Phase Behavior of Liquid Crystals with CO 2

Liquid crystals are being considered as novel process solvents for CO2 capture. The solubility of CO2 is higher in the isotropic phase than in the structured (e.g., nematic) phase. CO2 can be captured in the isotropic phase, and regeneration of the solvent is achieved by cooling down the mixture a few degrees until a phase transition to the structured phase occurs. This CO2 capture process has the potential to consume less energy than the conventional amine-based processes. To address the potential of liquid crystals to efficiently capture CO2 experimentally obtained P,T-phase diagrams of five liquid crystals with 5 mass % CO2 are reported. The liquid crystals used in this study are 4'-(pentyloxy)-4-biphenylcarbonitrile, 4'-pentyl-4-biphenylcarbonitrile, 4-ethyl-4'-propyl-bicyclohexyl, 4-propyl-4'-butyl-bicyclohexyl, and 4'-(octyloxy)-4-biphenylcarbonitrile. It is found that a weakly polar liquid crystal had a higher CO2 solubility than apolar and more polar liquid crystals.