Tuning the composition of deep eutectic solvents consisting of tetrabutylammonium chloride and n-decanoic acid for adjustable separation of ethylene and ethane

The separation of ethylene (C2H4) and ethane (C2H6) is crucial for the production of down-stream chemicals in petrochemical industry. However, the currently used technologies still suffer from high energy intensity and contamination of volatile organic compounds (VOCs), stimulating researches on screening greener separation media and methods. Developed in this work were deep eutectic solvents (DESs) containing tetrabutylammonium chloride (TBACl) and n-decanoic acid (DA) for absorption separation of C2H4 and C2H6. Chemical structures of DESs were confirmed using NMR and FT-IR spectroscopy, and their densities, viscosities as well as the thermal decomposition temperatures were measured. C2H4 solubilities in these DESs were found to be higher than 0.0632 mol/kg at 313 K and 1.0 bar, higher than most ionic liquids reported so far. Importantly, tunable C2H4/C2H6 selectivities ranging from 0.86 to 1.31 were achieved via altering the ratio of compositions in DESs, while the absolute C2H4 solubilities remained almost unchanged, which was attributed to the change in free volume and better affinity of DA and TBACl for C2H4. The Henry's constants (Hm), enthalpy changes (ΔsolH), Gibbs free energy changes (ΔsolG) and entropy changes (ΔsolS) for the dissolution of C2H4 and C2H6 in the DESs were calculated through thermodynamic analysis. In addition, quantum chemical calculations elucidated the origin of the different dissolution behaviors of the two gases in DESs. Furthermore, energy-efficient regeneration of DESs could be carried out by only depressurizing without heating, and no visible decrease in separation performance was observed after ten consecutive absorption–desorption cycles. Overall, the findings in this work may provide utility guidance for designing absorbents targeting separation of C2H4/C2H6 with variable composition.