DIFFUSION ANISOTROPY IN MOLECULAR-SIEVES - A FOURIER-TRANSFORM PFG NMR-STUDY OF METHANE IN ALPO4-5

By analyzing the shape of the H-1 PFG NMR spin echo decay of methane i n large crystals of AlPO4-5, we demonstrate that diffusion of methane is unidirectional but not single file. The intracrystalline diffusion coefficient is 2.9 X 10(-9) m(2)s(-1). The same experiment, when perfo rmed 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 .