Inorganic membranes are becoming increasingly interesting for CO2 gas separation processes considering their potential in CO2 capture and storage for mitigating global warming. Porous alumina filters with the ability of CO2 gas separation were prepared by modifying four types of silane coupling agents: 3-Aminopropyltriethoxysilane (APTES), 3-Ureidopropyltriethoxysilane (Ureido), Octadecyltrimethoxysilane (ODS), and Heptadecafluorodecyltrimethoxysilane (FAS17). CO2/N-2 gas selectivity was less than 1.0 for all the filters, meaning that less CO2 gas could permeate the filter, compared with N-2 gas. The difference of gas selectivity among the prepared filters was ascribed to two main factors. The first was the relative electric charges of the atoms in both the gas molecules and the functional groups of the silane coupling agents. Oppositely charged atoms result in a physical interaction between CO2 molecule and the silane coupling agent at the surface of porous alumina structure. The second factor was the configuration of the CO2 molecule. It could be considered that the oxygen atoms at the exterior of the CO2 molecule have a stronger physical interaction with other charged species than the carbon atoms located at the center of the molecule. (C) 2015 Elsevier B.V. All rights reserved.