Resonant diffusion of normal alkanes in zeolites: Effect of the zeolite structure and alkane molecule vibrations

TL;DR

This study models how the diffusion of normal alkanes in zeolites depends on molecular vibrations and pore structure, revealing conditions for resonant diffusion and the impact of molecule flexibility on diffusion rates.

Contribution

It introduces a theoretical framework that explains resonant diffusion phenomena considering molecule vibrations and zeolite structure, providing criteria for observable resonance.

Findings

Resonant diffusion occurs when pore structure dominates interaction potential.

Resonance is absent in zeolites without pore structure.

Molecular vibrations can suppress diffusion compared to rigid molecules.

Abstract

Diffusion of normal alkanes in one-dimensional zeolites is theoretically studied on the basis of the stochastic equation formalism. The calculated diffusion coefficient accounts for the vibrations of the diffusing molecule and zeolite framework, molecule-zeolite interaction, and specific structure of the zeolite. It is shown that when the interaction potential is predominantly determined by the zeolite pore structure, the diffusion coefficient varies periodically with the number of carbon atoms of the alkane molecule, a phenomenon called resonant diffusion. A criterion for observable resonance is obtained from the balance between the interaction potentials of the molecule due to the atomic and pore structures of the zeolite. It shows that the diffusion is not resonant in zeolites without pore structure, such as ZSM-12. Moreover, even in zeolites with developed pore structure no resonant dependence of the diffusion constant can be detected if the pore structure energy barriers are not at least three times higher than the atomic structure energy barriers. The role of the alkane molecule vibrations is examined as well and a surprising effect of suppression of the diffusion in comparison with the case of a rigid molecule is observed. This effect is explained with the balance between the static and dynamic interaction of the molecule and zeolite.