Observing Diffusion Anisotropy in Zeolites by Pulsed Field Gradient NMR

“Two-step” model of molecular diffusion in silicalite

“…When a molecule enters an intersection, it will proceed to one of the four adjacent ones with a probability independent of the channel section from which it has come from. This assumption was not in conflict with the experimental data [7][8][9] and has been, moreover, supported by the evidence of molecular dynamics ͑MD͒ simulations [10][11][12] for simple, spherical molecules like xenon and methane. For sufficiently long alkanes, however, as a consequence of the spatial extension of the molecule, molecular propagation from one channel intersection to the next one cannot be expected anymore to be independent of the trajectory.…”

“…As a consequence of the small size of zeolite crystallites, the measurement of diffusion anisotropy either by transient uptake techniques [4][5][6] or by the pulsed field gradient ͑PFG͒ nuclear magnetic resonance ͑NMR͒ method [7][8][9] is subjected to substantial experimental uncertainty, which may be expected to be on the order of magnitude of the effects of correlated motion. Quantitatively, the effect of correlated motion results in an interdependence of the main elements of the diffusion tensor.…”

NMR methods applied to anisotropic diffusion

“…40 The third example is water diffusion in oriented single crystals of the naturally occurring zeolite chabazite. 41 Oriented samples of a solid (zeolite), produced by orientating crystallites with anisomeric shape within narrow capillaries, 42,43 or oriented samples of hydrated DNA fibers 44 have also been investigated. However, the majority of diffusion experiments in macroscopically oriented samples were performed in thermotropic, lyotropic or polymeric liquid crystals.…”

Diffusion Measurements by NMR Techniques

“…However, by introducing large zeolite crystals of type ZSM-5 into an array of parallel capillaries [119,120], it was possible to align a sufficiently large amount of crystallites with respect to the crystallographic z axis (Fig. 12).…”

“…12 Schematic representation of the orientation of the internal channel system within zeolite crystallites of type ZSM-5. From [119] with permission rection can only proceed by interchanging periods of migration along the segments of the straight and sinusoidal channels. As therefore to be expected, the diffusivities in PFG NMR measurements with field gradients oriented perpendicular to the capillaries of the "container" are larger than in the case with field gradients parallel to the capillaries.…”