Selective Binding of O₂ Over N₂ in a Redox-Active Metal-Organic Framework with Open Iron(II) Coordination Sites


The air-free reaction between FeCl2 and H4dobdc (dobdc4- = 2,5-dioxido-1,4-benzenedicarboxylate) in a mixture of N,N-dimethylformamide (DMF) and methanol affords Fe2(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 m2/g and features a hexagonal array of one-dimensional channels lined with coordinatively unsaturated FeII centers. Gas adsorption isotherms at 298 K indicate that Fe2(dobdc) binds O2 preferentially over N 2, with an irreversible capacity of 9.3 wt %, corresponding to the adsorption of one O2 molecule per two iron centers. Remarkably, at 211 K, O2 uptake is fully reversible and the capacity increases to 18.2 wt %, corresponding to the adsorption of one O2 molecule per iron center. Mössbauer and infrared spectra are consistent with partial charge transfer from iron(II) to O2 at low temperature and complete charge transfer to form iron(III) and O2 2- at room temperature. The results of Rietveld analyses of powder neutron diffraction data (4 K) confirm this interpretation, revealing O2 bound to iron in a symmetric side-on mode with dO-O = 1.25(1) Å at low temperature and in a slipped side-on mode with dO-O = 1.6(1) Å when oxidized at room temperature. Application of ideal adsorbed solution theory in simulating breakthrough curves shows Fe2(dobdc) to be a promising material for the separation of O2 from air at temperatures well above those currently employed in industrial settings.



Keywords and Phrases

Break Through Curve; Brunauer-emmett-teller Surface Areas; Capacity Increase; Co-ordinatively Unsaturated; Coordination Sites; Hexagonal Arrays; Ideal Adsorbed Solution Theory; Industrial Settings; Infrared Spectrum; Iron Centers; Irreversible Capacity; Low Temperatures; Metal Organic Framework; Mössbauer; N ,N-Dimethylformamide; One-dimensional Channels; Partial Charge Transfer; Powder Neutron Diffraction Data; Redox-active; Room Temperature; Selective Binding; Structure Type; Charge Transfer; Dimethylformamide; Gas Adsorption; Iron Compounds; Metals; Methanol; Molybdenum; Organic Solvents; Rietveld Analysis; Spectroscopy; Iron; Iron; Metal; Nitrogen; Organic Compound; Oxygen; Air Temperature; Electron Transport; Infrared Spectroscopy; Isotherm; Low Temperature; Neutron; Oxidation Reduction Reaction; Oxygen Consumption; Potassium Transport; Room Temperature; Surface Property

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© 2011 American Chemical Society (ACS), All rights reserved.

Publication Date

01 Sep 2011