New Functionalized
Metal–Organic Frameworks
MIL-47‑X (X = −Cl, −Br, −CH<sub>3</sub>, −CF<sub>3</sub>, −OH, −OCH<sub>3</sub>): Synthesis,
Characterization, and CO<sub>2</sub> Adsorption Properties
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Abstract
Six new functionalized vanadium hydroxo
terephthalates [V<sup>III</sup>(OH)(BDC-X)]·n(guests) (MIL-47(V<sup>III</sup>)-X-AS) (BDC =
1,4-benzenedicarboxylate; X = −Cl, −Br,
−CH<sub>3</sub>, −CF<sub>3</sub>, −OH, −OCH<sub>3</sub>; 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 H<sub>2</sub>BDC-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(V<sup>IV</sup>)-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 (<i>S</i><sub>BET</sub> in the 305–897
m<sup>2</sup> g<sup>–1</sup> range), as verified by the N<sub>2</sub> and CO<sub>2</sub> sorption analysis. Among the six functionalized
compounds, MIL-47(V<sup>IV</sup>)-OCH<sub>3</sub> shows the highest
CO<sub>2</sub> uptake, demonstrating the determining role of functional
groups on the CO<sub>2</sub> sorption behavior. For this compound
and pristine MIL-47(V<sup>IV</sup>), 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 CO<sub>2</sub> affinity is due to two effects:
(i) the interaction between the methoxy group and CO<sub>2</sub> and
(ii) the collapse of the MIL-47(V<sup>IV</sup>)-OCH<sub>3</sub> framework