High ionic liquid content polymeric gel membranes: Correlation of membrane structure with gas and vapour transport properties

In this paper the transport properties of ionic liquid polymeric gel membranes, containing up to 80 wt.% of the ionic liquid 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([EMIM][TFSI]) in poly(vinylidene fluoride-co-hexafluoropropylene) (p(VDF-HFP)), are investigated. Gas permeability significantly increases in the presence of [EMIM][TFSI], especially for CO2. This suggests a potential application in gas separation membranes, for instance for natural gas sweetening and for CO2 sequestration from flue gas. A correlation of the transport properties with Young's modulus is proposed for the first time. It reveals a transition from diffusion-controlled transport to solubility-controlled transport with increasing IL content in the membrane. Vapour permeation experiments with the most permeable membrane containing 80 wt.% of ionic liquid were carried out. Permeability, diffusion and solubility coefficients were correlated with molecular kinetic diameter or critical temperature and volume, respectively. These correlations show clear and distinct trends for water, alcohols, linear and cyclic hydrocarbons. Polar compounds (linear C-1-C-4 alcohols, water) are more permeable and more sorbing than the corresponding hydrocarbons. The two classes show the opposite trends in permeability as a function of the critical volume, with an increase for the alkanes and a decrease for the alcohols. The same trends are observed for the solubility as a function of the critical temperature, evidencing that at such high IL concentrations in the membrane the mass transport is solubility controlled. On the other hand, diffusion is mainly correlated with the critical volume and the molecular size, regardless the chemical nature of the permeants. Significant differences in the transport of toluene and cyclohexane suggest that these membranes are also suitable for the separation of alkanes and aromatics. The manuscript intends to give fundamental information on how the presence of IL influences the transport properties of polymeric membranes. (c) 2012 Elsevier B.V. All rights reserved.