(59f) Transport Properties of Methane Confined in Nanoporous Carbons

Nanoporous carbons are used widely in industry as adsorbents for gas- and liquid-phase separations due to their excellent surface properties, which arise from their high specific surface area, porosity and nano-scale pores.  Although significant efforts have been made to optimize the adsorptive properties of these materials for separations, the transport properties of guest phases confined within nanoporous carbons have received less attention.  Using molecular simulation techniques, we have systematically examined the diffusive behavior of confined methane molecules inside a model nanoporous carbon in order to understand how the behavior is influenced by structural characteristics such as pore size, tortuosity, and pore connectivity. We report results from our study, beginning with our analysis of high pressure adsorption isotherms of methane calculated using Grand Canonical Monte Carlo simulations.  Self, collective and transport diffusion coefficients calculated using microcanonical molecular dynamics are also reported at different state conditions along the isotherms.  We find that these transport coefficients have a non-monotonic dependence on the loading and bulk pressure, indicating that the rate of transport is significantly altered by the state conditions. An analysis of first passage and mean survival times is also presented in order to examine how methane molecules are transported between spatially segregated regions within the heterogeneous porous network.  Finally, we show that there are significant differences in these properties depending on whether methane is described using a united-atom or all-atom force field,  suggesting that the latter is more appropriate for studying methane in confinement.