TEMPERATURE-DEPENDENCE OF THE LEVITATION EFFECT - IMPLICATIONS FOR SEPARATION OF MULTICOMPONENT MIXTURES

Sufficiently long molecular dynamics simulations have been carried out on spherical monatomic sorbates in NaY zeolite, interacting via simpl e Lennard-Jones potentials, to investigate the dependence of the levit ation effect on the temperature. Simulations carried out in the range 100-300 K suggest that the anomalous peak in the diffusion coefficient (observed when the levitation parameter, gamma, is near unity) decrea ses in intensity with increase in temperature. The rate of cage-to-cag e migrations also exhibits a similar trend. The activation energy obta ined from Arrhenius plots is found to exhibit a minimum when the diffu sion coefficient is a maximum, corresponding to the gamma approximate to 1 sorbate diameter. In the linear or normal regime, the activation energy increases with increase in sorbate diameter until it shows a sh arp decrease in the anomalous regime. Locations and energies of the ad sorption sites and their dependence on the sorbate size gives interest ing insight into the nature of the underlying potential-energy surface and further explain the observed trend in the activation energy with sorbate size. Cage residence times, tau(c), show little or no change w ith temperature for the sorbate with diameter corresponding to gamma a pproximate to 1, whereas there is a significant decrease in tau(c) wit h increase in temperature for sorbates in the linear regime. The impli cations of the present study for the separation of mixtures of sorbate s are discussed.