Molecular self-diffusion of methane in zeolite ZSM-5 by quasi-elastic neutron scattering and nuclear magnetic resonance pulsed field gradient technique

Abstract

The translational mobility of methane sorbed on an NaZSM-5 sample of proven crystallinity has been studied by quasi-elastic neutron scattering (QENS) and an n.m.r. pulsed field gradient technique (n.m.r. PFGT) at different loadings and temperatures. In both methods, long-range self-diffusion is detected. However, owing to the different timescales of the two experimental methods applied, the mean diffusion paths followed are of different magnitude: in QENS, molecular translation is measured up to 6nm; in n.m.r. PFGT, the mean molecular displacements amount to some µm. Nevertheless, since the r.m.s. molecular displacements followed by both methods considerably exceed the distances between the pore intersections of the ZSM-5 channel network, in both techniques the translational self-diffusion coefficient of guest molecules inside the zeolite pores is detected. For the intracrystalline self-diffusion coefficient, D, as well as for the activation energy of self-diffusion, E_a, the values determined by the two independent methods agree very well. In the temperature region 200–250 K, the intracrystalline self-diffusion coefficients of methane in ZSM-5 are found to be of the order of 10^–5–10^–4 cm² s^–1, with only slight concentration and temperature dependence. The activation energy determined by both methods amounts to 4–5 kJ mol^–1. Further agreement between QENS and n.m.r. PFGT is obtained by comparing the mean molecular jump lengths, which by both techniques are found to be ca. 1 nm for light hydrocarbons in ZSM-5, slightly decreasing with increasing loading.