Solubility of Nitrogen in Ionic Liquids at 295–353 K and Pressures to 140 bar
Caitlin L. Bentley, Tangqiumei Song, Oscar Morales‐Collazo, Joan F. Brennecke
Abstract
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, [emim][BF 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.