NH3 absorption in Brønsted acidic imidazolium- and ammonium-based ionic liquids

Abstract

This paper reports the NH₃ solubilities and absorption mechanisms in the imidazolium- and ammonium-based ionic liquids. The cations were 1-methyl-3-(4-sulfobutyl)-imidazolium, 1-(2-hydroxyethyl)-3-methylimidazolium, 1-ethyl-3-methyl-imidazolium, N-carboxymethyl-N,N,N-trimethylammonium, N-(2-hydroxyethyl)-N,N,N-trimethylammonium, and N-butyl-N,N,N-trimethylammonium, whereas the anions were bis(trifluoromethanelsulfonyl)amide, tris(pentafluoroethyl)trifluorophosphate, trifluoromethylsulfonate, and hydrogensulfate. The Brønsted acidic ionic liquids, consisting of sulfo and carboxy groups and hydrogensulfate anion, absorbed larger amounts of NH₃, followed by the hydroxy-functionalized and nonfunctionalized ionic liquids. The NMR and Raman spectra indicated that the Brønsted acidic ionic liquids absorbed NH₃ as both the molecular ammonia (physical absorption) and the ammonium cation (chemical absorption). Although the hydroxy group strongly interacted with NH₃, the hydroxy-functionalized ionic liquids dissolved NH₃ only physically, similar to the nonfunctionalized ionic liquids. In conclusion, the chemical absorption brought about the higher NH₃ solubilities in the Brønsted acidic imidazolium- and ammonium-based ionic liquids.