Heteropoly Acid Solutions Research Articles

Homogeneous Catalysts of RedOx Processes Based on Heteropolyacid Solutions. VI: Developing a Process for Low-Temperature Oxidation of CO with Oxygen

Research on the development of a homogeneous process for low-temperature oxidation of carbon monoxide in the presence of a “platinum group metal + vanadium-containing heteropolyacid (HPA)” catalytic system has been presented. The optimal reaction conditions, ensuring the maximum rate of CO oxidation to CO2, have been determined; for this reaction the kinetic features have been established, and its mechanism has been proposed. It has been shown that homogeneous systems based on PdII complex exhibit high activity and productivity, but have low stability and operate only at pH below 1,5. The stability of the catalyst can be increased by simultaneous introduction of σ- and π-donor ligands into the system; however, it is more effective to use the PtIV complex in the presence of catalytic amounts of palladium salt at a ratio of 100/1. The transition from HPA solutions with a low content of vanadium atoms (H7PMo8V4O40) to solutions of modified compositions (H10P3Mo18V7O84) ensures an increase in the activity and productivity of the system, with the kinetics of CO oxidation being maintained. The combined homogeneous catalyst PtIV + PdII + H10P3Mo18V7O84 remains stable during multi-cycle use without reducing activity, operates without an induction period and can be used at pH range of 1,7–2,0, which simplifies the process equipment.

Homogeneous Redox Catalysts Based on Heteropoly Acid Solutions: 5. Developing a Two-Stage Process for Propylene Oxidation to Acetone: A Review

Homogeneous Redox Catalysts Based on Heteropoly Acid Solutions: 5. Developing a Two-Stage Process for Propylene Oxidation to Acetone: A Review

Homogeneous Catalysts for Redox Processes Based on Heteropolyacids Solutions. V. Developing Two-Stage Process for Oxidation of Propylene to Acetone

Investigations on the development of homogeneous chloride-free two-stage process (1st stage – target reaction, 2nd stage – catalyst regeneration) for oxidizing С3Н6 to acetone with oxygen in the presence of catalyst \({\text{Pd}}_{{{\text{aq}}}}^{{{\text{2 + }}}}\) + Mo-V-Р heteropoly acid (HPA-х, х is the number of V atoms) have been presented. The kinetic equation of the target reaction has been obtained, and its mechanism has been proposed. It has been shown that catalysts based on high-vanadium modified (non-Keggin) compositions HPA-xm are the most efficient. The kinetics of C3H6 oxidation in the presence of Keggin HPA-x and HPA-xm solutions has been the same, but only the \({\text{Pd}}_{{{\text{aq}}}}^{{{\text{2 + }}}}\) + HPA-xm catalysts have turned out to be technologically feasible. These have a high thermal stability (up to 180°C), which allows the catalyst to be rapidly regenerated with oxygen. This favorably distinguishes the catalysts based on HPA-xm from Keggin HPA-x, the thermal stability of which is limited to 140°C. The possibility of rapid regeneration of the catalyst has made it possible to close the 2-stage catalytic cycle of C3H6 oxidation to acetone with oxygen and has opened up prospects for the practical use of the process in the presence of \({\text{Pd}}_{{{\text{aq}}}}^{{{\text{2 + }}}}\) + HPA-xm. The catalyst has been successfully tested for stability.

Homogeneous Redox Catalysts Based on Heteropoly Acid Solutions: IV. Tests of Methyl Ethyl Ketone Synthesis Catalysts in the Presence of Equipment Corrosion Products (Metal Cations)

Homogeneous Redox Catalysts Based on Heteropoly Acid Solutions: IV. Tests of Methyl Ethyl Ketone Synthesis Catalysts in the Presence of Equipment Corrosion Products (Metal Cations)

Influence of chemical etching on bond strength of translucent zirconia to resin cement

Purpose: To assess the effect of chemical pre-treatments (piranha, hot acid, and heteropolyacid) on the bond strength (μTBS) to zirconia. Materials and Methods: A total of 8 sintered zirconia blocks were randomly assigned into 4 groups; Group I: Air abrasion, Group II: Air abrasion and etching using piranha solution (3H2SO4: 1H2O2), Group III: Air abrasion and etching using hot acid solution (1HNO3: 1HF), and Group IV: Air abrasion and etching using heteropolyacid solution (2gm/50 mL). Zirconia blocks were bonded to their corresponding pre-constructed composite blocks using adhesive resin cement. For each group, each assembly was sectioned to obtain 20 microbars. All microbars were thermocyled (10,000), then the μTBS test was done with the aid of a Universal Testing machine. To determine the mode of failure, each specimen was examined under stereomicroscope (x40). One-way ANOVA and Post Hoc Tukey tests were used. Results: The piranha pre-treatment displayed the lowest bond strength (16.82 ±5.97 MPa), while heteropolyacid pre-treatment had higher bond strength (26.22 ±4.05 MPa). One-way ANOVA test indicated statistical significant difference between the studied groups (P<.001). No significant difference was observed between control and piranha groups, also between hot acid and heteropolyacid groups. However, there were statistical significant difference between the control and heteropolyacid groups, and between piranha and heteropolyacid groups. Conclusions: According to the current findings; The novel surface pre-treatment with heteropolyacid (2gm/50 mL) performed well to improve bonding to air-abraded zirconia-based ceramics. The hot acids etching pre-treatment enhanced the bonding to zirconia.

Study of hydrotreating performance of trimetallic NiMoW/Al2O3 catalysts prepared from mixed MoW Keggin heteropolyanions with various Mo/W ratios

Trimetallic NiMoW/Al2O3 catalysts based on mixed H4[SiMonWn-12O40] (n = 1, 3, 6, and 9) Keggin-type heteropolyacids (HPAs) were synthesized by incipient wetness impregnation of alumina with aqueous solutions of mixed HPAs. For comparison purposes, trimetallic samples were prepared from a mixture of monometallic H4[SiMo12O40] and H4[SiW12O40] HPAs with a Mo/W ratio corresponding to the mixed MoW HPAs. The catalysts were sulfided by a liquid phase method and tested in the model reactions of dibenzothiophene hydrodesulfurization and naphthalene hydrogenation, with subsequent addition of quinoline to study the effect of inhibition of target reactions. Further, the catalysts were tested in the hydrotreating of straight-run gas oil to evaluate the efficiency of catalysts on real feedstocks. In order to link catalytic performances with the preparation method and Mo/W ratio, the catalysts were fully characterized by high-resolution transmission electron microscopy, X-ray photoelectron spectroscopy, high-angle annular dark-field imaging, and quick X-ray absorption spectroscopy. It was found that the Mo/W atomic ratio of the structure-forming metals in the active phase, deriving from the Mo/W ratio in the HPA precursor, directly affects the ratio of the hydrogenation and hydrodesulfurization performances. For feedstock with a high concentration of N-containing compounds, it is necessary to use mixed NiMoW systems with a high tungsten percentage to reduce the inhibitory effect and ensure the quality of the products. The use of new mixed MoW Keggin HPAs has made it possible to enhance the synergistic effect in trimetallic NiMoW catalysts due to the closer interaction between Mo and W, which increased the sulfidation degree of metals and also contributed to the formation of highly active mixed NiMoWS sites.

Additive Effect of Iron and Copper Sulfates in Heteropolyacid Solution As a Cathode Reaction Mediator on the Performance of Redox Flow PEFCs

Since the Industrial Revolution, we have relied heavily on fossil resources. Even today, they are indispensable for our cultural activities, but environmental problems such as global warming and soil pollution caused by their combustion have been concerned. These issues have grown an interest in alternative fuels. In this situation, hydrogen has been regarded as promising next-generation energy sources in terms of energy content and environmental load. Moreover, fuel cells, which directly convert the chemical energy of hydrogen into electrical energy, are cleaner and more efficient than conventional power generation devices.Polymer electrolyte fuel cells (PEFCs) have been already commercialized as power sources for automobiles due to low operating temperature and high current density. However, the carbon-supported platinum or platinum-based alloy is widely applied as the cathode. Therefore, these fuel cells are very costly and required to reduce the platinum usage for more widespread application of PEFCs.We proposed a redox flow PEFC as one of the solutions to this problem [1]. The redox flow PEFC employing the heteropolyacid solution of H6PV3Mo9O40 as a redox mediator for the cathode reaction was constructed. In this system, the cathode is free from noble metals for electrocatalyst. The electrochemical reduction of heteropolyanion over the carbon cathode and the subsequent re-oxidation of the partially reduced heteropolyanion by exposure to the dissolved oxygen in the regenerator are important processes for the continuous power generation. Especially, the improvement of re-oxidation rate contributes to a stable operation with higher current density. Therefore, the development of catalyst to promote this process will lead to the improved performance.For the heteropolytungstates of H3PW12O40 and H5SiVW11O40, it has been reported that the re-oxidation of their reduced anion species are efficiently catalyzed by a combination of copper ion and iron ion [2].The catalytic pathway consists of a reversible reaction between Cu(II) and Fe(II), followed by fast oxidation of reduced heteropolyanion and Cu(I) by Fe(III) and O2, respectively, to regenerate Fe(II) and Cu(II). In this study, therefore, the influence of the addition of iron and copper sulfates to the H6PV3Mo9O40 solution on the cell performance was evaluated. As a result, the obtained current density and the re-oxidation rate were improved by the addition. Also, in the discharge operation at a constant current the terminal voltage remained higher in using the solution with these sulfates than without additives. Accordingly, the addition of iron and copper sulfates to H6PV3Mo9O40 solution contributed to the improvement of the performance of the redox flow PEFCs.-------------------------------------------------------------------------------------[1] T. Matsui, E. Morikawa, S. Nakada, T. Okanishi, H. Muroyama, Y. Hirao, T. Takahashi, K. Eguchi. ACS Appl. Mater. Inter.18119-18125 (2016).[2] M. Kim, M. Chamack, Y. V. Geletii, C. L. Hill, Inorg. Chem. 311-318 (2018)

Homogeneous Catalysts of Redox Processes Based on Heteropolyacid Solutions III: Developing Effective Ways for the Preparation of 2,3,5-Trimethyl-1,4-Benzoquinone

Different ways of preparing 2,3,5-trimethyl-1,4-benzoquinone (TMQ), a key intermediate in the synthesis of vitamin E, via the catalytic oxidation of 2,3,6-trimethylphenol (TMP) are considered. It is shown that aqueous solutions of Mo–V phosphoric heteropolyacids (HPAs) are promising catalysts for the oxidation of TMP with oxygen. Different technological options are analyzed for using HPAs in this reaction: the one-stage homogeneous acetic acid oxidation of TMP and a biphasic two-step procedure. The efficiency of a biphasic version that ensures high (up to 99.5%) selectivity for TMQ and high catalyst productivity in the target reaction (≈600–800 g TMQ $${\text{L}}_{{{\text{cat}}}}^{{ - 1}}$$ h−1) is proved.

Homogeneous Catalysts Based on Heteropoly Acid Solutions for Redox Processes. III. Development of an Effective Method for Synthesis of 2,3,5-trimethyl-1,4-benzoquinone

Various methods for preparation of 2,3,5-trimethyl-1,4-benzoquinone (TMQ) – the key semiproduct for synthesis of vitamin E – through catalytic oxidation of 2,3,6-trimethylphenol (TMP) are considered. Potentials of aqueous solutions of Mo-V-P heteropoly acids (HPA) as catalysts for oxidation of TMP with oxygen are shown. Various technological modes of application of HPA for the said reaction are analyzed, among which are a one-stage homogeneous acetic acid oxidation of TMP and a two-phase two-stage method. The efficiency of the two-stage method is demonstrated; it provides a high (up to 99.5 %) selectivity to TMQ and a high productivity of the catalyst for the target reaction (ca 600–800 g TMQ·Lcat–1·h–1).

Highly efficient hydrolysis of plant hemicelluloses by mixed-addenda Keggin-type (Mo-V-P)-heteropolyacids in diluted aqueous solution

Polyoxometalate (POM) catalyzed hydrolysis of wood and agro-crop hemicelluloses by mixed-addenda molybdic α-Keggin-type heteropolyacids (HPAs), such as molybdovanadophosphate heteropolyacids (Mo-V-P)-HPAs of series H(n+3)[PMo(12-n)VnO40] with n = 3 and 4 (so-called HPA-3 and HPA-4), have been assessed in comparison with conventional sulfuric acid catalysis. In very diluted (20 mM) aqueous solutions of (Mo-V-P)-HPAs, up to 90% of total biomass xylan was readily converted into soluble sugars (mono and oligosaccharides) after 60 min hydrolysis reaction at 130 °C, vs only ca. 25% conversion (as a max.) for sulfuric acid solutions. Low cellulose degradation in (Mo-V-P)-HPA solutions resulted in a significant improvement in the selectivity of hydrolysis (by up to 70%) compared with sulfuric acid. The oxidative capacity of (Mo-V-P)-HPAs caused increased dissolution of lignin during the hydrolysis reaction (by up to 13% over the sulfuric acid), thus providing favorable conditions for the sustainable recovery of cellulose fibers from the xylan-free residues in subsequent biorefining stages.

Multicycle Testing of P‐Mo–V Heteropoly Acid Catalysts in Oxidation of Substituted Phenols

AbstractA series of valuable para‐quinones, such as 2,3‐ and 2,6‐dimethyl‐1,4‐benzoquinones (Me2BQs), were synthesized via oxidation of the corresponding 2,3‐ and 2,6‐dimethylphenols by using homogeneous high‐vanadium solutions of heteropoly acids: H11P3Mo18V8’O87 (HPA‐8’) and H17P3Mo16V10’O89 (HPA‐10’). These catalysts were shown to be capable of interdependent and reversible change of their acid and oxidative properties, the efficient regeneration of which can be performed for 25 min. The complete optimization of reaction conditions allowed one to reach the selectivity of the desired Me2BQs in 95–97% and shorten the oxidation time. The structural integrity of heteropoly anions present in the catalyst solutions was found to persist during the repeated performing substrate oxidation and catalyst regeneration. The catalysts showed the preservation of stability and efficiency for 25 cycles, demonstrating the prospectivity of high‐vanadium heteropoly acids in oxidation of organic substances.

Recent Advances in Regeneration of Aqueous Low-Vanadium Mo-V-Phosphoric Heteropoly Acid Solutions with O2

At present, the aqueous solutions of low-vanadium Mo−V-phosphoric heteropoly acids (HPA-1 and HPA-2) are widely used in various areas of catalysis owing to their oxidizing ability. It has been shown that in the presence of concentrated acetic acid the reduced solutions of these HPAs can be almost completely oxidized with oxygen at high reaction rate at a temperature below 100 °C and atmospheric pressure. This opens broad prospects for the development of new catalytic oxidation technologies in the presence of low-vanadium heteropoly acid solutions as reversible oxidants.

A new method for producing substituted anthraquinones via diene synthesis in the presence of Mo-V-P heteropoly acid solution: Catalyst regeneration

The possibilities of developing catalytic processes for the synthesis of 2-methylanthraquinone from 1,4-naphthaquinone (NQ) and isoprene, and synthesizing 2,3-dimethylanthraquinone from NQ and 2,3-dimethylbutadiene in the presence of aqueous solutions of high-vanadium heteropoly acid (HPA) as a bifunctional catalyst are examined. Two methods for catalyst regeneration are discussed: oxidation with oxygen at \({P_{o{}_2}}\) = 0.3–0.4 MPa and with concentrated nitric acid at atmosphere pressure. It is shown that regeneration with nitric acid ensures deeper oxidation of the reduced catalyst. Complete restoration of the properties of the catalyst following regeneration offers the possibility of its repeated use in the processes discussed in this work.

Aggregation of Heteropolyanions Implicates the Presence of Zundel Ions Near Air‐Water Interfaces

AbstractProtons play crucial roles in the interactions between heteropolyanions (HPAs) in aqueous solutions and solid acid salts. We report the aggregation behaviours of Keggin HPAs near the surfaces of heteropolyacid solutions. The structure of the aggregated HPA layer near the solution–vapour phase boundary closely resembles the solid‐state crystal structure of the heteropolyacids in which the HPAs are connected by Zundel ions. The resemblance not only implicates the presence of protons in the form of planar Zundel ions near the air‐water interface but, also, suggests that these align parallel to the interface. This study demonstrates an indirect means of assessing the impact of protons on HPA interactions near air‐water interfaces and, in general, provides new insights about interfacial proton chemistry of heteropolyacids.

Crystallization of Keggin Heteropolyanions via a Two-Step Process in Aqueous Solutions.

Although the analytical simplicity of the one-step classical theory of nucleation facilitates its use to understand crystallization processes, recent experiments and simulations have shown that many occur via multiple steps. According to the contemporary two-stage theory of nucleation, the onset of crystallization in a solution is preceded by large density fluctuations in the mother liquor that results in the formation of dense liquid-like correlated structures of the constituent solute particles. Here we report the observation of dense liquid-like correlated structures of heteropolyacid salts of α-Keggin anions (heteropolyanions) in aqueous solutions as volume is decreased long before the onset of crystallization by in situ time-dependent small-angle X-ray scattering measurements. Experiments were performed on drying drops of solutions of heteropolyacids to monitor their ordering before and during the onset of their crystallization. A continuous change in the density of the correlated structures is observed up to the onset of crystallization. Moreover, the correlated structures and the onset of crystallization are found to depend upon the charge of the heteropolyanions. The crystals formed within the drying drops of solutions during the crystallization process are found to be metastable polymorphic structures that are different from the stable crystal structures obtained after complete drying of the drops. Our results support a two-step process and Ostwald's rule of stages for the crystallization of heteropolyanions in their aqueous solutions upon evaporation.

New Method for Production of Substituted Anthraquinones via Diene Synthesis in the Presence of Mo-V-P Heteropoly Acid Solution: Catalyst Regeneration

Feasibility of catalytic synthesis of 2-methylanthraquinone from 1,4-naphthaquinone (NQ) and isoprene and of synthesis of 2,3-dimethylanthraquinone from NQ and 2,3-dimethylbutadiene in the presence of an aqueous solution of high-vanadium heteropoly acid (HPA) as the bifunctional catalyst was demonstrated. Two methods for the catalyst regeneration are discussed: oxidation with oxygen at PO2 = 0,3÷0,4 MPa and with nitric acid at atmospheric pressure. It is shown that the regeneration with nitric acid provides deeper oxidation of the reduced catalyst. The full catalyst recovery after the regeneration allows its recycling in the processes under discussion.

Prospects of application of Mo–V-phosphoric heteropolyacid solutions as bifunctional catalysts for syntheses of anthraquinones and their substituted derivatives

The acid-catalyzed condensation of 1,3-butadienes with p-quinones and oxidation of the obtained adducts can be carried out as one-pot process in the presence of aqueous solutions of Mo–V–P-heteropolyacids (HPA) of the general composition HaPzMoyVxOb. These solutions act as bifunctional catalysts, since they simultaneously behave as strong Bronsted acids and fairly strong reversible oxidants. The condensation of 1,4-naphthoquinone (NQ) with 1,3-butadiene in solutions of high-vanadium HPA of the empirical compositions H15P4Mo18V7O89 and H17P3Mo16V10O89 in the presence of water-miscible organic solvents (acetone, 1,4-dioxane) affords 9,10-anthraquinone in a yield of ~70% and purity up to 97% at a complete conversion of NQ. Under similar conditions, the reactions of NQ and substituted 1,3-butadienes afford substituted anthraquinones in the yields up to 90% and purity up to 99%. The catalysts are regenerated with oxygen in an individual step and can be reused.

Self-assembly heteropoly acid catalyzed oxidative desulfurization of fuel with oxygen

The self-assembly heteropoly acid solution can be directly used for catalyzing oxidative desulfurization (ODS) of model fuel with O2 as oxidant in high sulfur removal. The sulfur compounds in model fuels can all be nearly completely oxidized. The oxidation mechanism of thiophenic sulfurs is proposed. In the ODS process, the self-assembly heteropoly acid solution acts as both the catalyst and the excellent extracting agent, and the oxidation and extraction are performed simultaneously.

Homogeneous catalysts of redox reactions based on heteropoly acid solutions. II. Synthesis of catalyst for pilot industrial production of methylethylketone

The homogeneous catalyst used in the 2005–2006 scale-up of the oxidation of n-butylenes into methylethylketone to the industrial level is a chloride-free aqueous solution of palladium complexes in a modified 0.25 M solution of Mo-V-P heteropoly acid with the molecular formula H12P3Mo18V7O85 (HPA-7′). Its advantages are increased oxidative capacity in the primary oxidation of n-C4H8 and increased thermal stability, which allows quick catalyst regeneration with atmospheric oxygen at 160–170°C. The paper describes the preparation of a pilot lot of the catalyst with a total volume of 50 L; the starting substances were V2O5, MoO3, and H3PO4. The key point of the synthesis was dissolving V2O5 while stirring in a dilute and cooled H2O2 solution. This formed V(V) peroxide complexes, which decompose at elevated temperature to give a 0.0175 M H6V10O28 solution. This solution was stabilized by adding a calculated amount of H3PO4 to give a more stable 0.0125 M H9PV14O42 solution. Since the H9PV14O42 solution occupied a large volume, its synthesis was performed three times in a 300-L reactor. In the main 500-L reactor, MoO3 was dissolved in water with stirring, while adding the remaining portion of H3PO4. The resulting mixture was evaporated, gradually introducing all of the previously obtained portions of the dilute H9PV14O42 solution. The resulting HPA-7′ solution was evaporated to ∼100 L and filtered twice, separating the insignificant amount of the precipitate. The filtered solution was again evaporated to 50 L, and a calculated amount of PdCl2 was added to it while stirring at 70–80°C. A total of 27 lots of the (Pd + 0.25 M HPA-7′) catalyst with a total volume of 1350 L were obtained. All apparatuses of the pilot industrial unit for MEK synthesis were filled with this catalyst.

One-pot process for preparing substituted anthraquinones via diene synthesis in the presence of solutions of Mo-V-P heteropoly acids

The acid catalyzed condensation of substituted 1,3-butadienes with para-quinones and the oxidation of the resulting adducts can be conducted as one operation using aqueous solutions of Mo-V-P heteropoly acids (HPAs) of the general composition HaPzMoyVxOb. Being strong Bronsted acids and at the same time reverse oxidizers, these solutions have bifunctional catalytic properties. The condensation of 1,4-naphthoquinone (NQ) with 1,3-butadiene in solutions of high-vanadium HPAs with the molecular formulas H15P4Mo18V7O89 and H17P3Mo16V10O89 using a water-miscible organic solvents (acetone and 1,4-dioxane) gives 9,10-anthraquinone (AQ) with yields of 70% and purities of up to 97% upon the complete conversion of NQ. The reaction between NQ and substituted 1,3-butadienes under similar conditions allows us to obtain substituted anthraquinones with yields of up to 90% and purities of up to 99%. The catalysts are regenerated with oxygen in a separate stage and used multiple times.