Diffusion of Water in Zeolites Na A and NaCa A: A Molecular Dynamics Simulation Study (original) (raw)
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Molecular Dynamics Simulation of Water Diffusion in MFI-Type Zeolites
Journal of Physical Chemistry B, 2009
Molecular dynamics simulation using COMPASS force field has been employed to understand the dynamics of water diffusion and structuring in silicalite-1 and Na-ZSM-5 (Si/Al ) 95 and 191) samples at three different temperatures, 297, 354, and 393 K, at a water loading of 8 molecules per unit cell, in canonical ensemble. Diffusion coefficients were significantly reduced upon the introduction of aluminum atoms into the framework, together with charge balancing cations placed in their vicinity, since the ion-dipole interactions dominant in ZSM-5 samples are stronger than the H-bond interactions in silicalite-1. The activation energy of diffusion increased with decreasing Si/Al ratio. In the silicalite-1 and ZSM-5 samples, straight channels were observed to be preferred than the sinusoidal ones and the channel preference was not observed to be a strong function of either temperature or the Si/Al ratio. The ordered structures of the water molecules, forming clusters in the channels of silicalite-1 at low temperature was observed to be broken to some extent by increased temperatures, and decreased Si/Al ratio, resulting in less ordered structures. The positions of the water molecules in the straight and sinusoidal channels for the ZSM-5 samples were mainly determined by the location of the charge compensating cation(s) in the structure, as was shown by the concentration profiles.
The Journal of Physical Chemistry C, 2017
Molecular dynamics simulations were carried out to explore the kinetic and structural properties of water diffusing through several types of zeolite with pore sizes that make them suitable for application as membranes for small ion filtering. Zeolites with pores in one and three dimensions were considered and insights into the effect of the structures of the zeolites were obtained by studying diffusion and ordering of the confined water. Interestingly, water molecules in the zeolites with pores in one-dimension showed up to 2.5-fold higher diffusivities than in those with pores in three-dimensions for a given water density and a similar pore diameter. The distribution of water molecules across pores and the number of water molecules in a specified region were also investigated to determine the effects of pore shape and size on the water assembly. The arrangement and the number of water molecules in a cluster were observed to depend heavily on the shape and size of pores. The present study provides a deeper understanding of how various structural features affect the dynamics and structure of the water within zeolites.
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.
Molecular simulation studies of water physisorption in zeolites
Physical Chemistry Chemical Physics, 2006
We report a series of Grand Canonical Monte Carlo simulations of water adsorption in NaY and NaX faujasite, as well as in silicalite-1. Computed adsorption isotherms and heats of adsorption were in good agreement with the available experiments. The existence of cyclic water hexamers in NaX located in the 12-ring windows, recently disclosed by neutron diffraction experiments (Hunger et al., J. Phys. Chem. B, 2006, 110, 342-353) was reproduced in our simulations. Interestingly enough, such cyclic hexamer clusters were also observed in the case of NaY, in which no stabilizing cation is present in the 12-ring window. We also report cation redistribution upon water adsorption for sodium faujasite with varying cation contents (Si : Al ratio in the range 1.53-3). A simple and transferable forcefield was used, that enabled to reproduce the different aspects of water physisorption in stable zeolites. The high pressure water condensation in hydrophobic silicalite-1 was reproduced without any parameter readjustment. The method and forcefield used here should be useful for engineering oriented applications such as the prediction of multi-component mixture adsorptive separations in various stable zeolites. It allows to address the issue of the effect of the small amounts of water that are almost inevitably present in zeolitebased separation processes.
Journal of Membrane Science, 2015
Molecular dynamics simulations were employed to investigate diffusion and structural properties of water molecules confined in one-dimensional zeolites. Several water loadings and thermostatting methods were used, and insight into the effects of these was obtained by comparing diffusion and structural properties. Water diffusion was characterised via mean square displacements (self and collective diffusivities) and radial distribution functions enabled the structural ordering of water for different pore sizes and loadings to be compared. Interestingly at lower loadings, molecules tend to form clusters and move collectively, while at higher loadings, the self-diffusion coefficient in the pores is similar to that in bulk water. The length of the simulation cell was varied to determine the system size effects on the results, and narrow pores were also investigated in order to examine how this affected the effectiveness of water transport through the zeolite.
Dynamics of water diffusion in mesoporous zeolites
Microporous and Mesoporous Materials, 2011
Pulsed field gradient NMR has been applied to investigate water diffusion in Na + -form zeolites beta and LTA if, in addition to the micropores, the intracrystalline space is traversed by a network of mesopores. In zeolite beta, the presence of the mesopores is found to enhance the rate of molecular diffusion by a factor of 3. The measurements with mesoporous zeolite LTA yield diffusivities which reproduce the order of magnitude of the diffusivities found in previous studies with genuine microporous specimens.
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.
Molecular Modeling of Univalent Cation Exchange in Zeolite N
The Journal of Physical Chemistry C
Molecular dynamics (MD) simulations are used to investigate the hydration energy and ion exchange properties of a synthetic zeolite, Zeolite N with composition |K 10 (H 2 0) 8 Cl | [Al Si O ]. The exchange of K + ions with univalent ions such as NH 4 + , Na + , Rb + and Cs + is investigated under a range of simulation conditions using a three dimensional membrane in an electrolyte box containing explicit water molecules. Hydration energy calculations indicate that Zeolite N prefers eight water molecules per cage which is consistent with X-ray and neutron diffraction determination of the structure. Ion density profiles and calculated self-diffusion coefficients show that univalent ion exchange by Zeolite N is selective towards NH 4 + in preference to other ions. The methodology used here to simulate the uptake of ions from an electrolyte within the zeolite N membrane produces results that are consistent with experimental data and implements a low computational overhead.
Mobility of adsorbed species in zeolites: a molecular dynamics simulation of xenon in silicalite
Journal of physical chemistry, 1990
Results from a molecular dynamics simulation of xenon in silicalite at 298 K and 4 atoms per unit cell (A&,=-26.9 kJ/mol, D= 1.86 X m2/s) are in good agreement with the experimental value of-24.5 kJ/mol and the diffusion coefficient derived from the NMR pulsed field-gradient method (4.00 X m2/s). The diffusivity is predicted to be negligible at temperatures around 77 K and then increases over the investigated range to D= 3.25 X m2/s at 450 K, yielding an activation energy of 5.5 kJ/mol. Increasing the concentration from 4 to 16 atoms per unit ...