Insight into CO2/CH4 separation performance in ionic liquids/polymer membrane from molecular dynamics simulation

Abstract

Owing to the advantages of both ionic liquids (ILs) and polymer membranes, ILs/polymer hybrid membranes are becoming one of the research hotspots in the CO2 separation. However, the lack of molecular insight into the CO2 separation mechanism restricts the development of ILs/polymer hybrid membranes. Herein, the separation process of CO2/CH4 through the membrane composed by poly(vinylidene fluoride) (PVDF) with 1-butyl-3-methylimidazolium hexafluorophosphate ([Bmim][PF6]) and 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([Bmim][NTf2]) were studied by molecular dynamics simulations. Structure analysis indicates that PVDF chains tend to aggregate on the surface with ILs being confined inside the membrane, but the ILs within the interfacial regions can dramatically weaken the crystallinity of PVDF and improve the flexibility of PVDF chains, thus benefiting the gas permeation and diffusion. In addition, the calculated results of the permeation selectivity, the solubility selectivity and the diffusion selectivity confirm that [Bmim][NTf2]/PVDF membrane shows better performance in CO2/CH4 separation than [Bmim][PF6]/PVDF membrane, and the solubility selectivity plays a dominant role. The high selectivity in the hybrid membranes is due to the stronger interaction between CO2 and ILs/PVDF compared to that for CH4. Detailed analysis on the CO2 transport process suggests that the driving force for CO2 penetrating onto the interface is about −4.11 kJ·mol−1, and most of CO2 at the interface are nearly parallel to the membrane. Moreover, the selectivity of CO2/CH4 increases as the thickness of membranes decreases.