Metal Organic Framework Nodes Research Articles

Synergetic Ni–Ce Active Sites in Mixed Cerium/Zirconium Metal–Organic Framework Nodes for Selective Methane Oxidation into Ethanol

Synergetic Ni–Ce Active Sites in Mixed Cerium/Zirconium Metal–Organic Framework Nodes for Selective Methane Oxidation into Ethanol

Accurately Modulating Binuclear Metal Nodes of Metal-Organic Frameworks for Oxygen Evolution.

The accurate manipulation of the species and locations of catalytic centers is crucial for regulating the catalytic activity of catalysts, which is essential for their efficient design and development. Metal-organic frameworks (MOFs) with coordinated metal sites are ideal materials for investigating the origin of catalytic activity. In this study, we present a Ni2-MOF featuring novel Ni-based binuclear nodes with open metal sites (OMSs) and saturated metal sites (SMSs). The nickel was replaced by iron to obtain Ni1Fe1-MOF. In the electrocatalytic oxygen evolution reaction, Ni1Fe1-MOF exhibited an overpotential and Tafel slope of 370 mV@10 mA cm-2 and 87.06 mV dec-1, respectively, which were higher than those of Ni2-MOF (283 mV@10 mA cm-2 and 39.59 mV dec-1, respectively), demonstrating the superior performance of Ni1Fe1-MOF. Furthermore, theoretical calculations revealed that iron as an SMS may effectively regulate the electronic structure of the nickel catalytic center to reduce the free energy barrier ΔG*OH of the rate-determining step.

Regulated Dual Defects of Bridging Organic and Terminal Inorganic Ligands in Iron-based Metal-Organic Framework Nodes for Efficient Photocatalytic Ammonia Synthesis.

Engineering advantageous defects to construct well-defined active sites in catalysts is promising but challenging to achieve efficient photocatalytic NH3 synthesis from N2 and H2O due to the chemical inertness of N2 molecule. Here, we report defective Fe-based metal-organic framework (MOF) photocatalysts via a non-thermal plasma-assisted synthesis strategy, where their NH3 production capability is synergistically regulated by two types of defects, namely, bridging organic ligands and terminal inorganic ligands (OH- and H2O). Specially, the optimized MIL-100(Fe) catalysts, where there are only terminal inorganic ligand defects and coexistence of dual defects, exhibit the respective 1.7- and 7.7-fold activity enhancement comparable to the pristine catalyst under visible light irradiation. As revealed by experimental and theoretical calculation results, the dual defects in the catalyst induce the formation of abundant and highly accessible coordinatively unsaturated Fe active sites and synergistically optimize their geometric and electronic structures, which favors the injection of more d-orbital electrons in Fe sites into the N2 π* antibonding orbital to achieve N2 activation and the formation of a key intermediate *NNH in the reaction. This work provides a guidance on the rational design and accurate construction of porous catalysts with precise defective structures for high-performance activation of catalytic molecules.

Photoinduced Dynamic Ligation in Metal-Organic Frameworks.

Metal organic frameworks (MOFs), a class of porous crystalline materials consisting of metal-based nodes and organic linkers, have emerged as a promising platform for photocatalysis due to their ultrahigh functional surface area, customizable topologies, and tunable energetics. While interesting photochemistry has been reported, the related photoinduced structural dynamics of MOFs remains unclear. The consensus is that the coordination bonds between MOF nodes and linkers are considered static during photoexcitation, while the open-metal sites on the nodes are taken as the key active sites for catalysis. In this work, through a complementary time-resolved visible and infrared (IR) spectroscopic investigation, along with computational studies, we report for the first time light-induced structural bond dissociation (COO-M) and reformation in an iron-oxo framework, MIL-101(Fe). The probed excited state displayed ligand-to-metal charge transfer (LMCT) characteristics and exhibited a ca. 30 μs lifetime. The incredibly long excited-state lifetime led us to probe potential structural rearrangements that facilitated charge separation in MIL-101(Fe). By probing the vibrational fingerprints of the carboxylate linker upon LMCT photoexcitation, we observed the reversible transition of the carboxylate-Fe bond from a bidentate bridging mode to a monodentate mode, indicating the partial dissociation of the carboxylate ligand. Importantly, the bidentate configuration is recovered on the same time scale of the excited state lifetimes as probed via visible transient absorption spectroscopy. The elucidated photoinduced configurational dynamics provides a foundation for an in-depth understanding of MOF-based photocatalytic mechanisms.

Characterization, Structure, and Reactivity of Hydroxyl Groups on Metal-Oxide Cluster Nodes of Metal-Organic Frameworks: Structural Diversity and Keys to Reactivity and Catalysis.

Among the most stable metal-organic frameworks (MOFs) are those incorporating nodes that are metal oxide clusters with frames such as Zr6 O8 . This review is a summary of the structure, bonding, and reactivity of MOF node hydroxyl groups, emphasizing those bonded to nodes containing aluminum and zirconium ions. Hydroxyl groups are often present on these nodes, sometimes balancing the charges of the metal ions. They arise during MOF syntheses in aqueous media or in post-synthesis treatments. They are identified with infrared and 1 H nuclear magnetic resonance spectroscopies and characterized by their reactivities with polar compounds such as alcohols. Terminal OH, pairedµ2 -OH, and aqua groupson nodes are catalytic sites in numerous reactions. Relatively unreactive hydroxyl groups (such as isolatedµ2 -OH groups) may replace reactive groups and inhibit catalysis; some node hydroxyl groups (e.g.,µ3 -OH) are mere spectators in catalysis. There are similarities between MOF node hydroxyl groups and those on the surfaces of bulk metal oxides, zeolites, and enzymes, but the comparisons are mostly inexact, and much remains to be understood about MOF node hydroxyl group chemistry. It is posited that understanding and controlling this chemistry will lead to tailored MOFs and improved adsorbents and catalysts.

Introducing Metal–Organic Frameworks to Melt Electrowriting: Multifunctional Scaffolds with Controlled Microarchitecture for Tissue Engineering Applications

AbstractScaffolds with multiple advantageous biological and structural properties are still a challenge in the field of tissue engineering. The convergence of advanced fabrication techniques and functional materials is key to fulfill this need. Melt electrowriting (MEW) is an additive manufacturing technique that enables the fabrication of microfibrous scaffolds with precisely defined microarchitectures. Here, it is proposed to exploit metal–organic frameworks (MOFs) to efficiently introduce multifunctionalities by combining polycaprolactone (PCL), the gold standard material in MEW, with a silver‐/silver‐chloride‐decorated iron‐based MOF (NH2‐MIL‐88B(Fe)). This results in highly ordered constructs with antibacterial properties and magnetic resonance imaging (MRI) visibility. Scaffolds with up to 20 wt% MOF are successfully melt‐electrowritten with a fiber diameter of 50 µm. Among these, 5 wt% MOF proves to be the optimal concentration as it exhibits silver‐induced sustained antibacterial efficacy while maintaining PCL cytocompatibility and in vitro immune response. The iron component of the MOF (Fe(III) nodes) renders the composite visible with MRI, thereby enabling scaffold monitoring upon implantation with a clinically accepted method. The combination of MEW and MOFs as tunable additives and cargo carriers opens the way for designing advanced multifunctional scaffolds with a wide range of applications in, e.g., tissue engineering, biosensing and drug delivery.

Remote substituent effects on catalytic activity of metal-organic frameworks: a linker orbital energy model

The Hammett equation is commonly used to theoretically depict the remote electronic effects of substituents on catalytic activities of metal nodes of metal-organic frameworks (MOFs). However, the application of the theory to MOF catalysts usually encounters problems because it relies heavily on empirical parameters with unknown transferability. To develop an alternative prediction theory, the linker orbital energy model has been proposed by density functional theory calculations. The model provides a simple method to approximately depict the remote electronic substituent effects on catalytic activities of metal nodes of MOFs, and its general applicability to MOFs is supported by extensively revisiting the structure-activity relationships reported in the literatures. The model can be used to design catalytic activity of metal nodes of MOFs by engineering the electronic properties of linkers and substituents.

Bioinspired inhibition of aggregation in metal-organic frameworks (MOFs)

Different from traditional procedures of using solid stabilizers like polymers and surfactants, here we demonstrate that water, as a very "soft" matter, could function as a "spacer" to prevent the aggregation of metal-organic frameworks (MOFs) in aqueous dispersions. Our theoretical calculations reveal in case of an excess of positively charged metal nodes of MOFs, where water molecules are ligated to metal nodes that greatly enhance MOFs' solution dispersibility through electrostatic stabilization. This discovery has motivated us to develop a facile experimental approach for producing a category of "clean" MOF dispersions without foreign additives. Potential application has been demonstrated for the size fractionation of MOFs, which results in small-size MOFs (50-80nm) characteristic of superior electrocatalytic oxygen evolution activities (256mV at 10mA cm-2, Tafel slope of 49mV dec-1 and durability >30h). This work would provide new clues for aqueous processing of MOFs for many emerging applications.

Enhanced Catalytic Activity of TEMPO-Mediated Aerobic Oxidation of Alcohols via Redox-Active Metal-Organic Framework Nodes.

Metal-organic frameworks (MOFs) are outstanding platforms for heterogeneous catalysis due to their tunable pore size, huge surface area, large porosity, and potential active sites. The design and synthesis of MOF/organocatalyst co-catalytic systems have attracted considerable interest owing to their high catalytic activity, low toxicity, and mild reaction conditions. Herein, we reported the synthesis of a bifunctional TEMPO-IsoNTA organocatalyst featuring a pyridyl group as an anchoring site and a TEMPO radical as a catalytic active site. By using the topologically isomorphic structures of MIL-101(Fe) and MIL-101(Cr) as co-catalysts, these MOF/TEMPO-IsoNTA systems enable the efficient aerobic oxidation of various alcohols to their corresponding aldehydes or ketones under mild conditions. Notably, the MIL-101(Fe)/TEMPO-IsoNTA system exhibits superior catalytic activity, thanks to their redox-active FeIII-oxo nodes, which facilitate the regeneration of TEMPO-IsoNTA. Our research not only solves the problem of potential heavy metal contamination in the TEMPO-based homogeneous catalytic system, but also enriches the understanding of synergism of MOFs/organocatalysts.

Transformation of Organonitrogen‐Encapsulated MOFs into N‐Doped Fe3O4@C Nanopolyhedron via CVD Super‐Assembly for Photochemical Oxidation

AbstractDevelopment of nano‐structured metal oxides/heteroatom composites with controlled components and structure for photochemical oxidation still remains a great challenge. Here, a new and versatile strategy is reported for transformation of organonitrogen‐encapsulated metal‐organic frameworks (MOFs) into N‐doped Fe3O4@C nanopolyhedron by chemical vapor deposition‐induced super‐assembly method. Strong confined interaction between organonitrogen guests (urea, thiourea, melamine, and dimethylimidazole) and Fe nodes of MOFs realizes reconstruction of crystal structure and introduction of N species. With the novel approach, the uniform dispersion of guests and perfect metallic/heteroatom interfacial is obtained. Compared with MOFs‐derived Fe2O3/C, the heteroatom/defect‐to‐metal cluster charge transfer excitations lead N‐doped Fe3O4@C to exhibit more superior activity for photocatalytic oxidation (turn‐over frequencies as high as 3.72 h−1). It demonstrates that the introduction of abundant pyrrole‐N and oxygen vacancies on carbon interface boosts the advance of photo‐generated carrier transfer. The study offers a simple and promising strategy for the design of novel metal oxides/heteroatom composite with adjustable structure and functions.

High‐Efficiency Electrogenerated Chemiluminescence of Novel Zr‐Based Metal–Organic Frameworks through Organic Linkers Regulation

AbstractExploiting efficiently luminescent metal‐organic frameworks (MOFs) for electrogenerated chemiluminescence (ECL) in aqueous solution is very intriguing and challenging. Described here are novel Zr‐based metal–organic frameworks (MOFs) electrochemiluminophores (ECLphores) synthesized through a facile solvothermal method by using polycyclic aromatic hydrocarbons (PAHs) as organic linkers. The photoluminescence (PL) and ECL of Zr‐based MOFs are affected by the actual organic linkers’ inherent properties and activities. The tetraphenyl‐carboxylate‐pyrene derived NU‐1000 shows the best ECL performance and reveals an enormous ECL quantum efficiency of 1.42‐fold higher than that of the standard Ru(bpy)32+by using K2S2O8as the coreactant. Due to its providential energy gap, it facilitated the feasibility of electron removal and injection identified by experiments and density functional theory (DFT) calculations. Moreover, the ECL mechanisms of NU‐1000 were elucidated based on the voltammetric ECL method, photoluminescence, and spooling ECL spectroscopies. The exceptional porosity and high surface area for the acceleration of mass and electron transfer during electrochemical reactions, along with the coordination immobilization of ECL active PAHs onto the MOF nodes for abolishing energy loss caused by notorious π‐π stacking and intramolecular rotation, representing significant advantages in the development of Zr‐based MOFs for ECL applications.

Unveiling the lattice distortion and electron-donating effects in methoxy-functionalized MOF photocatalysts for H2O2 production

Semiconducting metal-organic frameworks (MOFs) are emerging photocatalysts for H2O2 production via the two-electron oxygen reduction (2e-ORR) process. Whether and how the functional groups on MOF nodes affect the 2e-ORR mechanism and performance remain largely unexplored. Herein, we report that partial replacement of formate (-COO-) by methoxy (-OMe) coordinated Zr-oxo cluster in UIO-66-NH2 significantly enhances the H2O2 production. The introduction of -OMe groups induces lattice distortion in UIO-66-NH2, resulting in the change of proton donor for the protonation of OOH* intermediate from -COOH to -NH3+ with a lowered kinetic energy barrier. The -OMe groups with electron-donating effect contribute to promoted light harvesting and charge separation. The 2e-ORR selectivity of -OMe functionalized UIO-66-NH2 is also enhanced, evidenced by rotating ring-disk electrode test and theoretical calculation. As a result, an improved photocatalytic ability is achieved with an H2O2 yield of 312.9 mM g−1 h−1, ~4.7 times higher than that of the original UIO-66-NH2.

Zirconium-oxo Nodes of MOFs with Tunable Electronic Properties Provide Effective ⋅OH Species for Enhanced Methane Hydroxylation.

Direct conversion of methane to high value-added oxygenates under mild conditions has attracted extensive interest. However, the over-oxidation of target products is usually unavoidable due to the easily excessive activation of C-H bond on the sites of supported metal species. Here, we identified the most efficient Zr-oxo nodes of UiO-66 metal-organic frameworks (MOFs) catalysts for the selective oxidation of methane with H2 O2 . These nodes were modified by three types of benzene 1, 4-dicarboxylates (NH2 -BDC, H2 BDC, and NO2 -BDC). Detailed characterizations and DFT calculations revealed that these ligands can effectively tune the electronic properties of Zr-oxo nodes and the H2 BDC ligand led to optimal electronic density of Zr-oxo nodes in UiO-66. Thus the UiO-66-H catalyst promoted the formation of ⋅OH species that adsorbed on Zr-oxo nodes, and facilitated the activation of methane with a lower energy barrier and subsequent conversion to hydroxylation oxygenates with 100 % selectivity.

Zirconium‐oxo Nodes of MOFs with Tunable Electronic Properties Provide Effective ⋅OH Species for Enhanced Methane Hydroxylation

Direct conversion of methane to high value-added oxygenates under mild conditions has attracted extensive interest. However, the over-oxidation of target products is usually unavoidable due to the easily excessive activation of C-H bond on the sites of supported metal species. Here, we identified the most efficient Zr-oxo nodes of UiO-66 Metal-Organic Frameworks (MOFs) catalysts for the selective oxidation of methane with H2O2 oxidant, which adopted the modification strategy by three types of benzene 1, 4-dicarboxylates (NH2-BDC, H2BDC, and NO2-BDC, respectively). Detailed characterizations and DFT calculations revealed that these ligands can effectively tune the electronic property of Zr-oxo nodes and the H2BDC ligand led to proper electronic density of Zr-oxo nodes in UiO-66. Consequently, this UiO-66-H catalyst can promote the formation of effective •OH species that adsorbed on Zr-oxo nodes, which facilitated the activation of methane with a lower energy barrier and subsequent conversion to hydroxylation oxygenates with 100% selectivity.

Tailoring Lewis Acid Properties of Metal–Organic Framework Nodes via Anion Post-Replacement for Gas Adsorption and Separation

Tailoring Lewis Acid Properties of Metal–Organic Framework Nodes via Anion Post-Replacement for Gas Adsorption and Separation

Aqueous-Phase Destruction of Nerve-Agent Simulants at Copper Single Atoms in UiO-66.

Metal-organic frameworks (MOFs) have shown great success in aqueous-phase hydrolysis of nerve agents, with some even showing promise in the gas phase. However, both aqueous-phase reactivity and gas-phase reactivity are hindered because of the binding of the hydrolyzed products to the MOF nodes in a stable, bridging configuration, which limits turnover. Single transition-metal atoms in MOFs have been a growing field of interest for catalytic applications, and single atoms have been proposed to prevent the unwanted bridged conformation and increase catalytic turnover. To date, there has been little experimental evidence to support the hypothesis. Herein, we report two copper single atom-modified UiO-66 MOFs for nerve-agent simulant degradation. Despite the capping of highly active Zr4+ nodes with fewer Lewis acidic Cun+ atoms, the reactivity of both CuMOFs approaches that of native UiO-66 under aqueous conditions. Computational studies reveal that the Cu coordination environment impairs product inhibition with respect to the native MOF.

Homologous MXene‐Derived Electrodes for Potassium‐Ion Full Batteries

AbstractThe development of potassium‐ion battery (PIB) electrode materials is critical for promoting their use in next‐generation energy storage systems. Although metal–organic frameworks (MOFs) are appealing electrode materials, their performance in PIBs remains unsatisfactory. The low K+ adsorption energy (ΔEa) on the saturated coordination of MOFs can explain the limited capacity. Herein, MXene‐derived MOF nodes (NMD‐MOF) are unlocked and used as anodes in PIB. The NMD‐MOF anode exhibits substantially increased capacity (250 mA h g−1 at 0.05 A g−1), as well as good rate‐performance and excellent capacity retention. Density functional theory calculations reveal that the ΔEa at unlocked node sites is significantly higher than at intact node sites of the pristine MOF. Furthermore, the NMD‐MOF anode and homologous MXene‐derived K+‐intercalated vanadium oxide (MD‐KVO) cathode combine to assemble a PIB, which delivers an encouraging capacity of 63 mA h g−1 at 50 mA g−1 and high energy (143 Wh kg−1) and power density (440 W kg−1). The fabrication of MXene‐derived electrode materials and the unlocking node strategy for binding site activation may spur further research into highly active electrode materials for energy storage devices.

Modulating Photo- and Radioluminescence in Tb(III) Cluster-Based Metal–Organic Frameworks

Luminescent metal–organic frameworks (MOFs) are of interest for sensing, theranostics, dosimetry, and other applications. The use of lanthanoids in MOF nodes allows for intrinsic metal-based luminescence. In this work, a facile route for modulating the photoluminescent and radioluminescent properties of Tb(III)-based MOFs is reported. By using Tb(III) cluster nodes as X-ray attenuators, and organic linkers with varying excited state energies as sensitizers, MOFs with metal-based, linker-based, and metal+linker-based photo- and radioluminescence are reported.

Reversible Dissociation for Effective Storage of Diborane Gas within the UiO-66-NH2 Metal-Organic Framework.

There is an acute need for materials that can store the toxic and highly reactive diborane gas at room temperature. In this work, the interfacial chemistry leading to safe and reversible storage of diborane (B2H6) in the UiO-66-NH2 metal-organic framework (MOF) was investigated via in situ transmission infrared (IR) spectroscopy, temperature-programmed desorption (TPD), and electronic structure calculations. The infrared spectrum of B2H6 adsorbed within UiO-66-NH2 indicates hydrogen bonding with the μ3-OH groups of the MOF nodes and chemisorption at the -NH2 groups of the MOF linkers. The conversion of physisorbed to chemisorbed diborane, as observed through a spectroscopically unique intermediate species, occurred over a broad temperature regime from 80 to 410 K. During B2H6-TPD studies, both the weakly and strongly bound species were found to desorb exclusively as molecular B2H6. Infrared spectroscopic studies, performed during diborane adsorption and reaction, combined with electronic structure calculations, revealed that chemisorption occurred via a reversible dissociation reaction involving a "half-open" B2H6 intermediate and resulted in the formation of two NH2-bound BH3 units, which leave the MOF as B2H6 via recombinative desorption. The close spacing of -NH2 groups in the UiO-66-NH2 MOF is key to enabling high-temperature chemisorptive storage of B2H6, and the spatial arrangement of the amine groups has a significant effect on the dissociation energy profile. This work demonstrates that reversible dissociation of B2H6 on precisely engineered, nucleophile-rich materials represents a promising pathway to diborane stabilization and long-term storage.

Topology- and wavelength-governed CO2 reduction photocatalysis in molecular catalyst-metal-organic framework assemblies.

Optimising catalyst materials for visible light-driven fuel production requires understanding complex and intertwined processes including light absorption and catalyst stability, as well as mass, charge, and energy transport. These phenomena can be uniquely combined (and ideally controlled) in porous host-guest systems. Towards this goal we designed model systems consisting of molecular complexes as catalysts and porphyrin metal-organic frameworks (MOFs) as light-harvesting and hosting porous matrices. Two MOF-rhenium molecule hybrids with identical building units but differing topologies (PCN-222 and PCN-224) were prepared including photosensitiser-catalyst dyad-like systems integrated via self-assembled molecular recognition. This allowed us to investigate the impact of MOF topology on solar fuel production, with PCN-222 assemblies yielding a 9-fold turnover number enhancement for solar CO2-to-CO reduction over PCN-224 hybrids as well as a 10-fold increase compared to the homogeneous catalyst-porphyrin dyad. Catalytic, spectroscopic and computational investigations identified larger pores and efficient exciton hopping as performance boosters, and further unveiled a MOF-specific, wavelength-dependent catalytic behaviour. Accordingly, CO2 reduction product selectivity is governed by selective activation of two independent, circumscribed or delocalised, energy/electron transfer channels from the porphyrin excited state to either formate-producing MOF nodes or the CO-producing molecular catalysts.