The role of metal-organic frameworks (MOFs) in the field of catalysis is discussed, and special focus is placed on their assets and limits in light of current challenges in catalysis and green ...chemistry. Their structural and dynamic features are presented in terms of catalytic functions along with how MOFs can be designed to bridge the gap between zeolites and enzymes. The contributions of MOFs to the field of catalysis are comprehensively reviewed and a list of catalytic candidates is given. The subject is presented from a multidisciplinary point of view covering solid-state chemistry, materials science, and catalysis.
This
review article
presents the fundamental and practical aspects of water adsorption in Metal-Organic Frameworks (MOFs). The state of the art of MOF stability in water, a crucial issue to many ...applications in which MOFs are promising candidates, is discussed here. Stability in both gaseous (such as humid gases) and aqueous media is considered. By considering a non-exhaustive yet representative set of MOFs, the different mechanisms of water adsorption in this class of materials are presented: reversible and continuous pore filling, irreversible and discontinuous pore filling through capillary condensation, and irreversibility arising from the flexibility and possible structural modifications of the host material. Water adsorption properties of more than 60 MOF samples are reported. The applications of MOFs as materials for heat-pumps and adsorbent-based chillers and proton conductors are also reviewed. Some directions for future work are suggested as concluding remarks.
MOF and water, friend or enemy?
The one-pot postfunctionalization allows anchoring a molecular nickel complex into a mesoporous metal–organic framework (Ni@(Fe)MIL-101). It is generating a very active and reusable catalyst for the ...liquid-phase ethylene dimerization to selectively form 1-butene. Higher selectivity for 1-butene is found using the Ni@(Fe)MIL-101 catalyst than reported for molecular nickel diimino complexes.
A set of 15 metal–organic frameworks (MIL-53, MIL-68, MIL-125, UiO-66, ZIF) exhibiting different pore size, morphology, and surface chemistry is used to unravel the numerous behaviors of water ...adsorption at room temperature in this class of materials. Outstanding “S”-shaped (type V) adsorption isotherms are observed for MIL-68 type solids. We show that the underlying mechanism of water adsorption can be rationalized using a simple set of three parameters: the Henry constant ( i.e. the slope of the adsorption pressure in the low pressure range), the pressure at which pore filling occurs, and the maximum water adsorption capacity. While the Henry constant and pore filling pressure mostly depend on the affinity of water for the surface chemistry and on pore size, respectively, these two parameters are correlated as they both reflect different aspects of the hydrophobicity–hydrophilicity of the material. For a given type of porous structure, the functionalization of the material by hydrophilic moieties such as hydrogen bonding groups (amine or aldehyde) systematically leads to an increase in the Henry constant concomitantly with a decrease in the pore filling pressure. As for the adsorption mechanism, we show that, for a given temperature, there is a critical diameter ( D c ∼ 20 Å for water at room temperature) above which pore filling occurs through irreversible capillary condensation accompanied by capillary hysteresis loops. Below this critical diameter, pore filling is continuous and reversible unless the material exhibits some adsorption-induced flexibility.
Hollow capsules with dimensions below 1 μm have recently attracted much attention due to their potential applications as catalysts as well as biomedical and pharmaceutical vectors for controlled drug ...delivery. Among them, hollow zeolites are particularly interesting because they possess (i) a crystalline structure, which greatly improves their hydrothermal and chemical stability as compared to amorphous silica analogs and (ii) a microporous network that acts as a shape-selective membrane. Moreover, their properties can be continuously tuned by changing their composition, in particular the framework aluminum content. In this perspective review, we examine the recent progress in the development of synthetic methods for the preparation of hollow zeolite and zeotype structures, from templating routes providing large polycrystalline capsules to controlled dissolution methods leading to nanometer-sized hollow single crystals. The applications of these materials will be illustrated and discussed, namely their main potential as catalytic nanoreactors, these being materials particularly adapted for the encapsulation and the confinement of metal nanoparticles. Critical perspectives on future materials with specific properties are also addressed, particularly those with less common zeolite structures and/or compositions.
Transition-metal nanoparticles (Co, Ni, and Cu) encapsulated in hollow zeolite single crystals were prepared by recrystallization of impregnated bulk MFI crystals in the presence of ...tetrapropylammonium (TPAOH) solutions. The size and number of particles in hollow MFI depended mainly on the aluminum content. The encapsulation of the nanoparticles prevented them from growing, thus enabling the control of particle size even after high temperature treatments. For low metal loadings (<3 wt %), the mean particle sizes for Co, Ni, and Cu in hollow silicalite-1 were 3.5 ± 0.3, 3.1 ± 0.5, and 1.5 ± 0.2 nm, respectively. In the case of hollow ZSM-5, higher loadings (∼8 wt %) could be obtained with mean particle sizes of 17 ± 2 nm, 13 ± 2 nm, and 15 ± 2 nm for Co, Ni, and Cu systems. The mechanism of transition metal nanoparticle formation was markedly different from that of noble metals. At high pH values, transition-metal cations first reacted with dissolved silica species yielding fibrous metal phyllosilicates that were located inside the crystal cavities. The metal phyllosilicates were then converted into nanoparticles upon reduction under H2 at high temperature (500–750 °C). Silicalite-1 encapsulated Ni particles were used in the catalytic hydrogenation of substituted benzenes and showed an outstanding size-selectivity effect. Ni particles were accessible to toluene but not to mesitylene, confirming that the activity is directly related to the diffusion properties of molecules through the zeolite membrane.
The synthesis of dimethyl carbonate (DMC) from carbon dioxide and methanol was studied over a ceria- zirconia mixed metal oxide. When using oxygen-18 labeled ceria-zirconia, no oxygen-18 labeled DMC ...or water was observed. On the other hand, using oxygen-18 labeled methanol led almost exclusively to the formation of double labeled DMC, CH318O (C16O) 18OCH3. These results were used to further elaborate the proposed reaction mechanisms for DMC synthesis. The rate-controlling step identified was that of a nucleophilic attack of gas phase methanol on surface monomethyl carbonate species to form DMC.
Display omitted
•Dimethyl carbonate synthesis over oxides does not follow a Mars-Van Krevelen route.•The rate controlling step is a nucleophilic attack of methanol/methoxy species.•Zirconia promotion of ceria weakens the methyl carbonate species metal‑oxygen bond.
Heterogenization of molecular catalysts via their immobilization within extended structures often results in a lowering of their catalytic properties due to a change in their coordination sphere. ...Metal–organic polyhedra (MOP) are an emerging class of well-defined hybrid compounds with a high number of accessible metal sites organized around an inner cavity, making them appealing candidates for catalytic applications. Here, we demonstrate a design strategy that enhances the catalytic properties of dirhodium paddlewheels heterogenized within MOP (Rh-MOP) and their three-dimensional assembled supramolecular structures, which proved to be very efficient catalysts for the selective photochemical reduction of carbon dioxide to formic acid. Surprisingly, the catalytic activity per Rh atom is higher in the supramolecular structures than in its molecular sub-unit Rh-MOP or in the Rh-metal–organic framework (Rh-MOF) and yields turnover frequencies of up to 60 h–1 and production rates of approx. 76 mmole formic acid per gram of the catalyst per hour, unprecedented in heterogeneous photocatalysis. The enhanced catalytic activity is investigated by X-ray photoelectron spectroscopy and electrochemical characterization, showing that self-assembly into supramolecular polymers increases the electron density on the active site, making the overall reaction thermodynamically more favorable. The catalyst can be recycled without loss of activity and with no change of its molecular structure as shown by pair distribution function analysis. These results demonstrate the high potential of MOP as catalysts for the photoreduction of CO2 and open a new perspective for the electronic design of discrete molecular architectures with accessible metal sites for the production of solar fuels.
•A model for an all-porous solid oxide fuel cell is developed for the first time.•Good carbon resistance of the all porous fuel cell can be achieved.•High performance of the all porous fuel cell can ...be achieved with CH4 fuel.•Co-generation of electricity and syngas in all porous fuel cell is proposed.
The all porous solid oxide fuel cell concept is proposed to solve the carbon deposition problem of solid oxide fuel cells. The transport of oxygen molecules from the cathode to the fuel side through the porous electrolyte can resist carbon deposition but could reduce the fuel cell performance. In this paper, a two-dimensional model for all porous solid oxide button cells is developed for the first time. After model validation with experimental data, the model is then extended for a tubular cell for parametric simulations. The effects of operating conditions and the electrolyte microstructure properties on carbon resistance and electrochemical performance of all porous solid oxide fuel cells are examined. The good carbon resistance of all porous solid oxide fuel cell is numerically demonstrated. It is found that the electrochemical performance and anode surface O/C ratio is significantly affected by anode inlet gas composition and flowrate. In addition, the anode supported all porous solid oxide fuel cell shows a great potential in terms of both power generation and coking resistance. The results of this study form a solid foundation to understand the mechanism and promising future of all porous solid oxide fuel cells.
A substituted imidazolate-based MOF (SIM-1), easily shaped in situ on and in millimetre-sized alumina beads, is useful for catalytic applications and undergoes no modification of its intrinsic ...properties.
PDF
