Grand canonical Monte Carlo simulations have been conducted to study the adsorption of carbon dioxide and methane, as well as their binary mixtures on Na-montmorillonite clay material. It was found that the adsorption behavior near the clay structure for the two species is distinctively different. The Namontmorillonite clay shows obviously high adsorption capacity for CO2, as compared with CH4. The adsorption behavior and mechanism have been characterized by the interlayer interfacial structures and isosteric heats of adsorption. Meanwhile, the mixture adsorption demonstrates that CO2 molecules with enhanced adsorption strength are able to competitively replace CH4 molecules within the clay structure. The high separation selectivity of CO2 over CH4 implies the possibility of separating CO2 from natural gas mixtures using the clay minerals. The interlayer sodium cations and negatively charged clay surface can provide enhanced interaction with CO2 molecules that have high quadrupole moment, which is responsible for the higher adsorption loading of CO2. (C) 2015 Elsevier B.V. All rights reserved.