Heterogenization of polyoxometalates as solid catalysts in aerobic oxidation of glycerol

A series of heterogeneous catalysts LnPMo₁₂O₄₀ (L = Al³⁺, Fe³⁺, Cr³⁺, Ti⁴⁺, Zr⁴⁺, Zn²⁺) and HxPMo₁₁LO₃₉ (L = Zn²⁺, Cr³⁺, Fe³⁺, Al³⁺, Ti⁴⁺) were prepared using a simple calcination treatment and were evaluated in aerobic oxidation of glycerol.

Unraveling the role of the lacunar Na7PW11O39 catalyst in the oxidation of terpene alcohols with hydrogen peroxide at room temperature

The Na₇PW₁₁O₃₉ lacunar salt was the most active among Keggin lacunar salts (Na₈SiW₁₁O₃₉ and Na₇PMo₁₁O₃₉), a saturated salt (Na₇PW₁₂O₄₀) and heteropolyacid catalysts.

An unprecedented cobalt(ii)-containing Wells–Dawson-type tungstovanadate-based metal–organic framework as an efficient catalyst for ring-opening polymerization of ε-caprolactone

An unprecedented organic–inorganic hybrid material based on Wells–Dawson-type tungstovanadate building blocks and cobalt(ii)–organic framework with a bis(triazole) ligand was prepared and employed to catalyze solvent-free ROP of caprolactone.

Synergistic Catalysis of Ruthenium Nanoparticles and Polyoxometalate Integrated Within Single UiO-66 Microcrystals for Boosting the Efficiency of Methyl Levulinate to γ-Valerolactone

The synthesis of heterogeneous cooperative catalysts in which two or more catalytically active components are spatially separated within a single material has generated considerable research efforts. The multiple functionalities of catalysts can significantly improve the efficiency of existing organic chemical transformations. Herein, we introduce ruthenium (Ru) nanoparticles (NPs) on the surfaces of a metal–organic framework pre-encapsulated with polyoxometalate silicotungstic acid (SiW) UiO−66 (University of Oslo [UiO]) and prepared a 2.0% Ru/11.7% SiW@UiO−66 porous hybrid using the impregnation method. The close synergistic effect of metal Ru NPs, SiW, and UiO-66 endow 2.0% Ru/11.7% SiW@UiO-66 with increased activity and stability for complete methyl levulinate (ML) conversion and exclusive γ-valerolactone (GVL) selectivity at mild conditions of 80°C and at a H₂ pressure of 0.5 MPa. Effectively, this serves as a model reaction for the upgrading of biomass and outperforms the performances of the constituent parts and that of the physical mixture (SiW + Ru/UiO−66). The highly dispersed Ru NPs act as active centers for hydrogenation, while the SiW molecules possess Brønsted acidic sites that cooperatively promote the subsequent lactonization of MHV to generate GVL, and the UiO−66 crystal accelerates the mass transportation facilitated by its own porous structure with a large surface area.

Assembling [M(P4Mo6)2] (M = Na, Mn, Na/Cu) dimeric clusters via transition metal/sodium ions into 0D to 3D phosphomolybdates

Six novel molybdenum(v) phosphates with structures ranging from a zero-dimensional cluster to three-dimensional frameworks have been solvothermally synthesized.

Selective Hydrolysis of Ovalbumin Promoted by Hf(IV)-Substituted Wells-Dawson-Type Polyoxometalate

The reactivity and selectivity of Wells-Dawson type polyoxometalate (POM), K₁₆[Hf(α₂-P₂W₁₇O₆₁)₂]·19H₂O (Hf1-WD2), have been examined with respect to the hydrolysis of ovalbumin (OVA), a storage protein consisting of 385 amino acids. The exact cleavage sites have been determined by Edman degradation experiments, which indicated that Hf1-WD2 POM selectively cleaved OVA at eight peptide bonds: Phe13-Asp14, Arg85-Asp86, Asn95-Asp96, Ala139-Asp140, Ser148-Trp149, Ala361-Asp362, Asp362-His363, and Pro364-Phe365. A combination of spectroscopic methods including ³¹P NMR, Circular Dichroism (CD), and Tryptophan (Trp) fluorescence spectroscopy were employed to gain better understanding of the observed selective cleavage and the underlying hydrolytic mechanism. ³¹P NMR spectra have shown that signals corresponding to Hf1-WD2 gradually broaden upon addition of OVA and completely disappear when the POM-protein molar ratio becomes 1:1, indicating formation of a large POM/protein complex. CD demonstrated that interactions of Hf1-WD2 with OVA in the solution do not result in protein unfolding or denaturation even upon adding an excess of POM. Trp fluorescence spectroscopy measurements revealed that the interaction of Hf1-WD2 with OVA (K_q = 1.1 × 10⁵ M⁻¹) is both quantitatively and qualitatively slightly weaker than the interaction of isostructural Zr-containing Wells-Dawson POM (Zr1-WD2) with human serum albumin (HAS) (K_q = 5.1 × 10⁵ M⁻¹).

A Versatile Polyoxovanadate in Diverse Cation Matrices: A Supramolecular Perspective

A series of decavanadate based compounds, formulated as [Co(H₂O)₆][{Na₄(H₂O)₁₄}{V₁₀O₂₈}]·4H₂O (1), [Zn(H₂O)₆][Na₃(H₂O)₁₄] [HV₁₀O₂₈]·4H₂O (2), [HMTAH]₂ [{Zn(H₂O)₄}₂{V₁₀O₂₈}]·2H₂O (3), [{Co(3-amp)(H₂O)₅}]₂ [3-ampH]₂ [V₁₀O₂₈] · 6H₂O (4), [4-ampH]₁₀[{Na(H₂O)₆}{HV₁₀O₂₈}][V₁₀O₂₈]·15H₂O (5), [{4-ampH}₆ {Co(H₂O)₆}₃][V₁₀O₂₈]₂·14H₂O (6), and [{4-ampH}₁₀{Zn(H₂O)₆}][V₁₀O₂₈]₂·10H₂O (7), have been synthesized (where HMTAH = mono-protonated hexamethylenetetramine, 3-ampH = protonated 3-amino pyridine and 4-ampH= protonated 4-aminopyridine) from the relevant aqueous sodium-vanadate solution, by varying the pH of the solution and amino pyridine/hexamine derivatives as well as transition metal salts (Co(II)- and Zn(II)-salts). In this series of compounds 1-7, the polyoxovanadate (POV) cluster [V₁₀O₂₈]^6- is the common cluster anion, stabilized by diverse cations. The diverse supramolecular patterns around the decavanadate cluster anion in different cationic matrices have been described to understand the microenvironment in the decavanadate-based minerals. All of these compounds have solvent water molecules in their respective crystal lattices. Since water can interact directly with cations and anions, providing an additional stability and structural diversity, we have analyzed supramolecular water structures in all these compounds to comprehend the role of the lattice water in the formation of natural decavanadate containing minerals. Compounds 1-7, that are isolated at an ambient condition from aqueous solution, are characterized by routine spectral analysis, elemental analyses and finally unambiguously by single crystal X-ray crystallography.

Vanadium-based polyoxometalate complex as a new and efficient catalyst for phenol hydroxylation under mild conditions

A polyoxovanadate complex, synthesized under mild conditions, served as the catalyst for clean conversion of phenol to catechol and hydroquinone in water.

8-Hydroxy-2-methylquinoline-modified H4SiW12O40: a reusable heterogeneous catalyst for acetal/ketal formation

A heteropoly acid based organic hybrid heterogeneous catalyst, HMQ-STW, was prepared by combining 8-hydroxy-2-methylquinoline (HMQ) with Keggin-structured H₄SiW₁₂O₄₀ (STW).

Layered double hydroxide anchored ionic liquids as amphiphilic heterogeneous catalysts for the Knoevenagel condensation reaction

In this paper, three solid base catalysts of LDH-IL-Cn (n = 4, 8, 12) were synthesized by adopting an exfoliation/assembly approach. The as-prepared catalyst showed excellent activity and selectivity for the Knoevenagel reaction in aqueous solution.

Polyoxometalate LUMO engineering: a strategy for visible-light-responsive aerobic oxygenation photocatalysts

We report efficient visible-light-responsive oxygenation photocatalysis via the strategy of LUMO engineering of polyoxometalates.

Heterogeneous catalysis by tungsten-based heteropoly compounds

In this review, the recent works on heterogeneous catalytic applications of polyoxotungstates in liquid-phase organic reactions are reviewed.

A robust polyoxometalate-templated four-fold interpenetrating metal–organic framework showing efficient organic dye photodegradation in various pH aqueous solutions

A polyoxometalate-templated four-fold interpenetrating 3-D MOF of 1 acts as a photocatalyst, which can efficiently degrade the organic dyes under UV-Vis light irradiation in aqueous solutions over a wide pH range.

Novel tetra-propyl/butylammonium encapsulated Keggin-type polyoxotungstates: synthesis, structural characterization, and catalytic capability in oxidative cleavage of unsaturated fatty acids

Hybrid organic–inorganic Keggin compounds with organo-protected acidity resulted in high efficiency in oxidative cleavage of UFAs.

Crystal Engineering in Supramolecular Polyoxometalate Hybrids through pH Controlled in Situ Ligand Hydrolysis

A family of five different three-dimensional polyoxometalate (POM) based supramolecular hybrids were synthesized by a hydrothermal route under different pH using a hydrolyzable naphthalene diimide ligand. The mechanism of crystallographic phase variation of the POM-amino pyridine hybrids under different pH was studied through controlled experiments where the final hydrolyzed products were analyzed through NMR and single crystal X-ray diffraction. Different pH conditions led to variation in the extent of protonation and hydrolyzation of the ligand, yielding different phases. All of these were identified, and the structures of the supramolecular hybrids were characterized extensively. Mechanistic study proved that only the reaction conditions are responsible for the hydrolysis of the ligand and the in situ generated POM species do not have any role in it. Magnetic measurements confirmed the hexavalent oxidation states of the transition metal center (Mo) in the POM. Optical band gap measurements revealed that these hybrids are semiconducting in nature. Two of the compounds were studied for hydrogen peroxide mediated selective oxidation catalysis of small organic molecules and found to exhibit very good activity with high percentage of selectivity for the desired products of industrial importance.