| Membrane-based gas separation has shown advantages for a large variety of applications,e.g.,low energy cost,high efficiency and simple operation,but suffers from the problem of permeability-selectivity trade-off.The emerging 2D material-based membranes,which possess the merits of traditional membranes,yet solve the permeability-selectivity trade-off problem.Very recently,2D lamellar membranes were fabricated experimentally with an emerging 2D material,MXene.It showed unprecedented gas separation performance and its great application potential in the field of gas separation.In this paper,the molecular simulations are employed to investigate the process of adsorption,diffusion and separation of gas molecules in 2D MXene membrane.Firstly,the Grand Canonical Monte Carlo simulations are employed to investigate the adsorption of pure gas molecules H2,CH4,CO2 and N2 in MXene sheets,and study the effects of pressure,temperature,layer spacing and functional groups on gas adsorption.The simulation results show that the increasement of pressure enhances gas adsorption.The interlayer spacing has a great influence on gas adsorption,at small layer spacing,the increasement of the interlayer spacing reduces the adsorption quantity of gas,but the effect weakens at large layer spacing.The increasement of temperature reduces the adsorption quantity of gas.Finally,the composition of functional groups has little effect on the adsorption quantity of gas,which can be neglected.Then,the diffusion behavior of five different gases H2,He,CH4,CO2 and N2 in MXene nanochannels is studied by means of all-atom molecular dynamics simulation.The structural factors of MXene lamellar membranes such as layer spacing,interlayer water molecules and so on are fully considered.The results show that the size,mass and polarity of the gas molecules,as well as the layer spacing of the MXene sheets and the interlayer water have a great influence on the gas diffusion,and thus lead to different diffusion mechanisms and diffusion coefficients.In the design of lamellar membranes,these factors can be utilized to overcome the permeability-selective trade-off of conventional separation membranes.Finally,all-atom molecular dynamics simulations are performed to investigate the separation of H2/CH4 and H2/N2 gases in MXene lamellar membranes,and study the effects of the interlayer spacing,water content,channel length,functional group,molecular size,pressure,and temperature on gas permeation in MXenes 2D channels.The simulation results indicate that the mechanism of gas diffusion varies at different levels of interlayer spacing,the configuration diffusion and the Knudsen diffusion occur at small and large layer spacing,respectively,and the former of which leads to higher selectivity of the gas pair.Interlayer water molecules scatter the gas diffusion,and increase the stroke of gas molecules across the membrane,thereby reducing the flux of gas molecules and increasing selectivity.The increased channel length strengthens the effect of membrane walls on gas molecules,thus expands the difference in flux between the CH4,N2 and H2 gas molecules,and therefore increases the selectivity while reducing the flux of each gas molecule.The functional groups have less influence on gas permeation.The increasement in pressure increases the flux of gas molecules,but the effect on selectivity is weak.When the temperature is lowered,the gas flux is reduced by a small amount,while the selectivity of gas separation is somewhat improved.Therefore,the lowering the temperature is beneficial to the operation of gas separation. |
