Mass transport of O2 and N2 in nanoporous carbon (C168 schwarzite) using a quantum mechanical force field and molecular dynamics simulations

A hierarchical approach is used to calculate the single-component fluxes of N-2 and O-2 in nanoporous carbon molecular sieves (represented by C-168 schwarzite) over a wide range of pressures and pressure drops. The self- and corrected diffusivities are calculated using equilibrium molecular dynamics simulations with force fields for the gas-carbon interactions obtained from quantum mechanical calculations. These results are combined with previously reported adsorption isotherms of N-2 and O-2 in C-168 to obtain transport diffusivities and, by use of the Fick's equation of mass transport, to obtain single-component fluxes across the membrane. The diffusion coefficients and fluxes are also calculated using all empirical potential, which has been obtained by fitting low coverage adsorption data of N-2 and O-2 on a planar graphite sheet. By analyzing the diffusivities calculated with the ab initio potential in the limit of infinite dilution over the temperature range front 80 to 450 K, it is observed that the N-2/O-2 separation is energetically driven and a high selectivity of O-2 over N-2 call be obtained at low temperatures. However, with the empirical potential both the energetic and entropic contributions to selectivity were found to be close to unity. Similarly, by calculating single-component fluxes and ideal selectivities at 300 K and finite pressures it is found that the ab initio potential better explains the large O-2/N-2 selectivities of similarly sized molecules that have been observed experimentally. An interesting reversal in ideal selectivity is observed by adjusting the pressure at the two ends of the membrane. As a consequence, we predict that a highly selective kinetic separation in favor of either nitrogen or oxygen could be obtained with the same membrane depending on the operating conditions.