Ionic Liquid Mixtures for Direct Air Capture: High CO 2 Permeation Driven by Superior CO 2 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 N -(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 2 absorption rates, larger CO 2 solubilities, and lower absolute enthalpies of CO 2 absorption compared to those of single ILs. NMR spectroscopy of the IL mixture after exposure to 13 CO 2 allowed elucidation of the chemisorbed species, wherein [AEEA][X] reacts with CO 2 to form CO 2 -[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 2 permeability of 25,983 Barrer and a CO 2 /N 2 selectivity of 10,059 at 313.2 K and an applied CO 2 partial pressure of 40 Pa without water vapor. These values are higher than those reported for known facilitated transport membranes.