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.