Molecular dynamics of n-pentane in NaX zeolite studied by quasi-elastic neutron scattering

Abstract

Quasi-elastic neutron scattering (QENS) has been used to study the dynamics of n-pentane in NaX zeolite. The long range translational motion has been observed, the self-diffusion coefficient being 2.4×10^-9 s^-1, at 300 K. This value is in good agreement with pulsed-field gradient NMR (PFG NMR) data but it is two orders of magnitude larger than the diffusivities reported by the zero-length column (ZLC) method. The activation energy obtained from QENS is 6 kJ mol^-1, which is 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 is found to be independent of the temperature and equal to ≈7 Å. The increase of the self-diffusion coefficient with increased temperature is due to the decrease of the residence times. The rotational motion of n-pentane in NaX is isotropic, in contrast to n-butane or n-hexane in silicalite.