Translational diffusion of molecules in one-dimensional channel systems was measured by means of quasi-elastic neutron scattering. The scattering function for a single-file diffusion model was derived in order to differentiate this transport model from normal one-dimensional diffusion. In the AlPO4-5 structure, ordinary one-dimensional diffusion was observed for methane and ethane, whatever the loading, which indicated that the molecules were able to pass each other. The diffusion coefficients for both molecules were of the order of 10-9m2/s. On the other hand, a very different regime was obtained by varying the cyclopropane concentration in the same structure. At low loadings, normal one-dimensional diffusion was observed because the molecules could be considered to be isolated but, at higher concentrations, cyclopropane was found to follow single-file diffusion. However, the mobility was too small to be determined. In the less open channel system of ZSM-48, ordinary one-dimensional diffusion was found for small methane concentrations, the diffusion coefficient being 2.5 x 10-9m2at 155K. At medium loading, single-file diffusion was observed with a mobility factor of 2 x 10-12m2/s1/2. This was the first time that this technique had provided experimental evidence of single-file diffusion in zeolites.

Unidirectional and Single-File Diffusion of Molecules in One-Dimensional Channel Systems. A Quasi-Elastic Neutron Scattering Study. H.Jobic, K.Hahn, J.Kärger, M.Bée, A.Tuel, M.Noack, I.Girnus, G.J.Kearley: Journal of Physical Chemistry B, 1997, 101[30], 5834-41