STUDY OF CARBON-DIOXIDE DIFFUSION IN ZEOLITES WITH ONE-DIMENSIONAL AND 3-DIMENSIONAL CHANNEL NETWORKS BY MD SIMULATIONS AND FR METHODS

Molecular dynamics (MD) simulations and frequency response (FR) method s have been used to study the diffusion of carbon dioxide in the 10-ri ng pore zeolites, theta-1 and silicalite-1, which have one- and three- dimensional channel networks, respectively. A good agreement on the di ffusivities in silicalite-1 at various coverges has been obtained betw een MD simulations, FR and PFG NMR methods. The diffusivities in theta -1 calculated from the MD simulations using one molecule were about on e order of magnitude faster than when two or more molecules were invol ved in the simulations. The diffusivities in theta-1 calculated from t he MD simulations when more than one molecule was used in the simulati ons were in reasonable agreement with those determined by the FR metho d. For CO2 diffusion in theta-1 the MD simulations showed a transition from normal to single-file diffusion when more than one molecule were involved, while the FR measurements showed a surface resistance at po re entrances due to single-file diffusion, but these effects were not observed in silicalite-1.