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Exploring Mass Transfer in Mesoporous Zeolites by NMR Diffusometry
Materials 2012, 5(4), 721-740; doi:10.3390/ma5040721

Micro-Imaging by Interference Microscopy: A Case Study of Orientation-Dependent Guest Diffusion in MFI-Type Zeolite Host Crystals

1 Department of Interface Physics, University of Leipzig, Leipzig 04109, Germany 2 Department of Chemical and Biological Engineering, University of Maine, Orono, ME 04473, USA
* Author to whom correspondence should be addressed.
Received: 28 February 2012 / Revised: 10 April 2012 / Accepted: 13 April 2012 / Published: 24 April 2012
(This article belongs to the Special Issue Diffusion in Micropores)
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Because of the small particle size, orientation-dependent diffusion measurements in microporous materials remains a challenging task. We highlight here the potential of micro-imaging by interference microscopy in a case study with MFI-type crystals in which, although with different accuracies, transient concentration profiles in all three directions can be observed. The measurements, which were performed with “rounded-boat” shaped crystals, reproduce the evolution patterns of the guest profiles recorded in previous studies with the more common “coffin-shaped” MFI crystals. The uptake and release patterns through the four principal faces (which in the coffin-shaped crystals extend in the longitudinal direction) are essentially coincident and there is no perceptible mass transfer in the direction of the long axis. The surface resistances of the four crystal faces through which mass transfer occurs are relatively small and have only a minor effect on the mass transfer rate. As a result of the pore structure, diffusion in the crystallographic c direction (which corresponds to the direction of the long axis) is expected to be much slower than in the transverse directions. This could explain the very low rate of mass transfer observed in the direction of the long axis, but it is also possible that the small end faces of the crystal may have high surface resistance. It is not possible to distinguish unequivocally between these two possibilities. All guest molecules studied (methyl-butane, benzene and 4-methyl-2-pentyne) show the same orientation dependence of mass transfer. The long 4-methyl-2-pentyne molecules would be expected to propagate at very different rates through the straight and sinusoidal channels. The coinciding patterns for uptake through the mutually perpendicular crystal faces therefore provide clear evidence that both the coffin shaped crystals and the rounded-boat-shaped crystals considered in this study, must be intergrowths rather than pure single crystals.
Keywords: MFI (mordenite framework inverted); diffusion; anisotropy; surface resistance; interference microscopy MFI (mordenite framework inverted); diffusion; anisotropy; surface resistance; interference microscopy
This is an open access article distributed under the Creative Commons Attribution License (CC BY) which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

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Gueudré, L.; Binder, T.; Chmelik, C.; Hibbe, F.; Ruthven, D.M.; Kärger, J. Micro-Imaging by Interference Microscopy: A Case Study of Orientation-Dependent Guest Diffusion in MFI-Type Zeolite Host Crystals. Materials 2012, 5, 721-740.

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