A "coarse-grained" model based on a cellular automaton for the study of diffusion in microporous materials

It is well-known that in molecular liquids diffusion coefficients are in the range 10-9- 10-8m2s-1, while in microporous materials such as zeolites this range is much larger, spanning from 10-8m2s-1(for example methane in silicalite-1) to 10-19m2s-1(for example benzene in zeolite Ca Y) or even smaller. The phenomenon is made complex if the concentration (or loading) is considered: high loadings make the diffusion coefficient smaller and smaller, following different trends. Classical Molecular Dynamics (MD) computer simulation technique made it possible to study diffusion of small molecules in zeolites for times not exceeding some tens of nanoseconds and for systems including not more than 10000 particles. These limitations stimulated the interest to develop models able to reach mesoscopic scales through acoarse-grainingprocess of atomic scale models. Actually the coarse-graining scheme usually involves the evaluation of the free energy of an adsorbed molecules using detailed atomistic models to characterize adsorption sites and the possible presence of energy barriers to diffusion, and to use these results as an input for a lattice model of the considered system. This permits to study a much larger system (with a large number of adsorption sites) for a much larger time, because the calculation is greatly simplified as the integration of equations of motion is avoided.