Low viscosity superbase protic ionic liquids for the highly efficient simultaneous removal of H2S and CO2 from CH4

CO2 and H2S are considered impurities in natural gas that moderate the pace of environmental health. Developing highly efficient and cost-effective absorbents is of significance for sweetening natural gas. Herein, four superbase protic ionic liquids (SPILs) were prepared in this study. Typical physical properties (density, viscosity, and decomposition temperature) were also examined at ambient pressure. These SPILs show low viscosity values at ranging from 5.1 cP to 18.3 cP at 313.2 K. The solubility of H2S (0-1.0 bar), CO2 (0-1.0 bar), and CH4 (0-5.0 bar) was systematically measured at temperatures in the range of 298.2 K to 333.2 K. Impressively, the absolute solubility of H2S under 0.1 bar was found to be located at 4.31-5.15 mol/kg at 313.2 K, which is the highest value obtained among the reported results in the literature. The ideal selectivity of H2S/CH4 and CO2/CH4 (H2S or CO2 solubility at 0.1 bar vs CH4 solubility at 1 bar) at 313.2 K are in the ranges of 367-653 and 92.4-240, respectively. The interaction mechanism we explored was characterized by NMR and FT-IR spectroscopy. Thermodynamic parameters, such as enthalpy, Gibbs free energy, and entropy of dissolution were calculated from the temperature dependent solubility using the "deactivated model". These SPILs are promising alternative candidates for the simultaneous removal of H2S and CO2 in natural gas upgrading.