Titanium-monosubstituted polyoxometalates: relation between homogeneous and heterogeneous Ti-single-site-based catalysis

Tuning Reactivity of Zr-Substituted Keggin Phosphotungstate in Alkene Epoxidation through Balancing H2O2 Activation Pathways: Unusual Effect of Base

The Zr-monosubstituted Keggin-type dimeric phosphotungstate (Bu4N)8[{PW11O39Zr(μ-OH)(H2O)}2] (1) efficiently catalyzes epoxidation of C═C bonds in various kinds of alkenes, including terminal ones, with aqueous H2O2 as oxidant. Less sterically hindered double bonds are preferably epoxidized despite their lower nucleophilicity. Basic additives (Bu4NOH) in the amount of 1 equiv per dimer 1 suppress H2O2 unproductive decomposition, increase substrate conversion, improve yield of heterolytic oxidation products and oxidant utilization efficiency, and also affect regioselectivity of epoxidation, enhancing oxygen transfer to sterically hindered electron-rich C═C bonds. Acid additives produce a reverse effect on the substrate conversion and H2O2 efficiency. The reaction mechanism was explored using a range of test substrates, kinetic, and spectroscopic tools. The opposite effects of acid and base additives on alkene epoxidation and H2O2 degradation have been rationalized in terms of their impact on hydrolysis of 1 to form monomeric species, [PW11O39Zr(OH)(H2O)x]4- (1-M, x = 1 or 2), which favors H2O2 homolytic decomposition. The interaction of 1 with H2O2 has been investigated by HR-ESI-MS, ATR-FT-IR, and 31P NMR spectroscopic techniques. The combination of spectroscopic studies and kinetic modeling implicated the existence of two types of dimeric peroxo complexes, [Zr2(μ-η2:η2-O2){PW11O39}2(H2O)x]]8- and [{Zr(μ-η2-O2)}2(PW11O39)2(H2O)y]10-, along with monomeric Zr (hydro)peroxo species that begin to dominate at a high excess of H2O2. Both dimeric μ-η2-peroxo intermediates are inert toward alkenes under stoichiometric conditions. V-shape Hammett plots obtained for epoxidation of p-substituted styrenes suggested a biphilic nature of the active oxidizing species, which are monomeric Zr-hydroperoxo and peroxo species. Small basic additives increase the electrophilicity of the catalyst and decrease its nucleophilicity. HR-ESI-MS has identified a dimeric, most likely, bridging hydroperoxo species [{PW11O39Zr}2(μ-O)(μ-OOH)]9-, which may account for the improved epoxidation selectivity and regioselectivity toward sterically hindered C═C bonds.

Ru-W modified graphitic carbon nitride by a monomer complexation synthesis approach from a tailored polyoxometalate: towards electrochemical detection of hydrogen peroxide released by cells

Detection of hydrogen peroxide (H₂O₂) from cell cultures is important for monitoring different diseases. Here, g-C₃N₄ (gCN) was incorporated into well-defined clusters of RuW (RuW-gCN) through monomer complexation of Ru-substituted phosphotungstate and melamine for electrochemical detection of H₂O₂. RuW-gCN exhibited enhanced electrochemical sensing properties in comparison to its constituents due to the synergic effects between RuW and gCN. The characterization of RuW-gCN revealed successful complexation to form the composite in addition to the presence of a layered structure of gCN. The electrochemical sensor made of RuW-gCN was able to detect H₂O₂ with a detection limit of 46 nM in the linear ranges from 100 nM to 50 μM and from 50 μM to 1 mM. The developed sensor was employed for the selective detection of H₂O₂ in the presence of analytes like ascorbic acid (AA), dopamine, and glucose in addition to being stable even after a week of storage at room temperature. It has also been verified for real sample application by detecting H₂O₂ produced by cancer cells as a result of an AA trigger.

Heterolytic alkene oxidation with H2O2 catalyzed by Nb-substituted Lindqvist tungstates immobilized on carbon nanotubes

Immobilization of Nb-substituted Lindqvist tungstates on carbon nanotubes stabilizes a monomeric form, Nb(OH)W₅, and leads to efficient and recyclable catalysts for heterolytic alkene oxidation with H₂O₂.

Visual detection of H2O2 and melamine based on PW11MO39n− (M = Cu2+, Co2+, Mn2+, Fe3+) and PW9M3O34n− (M = Cu2+, Co2+, Mn2+, Fe3+)

The peroxidase-like activity of transition metal-substituted polyoxometalates (PW₁₁MO₃₉ⁿ⁻, PW₉M₃O₃₄ⁿ⁻, abbreviated as PW₁₁M and PW₉M₃, where M = Cu²⁺, Co²⁺, Mn²⁺, Fe³⁺) in the oxidation of 3,3′,5,5′-tetramethylbenzidine by H₂O₂ was valuated.

Modeling the Oxygen Vacancy at a Molecular Vanadium(III) Silica-Supported Catalyst

Here we report on the use of a silanol-decorated polyoxotungstate, [SbW₉O₃₃( ^tBuSiOH)₃]^3- (1), as a molecular support to describe the coordination of a vanadium atom at a single-site on silica surfaces. By reacting [V(Mes)₃·thf] (Mes = 2,4,6-trimethylphenyl) with 1 in tetrahydrofuran, the vanadium(III) derivative [SbW₉O₃₃( ^tBuSiO)₃V(thf)]^3- (2) was obtained. Compound 2 displays the paramagnetic behavior expected for a d²-V^III high spin complex (SQUID measurements) with a triplet electronic ground state (ca. 30 kcal·mol^-1 more stable than the singlet, from DFT calculations). Compound 2 proves to be a reliable model for reduced isolated-vanadium atom dispersed on silica surfaces [(≡Si-O)₃V^III(OH₂)], an intermediate that is often proposed in a Mars-van Krevelen type mechanism for partial oxidation of light alcohols. Oxidation of 2 under air produced the oxo-derivative [SbW₉O₃₃( ^tBuSiO)₃VO]^3- (3). In compound 2, the d²-electrons are localized in degenerated d(V) orbitals, whereas in the electronically analogous bireduced-[SbW₉O₃₃( tBuSiO)₃VO]^5-, 3·(2e), one electron is localized on d(V) orbital and the second one is delocalized on the polyoxotungstic framework, leading to a unique case of a bireduced heteropolyanion derivative with completely decoupled d¹-V(IV) and d¹-W(V). Our body of experimental results (EPR, magnetic measurements, spectroelectrochemical studies, Raman spectroscopy) and theoretical studies highlights (i) the role of the apical ligand coordination, i.e., thf (σ-donor) vs oxo (π-donor), in destabilizing or stabilizing the d(V) orbitals relative to the d(W) orbitals, and (ii) a geometrical distortion of the O₃VO entity that causes a splitting of the degenerated orbitals and the stabilization of one d(V) orbital in the bireduced compound 3·(2e).

Heterogeneous epoxidation of alkenes with H2O2 catalyzed by a recyclable organic–inorganic polyoxometalate-based framework catalyst

PW-MOF as an efficient, recyclable and stable catalyst for the selective oxidation of alkenes with hydrogen peroxide.

Simple procedure for vacant POM-stabilized palladium (0) nanoparticles in water: structural and dispersive effects of lacunary polyoxometalates

Metallic palladium nanoparticles have been generated by hydrogenation in very mild conditions of aqueous solutions of non-organometallic PdII-derivatives of heteropolytungstates.

The societal significance of catalysis and the growing practical importance of single-site heterogeneous catalysts

The concept of single-site heterogeneous catalysis, herein defined and extensively illustrated, offers a strategy for the design of new solid catalysts. By capitalizing on the opportunities presented by nanoporous materials to assemble a wide range of new, well-defined, catalytically active centres, it is possible to bring about numerous environmentally benign processes that can replace traditional methods of chemical production. The latter often employs aggressive, corrosive or hazardous reagents. By using both microporous (less than 20 Å diameter) and mesoporous solids (20–500 Å diameter), abundant scope exists for the construction and application of shape-selective, regio-selective and enantioselective catalysts.

Metal–organic frameworks of the MIL-101 family as heterogeneous single-site catalysts

In this short review paper, we survey our recent findings in the catalytic applications of mesoporous metal–organic frameworks of the MIL-101 family (Fe- and Cr-MIL-101) and demonstrate their potential in two types of liquid-phase processes: (i) selective oxidation of hydrocarbons with green oxidants—O 2 and tert -butyl hydroperoxide—and (ii) coupling reaction of organic oxides with CO 2 . A comparison with conventional single-site catalysts is made with special attention to issues of the catalyst's resistance to metal leaching and the nature of catalysis.