Liquid crystals are rod-like molecules, consisting of a rigid core and flexible tail, with a structured phase between the usual liquid and solid phase which is still fluid. This structured phase is called nematic if the molecules are orientationally ordered. Currently, liquid crystals are examined as novel solvents for capturing CO2.1,2 As CO2 is less soluble in the structured phase than in the isotropic phase, gaseous CO2 can be separated from the structured phase by cooling down a saturated isotropic mixture a few degrees at constant pressure. The solvent can be regenerated by heating the CO2 lean mixture a few degrees again.
To obtain the optimal liquid crystal for the CO2 capture process, binary mixtures of a liquid crystal and CO2 have been examined with different concentrations of CO2. Liquid crystals examined had either two phenyl rings, two cyclohexyl rings or one phenyl and one cyclohexyl ring. We can conclude, based on experimental results, that molecules with both a cyclohexyl and phenyl ring have highest solubility. The solubility difference between the isotropic and nematic phase is mainly determined by the phase transition enthalpy of the isotropic-nematic phase transition.
However, no molecule has been found yet with optimal properties. Therefore, mixtures of two liquid crystals and CO2 are investigated.3 Experiments showed that in binary mixtures of two liquid crystals, the nematic phases form a nematic homogeneous solution and the solid phases form an eutectic system. This leads to a solvent with improved properties for CO2 capture, as the temperature range in which the nematic phase is stable is increased, leading to an increased solubility difference of CO2 between the nematic and isotropic phase. The ternary mixtures 4,4’-pentyloxycyanobiphenyl + 4,4’-heptyloxycyanobiphenyl + CO2 and 4,4’-propylcyclohexylbenzonitrile + 4,4’-heptylcyclohexylbenzonitrile + CO2, were investigated. The ternary mixture of 4,4’-propylcyclohexylbenzonitrile + 4,4’-heptylcyclohexylbenzonitrile + CO2 showed an increased solubility of CO2 compared to the binary subsystems.
Furthermore, mixtures can be used for fine-tuning the phase transition temperature.
1 Gross, J. Jansens, P.J., A method for the removal of a gas from a process gas stream by means of liquid crystals, 2008, Patent WO2008147181-A1; NL2000654-C2.
2 de Groen, M. Vlugt, T.J.H. de Loos, T.W., Phase behavior of liquid crystals with CO2, J. Phys. Chem. B, 2012, 116 (30), 9101-9106.
3 de Groen, M. Vlugt, T.J.H. de Loos, T.W., Binary and Ternary Mixtures of Liquid Crystals with CO2, AIChE journal, accepted