Theoretical study of strain-controlled C2X (X = N, O) membrane for CO2/C2H2 separation

Membrane separation technology for CO2/C2H2 separation is challenging due to the same kinetic diameters and molecular shapes of these two molecules. In this paper, we reveal that strain-controlled C2X (X = N, O) membrane exhibits high CO2 permeability and high CO2/C2H2 selectivity. With the increasement of strain level, the CO2 permeability is dramatically improved, while the CO2/C2H2 selectivity is not decreased until the strain level surpassing 7.5%. Then the CO2/C2H2 separation mechanism is systematically discussed. It is found that when gas molecules approach the pores of C2X membrane, CO2 molecules interact with C2X membrane attractively while C2H2 molecules interact with C2X membrane repulsively, owing to the electropositivity of C atoms in CO2 molecules, electronegativity of C atoms in C2H2 molecules and electronegativity of X atoms in C2X membrane. In addition, the charge transferring from C2X membrane to CO2 molecule is less than that to C2H2 molecule, resulting in a lower energy barrier for CO2 molecule passing through C2X membrane.