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.