Cyanopyridinium-Based Ionic Liquids and Their Mixtures for Ethylene and Ethane Separation

The solubility of ethane and ethylene was determined in a series of cyanopyridinium ionic liquids known to form charge-transfer complexes with polyaromatic hydrocarbons to determine their potential to form specific interactions with the unsaturated gas. The solubilities of ethylene and ethane in 1-butyl-4-cyanopyridinium bis-(trifluoromethane)-sulfonimide ([C₄ ⁴CNPy]-[NTf₂]) and 1-butyl-3-cyanopyridinium bis-(trifluoromethane)-sulfonimide ([C₄ ³CNPy]-[NTf₂]) were measured using an isochoric saturation method. The mole fraction solubility of ethane in the ionic liquids ranged from 6.0 × 10^-3 to 7.2 × 10^-3 and from 7.5 × 10^-3 to 9.9 × 10^-3 for ethylene in [C₄ ³CNPy]-[NTf₂] and [C₄ ⁴CNPy]-[NTf₂] at 0.1 MPa and 313 K, respectively. The small preferential solubility of ethylene over ethane in the ionic liquids results in ideal ethylene separation selectivities between 1.2 and 1.4, which is in the same range as typical physisorbent ionic liquids of the same type and molecular weight, indicating that there is no significant preferential interaction between the ionic liquids and ethylene. The calculated thermodynamic properties of solvation reveal that the solvation of both gases is entropically driven. To promote cyanopyridinium-ethylene interactions and decrease the possibility of steric constrictions to the interactions, 1-butyl-4-methylimidazolium bis-(trifluoromethane)-sulfonimide ([C₄C₁Im]-[NTf₂]) was added as diluent to [C₄ ⁴CNPy]-[NTf₂]. This IL mixture was found to behave almost ideally based on isothermal titration nanocalorimetry results. The solubility of ethylene or ethane in the mixture was found to be the weighted average of the corresponding solubilities in the two pure ionic liquids, still indicating that no specific ethylene-ionic liquid interactions were formed. Molecular dynamics (MD) simulations of the systems were performed and revealed that the slightly higher ethylene solubility in [C₄ ⁴CNPy]-[NTf₂] is due to a slightly stronger association with this cation compared to the 3-isomer.