Statics and dynamics of ethane molecules in AlPO(4)-5: a molecular dynamics simulation study

Large-scale computational screening of zeolites for ethane/ethene separation

Large-scale computational screening of thirty thousand zeolite structures was conducted to find optimal structures for separation of ethane/ethene mixtures. Efficient grand canonical Monte Carlo (GCMC) simulations were performed with graphics processing units (GPUs) to obtain pure component adsorption isotherms for both ethane and ethene. We have utilized the ideal adsorbed solution theory (IAST) to obtain the mixture isotherms, which were used to evaluate the performance of each zeolite structure based on its working capacity and selectivity. In our analysis, we have determined that specific arrangements of zeolite framework atoms create sites for the preferential adsorption of ethane over ethene. The majority of optimum separation materials can be identified by utilizing this knowledge and screening structures for the presence of this feature will enable the efficient selection of promising candidate materials for ethane/ethene separation prior to performing molecular simulations.

Toward observation of single-file diffusion using the tracer zero-length column method

The tracer zero-length column (ZLC) method has been employed to study the diffusion of toluene in one-dimensional ZSM-12 and SAPO-5 zeolites. A significant deviation in the shape of the measured tracer exchange curves from monoexponential behavior was observed for toluene diffusion in both adsorbents in the limit of long-time asymptotes. In contrast, water/ZSM-12 and acetylene/SAPO-5 systems exhibit tracer exchange curves that are close to monoexponential behavior. Monoexponential curves are usually observed for systems obeying normal (Fickian) diffusion. Such diffusion is expected for the latter two systems because the diameters of both sorbates are less than the radii of their corresponding host channels. The differences in the shape of the tracer exchange curves for large and small sorbates can be explained by assuming the occurrence of anomalous, single-file diffusion for large sorbates in narrow, one-dimensional channels.

On the Knudsen transport of gases in nanochannels

We investigate the diffusion of gas molecules in nanochannels under the combinational effect of the vibration of the channel, gas-wall binding energy, and channel size through molecular dynamics simulations. It is found that the molecular vibration of the channel plays a critical role in gas transport process when the gas-wall binding energy is strong. For small binding energies, the influence of the flexibility of the wall can be neglected. In rigid channels, the gas self-diffusion coefficient increases with increasing gas-wall binding energy, while it decreases in nonrigid channels. The effect of the channel size on the self-diffusion coefficient is not significant except that a local maximum in the gas self-diffusion coefficient is found in 2 nm channels due to the strong repulsive force caused by the surface curvature of the channels.

Loading dependence of the diffusion coefficient of methane in nanoporous materials

In this work, we use molecular simulations to study the loading dependence of the self-and collective diffusion coefficients of methane in various zeolite structures. To arrive at a microscopic interpretation of the loading dependence, we interpret the diffusion behavior in terms of hopping rates over a free-energy barrier. These free-energy barriers are computed directly from a molecular simulation. We show that these free-energy profiles are a convenient starting point to explain a particular loading dependence of the diffusion coefficient. On the basis of these observations, we present a classification of zeolite structures for the diffusion of methane as a function of loading: three-dimensional cagelike structures, one-dimensional channels, and intersecting channels. Structures in each of these classes have their loading dependence of the free-energy profiles in common. An important conclusion of this work is that diffusion in nanoporous materials can never be described by one single effect so that we need to distinguish different loading regimes to describe the diffusion over the entire loading range.

129Xe Nuclear Magnetic Resonance Study of Pitch-Based Activated Carbon Modified by Air Oxidation/Pyrolysis Cycles: A New Approach to Probe the Micropore Size

(129)Xe NMR has been used to study a series of homologous activated carbons obtained from a KOH-activated pitch-based carbon molecular sieve modified by air oxidation/pyrolysis cycles. A clear correlation between the pore size of microporous carbons and the (129)Xe NMR of adsorbed xenon is proposed for the first time. The virial coefficient delta(Xe)(-)(Xe) arising from binary xenon collisions varied linearly with the micropore size and appeared to be a better probe of the microporosity than the chemical shift extrapolated to zero pressure. This correlation was explained by the fact that the xenon collision frequency increases with increasing micropore size. The chemical shift has been shown to vary very little with temperature (less than 9 ppm) for xenon trapped inside narrow and wide micropores. This is indicative of a smooth xenon-surface interaction potential.

Sticking Probability on Zeolites

The sticking coefficient, i.e., the probability that, on hitting the surface of a nanoporous particle (zeolite), a molecule shall be able to enter the intracrystalline space, is a key quantity for the application of such materials in heterogeneous catalysis and molecular sieving. On the basis of pulsed field gradient NMR diffusion measurements and molecular dynamics simulations, typical values of this probability are found to be close to one. They exceed previous estimates on the basis of IR uptake measurements by many orders of magnitude.

Dynamical behavior of one-dimensional water molecule chains in zeolites: Nanosecond time-scale molecular dynamics simulations of bikitaite

Nanosecond scale molecular dynamics simulations of the behavior of the one-dimensional water molecule chains adsorbed in the parallel nanochannels of bikitaite, a rare lithium containing zeolite, were performed at different temperatures and for the fully and partially hydrated material. New empirical potential functions have been developed for representing lithium-water interactions. The structure and the vibrational spectrum of bikitaite were in agreement both with experimental data and Car-Parrinello molecular dynamics results. Classical molecular dynamics simulations were extended to the nanosecond time scale in order to study the flip motion of water molecules around the hydrogen bonds connecting adjacent molecules in the chains, which has been observed by NMR experiments, and the dehydration mechanism at high temperature. Computed relaxation times of the flip motion follow the Arrhenius behavior found experimentally, but the activation energy of the simulated system is slightly underestimated. Based on the results of the simulations, it may be suggested that the dehydration proceeds by a defect-driven stepwise diffusion. The diffusive mechanism appears as a single-file motion: the molecules never pass one another, even at temperatures as high as about 1000 K, nor can they switch between different channels. However, the mean square displacement (MSD) of the molecules, computed with respect to the center of mass of the simulated system, shows an irregular trend from which the single-file diffusion cannot be clearly evidenced. If the MSDs are evaluated with respect to the center of mass of the molecules hosted in each channel, the expected dependence on the square root of time finally appears.

n-Pentane and isopentane in one-dimensional channels

Molecular dynamics studies of n-pentane and isopentane in one-dimensional channels of AlPO(4)-5 and a carbon nanotube are reported. Variation of the structure and energetics in AlPO(4)-5 along the channel axis of isopentane is similar to what has been found for other rigid molecular systems. In n-pentane, these properties exhibit more frequent undulations along the channel due to flexibility. The end-to-end distance of n-pentane is a function of its position along the channel in AlPO(4)-5, suggesting that n-pentane has to alternately stretch in the narrow part and destretch or coil in the broader part of the channel. n-Pentane lies flat instead of upright on the inner surface of the carbon nanotube. Both of the species exhibit diffusive motion in AlPO(4)-5, and the self-diffusivity is higher than that in bulk. Isopentane has a higher diffusivity than does n-pentane. This is attributed to the higher cross section of isopentane, which is closer to the void cross section. Further, the coupling of the translational motion with the slower dihedral angle reorientation in the case of n-pentane decreases its mobility. Superdiffusive motion is seen for both species in the carbon nanotube. These results can be understood in terms of the levitation effect.

Diffusion anomaly as a function of molecular length of linear molecules: levitation effect

Previous work on monatomic spherical sorbates has shown the existence of an anomalous peak in self-diffusivity (D) when plotted as a function of size of the diffusant. Molecular dynamics studies on linear molecules of different lengths l in zeolite NaY at 140 and 200 K are reported. It is seen that there is a peak in D as a function of l, suggesting that the levitation effect exists for linear molecules, the simplest member of polyatomics. This is confirmed by the lowering of the activation energy for the molecule whose length l exhibits highest D. Related quantities of interest such as the guest-host interaction energy and preexponential factor are discussed.