(185d) Nano-Confinement of Ionic Liquids for the Enhanced Performance of Gas Separation Membranes

Recently published evidence demonstrate that confined Ionic Liquids (ILs) have enhanced gas permeability and selectivity. However, a systematic membrane science investigation of this improvement has not been conducted; in part because of the lack of a stable, tunable platform for studying gas separations using confined ILs. We are at the start of a program to produce a biphasic membranes with the IL phase encapsulated in a continuous polymer phase eliminating capillary force destabilization and enhancing gas separation performance through confinement effects. Review of the literature found no clear consensus on the ramifications of IL confinement on gas separation performance. Literature includes some reports about the increase in CO₂ permeability and solubility in ILs due to confinement. These reports run the range of pore sizes from nanometer to micrometer. Some reports even comment that changing the chemistry of the confining material can increase the CO₂-permeabilities by a factor of 2, even when the confining pore lengths and IL are held constant. The literature verdict on the impact of confinement on selectivity is not as clear cut, with reports of selectivity improvements mixed in with reports of no improvement. In this paper we present a systematic study of the IL confinement impact on CO₂/CH₄ membrane separations by varying the polarity of the IL, the polarity of the confining polymer, and the confining pore diameter. The study combined both laboratory and molecular simulation experiments. For example, we produced [emim][SCN]-membranes with both hydrophilic and hydrophobic PVDF confinement. In the laboratory testing, the mixed gas selectivity improved with decreasing nominal pore size of the confinement. This improvement could come from an increase in membrane stability (increased capillary forces) or a confinement effect. The data are consistent with, but insufficient to prove, a confinement effect. However, the 100 nm confinement of the IL produced a selectivity of 73 that is high compared to the expected unconfined value of 50. Furthermore, combining this laboratory data with MD simulations provided a fundamental understanding of the confinement-architecture to aid in the interpretation of the solute transport through the confined ILs. Our MD data show changes in the IL atomic volumes near the PVDF pore wall and differences in the free energy of solvation of CO₂ in the PVDF-confined IL versus that of the bulk IL. The results of the MD-simulations further supports our findings and aids in the conclusions about how to optimize the impact of IL-confinement.