Direct Air Capture of CO ₂ Using a Liquid Amine–Solid Carbamic Acid Phase-Separation System Using Diamines Bearing an Aminocyclohexyl Group

Abstract

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.