Molecular Dynamics of C7 Hydrocarbon Diffusion in ITQ-2. The Benefit of Zeolite Structures Containing Accessible Pockets

The diffusion of C7 alkanes in the microporous material ITQ-2 has been investigated by molecular dynamics techniques and the results were compared with those obtained for MCM-22. In MCM-22, differences in diffusivity are observed between the isomers n-heptane and 2-methylhexane, especially in the sinusoidal 10 member ring (MR) system. Similar features in the diffusivity are observed in the supercages, and no intercage diffusion has been observed at any of the temperatures of the simulation (ranging from 450 to 650 K), which would suggest that a much longer time scale is needed for intercage diffusion to be observed. Intracage diffusion is a temperature-activated process. In ITQ-2 the diffusion in the supercage system has been studied using 2-methylhexane as the probe molecule. Important differences appear with respect to MCM-22 because of the opening of the supercages giving chalice-type pores. The molecules have a tendency to penetrate into the chalice, but the subsequent out-diffusion is not restrained by the 10 MR windows interconnecting the supercages, as these windows, present in MCM-22, do not exist in ITQ-2. This allows the 2-methylhexane to escape from the 12 MR system more easily than in MCM-22. Possible catalytic applications of ITQ-2 are analyzed in view of the results.