Metal-Organic Frameworks as Electrocatalysts

Transition metal complexes are well-known homogeneous electrocatalysts. In this regard, metal-organic frameworks (MOFs) can be considered as an ensemble of transition metal complexes ordered in a periodic arrangement. In addition, MOFs have several additional positive structural features that make them suitable for electrocatalysis, including large surface area, high porosity, and high content of accessible transition metal with exchangeable coordination positions. The present review describes the current state in the use of MOFs as electrocatalysts, both as host of electroactive guests and their direct electrocatalytic activity, particularly in the case of bimetallic MOFs. The field of MOF-derived materials is purposely not covered, focusing on the direct use of MOFs or its composites as electrocatalysts. Special attention has been paid to present strategies to overcome their poor electrical conductivity and limited stability.

Self-assembled mononuclear complexes: open metal sites and inverse dimension-dependent catalytic activity for the Knoevenagel condensation and CO2 cycloaddition

To lessen the greenhouse effect, measures such as improving the recovery of crude oil and converting carbon dioxide (CO₂) into valuable chemicals are necessary to create a sustainable low-carbon future. To this end, the development of efficient new oil-displacing agents and CO₂ conversion has aroused great interest in both academia and industry. The Knoevenagel condensation and CO₂ cycloaddition are the key reactions to solve the above problems. Four Cu- or Zn-based molecular complexes built from different ligands possessing hydrophilic-hydrophobic layers and different dimensionalities were chosen as solid catalysts for this study. Structural analysis revealed the presence of hydrophilic-hydrophobic layers and open metal sites in the low-dimensional complexes. To obtain deep insight into the reaction mechanism, first-principles density functional theory (DFT) calculations were carried out. These calculations confirmed that in the Knoevenagel condensation reaction, the final formation of benzylidenemalononitrile is the rate-determining step (an energy barrier (ΔE) value of 73.2 kJ mol^-1). The zero-dimensional (0D) Cu molecular complex with unsaturated metal centers, hydrophilic and hydrophobic layers, exhibited higher catalytic activity (yield: 100%, temperature: room temperature, and time: 2 h) compared with one- and two-dimensional Cu complexes. In the presence of a 0D Zn complex co-catalyzed with Br^- in the CO₂ cycloaddition reaction, the ΔE value reduces to 35.5 kJ mol^-1 for the ring opening of styrene oxide (SO), which is significantly lower than Br^- catalyzed (80.9 kJ mol^-1) reactions. The roles of unsaturated metal centers, hydrophilic-hydrophobic layers and dimensionality in the Knoevenagel condensation and CO₂ cycloaddition were explained in the results of structure-activity relationships.

Recent advances in metal-based nanoporous materials for sensing environmentally-related biomolecules

Highly sensitive, stable, selective, efficient, and short reaction time sensors play a substantial role in daily life/industry and are the need of the day. Due to the rising environmental issues, nanoporous carbon and metal-based materials have attracted significant attention in environmental analysis owing to their intriguing and multifunctional properties and cost-effective and rapid detection of different analytes by sensing applications. Environmental-related issues such as pollution have been a significant threat to the world. Therefore, it is necessary to fabricate highly promising performance-based sensor materials with excellent reliability, selectivity and good sensitivity for monitoring various analytes. In this regard, different methods have been employed to fabricate these sensors comprising metal, metal oxides, metal oxide carbon composites and MOFs leading to the formation of nanoporous metal and carbon composites. These composites have exceptional properties such as large surface area, distinctive porosity, and high conductivity, making them promising candidates for several versatile sensing applications. This review covers recent advances and significant studies in the sensing field of various nanoporous metal and carbon composites. Key challenges and future opportunities in this exciting field are also part of this review.

Analysis of Mn2+ and Zn2+ Ions in Macroalgae with Heteroelement-Doped Carbon-Based Fluorescent Probe

Kelp and laver are large economic macroalgae in China, which are rich in nutrients, especially Mn and Zn. Excessive intake of Mn and Zn can be harmful to the human body. Therefore, it is necessary to develop a convenient and efficient method to detect the contents of Mn and Zn in macroalgae. In this experiment, red carbon dots (R-CDs) doped with N and S elements were prepared by the thermal solvent method. The obtained R-CDs displayed excitation wavelength-independent fluorescent emission in the red spectral region. The R-CDs were used to construct a fluorescent probe for specific recognition of Mn²⁺ and Zn²⁺, achieving high-sensitivity detection of Mn²⁺ and Zn²⁺. The detection results showed a good linear relationship between fluorescence intensity and Mn²⁺ concentration, and the calculated detection limit was 0.23 nmol/L. For the detection of Zn²⁺, the detection limit was estimated as 19.1 nmol/L. At the same time, the content distribution of Mn and Zn elements in macroalgae produced in Fujian was investigated by the constructed fluorescence probe. It was found that kelp, laver, and their products are rich in Mn and Zn elements, and the content of Mn and Zn elements in laver is higher than that in kelp, which can be used as the optimal food supplement for Mn and Zn elements.

Two chemically robust Cd(II)-frameworks for efficient sensing of levofloxacin, benzaldehyde, and Fe3+ ions

Two luminescent Cd(II)-organic frameworks [Cd₂(L1)(tdc)₂(H₂O)]_n (1) and [Cd(L2)_0.5(tdc)]_n (2) (L1 = 1,5-bis(1-(pyridine-4-ylmethyl)-1H-benzo[d]imidazol-2-yl)pentane, L2 = 1,6-bis(1-(pyridine-4-ylmethyl)-1H-benzo[d]imidazol-2-yl)hexane, and H₂tdc = 2,5-thiophenedicarboxylic acid) were hydrothermally synthesized and characterized. 1 displays a rare binodal (3,4)-connected 2D cem-d network, while 2 exhibits a 3D mog (moganite) framework. The two MOFs are highly thermally durable and water-stable in a wide pH range from 3 to 12. Interestingly, 1 and 2 represent the first reported examples of multi-responsive probes based on MOFs for selectively detecting levofloxacin, benzaldehyde, and Fe³⁺ ions with reusability. The luminescence sensing mechanisms of the two CPs were explored in detail.

Conductive Metal-Organic Frameworks: Electronic Structure and Electrochemical Applications

Smarter and minimization of devices are consistently substantial to shape the energy landscape. Significant amounts of endeavours have come forward as promising steps to surmount this formidable challenge. It is undeniable that material scientists were contemplating smarter material beyond purely inorganic or organic materials. To our delight, metal-organic frameworks (MOFs), an inorganic-organic hybrid scaffold with unprecedented tunability and smart functionalities, have recently started their journey as an alternative. In this review, we focus on such propitious potential of MOFs that was untapped over a long time. We cover the synthetic strategies and (or) post-synthetic modifications towards the formation of conductive MOFs and their underlying concepts of charge transfer with structural aspects. We addressed theoretical calculations with the experimental outcomes and spectroelectrochemistry, which will trigger vigorous impetus about intrinsic electronic behaviour of the conductive frameworks. Finally, we discussed electrocatalysts and energy storage devices stemming from conductive MOFs to meet energy demand in the near future.

NiCo2O4 nanoparticles inlaid on sulphur and nitrogen doped and co-doped rGO sheets as efficient electrocatalysts for the oxygen evolution and methanol oxidation reactions

The present work depicts the fabrication of NiCo₂O₄ decorated on rGO, and doped and co-doped rGO and its electrocatalytic activity towards the oxygen evolution reaction and methanol oxidation reaction. The NiCo₂O₄ catalyst with S-doped rGO outperformed the other catalysts, indicating that the sulphur atoms attached on rGO possess low oxophilicity and optimum free energy. This results in facile adsorption of the intermediate products formed during the OER and a rapid release of O₂ molecules. The same catalyst requires an overpotential of 1.51 V vs. RHE to attain the benchmark current density value of 10 mA cm^-2 and shows a Tafel slope of 57 mV dec^-1. It also reveals outstanding stability during its operation for 10 h with a minimum loss in potential. On the other hand, NiCo₂O₄/S,N-rGO reveals superior activity with high efficiency and stability in catalyzing methanol oxidation. The catalyst delivered a low onset potential of 0.12 V vs. Hg/HgO and high current density of 203.4 mA cm^-2 after addition of 0.5 M methanol, revealing the outstanding performance of the electrocatalyst.

Heterometallic Actinide-Containing Photoresponsive Metal-Organic Frameworks: Dynamic and Static Tuning of Electronic Properties

Acquiring fundamental knowledge of properties of actinide-based materials is a necessary step to create new possibilities for addressing the current challenges in the nuclear energy and nuclear waste sectors. In this report, we established a photophysics-electronics correlation for actinide-containing metal-organic frameworks (An-MOFs) as a function of excitation wavelength, for the first time. A stepwise approach for dynamically modulating electronic properties was applied for the first time towards actinide-based heterometallic MOFs through integration of photochromic linkers. Optical cycling, modeling of density of states near the Fermi edge, conductivity measurements, and photoisomerization kinetics were employed to shed light on the process of tailoring optoelectronic properties of An-MOFs. Furthermore, the first photochromic MOF-based field-effect transistor, in which the field-effect response could be changed through light exposure, was constructed. As a demonstration, the change in current upon light exposure was sufficient to operate a two-LED fail-safe indicator circuit.

A Facile Reaction Strategy for the Synthesis of MOF-Based Pine-Needle-Like Nanocluster Hierarchical Structure for Efficient Overall Water Splitting

Solvothermal reactions of Co(NO₃)₂·6H₂O, 3-amino-1,2,4-triazole, and 1,2,4,5-benzenetetracarboxylic acid afforded a Co-MOF: {[Co₂(Hatz)(bta)]·H₂O}_n. Furthermore, a unique metal-organic-framework-based pine-needle-like nanocluster hierarchical architecture has been rationally designed and prepared on a nickel foam skeleton via a simple solvothermal method based on the Co(OH)F intermediate and directly adopted as an optimum bifunctional electrocatalyst for overall water splitting. The Co-MOF/NF exhibited enhanced catalytic performance for both the hydrogen evolution reaction (HER) and the oxygen evolution reaction (OER). The optimized catalyst reveals the highest electrocatalytic characteristics, affording current densities of 50 mA cm^-2 at an overpotential of 266 mV for the OER and 10 mA cm^-2 at an overpotential of 115 mV forthe HER in 1 M KOH. Meanwhile, the catalyst exhibits an ultrastability in the OER process and long-term test at 20 mA cm^-2 for 100 h led to only a 9.4% increase in overpotential. Furthermore, an electrolytic cell assembled from the bifunctional Co-MOF/NF delivers a current density of 10 mA cm^-2 at a cell voltage of 1.548 V. This excellent performance is believed to be the result of the exotic pine-needle-like nanocluster structure with effective accessibility of dense catalytically active sites, as well as the high specific surface area and the promotion of reversible chemisorption for oxygen species due to the linkers interacting with Co ions. Further SEM, TEM, and XPS analyses of the catalyst after OER stability tests reveal that the formation of Co₃O₄ on the surface and unconsolidated architecture withinthe electrode materials are responsible for the high catalytic activity. This work extends the applications of MOFs in the field of electrocatalysis.

A nickel ion-incorporating zinc-mesoporous metal organic framework thin film nanocomposite modified glassy carbon electrode for electrocatalytic oxidation of methanol in alkaline media

In this work, we have successfully synthesized a nickel ion-incorporating zinc-mesoporous metal–organic framework thin films (Zn-mMOFTFs) modified glassy carbon electrode (GCE), Ni/Zn-mMOFTFs/GCE, for electrooxidation of methanol in alkaline solution.

Crystal structure of poly[diaqua-(μ2-5-isopropoxyisophthalato-κ2 O:O′)-(μ2-(1,3-bis(3,5-di(1H-imidazol-1-yl)pyridine))-κ2 N:N′)cobalt(II)] monohydrate, C22H25N5O8Co

C22H25N5O8Co, monoclinic, P21/c (no. 2), a = 7.8425(3) Å, b = 18.1807(10) Å, c = 17.0436(7) Å, β = 95.133(3)°, V = 2420.37(19) Å3, Z = 4, R gt(F) = 0.0538, wR ref(F 2) = 0.1069, T = 293.18(10) K.

High-efficiency methanol oxidation electrocatalysts realized by ultrathin PtRuM-O (M = Ni, Fe, Co) nanosheets

A general method for controlling the synthesis of a class of ultrathin PtRuM-O (M = Ni, Fe, Co) NSs is reported for the first time. By optimizing the metal ratio, the Pt₇RuNi₂-O NS catalyst is found to have the highest electrocatalytic activity (mass activity, 3.57 A mg_Pt^-1) for the MOR among PtRuM-O NSs and PtRu-O NSs, which is 10.5 times higher than that of commercial Pt/C (0.34 A mg_Pt^-1). And the Pt₇RuNi₂-O NSs also have better stability and CO anti-poisoning properties in the prepared materials. In addition, the ultrathin Pt₇RuNi₂-O NS catalyst also shows the highest performance among reported Pt-based catalysts for the MOR in acidic medium.

Cubes on a string: a series of linear coordination polymers with cubane-like nodes and dicarboxylate linkers

A series of semicrystalline and amorphous one-dimensional (1D) polymeric chains consisting of cubane-like CoII4L4 units (L = S-1,2-bis(benzimidazol-2-yl)ethanol) and dicarboxylates were synthesized and characterized by single crystal diffraction and X-ray total scattering. The polycationic chains are composed of [Co4L4(dicarboxylate)]2+ monomeric units, while one molecular dicarboxylate counterion is balancing the charge of each monomer. The linear compound series has five members, and the crystal structures were solved for [Co4L4(tph)](tph) and [Co4L4(ndc)](ndc), where tph = terephthalate and ndc = 2,6-naphthalenedicarboxylate. Partly crystalline compounds were produced by slow assembly at elevated temperature (over days), while the amorphous compounds were formed by fast precipitation (within minutes). Pair distribution function (PDF) analysis based on X-ray total scattering data reveals the presence of the cubane-like entity in both the amorphous and semicrystalline samples. While the powders are non-porous, precipitation is a fast and versatile method to produce compounds with cubane-like centres with moderate surface areas of 17-49 m2 g-1 allowing for surface chemical reactions. The powders have a high concentration of Lewis base sites as verified by their selective adsorption of CO2 over N2. The use of an amorphous cubane-like polymer for the electrocatalytic oxygen evolution reaction was demonstrated.

Multimetal Incorporation into 2D Conductive Metal-Organic Framework Nanowires Enabling Excellent Electrocatalytic Oxidation of Benzylamine to Benzonitrile

As an important organic intermediate, benzonitrile (BN) is widely involved in organic synthetic chemistry and pharmaceutical and dyestuff industries. However, the exploration of a more efficient and controllable synthesis technique and the corresponding greener catalysts for the synthesis of BN still poses a great challenge. Herein, with multimetallic two-dimensional conductive metal-organic frameworks (2D cMOF) as anodic electrocatalysts, we develop a green, convenient, and highly efficient electrochemical synthesis strategy for BN. Thanks to the intrinsic 2D electrically conductive structure and the optimized the multimetallic coupling catalytic effect, the resulting multimetallic 2D cMOFs exhibit excellent benzylamine (BA) electrooxidation performance. Especially, the trimetallic 2D cMOF (NiCoFe-CAT) requires an ultralow potential of 1.29 V vs reversible hydrogen electrode (RHE) to achieve a 10 mA·cm^-2 current density, which indicates the fastest reaction and the most favorable thermodynamic condition. A very high yield (0.058 mmol·mg^-1·h^-1) and faradic efficiency (∼87%) of benzonitrile are both achieved during the BA electrooxidation reaction at 1.45 V. The reaction mechanism investigations indicated that the various redox mediators of M^II/M^III (Ni, Co, Fe) may be regarded as multimetal active species to promote BA conversion. Also, the excellent cycling durability of multimetallic 2D cMOFs further promotes their potential practical applications. These electrocatalytic performances are considered excellent and nearly surpass all other reported Ni-based inorganics or MOF-based electrocatalysts for the electrocatalytic oxidation of benzylamine.

Crystal structure of catena-poly[diaqua-(μ2-4,4′-bipyridine-κ2 N:N′)-bis(2,3,4,5-tetrabromo-6-carboxybenzoato-κ1 O)-nickel(II)], C26H14Br8NiN2O10

C26H14Br8NiN2O10, orthorhombic, Pbcn (no. 60), a = 24.1740(4) Å, b = 11.3727(2) Å, c = 11.7918(2) Å, V = 3241.84(10 Å3, Z = 4, R gt(F) = 0.0300, wR ref(F 2) = 0.0804, T = 293 K.

Crystal structure of poly[diaqua-(μ2-4,4′-bipyridine-κ2 N:N′)-(μ2-3,4,5,6-tetrafluorophthalato-κ2 O:O′)nickel(II)], C18H12F4NiN2O6

C18H12F4NiN2O6, orthorhombic, Pccn (no. 56), a = 7.4650(2) Å, b = 10.6954(2) Å, c = 22.7513(5) Å, V = 1816.49(7) Å3, Z = 4, R gt(F) = 0.0304, wR ref(F 2) = 0.0898, T = 293 K.

Crystal structure of poly[diaqua-(μ3-3,4,5,6-tetrafluoro-phthalato-κ3O:O′:O′′)-(μ2-1,2-bis(4-pyridyl)ethene-κ2 N:N′)cobalt(II)], C14H9CoF4NO6

C14H9CoF4NO6, monoclinic, P21/n (no. 14), a = 6.1183(2) Å, b = 7.5688(2) Å, c = 32.5016(7) Å, β = 90.672(2)°, V = 1504.99(7) Å3, Z = 4, R gt(F) = 0.0398, wR ref(F 2) = 0.1037, T = 293 K.

Inter-chain double-site synergistic photocatalytic hydrogen evolution in robust cuprous coordination polymers

We evaluate photocatalytic H₂ generation of three Cu-based coordination polymers, a record-high rate of 57.64 mmol g⁻¹ h⁻¹ for Cu–MIM, and reveal that the excellent performances are due to the synergies of the inter-chain double-site Cu atoms.