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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 were carried out on 138-molecule clusters freezing isothermally at 130, 120, and 80 K. At 120 K, the nucleation rate was the same as in our prior simulations performed adiabatically but the final product was different. During the nanosecond period of the runs, clusters transforming adiabatically had frozen to bcc crystals while warming from 120 K to about 130 K. On the other hand, isothermal clusters at 120 and 130 K changed to monoclinic clusters after passing through the bcc phase. Clusters cooled to 80 K froze to a variety of structures. The number of molecules whose Voronoi polyhedra qualified them as being in bcc embryos grew in size erratically, and in most runs it was difficult to use the Voronoi information by itself to identify a well-characterized nucleation time. Therefore, a more discriminating criterion for the onset of nucleation was devised. The 138-molecule clusters proved to be too small to yield definitive profiles of the several order parameters characterizing the change fro...

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 ...

To extend our molecular dynamics studies of nucleation in molecular clusters undergoing phase changes, we selected the acetylene system whose clusters had already been examined experimentally in supersonic flow. Because molecular dynamics simulations based on the several existing model potential functions were incapable of accounting for the structure of the low-temperature orthorhombic phase, a new interaction potential was developed which successfully yielded the correct molecular packing at low temperatures as well as the cubic structure observed at high temperatures. The results obtained in heating and cooling runs for the model system were unlike any seen in our prior simulations. There was no nucleation barrier inhibiting the passage from the cubic to the orthorhombic phase on cooling, nor were there discernible volume or enthalpy changes in the transformation. In clusters of several hundred molecules the apparently second-order or continuous transition was spread over a much wider range of temperat...

Recent computer simulations on zeolites Y and A have found that the diffusion coefficient and the rate of intercage diffusion exhibit, apart from a linear dependence on the reciprocal of the square of the sorbate diameter, an anomalous peak as sorbate diameter approaches the window diameter. Here we report molecular dynamics simulations of zeolite NaA incorporating framework flexibility as a function of sorbate diameter in order to verify the existence of anomalous diffusion. Results suggest persistence of anomalous diffusion or ring effect. This suggests that the anomalous behavior is a general effect characteristic of zeolites 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. The effect of sorbate on the cage motion has also been investigated. Results suggest that the window expands during intercage migration only if the sorbate size is comparable to the window diameter. Flexible cage simulations yield a higher value for the diffusion coefficient and also the rate of intercage diffusion. This increase has been shown to be due to an increase in the intercage diffusions via the centralized diffusion mode rather than the surface-mediated mode. It is shown that this increase arises from an increase in the single particle density distribution in the region near the cage center.

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.

Various geometrical and energetic distribution functions and other properties connected with the cage-to-cage diffusion of xenon in sodium Y zeolite have been obtained from long molecular dynamics calculations. Analysis of diffusion pathways reveals two interesting mechanisms-surface-mediated and centralized modes for cage-to-cage diffusion. The surface-mediated mode of diffusion exhibits a small positive barrier, while the centralized diffusion exhibits a negative barrier for the sorbate to diffuse across the 12-ring window. In both modes, however, the sorbate has to be activated from the adsorption site to enable it to gain mobility. The centralized diffusion additionally requires the sorbate to be free of the influence of the surface of the cage as well. The overall rate for cage-to-cage diffusion shows an Arrhenius temperature dependence with E(a) = 3 kJ/mol. It is found that the decay in the dynamical correction factor occurs on a time scale comparable to the cage residence time. The distributions of barrier heights have been calculated. Functions reflecting the distribution of the sorbate-zeolite interaction at the window and the variations of the distance between the sorbate and the centers of the parent and daughter cages are presented.

Molecular dynamics simulations on Xe in NaY and Ar in NaCaA zeolite are reported. Rates of cage-to-cage crossovers in the two zeolites exhibit trends which are contrary to that expected from geometrical considerations. The results suggest the important role of the sorbate-zeolite interactions in determining the molecular sieve properties of zeolites for small sized sorbates. The results are explained in terms of the barrier height for cage-to-cage crossover in the two zeolites.

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.

Molecular dynamics calculations of Ne and Ar in the cages of β-hydroquinone have been carried out. Radial distribution of the guest atoms, energy distribution functions, mean square displacements and power spectra have been obtained. The guest atoms are found to be completely entrapped in the cages, with Ar performing restricted one-dimensional motion and Ne performing restricted three-dimensional motion.

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.