The diffusion of a binary mixture of o- and p-xylene in purely siliceous CIT-1, a microporous solid with two channel systems of 10- and 12-membered rings, was simulated. Diffusion of the ortho isomers revealed a non-linear mean square displacement plot due to geometrical limitations imposed by appreciable interactions of o-xylene molecules with 10-member ring channels. Para-xylene followed long diffusion paths, via the 10-member ring channels, as a result of the impossibility of rotation in the narrower channels; which largely contributed to the increase in the self-diffusivity of this isomer with respect to o-xylene. The o-xylene self-diffusivity in the mixture was lower than in the pure system, while the para-xylene self-diffusivity was increased in the mixture with respect to the pure system. The differing mobilities of the isomers explained the differences in self-diffusivities in binary, as compared to pure, systems.

Diffusion of a Para- and Ortho-Xylene Mixture in CIT-1 Zeolite: a Molecular Dynamics Study. Sastre, G., Corma, A., Catlow, C.R.A.: Topics in Catalysis, 1999, 9[3-4], 215-24