Molecular Design of High CO2 Reactivity and Low Viscosity Ionic Liquids for CO2 Separative Facilitated Transport Membranes

Abstract

The viscosities of ionic liquids (ILs) that chemically react with CO2 and contain an aprotic heterocyclic anion (AHA) change very little after CO2 absorption, whereas the viscosities of other kinds of reactive ILs increase dramatically after CO2 absorption. This unique property has overcome a major problem with IL-based facilitated transport membranes (FTMs), namely, the low CO2 diffusivity caused by the extremely high liquid viscosity. This problem is especially severe at low temperature. In our preliminary experiments, the AHA IL tetrabutylphosphonium 2-cyanopyrrolide ([P4444][2-CNpyrr]) was studied in a FTM and it exhibited good CO2 permeability and CO2/N2 selectivity. [P4444][2-CNpyrr] does not react as strongly with CO2 as other reactive ILs, however, and its viscosity is still somewhat high. The CO2 separation performance of an IL-based FTM is expected to be better if a lower viscosity IL is used that binds CO2 more strongly. In this work, several AHA ILs were studied and their CO2 reactivity and viscosity were calculated using molecular simulation. The IL triethyl(methoxymethyl)phosphonium pyrrolide ([P222(1o1)][pyrr]) was predicted to have the highest reactivity and the lowest viscosity of the investigated AHA ILs, suggesting that its use in a FTM will lead to much higher CO2 permeability than previously reported IL-FTM systems such as [P4444][2-CNpyrr]).