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Molecules 2018, 23(3), 668; https://doi.org/10.3390/molecules23030668

Anomaly in the Chain Length Dependence of n-Alkane Diffusion in ZIF-4 Metal-Organic Frameworks

1
Faculty of Physics and Earth Sciences, Universität Leipzig, Linnéstraße 5, 04103 Leipzig, Germany
2
Department of Chemical and Biological Engineering, Northwestern University, 2145 Sheridan Road, Evanston, IL 60208-3109, USA
3
Erlangen Catalysis Resource Center, Friedrich-Alexander-Universität Erlangen-Nürnberg, Egerlandstraße 3, 91058 Erlangen, Germany
*
Author to whom correspondence should be addressed.
Received: 7 February 2018 / Revised: 8 March 2018 / Accepted: 13 March 2018 / Published: 15 March 2018
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Abstract

Molecular diffusion is commonly found to slow down with increasing molecular size. Deviations from this pattern occur in some host materials with pore sizes approaching the diameters of the guest molecules. A variety of theoretical models have been suggested to explain deviations from this pattern, but robust experimental data are scarcely available. Here, we present such data, obtained by monitoring the chain length dependence of the uptake of n-alkanes in the zeolitic imidazolate framework ZIF-4. A monotonic decrease in diffusivity from ethane to n-butane was observed, followed by an increase for n-pentane, and another decrease for n-hexane. This observation was confirmed by uptake measurements with n-butane/n-pentane mixtures, which yield faster uptake of n-pentane. Further evidence is provided by the observation of overshooting effects, i.e., by transient n-pentane concentrations exceeding the (eventually attained) equilibrium value. Accompanying grand canonical Monte Carlo simulations reveal, for the larger n-alkanes, significant differences between the adsorbed and gas phase molecular configurations, indicating strong confinement effects within ZIF-4, which, with increasing chain length, may be expected to give rise to configurational shifts facilitating molecular propagation at particular chain lengths. View Full-Text
Keywords: ZIF-4; n-alkanes; transport diffusivity; commensurate/incommensurate adsorption; GCMC simulation ZIF-4; n-alkanes; transport diffusivity; commensurate/incommensurate adsorption; GCMC simulation
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This is an open access article distributed under the Creative Commons Attribution License which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. (CC BY 4.0).
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Hwang, S.; Gopalan, A.; Hovestadt, M.; Piepenbreier, F.; Chmelik, C.; Hartmann, M.; Snurr, R.Q.; Kärger, J. Anomaly in the Chain Length Dependence of n-Alkane Diffusion in ZIF-4 Metal-Organic Frameworks. Molecules 2018, 23, 668.

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