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The process of carbon capture and sequestration has been proposed as a method of mitigating the build-up of greenhouse gases in the atmosphere. If implemented, the cost of electricity generated by a fossil fuel-burning power plant would rise substantially, owing to the expense of removing CO2 from the effluent stream. There is therefore an urgent need for more efficient gas separation technologies, such as those potentially offered by advanced solid adsorbents. Here we show that diamine-appended metal-organic frameworks can behave as phase-change adsorbents, with unusual step-shaped CO2 adsorption isotherms that shift markedly with temperature. Results from spectroscopic, diffraction and computational studies show that the origin of the sharp adsorption step is an unprecedented cooperative process in which, above a metal-dependent threshold pressure, CO2 molecules insert into metal-amine bonds, inducing a reorganization of the amines into well-ordered chains of ammonium carbamate. As a consequence, large CO2 separation capacities can be achieved with small temperature swings, and regeneration energies appreciably lower than achievable with state-of-the-art aqueous amine solutions become feasible. The results provide a mechanistic framework for designing highly efficient adsorbents for removing CO2 from various gas mixtures, and yield insights into the conservation of Mg 2+ within the ribulose-1,5-bisphosphate carboxylase/oxygenase family of enzymes.
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Acknowledgements We thank A. S. Bhown and A. H. Berger of EPRI, H. Krutka, C. M. Brown and. K. S. Suslick for discussions, and L. Ribaud and the 11-BM staff at the Advanced Photon Source at Argonne National Laboratory for assisting with powder X-ray diffraction experiments. The work presented here pertaining to the synthesis and gas adsorption properties of metal-organic frameworks was funded by the Advanced Research Projects Agency–Energy (ARPA-E), US Department of Energy (DOE), under award numbers DE-AR0000103 and DE-AR0000402. Funding pertaining to the characterization of materials by spectroscopy and X-ray diffraction and the computational work performed by W.S.D., B.V., R.P., S.K.S., K.L., J.B.N., B.S. and J.B.K. was providedbythe Center for Gas Separations Relevant toClean EnergyTechnologies, an Energy Frontier Research Center funded by the DOE, Office of Science, Office of Basic Energy Sciences under award DE-SC0001015. Experiments performed in Turin were supported by grant MIUR-PRIN 2010-2011. Work at SIMAP was performed using computer resources from GENCI (CINES grant 2014-c2015097211). The computational work performed by S.O.O., A.L.D., N.P. and L.G. was supported through the Nanoporous Materials Genome Center of the DOE, Office of Basic Energy Sciences, Division of Chemical Sciences, Geosciences, and Biosciences, under award number DE-FG02-12ER16362.This researchused resources of the Advanced Photon Source, a DOE Office of Science User Facility operated for the DOE Office of Science by Argonne National Laboratory under contract no. DE-AC02-06CH11357. Portions of this work (use of beamline 6.3.2 at the Advanced Light Source; a user project at The Molecular Foundry, facilitated by T.P., L.F.W. and D.P., and use of its computer cluster vulcan, managed by the High Performance Computing Services Group; use of the National Energy Research Scientific Computing Center) were performed at Lawrence Berkeley National Laboratory, which is supported by the Director, Office of Science, Office of Basic Energy Sciences, of the DOE under contract no. DE-AC02-05CH11231. For fellowship support, we further thank the National Science Foundation (J.A.M.), Gerald K. Branch and Arkema (E.D.B.) and the Research Council of Norway (grant 230534 to S.K.S.).
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