Phosphonium-based poly(Ionic liquid) membranes: The effect of cation alkyl chain length on light gas separation properties and Ionic conductivity

By Cowan, Matthew G.; Masuda, Miyuki; McDanel, William M.; Kohno, Yuki; Gin, Douglas L.; Noble, Richard D.
Published in Journal of Membrane Science NULL 2016

Abstract

Abstract Phosphonium poly(ionic liquid)s (PILs) have been studied as alternatives to more common ammonium and imidazolium {PILs} for potential transport and separation applications. This work characterizes the CO2, H2, N2, O2, CH4, and {C2H4} single-gas permeability, diffusivity, solubility, and selectivity of free-standing films of poly([(tri-n-alkyl)vinylbenzylphosphonium][bis(trifluoromethylsulfonyl)imide]) {PILs} (i.e., poly([PnnnVB][Tf2N]) where n=4, 6, 8). The gas permeability was found to increase approximately linearly with increasing alkyl chain length on the phosphonium group. To our knowledge, the {CO2} permeability of 186 barrers observed for poly([P888VB][Tf2N]) is the highest reported for neat {PIL} materials. In contrast, gas selectivity was observed to decrease with an increase in phosphonium alkyl chain length from n=4 to n=6, then remain approximately constant between n=6 and n=8. Additionally, the ionic conductivity of these materials was observed to increase from ca. 10?8 to ca. 10?5 S cm?1 as the measurement temperature was increased from 25 to 105

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