Self-diffusion of n-alkanes in type a zeolite

Abstract

Self-diffusion of n-pentane, n-hexane, n-heptane and n-nonane has been measured in (Ca, Na)-A-zeolites containing 30–35% of the exchangeable Na⁺ ions replaced by Ca²⁺ ions. The n-alkane vapour phase existing over the sorbent at equilibrium was partially replaced by the corresponding monodeuterated alkane and the rate of exchange between vapour and sorbed phases monitored using an on-line mass-spectrometer. Self-diffusion coefficients obtained from these isotope exchange experiments refer to saturation or near saturation of alkane in the zeolite channels.

A complementary study of intrinsic diffusion of these alkanes in these zeolites has also been made.

A comparison of energies and entropies of activation for the self- and intrinsic diffusion processes has allowed an assessment of the importance of molecule-molecule interaction in these small-pore sorbents under saturation conditions. For a given n-alkane the energy of activation for self-diffusion is greater than that for intrinsic diffusion and the difference increased with chain length. A similar trend was observed for the entropies of activation for these transport processes.