Absorption of H₂S and CO₂ in Aqueous Solutions of Tertiary-Amine Functionalized Protic Ionic Liquids

The development of ionic liquid-based absorbents for selective separation of H₂S from CO₂ is of interest to the natural gas industry. In this work, a series of tertiary-amine functionalized protic ionic liquids (TA-PILs) were synthesized by neutralizing one of the tertiary amine groups in diamine compounds with acetic acid, leaving the other tertiary amine group as the functional site for selective absorption of H₂S. To overcome the high viscosity of pure ionic liquids, TA-PILs were dissolved in water to give mixed absorbents. The effects of TA-PIL species, TA-PIL concentration, and experimental temperature on the solubilities of H₂S and CO₂, and absorption kinetics of H₂S and CO₂ in aqueous TA-PILs were investigated systematically. The reaction equilibrium constants and absorption enthalpy of H₂S and CO₂ in aqueous TA-PILs were also calculated by correlating the solubility data with a reaction equilibrium thermodynamic model. Results demonstrated that aqueous TA-PILs have high absorption capacities for H₂S. Although the equilibrium selectivities of H₂S/CO₂ in aqueous TA-PILs are minimized, they still display significant kinetic selectivities for the two gases. More importantly, the TA-PILs designed are featured with low cost and facile synthesis, making them more attractive than other functionalized ionic liquids reported in the literature for application in selective separation of H₂S from CO₂.