Selective Diffusion of C8 Aromatics in a 10 and 12 MR Zeolite. A Molecular Dynamics Study

Molecular dynamics simulations employing a flexible framework are used to simulate the diffusion of o- and p-xylene in purely siliceous zeolite CIT-1. The simulations are performed at 500 K, investigating two loadings (corresponding to 1 molecule/unit cell and 0.25 molecule/unit cell) for the ortho isomer and one loading (0.25 molecule/unit cell) for the para isomer. For the former system the diffusion coefficient decreases from 7.79 × 10-6 cm2/s for the lower to 3.56 × 10-6 cm2/s for the higher loading. The diffusion coefficient for the para isomer (25.18 × 10-6 cm2/s) is substantially greater. Graphical analyses reveal a jump diffusion mechanism, which in the case of the ortho isomer takes place in the 12 MR channels of the structure, while for the para isomer, incursions into the 10 MR channel are observed. The results of the MD simulations are complemented and reinforced by calculations of the activation energies for the diffusion of the two isomers in the two channels. Diffusivity measurements of both isomers in B-CIT-1 (Si/Al = 35) by FTIR have also been carried out in order to compare the values obtained theoretically and experimentally.