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

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

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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