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Rate of intercage diffusion (k c ) along with rate of cage visits (k v ) and diffusion coefficient (D) of sorbates in zeolite Y have been calculated. We show that as a consequence of the dependence of rate of intercage diffusion on short-time behaviour, significant deviation from the Arrhenius behaviour at suffciently high temperatures occurs.

Simulations of phase changes provide a way to investigate nucleation at a depth of supercooling so far not accessible to laboratory experiments. One helpful aspect of this regime is that deficiencies of theory are substantially magnified, helping to call attention to the weaknesses of existing treatments. In the present molecular dynamics simulations, a system of small liquid clusters of SeF6 spontaneously froze to single body-centered cubic (bcc) crystals when cooled to 120 K. The transition was mediated by homogeneous nucleation rather than by spinodal decomposition. Its nucleation rate of 6.6 × 1035 m-3 s-1 was analyzed in both terms of classical nucleation theory (to derive the interfacial free-energy parameter) and Granasy's diffuse-interface theory (to derive the interface thickness parameter). In each case, both the classical prefactor and the Grant−Gunton (GG) prefactor were applied. Large differences between the various treatments were found. Derived interfacial free energies of 13−17 mJ/m2 were ...

A systematic investigation of monatomic spherical sorbates in the supercages of zeolites Y and A by molecular dynamics technique is presented. Rates of intercage diffusion, rates of cage visits, and the diffusion coefficients have been calculated as a function of the sorbate-zeolite interaction strength. These properties exhibit markedly different dependences on interaction strength for the two zeolites. The observed behavior is shown to be a consequence of the two principal mechanisms of intercage diffusion and the energetic barrier associated with them. The diffusion coefficient and other properties associated with intercage diffusion are found to be directly proportional to the reciprocal of the square of the sorbate diameter when the sorbate diameter is significantly smaller than the window diameter. As the sorbate diameter increases, a peak is observed in all the transport properties investigated including the diffusion coefficient. We call this surprising effect as the ring or levitation effect and it explains several anomalous results reported in the literature and suggests a breakdown of the geometrical criterion for diffusion of sorbates. It shows that under certain conditions nongeometrical factors play a major role and geometrical factors become secondary in the determination of the molecular sieve property. A generalized parameter has been proposed which suggests conditions under which one can expect the ring or levitation effect in any porous medium. Inverse size selectivity becomes operative under these conditions.

Molecular dynamics calculations on sorbate atoms of varying diameters in zeolites A and Y are reported. Rates of cage-to-cage diffusions, rate of cage visits, and diffusion coefficients show unexpected behavior. A generalized parameter has been defined that exhibit a peak in diffusion coefficient around 0.84 for both the zeolites investigated.

Detailed molecular dynamics simulations of argon in zeolite NaCaA are reported. Thermodynamic, structural, and dynamical properties of the sorbate as a function of temperature have been obtained. The properties calculated include various site-site radial distribution functions, different energy distribution functions, selfdiffusion coefficients, the power spectra, and properties relating to cage-to-cage diffusion. The results suggest that sorbate is delocalized above 300 K. Both modes of cage-to-cage diffusion-the surface-mediated and centralized diffusion-are associated with negative barrier heights. Surprisingly, rate of cage-to-cage diffusion is associated with negative and positive activation energies below and above 500 K. The observed differences in the behavior of the rate of cage-to-cage diffusion between Xe-NaY and Ar-NaCaA systems and the nature of the potential energy surface are discussed. Presence of sorbatezeolite interactions results in significant enhancement in the rate of cage-to-cage diffusion and rate of cage visits. It is shown that properties dependent on the long-time behavior such as the diffusion coefficient and the rate of cages visited exhibit the expected Arrhenius dependence on temperature.

The sorption properties of xenon sorbed in sodium Y zeolite as a function of adsorbate concentration and temperature have been obtained from molecular dynamics simulations. The properties reported include the various site-site a center of cage-center of mass radial distribution functions, distribution of guest-host energy, guest-guest pair and bonding energy, self-diffusion coeffcients, the power spectra, and the distribution of site residence times. The location of the physical adsorption site for xenon is the same as for methane. The guest-host energy distribution function for xenon differs significantly from the bimodal function observed for methane. It is shown that the mean square displacement shows a crossover from ballistic to diffusive behaviour and the activation energy for diffusion is 4.1 kJ/mol. We suggest that the $15-cm^{-l}$ frequency mode seen in the power spectra of the autocorrelation function corresponds to the Xe-Xe dimers. The nonavailability of sorption sites at higher adsorbate concentrations is reflected in the guest-host distribution function, center of cage-center of mass radial distribution function, power spectra, and other properties.

A molecular-dynamics calculation of diffusion of xenon in the micropores of zeolite Y shows several unusual features. Xenon atoms migrating from one cage to another through a twelve-membered ring exhibit a temperature-dependent barrier height and a breakdown of Arrhenius behavior. Barrierless trajectories for cage-to-cage migration have been found. The mechanism of migration as a function of temperature is discussed. The activation-energy spectra show the existence of both positive and negative barrier heights.

A molecular dynamics calculation of argon in NaCaA zeolite at 393 K and 1 atom per cage is reported. Equilibrium properties such as guest–host interaction energy, guest–guest dimerization and bonding energy, various guest–host and guest–guest radial distribution functions and dynamical properties such as the mean-square displacement, power spectra and diffusion coefficient have been obtained.