Hernández, Eduardo Rafael Robert; (1994) Computational studies of diffusion in zeolites. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

In this thesis the subject of molecular self-diffusion in zeolites is addressed using the Molecular Dynamics simulation technique. We have focused our attention on the phenomenon of the intra-crystalline diffusion of linear hydrocarbon molecules in the zeolite Silicalite, and, to a lesser extent, also in the zeolite Chabazite. Two different models have been considered. In the first we included the full mobility of the lattice, and no restrictions were imposed on the molecular degrees of freedom. Simulations of ethane, propane, n-butane and n-hexane in silicalite show a clear trend of decrease in the diffusion coefficients as the molecular size is increased. It was also seen that the diffusion takes place by means of a hopping mechanism. However, due to the high computational cost of these simulations, other trends such as the dependence of the diffusivity on the adsorbate loading and system temperature could not be established without longer production runs. Therefore a second set of simulations was performed using a simpler model, which did not include the framework mobility. This allowed us to undertake long (1000 ps) simulations on bigger systems, thus increasing the accuracy of the resulting statistical averages, n-butane and n-hexane were simulated in Silicalite under a wide variety of conditions of sorbate loading and temperature. The trend of the diffusion with increasing loading could then be clearly established, and activation energies in good agreement with experimental values were obtained. Also, by comparing trajectories in the rigid and mobile framework models, it was possible to extract a number of conclusions on the role played by the framework dynamics in the diffusion process. A general Purpose FORTRAN Molecular Dynamics program, FUNGUS, was developed to perform the simulations that are described in this thesis. This program is based on a previously existing code, and was written in collaboration with G.E. Mills. Its generality allow it to be used in simulations of systems as varied as fast ionic conductors, polymer electrolytes and zeolites. FUNGUS is now being used by a number of groups worldwide, a fact that testifies to its wide applicability.

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