Theoretical studies on CO2 capture behavior of quaternary ammonium-based polymeric ionic liquids

Abstract

Quaternary ammonium-based polymeric ionic liquids (PILs) are novel CO₂ sorbents as they have high capacity, high stability and high binding energy. Moreover, the binding energy of ionic pairs to CO₂ is tunable by changing the hydration state so that the sorbent can be regenerated through humidity adjustment. In this study, theoretical calculations were conducted to reveal the mechanism of the humidity swing CO₂ adsorption, based on model compounds of quaternary ammonium cation and carbonate anions. The electrostatic potential map demonstrates the anion, rather than the cation, is chemically preferential for CO₂ adsorption. Further, the proton transfer process from water to carbonate at the sorbent interface is successfully depicted with an intermediate which has a higher energy state. By determining the CO₂ adsorption energy and activation energy at different hydration states, it is discovered that water could promote CO₂ adsorption by reducing the energy barrier of proton transfer. The adsorption/desorption equilibrium would shift to desorption by adding water, which constitutes the theoretical basis for humidity swing. By analyzing the hydrogen bonding and structure of the water molecules, it is interesting to find that the CO₂ adsorption weakens the hydrophilicity of the sorbent and results in release of water. The requirement of latent heat for the phase change of water could significantly reduce the heat of adsorption. The special “self-cooling” effect during gas adsorption can lower the temperature of the sorbent and benefit the adsorption isotherms.