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Observing Diffusion Anisotropy in Zeolites by Pulsed Field Gradient NMR

Pulsed Held gradient (PFG) NMR spectroscopy is shown to provide direct evidence of the diffusion anisotropy of guest molecules in zeolite crystallites of non-cubic structure. Orientation dependent diffusivities are determined in two different ways: with crystallites aligned within an array of parallel capillaries by changing the direction of the field gradients with respect to the capillaries, and in powder samples by an analysis of the shape of the NMR signal decay. For methane in zeolite ZSM-5, the diffusivities in the straight channels (direction of the crystal y coordinate) is found to be by a factor of the order of 2 to 3 larger than in the sinusoidal channels (x coordinate), and by a factor of about 8 larger than in the direction perpendicular to the two channel systems (. coordinate). These results are in satisfactory agreement with the predictions of molecular dynamics simulations.