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.

Sorbents for the Direct Capture of CO2 from Ambient Air

“…The decrease in water availability results in a rearrangement of the hydration water, which in turn affects the equilibrium of the chemical reaction. The reaction pathway of CO 2 sorption/desorption is explained in Figure and Equations (5)–(8) . For a wet resin without CO 2 loading, which we label State 1, the counterions to the positive quaternary ammonium ions are carbonate ions.…”

“…Thereaction pathway of CO 2 sorption/desorption is explained in Figure 17 and Equations ( 5)-( 8). [80,250,254] Forawet resin without CO 2 loading,which we label State 1, the counterions to the positive quaternary ammonium ions are carbonate ions. Thec arbonate ion is stabilized by the presence of sufficient water.…”

The Effect of Moisture on the Hydrolysis of Basic Salts

“…In other words, the studied trivalent and divalent ions can hydrolyze much more H 2 O into OH − ions, whereas monovalent ions can hydrolyze much less H 2 O into OH − ions under relatively dry condition. The hydrolysis degree of all basic ions holding only a few water molecules is significantly different from the degree of hydrolysis of basic salts in bulk water; in other words, the hydrolysis reaction has a moisture‐effect characteristic, which was first found in anionic, strong based exchange resins, the affinity of which to CO 2 is strongly affected by the presence or absence of water …”

Sorbenten zur direkten Gewinnung von CO2 aus der Umgebungsluft

“…Die Verringerung der Wasserverfügbarkeit führt zu einer Umlagerung des Hydratwassers, was sich wiederum auf das Gleichgewicht der chemischen Reaktion auswirkt. Der Reaktionsweg der CO 2 ‐Sorption‐/Desorption wird in Abbildung sowie durch die Gleichungen (5)–(8) erläutert . Für ein nasses Harz ohne eine CO 2 ‐Ladung, das wir als Zustand 1 bezeichnen, sind die Gegenionen zu den positiven quartären Ammoniumionen Carbonationen.…”