Influence of the Intermolecular Interactions on the Mobility of Heptane in the Supercages of MCM-22 Zeolite. A Molecular Dynamics Study

Molecular dynamics techniques have been used to simulate the diffusion of n-heptane in the supercage system of purely siliceous MCM-22 zeolite at 750 K. Diffusion through the 12-member-ring (MR) large cavities is dominated by intracage motion due to the presence of preferential minimum-energy positions at the top and bottom of the supercage forming the 12-MR pockets. Molecules in singly occupied supercages show higher diffusivity because there is no impediment for intracage motions other than the small activation energy. Molecules in doubly occupied supercages show lower diffusivity and intermolecular contacts preclude intracage jumps. Finally, higher loadings allow the intercage region to be more populated and the possibility of intercage diffusion when the n-heptane orientation is favorable. The intercage diffusion is analyzed on the basis of the energy path for the intercage motion and on the orientational probability for intercage jumps.