Heteropoly Anion Research Articles

Reversible phase transformation gel-type ionic liquid compounds based on tungstovanadosilicates

A series of new reversible phase transformation gel-type ionic liquid compounds, [PyPS]5SiW11VO40, [PyPS]7SiW9V3O40, [TEAPS]5SiW11VO40 and [TEAPS]7SiW9V3O40, have been synthesized from two organic ammoniums 1-(3-sulfonic group) propyl-pyridine (PyPS), 1-(3-sulfonic group) propyl-triethylammonium (TEAPS) and vanadium-substituted heteropoly acids H5SiW11VO40 and H7SiW9V3O40. The products can undergo a phase transformation from viscous gel-state to liquid-state below 100 °C, and ionic conductivity up to 10−3 S cm−1 was observed at 110 °C for these gel-type POM-ILs. The relationship between the component elements of the products and their physicochemical property has been studied. Their thermostability was measured using thermogravimetric and differential thermal analysis (TG-DTA), and the result indicates that the less vanadium atoms there are inside the heteropoly anion, the more stable the POM-IL is at high temperature. Cyclic voltammetry is carried out to study their electrochemical properties in organic solution. The potential values of redox waves confirm that the oxidability of these gel-type POM-ILs can be controlled upon changing the number of vanadium atoms in the heteropolyanions.

Nitrate and Nitrite Electrocatalytic Reduction at Layer-by-Layer Films Composed of Dawson-type Heteropolyanions Mono-substituted with Transitional Metal Ions and Silver Nanoparticles

A series of Dawson-type heteropolyanions (HPAs) mono-substituted with transitional metal ions (α2-[P2W17O61FeIII]8−, α2-[P2W17O61CuII]8− and α2-[P2W17O61NiII]8−) have exhibited electrocatalytic properties towards nitrate and nitrite reduction in slightly acidic media (pH 4.5). The immobilization of these HPAs into water-processable films developed via layer-by layer assembly with polymer-stabilized silver nanoparticles led to the fabrication of the electrocatalytic interfaces for both nitrate and nitrite reduction. The LBL assembly as well as the changes in the HPA properties by immobilization has been characterized by electrochemical methods. The effects of the substituent ions, outer layers and the cationic moieties utilized for the films assembly of the developed film on the performances of nitrate electrocatalysis has been elucidated.

Direct catalytic synthesis of ε-caprolactam from cyclohexanol using [n-C16H33N (CH3)3]H2PW12O40 as a catalyst

ε-Caprolactam was synthesized directly from cyclohexanol via a tandem catalytic process using [n-C16H33N(CH3)3]H2PW12O40 as a catalyst. The highly efficient performance of the catalysts is due to the phase-transfer function of cation, improved coordination with peroxotungsten during oxidation and stabilization function of heteropoly anion on the intermediate formed during Beckmann rearrangement. A ε-caprolactam yield of 73.9% was obtained with a cyclohexanol conversion of 97.1% under optimized conditions.

Enhancement of nitrite and nitrate electrocatalytic reduction through the employment of self-assembled layers of nickel- and copper-substituted crown-type heteropolyanions.

Multilayer assemblies of two crown-type type heteropolyanions (HPA), [Cu20Cl(OH)24(H2O)12(P8W48O184)](25-) and Ni4(P8W48O148)(WO2)](28-), have been immobilized onto glassy carbon electrode surfaces via the layer-by-layer (LBL) technique employing polycathion-stabilized silver nanoparticles (AgNP) as the cationic layer within the resulting thin films characterized by electrochemical and physical methods. The redox behaviors of both HPA monitored during LBL assembly with cyclic voltammetry and impedance spectroscopy revealed significant changes by immobilization. The presence of AgNPs led to the retention of film porosity and electronic conductivity, which has been shown with impedance and voltammeric studies of film permeabilities toward reversible redox probes. The resulting films have been characterized by physical methods. Finally, the electrocatalytic performance of obtained films with respect to nitrite and nitrate electrocatalytic reduction has been comparatively studied for both catalysts. Nickel atoms trapped inside HPA exhibited a higher specific activity for reduction.

Study of ionic equilibria and computation of thermodynamic characteristics of heteropoly anion formation in GdW10O 36 9− solution

Interaction in GdW10O369−-H+(OH−)-H2O system (\(C_{GdW_{10} O_{36}^{9 - } } \) = 1 × 10−3 mol/L) was studied by pH potentiometry at 25 ± 0.1°C, and a model that describes equilibrium processes in acid and alkaline regions was selected. Logarithms of concentrational and thermodynamic constants, values of Gibbs energy of monomeric ions reactions, and standard Gibbs energies of formation (ΔGfo) of heteropoly anions HnGdW10O36(9−n)− and HmGdW5O18(3−n)− were calculated. A series-parallel scheme of ion transitions was pro-posed, ion distribution diagrams in aqueous solutions were built, the regions of preferable anion content were found, and heteropoly salts were synthesized.

Immobilization of polyoxometalate-based ionic liquid on carboxymethyl cellulose for epoxidation of olefins

A novel ionic liquid consisting of a PEG-functionalized ammonium cation and a lacunary-type phosphotungstate anion was synthesized and characterized structurally. The ionic liquid was then immobilized onto environmentally benign polymer-carboxymethyl cellulose by two different routes: the impregnation method and the co-precipitation method. The immobilized ionic liquid can be used as a catalyst for olefin epoxidation with aqueous hydrogen peroxide in ethyl acetate. It was found that both of the immobilized ionic liquid catalysts showed better catalytic activities and stability than the homogeneous analogue in consecutive runs. In particular, the ionic liquid catalyst immobilized by the co-precipitation method afforded a higher catalytic stability than the catalyst immobilized by the impregnation method. It is suggested that the crucial factor influencing the catalytic performance is the difference in the interactions between the heteropoly anions and the polymer supports.

Dual-sulfonated dipyridinium phosphotungstate catalyst for liquid-phase Beckmann rearrangement of cyclohexanone oxime

The heteropolyanion (HPA)-based ionic liquid (IL) hybrid was prepared by pairing Keggin-structured HPA of the phosphotungstate anion PW12O403− (PW) with dual-sulfonated 4,4′-dipyridinium IL-cation [DPySO3H]2+. The obtained powdered IL–HPA hybrid material [DPySO3H]1.5PW was characterized and catalytically assessed in Beckmann rearrangement of cyclohexanone-oxime to e-caprolactam. Under the optimized reaction conditions: 10 mol% catalyst, 130 °C, 2 h, benzonitrile as the solvent, and without the environmentally harmful cocatalyst ZnCl2, [DPySO3H]1.5PW exhibits 100% conversion and 73.0% selectivity, remarkably higher than control catalysts involving other HPA-anions or the partially proton-substituted IL-cation. Moreover, the catalyst [DPySO3H]1.5PW can be isolated and reused via filtration from the liquid–solid heterogeneous reaction mixture demonstrated by the four-run recycling test.

Complexation of Cobalt with a Heteropolyanion of Dawson Type and Recovery by Emulsified Liquid Membrane

In this study, a lacunar heteropolyanion (HPA) of Dawson type (P2W15Mo2O61) 10- was prepared and tested in the complexation of aqueous (Co2+). The complex formed was recovered after that using an emulsified liquid membrane (ELM). The results showed that the complex (P2W15Mo2O61Co) 8- was very stable with constant stability of β = 4,073.103. The recovery of this complex formed by (ELM) attained the yield of 70%.

Microporous cesium salts of tetravalent Keggin-type polyoxotungstates Cs4[SiW12O40], Cs4[PW11O39(Sn-n-C4H9)], and Cs4[PW11O39(Sn-OH)] and their adsorption properties

Microporous cesium salts of modified and unmodified tetravalent Keggin-type polyoxometalates, including Cs4[SiW12O40], Cs4[PW11O39(Sn-n-C4H9)], and Cs4[PW11O39(Sn-OH)], were synthesized. The crystalline structures, which had body-centered cubic (bcc) arrangements, the lattice constants, and the pore-size distributions of the three Cs salts were similar, regardless of the presence or absence and types of functional groups introduced. The Cs salts had only micropores and no mesopores. The micropore size distributions were determined from adsorption isotherms of Ar, which showed a sharp peak at 0.59nm and a shoulder at 0.62nm. The fractions of the external surface areas to the total surface areas of the Cs salts were less than 6%. It is plausible that the micropores originate from the heteropoly anion defects in the crystallite, which form to avoid mismatches in the Cs+/(heteropoly anion) ratio required for charge balance (=4) and for a bcc structure (=3). The surface of the Cs salt introduced with n-butyl groups was hydrophobic, although the surface density of the n-butyl groups was low. On the other hand, the hydroxyl groups present on the surface of Cs4[PW11O39(Sn-OH)] had little effect on the adsorption of water, methanol, ethanol, and hexane but a great impact on that for benzene due to the interactions between the -OH groups and the aromatic rings (-OH···π).

Characterization of Immobilized Composite Catalysts of Cationic Metalloporphyrin and Heteropoly Anion in Oxidation of Ethyl Benzene by Molecular Oxygen

An immobilized cationic phenyl porphyrin was prepared with crosslinked polystyrene (CPS) microspheres as carriers via synthesis and immobilization.A kind of immobilized cationic cobalt porphyrin was obtained through a coordination reaction between the immobilized cationic phenyl porphyrin and the cobalt salt.In the composite reactions,phosphotungstic acid (HPW) and phosphomolybdic acid (HPMo) with Keggin structures were used as reagents,respectively,and the combination of the cationic cobalt porphyrin (CoP) with the heteropoly anions by electrostatic interaction led to the formation of the immobilized composite catalysts of CoPPW-CPS and CoPPMo-CPS.We then characterized these immobilized composite catalysts.The immobilized composite catalysts were used in the oxidation of ethylbenzene by molecular oxygen,and their catalytic characters were investigated.The results show that the immobilized composite catalysts possess high catalyst activity.They can transform ethyl benzene into acetophenone with high selectivity,and the yield of acetophenone after 12 h reaches 30.1%.The catalytic activity of the immobilized composite catalysts is 75% higher than that of the immobilized cobalt porphyrin.The catalytic activity of CoPPW-CPS is higher than that of CoPPMo-CPS.The cobalt porphyrin in the composite is the catalyst component as the heteropoly has no catalytic activity.However,the heteropoly anion has a protective effect on the metalloporphyrin against its deactivation during the catalytic reaction.In the oxidation reaction,there is an optimum addition for catalyst,and adding excessive catalyst inhibits its activity.The composite catalysts show good recycle properties.

Redox Switching of Heteropolyanions Entrapped in Polypyrrole Films Investigated by ac Electrogravimetry

In this work, the global kinetics and the exchange of protons and free solvent at the film/electrolyte interface were investigated when a PPy film doped with HPA (heteropolyanions) is polarized. The ionic and electronic transfers for the PPy-HPA system are fast because the determined values of the resistances are relatively low (a few ohms). Depending on the applied potential, the global kinetics is controlled by both the ionic and electronic transfers. The transport effects were neglected here because the prepared films were very thin. Protons play an important role in charge compensation whatever the imposed potential on the PPy-HPA film. This phenomenon results from the acidic-basic properties of the SiMo(12)O(40)(4-) reduced species. A small quantity of water was also shown to be involved in the film reaction process.

Active phase genesis of NiW hydrocracking catalysts based on nickel salt heteropolytungstate: Comparison with reference catalyst

The sulfidation of supported NiW hydrotreating catalysts is followed by a combination of X-ray photoelectron spectroscopy (XPS), high resolution transmission electron microscopy (HRTEM), Raman and electron paramagnetic resonance (EPR) spectroscopy. In this work, we compared two catalysts prepared by incipient wetness impregnation from different precursors. The first one is an innovating catalyst prepared with the nickel salt of the derived lacunary Keggin heteropolyanion (HPA) Ni4SiW11O39 and the second one is a conventional catalyst synthesized from ammonium metatungstate and nickel nitrate. The results show for both catalysts that the sulfidation of nickel and tungsten elements does not occur at the same temperature. However, a better sulfidation of tungsten and nickel atoms respectively to WS2 and NiWS phases is observed at 400°C for HPA precursor compared to conventional catalyst, highlighting the benefit of HPA starting material.

ChemInform Abstract: Highly Selective Sorption of Small Polar Molecules by a Nonporous Ionic Crystal of a Lacunary Keggin‐Type Heteropoly Anion and Alkali Metal Cations.

AbstractCs6.1K0.9[PW11O39] is synthesized from an aqueous solution of K7PW11O39 and CsCl (room temperature, 1 d, followed by treatment at 473 K for 2 h).

Study on Catalytic Oxidatio Benzaldehyde to Benzoic Acid with Keggin Polyoxometalate [(CH<sub>2</sub>)<sub>5</sub>NH<sub>2</sub>]<sub>4</sub>SiW<sub>12</sub>O<sub>40</sub>

The organic–inorganic hybrid catalyst [(CH2)5NH2]4SiW12O40 was prepared by matrix acid and piperidine. The matrix acid was synthesized by sodium molybdate and sodium silicate, and the piperidine was organic ligand. The proposed composition and structure of the catalyst were evidenced by XPS, FT-IR, XRD, TG-DTA and elemental analysis. The results indicated that the heteropoly anions still reserved their Keggin structure in the compound. Its catalytic performance was evaluated in the oxidation of benzaldehyde to benzoic acid. Various reaction parameters were changed to attain the optimal conditions. The optimal reaction conditions were found to be: n(catalyst): n(benzaldehyde)=3.1×10-3:1; n(H2O2)： n(benzaldehyde)=4.5：1; reaction temperature was 80°C; reaction time was 4h. The yield of benzoic acid achieved above 85%.

Highly Selective Sorption of Small Polar Molecules by a Nonporous Ionic Crystal of a Lacunary Keggin-type Heteropoly Anion and Alkali Metal Cations

Abstract Cesium–potassium salt of the lacunary phosphotungstate Cs6.1K0.9[PW11O39], which is a nonporous ionic solid, absorbed only water and methanol, whereas larger alcohols (C ≥ 2) and hydrocarbons were completely excluded. Cs6.1K0.9[PW11O39] selectively removed water from water–ethanol mixed gas and was reusable by vacuum treatment at room temperature.

Sequential injection determination of orthophosphate as ion associate of 12-molybdophosphate with Astra Phloxine

A simple and fast reaction between 12-molybdophosphate heteropoly anion and the polymethine dye Astra Phloxine was used for the development of a new SIA method for the determination of orthophosphate. The suggested method has higher sensitivity and a broader calibration range than existing SIA methods. It can be used to detect phosphate in the range from 0.2 to 10μmolL−1 with a detection limit of 0.1μmolL−1 and an acceptable throughput of 20 samples h−1. The comparably low molybdate (5.6mmolL−1) and dye (0.1mmolL−1) concentrations led to an improvement in the stability of the base line. Inter-day reproducibility was better than 5%, while the intra-day RSD was in the range 0.8–3.5%. The method was used for the analysis of natural water samples and phosphorus-containing chemicals having a low content of orthophosphate.

Interaction of Molybdo-Phosphoric and Molybdo-Antimono-Phosphoric Heteropoly Acids with Silica Gels Modified with Aliphatic and Heterocyclic Quaternary Ammonium Salts

The interaction of molybdo-phosphoric and molybdo-antimonophosphoric heteropoly acids with aliphatic (tetradecylammonium nitrate and trinonyloctadecylammonium iodide) and heterocyclic (lucigenine and safranine) quaternary ammonium salts immobilized onto silica has been studied using adsorption and spectroscopic methods. It has been shown that the formation of ionassociates between the quaternary ammonium cations and the heteropoly anions is accompanied by their interaction with surface silica groups which accounts for the tight binding of heteropoly acids to the surface. Such interfacial reactions can successfully be applied to the development of spectroscopic and semi-quantitative visual test techniques for detecting trace amounts of phosphorus and antimony.

ChemInform Abstract: Preparation and Properties of Heteropoly Molybdovanadate(V) Complexes.

Tetrabutylammonium salts of the mixed heteropoly molybdovanadate(V) anions, [Hx−1V(VxMo12−x)O40]4− (x=1–3) were prepared from a Mo(VI)–V(V)–CH3CN system at room temperature. These mixed heteropoly anions were distinguished by their voltammetric behavior; the anions with x=1, 2, and 3 were electroreduced by one-, two-, and three-electrons due to the peripheral V atoms at a glassy carbon electrode. The spectroscopic properties were also reported. Their formation conditions were compared with those of 12-molybdovanadate (V), [VMo12O40]3−, especially regarding the concentrations of Mo(VI), V(V), acid, and organic solvents, the nature of organic solvents, and the ionic strength of the system. It should be emphasized that the 12-molybdovanadate(V) and mixed molybdovanadate(V) complexes are not formed in the absence of organic solvents.

New Mo–V based oxidic precursor for the hydrotreatment of residues

In order to understand the role of vanadium and its interaction with molybdenum atoms in the catalysts used for the hydrotreatment of residues, mixed heteropolyanion (HPA) precursors PMo(12−x)VxO40(3+x)− have been prepared and deposited by incipient wetness impregnation on an alumina support. The evolution of the compounds in the impregnating solutions till their deposition on the support has been analysed using various spectroscopic techniques such as Raman, 31P and 51V NMR, XPS as well as TEM. Activity measurements on model molecules (toluene and cyclohexane) and on real feedstock (vacuum residue) have been performed after activation step (ex situ sulphidation). Hydrogenation (HYD), isomerisation (ISOM), hydrodemetallation (HDM), hydrodesulphurisation (HDS), hydrodeasphaltenisation (HDAsC7) were assessed. During tests (2h) on a Safaniya vacuum residue feed, interesting HDM activity was observed compared to reference promoted and unpromoted catalysts (NiMo and Mo/Al2O3). It can be concluded that the use of vanadium in the impregnating solution may promote HDM and HDAsC7 (increase of vanadium removal rate around 15% compared to Mo-catalyst without vanadium). Furthermore HDM and HDAsC7 catalytic performances turned out to be close to the ones of NiMo catalyst. The increase in HDM/HDAsC7 activity may be related to the improvement of hydrogenation and/or isomerisation, the latter being improved by the presence of vanadium.

Photometric cyclic-injection trace determination of phosphate ions in natural waters as an ion associate of phosphomolybdate with Astra Phloxine

A procedure for the automatic photometric determination of phosphate ions in natural waters was developed, based on the formation of a colored ion associate of the reduced form of phosphomolybdic heteropoly anion with a polymethine dye, Astra Phloxine. The detection limit was 5 μg P/L at the sample volume of 0.3 mL.