Direct Simulation of Ternary Mixture Separation in a ZIF-8 Membrane at Molecular Scale

Separation of H-2 in a ZIF-8 membrane from a syngas mixture composed of CO2, H-2, and N-2 at 300 K and 35 atm is simulated with the concentration gradient driven molecular dynamics (CGD-MD) method. Steady-state fluxes are computed to predict the H-2 selectivity of the ZIF-8 membrane using four different flexible force fields developed for ZIF-8. The permselectivities predicted by the CGD-MD method are compared with those obtained from the grand canonical Monte Carlo+equilibrium molecular dynamics (GCMC+EMD) approach, which is based on the solution-diffusion model and widely used to predict permselectivities for mixture separations. The permselectivities obtained by using the CGD-MD method accurately predict that ZIF-8 is H-2 selective over CO2 and N-2. On the other hand, permselectivities predicted with the GCMC+EMD approach are found to be incorrect, that is, ZIF-8 not selective for H-2. The study suggests that a reliable non-equilibrium molecular dynamics approach should be employed in order to obtain accurate predictions for the permselectivity of a membrane for a multicomponent mixture separation process which happens at moderate or high pressure conditions.