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

Abstract

Separation of H 2 in a ZIF-8 membrane from a syngas mixture composed of CO 2 , 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 CO 2 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.

Recent advances in simulating gas permeation through MOF membranes

“…It is important to note that since GCMC + EMD approach assumes that the molar volume of the fluid in the membrane phase is the same with the one exists in the bulk phase, this method is more reliable when different gases in the mixture do not have a strong interaction with each other at low pressures. 65 Single-component self-diffusivities ( D 0 i , D 0 j ) of gases i and j are then calculated by performing equilibrium molecular dynamics (EMD) simulations at infinite dilution. Gas-gas intermolecular interactions are generally switched-off in EMD simulations to mimic infinite dilution.…”

“…Namsani et al 65 developed a new NEMD simulation method, concentration gradient driven molecular dynamics (CGD-MD), 84 to investigate H 2 /N 2 and H 2 /CO 2 separation performances of ZIF-8. Both GCMC + EMD and CGD-MD methods were applied by using four different force fields for ZIF-8.…”

Bridging the gap between mesoscopic and molecular models of solid/liquid interfaces out-of-equilibrium

“…An alternative approach, known as Constant Chemical Potential Molecular Dynamics (CµMD), was recently introduced to overcome these limitations that mimics open-boundary, constant composition, out-of-equilibrium conditions. [36][37][38][39][40][41][42] CµMD, similarly to adaptive resolution simulation methods [43][44][45][46] , introduces an internal reservoir of molecules in the fluid phase, and applies ad-hoc forces to regulate the flux to/from the reservoir and the 'bulk' solution in contact with the interface. This technique has already proven to be very effective to understand the molecular-scale processes associated with crystallization, surface adsorption and permeation, and its application is not limited by the density of the fluid phase.…”

Adsorption of Aromatics in MFI-Type Zeolites: Experiments and Framework Flexibility in Monte Carlo Simulations

“…Framework flexibility has also been studied in other porous materials. − Witman et al studied the effect of framework flexibility on the separation of Xe/Kr mixtures in ∼3000 metal–organic frameworks (MOFs). A model that predicts the Henry regime adsorption of each framework and selectivity as a function of framework flexibility is used.…”