Analytical theory of benzene diffusion in Na-Y zeolite

We have developed an analytical expression for the diffusion coefficient of benzene in Na-Y at infinite dilution in terms of fundamental rate coefficients, which has beep confirmed by extensive kinetic Monte Carlo simulations. This model assumes that benzene jumps among S-II and W binding sites, located near Na+ ions in 6-rings and in 12-ring windows, respectively. Our diffusion theory is based on D = 1/6ka(2) where a congruent to 11 Angstrom is the intercage length, and k is the cage-to-cage rate coefficient. We have determined that k = k(S-II--> W).1/2.3[1 + k(W --> W)/k(W --> S-II)], a finding that has resolved discrepancies between theory and simulation and has suggested new interpretations of benzene diffusion in Na-Y. When alpha(T) = k(W --> W)/k(W --> S-II) is between 0 and 1, the factor 3[1 + alpha(T)] counts the number of thermally allowed target sites for cage-to-cage motion. Alternatively, when alpha(T) much greater than 1, benzene mobility is interpreted as interstitial diffusion, wherein k is controlled by the probability of W site occupancy multiplied by the rate of W --> W jump processes. This limit is expected to arise with benzene loadings of four molecules per supercage. (C) 1997 American Institute of Physics.