Green Oxidation Reactions by Polyoxometalate-Based Catalysts: From Molecular to Solid Catalysts

Polyoxometalate-Modified Amphiphilic Polystyrene-block-poly(2-(dimethylamino)ethyl methacrylate) Membranes for Heterogeneous Glucose to Formic Acid Methyl Ester Oxidation

Herein, we present a new heterogeneous catalyst active toward glucose to formic acid methyl ester oxidation. The catalyst was fabricated via electrostatic immobilization of the inorganic polyoxometalate HPA-5 catalyst H8[PMo7V5O40] onto the pore surface of amphiphilic block copolymer membranes prepared via non-solvent-induced phase separation (NIPS). The catalyst immobilization was achieved via wet impregnation due to strong coulombic interactions between protonated tertiary amino groups of the polar poly(2-(dimethylamino)ethyl methacrylate) block and the anionic catalyst. Overall, three sets of five consecutive catalytic cycles were performed in an autoclave under 90 °С and 11.5 bar air pressure in methanol, and the corresponding yields of formic acid methyl ester were quantified via head-space gas chromatography. The obtained results demonstrate that the membrane maintains its catalytic activity over multiple cycles, resulting in high to moderate yields in comparison to a homogeneous catalytic system. Nevertheless, presumably due to leaching, the catalytic activity declines over five catalytic cycles. The morphological and chemical changes of the membrane during the prolonged catalysis under harsh conditions were examined in detail using different analytic tools, and it seems that the underlying block copolymer is not affected by the catalytic process.

H8 [PV5 Mo7 O40 ] - A Unique Polyoxometalate for Acid and RedOx Catalysis: Synthesis, Characterization, and Modern Applications in Green Chemical Processes

Polyoxometalates (POMs) are a fascinating group of anionic metal-oxide clusters with a broad variety of structural properties and several catalytic applications, especially in the conversion of bio-derived platform chemicals. H8 [PV5 Mo7 O40 ] (HPA-5) is a unique POM catalyst that ideally links numerous fascinating research fields for the following reasons: a) HPA-5 can be synthesized by rational design approaches; b) HPA-5 can be well characterized using multiple analytical tools explaining its catalytic properties; and c) HPA-5 is suitable for multiple important catalytic transformations of bio-based feedstock. This Review combines the fields of synthesis, spectroscopic, electrochemical, and crystallographic characterization of HPA-5 with those of sustainable catalysis and green chemistry. Selected catalytic applications include esterification, dehydration, and delignification of biomass as well as selective oxidation and fractionation of bio-based feedstock. The unique HPA-5 is a fascinating POM that has a broad application scope for biomass valorization.

Immobilization of Polyoxometalates on Carbon Nanotubes: Tuning Catalyst Activity, Selectivity and Stability in H2O2-Based Oxidations

In recent years, carbon nanotubes (CNTs), including N-doped ones (N-CNTs), have received significant attention as supports for the construction of heterogeneous catalysts. In this work, we summarize our progress in the application of (N)-CNTs for immobilization of anionic metal-oxygen clusters or polyoxometalates (POMs) and use of (N)-CNTs-supported POM as catalysts for liquid-phase selective oxidation of organic compounds with the green oxidant–aqueous hydrogen peroxide. We discuss here the main factors, which favor adsorption of POMs on (N)-CNTs and ensure a quasi-molecular dispersion of POM on the surface and their strong attachment to the support. The effects of the POM nature, N-doping of CNTs, acid additives, and other factors on the POM immobilization process and catalytic activity/selectivity of the (N)-CNTs-immobilized POMs are analyzed. Particular attention is drawn to the critical issue of the catalyst stability and reusability. The scope and limitations of the POM/(N)-CNTs catalysts in H2O2-based selective oxidations are discussed.

Oxidation of amine α-carbon to amide: a review on direct methods to access the amide functionality

A number of methods have been adopted for the synthesis of amides; among these methods, the oxidation of an amine to an amide is growing in interest as a means to prepare this imperative functional group.

Mono-transition-metal-substituted polyoxometalate intercalated layered double hydroxides for the catalytic decontamination of sulfur mustard simulant

The mono-transition-metal-substituted polyoxometalate intercalated layered double hydroxides Zn₂Cr-LDH-PW₁₁M can effectively catalyze the oxidative decontamination of a sulfur mustard simulant.

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.

Gallium(III)-Containing, Sandwich-Type Heteropolytungstates: Synthesis, Solution Characterization, and Hydrolytic Studies toward Phosphoester and Phosphoanhydride Bond Cleavage

The gallium(III)-containing heteropolytungstates [Ga4(H2O)10(β-XW9O33)2](6-) (X = As(III), 1; Sb(III), 2) were synthesized in aqueous acidic medium by reaction of Ga(3+) ions with the trilacunary, lone-pair-containing [XW9O33](9-). Polyanions 1 and 2 are isostructural and crystallized as the hydrated sodium salts Na6[Ga4(H2O)10(β-AsW9O33)2]·28H2O (Na-1) and Na6[Ga4(H2O)10(β-SbW9O33)2]·30H2O (Na-2) in the monoclinic space group P21/c, with unit cell parameters a = 16.0218(12) Å, b = 15.2044(10) Å, c = 20.0821(12) Å, and β = 95.82(0)°, as well as a = 16.0912(5) Å, b = 15.2178(5) Å, c = 20.1047(5) Å, and β = 96.2(0)°, respectively. The corresponding tellurium(IV) derivative [Ga4(H2O)10(β-TeW9O33)2](4-) (3) was also prepared, by direct reaction of sodium tungstate, tellurium(IV) oxide, and gallium nitrate. Polyanion 3 crystallized as the mixed rubidium/sodium salt Rb2Na2[Ga4(H2O)10(β-TeW9O33)2]·28H2O (RbNa-3) in the triclinic space group P1̅ with unit cell parameters a = 12.5629(15) Å, b = 13.2208(18) Å, c = 15.474(2) Å, α = 80.52(1)°, β = 84.37(1)°, and γ = 65.83(1)°. All polyanions 1-3 were characterized in the solid state by single-crystal XRD, FT-IR, TGA, and elemental analysis, and polyanion 2 was also characterized in solution by (183)W NMR and UV-vis spectroscopy. Polyanion 2 was used as a homogeneous catalyst toward adenosine triphosphate (ATP) and the DNA model substrate 4-nitrophenylphosphate, monitored by (1)H and (31)P NMR spectroscopy. The encapsulated gallium(III) centers in 2 promote the Lewis acidic synergistic activation of the hydrolysis of ATP and DNA model substrates at a higher rate in near-physiological conditions. A strong interaction of 2 with the P-O bond of ATP was evidenced by changes in chemical shift values and line broadening of the (31)P nucleus in ATP upon addition of the polyanion.

Influence of the amino acid side chain on peptide bond hydrolysis catalyzed by a dimeric Zr(iv)-substituted Keggin type polyoxometalate

Structurally different dipeptides were hydrolyzed by [{α-PW₁₁O₃₉Zr-(μ-OH)(H₂O)}₂]⁸⁻. The rate constants were dependent on bulkiness and chemical nature of the dipeptide.

Asymmetric catalytic sulfoxidation by a novel VIV8 cluster catalyst in the presence of serum albumin: a simple and green oxidation system

Enantioselective oxidation of a series of alkyl aryl sulfides catalyzed by a novel VIV8 cluster is tested in an aqueous medium in the presence of serum albumin. The procedure is simple, environmentally friendly, selective, and highly reactive.

The Synthesis and Characterization of Aromatic Hybrid Anderson-Evans POMs and their Serum Albumin Interactions: The Shift from Polar to Hydrophobic Interactions

Four aromatic hybrid Anderson polyoxomolybdates with Fe³⁺ or Mn³⁺ as the central heteroatom have been synthesized by using a pre-functionalization protocol and characterized by using single-crystal X-ray diffraction, FTIR, ESI-MS, ¹H NMR spectroscopy, and elemental analysis. Structural analysis revealed the formation of (TBA)₃[FeMo₆O₁₈{(OCH₂)₃CNHCOC₆H₅}₂]⋅3.5 ACN (TBA-FeMo₆-bzn; TBA=tetrabutylammonium, ACN=acetonitrile, bzn=TRIS-benzoic acid alkanolamide, TRIS–R=(HOCH₂)₃C–R)), (TBA)₃[FeMo₆O₁₈{(OCH₂)₃CNHCOC₈H₇}₂]⋅2.5 ACN (TBA-FeMo₆-cin; cin=TRIS-cinnamic acid alkanolamide), (TBA)₃[MnMo₆O₁₈{(OCH₂)₃CNHCOC₆H₅}₂]⋅3.5 ACN (TBA-MnMo₆-bzn), and (TBA)₃[MnMo₆O₁₈{(OCH₂)₃CNHCOC₈H₇}₂]⋅2.5 ACN (TBA-MnMo₆-cin). To make these four compounds applicable in biological systems, an ion exchange was performed that gave the water-soluble (up to 80 mm) sodium salts Na₃[FeMo₆O₁₈{(OCH₂)₃CNHCOC₆H₅}₂] (Na-FeMo₆-bzn), Na₃[FeMo₆O₁₈{(OCH₂)₃CNHCOC₈H₇}₂] (Na-FeMo₆-cin), Na₃[MnMo₆O₁₈{(OCH₂)₃CNHCOC₆H₅}₂] (Na-MnMo₆-bzn), and Na₃[MnMo₆O₁₈{(OCH₂)₃CNHCOC₈H₇}₂] (Na-MnMo₆-cin). The hydrolytic stability of the sodium salts was examined by applying ESI-MS in the pH range of 4 to 9. Sodium dodecylsulfate–polyacrylamide gel electrophoresis (SDS-PAGE) showed that human and bovine serum albumin (HSA and BSA) remain intact in solutions that contain up to 100 equivalents of the sodium salts over more than 4 d at 20 °C. Tryptophan (Trp) fluorescence quenching was applied to study the interactions between the sodium salts and HSA and BSA at pH 5.5 and 7.4. The quenching constants were extracted by using Stern–Volmer analysis, which suggested the formation of a 1:1 POM–protein complex in all samples. It is suggested that the aromatic hybrid POM approaches subdomain IIA of HSA and exhibits hydrophobic interactions with its hydrophobic tails, whereas the Anderson core is stabilized through electrostatic interactions with polar amino acid side chains from, for example, subdomain IB.

Selective hydrolysis of oxidized insulin chain B by a Zr(IV)-substituted Wells-Dawson polyoxometalate

We report for the first time on the selective hydrolysis of a polypeptide system by a metal-substituted polyoxometalate (POM). Oxidized insulin chain B, a 30 amino acid polypeptide, was selectively cleaved by the Zr(IV)-substituted Wells-Dawson POM, K15H[Zr(α2-P2W17O61)2]·25H2O, under physiological pH and temperature conditions in aqueous solution. HPLC-ESI-MS, LC-MS/MS, MALDI-TOF and MALDI-TOF MS/MS data indicate hydrolysis at the Phe1-Val2, Gln4-His5, Leu6-Cys(SO3H)7, and Gly8-Ser9 peptide bonds. The rate of oxidized insulin chain B hydrolysis (0.45 h(-1) at pH 7.0 and 60 °C) was calculated by fitting the integration values of its HPLC-UV signal to a first-order exponential decay function. (1)H NMR measurements show significant line broadening and shifting of the polypeptide resonances upon addition of the Zr(IV)-POM, indicating that interaction between the Zr(IV)-POM and the polypeptide takes place in solution. Circular dichroism (CD) measurements clearly prove that the flexible unfolded nature of the polypeptide was retained in the presence of the Zr(IV)-POM. The thermal stability of the Zr(IV)-POM in the presence of the polypeptide chain during the hydrolytic reaction was confirmed by (31)P NMR spectroscopy. Despite the highly negative charge of the Zr(IV)-POM, the mechanism of interaction appears to be dominated by a strong metal-directed binding between the positively charged Zr(IV) center and negatively charged amino acid side chains.

Self-assembled polyoxometalates nanoparticles as pickering emulsion stabilizers

We easily produced a series of polyoxometalate (POM) nanoparticles by taking benefit from electrostatic attractions between various POMs and alkylammonium cations. These self-assembled supramolecular nanoparticles are fully characterized in terms of shape, nanostructure, and physicochemical properties. The nanoparticle differences are discussed on the basis of the chemical composition of the initial POM. Moreover, such particles have the ability to stabilize water-in-oil Pickering emulsions. Using a gel-trapping technique coupled to atomic force microscopy (AFM) observations, we determined their affinity toward oil by the contact angle of adsorbed nanoparticles. We show that the emulsion droplet size and stability can be directly linked to the nanoparticle hydrophobicity, which is tuned by the charge localization and molecular packing of POMs with the ammonium cations. Such particles are of special interest as they open large possibilities for Pickering interfacial catalysis.

Polyoxometalate-based phase transfer catalysis for liquid–solid organic reactions: a review

Recent progress in POM-based phase transfer catalysis for liquid–solid organic reactions is summarized in this review.

Heterogeneous catalysis and the challenges of powering the planet, securing chemicals for civilised life, and clean efficient utilization of renewable feedstocks

This article reviews, first, the prospects, practices and principles of generating solar fuels. It does so with an analysis of recent progress in the light-driven emission of H2 (and other fuels) as well as O2 from water. To place this challenge in perspective, some current practices entailing the use of well-proven solid catalysts developed for fossil-based feedstocks, are described. The massive differences between proven methods of generating fuel and chemicals from non-renewable and from solar radiation are emphasized with the aid of numerous quantitative examples. Whilst it is acknowledged that a key action in reducing the liberation of greenhouse gases (GHG) is to tackle the challenge of decreasing their evolution in power generation and in the production of steel, aluminium and other bulk commodities (metals, alloys, concrete and ceramics), nevertheless much can be done to diminish the emission of CO2 (and to use it as feedstock) through the agency of new, designed solid catalysts and microalgae. Solar-thermal converters are also attractive alternatives, even though they are more likely to be used centrally rather than in small modular units like 'artificial leaves,' some of which are promising for the purposes of generating energy (and perhaps fuel) in a delocalized, modular manner.

Molecular interactions between serum albumin proteins and Keggin type polyoxometalates studied using luminescence spectroscopy

The interaction between the plenary Keggin H3PW12O40, lacunary Keggin K7PW11O39 and the Eu(III)-substituted Keggin K4EuPW11O39 (Eu-Keggin) type polyoxometalates (POMs), and the proteins human and bovine serum albumin (HSA and BSA) was studied using steady state and time-resolved Eu(III) luminescence and tryptophan (Trp) fluorescence spectroscopy. The excitation spectrum of the Eu-Keggin POM is dominated by a ligand-to-metal charge transfer band at 291 nm. In the absence of proteins, the number of water molecules coordinated in the first coordination sphere of the Eu(III) center of Eu-Keggin was determined to be 4, indicating that Eu(III) occurs as a 1 : 1 isomer in solution. In the presence of HSA or BSA, the number of coordinated water molecules decreased to 0 and 1, respectively, suggesting interaction between the Eu-Keggin POM and the protein surface. As a result of this interaction, a five-fold increase of the hypersensitive (5)D0 → (7)F2 transition in the luminescence intensity was observed for the Eu-Keggin-HSA complex. The association constants were calculated to be 1.5 × 10(2) M(-1) and 2.0 × 10(3) M(-1) for the Eu-Keggin-HSA and Eu-Keggin-BSA complexes, respectively. Tryptophan fluorescence quenching studies were performed and the quenching constants were calculated using a Stern-Volmer analysis. The obtained values of the quenching constants were 6.1 × 10(4) M(-1) and 2.0 × 10(6) M(-1) for the Eu-Keggin-HSA and Eu-Keggin-BSA complexes, respectively. The surface map of both proteins shows that the cavity containing the tryptophan has a positive surface potential, providing a specific binding site at the surface of albumin proteins for the negatively charged POM.