The highly selective composite membranes consisting of ionic liquid 1-butyl-3-methyl imidazolium tetrafluoroborate (BMIIVI+BE4-)/Ag2O/KF were prepared for CO2 separation. When Ag2O and KF were incorporated into BMIM+CB4-, the ideal selectivities of CO2/N-2 and CO2/CH4 reached 105 and 52, respectively, with a CO2 gas permeance of 31 GPU. These enhanced separation performance were attributable to synergistic effect of (1) K+ cations in KF as CO2 carriers for facilitated transport and (2) the oxide layer of the well-dispersed Ag2O and free ions in ionic liquid for improvement in CO2 solubility. The coordinative interactions in the BMIM+BE4-/AB(2)O/KF membrane were characterized by TGA, XPS, and FT-Raman spectroscopy. (C) 2017 Elsevier B.V. All rights reserved.