Molecular simulation of adsorption of alkanes in sodium MOR-type zeolites using a new force field (original) (raw)
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The Journal of Physical Chemistry B, 2002
Molecular simulations were performed for the adsorption of methane, ethane, and propane in MFI-and MORtype zeolites with various nonframework sodium and framework aluminum densities. The position of the nonframework sodium cations determined by Monte Carlo simulations is in agreement with positions determined by X-ray diffraction. The position and density of the sodium and aluminum atoms in the zeolite have a large influence on the adsorption of alkanes. The computed adsorption isotherms and Henry coefficients agree well with those obtained experimentally. Finally, we show that Configurational Bias Monte Carlo (CBMC) simulations are able to provide a better understanding of the effect of nonframework sodium on the selective adsorption of binary mixtures of isomers by these structures. Our results show that increasing the nonframework sodium density in MFI-type zeolites increasingly blocks the intersections and thereby increases the selectivity of MFI-type zeolites for adsorbing linear alkanes. By contrast, increasing the nonframework sodium density in MOR-type zeolites increases the number of sites favorable for adsorbing small linear alkanes.
A New United Atom Force Field for Adsorption of Alkenes in Zeolites
Journal of Physical Chemistry C, 2008
A new united atom force field was developed that accurately describes the adsorption properties of linear alkenes in zeolites. The force field was specifically designed for use in the inhomogeneous system and therefore a truncated and shifted potential was used. With the determined force field, we performed a comparative study on the adsorption behaviors of ethene and propene in four pure-silica small-pore eight-membered-ring zeolites, CHA, DDR, ITE, and IHW (named Chabazite, DD3R, ITQ-3, and ITQ-32, respectively), characterized for their paraffin/olefin separation capability. The different macroscopic adsorption behaviors of alkenes in the four zeolites were elucidated and related to their structures with the microscopic information obtained from the molecular simulations providing useful information for further rational design of such zeolites with tailored properties.
The Journal of Physical Chemistry B, 2006
The recently proposed united atom force field by Dubbeldam et al. (Phys. ReV. Lett. 2004, 93, 088302) for the adsorption of alkanes in MFI-type zeolites was extended to other zeolites in this work. Its applicability to FER-type zeolites was evaluated in detail, for which the Henry coefficients, the isosteric heat of adsorption, the adsorption isotherms, as well as the locations of alkanes in the FER-type zeolites were computed and compared to experimental values. The results show that the new force field works well for FER zeolites. Furthermore, its applicability to MWW-, MTW-, CFI-, LTA-, and STF-type zeolites was investigated, and we found that the experimental isotherms could be accurately predicted except for STF-type zeolites. This work shows that the new united atom force field proposed by Dubbeldam et al. is applicable to most pure silica zeolites.
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.
Adsorption in zeolites: intermolecular interactions and computer simulation
Advances in Colloid and Interface Science, 1998
The input to a simulation is the potential energy model. In microporous solids, the adsorbate adsorbent interaction is the most significant part of the total potential energy. The full scale semi-empirical PN model is summarised as an example of a state of the art potential model. The potential includes damped two-body dispersion interactions, three-body Ž . interactions, induced interactions often negligibly small and long-range electrostatic interactions. Repulsive interaction is modelled as a Born᎐Mayer function. All the contributions can be calculated from a knowledge of four parameters that depend on partial charges at sites. Of these the dipole polarizabilities and excitation energies can be calculated from independent information. A guide to the magnitude of the repulsive parameters A and b comes from quantum mechanical calculations, and these are then tuned against experiment. They maintain a high degree of transferability through combination rules. Several systems are discussed as examples. For argon adsorption in silicalite at 77 K it is found that a transition, similar in magnitude to that observed experimentally, occurs in the simulation, but only at very much higher pressure. Detailed evidence suggests that adsorbent lattice distortion, not permitted in the simulation, may occur in the real system. A similar conclusion applies to nitrogen in silicalite at 77 K, where the first, but not the second, of the experimental transitions is observed in simulation. In AlPO -5, both argon and methane 4 adsorption have been studied, good agreement with experiment can be obtained when methane is modelled as a 20᎐6, rather than a 12᎐6, molecule, and it seems improbable that lattice distortion occurs in these systems. A third zeolite studied using the PN potential is NaY. Here xenon, methane and p-and m-xylene adsorption have been simulated. Good agreement with experiment is attained for the first two adsorbates, but it is again found to Ž . P I I S 0 0 0 1 -8 6 8 6 9 8 0 0 0 4 6 -3 ( ) D. Nicholson, R. J-M. Pellenq r Ad¨. Colloid Interface Sci. 76᎐77 1998 179᎐202 180 be necessary to use a more realistic model than the 12᎐6 potential for methane interactions. A notable conclusion is that Na migration may occur at high loading of p-xylene. In many of these examples, comparison with the frequently used oxygen-only potentials are made. It is concluded that full-scale modelling of the PN type is beneficial in the characterisation of microporous zeolites. Nevertheless, full transferability cannot be claimed for the PN model in its present form. ᮊ
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.
Relationships between the structure of a zeolite and its adsorption properties
Surface Science, 1998
The adsorptmn of N 2 has been stu&ed m different Na-LSX Zeohtes, using an embedded cluster approach. Embedded catmn-molecule systems modeling sates II and sites III of Ideal LSX zeolltes (Si/A1 = 1 ) have been treated quantum chemically, using a method based on density functmnal theory. Two experlmental structures have been compared with two artificially modafied structures The adsorptaon strength of the caUomc sites is correlated with the values of the electrac field and electric field gradaent in the supercages © 1998 Elsevxer Science B V