Browsing by Author "Brown, CM"
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- ItemComprehensive study of carbon dioxide adsorption in the metal–organic frameworks M2(dobdc) (M = Mg, Mn, Fe, Co, Ni, Cu, Zn)(Royal Society of Chemistry, 2014-08-28) Queen, WL; Hudson, MR; Bloch, ED; Mason, JA; Gonzalez, MI; Lee, JS; Gygi, D; Howe, JD; Lee, K; Darwish, TA; James, M; Peterson, VK; Teat, SJ; Smit, B; Neaton, JB; Long, JR; Brown, CMAnalysis of the CO2 adsorption properties of a well-known series of metal–organic frameworks M2(dobdc) (dobdc4− = 2,5-dioxido-1,4-benzenedicarboxylate; M = Mg, Mn, Fe, Co, Ni, Cu, and Zn) is carried out in tandem with in situ structural studies to identify the host–guest interactions that lead to significant differences in isosteric heats of CO2 adsorption. Neutron and X-ray powder diffraction and single crystal X-ray diffraction experiments are used to unveil the site-specific binding properties of CO2 within many of these materials while systematically varying both the amount of CO2 and the temperature. Unlike previous studies, we show that CO2 adsorbed at the metal cations exhibits intramolecular angles with minimal deviations from 180°, a finding that indicates a strongly electrostatic and physisorptive interaction with the framework surface and sheds more light on the ongoing discussion regarding whether CO2 adsorbs in a linear or nonlinear geometry. This has important implications for proposals that have been made to utilize these materials for the activation and chemical conversion of CO2. For the weaker CO2 adsorbents, significant elongation of the metal–O(CO2) distances are observed and diffraction experiments additionally reveal that secondary CO2 adsorption sites, while likely stabilized by the population of the primary adsorption sites, significantly contribute to adsorption behavior at ambient temperature. Density functional theory calculations including van der Waals dispersion quantitatively corroborate and rationalize observations regarding intramolecular CO2 angles and trends in relative geometric properties and heats of adsorption in the M2(dobdc)–CO2 adducts. © 2014, The Royal Society of Chemistry.
- ItemHydrocarbon 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, JRThe 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)
- ItemHydrogen adsorption in HKUST-1: a combined inelastic neutron scattering and first-principles study(Institute of Physics, 2009-05-20) Brown, CM; Liu, Y; Yildirim, T; Peterson, VK; Kepert, CJHydrogen adsorption in high surface area nanoporous coordination polymers has attracted a great deal of interest in recent years due to the potential applications in energy storage. Here we present combined inelastic neutron scattering measurements and detailed first-principles calculations aimed at unraveling the nature of hydrogen adsorption in HKUST-1 (Cu-3(1,3,5-benzenetricarboxylate)(2)), a metal-organic framework (MOF) with unsaturated metal centers. We reveal that, in this system, the major contribution to the overall binding comes from the classical Coulomb interaction which is not screened due to the open metal site; this explains the relatively high binding energies and short H-2-metal distances observed in MOFs with exposed metal sites as compared to traditional ones. Despite the short distances, there is no indication of an elongation of the H-H bond for the bound H-2 molecule at the metal site. We find that both the phonon and rotational energy levels of the hydrogen molecule are closely similar, making the interpretation of the inelastic neutron scattering data difficult. Finally, we show that the orientation of H-2 has a surprisingly large effect on the binding potential, reducing the classical binding energy by almost 30%. The implication of these results for the development of MOF materials for better hydrogen storage is discussed. © 2009, Institute of Physics
- ItemInelastic neutron scattering of H-2 adsorbed in HKUST-1(Elsevier, 2007-10-31) Liu, Y; Brown, CM; Neumann, DA; Peterson, VK; Kepert, CJA series of inelastic neutron scattering (INS) investigations of hydrogen adsorbed in activated HKUST-1 (Cu-3(1,3,5-benzenetricarboxylate)(2)) result in INS spectra with rich features, even at very low loading (< 1.0 H-2:Cu). The distinct inelastic features in the spectra show that there are three binding sites that are progressively populated when the H-2 loading is less than 2.0 H-2:Cu, which is consistent with the result obtained from previous neutron powder diffraction experiments. The temperature dependence of the INS spectra reveals the relative binding enthalpies for H-2 at each site. © 2007, Elsevier Ltd.
- ItemMetal-organic frameworks with exceptionally high methane uptake: where and how is methane stored?(Wiley-VCH Verlag Berlin, 2010-05-03) Wu, H; Simmons, JM; Liu, Y; Brown, CM; Wang, XS; Ma, S; Peterson, VK; Southon, PD; Kepert, CJ; Zhou, HC; Yildirim, T; Zhou, WMetal–organic frameworks (MOFs) are a novel family of physisorptive materials that have exhibited great promise for methane storage. So far, a detailed understanding of their methane adsorption mechanism is still scarce. Herein, we report a comprehensive mechanistic study of methane storage in three milestone MOF compounds (HKUST-1, PCN-11, and PCN-14) the CH4 storage capacities of which are among the highest reported so far among all porous materials. The three MOFs consist of the same dicopper paddlewheel secondary building units, but contain different organic linkers, leading to cagelike pores with various sizes and geometries. From neutron powder diffraction experiments and accurate data analysis, assisted by grand canonical Monte Carlo (GCMC) simulations and DFT calculations, we anambiguously revealed the exact locations of the stored methane molecules in these MOF materials. We found that methane uptake takes place primarily at two types of strong adsorption site: 1) the open Cu coordination sites, which exhibit enhanced Coulomb attraction toward methane, and 2) the van der Waals potential pocket sites, in which the total dispersive interactions are enhanced due to the molecule being in contact with multiple “surfaces”. Interestingly, the enhanced van der Waals sites are present exclusively in small cages and at the windows to these cages, whereas large cages with relatively flat pore surfaces bind very little methane. Our results suggest that further, rational development of new MOF compounds for methane storage applications should focus on enriching open metal sites, increasing the volume percentage of accessible small cages and channels, and minimizing the fraction of large pores. © 2010, Wiley-VCH Verlag Berlin
- ItemSelective 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, JRThe 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
- ItemSpectroscopic identification of hydrogen spillover species in ruthenium-modified high surface area carbons by diffuse reflectance infrared fourier transform spectroscopy(American Chemical Society., 2012-12-27) Blackburn, JL; Engtrakul, C; Bult, JB; Hurst, K; Zhao, Y; Xu, Q; Parilla, PA; Simpson, LJ; Rocha, JDR; Hudson, MR; Brown, CM; Gennett, TIn recent years, carbon-based sorbents have been recognized for their potential application within vehicular hydrogen storage applications. One method by which sorbents have been reported to store appreciable hydrogen at room temperature is via a spillover process: where molecular hydrogen is first dissociated by metal nanoparticle catalysts and atomic hydrogen subsequently migrates onto the carbon substrate. Many reports have invoked the spillover mechanism to explain enhancements in reversible room temperature hydrogen uptake for metal-decorated sorbents. However, there is a lack of experimental evidence for the proposed chemical species formed as well as several differing theoretical explanations describing the process. In this report, we utilize diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) to identify the various chemical species formed upon room temperature H-2 charging of ruthenium-decorated high surface area carbons. Room temperature H-2 loading of a control sample with no ruthenium nanoparticles (Ru NPs) leads to broad reversible peaks in the DRIFTS spectrum that correspond to the vibration-rotation transitions of weakly bound physisorbed hydrogen molecules. In contrast, the sample modified with Ru NPs shows a variety of reversible and irreversible peaks in addition to the physisorbed H-2 peaks. Rigorous experimental and theoretical analysis enables the assignment of the peaks to ruthenium-mediated formation of water, surface hydroxyl groups (R-OH, where R = carbon or ruthenium), and C-H bonds. The low-energy DRIFTS peaks assigned to spillover C-H bonds were additionally confirmed using inelastic neutron spectroscopy. Reversible vibrational peaks consistent with ruthenium-mediated formation of C-H bonds provide much-needed spectroscopic evidence for the spillover process. The results demonstrated here should facilitate future mechanistic investigations of hydrogen sorption on transition metal nanoparticles and high surface area activated carbons. © 2012, American Chemical Society.
- ItemStructural study of D(2) within the trimodal pore system of a metal organic framework(American Chemical Society, 2011-05-05) Peterson, VK; Brown, CM; Liu, Y; Kepert, CJD-2 loaded to saturation at 25 K into the porous coordination framework Cu-3(1,3,5-benzenetricarboxylate)(2) was studied using neutron powder diffraction. A saturation loading equivalent to 6.1 wt % H-2 is reached at 25 K. D-2 is located at up to nine distinct sites with six stable at saturation and three "metastable" sites at doses intermediate to saturation. Filling of the trimodal pore system is complex, and a concentration dependency on the optimal D-2 arrangement is noted. A dynamic rearrangement of the D-2 in the pores to a cubic-closed packed equilibrium structure occurs at doses close to saturation, with 4 D-2 molecules arranged tetrahedrally in the smallest pore, 32 D-2 arranged in a truncated octahedron with capped hexagonal faces in the intermediate sized pore, and 48 D-2 arranged in a face-capped rhombic dodecahedron in the largest pore. The equilibrium structure of D-2 in the largest pore at doses close to and at saturation was found to be the same arrangement that is optimal for 48 circles on a sphere. The structural response of the framework to D-2 adsorption is dependent on the amount of D-2, where the host lattice expands with increasing amount of adsorbed D-2 at low and high D-2 doses, and contracts upon adsorption of intermediate amounts of D-2, commensurate with the known flexibility exhibited by this framework. © 2011, American Chemical Society