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Molecular diffusion in zeolites of unidimensional channel structure is studied by pulsed field gradient NMR. In contrast to ordinary diffusion, the mean square displacement increases in proportion with the square root of the observation time. This behavior is explained by assuming that the molecules are unable to pass each other (“single-file” diffusion). The diffusivity of an isolated molecule estimated from the mobility in the single-file system is 2 orders of magnitude larger than the largest intracrystalline diffusivities in zeolites so far observed. The finding is supported by both the observed concentration dependence and molecular dynamics simulations.

Single-file diffusion is the restricted propagation of particles that cannot pass each other. The occurrence of this phenomenon should be reflected by a change in the time dependence of the mean particle displacement in comparison with ordinary diffusion. Although this process is considered to be the rate-controlling mechanism in a large variety of processes, so far no direct evidence of this phenomenon has been provided. Diffusion measurements made with pulsed field gradient nuclear magnetic resonance (NMR) in unidimensional pore systems (zeolites AlPO₄-5 and Theta-1) reflect the expected time dependence of single-file diffusion.

This work provides an overview of different experimental techniques of diffusion measurements in porous materials and discusses transport properties of several classes of hierarchically organized micro-mesoporous materials.