Determination of the intracrystalline diffusion coefficient of methane in A zeolites by means of neutron spin-echo experiments

Abstract

The intracrystalline motion of methane molecules adsorbed on CaNaA (5A) and NaA (4A) zeolites have been studied by means of spin-echo neutron scattering spectroscopy over distances from 15 to 30 Å within the time-scale from 10^–10 to 10^–8 s. In CaNaA zeolite we were able to measure a mean lifetime of 4 × 10^–10 s for the molecule in one cavity. In NaA this lifetime is > 10^–8 s. The increase in residence time on going from CaNaA to NaA shows the influence of the cations located in the eight-membered oxygen windows. The interpretation of the results in terms of a diffusion-jump model involving discrete sites on a cubic lattice leads to a value of the intracrystalline diffusion coefficient in agreement with that obtained from n.m.r. pulse field-gradient measurement. This shows the influence of crystallinity on the diffusivity.