Ionic Liquid Mixtures for Direct Air Capture: High CO Permeation Driven by Superior CO Absorption with Lower Absolute Enthalpy.

This paper reports a series of liquid materials suitable for use as high-performance separation membranes in direct air capture. Upon mixing two ionic liquids (ILs), namely -(2-aminoethyl)ethanolamine-based IL ([AEEA][X]) and 1-ethyl-3-methylimidazolium acetate ([emim][AcO]), the resulting mixtures with a specific range of their composition showed higher CO absorption rates, larger CO solubilities, and lower absolute enthalpies of CO absorption compared to those of single ILs. NMR spectroscopy of the IL mixture after exposure to CO allowed elucidation of the chemisorbed species, wherein [AEEA][X] reacts with CO to form CO-[AEEA] complexes stabilized by hydrogen bonding with acetate anions. Supported IL membranes composed of [AEEA][X]/[emim][AcO] mixtures were then fabricated, and the membrane with a suitable mixing ratio showed a CO permeability of 25,983 Barrer and a CO/N selectivity of 10,059 at 313.2 K and an applied CO partial pressure of 40 Pa without water vapor. These values are higher than those reported for known facilitated transport membranes.