Separation of carbon dioxide and nitrogen gases through modified boron nitride nanosheets as a membrane: insights from molecular dynamics simulations

Boron nitride nanosheets (BNNSs) can be a suitable membrane for gas separation with high permeability and selectivity. Molecular dynamics (MD) simulations were performed to investigate the CO2/N-2 molecules separation using modified BNNSs. The simulated systems were comprised of modified BNNSs, CO2 and N-2 molecules. A series of BNNSs with different pores as the separation membrane of CO2/N-2 was designed. The selectivity and permeability of these molecules can be controlled by drilling various pores with different sizes and functionalized factors in the edge of the pores. The permeation mechanisms of the carbon dioxide and nitrogen molecules through the BNNS pores were different. Non-functionalized pores (pores 1 and 2) provided a high permeation rate and moderate selectivity for N2 over CO2 based on a simple size sieving mechanism. Modifying the pores by attaching functional groups to the boron and nitrogen atoms at the edge of the pores (pores 3 and 4) leads to very different outcomes. Using hydroxyl groups and hydrogen atoms leads to a substantial increase in the selectivity for N-2 over CO2 and using fluoride atoms actually inverts the selectivity, which is strongly in favour of CO2. When the pore size is further increased, selective separation of molecules does not happen and both molecules propagate through the pores. Due to the interactions between the molecules and membrane pores, the energy barrier for the two molecules in various pores was different. As a consequence, with a low energy barrier more gas molecules permeated the pore. If the energy barrier difference between the two types of molecules is high, a complete separation occurs. The results show that the separation of molecules using the BNNSs was dependent on the pore diameter of the BNNS and the functionalized group used in the edge of the pore. The present study is valuable for designing novel nanostructure membranes for gas separation.