Mechanistic Investigation Of CO₂/N₂ Separation Based On Graphene-based Membrane

Graphene-based membrane acts as a high throughput and energy efficient gas separation membrane,providing a simple and efficient solution for CO₂/N₂ separation.However,the mechanism of CO₂/N₂ separation by graphene-based membranes is not yet clear.Since experimental measurements are insufficient to investigate the effect of graphene membrane microstructure on gas transport,it has become an urgent scientific problem to investigate the mechanism of gas separation by graphene membranes at the molecular level.Molecular dynamics（MD）Simulation is an effective tool for describing molecular interactions and molecular trajectories.In this paper,we will investigate the influence of the microstructure of graphene selective layer on the selective gas permeation and reveal the gas separation mechanism with the support of molecular dynamics simulation.Based on actual graphene stacked membrane,two types of systems,the single-layer gap model and the channel model,are constructed to replicate the internal structure of graphene membrane.It has been found that gas molecules pass through graphene membrane gaps by both surface diffusion and direct transfer.Gases adsorbed to the graphene surface can cross the gap by surface diffusion with a comparable or greater flux compared to direct transfer.CO₂ relies on surface diffusion and N₂ on direct transfer,with the preferential adsorption of CO₂resulting from its stronger van der Waals interaction with the wall.For pure gases,enhanced van der Waals interactions between the wall and the gas increase adsorption,effectively enhancing surface diffusion and enhancing permeation.For CO₂/N₂ mixtures,the difference in adsorption capacity is an important reason for the separation of CO₂ and N₂.When the van der Waals interaction is enhanced to 3?kcal·mol^-1,CO₂ is confined to the graphene surface with reduced motility,but the surface diffusion flux of N₂ is effectively enhanced and the amount of permeated N₂ is increased,resulting in the effective separation of CO₂ and N₂mixtures.By constructing a graphene channel membrane model,the permeation behaviour of pure CO₂,pure N₂ and mixed gas in the channels were investigated,the effects of interlayer spacing and van der Waals interactions on CO₂/N₂ separation were explored.Only surface diffusion can occur in small size channels,and Knudsen diffusion appears when the channel size is further expanded.The channel is 6?and can be used as a molecular sieve,through which small molecules of CO₂ can pass.Gas transport within the channel is significantly influenced by the van der Waals interactions between the graphene channel walls and the gas.At the right size（12?）of the nanochannel,enhanced graphene wall-gas interactions will restrict CO₂ and will not impede N₂ transport,thus effectively separating a mixture of CO₂and N₂.This paper reveals the mechanism of CO₂/N₂ separation in graphene-based membranes,i.e.,the intermolecular interaction force between CO₂ and graphene walls is higher than that of N₂.Therefore,the intermolecular interaction force on the graphene wall can be enhanced to regulate the diffusion ability of the gas surface by using the difference in intermolecular interaction force between the gas and the wall,so that the membrane material has high permeability and high selectivity.