Constructing stable protic ionic liquids with cuprous site and long alkyl chain for highly efficient separation of propylene and propane

The separation of C3H6 and C3H8 has been a technical challenge owing to their similar physical and chemical properties. The currently used separation technologies still suffer from the low efficiency, high operating cost and volatility of media, which urges researchers to develop greener and more efficient media. To achieve this, a series of protic cuprous-based ionic liquids (PCILs) combining both cuprous site and long alkyl chain were designed in this work for the absorption separation of C3H6 and C3H8. Surprisingly, the solubility of C3H6 in [TOAH][CuCl2] is as high as 0.238 mol/kg at 313 K and 1.0 bar, which exceeds most of results reported so far. The experimental data and thermodynamic analysis showed that extending alkyl chain of ILs promoted the dissolution of C3H6 and C3H8, while the introduction of Cu(I) improved the selectivity of C3H6/C3H8. In addition, the interaction mechanism of C3H6 and C3H8 with ILs was further clarified by quantum chemical calculations and FT-IR spectroscopy. Furthermore, regeneration of [TOAH][CuCl2] can be easily realized under depressurizing condition without heating and loss in separation performance, and [TOAH][CuCl2] was found very stable to both moisture and oxygen for at least fifteen days. The strategy of combining long-chain alkyl and Cu(I) site in this work may provide an important guideline for enhancing C3H6/C3H8 selectivity, but more efforts on enhancing Cu(I) activity still need to be made for improving C3H6/C3H8 selectivity.