Diffusion and separation of CO2 and CH4 in silicalite, C168 schwarzite,and IRMOF-1:: A comparative study from molecular dynamics simulation

Recently we have investigated the storage and adsorption selectivity Of CO2 and CH4 in three different classes of nanoporous materials-silicalite, IRMOF-1, and C-168 schwarzite through Monte Carlo simulation (Babarao, R.; Hu, Z.: Jiang, J. Langmuir, 2007, 23, 659). In this work, the self-, corrected, and transport diffusivities Of CO2 and CH4 in these materials are examined using molecular dynamics simulation. The activation energies at infinite dilution are evaluated from the Arrhenius fits to the diffusivities at various temperatures. As loading increases, the self-diffusivities in the three frameworks decrease as a result of the steric hindrance; the corrected diffusivities remain nearly constant or decrease approximately linearly depending on the adsorbate and framework; and the transport diffusivities generally increase except for CO2 in IRMOF-1. The correlation effects are identified to reduce from MFI, C-168 to IRMOF-1, in accordance with the porosity increasing in the three frameworks. Predictions of self-, corrected, and transport diffusivities for pure CO2 and CH4 from the Maxwell-Stefan formulation match the simulation results well. In a CO2/CH4 mixture, the self-diffusivities decreases with loading, and good agreement is found between simulated and predicted results. On the basis of the adsorption and self-diffusivity in the mixture, the permselectivity is found to be marginal in IRMOF-1, slightly enhanced in MFI, and greatest in C-168 schwarzite. Although IRMOF-1 has the largest storage capacity for CH4 and CO, its selectivity is not satisfactory.