Quasi-elastic neutron scattering was used to study the dynamics of n-pentane in NaX zeolite. Long-range translational motion was observed, the self-diffusion coefficient being 2.4 x 10-9m2/s at 300K. This value was in good agreement with pulsed-field gradient NMR data, but it was two orders of magnitude larger than the diffusivities reported by using the zero-length column method. The activation energy obtained from quasi-elastic neutron scattering was 6kJ/mol; smaller than the values measured by PFG NMR or ZLC techniques (by a factor of two). A jump diffusion model with a fixed jump length was fitted to the spectra. The jump length was found to be independent of the temperature and equal to ≃7Å. The increase of the self-diffusion coefficient with increased temperature was due to the decrease of the residence times. The rotational motion of n-pentane in NaX was isotropic, in contrast to n-butane or n-hexane in silicalite.

Molecular Dynamics of n-Pentane in NaX Zeolite Studied by Quasi-Elastic Neutron Scattering. H.Jobic: Physical Chemistry Chemical Physics, 1999, 1[4], 525-30