Diffusion of aromatic guest molecules in zeolite NaX studied by pulsed field gradient NMR

The pulsed field gradient (PFG) NMR technique has been applied for studying molecular diffusion of a series of aromatics in zeolite NaX. For a loading of one molecule per supercage and a measuring temperature of 105 °C, the diffusivities of benzene and the diisopropylbenzene isomers are found to differ by one order of magnitude. Complementary measurements with zeolite specimens of different origin and comparison with literature data illustrate the scattering in the diffusivity data which may result from measurements with different measuring techniques and/or different zeolite material. Benzene is identified as a reliable probe molecule for comparison of the diffusion properties of different zeolite specimens. In this way, after more than 20 years, the diffusivity data in identical specimens could be reproduced and thus be confirmed to exceed the diffusivity in a specimen of apparently identical type, but of different origin, by a factor of about two, indicating the existence of additional intracrystalline resistances. Simultaneously with examples demonstrating the self-consistency of the attained data, a critical assessment of the limitations and possible pitfalls of the application of PFG NMR to diffusion measurements in zeolites are presented.