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

Abstract

Six new functionalized vanadium hydroxo terephthalates [V^III(OH)(BDC-X)]·n(guests) (MIL-47(V^III)-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(V^IV)-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 (S_BET in the 305–897 m² g^–1 range), as verified by the N₂ and CO₂ sorption analysis. Among the six functionalized compounds, MIL-47(V^IV)-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(V^IV), 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(V^IV)-OCH₃ framework.