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Homogeneous and biphasic cellulose acetate/room temperature ionic liquid membranes for gas separations: Solvent and phase-inversion casting vs. supported ionic liquid membranes

Khakpay, Amir, Scovazzo, Paul, Nouranian, Sasan
Journal of membrane science 2019 v.589 pp. 117228
ambient temperature, carbon dioxide, cellulose acetate, ionic liquids, liquid membranes, methane, models, polymers, thiocyanates
Standard casting methods, using polymer/RTIL solutions, can produce membranes with improved stability. We evaluated the performance of RTIL-membranes cast using either solvent casting (homogeneous polymer/RTIL film) or phase-inversion (biphasic polymer/RTIL films). The study used a model casting polymer, cellulose acetate (CA), and the RTIL 1-ethyl-3-methylimidazolium thiocyanate, ([emim][SCN]). The gas separation performances of the resulting homogeneous and biphasic CA/[emim][SCN] membranes were compared with a conventional supported ionic liquid membrane (SILM) of [emim][SCN]. The evaluations of CO2 removal from CH4 supported our hypotheses that the biphasic phase-inversion cast membranes have higher selectivities and membrane stability compared to the homogeneous film membranes. While the homogeneous membranes had the highest gas permeances, the biphasic membranes had three-fold higher selectivities. The highest CO2/CH4 mixed-gas selectivity reported was 89 ± 12 for the biphasic membrane. It appears that selectivity in cast membranes is a function of free RTIL content. The order of membrane stability vs. cross-membrane pressure was biphasic ≈ SILM > homogeneous, with the biphasic breakthrough pressure being two-fold higher than the homogeneous membrane's. Therefore, the phase-inversion casting method (biphasic membranes) is a viable alternative to SILM membranes for fabricating RTIL-membranes. Infrared spectra and atomic force micrographs characterized the homogeneous and biphasic membrane morphologies.