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 CO 2 capture solvents, with CO 2 solubility data available for a large number of ILs. However, data for the solubility of the less-soluble gas component, N 2, are sparse. Therefore, the solubility of N 2 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 (MeSO 3 − ), tetrafluoroborate BF 4 − ), and triflate (TfO − ) anions. Increasing the alkyl chain length from ethyl to hexyl to decyl in alkylimidazolium bis(trifluoromethanesulfonyl)imide ILs increased the N 2 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 − < MeSO 3 − < BF 4 − < TfO − < TFSI − . Generally, regardless of the IL structure, the N 2 solubility increased with increasing IL molar volume. For ILs with similar chemical composition and molar volume (i.e., ILs with tetraalkylammonium and pyrrolidinium cations), N 2 was less soluble in the IL with the cyclic cation. The absorption of N 2 in bis(trifluoromethylsulfonyl)imide ILs did not depend significantly on temperature. However, emimBF 4 had a significant positive enthalpy of N 2 absorption. These N 2 solubility data, along with the readily available CO 2 solubilities in ILs, is critical to evaluating ILs for post-combustion carbon capture processes.