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

The PFG NMR technique is applied to investigate the intracrystalline diffusion of methane and n-butane molecules in MFI-type zeolites at several temperatures (from 123 up to 383 K) and over a wide range of diffusion times (from 2 to 35 ms). The intracrystalline self-diffusion coefficients of the guest molecules recorded at low temperatures were observed to decrease with increasing root-mean-square displacements. The comparison of the experimental results with the results of the Monte Carlo simulations of diffusion allowed us to rule out the restriction of diffusion by crystal boundaries as a possible explanation of the observed dependencies of the diffusivities on the root-mean-square displacement. These dependencies are tentatively attributed to the existence of intracrystalline transport barriers in MFI-type crystals. The intersections between the elementary building blocks of the crystals and/or intergrowth sections of MFI crystals are suggested as the possible candidates for the transport barriers.