DYNAMICS OF ZEOLITE CAGE AND ITS EFFECT ON THE DIFFUSION PROPERTIES OF SORBATE - PERSISTENCE OF DIFFUSION ANOMALY IN NAA ZEOLITE

Recent computer simulations on zeolites Y and A have found that the di ffusion coefficient and the rate of intercage diffusion exhibit, apart from a linear dependence on the reciprocal of the square of the sorba te diameter, an anomalous peak as sorbate diameter approaches the wind ow diameter. Here we report molecular dynamics simulations of zeolite NaA incorporating framework flexibility as a function of sorbate diame ter in order to verify the existence of anomalous diffusion. Results s uggest persistence of anomalous diffusion or ring effect. This suggest s that the anomalous behavior is a general effect characteristic of ze olites Y and A. The barrier for diffusion across the eight-ring window is seen to be negative and is found to decrease with sorbate size. Th e effect of sorbate on the cage motion has also been investigated. Res ults suggest that the window expands during intercage migration only i f the sorbate size is comparable to the window diameter. Flexible cage simulations yield a higher value for the diffusion coefficient and al so the rate of intercage diffusion. This increase has been shown to be due to an increase in the intercage diffusions via the centralized di ffusion mode rather than the surface-mediated mode. It is shown that t his increase arises from an increase in the single particle density di stribution in the region near the cage center.