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During the formation of metal-organic frameworks (MOFs), metal centres can coordinate with the intended organic linkers, but also with solvent molecules. In this case, subsequent activation by removal of the solvent molecules creates unsaturated 'open' metal sites known to have a strong affinity for CO 2 molecules, but their interactions are still poorly understood. Common force fields typically underestimate by as much as two orders of magnitude the adsorption of CO 2 in open-site Mg-MOF-74, which has emerged as a promising MOF for CO 2 capture. Here we present a systematic procedure to generate force fields using high-level quantum chemical calculations. Monte Carlo simulations based on an ab initio force field generated for CO 2 in Mg-MOF-74 shed some light on the interpretation of thermodynamic data from flue gas in this material. The force field describes accurately the chemistry of the open metal sites, and is transferable to other structures. This approach may serve in molecular simulations in general and in the study of fluid-solid interactions. Most energy scenarios project a significant increase in the role of renewable energy sources 1 . These scenarios also predict an even higher increase in our energy needs. As a consequence, although the relative consumption of fossil fuels will be decreasing, in absolute terms we will continue to burn more coal. In such a scenario, carbon capture and sequestration will be one of the only viable technologies to mitigate CO 2 emissions 1,2 . At present the cost associated with the capture of CO 2 from flue gas is one of the bottlenecks in the large-scale deployment of this technology. Of particular concern is that the efficiency of a coal-fired power plant decreases by as much as 30-40% (ref. 3) because of the energy required to separate and compress CO 2 . The aim of decreasing this parasitic load has motivated the search for novel materials 4,5 .A promising class of materials is metal-organic frameworks (MOFs) 4,6 . MOFs are crystalline materials that consist of metal centres connected by organic linkers. These materials have an extremely large internal surface area and, compared to other common adsorbents, promise very specific customization of their chemistry. By changing the metal and the linker, one can in principle generate many millions of possible materials. In practice, however, we can synthesize only a very small fraction of these materials, and it is important to develop a theoretical method that supports the experimental efforts to identify an ideal MOF for carbon capture. A key aspect is the ability to predict the properties of a MOF before the material is synthesized. At present it is possible to carry out accurate quantum chemical calculations on these types of systems 7 . State-of-the-art density functional theory (DFT) calculations provide important insights into the energetics and siting of CO 2 at zero Kelvin 7 . The separation of flue gas, however, requires thermodynamic information (for example, adsorption isotherms) at flue-gas conditions (40...

Selected metalÀorganic frameworks exhibiting representative properties-high surface area, structural flexibility, or the presence of open metal cation sites-were tested for utility in the separation of CO 2 from H 2 via pressure swing adsorption. Single-component CO 2 and H 2 adsorption isotherms were measured at 313 K and pressures up to 40 bar for Zn 4 O(BTB) 2 (MOF-177, BTB 3À = 1,3,5-benzenetribenzoate), Be 12 (OH) 12 (BTB) 4 (Be-BTB), Co(BDP) (BDP 2À = 1,4-benzenedipyrazolate), H 3 [(Cu 4 Cl) 3 (BTTri) 8 ] (CuBTTri, BTTri 3À = 1,3,5-benzenetristriazolate), and Mg 2 -(dobdc) (dobdc 4À = 1,4-dioxido-2,5-benzenedicarboxylate). Ideal adsorbed solution theory was used to estimate realistic isotherms for the 80:20 and 60:40 H 2 /CO 2 gas mixtures relevant to H 2 purification and precombustion CO 2 capture, respectively. In the former case, the results afford CO 2 /H 2 selectivities between 2 and 860 and mixed-gas working capacities, assuming a 1 bar purge pressure, as high as 8.6 mol/kg and 7.4 mol/L. In particular, metalÀorganic frameworks with a high concentration of exposed metal cation sites, Mg 2 (dobdc) and Cu-BTTri, offer significant improvements over commonly used adsorbents, indicating the promise of such materials for applications in CO 2 /H 2 separations. C oal is an abundant resource that is heavily relied upon for global energy, and therefore emission-free coal-fired power plants are a necessary near-term component of a clean energy future.1 Integrated gasification and combined cycle (IGCC) systems equipped with precombustion CO 2 capture, wherein CO 2 is separated from H 2 and sequestered, are promising in this regard. 2The technology for separating H 2 and CO 2 is already well-developed due to the tremendous scale of H 2 production (50 million tons per year worldwide) and is primarily accomplished using pressureswing adsorption (PSA).3 Here, porous zeolites or activated carbons selectively and reversibly adsorb impurities in the presence of H 2 at high pressures. Methods for altering the current PSA specifications to meet the needs of precombustion CO 2 capture in an IGCC system are being explored, 4 although vast improvements in CO 2 /H 2 separations must be made to render this strategy economically viable.2f Regardless, optimizing the efficiency of adsorptive H 2 purification in industry is also of crucial importance to minimizing energy usage, 5 since, at current production rates, which are steadily rising, a 10% increase in the efficiency of the process could save the energy output of approximately 18 average US coal-fired power plants. 6 Much of the energy input for a PSA system is used in mass transport of the gas and regeneration of the adsorbents, and as a result improving adsorbent selectivity and the capacity for CO 2 would increase efficiency.3f Extensive experimental 7 and theoretical 8 investigations suggest that further optimization of zeolites and activated carbons will yield only modest improvements in CO 2 /H 2 separation performance. Thus, there is a need for new type...

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