Metal-organic frameworks (MOFs) have shown great promise in catalysis, mainly due to their high content of active centers, large internal surface areas, tunable pore size, and versatile chemical functionalities. However, it is a challenge to rationally design and construct MOFs that can serve as highly stable and reusable heterogeneous catalysts. Here two new robust 3D porous metal-cyclam-based zirconium MOFs, denoted VPI-100 (Cu) and VPI-100 (Ni), have been prepared by a modulated synthetic strategy. The frameworks are assembled by eight-connected Zr clusters and metallocyclams as organic linkers. Importantly, the cyclam core has accessible axial coordination sites for guest interactions and maintains the electronic properties exhibited by the parent cyclam ring. The VPI-100 MOFs exhibit excellent chemical stability in various organic and aqueous solvents over a wide pH range and show high CO uptake capacity (up to ∼9.83 wt% adsorption at 273 K under 1 atm). Moreover, VPI-100 MOFs demonstrate some of the highest reported catalytic activity values (turnover frequency and conversion efficiency) among Zr-based MOFs for the chemical fixation of CO with epoxides, including sterically hindered epoxides. The MOFs, which bear dual catalytic sites (Zr and Cu/Ni), enable chemistry not possible with the cyclam ligand under the same conditions and can be used as recoverable stable heterogeneous catalysts without losing performance.

The chronoamperometric response (I vs t) of three metallocene-doped metal–organic frameworks (MOFs) thin films (M-NU-1000, M = Fe, Ru, Os) in two different electrolytes (tetrabutyl­ammonium hexafluoro­phosphate [TBAPF6] and tetrabutylammonium tetrakis­(pentafluorophenyl)­borate [TBATFAB]) was utilized to elucidate the diffusion coefficients of electrons and ions (D e and D i, respectively) through the structure in response to an oxidizing applied bias. The application of a theoretical model for solid state voltammetry to the experimental data revealed that the diffusion of ions is the rate-determining step at the three different time stages of the electrochemical transformation: an initial stage characterized by rapid electron diffusion along the crystal-solution boundary (stage A), a second stage that represents the diffusion of electrons and ions into the bulk of the MOF crystallite (stage B), and a final period of the conversion dominated only by the diffusion of ions (stage C). Remarkably, electron diffusion (D e) increased in the order of Fe < Ru < Os using PF6 1– as the counteranion in all the stages of the voltammogram, demonstrating the strategy to modulate the rate of electron transport through the incorporation of rapidly self-exchanging molecular moieties into the MOF structure. The D e values obtained with larger TFAB1– counteranion were generally in agreement with the previous trend but were on average lower than those obtained with PF6 1–. Similarly, the ion diffusion coefficient (D i) was generally higher for TFAB1– than for PF6 1– as the ions diffuse into the crystal bulk, due to the high degree of ion-pair association between PF6 1– and the metallocenium ion, resulting in a faster penetration of the weakly associated TFAB1– anion through the MOF pores. These structure–function relationships provide a foundation for the future design, control, and optimization of electron and ion transport properties in MOF thin films.

The ditopic ligands 2,6-dicarboxy-9,10-anthraquinone and 1,4-dicarboxy-9,10-anthraquinone were used to synthesize two new UiO-type metal-organic frameworks (MOFs; namely, 2,6-Zr-AQ-MOF and 1,4-Zr-AQ-MOF, respectively). The Pourbaix diagrams (E vs pH) of the MOFs and their ligands were constructed using cyclic voltammetry in aqueous buffered media. The MOFs exhibit chemical stability and undergo diverse electrochemical processes, where the number of electrons and protons transferred was tailored in a Nernstian manner by the pH of the media. Both the 2,6-Zr-AQ-MOF and its ligand reveal a similar electrochemical pK value (7.56 and 7.35, respectively) for the transition between a two-electron, two-proton transfer (at pH < pK) and a two-electron, one-proton transfer (at pH > pK). In contrast, the position of the quinone moiety with respect to the zirconium node, the effect of hydrogen bonding, and the amount of defects in 1,4-Zr-AQ-MOF lead to the transition from a two-electron, three-proton transfer to a two-electron, one-proton transfer. The pK of this framework (5.18) is analogous to one of the three electrochemical pK values displayed by its ligand (3.91, 5.46, and 8.80), which also showed intramolecular hydrogen bonding. The ability of the MOFs to tailor discrete numbers of protons and electrons suggests their application as charge carriers in electronic devices.

Metal−organic cages are a class of supramolecular structures that often require the careful selection of organic linkers and metal nodes. Of this class, few examples of metal−organic cages exist where the nodes are composed of main group metals. Herein, we have prepared an aluminum-based metal−organic cage, H 8 [Al 8 (pdc) 8 (OAc) 8 O 4 ] (Al-pdc-AA), using inexpensive and commercially available materials. The cage formation was achieved via solvothermal self-assembly of solvated aluminum and pyridine-dicarboxylic linkers in the presence of a capping agent, acetic acid. The obtained supramolecular structure was characterized by single-crystal X-ray diffraction (SCXRD), thermogravimetric analysis, and NMR spectroscopy. Based on crystal structure and computational analyses, the cage has a 3.7 Å diameter electronrich cavity suitable for the binding of cations such as cesium (ionic radius of 1.69 Å). The host−guest interactions were probed with 1 H and 133 Cs NMR spectroscopy in DMSO, where at low concentrations, Cs + binds to Al-pdc-AA in a 1:1 ratio. The binding site was identified from the crystal structure of CsH 7 [Al 8 (pdc) 8 (OAc) 8 O 4 ] (Cs + ⊂Al-pdc-AA), and a binding affinity of ∼10 6 −10 7 M −1 was determined from NMR titration experiments. The Al-pdc-AA showed improved selectivity for cesium binding over alkali metal cations (Cs + > Rb + > K + ≫ Na + ∼ Li + ). Collectively, the study reports a novel aluminum cage that can serve as a promising host for efficient and selective cesium removal.

Abstract:Orange II decomposition was studied on a variety of iron/carbon supported catalysts and control studies of the supports alone (carbon), and iron/alumina (non-active support). Variables tested included the impact of UV radiation, inclusion of hydrogen peroxide, catalyst treatment methods (oven treated and plasma torch treated) and type of the support. Results obtained for Orange II degradation indicated that active sites on carbon are more active for the catalytic decomposition of Orange II molecules, than metal sites. Oven-treated iron catalysts showed higher OII removal than catalysts prepared by plasma torch due to the fact that iron blocks carbon catalytic sites. XRD experiment on the non-active support allowed concluding that the oxidation state of Fe on the catalyst is not the main factor in the photocatalytic degradation of Orange II.