Heteropolyacid Catalysts Research Articles

Research on catalytic oxidation of dimethyl ether to dimethoxymethane over MnCl2 modified heteropolyacid catalysts

The catalytic oxidation of dimethyl ether (DME) to dimethoxymethane (DMM) was carried out over MnCl2 modified different heteropolyacid catalysts. Kinds of supports and the catalyst packing structure were investigated. Besides, the possible reaction pathway of DME direct-oxidation to DMM was also discussed. The results showed H4SiW12O40/SiO2 modified with MnCl2 exhibited a better catalytic performance than other heteropolyacids. SiO2 is the better supporting materials than the materials used as support. The synergistic effects of the acid sites offered by H4SiW12O4 and the redox sites mainly offered by MnO2 significantly improve the catalytic activity of the catalyst.

Direct preparation of dichloropropanol (DCP) from glycerol using heteropolyacid (HPA) catalysts: A catalyst screen study

Abstract Direct preparation of dichloropropanol (DCP) from glycerol was carried out in a batch reactor using H3PMo12−XWXO40 (X = 0–12), H4SiMo12−XWXO40 (X = 0–12), H3+XPW12−XVXO40 (X = 0–3), and H3+XPMo12−XVXO40 (X = 0–3) heteropolyacid (HPA) catalysts. Acid properties of the HPA catalysts were then determined by NH3-TPD measurements in order to correlate the catalytic activity with the acid property of the catalysts. The acid strength of the HPA catalyst played a key role in determining the catalytic performance in the reaction. The selectivity for DCP increased with increasing acid strength of the catalyst. Among the catalysts tested, H3PW12O40 with the highest acid strength showed the best catalytic performance.

Photocatalytic decomposition of a perfluoroether carboxylic acid by tungstic heteropolyacids in water

The photochemical decomposition of perfluoroether carboxylic acid C2F5OC2F4OCF2COOH (EEA), which is being introduced in industry as a surfactant alternative to environmentally persistent and bioaccumulative perfluorocarboxylic acids, was investigated by use of water-soluble tungstic heteropolyacids H3PW12O40 and H4SiW12O40, and the results were compared with those of conventional techniques, UV–visible light irradiation in the presence of H2O2, treatment with Fenton's reagent, and heterogeneous TiO2 photocatalyst. H4SiW12O40 induced little photochemical decomposition of EEA. Irradiation with UV–visible light in the presence of H2O2 and treatment with Fenton's reagent also led to little decomposition of EEA. Alternatively, irradiation of EEA at wavelengths of >290nm in the presence of H3PW12O40 under an oxygen atmosphere efficiently decomposed EEA to F− and CO2. The reaction followed pseudo-first-order kinetics, and the decomposition rate was almost identical with that of the corresponding perfluorocarboxylic acid, that is, C5F11COOH. The proposed reaction mechanism involves redox reactions between EEA, the heteropolyacid catalyst, and oxygen, followed by CC bond cleavage between C2F5OC2F4OCF2 and COOH (induced by the photo-Kolbe mechanism), and subsequent formation of C2F5OCF2COOH and trifluoroacetic acid. EEA was also photocatalytically decomposed by TiO2 with a rate constant comparable with that by H3PW12O40. Although the reactivity of TiO2 toward perfluorocarboxylic acids was reported to be very low, EEA, which has ether linkages in the perfluoroalkyl chain, was clearly decomposed to F− and CO2. This fact suggests that oxidative active species other than OH radicals significantly participate in the reaction.

Improved catalytic activity using water for isomerization of linear butene to isobutene over heteropolyacid catalysts

Water minimized a decrease in conversion for the isomerization of linear butene to isobutene on silica-supported heteropolyacid (H 4SiW 12O 40) catalysts while maintaining high selectivity toward isobutene. The role of water was related to surface acidic properties (number of surface acid sites and acid strength) and characteristics of hydrocarbon deposits (extent of deposits and hydrogen/carbon ratio), as determined by benzonitrile and ammonia temperature-programmed desorption as well as temperature-programmed oxidation of fresh and spent catalysts. It appears that highly unsaturated hydrocarbon deposits consisting of 8–12 carbon units covered surface acid sites, especially strong acid sites, limiting the isomerization reaction. Water in the feed was helpful in preserving the acid sites and decreasing the number of carbon units in the hydrocarbon deposits. Thus, conversion occurred more readily in the presence of water than in the absence of water.

Chemical Immobilization of H5PMo10V2O40 (PMo10V2) Catalyst on Nitrogen-rich Macroporous Carbon (N-MC) for Use as an Oxidation Catalyst

Nitrogen-rich macroporous carbon (N-MC) was prepared using melamine-formaldehyde resin as a carbon precursor. The surface of N-MC was then modified to have a nitrogen-derived functional group (amine group), and thus, to provide an anchoring site for heteropolyacid (HPA) catalyst. By taking advantage of the overall negative charge of [PMo10V2O40]5−, H5PMo10V2O40 (PMo10V2) HPA catalyst was chemically immobilized on the N-MC support as a charge matching component. It was found that PMo10V2 was finely dispersed on the N-MC support. In the model vapor-phase 2-propanol conversion reaction, the PMo10V2/N-MC catalyst showed a remarkably enhanced oxidation catalytic activity and a suppressed acid catalytic activity compared to the mother catalyst. The PMo10V2/N-MC catalyst served as an efficient oxidation catalyst in the reaction where both acid and oxidation reactions occurred simultaneously.

Heterocyclic amine salts of Keggin heteropolyacids used as catalyst for the selective oxidation of sulfides to sulfoxides

A range of sulfides can be selectively oxidized to the corresponding sulfoxides in good yields using catalytic quantities of heterocyclic amine salts (quinoline, cinchonine, and cinchonidine) of Keggin heteropolyacids with different green oxidants. The cinchonine heteropolyacid catalyst is easily recoverable and reusable without loss of its catalytic activity, and an enantiomeric excess can be obtained.

Correlation between reduction potential of vanadium-containing heteropolyacid (HPA) catalysts and their oxidation activity for cyclohexanol dehydrogenation

Vanadium-containing H6+xP2Mo18−xVxO62 (x = 0, 1, 2 and 3) Wells-Dawson heteropolyacid (HPA) and H3+xPMo12−xVxO40 (x = 0, 1, 2 and 3) Keggin HPA catalysts were applied to the vapor-phase dehydrogenation of cyclohexanol. The catalytic oxidation activity showed a volcano-shaped curve with respect to vanadium substitution for both families of HPA catalysts. The Wells-Dawson HPA showed a better catalytic oxidation performance than the Keggin HPA at the same level of vanadium substitution.

Hydroxylation of Benzene with Hydrogen Peroxide over Highly Efficient Molybdovanadophosphoric Heteropoly Acid Catalysts

Keggin type molybdovanadophosphoric heteropoly acids, H 3+ n PMo 12- n V n O 40 ( n = 1–3), were prepared by a novel environmentally benign method, and their catalytic performances were evaluated via hydroxylation of benzene to phenol with hydrogen peroxide as oxidant in a mixed solvent of glacial acetic acid and acetonitrile. Various reaction parameters, such as reaction time, reaction temperature, ratio of benzene to hydrogen peroxide, concentration of aqueous hydrogen peroxide, ratio of glacial acetic acid to acetonitrile in solvent and catalyst concentration, were changed to obtain an optimal reaction conditions. H 3+ n PMo 12- n V n O 40 ( n = 1–3) are revealed to be highly efficient catalyst for hydroxylation of benzene. In case of H 5PMo 12- n V n O 40, a conversion of benzene of 34.5% with the selectivity of phenol of 100% can be obtained at the optimal reaction conditions.

Reduction potentials and UV–visible absorption edge energies of H6P2Mo W18−O62 heteropolyacid (HPA) catalysts as a probe of oxidation catalysis for ethanol conversion reaction

Reduction potentials of H6P2MoxW18−xO62 Wells–Dawson heteropolyacid (HPA) catalysts were measured electrochemically. UV–visible spectra of H6P2MoxW18−xO62 catalysts were then obtained in order to explore the reduction potentials of the HPA catalysts. Absorption edge energies shifted to lower values and reduction potentials increased with increasing molybdenum substitution. An HPA catalyst with a high reduction potential showed a low absorption edge energy in the UV–visible spectrum. The absorption edge energies could be utilized as a correlation parameter for the reduction potentials of the H6P2MoxW18−xO62 catalysts, and furthermore, as a probe of oxidation catalysis of the HPA catalysts.

Solvent‐Free Approach to 3,4‐Dihydropyrimidin‐2(1H)‐(thio)ones: Biginelli Reaction Catalyzed by a Wells–Dawson Reusable Heteropolyacid

The Biginelli reaction is performed in an efficient, simple, solvent‐free procedure, using a small amount of H6P2W18O62 · 24H2O, a reusable heteropolyacid catalyst with Wells–Dawson structure. Both aromatic and aliphatic aldehydes, and different β‐dicarbonyl compounds and urea or thiourea, were used as starting materials. Seventeen examples of dihydropyrimidinones and dihydropyrimidinethiones were prepared by heating the reactants either in refluxing acetonitrile or in the absence of solvent. Operational simplicity, the use of a noncorrosive, reusable catalyst in solventless conditions, short reaction times, and very good to excellent yields in most of the selected examples are the main advantages of the method.

Chemical Immobilization of Heteropolyacid Catalyst on Inorganic Mesoporous Material for use as an Oxidation Catalyst

Recent progress on the chemical immobilization of heteropolyacid (HPA) catalyst on inorganic mesoporous material is reported in this review. Mesostructured cellular foam silica, mesoporous carbon, and nitrogen-containing mesoporous carbon were used as supporting materials. The mesoporous materials were modified to have a positive charge, and thus, to provide sites for the immobilization of HPA catalyst. By taking advantage of the overall negative charge of heteropolyanion, the HPA catalyst was chemically immobilized on the surface-modified mesoporous material as a charge-compensating component. Characterization results showed that the HPA catalyst was finely and molecularly dispersed on the surface of mesoporous material via strong chemical immobilization, and that the pore structure of mesoporous material was still maintained even after the immobilization of HPA catalyst. The supported HPA catalysts were applied to the model vapor-phase ethanol conversion, 2-propanol conversion, and methacrolein oxidation reactions. The supported HPA catalyst showed a better oxidation catalytic activity than the unsupported HPA catalyst in the model reactions. The enhanced oxidation catalytic performance of the supported HPA catalyst was attributed to the finely dispersed HPA catalyst, which was chemically immobilized on the positive site of mesoporous material by sacrificing its proton (Bronsted acid site). The HPA catalyst chemically immobilized on mesoporous material served as an excellent oxidation catalyst.

Reduction potentials of H3+PMo12–V O40 and H6+P2Mo18−V O62 heteropolyacid (HPA) catalysts and their catalytic activity for the vapor-phase oxidative dehydrogenation of isobutyric acid

Abstract Vanadium-containing H6+xP2Mo18−xVxO62 (x = 0, 1, 2, and 3) Wells-Dawson heteropolyacid (HPA) catalysts were prepared for use in the oxidative dehydrogenation of isobutyric acid (IBA) to methacrylic acid (MAA). Vanadium-containing H3+xPMo12−xVxO40 (x = 0, 1, 2, and 3) Keggin HPA catalysts were also investigated for the purpose of comparison. The reduction potentials of H3+xPMo12−xVxO40 and H6+xP2Mo18−xVxO62 catalysts were determined by temperature-programmed reduction (TPR) measurements. The reduction potentials of H3+xPMo12−xVxO40 and H6+xP2Mo18−xVxO62 catalysts showed volcano-shaped curves and exhibited the same trend with respect to vanadium substitution. The conversions of IBA over H3+xPMo12−xVxO40 and H6+xP2Mo18−xVxO62 catalysts also showed volcano-shaped curves and exhibited the same trend with respect to vanadium substitution. However, the H6+xP2Mo18−xVxO62 catalysts showed a higher reduction potential and a higher conversion of IBA than the H3+xPMo12−xVxO40 catalysts at the same level of vanadium substitution. A correlation between reduction potential and catalytic activity of the HPA catalysts clearly demonstrated that the conversion of IBA increased monotonically with increasing reduction potential of the HPA catalysts, across both families of vanadium-containing HPA catalysts examined in this work.

Structure–activity relations in Cs-doped heteropolyacid catalysts for biodiesel production

A series of insoluble heteropolytungstate (H 3PW 12O 40 HPW) salts, Cs x H 3− x PW 12O 40 ( x = 0.9 – 3 ), were synthesized and characterized using a range of bulk and surface sensitive probes including N 2 porosimetry, powder XRD, FTIR, XPS, 31P MAS NMR, and NH 3 calorimetry. Materials with Cs content in the range x = 2.0 – 2.7 were composed of dispersed crystallites with surface areas ∼100 m 2 g −1 and high Brönsted acid strengths [ Δ H ads 0 ( NH 3 ) = − 150 kJ mol −1 ], similar to the parent heteropolyacid. The number of accessible surface acid sites probed by α-pinene isomerization correlated well with those determined by NH 3 adsorption calorimetry and surface area measurements. Cs x H 3− x PW 12O 40 were active toward the esterification of palmitic acid and transesterification of tributyrin, important steps in fatty acid and ester processing for biodiesel synthesis. Optimum performance occurs for Cs loadings of x = 2.0 – 2.3 , correlating with the accessible surface acid site density. These catalysts were recoverable with no leaching of soluble HPW.

Effect of different Mn salt precursors on Mn-H 4SiW 12O 40/SiO 2 used for dimethoxymethane synthesis from dimethyl ether oxidation

Mn-H 4SiW 12O 40/SiO 2 heteropolyacid catalysts were prepared by impregnation method from different Mn salt precursors, such as MnSO 4, Mn(NO 3) 2, MnCl 2 and Mn(CH 3COO) 2. The catalytic oxidation reaction of dimethyl ether (DME) to dimethoxymethane (DMM) was carried out in a continuous flow type fixed-bed reactor with a ratio of N DME/ N O 2 = 1:1. It is found that the sequence of catalytic activity for DMM synthesis is MnCl 2-H 4SiW 12O 40/SiO 2>Mn(NO 3) 2-H 4SiW 12O 40/SiO 2>MnSO 4-H 4SiW 12O 40/SiO 2>Mn(AC) 2-H 4SiW 12O 40/SiO 2. The effects of reaction temperature (573 −633 K) on the catalysts were also investigated. With the increase of reaction temperature, the DME oxidation reaction is more exquisite over MnSO 4-H 4SiW 12O 40/SiO 2 catalyst. 42.4% of DME conversion and 0.9% of DMM selectivity have also been obtained at 613 K. However, MnCl 2 modified H 4SiW 12O 40/SiO 2 catalyst obtains higher DMM selectivity (37.5%, at 593 K) than other three catalysts at mild reaction conditions. H 2-TPR profiles show that MnSO 4 modification demonstrates stronger oxidative performance at high temperature than other catalysts, while MnCl 2-H 4SiW 12O 40/SiO 2 catalyst exhibits better oxidative performance at low temperature. XRD patterns of the catalysts show that the diffraction peaks are strong and MnO 2 diffraction peak is also found over the MnCl 2 modified catalyst.

Heteropolyacid: An efficient and eco-friendly catalyst for the synthesis of 14-aryl-14H-dibenzo[a,j]xanthene

A simple and efficient catalytic procedure for the synthesis of 14-aryl-14H-dibenzo[a,j]xanthene is reported. Tungsten heteropoly acid catalysts are used both in bulk or supported on silica gel under solvent-free conditions at 100 °C. Good yields and short reaction time are advantages of this new method.

Ethylene and diethyl-ether production by dehydration reaction of ethanol over different heteropolyacid catalysts

Dehydration reaction of ethanol was investigated in a temperature range of 140–250 °C with three different heteropolyacid catalysts, namely tungstophosphoricacid (TPA), silicotungsticacid (STA) and molybdophosphoricacid (MPA). Very high ethylene yields over 0.75 obtained at 250 °C with TPA was highly promising. At temperatures lower than 180 °C the main product was diethyl-ether. Presence of water vapor was shown to cause some decrease of catalyst activity. Results showing that product selectivities did not change much with the space time in the reactor indicated two parallel routes for the production of ethylene and DEE. Among the three HPA catalysts, the activity trend was obtained as STA > TPA > MPA .

Immobilization of a heteropolyacid catalyst on the aminopropyl-functionalized mesostructured cellular foam (MCF) silica

Mesostructured cellular foam (MCF) silica with high surface area (>600 m 2/g) and large pore volume (≈1.16 cm 3/g) was synthesized via a surfactant templating method. The MCF silica was then modified by grafting 3-aminopropyl-triethoxysilane (APTES) to create a positive charge on the surface, and thus, to provide sites for the immobilization of H 3PMo 12O 40. By taking advantage of the overall negative charge of [PMo 12O 40] 3−, the H 3PMo 12O 40 catalyst was chemically immobilized on the aminopropyl group of the surface modified MCF silica as a charge matching component. The mesopore structure of MCF silica was maintained even after the surface modification step and the subsequent immobilization step of H 3PMo 12O 40. The H 3PMo 12O 40 species were finely and molecularly dispersed on the surface modified MCF silica via chemical immobilization.

A Method for the Acetylation of Alcohols Catalyzed by Heteropolyoxometallates

Esterifications of acetic acid with some linear, secondary, tertiary, and benzylic alcohols mediated by catalytic amounts of Keggin, Wells–Dawson, and Preyssler type heteropolyacids were carried out under reflux at mild reaction conditions with good to excellent yields. Among the examined catalysts, H3PW12O40 and H14NaP5W30O110 revealed better results than other heteropolyacids. This work was performed with the aim of simplifying the esterification process by omitting any solvents and mineral acid catalysts. Easy work-up, low cost, and acidic waste reduction, which are all important features from the environmental and economical points of view, are distinct aspects of this protocol. Heteropolyacid catalysts could be separated after a simple work-up and reused for several times.

Reduction potentials of heteropolyacid catalysts probed by scanning tunneling microscopy and UV-visible spectroscopy

Reduction potentials of heteropolyacid (HPA) catalysts were probed by scanning tunneling microscopy (STM) and UV-visible spectroscopy. Correlations between reduction potentials and NDR (negative differential resistance) peak voltages, and between reduction potentials and absorption edge energies of HPA catalysts were established. The reduction potentials of HPA catalysts have been shown to follow similar trends to the NDR peak voltages of selfassembled HPA monolayers and the absorption edge energies of bulk HPAs. In the UV-visible spectra of HPA catalysts, lower absorption edge energies corresponded to higher reduction potentials of the HPAs.

Preparation and Characterization of Surface Modified MCF (Mesostructured Cellular Foam) Silica for the Immobilization of Heteropolyacid Catalyst

Mesostructured cellular foam (MCF) silica was synthesized via surfactant templating route. The surface of MCF silica was then modified by grafting 3-aminopropyl-triethoxysilane (APTES) to have the positive charge, and thus, to provide sites for the immobilization of H3PMo12O40 (PMo12). By taking advantage of the overall negative charge of [PMo12O40]3-, PMo12 catalyst was chemically immobilized on the amine-functional group of surface modified MCF (SM-MCF) silica as a charge matching component. It was found that pore structure of MCF silica was maintained even after the surface modification and the subsequent immobilization of PMo12. It was also revealed that PMo12 species were finely and molecularly dispersed on the SM-MCF silica via chemical immobilization.