analytical formulas for the T and
dependencies of k1 and P1 , assuming that SII and W site occupancies are either 0 or 1 and that
