DIFFUSION OF COADSORBED MOLECULES IN ZEOLITES - A PULSED-FIELD GRADIENT NMR-STUDY

The Fourier transform pulsed field gradient NMR technique has been app lied to study the diffusion of coadsorbed molecules in large zeolite c rystals. The two systems studied were (i) methane/ethylene in NaY and (ii) ethane/ethylene in silicalite. The values obtained for these syst ems were compared with single-component diffusivities. In NaY, the sel f-diffusivity of methane, though higher than ethylene by an order of m agnitude, was reduced in comparison to its single-component value by a factor of 2 (at the same total sorbate loading). In silicalite, the d iffusivity of ethane was comparable to coadsorbed ethylene; having fal len 2 orders of magnitude below the single-component ethane diffusivit y in silicalite (at the same total sorbate loading). This result confi rms the expectation, due to Pfeifer et al. [J. Am. Chem. Sec. 1991, 11 3, 4812], that slowly diffusing sorbates act to reduce the diffusivity of otherwise very mobile sorbates. In NaY though, ethylene does not b lock diffusion as effectively as in silicalite, probably because (on a ccount of the larger pore size and higher connectivity in NaY) methane can find paths around the immobile ethylene.