1H pulse field-gradient spin-echo NMR was used to measure the diffusivity of methane in a polycrystalline MFI-type silicalite membrane. The measured diffusivities decreased with an increase in the diffusion distance, and converged to a constant value. This suggested the presence of a transport barrier in the membrane. The long-term diffusivity in the membrane was 3.7 x 10-9m2/s; a factor of 3 smaller than the values reported for a single crystal. The distance between the transport barriers was estimated to be much larger than 6μm, from the relationship of diffusivity to displacement. It was noted that the estimated distances were larger than the smallest dimension of the crystals appearing in the membrane surface. Gas permeation and pervaporation tests were carried out on the same sample on which NMR measurements were made. The estimated methane flux, using measured long-time diffusivity and the permeation theory, over-estimated the experimental value, although it was closer to the experimental value than the one estimated using the short-term diffusivity. The results showed that the methane diffusivity in a silicalite membrane was much lower than that in a single crystal.

Gas Diffusion in Polycrystalline Silicalite Membranes Investigated by 1H Pulse Field-Gradient NMR. H.Takaba, A.Yamamoto, K.Hayamizu, S.I.Nakao: Journal of Physical Chemistry B, 2005, 109[29], 13871-6