UNIDIRECTIONAL AND SINGLE-FILE DIFFUSION OF MOLECULES IN ONE-DIMENSIONAL CHANNEL SYSTEMS - A QUASI-ELASTIC NEUTRON-SCATTERING STUDY

Translational diffusion of molecules in one-dimensional channel system s has been measured by quasi-elastic neutron scattering. The scatterin g function for a single-file diffusion model has been derived in order to differentiate this transport model from normal 1D diffusion. In th e AlPO4-5 structure, ordinary 1D diffusion is observed for methane and ethane, whatever the loading, which indicates that the molecules are able to pass each other. The diffusion coefficients for both molecules are on the order of 10(-9) m(2) s(-1). On the other hand, a quite dif ferent regime is obtained by varying cyclopropane concentration in the same structure, At low loading, normal 1D diffusion is observed becau se the molecules can be considered as isolated, but at higher concentr ation cyclopropane is found to follow single-file diffusion. However, the mobility is too small to be determined. In the less open channel s ystem of ZSM-48, ordinary 1D diffusion is found for small methane conc entration, the diffusion coefficient being 2.5 x 10(-9) m(2) s(-1), at 155 K. At medium loading, single-file diffusion is observed with a mo bility factor of 2 x 10(-12) m(2) s(-1/2). This is the first rime that this technique has provided experimental evidence of single-file diff usion in zeolites.