Diffusion of a para- and ortho-xylene mixture in CIT-1 zeolite: a molecular dynamics study

Molecular dynamics techniques have been used to simulate the diffusion of a binary mixture of ortho- and para-xylene in the purely siliceous zeolite CIT-1, a microporous solid with two channel systems of 10 and 12 MR (membered rings). A loading of 0.125 molec/uc of each isomer was selected for the 200 ps simulation run at a temperature of 500 K. Diffusion of the ortho isomer shows a nonlinear MSD plot owing to geometrical restrictions caused by the appreciable interactions of ortho-xylene molecules with the 10 MR channels. Para-xylene molecules show long diffusion paths through the 10 MR channels due to the impossibility of rotation in these narrower channels which contributes significantly to the increase in the self-diffusivity of this isomer with respect to ortho-xylene. The ortho-xylene self-diffusivity in the mixture is lower than in the pure system, while the para-xylene self-diffusivity increases in the mixture with respect to the pure system. These differences in self-diffusivities in the binary compared to the pure systems are explained in terms of the different mobilities of the isomers.