  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.
