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

The viscosities of ionic liquids (ILs) that chemically react with CO 2 and contain an aprotic heterocyclic anion (AHA) change very little after CO 2 absorption, whereas the viscosities of other kinds of reactive ILs increase dramatically after CO 2 absorption. This unique property has overcome a major problem with IL-based facilitated transport membranes (FTMs), namely, the low CO 2 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 ([P 4444 ][2-CNpyrr]) was studied in a FTM and it exhibited good CO 2 permeability and CO 2 /N 2 selectivity. [P 4444 ][2-CNpyrr] does not react as strongly with CO 2 as other reactive ILs, however, and its viscosity is still somewhat high. The CO 2 separation performance of an IL-based FTM is expected to be better if a lower viscosity IL is used that binds CO 2 more strongly. In this work, several AHA ILs were studied and their CO 2 reactivity and viscosity were calculated using molecular simulation. The IL triethyl(methoxymethyl)phosphonium pyrrolide ([P 222(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 CO 2 permeability than previously reported IL-FTM systems such as [P 4444 ][2-CNpyrr]).