Highly efficient and selective absorption of H2S in phenolic ionic liquids: A cooperative result of anionic strong basicity and cationic hydrogen-bond donation

- Solubilities of H2S and CO2 in phenolic ionic liquids were determined.
- Phenolic ionic liquids show comparably high solubilities of H2S.
- Solubilities of CO2 are highly dependent on the cationic hydrogen-bond donating.
- Interactions of phenolic ionic liquids with H2S and CO2 were disclosed by DFT calculations.
- A strategy was proposed to design absorbents for the selective sweetening of natural gas.

A series of phenolic ILs containing different cations were synthesized and investigated for the absorption of H2S and CO2 in this work. It is interestingly found that the solubilities of H2S in these phenolic ILs are comparably high because of the strong interaction of basic phenolate anion with acidic H2S, while the solubilities of CO2 decrease significantly with the increase of cationic hydrogen-bond donation. Tetramethylguanidinium phenolate ([TMGH][PhO]), which is constructed with anion of strong basicity and cation of strong hydrogen-bond donating ability, is thus highlighted with both high solubilities of H2S (0.56 mol/mol at 313.2 K and 0.1 bar, and 0.85 mol/mol at 313.2 K and 1 bar) and high selectivities of H2S/CO2 (6.2 for the ratio of H2S solubility at 313.2 K and 0.1 bar vs. CO2 solubility at 313.2 K and 1 bar, and 9.4 for the ratio of H2S solubility at 313.2 K and 1 bar vs. CO2 solubility at 313.2 K and 1 bar). Owing to the small molecular size of [TMGH][PhO], the absolute solubilities of H2S in it (2.68 mol/kg at 313.2 K and 0.1 bar, and 4.08 mol/kg at 313.2 K and 1 bar) are particularly fascinating, and much higher than other absorbents reported in the literature. Furthermore, [TMGH][PhO] is cost-effective in comparison with other functionalized ILs specifically designed for H2S capture, since it can be facilely synthesized from the one-step neutralization of readily available 1,1,3,3-tetramethylguanidine and phenol. The results obtained in this work indicate that [TMGH][PhO] is a promising candidate for the selective sweetening of natural gas.