Ionic Liquids
Ionic liquids (ILs) are salts that are liquid near ambient temperature. Their very low vapor pressure, thermal stability and low flammability, together with the ability to tailor properties by choosing the cation and anion, make them candidate solvents for gas separations. For a given application the structure of the ions, including small substituents, can strongly change viscosity, thermal stability, water uptake and chemical reactivity, so systematic structure-property studies are central to IL design.
Ionic liquids as candidate capture solvents (2012)
Rubin et al. list ionic liquids among the laboratory-stage liquid solvents for post-combustion capture and also as candidate physical solvents for pre-combustion capture, where their stability (no evaporation up to about 250 C) might allow CO2 removal from syngas without cooling it (Rubin et al. 2012). They mention work on SO2 co-capture and on reversible ionic liquids that react with CO2, and identify the rise in viscosity on CO2 uptake (raising pumping energy and causing mass transfer problems) and the high cost of ionic liquids as the main obstacles. Summary: rubin2012.
Cation-anion interactions in [Emim][TCB] (2013)
Mao et al. used B3LYP/6-31+G(d,p) ion pair calculations together with IR and Raman spectra to characterize the weakly coordinating RTIL 1-ethyl-3-methylimidazolium tetracyanoborate (Mao et al. 2013). Four stable ion pairs were found, the most stable having the anion beneath a non-planar [Emim]+ cation, and their counterpoise-corrected interaction energies (about 280 to 287 kJ/mol in magnitude) were much weaker than those of [Emim][BF4] pairs (about 353 to 364 kJ/mol). Atoms-in-molecules analysis identified four weak inter-ionic hydrogen bonds, and natural energy decomposition analysis attributed the weakening relative to BF4- mainly to the electrical (electrostatic plus polarization) term, which the authors connect to larger free volume and better CO2 solubility. Summary: mao2013.
Substituent effects in triazolide ILs (2014)
Thompson et al. made five tetrabutylphosphonium 1,2,3-triazolide ILs that differ only in the substituent at the ring 4-position and compared their properties (Thompson et al. 2014). Electron-donating alkyl groups made the ring more electron rich (upfield H5 NMR shift) and lowered the thermal decomposition onset, while the phenyl and fluorinated ether substituents gave the most thermally stable ILs. Residual water after vacuum drying (up to about 54 mol%) raised viscosity several-fold, apparently through a hydrogen-bonding network with the anions, while dry ILs were much less viscous but thickened or even solidified after reacting with CO2. The ILs were hygroscopic yet insoluble in water. The CO2 consequences are described under CO2 capture. Summary: thompson2014.