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 29.09.2012   Карта сайта     Language По-русски По-английски
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29.09.2012

Nature Chemistry | Article


Selectivity and direct visualization of carbon dioxide and sulfur dioxide in a decorated porous host





Journal name:

Nature Chemistry

Year published:

(2012)

DOI:

doi:10.1038/nchem.1457


Received


Accepted


Published online



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Abstract



Understanding the mechanism by which porous solids trap harmful gases such as CO2 and SO2 is essential for the design of new materials for their selective removal. Materials functionalized with amine groups dominate this field, largely because of their potential to form carbamates through H2N(δ)···C(δ+)O2 interactions, thereby trapping CO2 covalently. However, the use of these materials is energy-intensive, with significant environmental impact. Here, we report a non-amine-containing porous solid (NOTT-300) in which hydroxyl groups within pores bind CO2 and SO2 selectively. In situ powder X-ray diffraction and inelastic neutron scattering studies, combined with modelling, reveal that hydroxyl groups bind CO2 and SO2 through the formation of O=C(S)=O(δ)···H(δ+)–O hydrogen bonds, which are reinforced by weak supramolecular interactions with C–H atoms on the aromatic rings of the framework. This offers the potential for the application of new ‘easy-on/easy-off’ capture systems for CO2 and SO2 that carry fewer economic and environmental penalties.



Figures at a glance


left


  1. Figure 1: Comparison of gas-binding interactions in amine-functionalized materials and in hydroxyl-functionalized NOTT-300.
    Comparison of gas-binding interactions in amine-functionalized materials and in hydroxyl-functionalized NOTT-300.

    In an amine-functionalized system, the first CO2 molecule attacks the NH2 group through a side-on mode, and the second CO2 molecule interacts with the first CO2 molecule through an end-on mode, resulting in high isosteric adsorption enthalpy (40–90 kJ mol−1)13, 21, 22. In hydroxyl-functionalized system NOTT-300, the first CO2 molecule attacks the OH group through an end-on mode, and the second CO2 molecule interacts with the first CO2 molecule through a side-on mode, resulting in a lower isosteric adsorption enthalpy (27–30 kJ mol−1). Similar weak interactions are also observed in binding of SO2.




  2. Figure 2: Structure of NOTT-300-solvate.
    Structure of NOTT-300-solvate.

    The structure was solved from high-resolution PXRD data by ab initio methods. a, Coordination environment for ligand L4− and the Al(III) centre. b, View of the corner-sharing extended octahedral chain of [AlO4(OH)2]. The μ2-(OH) groups are highlighted as a space-filling model, and linked to each other in a cis configuration. c, View of the three-dimensional framework structure with a channel formed along the c-axis. The free water molecules in the channel are omitted for clarity. d, View of the square-shaped channel. The μ2-(OH) groups protrude into the centre of the channel from four directions (aluminium, green; carbon, grey; oxygen, red; hydrogen, white; [AlO4(OH)2], green octahedron).




  3. Figure 3: Gas sorption isotherms and variation of thermodynamic parameters Qst and ΔS as a function of CO2 uptake in NOTT-300.
    Gas sorption isotherms and variation of thermodynamic parameters Qst and ΔS as a function of CO2 uptake in NOTT-300.

    a, Comparison of the gas adsorption isotherms for NOTT-300 at 273 K and 1.0 bar. b, Variation of isosteric enthalpy (Qst) and entropy (ΔS) of CO2 adsorption. NOTT-300 exhibits highly selective uptake for CO2 and SO2 compared with CH4, CO, N2, H2, O2 and Ar. The CO2 selectivities (calculated from the ratio of initial slopes of the isotherm) are 100, 86, 180, >105, 70 and 137 for CH4, CO, N2, H2, O2 and Ar, respectively. The SO2 selectivities (calculated from the ratio of the initial slopes of the isotherms) are 3,620, 3,105, 6,522, >105, 2,518 and 4,974 for CH4, CO, N2, H2, O2 and Ar, respectively. The thermodynamic parameters Qst and ΔS were calculated using the van't Hoff isochore on CO2 adsorption isotherms measured at 273–303 K. The Qst values lie in the range 27.5–28 kJ mol−1 for CO2 uptakes of 1–2 mmol g−1 and increase continuously thereafter to ∼30 kJ mol−1 at 4.5 mmol g−1. The error in Qst is estimated as 0.05–0.5 kJ mol−1 (as shown by the error bars). Overall, ΔS decreases continuously with increasing surface coverage over the whole loading range. The error in ΔS is estimated as 0.2–1.6 J K−1 mol−1 (as shown by the error bars).




  4. Figure 4: In situ INS and simulated CO2 positions in the pore channel of NOTT-300.
    In situ INS and simulated CO2 positions in the pore channel of NOTT-300.

    a, Comparison of the experimental (top) and DFT-simulated (bottom) INS spectra for bare and CO2-loaded NOTT-300. b, Difference plot for experimental INS spectra of bare and CO2-loaded NOTT-300. Two distinct energy transfer peaks are labelled as I and II. c, View of the structure of NOTT-300·3.2CO2 obtained using PXRD analysis. The adsorbed CO2 molecules in the pore channel are highlighted by the use of ball-and-stick mode. The carbon atom of the second CO2 site is highlighted in blue. The dipole interaction between CO2(I,II) molecules is highlighted in orange (O=C=O···CO2 = 3.920 Å). d, Detailed view of the roles of the OH and CH groups in binding CO2 molecules in a pocket-like cavity. The model was obtained from DFT simulation. The modest hydrogen bond between O(δ) of CO2 and H(δ+) from the Al OH moiety is highlighted in cyan (O···H = 2.335 Å). The weak cooperative hydrogen-bond interactions between O(δ) of CO2 and H(δ+) from CH are highlighted in purple (O···H = 3.029, 3.190 Å, each occurring twice). Each O(δ) centre therefore interacts with five different H(δ+) centres. (Aluminium, green; carbon, grey; oxygen, red; hydrogen, white.)




  5. Figure 5: In situ synchrotron X-ray powder diffraction patterns and refined SO2 positions in the pore channel of NOTT-300.
    In situ synchrotron X-ray powder diffraction patterns and refined SO2 positions in the pore channel of NOTT-300.

    a, Comparison of the powder diffraction patterns for original, evacuated, SO2-loaded and final desolvated samples at 273 K. b, View of the crystal structure of NOTT-300·4.0SO2 obtained from Rietveld refinement of data on SO2-loaded material at 1.0 bar. The adsorbed SO2 molecules in the pore channel are highlighted by the use of ball-and-stick mode. The sulfur atom of the second site of SO2 is highlighted in blue. c, Detailed view of the role of the OH and CH groups in binding SO2 molecules into a distorted pocket-like cavity. The moderate hydrogen bond between O(δ) of SO2(I) and H(δ+) of OH is highlighted in cyan [O···H = 2.376(13) Å]. The weak cooperative hydrogen-bond interaction between O(δ) of SO2 and H(δ+) of CH is highlighted in purple [O···H = 2.806(14), 2.841(17), 3.111(16), 3.725(18) Å]. Therefore, each O(δ) centre is interacting with five different H(δ+) centres simultaneously. The bond distance between S(δ+) of SO2(I) and O(δ) of SO2(II) is 3.34(7) Å. (Aluminium, green; carbon, grey; oxygen, red; hydrogen, white; sulfur, yellow.)




  6. Figure 6: In situ INS and simulated SO2 positions in the pore channel of NOTT-300.







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