Molecular dynamics simulation of small gas molecule permeation through CAU-1 membrane

CAU-1 is one of aluminum-based amine-functionalized Metal-Organic Frameworks(MOFs).Gas permeation and separation behaviors through CAU-1 membrane were simulated by the dual-control plane nonequilibrium molecular dynamics(DCP-NEMD)method.The thickness of membrane was 3.55 nm.Gases CO_(2),N_(2),CH_(4),H_(2),He,Kr and Xe were chosen for the calculation in both single component and binary mixtures.The permeation process was calculated in grand canonical(μVT)ensemble with periodic boundary conditions(PBC)in x-and y-directions at different temperatures.The calculated permeance of H_(2),CH_(4),N_(2),CO_(2) and Kr decreased with increasing temperature in both single and binary system,while that of Xe with kinetic molecule of 0.41 nm increased with increasing temperature.It shows Xe permeation is governed by activated diffusion.The simulated separation factors of CO_(2)/N_(2) and CO_(2)/CH_(4) of 4.2 and 1.3 respectively were lower than the experimental ones when only considering van der Waals interaction.Further consideration of electrostatic potential leads to improved calculation CO_(2)/N_(2) and CO_(2)/CH_(4) separation factor of 23.0 and 12.9 respectively that were consistent with the experimental ones of 26.2 and 14.8.It suggests the necessity of considering the Coulomb interactions between CO_(2) and NH_(2)-on the pore wall of CAU-1 for permeation of CO_(2).For H_(2)/N_(2) and H_(2)/CH_(4) the ideal selectivities also keep consistent with our experimental results.Interestingly,the simulated separation factor for noble Kr/Xe reaches infinite,predicting that CAU-1 membrane possesses potential separation properties for radioactive Kr/Xe.