Moisture Swing Ion-Exchange Resin-PO₄ Sorbent for Reversible CO₂ Capture from Ambient Air

Reversible CO₂ capture from ambient air by a humidity swing has shown great potential in mitigating the greenhouse effect. In this work, we developed a new humidity-swing absorbent based on PO₄^3-/HPO₄^2-/H₂PO₄^- ions exhibiting superior CO₂ absorption capacity and kinetics compared to that of CO₃^2-/HCO₃^--based absorbent. After ion exchange with PO₄^3- ions, the ion-exchange resin (IER-PO₄) containing positive quaternary ammonium groups and movable PO₄^3- ions is able to reversibly capture CO₂ from the ambient air by a humidity swing, which triggers the transformation between the PO₄^3- ions and HPO₄^2-/H₂PO₄^- ions in the resin. In a dry environment, PO₄^3- ions in IER-PO4 are hydrolyzed into OH^- ions and HPO₄^2- ions, which are further hydrolyzed into H₂PO₄^- ions and OH^- ions. Both hydrolysis reactions produce OH^- ions for CO₂ absorption, while the adsorbed CO₂ can be released in a humid environment. The results of quantum chemical calculation show that the hydrolysis of the ions is promoted by the reduction of water molecules in the nanoscale hydrated cluster. The adsorption capacity of IER-PO4 during the moisture swing is 80% larger than that of IER-CO₃, and the adsorption rate of the IER-PO₄ resin at a temperature range of 15-35 °C is much higher than that of the IER-CO₃ absorbent. A modified pseudo-first-order (MPFO) kinetic is developed, which can describe the experimental results well. The present study sheds light on the design of high performance moisture-swing absorbents with PO₄^3- ions. © 2019 American Chemical Society.