By analyzing the shape of the 1H PFG NMR spin echo decay of methane in large crystals of AlPO4-5, we demonstrate that diffusion of methane is unidirectional but not single file. The intracrystalline diffusion coefficient is 2.9 × 10-9 m2s-1. The same experiment, when performed on AlPO4-5 crystals of much smaller size, results in an isotropic diffusivity that is higher by nearly three orders of magnitude. This crystal size artifact emphasizes the importance of using large zeolite crystals in NMR measurements to obtain intracrystalline diffusivities.
Bibliographical noteFunding Information:
This work was supportedb y a grant from the Dow Chemical Company and by an NSF-PYI grant (CTS-9058387). The authors would like to thank Dr. Michael J. Annen (PPG Industries) for helpful dis-cussionsa nd Dr. Juan M. Garces and Dr. Michael M. Olken (Dow Chemical Company). We also thank Professor M.ED. avis (Caltech) and Dr. D. Young (University of Toronto) for providing the AlPOd-samples.
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