Effect of Inorganic Salt Blending on the CO2 Separation Performance and Morphology of Pebax1657/Ionic Liquid Gel Membranes

Gel membranes comprising inorganic salt, molten salt or ionic liquid (IL), and Pebax1657 were prepared by blending the three components in ethanol/water mixture, and their carbon dioxide separation properties were evaluated. To probe the effects of inorganic salt types and concentrations, a small amount of common inorganic salts, such as sodium chloride (NaCl), calcium chloride (CaCl2), and sodium sulfate (Na2SO4) were first doped into neat Pebax membranes. The thermal properties, surface chemistry, and gas separation performance of the free-standing membranes were evaluated using differential scanning calorimetry, Fourier-transform infrared spectroscopy, and gas permeation tests. CaCl2-doped membranes gave the best performance due to the high water content and cross-linking between the salt cation and polyamide segments, which resulted in lower polymer crystallinity. Further incorporation of 1-ethyl-3-methylimidazolium tetrafluoroborate IL into Pebax1657/inorganic salt mixture to form thin film composite gel membranes improved the inorganic salt solubility and enhanced CO2 permeance up to 899 GPU at the expense of CO2/N-2 selectivity and mechanical strength. Cyclic permeation tests using humidified feed containing a trace amount of contaminant showed stable performance although no improvement in gas pair selectivity was observed, confirming solution-diffusion mechanism through the gel membranes.