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.

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Molecular dynamics simulations have been performed to study the diffusion of octane in silicalite. When hydrocarbons that are larger than C6 diffuse through this zeolite, the length of the molecule is such that diffusion through the sinusoidal channels becomes difficult. We have investigated the relative diffusivity through each channel and give an interpretation of the effect of temperature on this process. Simulations at 300 K show greater diffusivity in the sinusoidal channels, whereas at 450 K the straight channels show higher diffusion rates. When the temperature increases from 300 to 450 K, the diffusion coefficient in the straight channels increases by a factor of 5.2, whereas the coefficient in the sinusoidal channels increases by only 1.3. The trajectory plots also show larger diffusion paths through the straight channels at higher temperatures.

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