Synthesis and properties of Polymeric ionic liquids (PILs) bearing hydrophilic PEO groups: Evaluation of gas and water vapor separation performance

Abstract New pyridinium based PILs containing PEO pendants were synthesized to be examined as CO2 and water selective membranes. A two-step synthetic procedure was followed including an N-methylation reaction of a precursor copolymer and a subsequent anion exchange metathesis to afford the corresponding PILs. The prepared PILs having different counter-anions form mechanically robust, flexible membranes, which exhibit high thermal stability and Tgs close to room temperature. It was examined the effect of anion nature on physicochemical as well as gas/water vapor separation properties. The conversion of the precursor copolymer to its corresponding PIL analogues led to Tg increase and thermal stability decrease of the latter. Surprisingly, the gas permeability for all PILs was decreased compared to precursor owing to the strong cation-anion interactions developed in the PIL structures which lead to a tighter polymer chain packing and consequently to gas diffusion hindering. The PIL containing the C(CN)3− anion (PIL750-C(CN)3) experiences the lowest CO2 permeability compared to TFSI− and MeSO4− which is accompanied by a very high CO2/CH4 selectivity value of 70 that is among the highest reported in the literature for PILs. This behavior can be attributed to its dense polymer chain packing and thus to its strong sieve-sieving ability associated with the diffusion-driven permeability decrease. Regarding water vapor permeability, it was evidenced that the PIL containing the MeSO4− anion (PIL750-MeSO4) showed the highest water vapor permeability (80,000 Barrer) compared to anions TFSI− and C(CN)3−, which is attributed to MeSO4− increased hydrophilicity and higher hydrogen bond basicity. The high water permeability is accompanied by a very high H2O/CO2 and H2O/N2 selectivity. The water vapor effect on gas permeation of PIL750-TFSI and PIL750-MeSO4 under process gas streams conditions using two different mixtures with compositions 3% H2O:14.5% CO2:82.5 %N2 and 3% H2O:48.5% CO2:48.5% CH4, respectively, was also investigated. The results demonstrate that gas permeability of all tested gases increased under humid conditions due to plasticization induced by water vapor. Accordingly, the separation factors decreased when compared with ideal selectivities in dry conditions for both PILs. However, PIL750-MeSO4 combines the high water vapor permeability with the high H2O/CO2 and HO2/N2 separator factors highlighting its potential to be used in flue gas dehydration and air dehydration.