A Memory Diffusion Model For Molecular Anisotropic Diffusion In Siliceousβ-zeolite

In this paper, we have studied the molecular diffusion on nanoporous membranes usingmolecular dynamics simulation, specifically, the anisotropic diffusion behavior of sphericalmolecules CH₄、CF₄and SF₆in theβzeolite. By analyzing the simulation results, a memorydiffusion model of molecules inβzeolite has been proposed. In this model, the movement ofmolecules is regarded as jumping among different adsorption sites and the probability of eachjump depends not only on the site to which the molecule jumps but also on the site where themolecules come from. With the aid of this assumption, we have obtained an analytic formulaof correlating between diffusion coefficients along two principal axes.We attain the results as follows:（1） A model of molecular diffusion in theβzeolite is proposed basing on diffusiontheory.（2） To test this model, by using molecules dynamic simulations on the diffusion ofbenzene, methane, CF₄, and SF₆inβzeolite are performed under the conditions of differenttemperatures.（3） The simulation results of the molecules diffusion were compared with the theoreticalmodel show that: the model fits simulations well for benzene, whose interaction diameter isthe biggest. For the three different interaction radiuses of spherical molecules, SF6molecule isquantificationally consistent with the model, but fails for CH₄and CF₄. This indicates that themodel requires the molecular size to be large enough.In conclusion, the anisotropic diffusion of spherical molecules in the pores ofβzeolitewas investigated by molecular dynamics simulation. These provide new ideas and methods tomeasure the physical, chemical phenomena and processes which use the modern experimentalmethods to study and to explore the physical and chemical mechanism are difficult.