The BMIM+NO3 -/Ag metal nanocomposite membranes were utilized for separation of propylene/propane mixtures. The selectivity of propylene/propane and the mixed gas permeance increased to 10 and 1.3 GPU, respectively. It is anticipated that the interactions between NO3 - of BMIM+NO3 - and the surface of silver nanoparticles causes the Ag surface to be partially positively charged, resulting in the reversible complexation with propylene and consequently facilitated propylene transport. The surface positive charge was confirmed from the increase in the binding energy of the d5/2 orbital of the silver nanoparticles by x-ray photoelectron spectroscopy. It increased from 368.26 for the neat silver nanoparticles to 368.47 eV for BMIM+NO3 -/Ag metal composite. The positively charged surface of silver nanoparticles can be utilized as a new durable olefin carrier for facilitated transport.