The diffusion of C7 alkanes in microporous ITQ-2 was investigated using molecular dynamics, and the results (table 24) were compared with those obtained for MCM-22. In MCM-22, differences in diffusivity were observed between the isomers n-heptane and 2-methylhexane, especially in the sinusoidal 10-member ring system. Similar features of diffusivity were observed for super-cages, and no inter-cage diffusion was observed at any of the simulated temperatures (450 to 650K). This suggested that a much longer time-scale was required for inter-cage diffusion to be observed. Intracage diffusion was a temperature-activated process. In ITQ-2, diffusion in the super-cage system was studied using 2-methylhexane as a probe molecule. Large differences appeared with respect to MCM-22 because of opening of the super-cages to give chalice-type pores. The molecules had a tendency to penetrate into the chalice, but subsequent out-diffusion was not restrained by the 10-member ring windows interconnecting the super-cages. These windows, present in MCM-22, did not exist in ITQ-2. This allowed 2-methylhexane to escape from the 12-member ring system more easily than in the case of MCM-22.

Molecular Dynamics of C7 Hydrocarbon Diffusion in ITQ-2. the Benefit of Zeolite Structures Containing Accessible Pockets. G.Sastre, C.R.A.Catlow, A.Chica, A.Corma: Journal of Physical Chemistry B, 2000, 104[3], 416-22

Table 24

Diffusivities in MCM-22 Zeolite