Ionic-liquid-gated porous graphene membranes for efficient CO2/CH4 separation

Abstract

Ion-gated graphene membranes coated with monolayer ionic liquids (ILs) on porous graphene have shown good separation performance. However, it is necessary to match the IL with the appropriate pore size of porous graphene for practical applications. In this study, we investigated the effects of different ILs and graphene pore sizes on the CO2/CH4 separation performance using non-equilibrium molecular dynamics simulations. When the same anion was used as a gating ion, the different cation sizes influenced the separation performance of the membrane. The selectivity of [BMIM][BF4] for CO2 reached 104.2, which was the highest among the [EMIM][BF4], [BMIM][BF4], and [HMIM][BF4] systems for 6 Å pores. With increasing pore size, larger anions were required as the gating ions. [EMIM][PF6] was found to be a suitable gating ion for 7 Å pores and led to higher selectivity and CO2 membrane permeability. We also studied the effects of the number of IL molecules and the charges of the pore atoms for [EMIM][PF6] with 7 Å pore systems. The results showed that increasing the charge of the pore atoms could significantly improve the selectivity for CO2, reaching as high as 231.82. Although increasing the amount of IL molecules can enhance selectivity, it may decrease the CO2 membrane permeability. Further, non-spherical symmetric anion gating was studied. When [EMIM][NTF2] was coated on porous graphene with 6 Å pores, neither CO2 nor CH4 could pass through the membrane pores. For 7 Å pores, the CO2 separation selectivity reached 88, and the permeation number of CO2 was 22. [EMIM][PF6] and [EMIM][NTF2] were determined to be unsuitable gating ions for 8.5 Å pores. They showed poor selectivity and significant permeation of both CO2 and CH4 for porous graphene with 8.5 Å pores.