Insight into CO2/CH4 separation by ionic liquids confined in MXene membrane from molecular level

Abstract

Composite membranes incorporating ionic liquids (ILs) within MXene demonstrate promising potential for CO2 separation. However, studies on the separation of CO2/CH4 using MXene-confined ILs membranes are limited, especially in terms of understanding the mechanisms at the molecular level. In this work, the system of CO2/CH4 in MXene-confined ILs membranes was studied by molecular dynamic simulations. The number density results reveal that MXene stratifies the ILs between the layers, with higher concentrations of ILs near MXene and lower concentrations in the middle layer. Notably, MXene has a greater impact on cations distribution compared to anions. As the layer spacing of MXene expands from 1.5 to 3 nm, the interaction between MXene and IL weakens, while that between the cations and anions strengthens. The confined ILs enhance gas solubility capability but impede gas diffusion. CO2 is distributed closer to anions, while CH4 tends to be closer to cations, with the distance between CH4 and cations decreasing as the layer spacing increases. Additionally, with the increase of layer distance, the proportion of confined ILs gradually decreases, and the gas diffusion coefficient gradually increases. Furthermore, compared to 1-Ethyl-3-methylimidazolium tetrafluoroborate ([EMIM][BF4]) and 1-Ethyl-3-methylimidazolium hexafluorophosphate ([EMIM][PF6]), MXene-confined 1-Ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([EMIM][TF2N]) is identified as the most effective for CO2/CH4 separation, owing to its superior CO2 solubility and highest diffusion selectivity.