Pulse Gas Chromatographic Study of Adsorption of Substituted Aromatics and Heterocyclic Molecules on MIL-47 at Zero Coverage

The low coverage adsorptive properties of the MIL-47 metal organic framework toward aromatic and heterocyclic molecules are reported in this paper. The effect of molecular functionality and size on Henry adsorption constants and adsorption enthalpies of alkyl and heteroatom functionalized benzene derivates and heterocyclic molecules was studied using pulse gas chromatography. By means of statistical analysis, experimental data was analyzed and modeled using principal component analysis and partial least-squares regression. Structure–property relationships were established, revealing and confirming several trends. Among the molecular properties governing the adsorption process, vapor pressure, mean polarizability, and dipole moment play a determining role

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

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 (<i>S</i>_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

New V^IV-Based Metal–Organic Framework Having Framework Flexibility and High CO₂ Adsorption Capacity

A vanadium based metal–organic framework (MOF), VO­(BPDC) (BPDC^2– = biphenyl-4,4′-dicarboxylate), adopting an expanded MIL-47 structure type, has been synthesized via solvothermal and microwave methods. Its structural and gas/vapor sorption properties have been studied. This compound displays a distinct breathing effect toward certain adsorptives at workable temperatures. The sorption isotherms of CO₂ and CH₄ indicate a different sorption behavior at specific temperatures. In situ synchrotron X-ray powder diffraction measurements and molecular simulations have been utilized to characterize the structural transition. The experimental measurements clearly suggest the existence of both narrow pore and large pore forms. A free energy profile along the pore angle was computationally determined for the empty host framework. Apart from a regular large pore and a regular narrow pore form, an overstretched narrow pore form has also been found. Additionally, a variety of spectroscopic techniques combined with N₂ adsorption/desorption isotherms measured at 77 K demonstrate that the existence of the mixed oxidation states V^III/V^IV in the titled MOF structure compared to pure V^IV increases the difficulty in triggering the flexibility of the framework