Direct Air Capture of CO2 Using a Liquid Amine–Solid Carbamic Acid Phase-Separation System Using Diamines Bearing an Aminocyclohexyl Group

The phase separation between a liquid amine and the solid carbamic acid exhibited >99% CO2 removal efficiency under a 400 ppm CO2 flow system using diamines bearing an aminocyclohexyl group. Among them, isophorone diamine [IPDA; 3-(aminomethyl)-3,5,5-trimethylcyclohexylamine] exhibited the highest CO2 removal efficiency. IPDA reacted with CO2 in a CO2/IPDA molar ratio of ≥1 even in H2O as a solvent. The captured CO2 was completely desorbed at 333 K because the dissolved carbamate ion releases CO2 at low temperatures. The reusability of IPDA under CO2 adsorption-and-desorption cycles without degradation, the >99% efficiency kept for 100 h under direct air capture conditions, and the high CO2 capture rate (201 mmol/h for 1 mol of amine) suggest that the phase separation system using IPDA is robust and durable for practical use.