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Oct 28

New Functionalized Metal-Organic Frameworks MIL-47-X (X = -Cl, -Br, -CH3, -CF3, -OH, -OCH3): Synthesis, Characterization and CO2 Adsorption Properties

Authors: Shyam Biswas, Danny E. P. Vanpoucke, Toon Verstraelen, Matthias Vandichel, Sarah Couck, Karen Leus, Ying-Ya Liu, Michel Waroquier, Veronique Van Speybroeck, Joeri F. M. Denayer, and Pascal Van Der Voort
Journal: J. Phys. Chem. C 117(44), 22784-22796 (2013)
doi: 10.1021/jp406835n
IF(2013): 4.835
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pdf: <J.Phys.Chem.C>

Abstract

Six new functionalized vanadium hydroxo terephthalates [VIII(OH)(BDC-X)]·n(guests) (MIL-47(VIII)-X-AS) (BDC = 1,4-benzenedicarboxylate; X = −Cl, −Br, −CH3, −CF3, −OH, −OCH3; AS = as-synthesized) along with the parent MIL-47 were synthesized under rapid microwave-assisted hydrothermal conditions (170 °C, 30 min, 150 W). The unreacted H2BDC-X and/or occluded solvent molecules can be removed by thermal activation under vacuum, leading to the empty-pore forms of the title compounds (MIL-47(VIV)-X). Except pristine MIL-47 (+III oxidation state), the vanadium atoms in all the evacuated functionalized solids stayed in the +IV oxidation state. The phase purity of the compounds was ascertained by X-ray powder diffraction (XRPD), diffuse reflectance infrared Fourier transform (DRIFT) spectroscopy, Raman, thermogravimetric (TG), and elemental analysis. The structural similarity of the filled and empty-pore forms of the functionalized compounds with the respective forms of parent MIL-47 was verified by cell parameter determination from XRPD data. TGA and temperature-dependent XRPD (TDXRPD) experiments in an air atmosphere indicate high thermal stability in the 330–385 °C range. All the thermally activated compounds exhibit significant microporosity (SBET in the 305–897 m2 g–1 range), as verified by the N2 and CO2 sorption analysis. Among the six functionalized compounds, MIL-47(VIV)-OCH3 shows the highest CO2 uptake, demonstrating the determining role of functional groups on the CO2 sorption behavior. For this compound and pristine MIL-47(VIV), Widom particle insertion simulations were performed based on ab initio calculated crystal structures. The theoretical Henry coefficients show a good agreement with the experimental values, and calculated isosurfaces for the local excess chemical potential indicate the enhanced CO2 affinity is due to two effects: (i) the interaction between the methoxy group and CO2 and (ii) the collapse of the MIL-47(VIV)-OCH3 framework.