Polyoxometalate-based Metal Research Articles

Enhanced C(sp3)–H Selective Oxidation Using Efficient Polyoxometalate-Based Metal–Organic Framework Photocatalysts Synergized with Ir Metalloligands and Polyoxometalates

Enhanced C(sp³)–H Selective Oxidation Using Efficient Polyoxometalate-Based Metal–Organic Framework Photocatalysts Synergized with Ir Metalloligands and Polyoxometalates

Two novel 3D polyoxometalate-based metal–organic frameworks for structure-directed selective adsorption and photodegradation of organic dyes

Degradation of organic dyes from industrial wastewater is crucial for human health and ecological balance. Synergy adsorption and photodegradation is one of the effective methods to degrade organic dyes at present. The former is usually limited by the challenge of its the recovery and the poor adsorption capacity of the adsorbent, whereas the latter frequently suffers from sluggish reaction kinetics. Herein, the introduction of the unique electron-rich Keggin polyoxometalates (CuW12 and SiW12) into electron-deficient metal photosensitive viologen framework to construct two polyoxometalate(POM)-based metal organic frameworks (POMOFs) of 3D porous layered structures, namely [Cu2(ipbp)2(H2O)2][CuW12O40]·3H2O (1) and [Co2(H2ipbp)3(H2O)2(CoO6)2][SiW12O40]·H2O (2); (H2ipbp·Cl = 1-(3,5-dicarboxyphenyl)-4,4′-bipyridinium), which enable selective adsorption of methylene blue (MB) and effectively photocatalytic degradation cationic dyes. In addition, POMs and metal-organic frameworks act in synergy to result in much improved adsorption of certain cationic dyes as well as enhanced photocatalytic activity that allow the catalyst to show better excellent reusability and stability. The adsorption degradation rate of MB and rhodamine B (RhB) of 2 is faster than that of 1 because of the lower Zeta potential of 2 suggesting that there are electrostatic interactions in the adsorption process. This work adopted a novel view to comprehening the technology.

Conjugated acid-base pairs in Keggin-type polyoxometalate-based metal-organic frameworks enhance proton conduction

To achieve high proton conductivities, it is pivotal to design and select appropriate functional organic ligands for constructing polyoxometalate-based metal–organic frameworks (POMOFs). In this work, the effects of functional groups upon the molecular electrostatic potentials and pKa of two organic ligands (4-abpt, theoretical pKa value of 25.13; Hdpt, theoretical pKa value of 9.24) are first investigated by theoretical calculations. And then, two new proton-conductive decorated POMOFs, {[Cu(4-abpt)2(H2O)][Cu(4-abpt)(H2O)][SiW12O40]}·6H2O (CUST-875) and {[Cu(Hdpt)(H2O)2] [Cu(Hdpt)][SiW12O40]}·2H2O (CUST-876), have been designed and obtained via hydrothermal method. Besides, their proton conduction performances and influencing factors also have been investigated in more details. It is found that more water molecules adsorption and the conjugated acid-base pairs played substantial roles in proton conduction. As a consequence, the three-dimensional framework CUST-875, composed of the higher theoretical pKa value organic ligand 4-abpt, exhibited a higher proton conductivity of 1.07 × 10−4 S cm−1 at 95 °C and 98 % relative humidity (RH), surpassing that of CUST-876 by three orders of magnitude. Additionally, CUST-875 possesses the ability to degrade H2O2, which readily generates during charging and discharging of battery and has adverse effects on the battery life. This work contributes new insights into the high efficiency preparation and structure–property relationships of new polyoxometalate (POM)-based crystalline proton conducting materials with relatively high intrinsic proton conductivities.

Ultrafine Co-MoC particles in porous carbon derived from polyoxometalate-based metal organic framework for efficient hydrogen evolution reaction

Accurately regulating ultrafine molybdenum carbide (MoC)-based catalysts is a significant challenge in the rational design of hydrogen evolution reaction (HER) electrocatalysts. Herein, under the guidance of the first principle calculations, we proposed an in-situ polyoxometalate-confined strategy for creating uniformly distributed ultrafine Co-MoC bimetallic nanoparticles in porous carbon nanostars, with the assistance of precisely designed metal–organic framework (MOF). The Co-MoC@C electrocatalyst has a high specific surface area of 969 m2·g−1 because of the conductive carbon substrate with abundant mesopores, which makes for exposing more active sites of Co-MoC nanocrystals (∼1.5 nm) and facilitating electron/ion transport. Thus, Co-MoC@C electrocatalyst shows the excellent electrochemical activity with overpotentials of 88.4 mV and 66.6 mV at a current density of 10 mA·cm−2 under acidic and alkaline conditions, respectively. The in-situ polyoxometalate-confined strategy will provide a new guideline for the design and preparation of efficient HER electrocatalysts.

Vanadate-based Fe-MOFs as promising negative electrode for hybrid supercapacitor device

In the supercapacitor field, negative electrodes are mainly concentrated in carbon-based materials, such as activated carbon, carbon nanotubes, graphene, and so forth. However, materials based on metal–organic frameworks (MOFs) as negative active components are relatively rare. Herein, a series of composite materials based on graphene oxide (GO) and vanadate-based Fe-organic frameworks have been prepared by hydrothermal method namely GO/Fe-VO4-BIPY. The deposition amount of polyoxometalate-based metal–organic frameworks (POMOFs) on the surface of graphene is adjusted by changing the content of POMOFs. Through the deposition, it can effectively reduce the accumulation between graphene, and increase the dispersion of POMOFs. As a result, the charge storage performance of the as-obtained materials is greatly improved. Among these materials, GO/Fe-VO4-BIPY-1 has the most prominent performance, with a specific capacitance of 190 F g−1 at 0.5 A g−1, which is attributed to the excellent synergistic effect between the Faraday chemical reaction and electric double-layer capacitance. In comparison with pristine Fe-VO4-BIPY, GO/Fe-VO4-BIPY-1 delivers more excellent surface area and therefore exhibits abundant redox reaction sites, achieving better electrochemical performance the best. After assembly with the positive Ni(OH)2 electrode, the maximum energy density of 46.84 W h kg−1 at a power density of 850 W kg−1 is achieved.

Zirconium-rich magnetic polyoxometalate-based metal–organic framework: Tailored for phosphopeptide analysis from lung cancer A549 cells

Polyoxometalate-based metal–organic frameworks (POMOFs) have become a promising affinity material for separation and enrichment. The analysis of protein phosphorylation represents a challenge for the development of efficient enrichment materials. Here, a novel zirconium-rich magnetic POMOF was successfully designed and prepared for the enrichment of phosphopeptides. The binding affinity of the nanomaterial partly came from Fe–O clusters in the MOF. The Lewis acid–base interactions between V–O clusters and zirconium ions in V10O28-Zr4+ and phosphate groups in phosphopeptides further strengthened the enrichment ability. The zirconium-rich magnetic POMOF was employed to capture phosphopeptides from non-fat milk, human saliva, and serum. Additionally, 748 unique phosphopeptide peaks were detected from the tryptic digests of lung cancer A549 cell proteins with a high specificity (86.9 %). POMOFs will become an active competitor for the design of protein affinity materials and will provide a new approach for phosphopeptide analysis.

Revealing the roles of the dual active-sites on a polyoxometalate-based metal–organic framework in catalyzing Knoevenagel condensations

The efficient catalysis of Knoevenagel condensations under mild conditions is a significant way for obtaining olefin compounds, which can be used to produce a wide range of biologically active heterocyclic chemicals and fine chemical raw materials. Herein, a novel polyoxometalate-based metal–organic framework (POMOF), [Co(bix)(H2O)]{V2O6} (VMOF-1, bix = 1,4-bis(imidazol-1-ylmethyl)benzene), is synthesized and structurally characterized. After removal of the coordinated water molecules of CoII ions by vacuum drying, VMOF-1a with both Lewis acid and Lewis base catalytic active-centers is achieved. VMOF-1a as the heterogeneous catalyst for the conversion of benzaldehyde and malononitrile to high value-added olefinic compounds under mild conditions, exhibits excellent conversion and selectivity, good recyclability and broad substrate applicability. Experimental and theoretical studies have revealed that CoII ion and {V2O6}2− unit in VMOF-1a synergistically catalyze the Knoevenagel condensation reaction: (1) {V2O6}2− is used as a Lewis base to pull out the proton of the active methylene group on malononitrile; (2) CoII ion acts as a Lewis acid to activate the carbonyl oxygen on benzaldehyde. Furthermore, it is demonstrated for the first time that the Lewis acid sites in POMOF can play the role of stabilize the alcohol hydroxyl anion intermediates generated during the reaction process, which further elucidates the reason for the high catalytic activity of VMOF-1a in the Knoevenagel condensation. This study provides theoretical guidance for the design of efficient and sustainable catalysts for Knoevenagel condensation.

Two new 6-nitrobenzimidazole ligand-modified polymolybdate-based metal–organic complexes with excellent capacitive and electrocatalytic performances

6-Nitrobenzimidazole ligand was first introduced into polyoxometalate-based metal–organic architecture, and two unique butterfly-like complexes were obtained. Their applications in electrochemical energy storage and electrocatalysis were studied.

Advancing biomedical applications of polyoxometalate-based metal–organic frameworks: from design to therapeutic potential

The review covers POMOF materials synthesis, modification, and unique properties, focusing on biomedical sensors and therapies. Finally, the review summarises the challenges and developments of POMOFs in biomedicine.

Photocatalytic oxidative coupling of terminal alkynes with p-benzoquinone to diaryl ketones by the Cu(I)-PS-POM assembling system through synergistic effect

Photocatalytic C–C bonds formation is an effective approach to synthesize the significant drug intermediates diaryl ketones. Herein, a new polyoxometalate-based metal–organic framework (POMOF), CuW6–TPT, was synthesized by assembling an oxidation catalyst [W6O19]2− into photoactive MOF constructed by photosensitizer (PS) 4,6-tri(4-pyridyl)-1,3,5-triazine (TPT) and binuclear Cu(I) units. CuW6–TPT exhibits excellent light absorption ability and unique conductivity due to the coordination bond, strong π∙∙∙π interactions and H-bonding among Cu(I) ions, TPT moieties and [W6O19]2− in the defined spatial. The abundant exposed Cu(I) catalytic centres are conductive to activate phenylacetylene in situ formation Cu(I)-phenylacetylide, which then undergoes a reductive single electron transfer (SET) process to obtain the key intermediate Cu(II)-phenylactetylide for the Paterno-Buchi (2+2) cycloaddition with benzoquinone to form the labile Cu(II)-oxetane ring. It is a direct route to synthesize diaryl ketones with a hydroxyl group from simple starting materials such as terminal alkynes with p-benzoquinone.

Improved supercapacitors and water splitting performances of Anderson-type manganese(III)-polyoxomolybdate through assembly with Zn-MOF in a host–guest structure

Enhancing performance through the combination of polyoxometalates (POMs) clusters with metal–organic frameworks (MOFs) that contain various transition metals is a challenging task. In this study, we synthesized a polyoxometalate-based metal–organic framework (POMOF) named HRBNU-5 using a solvothermal method. HRBNU-5 is composed of Zn[N(C4H9)4][MnMo6O18{(OCH2)3CNH2}2]@Zn3(C9H3O6)2·6C3H7NO, which includes two components: Zn[N(C4H9)4][MnMo6O18{(OCH2)3CNH2}2]·3C3H7NO ({Zn[MnMo6]}) and Zn3(C9H3O6)2·3C3H7NO (Zn-BTC). Structural characterization confirmed the host–guest structure, with Zn-BTC encapsulating {Zn[MnMo6]}. In a three-electrode system, HRBNU-5 exhibited a specific capacitance of 851.3 F g−1 at a current density of 1 A/g and retained high stability (97.2 %) after 5000 cycles. Additionally, HRBNU-5 performed well in aqueous-symmetric/asymmetric supercapacitors (SSC/ASC) in terms of energy density and power density in a double-electrode system. Moreover, it demonstrated excellent catalytic performance in a 1.0 M KOH solution, with low overpotentials and Tafel slopes for hydrogen and oxygen evolution reactions: 177.1 mV (η10 HER), 126.9 mV dec–1 and 370.3 mV (η50 OER), 36.3 mV dec–1, respectively, surpassing its precursors and most reported studies. HRBNU-5′s positive performance is attributed to its host–guest structure, high electron-transfer conductivity, and porous structure that enhances efficient mass transport. This work inspires the design of Anderson-type POMOF electrode materials with multiple active sites and a well-defined structure.

Reactive antistatic additive modified copper(II) azide as a primary explosive with simultaneously enhanced stability and energy

The emergence of micro-priming systems expedites the demand for balancing the inherent contradiction between energy and safety in primary explosives. This paper proposes a new strategy (metal organic framework/reactive antistatic agent strategy, MOF/RAA strategy) which introduces the reactive antistatic agent (WO3) into ternary copper azide /carbon activity system by carbonization and azide of polyoxometalate-based metal–organic frameworks. Subsequent characterization validates that the novel reactive multi-component system (ternary copper azide /carbon/tungsten trioxide) performs well with both high electrostatic safety and remarkable detonating ability (detonate CL-20 as low as 0.98 mg) due to the electrical conductivity and active participation in secondary reaction of reactive antistatic agent (WO3). In general, this discovery has obtained prominent achievements in the application of copper azide in micro-initiating systems, providing a safe yet energy simultaneously enhanced way for the design and synthesis of novel energetic compounds.

Enhanced Photoinduced Baeyer–Villiger Oxidation of Ketones by Introducing Trinuclear Ruthenium Clusters into Polyoxometalate-Based Metal–Organic Frameworks

The development of solar-driven organic synthesis processes as an alternative to conventional thermo-catalytic technologies is of great research significance in academia and industry. Herein, we looked into the photoinduced Baeyer–Villiger oxidation of cyclohexanone based on the Mukaiyama method for the first time. We fabricated a crystalline polyoxometalate-based metal–organic framework photocatalyst, CR-SiW12, which represents the first example of trinuclear ruthenium clusters involved in the construction of polyoxometalate-based metal–organic frameworks. The cyclohexanone was efficiently converted to ε-caprolactone under visible light (>400 nm) irradiation with CR-SiW12 as the catalyst, with a reaction turnover number and turnover frequency recorded as 941 and 274.5 h–1, respectively, and the apparent quantum yield at 465 nm was measured to be 8.4%. Moreover, CR-SiW12 exhibited high structural stability and reaction reusability where reaction yield remained high at 91.3% after five consecutive reaction cycles. This work offers a promising strategy for accomplishing Baeyer–Villiger oxidation reactions under green conditions.

Preparation of Single-Ni atom active points by anchoring Anderson polyoxometalate in HKUST-1 for efficient Symmetric/ Asymmetric device energy storage and electrocatalytic overall water splitting

We explore and synthesize hollow polyoxometalate-based metal–organic framework (POMOF) nanomaterial {Cu(NH4)2NiMo6O24H6}@HKUST-1 (HRBNU-2) with single-atom sites (SASs) by one-step solution method. It exhibits high specific capacitance (978.50F g−1) in neutral electrolyte. Symmetrical/Asymmetrical devices can light four Red Light Emitting Diodes (RLEDs)/one RLED on for 14 min. For electrocatalytic water splitting, the overpotentials of hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) in alkaline medium are 192.98 mV (η100) and 405.38 mV (η100). The double-layer capacitance (Cdl) of HRBNU-2 is 269.23 mF cm−2, reaching about 4.43 times of {Cu2(NH4)2Mo7O24}@HKUST-1 (HRBNU-3) and about 12.70 times of HKUST-1. Moreover, the device assembled by HRBNU-2 as cathode and anode, requires only 1.61 V cell voltage to achieve current density of 10 mA cm−2 and obtain stability of 100 h. Meanwhile, comparative study of HRBNU-3 system with similar structural composition is synthesized to illustrate that the synergy of SASs(Ni) and Mo in well-defined structures plays important role in the study of efficient overall water splitting and supercapacitance performance.

Structural Regulation of Two Polyoxometalate-Based Metal-Organic Frameworks for the Heterogeneous Catalysis of Quinazolinones.

Two dimeric {ε-Zn4PMo12}-based metal-organic frameworks (MOFs), [ε-PMo8VMo4VIO34(OH)6Zn4][LO] (SDUT-21, LO = [5-((4'-carboxybenzyl)oxy)isophthalic acid]) and [TBA]3[ε-PMo8VMo4VIO37(OH)3Zn4][LN] (SDUT-22, TBA+ = tetrabutylammonium ion, LN = [5-((4-carboxybenzyl)imino)isophthalic acid]), combining the advantages of polyoxometalates (POMs) and MOFs, were synthesized by the one-pot assembly strategy. The dimeric {ε-Zn4PMo12} units act as nodes that are linked by the flexible ligands and extended into two- or three-dimensional frameworks. The cyclic voltammetry and proton conductivity measurements of SDUT-21 and SDUT-22 were performed and indicated the high electron and proton transfer abilities. These materials also e xhibited the catalytic performance for the synthesis of quinazolinones in the heterogeneous state, and the different binding capacities toward the substrates caused the catalytic activity of SDUT-21 to be higher than that of SDUT-22 under the same conditions. In addition, the used catalysts could be readily recovered for five successive cycles and maintained high catalytic efficiency.

Nanomaterial with Core–Shell Structure Composed of {P2W18O62} and Cobalt Homobenzotrizoate for Supercapacitors and H2O2-Sensing Applications

Designing and preparing dual-functional Dawson-type polyoxometalate-based metal-organic framework (POMOF) energy storage materials is challenging. Here, the Dawson-type POMOF nanomaterial with the molecular formula CoK4[P2W18O62]@Co3(btc)2 (abbreviated as {P2W18}@Co-BTC, H3btc = 1,3,5-benzylcarboxylic acid) was prepared using a solid-phase grinding method. XRD, SEM, TEM et al. analyses prove that this nanomaterial has a core-shell structure of Co-BTC wrapping around the {P2W18}. In the three-electrode system, it was found that {P2W18}@Co-BTC has the best supercapacitance performance, with a specific capacitance of 490.7 F g-1 (1 A g-1) and good stability, compared to nanomaterials synthesized with different feedstock ratios and two precursors. In the symmetrical double-electrode system, both the power density (800.00 W kg-1) and the energy density (11.36 Wh kg-1) are greater. In addition, as the electrode material for the H2O2 sensor, {P2W18}@Co-BTC also exhibits a better H2O2-sensing performance, such as a wide linear range (1.9 μM-1.67 mM), low detection limit (0.633 μM), high selectivity, stability (92.4%) and high recovery for the detection of H2O2 in human serum samples. This study provides a new strategy for the development of Dawson-type POMOF nanomaterial compounds.

State-of-the-art advances in the syntheses, structures, and applications of polyoxometalate-based metal–organic frameworks

Polyoxometalate-based metal–organic frameworks (PMOFs) as extended solids assembled from metal-oxide cluster units and metal–organic groups have drawn wide research attention in recent years due to the unique advantages of containing both polyoxometalate (POM) and metal–organic framework (MOF) units, which allow their multifunctional applications in catalysis, sensing, and energy storage. In this review, the recent progress on the syntheses, structural diversity, and potential applications of PMOFs are summarized. In terms of structure, two categories of PMOFs, POM@MOFs and POM–MOFs, are discussed. POM@MOFs are PMOFs in which the POM units are not coordinated to the MOF, whereas POM–MOFs are PMOFs in which the POM units are coordinated to the MOF. In terms of properties, some selected investigations on the application of PMOFs in catalysis, dye adsorption and degradation, chemical sensing and energy storage are covered. To conclude, a personal outlook and viewpoints on this field are presented. It is expected that this review will inspire researchers and provide helpful tips for the future rational design of function-oriented PMOFs.

Polyoxometalate-based metal–organic frameworks directed fabrication of defective-1 T/2H-MoS2/ZnS heterostructured nanozyme for colorimetric determination of hydroquinone

1 T phase of MoS2-based nanomaterial is an ideal nanozymes. However, 1 T-MoS2 is easily converted into stable 2H-MoS2, so the realization of high-content 1 T-MoS2 materials is still a great challenge at present. In this paper, a new polyoxometalate-based metal–organic framework (POMOF) Zn6(C9H7N7)9(PMo12O40)4·4H2O (Zn-POM) was prepared and applied as a template to construct 1 T/2H-MoS2/ZnS heterostructured composites by hydrothermal method. The content of 1 T-phase MoS2 and defective surface in the composites under different hydrothermal reaction times were monitored by XPS spectroscopy, and a heterostructured nanomaterial with robust defects and high content of 1 T-MoS2 was obtained. The content of 1 T-MoS2 in the whole MoS2 reaches up to 73.52 % (the remaining 26.5 % is 2H-MoS2). The obtained 1 T/2H-MoS2/ZnS as a nanozyme reveals fantastic peroxidase-like activity by catalyzing the oxidation of peroxide substrate 3, 3, 5, 5-tetramylbenzidine (TMB) to blue oxidized TMB (oxTMB) with H2O2. Compared with horseradish peroxidase (HRP), 1 T/2H-MoS2/ZnS exhibited higher affinity and catalytic activity for H2O2 and TMB, respectively. The Michaelis Menten constant (Km) of 1 T/2H-MoS2/ZnS toward H2O2 is 77-fold lower than that of HRP. Colorimetric sensor properties for hydroquinone (HQ) detection were also exploited. 1 T/2H-MoS2/ZnS showed superior sensor performance to HQ, with a wide linear response ranging (from 1 to 50 μM) and a low detection limit (LOD) of 0.39 μM, exceeding most reported nanozymes. This study affords a feasible method to design defective-1 T-MoS2 heterostructured nanozymes with excellent peroxidase-like properties in colorimetric sensors and environmental monitoring.

Dynamic Construction and Maintenance of Confined Nanoregions via Hydrogen-Bond Networks between Acetylene Reactants and a Polyoxometalate-Based Metal-Organic Framework.

The nanoconfinement effect in catalysis has attracted much attention because it provides a novel means of regulating the molecular properties and related reactions. Confined nanoregions composed of both reactants and catalysts through weak interactions are expected to improve the catalytic performance and promote the mass transport of relevant molecules simultaneously. However, at reaction temperatures, the structural variation of such confined spaces constructed via weak interactions remains unclear. Herein, through density functional theory calculations combined with ab initio molecular dynamics simulations, we have systematically investigated the dynamic structural evolution of the confined space constructed by acetylene reactants and a polyoxometalate-based metal-organic framework (POMOF) via hydrogen-bond networks. It is found that, at the reaction temperature of acetylene semihydrogenation, the hydrogen-bond networks and generated confined nanoregions are not rigid but are constantly changing and dynamically maintained. The steering role played by the O atoms at the surfaces of the polyoxometalate clusters is essential for generation of the hydrogen-bond networks and maintenance of the nanoregions. Upon confinement, the acetylene reactants can be better activated than those in an unconstrained atmosphere, which is reflected by the different dynamic distributions of the ∠CHC bending magnitude. Moreover, from a comparison of the distinct interaction characteristics between acetylene/ethylene and POMOF, the different manifestations in the adsorption energy variations of the confined molecules can be interpreted. This work helps to elucidate the underlying mechanisms of confined catalysis and may promote its application in practical catalytic processes.

Hydroxyl radical-dominated selective oxidation of ethylbenzene over a photoactive polyoxometalate-based metal-organic framework.

Realizing photo-promoted saturated C-H functionalization is a significant challenge. [CuI3(H2O)6(TPT)2][H2BW12O40]·28H2O was assembled by combining electron reservoir [BW12O40]5- with photosensitizer TPT. The continuous coordination bonds and π-π stacking interactions facilitate hole-electron separation and electron transfer, and allow it to exhibit high photocatalytic activity toward ethylbenzene oxidation with O2/H2O as oxidants.