On the study of diffusion in zeolites using molecular dynamics techniques.

摘要

Molecular dynamics (MD) simulation is a powerful tool for studying diffusion in zeolites. In MD, the equations of motion are integrated forward in time to track the motions of atoms in order to obtain dynamic information. Here we use this information to calculate equilibrium self diffusivities and nonequilibrium transport diffusivities. Better understanding of diffusion leads to improved application of zeolites in catalysis, separations, and novel membranes.; Binary mixtures of CF4 and n-alkanes in the zeolite faujasite were studied and their self diffusivities obtained. The scaling of self diffusivities with mixture compositions was not pronounced for the given systems. Structural information revealed that considerable clustering occurred in mixtures of CF4 and long alkanes. In addition to the equilibrium self diffusivities, the nonequilibrium Fickian transport diffusivities for binary mixtures of CF4 and methane in the zeolite faujasite were also calculated from MD simulations. The main and cross term diffusivities were explicitly calculated for a range of loadings and compositions. In general, Fickian transport diffusivities may be complicated to obtain experimentally and often simplifying assumptions are invoked. Molecular dynamics allows for the explicit calculation of both the main and cross term diffusivities without assumptions other than those in the model. The diffusivities from the MD simulations were used to predict flux across a perfectly crystalline faujasite membrane and the results demonstrate the important role of cross terms, especially at higher loadings.; An accelerated molecular dynamics (AMD) technique, based on transition state theory but without requiring knowledge of the transition states, was adapted to study the self diffusion of xenon, SF6, and ethane in the zeolite silicalite at low temperatures. Diffusion at low temperature takes place on time scales that cannot be accessed by conventional MD. The results of the AMD compare favorably with MD at high temperatures and with transition state theory at lower temperatures where MD is intractable. A method for applying AMD to arbitrary adsorbate-zeolite systems was formulated.