Separation Efficiency of CO2 in Ionic Liquids/Poly(vinylidene fluoride) Composite Membrane: A Molecular Dynamics Study

Ionic liquids (ILs)/polymer composite membranes show great potential for CO2 separation. The main challenge is to select the appropriate combination of ILs and polymer in a limited time and cost. In this work, the microstructure, interactions and dynamic properties of a series of systems of 1-butyl-3-methylimidazolium hexafluorophosphate ([bmim]­[PF6]), 1-butyl-3-methylimidazolium bis­(trifluoromethylsulfonyl)­imide ([bmim]­[Tf2N]) and 1-butyl-3-methylimidazolium tetracyanoborate ([bmim]­[B­(CN)4] composited with poly­(vinylidene fluoride) (PVDF) with increasing ionic liquids (ILs) content were studied by molecular dynamics simulations for understanding the composite membrane at the molecular level. The radial distribution functions show that ILs aggregation is formed in the PVDF matrix, and the aggregation region continuously expands and finally becomes ionic channels with increased ILs content to 50 wt %. The weakest cations–anions interaction of [bmim]­[B­(CN)4] and the strongest interaction of PVDF-[B­(CN)4] as well as the weakest aggregation of [bmim]­[B­(CN)4] are favorable for forming the continuous ionic channels in PVDF matrix for CO2 diffusion. The increased self-diffusion coefficients of PVDF after the addition of ILs, which originates from the strong interaction of ILs-PVDF and the broken hydrogen bond network among PVDF chains, facilitate the transportation of CO2 among PVDF chains. Moreover, the free energy of solvation, Henry’s law constant and self-diffusion coefficients for CO2 in three ILs/PVDF systems suggest that [bmim]­[B­(CN)4]/PVDF composite membrane possesses better CO2 separation performance.