Selective Binding of O₂ Over N₂ in a Redox-active Metal-organic Framework with Open Iron(II) Coordination Sites
The air-free reaction between FeCl 2 and H 4dobdc (dobdc 4- = 2,5-dioxido-1,4-benzenedicarboxylate) in a mixture of N,N-dimethylformamide (DMF) and methanol affords Fe 2(dobdc) ·4DMF, a metal-organic framework adopting the MOF-74 (or CPO-27) structure type. The desolvated form of this material displays a Brunauer-Emmett-Teller (BET) surface area of 1360 m 2/g and features a hexagonal array of one-dimensional channels lined with coordinatively unsaturated Fe II centers. Gas adsorption isotherms at 298 K indicate that Fe 2(dobdc) binds O 2 preferentially over N 2, with an irreversible capacity of 9.3 wt %, corresponding to the adsorption of one O 2 molecule per two iron centers. Remarkably, at 211 K, O 2 uptake is fully reversible and the capacity increases to 18.2 wt %, corresponding to the adsorption of one O 2 molecule per iron center. Mössbauer and infrared spectra are consistent with partial charge transfer from iron(II) to O 2 at low temperature and complete charge transfer to form iron(III) and O 2 2- at room temperature. The results of Rietveld analyses of powder neutron diffraction data (4 K) confirm this interpretation, revealing O 2 bound to iron in a symmetric side-on mode with d O-O = 1.25(1) Å at low temperature and in a slipped side-on mode with d O-O = 1.6(1) Å when oxidized at room temperature. Application of ideal adsorbed solution theory in simulating breakthrough curves shows Fe 2(dobdc) to be a promising material for the separation of O 2 from air at temperatures well above those currently employed in industrial settings. © 2011 American Chemical Society.
E. D. Bloch and L. J. Murray and W. L. Queen and S. Chavan and S. N. Maximoff and J. P. Bigi and R. Krishna and V. K. Peterson and F. Grandjean and G. J. Long and B. Smit and S. Bordiga and C. M. Brown and J. R. Long, "Selective Binding of O₂ Over N₂ in a Redox-active Metal-organic Framework with Open Iron(II) Coordination Sites," Journal of the American Chemical Society, American Chemical Society (ACS), Jan 2011.
The definitive version is available at http://dx.doi.org/10.1021/ja205976v
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