Selective Separation of H2S and CO2 from Natural Gas Using Low-Viscosity, Highly Basic Protic Ionic Liquids: Absorption Characteristics and Molecular Mechanisms

To achieve efficient removal of CO2 and H2S from natural gas, four protic ionic liquids (PILs) were designed and systematically investigated in this work. Structural characterization and physicochemical property measurements were carried out. Among the four PILs, the cation and anion of [DBNH][PhO] exhibit planar structures that confer the lowest viscosity (20.30 cP at 313.2 K). At 313.15 K, its H2S solubility reaches 2.61 and 4.68 mol·kg–1 at H2S partial pressures of 0.1 and 1 bar, respectively. The solubility-based H2S/CO2 selectivity is 4.83 when defined as the ratio of H2S solubility at 0.1 bar to CO2 solubility at 1 bar, whereas the equal-pressure solubility ratio at 1 bar is 8.67. The experimentally determined gas solubility data are correlated using the reaction equilibrium thermodynamic model (RETM), from which key thermodynamic parameters, including the apparent equilibrium constant, Henry’s law constant, enthalpy, and entropy, are obtained. Furthermore, quantum chemistry calculations, including electrostatic potential (ESP) analysis, Atoms in Molecules (AIM) topological analysis, and energy decomposition analysis, are performed to elucidate the origin of the high H2S/CO2 selectivity of [DBNH][PhO]. The strong basicity of [PhO]− significantly enhances H2S solubility, whereas steric hindrance and π-conjugation effects hinder the effective insertion of CO2.