Molybdophosphoric Heteropoly Acid Research Articles

Hydrolysis-oxidation of starch to formic acid in the presence of vanadium-containing molybdophosphoric heteropoly acid (H3+xPMo12-xVxO40): Effect of acidity and vanadium content on the yield of formic acid

Formic acid (FA) is an important chemical commodity as well as a promising medium for hydrogen storage and hydrogen production. Herein, a one-pot hydrolysis-oxidation of starch to formic acid (FA) catalyzed by molybdovanadophosphoric heteropoly acids (H3+xPMo12-xVxO40, x = 1, 2, 4, and 5) (PMo12-xVx) as bifunctional catalysts was demonstrated. The reaction was investigated at 120–150 °C. 2–5 MPa of gas mixture (O2/N2 - 20/80 vol/vol) was used as the oxidant. The rate of starch hydrolysis to glucose was found to decrease with decreasing the strength of PMo12-xVx. The yield of FA was demonstrated to depend on the number of V atoms in the framework of heteropoly anion. The initial rate of FA formation rose with an increasing number of V atoms. However, multi-substituted HPA favors overoxidation of the reaction mixture. Maximum yield of formic acid reached 67 % in 7 h under optimal conditions.

Acid leaching–extraction–circulation process based on Mo(VI) coordination with H3PO4 to efficiently extract molybdenum from different components of molybdenum calcine

Ammonia nitrogen wastewater generated during the ammonia decomposition of molybdenum calcine causes serious environmental pollution. To address this issue, a novel acidic leaching–extraction–circulation process is proposed to efficiently extract molybdenum (Mo) from different molybdenum calcine components (MoO2, MoO3, and CaMoO4). This process differs from traditional alkali leaching methods as it relies on the coordination of Mo(VI) with H3PO4 to yield soluble H3PMo12O40 while adding sulfuric and nitric acids to achieve the efficient one-step leaching of complex raw ores. The addition of sulfuric acid not only enhances the oxidation of nitrate ion, effectively leading to the co-leaching of MoO2, which cannot be directly complexed by phosphoric acid, but also results in the decomposition of molybdophosphoric heteropoly acid into H3PO4 for cyclic leaching, thus reducing the production cost. Since most of the Mo in acidic lixivium is present as MoO22+ due to the depolymerization of H3PMo12O40, the acidic extractant P507 is employed to extract Mo in the organic phase via cation exchange with MoO22+. The H+ released during the extraction hinders the conversion of MoO22+ to PMo12O403– and regenerates sulfuric acid in the raffinate, which can be reused in cyclic leaching. This study provides an effective and environmentally friendly approach for the industrial extraction of Mo.

Hydrolysis-oxidation of cellulose to formic acid in the presence of micellar vanadium-containing molybdophosphoric heteropoly acids

OxFA-processing of inedible cellulose is considered as a perspective way for the production of biogenic formic acid (FA) which appeared to be a promising source for sustainable energy. In our work, the one-pot hydrolysis-oxidation of cellulose to FA was studied in the presence of the micellar acidic catalysts based on Mo–V–P heteropoly acids [Cn-MIM]y/H3+x-yPMo12-xVx (n = 6, 10 and 16; x = 1, 2 and 4; y = 2–6). The influence of amount of V, substitution of H+ by [Cn-MIM] and the length of alkyl group Cn of the cation was revealed. The optimization of the reaction conditions was also performed. To our knowledge, heterogeneous catalysts of such a composition were applied in cellulose hydrolysis-oxidation to FA for the first time. In the presence of C10-MIM/HPMo11V catalyst, the optimal reaction conditions were determined: 10 g/L of cellulose, 1.25–10 g/L of catalyst, 5 MPa of gas mixture (O2/N2 - 20/80 vol/vol) and 170–180 °C. Decreasing the alkyl group length of Cn-MIM+ cation or V content into framework of heteropoly anion were revealed to rise up the yield of the target product. These correlations were caused by the following factors. First, the decreasing acidity of micellar catalysts. Senond, the change in distribution of electron density in heteropoly anion due to the interaction between hydrogen atoms of alkyl group of Cn-MIM cation and oxygen atom of heteropoly anion. Third, blocking of V active sites for reagents. Influence of acidity on catalytic properties of micellar catalysts was also revealed in the course of screening samples with the different degree of protons substitution by C16-MIM+. The reaction rate was demonstrated to rise with increasing degree of protons substitution by C16-MIM + cation in C16-MIM/HyPMo11V and C16-MIM/HyPMo8V4 samples. The yield of FA up to 45 mol% can be reached over micellar HPA catalysts studied.

A dual-mode system based on molybdophosphoric heteropoly acid and fluorescent microspheres for the reliable and ultrasensitive detection of alkaline phosphatase.

A novel colorimetric and fluorescent dual-mode sensing system based on molybdophosphoric heteropoly acid (PMA) and fluorescent microspheres (FMs) was established for monitoring the activity of alkaline phosphatase (ALP). In the presence of ALP, L-ascorbic acid-2-phosphate (AAP) could be hydrolyzed catalytically to ascorbic acid (AA), which could reduce PMA to phosphorus molybdenum blue (PMB), accompanied by the generation of colorimetric signals depending on the level of ALP. Meanwhile, the fluorescence of FMs was quenched markedly by the PMB produced due to the inner-filter effect, which constituted the response mechanism for the dual-mode sensing systems of ALP. On this basis, a PMA-FMs based dual-mode sensing system was used for the detection of ALP, which not only possessed remarkable sensitivity, with a limit of detection of 0.27 U L-1 and 0.11 U L-1, but also exhibited good analytical performance in biological samples with satisfactory results. Moreover, a simple and portable test kit for the visual detection of ALP in real serum samples was fabricated utilizing a smartphone with image-recognition and data-processing capabilities as a visual-detection platform.

A comparative study on porous solid acid oxides as catalysts in the esterification of glycerol with acetic acid

Abstract Acid catalysts comprising of porous SiAl, as well as molybdophosphoric heteropolyacid, supported SiAl nanotubes were synthesized. The characterizations were through scanning electron microscopy coupled to energy-dispersive X-ray spectroscopy (SEM-EDS), transmission electron microscopy (TEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), termoprogrammed ammonia desorption (NH3-TPD) and Fourier transform infrared spectroscopy (FTIR). Esterification of glycerol into acetins reaction was evaluated over two series of SiAl-based solids. Both series displayed very good activity as well as selectivity towards the acetins within a short reaction time. However, porous SiAl deactivated due to acid sites leaching in long-term catalytic runs. The tuning of the loadings of the molybdophosphoric amount on SiAl halloysite nanotubes leads to an increase in the selectivity towards the mono, di, and triacetins. These catalysts were also recycled up to three times, and a 17 % of conversion and 100 % of selectivity for triacetin were obtained due to the combination of acidity, stable structure and porosity of the molybdophosphoric supported SiAl nanotubes.

Preparation, Characterization and Photoelectric Properties of Hybrid Langmuir–Blodgett Films of Alkynylplatinum(II)–Zinc(II) Porphyrinate/Heteropolyoxometalate

A new family of organometallic/inorganic hybrid Langmuir–Blodgett (LB) films, consisting of rigid-rod alkynylplatinum(II)–zinc(II) porphyrinate complex (OMA) as the π-conjugated donor–acceptor-type molecule, molybdophosphoric heteropolyacid and heteropolyacid salt (POM = H3PMo12O40, Na5IMo6O24, abbreviated as HPMo12 and NaIMo6, respectively) of the Keggin and Anderson structures as the inorganic composite, were prepared and characterized by π-A isotherms, UV–vis absorption and luminescence spectra, scanning tunneling microscopy and surface photovoltage spectroscopy. Our experimental results indicate that stable, well-defined and well-organized Langmuir and LB films have been formed in pure water and POM subphase. Luminescence spectra of these hybrid LB films show that HPMo12 can increase the emission intensity of OMA to some extent. These LB composites show good photovoltage responses and a photovoltage of 11.1 μV can be obtained for the OMA/HPMo12 system when it is excited by light. The 3-layer LB films on ITO wafer can also display intriguing electrical conductivity behavior and the tunneling current amounts to ±100 nA when the voltage is set at −0.7–2.5 and −1.3–0.8 V for OMA/HPMo12 and OMA/NaIMo6 hybrid LB films, respectively.

Spectrophotometric determination of orthophosphate ions for conducting indicator investigations in stratal waters

The methodologies for determining orthophosphate ions by spectrophotometry and flow-injection analysis for 1995–2011 in different features are briefly described in the paper. The method of the spectrophotometric determination of orthophosphate in the form of molybdophosphoric heteropolyacid via reduced ascorbic acid in the presence of potassium antimonyl tartrate has been modified. The method allows analysts to determine 0.1–1.0 mg/L of orthophosphate ions in natural waters and has been tested by analyzing a model solution similar to the subterraneous waters of the Arigol licensed area. The advantages of the developed method, compared to the certified method, consist of the possibility of the express assessment and lower consumption of chemical reagents. The method is suitable for indicator investigations in geophysics.

Determination of phosphate in freshwater samples by flow-injection with lucigenin chemiluminescence

Lucigenin chemiluminescence (CL) in conjunction with flow-injection analysis (FIA) is used for the determination of phosphate in freshwater samples. The procedure is based on the formation of molybdophosphoric heteropoly acid (MoP–HPA) by the reaction of phosphate and ammonium molybdate under acidic conditions. CL emission was observed as a result of oxidation of lucigenin in aqueous sodium hydroxide solution in the presence of MoP–HPA. Calibration was linear up to 500 µg L−1 (r 2 = 0.9998; n = 8), with a detection limit (S/N = 3) of 0.95 µg L−1. An injection throughput of 120 h−1, and relative standard deviation (RSD; n = 4) of 1.3–3.2% were achieved in the concentration range studied. An on-line chelating column was used to remove interfering cations. The method was applied to freshwater samples, and the results (51 ± 1.0 – 107 ± 2.0 µg L−1) did not differ significantly from results obtained using a spectrophotometric method (52.5 ± 1.0 – 102 ± 2.0 µg L−1) at 95% confidence level (t-test).

Bi-component redox systems obtained from iridium or rhodium chlorides and molybdophosphoric heteropoly acid

An XP, IR and UV–VIS spectroscopy study was carried out to characterize the compositions of molybdophosphate heteropoly anions with Ir(III) or Rh(III) hydroxo–chloro complexes, such as Ir1.1Cl2.4OxHy·(PMo12O40)·16.0 H2O, where x is the number of oxygen atoms in the coordination sphere of iridium, and also mixed-valent species containing partially reduced and dehydrated heteropoly acid of suggested composition (Ir0, IrIII)nClmOxHy(PMo12−pVIMopVO40−p/2) (on the average, n ≈ 1, m < 1, p ≥ 4). The mixed-valent Ir containing species showed stable activity in the gas-phase oxidation of benzene by O2–H2 mixture.

The influence of additives and their concentration on the selectivity of catalysts based on heteropoly compounds in the reaction of propane oxidation

The catalytic properties and X-ray phase structure of molybdophosphoric heteropoly acid modified with Sb, W, and Nb ions have been studied in the reaction of propane oxidation. The additive of Nb ions increases the total activity of catalyst and its selectivity to acrolein and acrylic acid and decreases the selectivity to propylene. The concentration of Nb ions in samples is varied from 0.025 to 1 mol %. The concentration of Nb ions at a level of 0.025–0.1 mol % has the most profound effect on the reaction selectivity and the yield of mild oxidation products. The reaction selectivity depends on both the concentration of added niobium and the reaction conditions. The growth in the reaction temperature increases the conversion and selectivity to acrolein and reduces the selectivity to acrylic acid and propylene. The use of reaction mixture with higher propane concentrations decreases sharply the selectivity to acrolein. The obtained data can be explained by the possibility of the rearrangement of Nb ions between the cationic and anionic sublattices of heteropoly acids and by the change in the ratio between the acrolein and acrylic acid formation rates. The development and application of heteropoly compounds modified with niobium permits to increase the yield of target products in the partial propane oxidation.

Effect of preparation technique on the properties of Mo-containing Al-MCM-41

The Al-MCM-41 has been used as support to prepare Mo-containing catalysts. The 12- molybdophosphoric heteropoly acid (HPMo) is used as initial compound. The catalysts are synthesized by two different methods: incipient impregnation with aqueous solution of the acid and mechanochemical synthesis. The samples were tested in the reaction of the thiophene hydrodesulfurization after activation with mixture H2 + H2S. The effect of the preparation method of the catalysts on their physicochemical and catalytic properties has been studied. A partial destruction of the loaded compound is observed in mechanochemically treated sample whereas the aggregates are formed from the particles of different size in the impregnated sample. The specific surface area of the sample prepared by mechanical–chemical treatment decreases 2–3 times, while the total pore volume is about four times lower. The HDS activity is higher on the impregnated sample than on the mechanochemically treated one.

Determination of phosphate in water by means of a new electrochemiluminescence technique based on the combination of liquid–liquid extraction with benzene-modified carbon paste electrode

In this paper, it was found that the hydrophobic ion-associated complex of the molybdophosphoric heteropoly acid with protonated butyl-rhodamine B (BRhB) could be formed and was further selectively extracted into the bulk of the paraffin oil-based carbon paste electrode (CPE). At the same time, compared with other modifiers, the benzene-modified CPE created a suitable electrochemiluminescence (ECL) reaction microenvironment for electro-oxidation BRhB to produce the stronger ECL signal when a 1.30 V electrolytic potential was applied to the CPE in the alkaline medium. Based on these findings, a selective and sensitive ECL method for indirectly detecting phosphate was developed. Under the optimum experimental conditions, the ECL intensity was linear with the concentration of phosphate in the range of 2.0 × 10 −10 to 1.0 × 10 −8 g mL −1. The detection limit was 8.0 × 10 −11 g mL −1. The proposed method has been applied successfully to the analysis of phosphate in the water samples.

Interaction of platinum and molybdophosphoric heteropoly acid under conditions of catalyst preparation for benzene oxidation to phenol with an O2-H2 gas mixture

The transformations of platinum and a heteropoly acid (HPA) in binary systems prepared from H2PtCl6 or H2PtCl4 and H3PMo12O40 were studied using IR and UV-VIS spectroscopy, elemental analysis, XPS, EXAFS, TPR, and HREM. The calcination of platinum chloride with the HPA to 450°C resulted in the formation of a platinum salt of the HPA along with decomposition products (mixture I). The reduction of calcined samples containing Pt: HPA = 1: 1 with hydrogen at 300°C (mixture II) followed by exposure to air resulted in the regeneration of the HPA structure. The resulting solid samples of Pt 1−n 0 Pt n II ClmOxHy) (H3+p PMo 12−p VI Mo p V O40) (III) contained platinum and molybdenum in both oxidized and reduced states. The following association species were isolated from mixtures I and II by dissolving in water: [Pt n II PMo12O40] (I s) (n = 0.3−0.8) and [Pt n 0 PMo 12 red O40] (II s) (n ≈ 1). Under exposure to air, the solutions of I s were stable (pH ∼2), whereas Ptmet was released from II s. After the drying of I s, the solid association species (Pt n II ClmOxHy). (H3PMo12O40), where n = 0.3−0.8, m = 0.2−1, and x = 3−0, (I solid) were obtained. The I solid/SiO2 supported samples were prepared by impregnating SiO2 with a solution of I s and drying at 100°C. Platinum metal particles of size ∼20 A and a mixed-valence association species of platinum with the HPA were observed after the reduction of I solid/SiO2 with hydrogen at 100–250°C. These samples were active in the gas-phase oxidation of benzene to phenol at 180°C with the use of an O2-H2-N2 mixture.

Flow injection analysis of ultratrace orthophosphate in seawater with solid-phase enrichment and luminol chemiluminescence detection

Solid-phase extraction technique had been applied to extract molybdophosphoric heteropoly acid (MoP) paired with cetyltrimethylammonium bromide (CTAB) from seawater matrix using C18 sorbent. Chemiluminescence emission could be generated via MoP reaction with alkaline luminol. Based on these, a novel on-line solid-phase extraction method coupled with flow injection (FI) analysis and luminol chemiluminescence detection had been established to determine ultratrace orthophosphate in seawater. The MoP–CTAB compound could be efficiently extracted on an in-line Sep-Pak C18 cartridge, and rapidly eluted by 0.3 mol l −1 sulphuric acid–ethanol solution. Then the compound was reduced by luminol to produce chemiluminescence light, which could be detected using a luminescence analyzer. Experimental parameters were optimized using a univariate experimental design. Using artificial seawater with salinity of 35 as a matrix, the standard curve with a linear range between 0.005 and 0.194 μmol l −1 had been obtained, and the recovery and the detection limit of the proposed method were found to be 92.5% and 0.002 μmol l −1, respectively. The relative standard deviation (R.S.D.), which was determined over eight hour, was 4.66% ( n = 7) for the artificial seawater at a concentration of 0.097 μmol l −1 orthophosphate. Si of 200 μmol l −1 would not interfere with the detection of 0.012 μmol l −1 orthophosphate compound. Three typical seawater samples were analyzed using both the proposed method and the magnesium hydroxide-induced coprecipitation (MAGIC) method, and the results of the two methods showed no significant difference using the t test. Compared to the MAGIC method, the proposed method was more sensitive, time saving and easy for on-line analysis.

Selective oxidation of isobutane over H xFe 0.12Mo 11VPAs 0.3O y heteropoly compound catalyst

Keggin-type molybdophosphoric heteropolyacid catalysts with iron cation partially substituted for proton and arsenic for phosphorus in a bulk form have been synthesized for the selective oxidation of isobutane into methacrylic acid (MAA). It was found that the addition of iron can obviously improve both the conversion of isobutane and selectivity to MAA, while arsenic substitution was mainly in favor of the selectivity to MAA. The best conversion and yield of MAA were 23.9% and 16.7%, respectively, which was obtained at 370 °C with a space velocity of 420 h −1 over H x Fe 0.12PVAs 0.3Mo 11O y heteropoly compound catalyst. The catalysts were characterized by XRD, SEM, FT-IR, and TPR.

Determination of nanomolar concentrations of phosphate in freshwaters using flow injection with luminol chemiluminescence detection

A simple and rapid flow injection (FI) method is reported for the determination of phosphate (as molybdate reactive P) in freshwaters based on luminol chemiluminescence (CL) detection. The molybdophosphoric heteropoly acid formed by phosphate and ammonium molybdate in acidic conditions generated chemiluminescence emission via the oxidation of luminol. The detection limit (3× standard deviation of blank) was 0.03 μg P l −1 (1.0 nM), with a sample throughput of 180 h −1. The calibration graph was linear over the range 0.032–3.26 μg P l −1 ( r 2=0.9880) with relative standard deviations ( n=4) in the range 1.2–4.7%. Interfering cations (Ca(II), Mg(II), Ni(II), Zn(II), Cu(II), Co(II), Fe(II) and Fe(III)) were removed by passing the sample through an in-line iminodiacetate chelating column. Silicate interference (at 5 mg Si l −1) was effectively masked by the addition of tartaric acid and other common anions (Cl −, SO 4 2−, HCO 3 −, NO 3 − and NO 2 −) did not interfere at their maximum admissible concentrations in freshwaters. The method was applied to freshwater samples and the results ( 26.1±1.1–62.0±0.4 μg P l −1 ) were not significantly different ( P=0.05) from results obtained using a segmented flow analyser method with spectrophotometric detection ( 24.4±4.45–84.0±16.0 μg P l −1 ).

Effect of iron counter-ions on the redox properties of the Keggin-type molybdophosphoric heteropolyacid: Part I. An experimental study on isobutane oxidation catalysts

Keggin-type molybdophosphoric heteropolyacid with protons partially substituted by iron cations in a bulk form (Fe 0.85H 0.45PMo 12O 40) or supported on the cesium salt (Cs 2Fe 0.2H 0.4PMo 12O 40) have been synthesized and characterized by different techniques like the Mössbauer spectroscopy and the electron spin resonance (ESR). The effect of iron on the redox and catalytic properties for the oxidation of isobutane into methacrylic acid (MAA) has also been studied. Iron has been shown to have a different effect whether acts as a counter-cation in the bulk acid or in the acid supported on the cesium salt. In the first case, it increases both the selectivity in methacrylic acid and methacrolein (MA) and the activity of the acid phase whereas in the second case, it increases only the selectivity. This difference has been explained by the existence of an electron transfer between iron and molybdenum occurring only in the bulk acid. This electron transfer was related to a combined hydration-oxidation mechanism which promotes the reducibility of the solid and consequently its catalytic activity.

Heteropoly compounds in ammoxidation of methylpyrazine

Catalytic properties of molybdophosphoric, mixed vanadium molybdophosphoric heteropoly acids H3+nPMo12−nVnO40 (n=0, 1, 2) and their salts Na7PV4Mo8O40 and Na8HPV6Mo6O40 have been studied in methylpyrazine ammoxidation. Partial vanadium substitution for molybdenum in heteropoly acids increases activity and selectivity towards cyanopyrazine. Sodium salts are somewhat less active and selective.

Characterization of silica-supported molybdenum catalysts by ESR, XPS and thiophene hydrodesulfurization

Two Mo/SiO2 samples have been prepared by impregnation of silica gel with aqueous solution of molybdophosphoric heteropoly acid or ammonium heptamolybdate. The catalysts were studied by ESR and IR spectroscopy, X-ray diffraction, and XPS. Their activity was measured in the reaction of thiophene hydrodesulfurization. The silica-supported P−Mo heteropoly acid, with a preserved Keggin structure of the heteropoly anion and sulfided by H2S, exhibited two times higher steady conversion in comparison with ammonium heptamolybdate supported on silica at pH=11 and calcined and sulfided at 500°C.

Redox mechanism of acrolein oxidation over the product of molybdophosphoric heteropolyacid thermolysis: PMo12O38.5

Redox mechanism of oxygen insertion into acrolein upon its oxidation by molecular oxygen on PMo12O38.5 catalyst has been studied. The latter is the product of thermolysis of the series 12 molybdophosphoric heteropolyacid. The heat of oxygen adsorption on the catalyst surface was measured by calorimetry as a function of catalyst reduction by carbon oxide. The value amounted to 72.5±2.5 kcal/mol O2 and did not depend on the reduction degree, which was about 3.5 oxygen monolayers.