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Processes 2018, 6(9), 147; https://doi.org/10.3390/pr6090147

Diffusion in Nanoporous Materials: Novel Insights by Combining MAS and PFG NMR

Fakultät für Physik und Geowissenschaften, Universität Leipzig, Linnéstraße 5, 04103 Leipzig, Germany
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Received: 6 August 2018 / Revised: 15 August 2018 / Accepted: 21 August 2018 / Published: 1 September 2018
(This article belongs to the Special Issue Transport of Fluids in Nanoporous Materials)
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Abstract

Pulsed field gradient (PFG) nuclear magnetic resonance (NMR) allows recording of molecular diffusion paths (notably, the probability distribution of molecular displacements over typically micrometers, covered during an observation time of typically milliseconds) and has thus proven to serve as a most versatile means for the in-depth study of mass transfer in complex materials. This is particularly true with nanoporous host materials, where PFG NMR enabled the first direct measurement of intracrystalline diffusivities of guest molecules. Spatial resolution, i.e., the minimum diffusion path length experimentally observable, is limited by the time interval over which the pulsed field gradients may be applied. In “conventional” PFG NMR measurements, this time interval is determined by a characteristic quantity of the host-guest system under study, the so-called transverse nuclear magnetic relaxation time. This leads, notably when considering systems with low molecular mobilities, to severe restrictions in the applicability of PFG NMR. These restrictions may partially be released by performing PFG NMR measurements in combination with “magic-angle spinning” (MAS) of the NMR sample tube. The present review introduces the fundamentals of this technique and illustrates, via a number of recent cases, the gain in information thus attainable. Examples include diffusion measurements with nanoporous host-guest systems of low intrinsic mobility and selective diffusion measurement in multicomponent systems. View Full-Text
Keywords: NMR; PFG; MAS; diffusion; adsorption; hierarchical host materials NMR; PFG; MAS; diffusion; adsorption; hierarchical host materials
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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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Kärger, J.; Freude, D.; Haase, J. Diffusion in Nanoporous Materials: Novel Insights by Combining MAS and PFG NMR. Processes 2018, 6, 147.

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