Newsome, D;
Coppens, M-O;
(2015)
Molecular dynamics as a tool to study heterogeneity in zeolites - Effect of Na cations on diffusion of CO and N in Na-ZSM-5.
Chemical Engineering Science
, 121
(6 Janu)
300 - 312.
10.1016/j.ces.2014.09.024.
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Abstract
Zeolites typically contain extra-framework cations to charge-compensate for trivalent Al atom substitutions in the SiO framework. These cations, such as Na, directly interact with quadrupolar guest molecules, such as CO and N, which move through their micropores, causing energetic heterogeneity. To assess the effects of heterogeneity in Na-ZSM-5 on diffusion of CO and N, 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 at T=200 and 300K, while the cations only have minimal impact on the diffusivity of N. The MD simulations allow us to identify energy basins or sites at which guest molecules spend a relatively long time, and construct a coarse-grained 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 at T=200K are long at low loading, but decrease with loading as additional CO molecules compete to stay close to a cation. In contrast, the residence times for N 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.
| Type: | Article |
|---|---|
| Title: | Molecular dynamics as a tool to study heterogeneity in zeolites - Effect of Na cations on diffusion of CO and N in Na-ZSM-5 |
| Open access status: | An open access version is available from UCL Discovery |
| DOI: | 10.1016/j.ces.2014.09.024 |
| Publisher version: | http://dx.doi.org/10.1016/j.ces.2014.09.024 |
| Language: | English |
| Additional information: | © 2015 The Authors.Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/3.0/). |
| UCL classification: | UCL UCL > Provost and Vice Provost Offices UCL > Provost and Vice Provost Offices > UCL BEAMS UCL > Provost and Vice Provost Offices > UCL BEAMS > Faculty of Engineering Science UCL > Provost and Vice Provost Offices > UCL BEAMS > Faculty of Engineering Science > Dept of Chemical Engineering |
| URI: | https://discovery.ucl.ac.uk/id/eprint/1463323 |
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