A comparative study of ammonia solubility in imidazolium-based ionic liquids with different structural compositions
Muhammad Salman, Ji Won Lee, Sang Hyuk Lee, Min Ho Lee, Van Duc Pham, Min‐Sik Kim, Daeheum Cho, Hye Jin Lee
Abstract
Four imidazolium-based ionic liquids (ILs) with two cations 1-pentyl-3-butylimidazolium [PBIM] + and 1-benzyl-3-butylimidazolium tetrafluoroborate [BzBIM] + , and two anions tetrafluoroborate (BF 4 − ) and trifluoromethanesulfonate (OTf − ) were synthesized for NH 3 solubility enhancement. The structural, thermal, and electrochemical stabilities, ionic conductivity, and viscosity of the four ILs, namely, [PBIM]BF 4 , [BzBIM]BF 4 , [PBIM]OTf, and [BzBIM]OTf, were investigated. Due to the intermolecular interaction of the benzyl group attached to the imidazolium ring, [BzBIM] + -based ILs exhibited higher thermal stability but lower ionic conductivity compared to [PBIM] + -based ILs. Further, the NH 3 solubility in all ILs was measured using a custom-made setup at temperatures ranging from 293.15 to 323.15 K and pressures ranging from 1 to 5 bar. The effects of the cation and anion structures of ILs, as well as pressure and temperature, on the NH 3 solubility in the ILs were also investigated. [PBIM]BF 4 showed the best solubility because of its high free volume and low viscosity. Density functional calculations validated the superior NH 3 solubility in [PBIM]BF 4 , attributable to the minimal reorganization of the [cation]anion complex geometry during the solvation process, yielding a low solvation free energy. The findings of this study suggest that ILs exhibit a high NH 3 solubility capacity and cation and anion structures considerably affect the NH 3 solubility in ILs.