Solubility of Nitrogen in Ionic Liquids at 295–353 K and Pressures to 140 bar

Ionic liquids (ILs) have been evaluated extensively as post-combustion CO2 capture solvents, with CO2 solubility data available for a large number of ILs. However, data for the solubility of the less-soluble gas component, N2, are sparse. Therefore, the solubility of N2 was measured gravimetrically at pressures up to 140 bar in 12 ILs containing imidazolium, ammonium, pyrrolidinium, and phosphonium cations paired with bis­(trifluoromethanesulfonyl)­imide ([TFSI]−), dicyanamide ([DCA]−), methanesulfonate ([MeSO3]−), tetrafluoroborate [BF4]−), and triflate ([TfO]−) anions. Increasing the alkyl chain length from ethyl to hexyl to decyl in alkylimidazolium bis­(trifluoromethanesulfonyl)­imide ILs increased the N2 solubility. The same effect was observed with phosphonium cations paired with the bis­(trifluoromethylsulfonyl)­imide anion. Nitrogen solubility in ILs with the 1-ethyl-3-methylimidazolium cation followed the order [DCA]− < [MeSO3]− < [BF4]− < [TfO]− < [TFSI]−. Generally, regardless of the IL structure, the N2 solubility increased with increasing IL molar volume. For ILs with similar chemical composition and molar volume (i.e., ILs with tetraalkylammonium and pyrrolidinium cations), N2 was less soluble in the IL with the cyclic cation. The absorption of N2 in bis­(trifluoromethylsulfonyl)­imide ILs did not depend significantly on temperature. However, [emim]­[BF4] had a significant positive enthalpy of N2 absorption. These N2 solubility data, along with the readily available CO2 solubilities in ILs, is critical to evaluating ILs for post-combustion carbon capture processes.