CO2/CH4 mixed-gas separation through carbon nitride membrane: A molecular dynamics simulation

We performed molecular dynamics simulations to study the performance of a pristine carbon nitride (C2N) membrane for separation of CO2/CH4 gas mixture. The influences of system temperature (298, 323, and 348 K), applied pressure difference (up to 2000 kPa), and feed composition on the CO2/CH4 separation were also explored. The permselectivity of the C2N membrane for separation of CO2/CH4 mixture was theoretically determined. The highest CO2 permeance of 1.32 x 106 GPU in the absence of applied external pressure was achieved. It is demonstrated that CO2 molecules can easily diffuse through the membrane, while no CH4 molecule was seen on the permeate side. Thus, an ultrahigh selectivity of CO2 over CH4 was achieved. The membrane indicated a superior performance, which easily exceeds the Robeson's upper bound. The investigation of the gas permeation mechanism revealed that the preferential adsorption of CO2 and suitable size of this molecule owing to proper orientation are two important factors for efficient separation of CO2 from CH4 by the C2N membrane. In fact, CO2 molecules tend to reorient themselves close to the membrane nanopores to diffuse perpendicularly through. To this end, the density map and the potential of the mean force analysis were also conducted, which confirmed the simulation results.