Ionic liquids for CO2 separation have been widely utilized, but performance related with permeance was very low, resulting in the prevention of practical applications. For enhanced separation performance, the ionic liquid 1-methyl-3-octylimidazolium tetrafluoroborate (MOIM+BF4-) existing as mostly free ions was utilized for separation of CO2/CH4 and CO2/N-2 mixtures. The free imidazolium ions in MOIM+BF4-, confirmed by FT-Raman, could be easily interacted with CO2 molecules, resulting in enhanced separation performance: the selectivity of CO2/N-2 and CO2/CH4 was 11.8 and 10.5, respectively, with a CO2 permeance of 20.3 GPU. Furthermore, when the positively polarized surface of Cu nanoparticles by free BF4- in MOIM+BF4-, confirmed by XPS, was generated in the composite membrane, the separation performance was enhanced: the selectivity of CO2/N-2 and CO2/CH4 was 25.2 and 24.3, respectively, with a CO2 permeance of 24.2 GPU. These highly permeable and selective membranes could be prepared by facilitated CO2 transport generated from both the highly positively polarized Cu nanoparticles and the free imidazolium group in ionic liquid MOIM+BF4-. (C) 2013 Elsevier B.V. All rights reserved.