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The performance of an ionic liquid with an aprotic heterocyclic anion (AHA-IL), trihexyl(tetradecyl)phosphonium 2-cyanopyrrolide ([P66614][2-CNPyr]), for CO2capture has been evaluated considering both the thermodynamics and the kinetics of the phenomena. Absorption gravimetric measurements of the gas–liquid equilibrium isotherms of CO2–AHA-IL systems were carried out from 298 to 333 K and at pressures up to 15 bar, analyzing the role of both chemical and physical absorption phenomena in the overall CO2solubility in the AHA-IL, as has been done previously. In addition, the kinetics of the CO2chemical absorption process was evaluated by in situ Fourier transform infrared spectroscopy-attenuated total reflection, following the characteristic vibrational signals of the reactants and products over the reaction time. A chemical absorption model was used to describe the time-dependent concentration of species involved in the reactive absorption, obtaining kinetic parameters (such as chemical reaction kinetic constants and diffusion coefficients) as a function of temperatures and pressures. As expected, the results demonstrate that the CO2absorption rate is mass-transfer-controlled because of the relatively high viscosity of AHA-IL. The AHA-IL was encapsulated in a porous carbon sphere (Encapsulated Ionic Liquid, ENIL) to improve the kinetic performance of the AHA-IL for CO2capture. The newly synthesized AHA-ENIL material was evaluated as a CO2sorbent with gravimetric absorption measurements. AHA-ENIL systems preserve the good CO2absorption capacity of the AHA-IL but drastically enhance the CO2absorption rate because of the increased gas–liquid surface contact area achieved by solvent encapsulation.

Mixtures of ionic liquids (ILs) present intriguing possibilities for modifying both physical and chemical properties relative to the pure ILs. Here we investigate the solubility of CO2and N2in mixtures of 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([emim][TFSI]) with 1-ethyl-3-methylimidazolium tetrafluoroborate ([emim][BF4]) at 313.2 K and 1-ethyl-3-methylimidazolium dicyanamide ([emim][DCA]) at 313.2 and 333.2 K. CO2solubilities are measured at pressures up to 15 bar, while N2solubilities were measured up to 140 bar. Both gases dissolve in these ILs by physical dissolution. For both mixtures the Henry’s Law constants for CO2and N2in the mixtures are lower (i.e., higher solubility) than a mole fraction weighted linear combination of the Henry’s Law constants in the pure ILs. In fact, the Henry’s Law constants for both gases in the mixtures are adequately fit with a mole fraction weighted linear combination of the natural logarithmof the pure IL Henry’s Law constants. However, in no case is the solubility of either gas in the IL mixtures greater than or less than the solubilities in the two pure ILs. As a result, the anticipated CO2/N2selectivity in the IL mixtures, based on pure gas solubility measurements in the IL mixtures, is always between the values of the CO2/N2selectivity for the pure ILs.

Aprotic-heterocyclic anion (AHA) ionic liquids (ILs) have the potential to be separation and reaction media for carbon capture and utilization (CCU) processes. While CO2capture may occur at moderate temperatures (e.g., between ambient and 50 °C), thermochemical conversion of the CO2requires good thermal stability of the ILs so that they can withstand reaction at elevated temperatures. ILs with noncoordinating anions such as bis(trifluoromethylsulfonyl)imide ([Tf2N]−) have shown improved thermal stability when the cation is modified to have 2+ charge, i.e., so-called dicationic ILs. To our knowledge, no previous studies have paired dications with AHAs. A series of dicationic ILs composed of di(tetraalkylphosphonium) cations with tributyl chains and linkers ranging from propyl to undecyl, paired with 2-cyanopyrrolide anions, do show improved thermal stability compared to their monocationic AHA IL equivalents. Interestingly, the dicationic ILs all have similar CO2uptake per anion, although they exhibit CO2capacity intermediate between those of monocationic ILs with the 2-cyanopyrrolide anion and tetraalkylphosphonium cations with longer (trihexyl(tetradecyl)) and shorter (triethyloctyl) chains. This is consistent with differences in the standard entropy of the reaction between the anion and CO2. Like their monocationic AHA counterparts, dicationic AHA ILs are sensitive to the presence of oxygen, which results in the formation of phosphine oxides, as determined by long-term isothermal stress tests with air purging.

Ionic liquids with aprotic heterocyclic anions (AHAs) have been developed for postcombustion CO2capture applications. The anions of AHA ILs play a significant role in tuning anion–CO2complexation. In addition, AHAs are able to trigger the abstraction of acidic protons located at the α position of phosphonium cations by forming hydrogen bonds between cations and anions, eventually leading to cation-driven CO2complexation. Here we investigate the role of the anion in cation–anion hydrogen bonding and ylide formation. Using CO2uptake measurements, 31P nuclear magnetic resonance (NMR), attenuated total reflection-Fourier transform infrared (ATR-FTIR) deuterium exchange equilibrium and rates, two-dimensional nuclear Overhauser effect spectroscopy (2D NOESY), and density functional theory calculations, we show that the key is the proximity of the negatively charged nitrogen atoms on the anion to the α protons, which is governed not just by anion basicity but by sterics. Thus, we show that triethyl(octyl)phosphonium 3-methyl-5-trifluoromethylpyrazolide is much more effective in hydrogen-bonding with and deprotonating the cation than the equivalent [P2228] ILs with more basic 2-cyanopyrrolide and 3-trifluoromethylpyrazolide anions.