POM-based Catalysts Research Articles

Highly efficient C16-PW9/SiO2 designed based on trivacant tungstophosphate ([A-PW9O34]9−) for rapid oxidative desulfurization under mild conditions

In order to achieve ultra-low sulfur fuel production, it is essential to design highly efficient catalysts for deep desulfurization. In this work, trivacant Keggin-type tungstophosphate PW9 (Na9[A-PW9O34]•7H2O) was innovatively utilized to synthesize polyoxometalate-based supported silica C16-PW9/SiO2. The intact presence of polyoxometalates (POMs) was evidenced by several techniques of describing the structure and composition of the obtained hybrid samples. Compared with the saturated tungstophosphate ([PW12O40]3−), the [A-PW9O34]9− showed better coordination, which can coordinate with five ILs (C16MIM). More ILs make it easier for the catalyst to attract DBT to the vicinity of PW9, and the abundance of metal oxide W = O on PW9 endows the catalyst’s excellent catalytic activity. Specifically, C16-PW9/SiO2 exhibited excellent oxidative desulfurization performance, achieving 100 % dibenzothiophene (DBT) conversion efficiency at 50 °C for 10 min, and the turnover frequency (TOF) number reached 723.4 h−1, which is higher than the catalysts of the POMs type that have been published. The strategy of combining lacunary polyoxometalates with ionic liquids offers a fresh viewpoint on the creation of POM-based catalysts.

Sustainable depolymerization of lignin into aromatic compounds using amphiphilic Anderson-type polyoxometalate catalysts

Lignin serves as a primary abundant source of renewable aromatic compounds. Achieving efficient breakdown of lignin and retaining its aromatic properties is highly desirable but remains a challenging task. To address this challenge, we synthesized Anderson-type polyoxometalate (POM) catalysts, particularly [CTAC]2[CoMo6]. We then investigated the effectiveness of the POM catalysts in the oxidative depolymerization of larch lignin. Under conditions of 160 °C, 1.0 MPa oxygen atmosphere, and a catalyst-to-substrate ratio of 1:5, we achieved a monomer yield of phenolic compounds at 12.43 wt%. The unsaturated coordination sites of Mo5+ within the catalysts were identified as active sites, facilitating enhanced O2 adsorption and activation. The enhanced O2 adsorption significantly influenced the production of aromatic monomers from lignin. We observed that the catalysts effectively cleaved CC bonds in β-O-4 dimer compounds using lignin dimer model compounds. Notably, the [CTAC]2[CoMo6] catalyst exhibited excellent stability across five cycles, maintaining its high efficiency in lignin depolymerization. This indicates that Anderson-type POM-based catalysts exhibit potential for sustainable conversion of biomass into valuable compounds and for enhancing lignin valorization processes.

Application of polyoxometalates and their composites for the degradation of antibiotics in water medium

This paper summarizes the potential of polyoxometalate (POM)-based catalysts in view of pharmaceutical wastewater treatment and recent advances that took place in this field. POMs are anionic clusters of transition metals, which exhibit unique characteristics such as high catalytic activity and multi-electron redox properties. Recently, they have been explored by some research groups for degrading antibiotics and pharmaceutical compounds (PCs) from contaminated water matrix. Several modifications of POM, along with their composite formation with new-age materials like g-C3N4 and reduced graphene oxide (RGO), have led to the formation of novel photocatalysts, which have also been reported as active materials to destroy the PCs. These promising catalysts have revealed the efficiency of complete destruction of these recalcitrant compounds within a short reaction time and showed good reusability characteristics. Among the widely used PCs, the notable ones include tetracycline (TC), sulfamethoxazole (SMX), ciprofloxacin (CIP), etc. Most of the articles cited here centered on TC degradation followed by other drugs. The effects of different operating conditions, degradation efficiency, and mechanism and stability aspects of various POM-based catalysts are discussed. The current knowledge gap in this area with bright future perspectives is also highlighted. The description will provide valuable insight to the research community regarding the capability of POM-based catalysts to eliminate antibiotics, as well as designing highly efficient catalysts for a sustainable future.

A series of polyoxometalate-based COF composites by one-pot mechanosynthesis of thioether to sulfone.

An effective combination of polyoxometalates (POMs) and porous materials is a feasible method to solve the homogeneity of POMs and synthesize extremely stable POM-based catalysts. Herein, by using simple mechanochemical synthesis, we fabricated a series of composites constructed by Keggin-POMs, p-phenylenediamine (Pa-1), and 1,3,5-triformylphloroglucinol (Tp), which in situ form a stable covalent organic framework (Keggin-POMs@TpPa-1). Notably, the different Keggin-POMs@TpPa-1 composites showed different catalytic effects on thioether oxidation reaction under mild conditions. From the comparison, the catalytic effect of PW12@TpPa-1 with its added amount of 27% H3PW12O40 is superior to that of other composites, whose catalytic efficiency can reach 99%. This study provides some inspiration for designing diverse POM-modified catalysts with outstanding stability and efficiency using COFs as supports.

Catalytic application of polyoxovanadates in the selective oxidation of organic molecules

The structural synthesis and catalytic application of polyoxovanadates (POVs), an important subfamily of polyoxometalates (POMs), have attracted significant attention, owing to their rich coordination chemistry and splendid redox properties. In this review, the advances in the past 10 years in the oxidative transformations catalyzed using all-inorganic POVs, organ functionalized POVs, and POV-based inorganic–organic hybrids have been summarized and are discussed in detail. In particular, we focus on the selective oxidation of organic molecules, including alkanes, alkenes, alcohols, sulfides, sulfur-mustard simulants, and organic dyes. For most oxidative transformations, the active V-peroxo species generated in the presence of hydrogen peroxide or tert-butyl hydroperoxide account for enhanced catalytic activities. In addition, the challenges and prospects in the development of POM-based catalysts are addressed.

Monosubstituted Keggin as heterogeneous catalysts for solvent-free cyanosilylation of aldehydes and ketones

Any successful catalyst should have a dedicated synthesizing route considering their high yields, time-efficient, and do not require any highly toxic chemicals. In this regard, two nano-sized monosubstituted polyoxometalates (POMs) with the well-known Keggin structure were synthesized and characterized: (TBA)4[Zr(OH)PW11O39]·22H2O (TBA-ZrPW11) and (TBA)5[Zr(OH)SiW11O39]·17H2O (TBA-ZrSiW11) (TBA+: tetrabutylammonium). Thus, the catalytic activities of both catalysts were tested in the cyanosilylation reaction without any solvents of various aldehydes and ketones with trimethylsilyl cyanide (TMSCN) over two heterogeneous catalysts. Finally, the recyclability and heterogeneity of these POM-based catalysts were also checked, showing that they remain active at least after three recycling procedures.

A Copper-Containing Polyoxometalate-Based Metal-Organic Framework as an Efficient Catalyst for Selective Catalytic Oxidation of Alkylbenzenes.

A copper-containing polyoxometalate-based metal-organic framework (POMOF), CuI12Cl2(trz)8[HPW12O40] (HENU-7, HENU = Henan University; trz = 1,2,4-triazole), has been successfully synthesized and well-characterized. In addition, the excellent catalytic ability of HENU-7 has been proved by the selective oxidation of diphenylmethane. Under the optimal conditions, the diphenylmethane conversion obtained over HENU-7 is 96%, while the selectivity to benzophenone is 99%, which outperforms most noble-metal-free POM-based catalysts. Moreover, HENU-7 is stable to reuse for five runs without an obvious loss in activity and also can catalyze the oxidation of different benzylic C-H with satisfactory conversions and selectivities, which implied the significant catalytic activity and recyclability.

Regulating the catalytic activity of multi-Ru-bridged polyoxometalates based on differential active site environments with six-coordinate geometry and five-coordinate geometry transitions.

Five-coordinate geometry around ruthenium with highly exposed active sites has attracted intensive scientific interest due to its superior properties and extensive applications. Herein, we report a series of structurally controllable multi-Ru-bridged polyoxometalates, K5NaH10[{Ru4(H2O)n}(WO2)4(AsW9O33)4]·mH2O {1, 1-dehyd-373K, 1-dehyd-473K, 1-dehyd-573K; n = 4, m = 36; n = 4, m = 6; n = 4, m = 0; n = 0, m = 0} fabricated through a feasible assembly strategy using arsenotungstate {2, KNa12H17Cl2(As4W40O140)·29H2O} as a structure-directing unit. Systematic characterization methods identified that the six-coordinate geometry can successfully transform into five-coordinate geometry about active sites (Ru) by removing aqua ligands under high reaction temperatures. All the multi-Ru-bridged polyoxometalates demonstrated strong stability and catalytic effectiveness in the transformation of 1-(4-chlorophenyl)ethanol to 4'-chloroacetophenone under very mild conditions. 1-dehyd-573K, specifically, achieves the best catalytic effectiveness with a turnover frequency (TOF) = 25 100·h-1 owing to its unique five-coordinate geometry on the Ru sites. To our knowledge, 1-dehyd-573K outperforms other POM-based catalysts in the oxidative catalysis of 1-(4-chlorophenyl)ethanol. The heterogeneous polyoxometalates were also proven to be strongly reusable, with their structural integrities well maintained after multiple-cycle catalytic reactions.

Biomass valorisation over polyoxometalate-based catalysts

POM-based catalysts have wide applications in catalytic biomass refinery.

Keggin-type polyoxometalate-based supramolecular complex for selective oxidation of styrene to benzaldehyde

A Keggin-type polyoxometalate (POM)-based supramolecular complex with the formula of [Co(H2O)3]2(ttb)2[HBW12O40]·2H2O (1) (ttb = 1,3,5-tri(1,2,4-triazol-1-ylmethyl)-2,4,6-trimethylbenzene) was synthesized via hydrothermal method and characterized by single-crystal X-ray diffraction, IR and powder X-ray diffraction analyses. In 1, [Co(H2O)3]2+ cations are connected with the adjacent ttb ligands to form a porous two-dimensional (2-D) cationic layered network, and the [HBW12O40]4− polyoxoanions are encapsulated between the layers through intermolecular interactions, generating a POM-capsulated 3-D supramolecular network. Integrating the advantage of Co2+ with [HBW12O40]4−, 1 as a heterogeneous catalyst displays a synergistically improved catalytic performance for oxidation of styrene. It can achieve almost 98% conversion of styrene in 4 h with about 99% selectivity of benzaldehyde product, which is about five times more than the catalytic activities of the single components (Co(OAc)2·4H2O or K5BW12O40). A series of comparative analyses showed that the high catalytic performance of 1 is mainly attributed to the synergistic effect of unsaturated metal sites and polyoxoanions. This work provides an approach for developing highly efficient POM-based heterogeneous catalysts toward the selective oxidation of styrene.

Step-by-Step Assembly of 2D Confined Chiral Space Endowing Achiral Clusters with Asymmetric Catalytic Activity for Epoxidation of Allylic Alcohols.

Endowing achiral polyoxometalates (POMs) with asymmetric catalytic properties is always an intriguing but challenging topic because of their high catalytic activities yet highly symmetrical molecular structures. In this work, a novel strategy was proposed to fabricate a series of two-dimensional chiral POM catalysts. Following the steps of exfoliation, covalent modification, and reassembly, the achiral POMs were orderly confined into the chiral interstitial domains of chiral ionic liquid (CIL)-modified layered double hydroxide materials with a decreased molecular symmetry. The chirality of POM molecules was induced by the l- or d-pyrrolidine-type CILs, and their asymmetric catalytic activity was enhanced by the confinement effect. Compared with the reported chiral POM-based catalysts [e.g., 8 turnover frequency (TOF) and 79% enantiomeric excess (ee) for chiral POM-based metal-organic frameworks], the constructed chiral POM catalysts showed a significantly higher TOF and enantioselectivity (up to 240 TOF and 93% ee) for the asymmetric epoxidation of allylic alcohols. The facilitated mass transfer in the IL channels and the increased binding efficiency between the chiral catalytic sites and reactants render this strategy highly promising for constructing efficient chiral catalysts from the catalytically active while achiral building blocks.

Confining Polyoxometalate Clusters into Porous Aromatic Framework Materials for Catalytic Desulfurization of Dibenzothiophene.

Removal of notorious sulfur compounds to produce low-sulfur-content (≤10 ppm) diesel is necessary and vital for modern industry and environmental protection. A new type of inorganic-organic hybrid material has been designed and synthesized via confining molybdenum-containing polyoxometalate (POM) clusters within porous aromatic framework-1 (PAF-1) cavities named POM-PAF-1. Deep oxidative desulfurization experiments reveal that POM-PAF-1 possesses excellent reactivity under mild conditions, exemplified by a sulfur removal degree of 98.5% dibenzothiophene within 30 min at 30 °C. The improvement in oxidative desulfurization reactivity from traditional porous POM-based catalysts is owing to uniform POMs and lipophilic and porous PAF-1. The high performance of POM-PAF-1 in terms of excellent reactivity and good stability means it has potential in new heterogeneous catalysis.

Copper-Containing Polyoxometalate-Based Metal-Organic Frameworks as Highly Efficient Heterogeneous Catalysts toward Selective Oxidation of Alkylbenzenes.

With a one-pot assembly method, two copper-containing Keggin-type polyoxometalate-based metal-organic frameworks (POMOFs), i.e., [CuI6(trz)6{PW12O40}2] (HENU-2, HENU = Henan University; trz = 1,2,4- triazole) and [CuI3(trz)3{PMo12O40}] (HENU-3), were successfully prepared and structurally characterized. These two compounds, which are generated by the extension of a crown-like {Cu6(trz)6} macrocycle-based sandwich-type structural unit, possess identical noninterpenetration 3D frameworks except for the polyanions difference. Additionally, both of themare assessed as highly effective heterogeneous catalysts in facilitating the oxidation of alkylbenzenes to ketone products in the presence of tert-butyl hydroperoxide. Under optimized conditions, HENU-2 can achieve a 95.2% conversion of diphenylmethane in 20 h with a 100% selectivity toward benzophenone, and it was reused for three runs with constant high activity, which outperforms most POM-based catalysts for this catalytic reaction.

Devisable POM/Ni Foam Composite: Precisely Control Synthesis toward Enhanced Hydrogen Evolution Reaction at High pH.

Polyoxometalates (POMs) are promising catalysts for the electrochemical hydrogen production from water owing to their high intrinsic catalytic activity and chemical tunability. However, poor electrical conductivity and easy detachment of the POMs from the electrode cause significant challenges under operating condition. Herein, a simple one‐step hydrothermal method is reported to synthesize a series of Dexter–Silverton POM/Ni foam composites (denoted as NiM‐POM/Ni; M=Co, Zn, Mn), in which the stable linkage between the POM catalysts and the Ni foam electrodes lead to high activity for the hydrogen evolution reaction (HER). Among them, the highest HER performance can be observed in the NiCo‐POM/Ni, featuring an overpotential of 64 mV (at 10 mA cm−2, vs. reversible hydrogen electrode), and a Tafel slope of 75 mV dec−1 in 1.0 m aqueous KOH. Moreover, the NiCo‐POM/Ni catalyst showed a high faradaic efficiency ≈97 % for HER. Post‐catalytic of NiCo‐POM/Ni analyses showed virtually no mechanical or chemical degradation. The findings propose a facile and inexpensive method to design stable and effective POM‐based catalysts for HER in alkaline water electrolysis.

Discovery of a New Family of Polyoxometalate-Based Hybrids with Improved Catalytic Performances for Selective Sulfoxidation: The Synergy between Classic Heptamolybdate Anions and Complex Cations.

A series of functional cation-regulated isopolymolybdate-based organic-inorganic hybrid compounds, Na2H2[Mo4O12(C8H17O5N)2]·10H2O (1), Na2[M(Bis-tris)(H2O)]2[Mo7O24]·10H2O [M = Cu, 2; Ni, 3; Co, 4; Zn, 5; Bis-tris = 2,2-Bis(hydroxymethyl)-2,2',2″-nitrilotriethanol], and (NH4)2[M(Bis-tris)(H2O)]2[Mo7O24]·6H2O (M = Zn, 6; Cu, 7), were synthesized and characterized toward advanced molecular catalyst design. Compound 1 is a covalently bonded adduct, and its self-assembly process can be probed by electrospray ionization mass spectrometry (ESI-MS). Compounds 2-7 are polyoxometalate (POM)-based hybrids containing classic heptamolybdate anions and complex cations with Bis-tris ligands. All of these compounds showed remarkable catalytic effects for selective sulfide oxidation. To the best of our knowledge, compound 5 presents the best catalytic activity so far among the reported hybrid materials with common easily synthesized small-molecule POM clusters and also exhibits outstanding reliability. The conclusion of the catalytic effect is drawn from the results that Zn-based compounds have better catalytic effects than other transition-metal-containing compounds and the compound constructed by Na+ has higher catalytic activity than that constructed by NH4+. The mechanism studies show that the improvements of the catalytic performance are caused by the synergy between classic heptamolybdate anions and complex cations. ESI-MS data and UV-vis spectra revealed that the POM anions can form intermediate peroxomolybdenum units during catalytic reaction. Further, the combination of the substrate thioanisole with complex cations was characterized by NMR experiments and UV-vis spectra. Thus, a new synergistic mechanism of anions and cations is proposed in which the activated thioanisole is used as a nucleophile to attack the peroxomolybdenum bonds, and this provides a new strategy in the design of reliable POM-based catalysts.

Self-Assembled Supramolecular Polyoxometalate Hybrid Architecture as a Multifunctional Oxidation Catalyst.

Polyoxometalates (POMs) are widely applied as tuneable and versatile catalysts for a variety of oxidation reactions in an aqueous/organic two-phase system. However, the practical applications of POMs-based biphasic catalysis are hampered by low space-time yields and mass-transport limitation between two layers due to extremely low solubility of the organic reactants in the aqueous phase. Here, we first introduced β-cyclodextrin (β-CD) as an inverse phase transfer agent and a supramolecular nanoreactor to construct a supramolecular POM inorganic-organic hybrid framework (KCl4)Na7[(β-CD)3(SiW12O40)]·9H2O {3CD@SiW12} for various oxidation catalyses. In contrast to free CD, Keggin [SiW12O40]4- catalysts, and their mixture, the {3CD@SiW12} catalyst, efficiently catalyze oxidation reactions of alcohol, alkene, and thiophene. A comprehensive strategy of experimental, crystallographic, and density functional theory (DFT) calculations elucidates that the catalytic pathway involved three combined aspects of supramolecular recognition, phase transfer property, and POM catalysis. The strategic combination of supramolecular characteristic and POM-based catalysts to fabricate supramolecular POM hybrid materials opens up new economic and green tuning options, thus paving the way to informed catalyst design.

ChemInform Abstract: Recent Advances in Polyoxometalate‐Based Heterogeneous Catalytic Materials for Liquid‐Phase Organic Transformations

AbstractReview: 220 refs.

Recent advances in polyoxometalate-based heterogeneous catalytic materials for liquid-phase organic transformations

Polyoxometalates (POMs) are a unique class of molecular metal oxides with a tunable structure at the atomic level. They have demonstrated various potential applications in various fields. In particular, POMs have been widely used as catalysts, due to their facilely modified acid–base properties and redox potential through molecular designing. Normally, POMs can be utilized as both homogeneous and heterogeneous catalysts, where the former favors high activity, while the latter benefits facile separation of the catalysts. Faced with the requirement for sustainable development, significant effort has been made in the preparation of POM-based heterogeneous catalysts. This review focuses on the recent developments in heterogeneous strategies of POM-based catalysts and their applications in liquid organic reactions.

ChemInform Abstract: Recent Trends in the Use of Polyoxometalate‐based Material for Efficient Water Oxidation

AbstractReview: 74 refs.

Recent trends in the use of polyoxometalate-based material for efficient water oxidation

Polyoxometalates (POMs), with their attractive topological and electronic properties, have been demonstrated over the last few decades to be efficient catalysts for many reactions. In a new direction, POMs having a purely inorganic framework are now being explored as efficient catalysts for water oxidation. This review focuses on very recent developments of POM-based catalysts for water oxidation to O2. In the very extensive family of POMs, tetra-core sandwich-type Ru- and Co-containing POM complexes, namely [Ru4O4(OH)2(H2O)4(γ-SiW10O36)2]10− and [Co4(H2O)2(γ-PW9O34)2]10− exhibit the ability to facilitate water oxidation under electrocatalytic and/or photocatalytic conditions. However, it is concluded that much work still needs to be done to explain the catalytic mechanisms and optimize these catalytic systems. In addition, recent trends in using ionic liquids as media to modify the electrolytic environment and enhance water oxidation are highlighted.