Post-synthesis modification of a MOF by replacing coordinated solvent molecules with highly polar ligands leads to considerable enhancement of CO 2 /N 2 selectivity. Concerns about greenhouse gas concentrations in the atmosphere are a strong motivation to reduce CO 2 emissions from industrial processes. Burning of fossil fuel to generate electricity is a major source of CO 2 in the atmosphere, but the capture and sequestration of CO 2 from flue gas emissions of power plants is a daunting chal-lenge. 1 Flue gases consist of nitrogen (typically more than two-thirds), CO 2 , water vapor, oxygen, and minor components such as carbon monoxide, nitrogen oxides, and sulfur oxides. Several technologies have been considered for CO 2 separation from nitrogen-rich streams, including absorption, membranes, and adsorption separations. 2 Adsorption-based separations such as pressure-swing adsorption (PSA) are attractive due to their low energy requirements. However, an adsorbent with high CO 2 selectivity and capacity is essential in PSA processes for CO 2 separations. 3 Recently, metal-organic frameworks (MOFs) have attracted great interest as adsorbents due to their extremely high surface areas, low densities, and uniform, tailorable pore structures. These properties make them promising candidates for adsorption separations, as well as gas storage, catalysis, and sensing. 4 Efforts to tune the pore size and provide desired surface chemistries in MOFs can be divided into two main strategies: (1) direct assembly of new MOFs from particular metal nodes and organic linkers and (2) post-synthesis modification of pre-constructed robust precursor MOFs. In the direct-assembly strategies, certain desirable functional groups may be hard to incor-porate into MOFs, either because of thermal instability under MOF synthesis conditions or due to competitive reaction with intended framework components. 5 Additionally, it is known that both the connectivity and the degree of catenation can be very sensitive to small changes in the organic ligands for the synthesis of MOFs through direct assembly. 6 Because of these complexities, post-synthesis modification strategies are emerging as an alternative method for tailoring MOFs toward specific applications. Several reports on this strategy have appeared recently. 7 Recently, Farha et al. proposed a new MOF strut (4,4 0 ,4 00 ,4%-benzene-1,2,4,5-tetrayltetrabenzoic acid, 1, Scheme 1) and used it to synthesize a 3D non-catenated Zn-paddlewheel MOF [Zn 2 (1)(DMF) 2 ] n (DMF) m (2) [DMF ¼ dimethylformamide]. 5 The single-crystal X-ray structure of 2 indicates that two DMF molecules are coordinated to the axial sites of the Zn(II) 2 units. Farha et al. showed that by heating 2 at 100 C under vacuum for 24 h, free non-coordinated DMF molecules (designated (DMF) m) were removed and a partially evacuated MOF, 3, was prepared. In 3, the coordi-nated DMF molecules remain in the pores. A DMF-free version, 4, was obtained by heating 2 at 150 C under vacuum for 24 h. In this case, all free and coordinated DMF molecules were removed and open metal sites were formed. By immersing 4 in CHCl 3 solutions of each of several pyridine derivatives (py-R), a collection of py-R-modified MOFs was obtained. 1 H NMR and TGA results showed the formation of highly porous cavity-modified MOFs, [Zn 2 (1)(py-R) 2 ] n , as well as the coordinative binding of the py-R ligands. 5 In this work, we compare adsorption in MOFs 3, 4, and 5, where 5 is the py-CF 3 modified version of 4, i.e. [Zn 2 (1)(py-CF 3) 2 ] n . Single-component adsorption isotherms for CO 2 , N 2 , and CH 4 were measured experimentally in all three MOFs. Then from the pure-component isotherms, the selectivities for CO 2 /N 2 and CO 2 /CH 4 mixtures were calculated using ideal adsorbed solution theory (IAST). 8 Many studies have shown that IAST can be used to successfully predict gas mixture adsorption in zeolites, 9–11 and recently the theory has been tested in MOFs using molecular simulations. 9,12,13 MOFs 3, 4, and 5 form an interesting series for elucidating the effects of different framework features on adsorption capacity and selectivity. MOF 4 has open-metal sites, which are expected to enhance adsorption, especially for CO 2 and N 2 , which are Scheme 1 Preparation methods of 3, 4, and 5. i) DMF/80 C/24 h, fol-lowed by evacuation while heating at 100 C; ii) evacuation while heating at 150 C; iii) soak in a solution of CHCl 3 /4-(trifluoromethyl)pyridine, followed by evacuation while heating at 100 C.