CO₂ Adsorption in M‑IRMOF-10 (M = Mg, Ca, Fe, Cu, Zn, Ge, Sr, Cd, Sn, Ba)

Metal–organic frameworks (MOFs) have been studied extensively for application in flue gas separation because of their tunability, structural stability, and large surface area. M-IRMOF-10 (M = transition metal or main-group atom) is a well-studied series of structures and is composed of saturated tetrahedral Zn₄O nodes and dicarboxylate linkers that form a cubic unit cell. We report the results of a computational study on the effects that changing the metal atoms within IRMOF-10 has on the affinity of the material towards CO₂. Force fields were parametrized using quantum mechanical calculations to systematically compare the effects of different metal centers on CO₂ adsorption at high and low pressure. Two different methods for the determination of partial charges (DDEC and CM5) and force field parameter sets (TraPPE and UFF) were employed. TraPPE parameters with fitted metal–CO₂ interactions and CM5 charges resulted in isotherms that were closer to experiment than pure UFF. The results indicate that exchanging the Zn²⁺ ions in the IRMOF-10 series with metals that have larger ionic radii (Sn²⁺ and Ba²⁺) can lead to an increase in CO₂ affinity due to the increased exposure of the positive metal charge to the oxygen atoms of CO₂ and the increased interaction from the more diffuse electrons

Computational Study of Structural and Electronic Properties of Lead-Free CsMI₃ Perovskites (M = Ge, Sn, Pb, Mg, Ca, Sr, and Ba)

Electronic structure calculations of five crystallography-imitated structures of CsMI₃ perovskites with M = Ge, Sn, Pb, Mg, Ca, Sr, and Ba were performed. The formation energy of different perovskite phases, their relative stability, and structural and electronic properties were explored. The sensitivity of the calculations to the choice of the density functional was investigated, and the predictions were compared with experimental results. The outcome of this study is that Mg and Ba perovskites are unlikely to form in the cubic, tetragonal, or orthorhombic phases because they have positive formation energies. Although Ca and Sr perovskites have negative formation energies with respect to the metal-iodide precursors, they exhibit wide band gaps and high hygroscopicity, making these unlikely candidates for applications in photovoltaic devices. Our results suggest that the performance of a local density functional with a nonseparable gradient approximation (NGA) is similar to that of hybrid functionals in terms of band gap predictions, when M in CsMI₃ is a p-block element (Pb, Sn, and Ge). However, local density functionals with NGA predictions for the band gap are similar to other local functionals with a generalized gradient approximation (PBE, PBEsol, and PBE-D3) and are worse than those of HSE06, when M is an s-block element (Mg, Ca, Sr, and Ba)

Single-Ion Magnetic Anisotropy and Isotropic Magnetic Couplings in the Metal–Organic Framework Fe₂(dobdc)

The metal–organic framework Fe₂(dobdc) (dobdc^4– = 2,5-dioxido-1,4-benzenedicarboxylate), often referred to as Fe-MOF-74, possesses many interesting properties such as a high selectivity in olefin/paraffin separations. This compound contains open-shell Fe^II ions with open coordination sites which may have large single-ion magnetic anisotropies, as well as isotropic couplings between the nearest and next nearest neighbor magnetic sites. To complement a previous analysis of experimental data made by considering only isotropic couplings [Bloch et al. <i>Science</i> <b>2012</b>, <i>335</i>, 1606], the magnitude of the main magnetic interactions are here assessed with quantum chemical calculations performed on a finite size cluster. It is shown that the single-ion anisotropy is governed by same-spin spin–orbit interactions (i.e., weak crystal-field regime), and that this effect is not negligible compared to the nearest neighbor isotropic couplings. Additional magnetic data reveal a metamagnetic behavior at low temperature. This effect can be attributed to various microscopic interactions, and the most probable scenarios are discussed

La Lanterne : journal politique quotidien

06 juillet 18901890/07/06 (N4824,A14)

Structural and Electronic Effects on the Properties of Fe₂(dobdc) upon Oxidation with N₂O

We report electronic, vibrational, and magnetic properties, together with their structural dependences, for the metal–organic framework Fe₂(dobdc) (dobdc^4– = 2,5-dioxido-1,4-benzenedicarboxylate) and its derivatives, Fe₂(O)₂(dobdc) and Fe₂(OH)₂(dobdc)species arising in the previously proposed mechanism for the oxidation of ethane to ethanol using N₂O as an oxidant. Magnetic susceptibility measurements reported for Fe₂(dobdc) in an earlier study and reported in the current study for Fe^II_0.26[Fe^III(OH)]_1.74(dobdc)­(DMF)_0.15(THF)_0.22, which is more simply referred to as Fe₂(OH)₂(dobdc), were used to confirm the computational results. Theory was also compared to experiment for infrared spectra and powder X-ray diffraction structures. Structural and magnetic properties were computed by using Kohn–Sham density functional theory both with periodic boundary conditions and with cluster models. In addition, we studied the effects of different treatments of the exchange interactions on the magnetic coupling parameters by comparing several approaches to the exchange-correlation functional: generalized gradient approximation (GGA), GGA with empirical Coulomb and exchange integrals for 3<i>d</i> electrons (GGA+U), nonseparable gradient approximation (NGA) with empirical Coulomb and exchange integrals for 3<i>d</i> electrons (NGA+U), hybrid GGA, meta-GGA, and hybrid meta-GGA. We found the coupling between the metal centers along a chain to be ferromagnetic in the case of Fe₂(dobdc) and antiferromagnetic in the cases of Fe₂(O)₂(dobdc) and Fe₂(OH)₂(dobdc). The shift in magnetic coupling behavior correlates with the changing electronic structure of the framework, which derives from both structural and electronic changes that occur upon metal oxidation and addition of the charge-balancing oxo and hydroxo ligands

Mechanism of Oxidation of Ethane to Ethanol at Iron(IV)–Oxo Sites in Magnesium-Diluted Fe₂(dobdc)

The catalytic properties of the metal–organic framework Fe₂(dobdc), containing open Fe­(II) sites, include hydroxylation of phenol by pure Fe₂(dobdc) and hydroxylation of ethane by its magnesium-diluted analogue, Fe_0.1Mg_1.9(dobdc). In earlier work, the latter reaction was proposed to occur through a redox mechanism involving the generation of an iron­(IV)–oxo species, which is an intermediate that is also observed or postulated (depending on the case) in some heme and nonheme enzymes and their model complexes. In the present work, we present a detailed mechanism by which the catalytic material, Fe_0.1Mg_1.9(dobdc), activates the strong C–H bonds of ethane. Kohn–Sham density functional and multireference wave function calculations have been performed to characterize the electronic structure of key species. We show that the catalytic nonheme-Fe hydroxylation of the strong C–H bond of ethane proceeds by a quintet single-state σ-attack pathway after the formation of highly reactive iron–oxo intermediate. The mechanistic pathway involves three key transition states, with the highest activation barrier for the transfer of oxygen from N₂O to the Fe­(II) center. The uncatalyzed reaction, where nitrous oxide directly oxidizes ethane to ethanol is found to have an activation barrier of 280 kJ/mol, in contrast to 82 kJ/mol for the slowest step in the iron­(IV)–oxo catalytic mechanism. The energetics of the C–H bond activation steps of ethane and methane are also compared. Dehydrogenation and dissociation pathways that can compete with the formation of ethanol were shown to involve higher barriers than the hydroxylation pathway

Tuning Zr₆ Metal–Organic Framework (MOF) Nodes as Catalyst Supports: Site Densities and Electron-Donor Properties Influence Molecular Iridium Complexes as Ethylene Conversion Catalysts

The Zr₆ nodes of the metal–organic frameworks (MOFs) UiO-66 and UiO-67 are metal oxide clusters of atomic precision and can be used as catalyst supports. The bonding sites on these nodesthat is, hydrogen-bonded H₂O/OH groups on UiO-67 and non-hydrogen-bonded terminal OH groups on UiO-66were regulated by modulation of the MOF syntheses. Ir­(C₂H₄)₂(C₅H₇O₂) complexes reacted with these sites to give site-isolated Ir­(C₂H₄)₂ complexes, each anchored to the node by two Ir–O_node bonds. The supported iridium complexes on these sites have been characterized by infrared (IR) and extended X-ray absorption fine structure (EXAFS) spectroscopies and density functional theory calculations. The ethylene ligands on iridium are readily replaced by CO, and the ν_CO frequencies of the resultant complexes and those of comparable complexes reported elsewhere show that the support electron-donor tendencies increase in the order HY zeolite ≪ UiO-66 < UiO-67 (= NU-1000) < ZrO₂ < MgO. The sharpness of the IR ν_CO bands shows that the degree of uniformity of the support bonding sites decreases in the order ZrO₂ ≈ UiO-67 ≈ NU-1000 < MgO < UiO-66 ≪ HY zeolite. The reactivity of supported Ir­(CO)₂ complexes with C₂H₄ to form Ir­(C₂H₄)­(CO) and Ir­(C₂H₄)₂(CO) is influenced by the support electron-donor properties, with the reactivity increasing in the order MgO = ZrO₂ = NU-1000 (not reactive) < UiO-66 < UiO-67 ≪ HY zeolite. Density functional theory calculations characterizing the complexes supported on NU-1000, UiO-66/67, and HY zeolite concur with the use of the calculated ν_CO bands as indicators of electron-donor properties of the supported metal catalysts. Our calculations also show that the reactivity of the supported Ir­(CO)₂ complexes with C₂H₄ is correlated with the electron-donor properties of the iridium center. The supported Ir­(C₂H₄)₂ samples are precatalysts for ethylene hydrogenation and ethylene dimerization, with the activity for each reaction increasing with increasing electron-withdrawing strength of the support

CO₂ Adsorption in Fe₂(dobdc): A Classical Force Field Parameterized from Quantum Mechanical Calculations

Carbon dioxide adsorption isotherms have been computed for the metal–organic framework (MOF) Fe₂(dobdc), where dobdc^4– = 2,5-dioxido-1,4-benzenedicarboxylate. A force field derived from quantum mechanical calculations has been used to model adsorption isotherms within a MOF. Restricted open-shell Møller–Plesset second-order perturbation theory (ROMP2) calculations have been performed to obtain interaction energy curves between a CO₂ molecule and a cluster model of Fe₂(dobdc). The force field parameters have been optimized to best reproduced these curves and used in Monte Carlo simulations to obtain CO₂ adsorption isotherms. The experimental loading of CO₂ adsorbed within Fe₂(dobdc) was reproduced quite accurately. This parametrization scheme could easily be utilized to predict isotherms of various guests inside this and other similar MOFs not yet synthesized

A Hafnium-Based Metal–Organic Framework as an Efficient and Multifunctional Catalyst for Facile CO₂ Fixation and Regioselective and Enantioretentive Epoxide Activation

Porous heterogeneous catalysts play a pivotal role in the chemical industry. Herein a new Hf-based metal–organic framework (Hf-NU-1000) incorporating Hf₆ clusters is reported. It demonstrates high catalytic efficiency for the activation of epoxides, facilitating the quantitative chemical fixation of CO₂ into five-membered cyclic carbonates under ambient conditions, rendering this material an excellent catalyst. As a multifunctional catalyst, Hf-NU-1000 is also efficient for other epoxide activations, leading to the regioselective and enantioretentive formation of 1,2-bifuctionalized systems via solvolytic nucleophilic ring opening

Correction to “Tuning Zr₆ Metal-Organic Framework (MOF) Nodes as Catalyst Supports: Site Densities and Electron-Donor Properties Influence Molecular Iridium Complexes as Ethylene Conversion Catalysts”

Correction to “Tuning Zr₆ Metal-Organic Framework (MOF) Nodes as Catalyst Supports: Site Densities and Electron-Donor Properties Influence Molecular Iridium Complexes as Ethylene Conversion Catalysts