Separation of propylene and propane by alkylimidazolium thiocyanate ionic liquids with Cu+ salt

Ionic liquids (ILs) coupled with Ag + or Cu + salts to form a new kind of reactive absorbent have been studied to separate light olefin from paraffin recently. In this work, we prepared two halogen-free alkylimidazolium thiocyanate ILs with cheaper cuprous thiocyanate, i.e., [Bmim]SCN–CuSCN and [Emim]SCN–CuSCN (Bmim, 1-butyl-3-methylimidazolium; Emim, 1-ethyl-3-methylimidazolium) and investigated their absorption capability for propylene, propane and mixture of both at 1–7   bar and 298–318   K. The effects of operating parameter including cation nature, temperature, pressure, Cu + concentration and reuse of absorbent were investigated. Propylene shows a chemical absorption while propane does a physical one, and increasing Cu + concentration effectively improves the absorption capability for propylene and the selectivity of propylene/propane. [Bmim]SCN–CuSCN has higher absorption capability and selectivity for propylene than [Emim]SCN–CuSCN, e.g., [Bmim]SCN–CuSCN-1.5   M can absorb 0.12   mol of propylene per liter while 0.012   mol of propane per liter at 1   bar and 298   K, with a selectivity of 10, which is comparable to some other ILs-Ag + salts and better than pure ILs. Such absorbents can be regenerated through temperature and pressure swing without remarkable activity loss. This work shows that alkylimidazolium thiocyanate ILs with Cu + salts are promising reactive absorbents to separate propylene from propane.