PFG NMR study of diffusion in MFI-type zeolites: Evidence of the existenceof intracrystalline transport barriers

The PFG NMR technique is applied to investigate the intracrystalline diffus ion of methane and n-butane molecules in MFI-type zeolites at several tempe ratures (from 123 up to 383 K) and over a wide range of diffusion times (fr om 2 to 35 ms). The intracrystalline self-diffusion coefficients of the gue st molecules recorded at low temperatures were observed to decrease with in creasing root-mean-square displacements. The comparison of the experimental results with the results of the Monte Carlo simulations of diffusion allow ed us to rule out the restriction of diffusion by crystal boundaries as a p ossible explanation of the observed dependencies of the diffusivities on th e root-mean-square displacement. These dependencies are tentatively attribu ted to the existence of intracrystalline transport barriers in MFI-type cry stals. The intersections between the elementary building blocks of the crys tals and/or intergrowth sections of MFI crystals are suggested as the possi ble candidates for the transport barriers.