Transport in Nanoporous Materials Including MOFs: The Applicability of Fick's Laws

• Published in: Angewandte Chemie International Edition Vol. 54; no. 48; pp. 14580 - 14583
• Main Authors: Titze, Tobias, Lauerer, Alexander, Heinke, Lars, Chmelik, Christian, Zimmermann, Nils E. R., Keil, Frerich J., Ruthven, Douglas M., Kärger, Jörg
• Format: Journal Article
• Language: English
• Published: Weinheim WILEY-VCH Verlag 23-11-2015; WILEY‐VCH Verlag; Wiley Subscription Services, Inc
• Edition: International ed. in English
• Subjects: Biological Transport; Diffusion; Fick's laws; Magnetic Resonance Spectroscopy; metal-organic frameworks; Microscopy; Microscopy - methods; Models, Chemical; nanoporous materials; Nanostructures; NMR; NMR spectroscopy; Nuclear magnetic resonance; Porosity; Supramolecular compounds; zeolites; nanoporous materials; diffusion; zeolites; Fick’s laws; metal-organic frameworks
• ISSN: 1433-7851; 1521-3773
• DOI: 10.1002/anie.201506954

Diffusion in nanoporous host–guest systems is often considered to be too complicated to comply with such “simple” relationships as Fick’s first and second law of diffusion. However, it is shown herein that the microscopic techniques of diffusion measurement, notably the pulsed field gradient (PFG) technique of NMR spectroscopy and microimaging by interference microscopy (IFM) and IR microscopy (IRM), provide direct experimental evidence of the applicability of Fick’s laws to such systems. This remains true in many situations, even when the detailed mechanism is complex. The limitations of the diffusion model are also discussed with reference to the extensive literature on this subject.
No more diffusion confusion: Although the elementary steps of molecular propagation in nanoporous materials may be quite intricate, overall mass transfer in the individual particles generally follows the simple Fickian laws of diffusion. Even in a complex system, the diffusion coefficient is a well‐defined quantity and the addition of qualifiers such as “effective”, “seeming”, or “apparent” is unnecessary and even misleading.