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Free energy barriers for CO2 and N-2 in zeolite NaKA: an ab initio molecular dynamics approach
Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).ORCID iD: 0000-0002-0323-0210
Stockholm University, Faculty of Science, Department of Materials and Environmental Chemistry (MMK).
2014 (English)In: Physical Chemistry, Chemical Physics - PCCP, ISSN 1463-9076, E-ISSN 1463-9084, Vol. 16, no 1, 166-172 p.Article in journal (Refereed) Published
Abstract [en]

Ab initio Molecular Dynamics (AIMD) is used with spatial constraints to estimate the free energy barriers of diffusion for CO2 and N-2 gas molecules in zeolite NaA and KA. We investigate the extent to which the diffusion of these gas molecules is hindered, in the two separate cases of a smaller Na+ ion or a larger K+ ion blocking the 8-ring pore window. In contrast to classical Molecular Dynamics, AIMD performs these computations accurately and unbiased in the absence of empirical parameterization. Our work has resulted in stable and reliable force profiles. The profiles show that the larger K+ ion effectively blocks the passage of both CO2 and N-2 molecules while the smaller Na+ ion will allow both molecules to pass. These results are a quantitative demonstration of the concept of pore blocking where we compute the effect, which the size of the respective cation occupying the pore window has on diffusive properties of each gas molecule. Hence, this effect can be altered through ion exchange to fine-tune the functionality of a specific zeolite as a molecular sieve.

Place, publisher, year, edition, pages
2014. Vol. 16, no 1, 166-172 p.
National Category
Chemical Sciences
Identifiers
URN: urn:nbn:se:su:diva-98265DOI: 10.1039/c3cp52821aISI: 000327667500018OAI: oai:DiVA.org:su-98265DiVA: diva2:686044
Note

AuthorCount:3;

Available from: 2014-01-10 Created: 2014-01-03 Last updated: 2017-12-06Bibliographically approved
In thesis
1. Multiscale Modeling of Molecular Sieving in LTA-type Zeolites: From the Quantum Level to the Macroscopic
Open this publication in new window or tab >>Multiscale Modeling of Molecular Sieving in LTA-type Zeolites: From the Quantum Level to the Macroscopic
2015 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

LTA-type zeolites with narrow window apertures coinciding with the approximate size of small gaseous molecules such as CO2 and N2 are interesting candidates for adsorbents with swing adsorption technologies due to their molecular sieving capabilities and otherwise attractive properties. These sieving capabilities are dependent on the energy barriers of diffusion between the zeolite pores, which can be fine-tuned by altering the framework composition. An ab initio level of theory is necessary to accurately describe specific gas-zeolite interaction and diffusion properties, while it is desirable to predict the macroscopic scale diffusion for industrial applications. Hence, a multiscale modeling approach is necessary to describe the molecular sieving phenomena exhaustively.

In this thesis, we use several different modeling methods on different length and time scales to describe the diffusion driven uptake and separation of CO2 and N2 in Zeolite NaKA. A combination of classical force field based modeling methods are used to show the importance of taking into account both thermodynamic, as well as, kinetic effects when modeling gas uptake in narrow pore zeolites where the gas diffusion is to some extent hindered. For a more detailed investigation of the gas molecules’ pore-to-pore dynamics in the material, we present a procedure to compute the free energy barriers of diffusion using spatially constrained ab initio Molecular Dynamics. With this procedure, we seek to identify diffusion rate determining local properties of the Zeolite NaKA pores, including the Na+-to-K+ exchange at different ion sites and the presence of additional CO2 molecules in the pores. This energy barrier information is then used as input for the Kinetic Monte Carlo method, allowing us to simulate and compare these and other effects on the diffusion driven uptake using a realistic powder particle model on macroscopic timescales.

Place, publisher, year, edition, pages
Stockholm: Department of Materials and Environmental Chemistry, Stockholm University, 2015. 76 p.
Keyword
CO2 separation, carbon capture, kinetic sieving, Zeolite A, Zeolite NaKA, cation exchange, temporal coarse graining, Kinetic Monte Carlo, Molecular Dynamics, AIMD, DFT
National Category
Physical Chemistry
Research subject
Physical Chemistry
Identifiers
urn:nbn:se:su:diva-113024 (URN)978-91-7649-081-5 (ISBN)
Public defence
2015-02-20, Magnélisalen, Kemiska övningslaboratoriet, Svante Arrhenius väg 16 B, Stockholm, 13:00 (English)
Opponent
Supervisors
Note

At the time of the doctoral defense, the following paper was unpublished and had a status as follows: Paper 4: Manuscript.

Available from: 2015-01-28 Created: 2015-01-21 Last updated: 2015-01-28Bibliographically approved

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