Structure and Dynamics of Carbon Dioxide, Nitrogen, Water, and Their Mixtures in Metal Organic Frameworks

The self-diffusion coefficients of carbon dioxide, nitrogen, and water in metal organic frameworks (IRMOF-1, Cu-BTC, and MIL-47) are calculated using molecualr dynamics simulations at various temperatures for pure gass and in mixtures. The structure of the adsorbates with respect to the metal active sites in the metal organic frameworks and their intermolecular structural features are investigated through radial distribution fuctions and related to the self-diffusivity behavior. It is found that while in IRMOF-1 the three species maintain their mobility when they are adsorbed as pure components or in mixutres, the diffusivities of CO2 and water in Cu-BTC are slower in the ternary mixture than when adsorbed individually, whereas the opposite behavior is observed in MIL-47 where the species diffuse faster in the mixture than as pure components. The behavior can be explained in terms of the strong interactions of water with the framework which slows down diffusion in Cu-BTC; however in MIL-47, the competition between CO2 and water for the active vanadium sites increase the mobilities of both adsorbates.