Efficient separation of CO2/CH4 by ionic liquids confined in graphene oxide: A molecular dynamics simulation

Ionic liquids (ILs)/graphene oxide (GO) membranes have been regarded as a prospective alternative in CO2 separation. However, the correlation between the microstructure of ILs/GO and CO2 separation performance is unclear. In this work, the dynamic properties and interactions for CO2/CH4 in 1-butyl-3-methylimidazolium hexafluorophosphate ([Bmim][PF6]), 1-butyl-3-methylimidazolium tetrafluoroborate ([Bmim][BF4]) and 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([Bmim][TF2N]) confined between two GO sheets with different layer spacings were studied using molecular dynamics simulation. The number density and angular orientation of cations suggest that there is a dense cation adsorption layer near GO with the imidazole rings and alkyl side chains mostly paralleled to GO surface. The strong interaction of GO-cations weakens the interaction of cations-anions, facilitating the adsorption of CO2. The overlapping distribution regions of the number density of CO2/CH4 with the cations and anions reveal that CO2/CH4 mainly distribute around the ILs area of the ILs/GO membrane, which improves the gas selectivity. The RDFs results of CO2/CH4-ILs indicate that the confined ILs/GO system is more favorable for capturing gases than bulk ILs. The stronger interaction of CO2-anions/cations and the faster diffusion of CO2 than CH4 reflect the high solubility selectivity and diffusion selectivity for CO2/CH4 in ILs/GO membrane. In addition, it was found that decreasing the layer spacing would increase the solubility selectivity, but could decrease the diffusion selectivity. However, solubility selectivity plays a dominant role, thus 2 nm is the optimal layer spacing. Furthermore, the low viscosity ILs were found to be beneficial to improve the diffusion selectivity. Finally, [Bmim][TF2N]/GO membrane is predicted to possess superior CO2/CH4 separation performance than [Bmim][PF6]/GO and [Bmim][BF4]/GO membranes.