Molecule-based water-oxidation catalysts (WOCs): cluster-size-dependent dye-sensitized polyoxometalates for visible-light-driven O2 evolution

MOF/POM hybrids as catalysts for organic transformations

Insertion of molecular metal oxides, e.g. polyoxometalates (POMs), into metal-organic frameworks (MOFs) opens up new research opportunities in various fields, particularly in catalysis. POM/MOF composites have strong acidity, oxygen-rich surface, and redox capacity due to typical characteristics of POMs and the large surface area, highly organized structures, tunable pore size, and shape are due to MOFs. Such hybrid materials have gained a lot of attention due to astonishing structural features, and hence have potential applications in organic catalysis, sorption and separation, proton conduction, magnetism, lithium-ion batteries, supercapacitors, electrochemistry, medicine, bio-fuel, and so on. The exceptional chemical and physical characteristics of POMOFs make them useful as catalysts in simple organic transformations with high capacity and selectivity. Here, the thorough catalytic study starts with a brief introduction related to POMs and MOFs, and is followed by the synthetic strategies and applications of these materials in several catalytic organic transformations. Furthermore, catalytic conversions like oxidation, condensation, esterification, and some other types of catalytic reactions including photocatalytic reactions are discussed in length with their plausible catalytic mechanisms. The disadvantages of the POMOFs and difficulties faced in the field have also been explored briefly from our perspectives.

Visible-Light-Driven Oxidation of Amines to Imines in Air Catalyzed by Polyoxometalate-Tris(bipyridine)ruthenium Hybrid Compounds

The development of visible-light photocatalysts for the selective oxidative coupling of amines to imines is an area of great interest. Herein, four hybrid compounds based on polyoxometalate anions and tris(bipyridine)ruthenium cations, Ru(bpy)₃[M₆O₁₉] (M = Mo, W) 1-2, [Ru(bpy)₃]₂[Mo₈O₂₆] 3, [Ru(bpy)₃]₂[W₁₀O₃₂] 4, are prepared and characterized by X-ray diffraction (single-crystal and powder), elemental analysis, energy-dispersive X-ray spectroscopy (EDS) analysis, infrared (IR) spectroscopy, and solid diffuse reflective spectroscopy. Single-crystal structural analysis indicates that polyoxometalate anions and tris(bipyridine)ruthenium cations interact with each other through extensive hydrogen bonds in these compounds. These hybrid species with strong visible-light-harvesting abilities and suitable photocatalytic energy potentials show excellent photocatalytic activity and selectivity for the oxidation of amines to imines at room temperature in air as an oxidant. Among them, compound 1 with the [Mo₆O₁₉]^2- anion has the highest catalytic activity, which can swiftly convert >99.0% of benzylamine into N-benzylidenebenzylamine with a selectivity of 98.0% in 25 min illumination by a 10 W 445 nm light-emitting diode (LED). Its turnover frequency reaches 392 h^-1, which is not only better than the homogeneous catalyst [Ru(bpy)₃]Cl₂ but also much superior to those achieved over most of reported heterogeneous catalysts. Moreover, it shows a wide generality for various aromatic amines, accompanied by the advantages of good recyclability and stability. The photocatalytic oxidation mechanism of amines to the corresponding imines over polyoxometalate-based hybrid compounds was fully investigated.

Organoamine-Directed Assembly of 5p-4f Heterometallic Cluster Substituted Polyoxometalates: Luminescence and Proton Conduction Properties

The assembly of heterometallic cluster substituted polyoxometalates (POMs) remains a great challenge for inorganic synthetic chemistry up to now. Herein, a series of 5p-4f heterometallic cluster substituted POMs were successfully isolated by a facile one-step hydrothermal reaction method, namely H₁₇(H₂en)₃[Sb^III₉Sb^VLn₃O₁₄(H₂O)₃][(SbW₉O₃₃)₃(PW₉O₃₄)]·28H₂O(1-Ln, Ln = Ce, Sm, Eu, Gd, Tb, Dy) (en = ethylenediamine). Interestingly, by replacing en with imidazole, another series of 5p-4f heterometallic cluster substituted POMs H₁₃(HIm)₄K₂Na₄(H₂O)₉[Sb^III₉Sb^VLn₃O₁₄(H₂O)₃][(SbW₉O₃₃)₃(PW₉O₃₄)]·26H₂O (2-Ln, Ln = Sm, Eu, Gd, Tb, Dy, Im = imidazole) were obtained. Structural analyses indicate that both 1-Ln and 2-Ln are made up of an unprecedented 5p-4f heterometallic {Sb₁₀Ln₃O₁₄(H₂O)₃} cluster stabilized simultaneously by mixed trilacunary heteropolyanions including {A-α-PW₉O₃₄} and {B-α-SbW₉O₃₃}. Impedance measurements indicate that both compounds exhibit different proton conduction properties, and the conductivity of 2 can reach up to 1.64 × 10^-2 S cm^-1 at 85 °C under 98% relative humidity. Moreover, the fluorescence emission behaviors of both compounds have been studied.

A Tellurium-Substituted Heteropolyniobate with Unique π-π Stacking and Ionic Conduction Property

A tetratellurium-substituted heteropolyoxoniobate with the formula K₂H[Cu(phen)(H₂O)]₄[Cu(phen)]₂[(LiNb₈Te₄O₄₀)]·34H₂O (1; phen = 1,10-phenanthroline) was hydrothermally prepared and structurally characterized. The compound represents the first hexavalent tellurium-substituted Keggin-type polyoxometalate (POM) so far. What is more, the POM cluster in 1 can elaborately self-assemble into a stable supramolecular framework with an ion conduction property through unique intermolecular π-π stacking.

A Rare 3D Porous Inorganic-Organic Hybrid Polyoxometalate Framework Based on a Cubic Polyoxoniobate-Cupric-Complex Cage with a High Water Vapor Adsorption Capacity

A rare 3D porous inorganic-organic polyoxoniobate framework based on the cubic polyoxoniobate-cupric-complex cage {[Cu(en)₂]@{[Cu₂(en)₂(trz)₂]₆(Nb₆₈O₁₈₈)}} (1a), has been successfully synthesized by a hydrothermal method. The cubic cages 1a are connected with 4-(tetrazol-5-yl)pyridine to form a 1D pillar-like chain structure, and every 1D pillar-like chain is further linked with four adjacent pillar-like chains by the [Cu(en)₂]²⁺ complex to form a 3D porous inorganic-organic polyoxoniobate framework with 4-connected CdSO₄-type topology. To our knowledge, it is the first time that three different types of organic ligands are simultaneously introduced into one polyoxoniobate. This material also exhibits a high vapor adsorption capacity and good ionic conductivity properties.

Efficient Solar Evaporation by [Ni(Phen)3 ][V14 O34 Cl]Cl Hybrid Semiconductor Confined in Mesoporous Glass

Efficient utilization of solar energy for water evaporation is an advanced and environmentally friendly technology to address the crisis of global drinking water shortages. This study concerns an efficient solar vapor generator comprised of a light-absorbing and photothermal hybrid compound [Ni(Phen)₃ ][V₁₄ O₃₄ Cl]Cl (NiV₁₄ ) confined in mesoporous and hydrophilic glass (meso-glass). The generator is floated in water by supporting it on a domestic melamine-formaldehyde (MF) foam to ensure evaporation at the water-air interface. The porous structures and poor thermal conductivities of the meso-glass and MF foam contribute to enabling a consistent water supply, strong solar thermal localization, and less heat dissipation and convection. Associated with the strong photothermal role of NiV₁₄ , these synergistic effects lead to a water evaporation rate of 14.38 kg m^-2  h^-1 with total water evaporation efficiency of 111.4% under 6 suns and a daily solar water purification yield of 42.00 L m^-2 under 1 sun irradiation. This solar evaporation system shows great promise for high-efficiency water purification application.

The Reactivity and Stability of Polyoxometalate Water Oxidation Electrocatalysts

This review describes major advances in the use of functionalized molecular metal oxides (polyoxometalates, POMs) as water oxidation catalysts under electrochemical conditions. The fundamentals of POM-based water oxidation are described, together with a brief overview of general approaches to designing POM water oxidation catalysts. Next, the use of POMs for homogeneous, solution-phase water oxidation is described together with a summary of theoretical studies shedding light on the POM-WOC mechanism. This is followed by a discussion of heterogenization of POMs on electrically conductive substrates for technologically more relevant application studies. The stability of POM water oxidation catalysts is discussed, using select examples where detailed data is already available. The review finishes with an outlook on future perspectives and emerging themes in electrocatalytic polyoxometalate-based water oxidation research.

Photoinduced Oxygen Evolution Catalysis Promoted by Polyoxometalate Salts of Cationic Photosensitizers

The insoluble salt Cs₁₅K[Co₉(H₂O)₆(OH)₃(HPO₄)₂(PW₉O₃₄)₃] (CsCo₉) is tested as heterogeneous oxygen evolution catalyst in light-induced experiments, when combined with the homogeneous photosensitizer [Ru(bpy)₃]²⁺ and the oxidant Na₂S₂O₈ in neutral pH. Oxygen evolution occurs in parallel to a solid transformation. Post-catalytic essays indicate that the CsCo₉ salt is transformed into the corresponding [Ru(bpy)₃]²⁺ salt, upon cesium loss. Remarkably, analogous photoactivated oxygen evolution experiments starting with the [Ru(bpy)₃]_(5+x)K_(6−2x)[Co₉(H₂O)₆(OH)₃(HPO₄)₂(PW₉O₃₄)₃]·(39+x)H₂O (RuCo₉) salt demonstrate much higher efficiency and kinetics. The origin of this improved performance is at the cation-anion, photosensitizer-catalyst pairing in the solid state. This is beneficial for the electron transfer event, and for the long-term stability of the photosensitizer. The latter was confirmed as the limiting process during these oxygen evolution reactions, with the polyoxometalate catalyst exhibiting robust performance in multiple cycles, upon addition of photosensitizer, and/or oxidant to the reaction mixture.

Shining a light on the photo-sensitisation of organic-inorganic hybrid polyoxometalates

Finding new ways of using visible light (or, more specifically, solar irradiation) to drive commercially significant and/or challenging chemical processes is an ongoing research goal. Polyoxometalates (POMs) are discrete, metal-oxide clusters which are cheap, robust and easily synthesised but can also act as versatile molecular building blocks, allowing for astonishing variety in their structures and properties. In particular, the rich redox chemistry and inherent photo-activity of POMs makes them attractive for use in a variety of photochemical applications, however POMs characteristically only absorb strongly in the UV region. In this perspective, we discuss the various strategies which have been employed in order to sensitise POMs to visible light, with a particular focus on hybrid inorganic-organic POM species. We will discuss the two clear photo-activation mechanisms which have been developed to date and provide an outlook on some of the possible future directions of the field.

Evolutionary Metal Oxide Clusters for Novel Applications: Toward High-Density Data Storage in Nonvolatile Memories

Because of current fabrication limitations, miniaturizing nonvolatile memory devices for managing the explosive increase in big data is challenging. Molecular memories constitute a promising candidate for next-generation memories because their properties can be readily modulated through chemical synthesis. Moreover, these memories can be fabricated through mild solution processing, which can be easily scaled up. Among the various materials, polyoxometalate (POM) molecules have attracted considerable attention for use as novel data-storage nodes for nonvolatile memories. Here, an overview of recent advances in the development of POMs for nonvolatile memories is presented. The general background knowledge of the structure and property diversity of POMs is also summarized. Finally, the challenges and perspectives in the application of POMs in memories are discussed.

From Enzymes to Functional Materials-Towards Activation of Small Molecules

The design of non-noble metal-containing heterogeneous catalysts for the activation of small molecules is of utmost importance for our society. While nature possesses very sophisticated machineries to perform such conversions, rationally designed catalytic materials are rare. Herein, we aim to raise the awareness of the overall common design and working principles of catalysts incorporating aspects of biology, chemistry, and material sciences.

Synthesis, Property Characterization and Photocatalytic Activity of the Polyaniline/BiYTi₂O₇ Polymer Composite

A new polyaniline/BiYTi₂O₇ polymer composite was synthesized by chemical oxidation in-situ polymerization method for the first time. The effect of polyaniline doping on structural and catalytic properties of BiYTi₂O₇ was reported. The structural properties of novel polyaniline/BiYTi₂O₇ have been characterized by X-ray diffraction, scanning electron microscopy, X-ray photoelectron spectroscopy and UV-Vis DRS. The results showed that BiYTi₂O₇ crystallized well with the pyrochlore-type structure, stable cubic crystal system by space group Fd3m. The lattice parameter or band gap energy of BiYTi₂O₇ was found to be a = 10.2132 Å or 2.349 eV, respectively. The novel polyaniline/BiYTi₂O₇ polymer composite possessed higher catalytic activity compared with BiYTi₂O₇ or nitrogen doped TiO₂ for photocatalytic degradation of Azocarmine G under visible light irradiation. Additionally, the Azocarmine G removal efficiency was boosted from 3.0% for undoped BiYTi₂O₇ to 78.0% for the 10% polyaniline-modified BiYTi₂O₇, after only 60 min of reaction. After visible light irradiation for 330 min with polyaniline/BiYTi₂O₇ polymer composite as photocatalyst, complete removal and mineralization of Azocarmine G was observed. The photocatalytic degradation of Azocarmine G followed first-order reaction kinetics. Ultimately, the promoter action of H₂O₂ for photocatalytic degradation of AG with BiYTi₂O₇ as catalyst in the wastewater was discovered.

Carbon Nitride Supramolecular Hybrid Material Enabled High-Efficiency Photocatalytic Water Treatments

Surface defects in relation to surface compositions, morphology, and active sites play crucial roles in photocatalytic activity of graphitic carbon nitride (g-C₃N₄) material for highly reactive oxygen radicals production. Here, we report a high-efficiency carbon nitride supramolecular hybrid material prepared by patching the surface defects with inorganic clusters. Fe (III) {PO₄[WO(O₂)₂]₄} clusters have been noncovalently integrated on surface of g-C₃N₄, where the surface defects provide accommodation sites for these clusters and driving forces for self-assembly. During photocatalytic process, the activity of supramolecular hybrid is 1.53 times than pure g-C₃N₄ for the degradation of Rhodamine B (RhB) and 2.26 times for Methyl Orange (MO) under the simulated solar light. Under the mediation of H₂O₂ (50 mmol L^-1), the activity increases to 6.52 times for RhB and 28.3 times for MO. The solid cluster active sites with high specific surface area (SSA) defect surface promoting the kinetics of hydroxide radicals production give rise to the extremely high photocatalytic activity. It exhibits recyclable capability and works in large-scale demonstration under the natural sunlight as well and interestingly the environmental temperature has little effects on the photocatalytic activity.

Efficient electrochemical water oxidation in neutral and near-neutral systems with a nanoscale silver-oxide catalyst

In electrocatalytic water splitting systems pursuing for renewable energy using sunlight, developing robust, stable and easily accessible materials operating under mild chemical conditions is pivotal. We present here a unique nanoparticulate type silver-oxide (AgOx-NP) based robust and highly stable electrocatalyst for efficient water oxidation. The AgOx-NP is generated in situ in a HCO3(-)/CO2 system under benign conditions. Micrographs show that they exhibit a nanoscale box type squared nano-bipyramidal configuration. The oxygen generation is initiated at low overpotential, and a sustained O2 evolution current density of >1.1 mA cm(-2) is achieved during prolonged-period water electrolysis. The AgOx-NP electrocatalyst performs exceptionally well in metal-ion free neutral or near-neutral carbonate, phosphate and borate buffers relative to recently reported Co-oxide and Ni-oxide based heterogeneous electrocatalysts, which are unstable in metal-ion free electrolytes and tend to deactivate with time and lose catalytic performance during long-term experimental tests.

Two New Armtype Polyoxometalates Grafted on Titanium Dioxide Films: Towards Enhanced Photoelectrochemical Performance

Two new carboxyethyltin-functionalized polyoxometalates (POMs) were successfully obtained and confirmed with physicochemical and spectroscopic methods including X-ray crystallography. The lowest unoccupied molecular orbitals of both compounds are higher in energy than that of TiO2 , and the optical band gaps of these compounds are smaller than that of TiO2 . Grafting them onto a TiO2 film created two kinds of novel photoanode materials that showed significantly enhanced photovoltaic and photocurrent responses, as well as improved photoelectrooxidation activities for methanol relative to that shown by a single TiO2 film. Further, P2 W15 -Co-SnR produced the largest photocurrent by exploring the photoelectric activities of a series of carboxyethyltin POM derivatives. This work provides new insight into the photoelectrochemical functionalization of POM-based organic-inorganic hybrids.

An unprecedented 3D POM–MOF based on (7,8)-connected twin Wells–Dawson clusters: synthesis, structure, electrocatalytic and photocatalytic properties

The first (7,8)-connected POM–MOF has been obtained, representing the highest connected POMs to any mixed-connected POM–MOF and a unique structure that contains organic–inorganic and all-inorganic networks.

Efficient dye-sensitized solar cell with a pure thin film of a hybrid polyoxometalate covalently attached organic dye as a working electrode in a cobalt redox mediator system

A Keggin-type hybrid polyoxometalate is used as a pure semiconductor thin film in the working electrode of a dye-sensitized solar cell fabricated with a D35 organic dye and a cobalt redox mediator system, giving a maximum IPCE of about 91% .

Mesoporous polyoxometalate cluster–crosslinked organosilica frameworks delivering exceptionally high photocatalytic activity

Mesoporous polyoxometalate (POM)-based organosilica frameworks with high catalytic activity, 3D open-pore structure, and uniform pores are prepared via a surfactant-assisted co-polymerization of organically-modified POM clusters with ethane-bridged silsesquioxane linkers.

Dye-sensitized polyoxometalate for visible-light-driven photoelectrochemical cells

A simple and facile one-step method for the synthesis of an organic dye-functionalized polyoxometalate (POM) hybrid with visible-light photo-response was reported.

The use of a vanadium species as a catalyst in photoinduced water oxidation

The first water oxidation catalyst containing only vanadium atoms as metal centers is reported. The compound is the mixed-valence [(V(IV)5V(V)1)O7(OCH3)12](-) species, 1. Photoinduced water oxidation catalyzed by 1, in the presence of Ru(bpy)3(2+) (bpy = 2,2'-bipyridine) and Na2S2O8, in acetonitrile/aqueous phosphate buffer takes place with a quantum yield of 0.20. A hole scavenging reaction between the photochemically generated Ru(bpy)3(3+) and 1 occurs with a bimolecular rate constant of 2.5 × 10(8) M(-1) s(-1). The time-resolved formation of the oxidized molecular catalyst 1(+) in bimolecular reactions is also evidenced for the first time by transient absorption spectroscopy. This result opens the way to the use of less expensive vanadium clusters as water oxidation catalysts in artificial photosynthesis schemes.