Browsing by Author "Krishna, R"

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    Hydrocarbon separations in a metal-organic framework with open iron(II) coordination sites
    (American Association for the Advancement of Science (AAAS), 2012-03-30) Bloch, ED; Queen, WL; Krishna, R; Zadrozny, JM; Brown, CM; Long, JR
    The energy costs associated with large-scale industrial separation of light hydrocarbons by cryogenic distillation could potentially be lowered through development of selective solid adsorbents that operate at higher temperatures. Here, the metal-organic framework Fe2(dobdc) (dobdc4– : 2,5-dioxido-1,4-benzenedicarboxylate) is demonstrated to exhibit excellent performance characteristics for separation of ethylene/ethane and propylene/propane mixtures at 318 kelvin. Breakthrough data obtained for these mixtures provide experimental validation of simulations, which in turn predict high selectivities and capacities of this material for the fractionation of methane/ethane/ethylene/acetylene mixtures, removal of acetylene impurities from ethylene, and membrane-based olefin/paraffin separations. Neutron powder diffraction data confirm a side-on coordination of acetylene, ethylene, and propylene at the iron(II) centers, while also providing solid-state structural characterization of the much weaker interactions of ethane and propane with the metal. © 2012, American Association for the Advancement of Science (AAAS)
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    Selective binding of O(2) over N(2) in a redox-active metal-organic framework with open iron(II) coordination sites
    (American Chemical Society, 2011-09-21) Bloch, ED; Murray, LJ; Queen, WL; Chavan, S; Maximoff, SN; Bigi, JP; Krishna, R; Peterson, VK; Grandjean, F; Long, GJ; Smit, B; Bordiga, S; Brown, CM; Long, JR
    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 N2, 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 O22– 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) Ã… at low temperature and in a slipped side-on mode with dO–O = 1.6(1) Ã… 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. © 2011, American Chemical Society