Ionic Liquid Mixtures for Direct Air Capture: High CO2Permeation Driven by Superior CO2Absorption with Lower Absolute Enthalpy

Abstract

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 CO2absorption rates, larger CO2solubilities, and lower absolute enthalpies of CO2absorption compared to those of single ILs. NMR spectroscopy of the IL mixture after exposure to 13CO2allowed elucidation of the chemisorbed species, wherein [AEEA][X] reacts with CO2to form CO2-[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 CO2permeability of 25,983 Barrer and a CO2/N2selectivity of 10,059 at 313.2 K and an applied CO2partial pressure of 40 Pa without water vapor. These values are higher than those reported for known facilitated transport membranes.