We prepared a new class of functional solutions by dissolving CuCl or CuBr into 1-butyl-3-methylimidazolium bromide ([BMIM] [Br]) or 1-ethyl-3-methylimidazolium bromide ([EMIM] [Br]). We determined experimentally the solubility of propylene and propane in such solutions respectively at 1-8 bar and 298-313 K, and investigated the separation of propylene from propylene/propane mixture. The absorption capacity and selectivity for propylene were enhanced with the addition of Cu(I) salt and were affected by cationic side chain and anionic species following this order [EMIM][Br]-CuCl < [EMIM][Br]-CuBr < [BMIM][Br]-CuCl < [BMIM][Br]-CuBr. It was observed that this absorption was a chemical nature through pi-interaction between Cu(I) cation and propylene. In a typical result, the absorption capacity for propylene of [BMIM][Br]-CuBr (Cu(I) cation, 1.5 M) is 0.13 mol/L at 298 K and 1 bar while it is 0.01 mol/L for propane with a selectivity of 13, which is comparable to those absorption systems with more expensive silver salts; the selectivity is high, up to 64.6 for 80 mol % propylene mixture with propane. The effects of temperature and pressure on absorption were investigated. Such systems of Cu(I) salt and ionic liquids are potential absorption reagents for separation of propylene and propane with competitive cost to silver systems.