Tracing Experimentally Compatible Dynamical and Structural Behavior of Atmospheric N2/O2 Binary Mixtures within Nanoporous Li–LSX Zeolite: New Insights to Influence of Extra-Framework Cations by MD Simulations (original) (raw)
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Modeling of N2 and O2 Adsorption in Zeolites
The Journal of Physical Chemistry, 1995
The fundamental aspects of N2 and 0 2 adsorption in zeolites have been investigated by density functional calculations on different models. Simple systems where these molecules interact with a positive point charge or with isolated Li+ and Na+ cations have led to a qualitative explanation for the N2/02 separation process.
Microporous and Mesoporous Materials, 2017
The dynamical and structural properties of hydrogen (H 2) guest gas inside nanoporous Li-LSX zeolite were studied by molecular dynamics (MD) simulation for different loadings (8, 12, 16, and 20) of H 2 per unit cell at temperatures of 200, 298, 400, and 500 K. Three equal mean-square displacement (MSD) components (in the x, y, and z-directions) for the center of mass of H 2 guest molecules show that the translational motion of H 2 in this zeolite medium is isotropic due to the high symmetry of the zeolite framework. At these conditions, H 2 guest molecules freely move without blocking each other's path into the supercages and channels of the Li-LSX zeolite. The order of calculated self-diffusion coefficient of H 2 guest molecules at different temperatures, in the range of 10-9 up to 10-7 m 2 •s-1 , and corresponding activation energy, ~ 2 kcal•mol-1 , followed using the Arrhenius equation is in good agreement with the pores size of Li-LSX zeolite (7.4-12 Å) and compatible with inter-region of wellknown Knudsen and Configurational diffusion. The H 2 self-diffusion coefficients increase with temperature, while showing no quantifiable changes with loading within this loading range. A further study with a broader guest loading range would be appropriate to fully understand the loading effect on the self-diffusion of H 2 guest molecules in the Li-LSX zeolite as the H 2 storage candidate. In addition to determining the temperature and loading effects on the H 2 guest behavior, current simulations also show that the Li-III cations are specific H 2 sorption sites and the structural correlation, dynamics, self-diffusion coefficient, and adsorption of H 2 molecules are strongly dependent on the mobility or immobility of the key extraframework Li-III cationic sites of Li-LSX zeolite. The results of the simulation help in the choice of favorable structure, best design, and operative manufacture of zeolites or other microporous materials for similar applications.
Thermal and Kinetic Performance of Water Desorption for N2 Adsorption in Li-LSX Zeolite
The Journal of Physical Chemistry C, 2014
The effect of small amounts of water in inhibiting the adsorption of N 2 gas on different cationic forms of LSX zeolite was investigated using thermogravimetric and derivative thermogravimetry (TG-DTG) techniques at different heating rates, which is shown to be a very effective way of studying the influence of strongly adsorbed water on adsorption of less-strongly adsorbed N 2 molecules. According to DTG profiles, the apparent activation energies (E) relating to the thermal desorption of water molecules existing in the skeleton of Na-and Li-LSX zeolites were estimated through both the Kissinger and Flynn−Wall−Ozawa methods. The E values calculated from these two equations were in close agreement, and the thermal dehydration mechanism was further studied on the basis of the Coats−Redfern method. Moreover, the observations for the exponential declination of N 2 adsorption amount with the loading of water revealed that the adsorption capacity of cationic zeolites was significantly influenced by water through reducing heterogeneity and strength of the electric field. More specifically, the E value at high temperature was higher than that at low temperature, implying that at high temperature, decomposition of bonded water belongs to dynamic-based control, whereas the dehydration process of physisorbed water belongs to diffusionbased control at low temperature.
Chemical Engineering Science, 2015
Zeolites typically contain extra-framework cations to charge-compensate for trivalent Al atom substitutions in the SiO 2 framework. These cations, such as Na + , directly interact with quadrupolar guest molecules, such as CO 2 and N 2 , which move through their micropores, causing energetic heterogeneity. To assess the effects of heterogeneity in Na-ZSM-5 on diffusion of CO 2 and N 2 , molecular dynamics (MD) simulations are carried out. In silicalite-1, the pure-silicon form of ZSM-5, the self-diffusivity exhibits a monotonic decrease with molecular loading, while the corrected diffusivity shows a relatively constant value. In contrast, the Na + cations cause a maximum or a flat profile over molecular loading for the self-and corrected diffusivities of CO 2 at T = 200 and 300 K, while the cations only have minimal impact on the diffusivity of N 2 . The MD simulations allow us to identify energy basins or sites at which guest molecules spend a relatively long time, and construct a coarsegrained lattice representation for the pore network. Average residence times at these sites are calculated for both species. The trends observed in the residence times correlate to the trends observed in the diffusivity. The residence times for CO 2 at T = 200 K are long at low loading, but decrease with loading as additional CO 2 molecules compete to stay close to a cation. In contrast, the residence times for N 2 are relatively insensitive to the cations, only mildly increasing near a cation. This difference in behavior can be associated to the quadrupole moments of these molecules.
Adsorption of NO in Li-exchanged zeolite A. A density functional theory study
Chemical Physics Letters, 2010
Density functional theory (DFT) calculations were applied for the modeling of the adsorption sites of nitric oxide in Li + exchanged zeolite A previously studied experimentally. Two model clusters, a 6T sixmembered ring, and a 3T fragment of an octagonal structure were examined. The obtained results showed that the geometry of the formed [Li-NO] + complex depended on the coordination of the exchangeable cation with the oxygen atoms of the zeolite framework. Calculated anisotropy of the g-factor and the magnetic parameters of 14 N and 7 Li are in the range of experimental values observed for those types of complexes.
Computer simulation of the gas separation properties of zeolite Li-X
Journal of Porous Materials, 1995
The criteria determining the effectiveness of a particular zeolite for gas separation are the physical pore size and the location, size, and charge of any cations present. To date the experimentalist has had to use a great deal of intuition when selecting a zeolite for a specific use. Computer modelling of such systems, using a Grand Canonical Monte Carlo method, has been successful in elucidating the behaviour of adsorbates in a wide range of systems. Successful predictions for adsorption isotherms for nitrogen, oxygen and argon have been previously reported by the authors for zeolites A, X and Y with calcium and sodium cations. The aim of the work reported in this paper is to investigate the air separation properties of a different, although similar system namely: zeolite X with lithium cations. The simulations performed using Cerius 2 molecular modelling software are able to predict adsorption isotherms for nitrogen and oxygen gases, both as single component, and as binary mixtures in Li-X. Further the predicted equilibrium separation factor is calculated to be in the range of 6 to 13 at room temperature, making this system ideal for the preferential adsorption of nitrogen and production of oxygen.
Applications of molecular simulations for separation and adsorption in zeolites
Microporous and Mesoporous Materials, 2017
Zeolites are fascinating and versatile materials which are vital for a wide range of industries, due to their unique structural and chemical properties, which are the basis of applications in gas separation, ion exchange, and catalysis. Given their economic impact, there is a powerful incentive for smart design of new materials with enhanced functionalities for maximizing their application performance. This review article intends to summarize the published reports on the applications of molecular simulation in adsorption, separation and diffusion. The theoretical aspects, adsorption thermodynamics, adsorption isotherm were comprehensively studied in relation to the adsorption applications and how the adsorbates' characteristics influence the adsorption. This review comprehensively discusses the theoretical and computational aspects of diffusion of pure components, long chain hydrocarbons or mixture diffusion, including the molecular dynamics simulations and kinetic Monte Carlo. Furthermore, the cation-zeoliteadsorbate interactions are thoroughly examined so as to elucidate the role of cations in zeolites applications and how the cation exchange influences structural dynamics and properties of zeolites. This study also focuses on the role of cations in gas/liquid adsorption and separations.
Modeling of Adsorption Properties of Zeolites: Correlation with the Structure
The Journal of Physical Chemistry B, 1997
The adsorption of N 2 and CO in Na X-zeolites has been studied for different framework structures and extraframework cation distributions. To this aim, the cation-molecule system modeling one site has been embedded in a set of external point charges which simulate the zeolite environment of the site and has been treated quantum chemically, using a method based on density functional theory. This procedure has been applied to the 64 cationic sites accessible for adsorption in a crystal unit cell of an ideal X-zeolite with a Si/Al ratio equal to 1. These calculations have shown that only a few cations are favorable for initial adsorption and that those cations are always of type III(III′). Their efficiency depends both on the framework geometry and on their location in the supercages. The analysis of the quantum chemical results in terms of a classical description involving electrostatic and induction interaction energies with the framework has led to the conclusion that the direction of the electric field vector created by the zeolite in the supercages is an important factor determining the zeolite adsorption properties.