Molecular dynamics simulations involving a flexible framework were used to simulate the diffusion of o- and p-xylene in purely siliceous CIT-1. The simulations were performed at 500K, investigating two loadings (corresponding to 1 or 0.25mol/unit-cell) for the ortho isomer and one loading (0.25mol/unit-cell) for the para isomer. In the former system, the diffusion coefficient decreased from 7.79 x 10-6cm2/s for the lower, to 3.56 x 10-6cm2/s for the higher loading (table 15). The diffusion coefficient for the para isomer (25.18 x 10-6cm2/s) was substantially greater. Graphical analyses revealed a jump diffusion mechanism which, in the case of the ortho isomer, took place in the 12 MR channels of the structure while, for the para isomer, incursions into the 10 MR channel were observed. The results of the molecular dynamics simulations were complemented by calculations of the activation energies for the diffusion of the two isomers in the two channels.

Selective Diffusion of C8 Aromatics in a 10 and 12 MR Zeolite. A Molecular Dynamics Study. G.Sastre, N.Raj, C.R.A.Catlow, R.Roque-Malherbe, A.Corma: Journal of Physical Chemistry B, 1998, 102[17], 3198–209

Table 14

Counter-Diffusion of 50:50 Xylene Mixtures in H-CIT-1