Keggin-type Heteropolyacids Research Articles

Vibration Spectroscopy Stability Investigation of 12-Tungstosilicic Acid Solution

Heteropoly acids (HPA) attract the attention of large variety of scientists, due to HPA’s extraordinary interesting properties and possible application fields. 12-tungstosilicic acid (WSiA), the Keggin type HPA, has some promising characteristic to be used in catalytic processes, but with not well-defined stability. Raman spectroscopy was used for in situ analysis of WSiA hydrolysis in detail in a wide pH range of 1–12. Raman spectroscopy is able to give an almost immediate response/spectrum as a representation of the exact profile/composition of the solution. This method and FTIR spectroscopy, as a complementary technique, enabled recording of the solid and liquid phases of the same sample under different conditions. Our results confirm that the decomposition pathways of WSiA in solution proceed via the formation of the lacunary monovacant anion at pH > 6.4. This anion is a major constituent in pH range up to 9.5. With further increases in pH this species convert to the trivacant lacunary anion. The total decomposition of the Keggin anion to silicate and tungstate occurs at pH > 11.0. The results of the performed study contribute to understand the behavior of WSiA in the water–methanol solution, as the model system of aqueous-organic system. It is concluded that addition of methanol in aqueous solution of WSiA leads to expansion of the pH region where Keggin anion is stable up to 8.1 and above this pH value, precipitation occurs. The obtained data clarify the stability range of WSiA in both water and water–methanol solutions, as well.

Importance of Support and Interactions with Noble Metal Nanocenters in Electrocatalysis

Recently there has been growing interest in low-temperature fuel cells as alternative sources of energy. The most commonly considered electrocatalytic systems (e.g. for oxygen reduction) utilize precious platinum and, therefore, there is a need to minimize contents of Pt (e.g. by activating it with certain metal oxo-species including polyoxometallates of tungsten) as well as to look for alternate molecular catalysts comprising metalloporphyrins or N-chelates combined with Ru-based chalcogenides, and bifunctional materials inducing reduction of both oxygen and hydrogen peroxide intermediate. Broad utilization of hydrogen-oxygen fuel cells would require development of effective means of hydrogen production and storage. In this context, the visible-light induced photelectrocatalytic water splitting is an important concept that will be addressed in this presentation as well. We have found that modification of a regular mesoporous WO3 film with colloidal gold nanoparticles resulted in a significant improvement of the delivered photocurrent. Enhanced optical properties of WO3 implemented with colloidal gold nanoparticles are induced by plasmonic excitation of gold nanoparticles.The problem of oxygen reduction is even more severe case of alcohol-utilizing fuel cells. In this respect, heat-treated carbon-supported macrocyclic complexes and their derivatives as well as chalcogen modified transition metals are promising and selective (e.g. methanol tolerant) platinum-free catalysts for oxygen reduction.Our research interests also concern development of electrocatalytic systems for the reduction of carbon dioxide. For example, as catalytic materials commercial palladium nanoparticles and the macromolecular complex of palladium, namely [Pd(C14H12N2O3)Cl2]2∙MeOH, were considered. The electrochemical data clearly imply that the process of electroreduction of carbon dioxide begins at the less negative potentials when pursued at the palladium complex in addition to the significant enhancement of the net (background-subtracted) CO2-reduction current densities relative to those observed at conventional nanostructured palladium. Among important issues are specific interactions between nitrogen coordinating centers and electrogenerated metallic palladium sites at the electrocatalytic interface.We have interest in the better understanding and controlling of electrocatalytic phenomena appearing in the direct alcohol fuel cells as alternative technology to hydrogen based electrochemical energy systems. The most common Pt anodes are readily poisoned by the strongly adsorbed intermediates, namely by CO-type species, requiring fairly high overpotentials for their removal. We have recently demonstrated that catalytic activity of platinum-based nanoparticles towards electrooxidation of ethanol has been significantly enhanced through interfacial modification with ultra-thin monolayer-type films of polyoxometallates particularly heteropolymolybdates. Also gold in combination with platinum has been demonstrated to produce novel high performance bimetallic catalysts. We are going to describe here a concept of utilization of functionalized or mixed titanium dioxide, tungsten oxide or zirconium oxide matrices for supporting and activating noble metal nanoparticles (Pt, Pt-Ru or Pt-Rh) during electrooxidation of ethanol. We are going to consider protonically/electronically conducting zeolite-type cesium salts of Keggin-type heteropolyacids of molybdenum and tungsten as matrices for Vulcan-supported Pt, PtRu and PtSn nanoparticles during electrooxidation of methanol and ethanol.

Development and Characterization of Nanostructured Hybrid Materials for Oxidation of Ethanol in Fuel Cells and Biofuel Cells

Our research interests concern development of the direct alcohol fuel cells as alternative technology to hydrogen based electrochemical energy systems. For example, ethanol can be ideally oxidized to carbon dioxide thus delivering 12 electrons.But realistically the reaction is rather slow at ambient conditions. Obviously, there is a need to develop novel electrocatalytic materials. Platinum has been recognized as the most active catalytic metal towards oxidation of ethanol at low and moderate temperatures. But Pt anodes are readily poisoned by the strongly adsorbed intermediates, namely by CO-type species, requiring fairly high overpotentials for their removal. To enhance activity of Pt catalysts towards methanol and ethanol oxidation, additional metals (one or two), such as Ru, Sn, Mo, W or Rh, are usually introduced as the alloying component. More recently it has been demonstrated that catalytic activity of platinum-based nanoparticles towards electrooxidation of ethanol has been significantly enhanced through interfacial modification with ultra-thin monolayer-type films of polyoxometallates, particularly heteropolymolybdates. Also gold in combination with platinum has been demonstrated to produce novel high performance bimetallic catalysts.We are going to describe a concept of utilization of functionalized titanium dioxide, tungsten oxide or zirconium oxide matrices (by admixing them with polyoxometallate-modified gold nanoparticles) for supporting and activating noble metal nanoparticles (Pt, Pt-Ru or Pt-Rh) during electrooxidation of ethanol. Remarkable increases of electrocatalytic currents measured under voltammetric and chronoamperometric conditions have been observed. The most likely explanation takes into account improvement of overall conductivity (due to the presence of nanostructured gold) at the electrocatalytic interface (formed by metal oxide support), as well as and possibility of specific Pt-metal oxide or Pt-Au electronic interactions and existence of active hydroxyl groups on transition metal oxo species.in the vicinity of catalytic Pt sites. Further, the capping layers of Keggin-type phosphomolybdates, which are known to undergo fast stepwise multi-electron redox processes as well as to activate Pt-based electrocatalysts during ethanol oxidation, may also contribute to the overall enhancement effect.Having in mind a wide spectrum of powerful catalytic activities of zeolites, it is reasonable to expect that application of certain protonically/electronically conducting zeolite-type matrices shall lead to enhancement of the overall reactivity of anode electrocatalysts. Among potential candidate materials for rigid and active matrices in electrocatalysis, there are cesium salts of Keggin-type heteropolyacids of molybdenum and tungsten. These salts are insoluble in water and organic solvents, and they posses micro and mesopores of high surface area (often exceeding 100 m2 g-1). Contrary to typical alkali-exchanged zeolites, the analogous heteropolyacid salts retain their strong acidity. In the present work, we describe preparation and physicochemical (including electrochemical) properties of cesium salts of selected Keggin-type heteropolyacids of molybdenum and tungsten, as well as address their influence on catalytic activity of Vuclcan-supported Pt, PtRu and PtSn nanoparticles (Pt40%/Vulcan XC-72) during electrooxidation of methanol and ethanol.Oxidation of ethanol can also be pursued under neutral conditions in so called biofuel cells. In our work, we have exploited unique characteristics of multi-walled carbon nanotubes (CNTs) to construct the efficient anodic bioelectrocatalytic system of potential utility for biofuel cells and biosensors. Some attention has been also paid to the choice of the modifying agent poly(diallyldimethylammonium chloride) (PDDA) used to stabilize CNTs. While PDDA seems to form stable ultra-thin layers on CNTs, it is not expected to serve as a good charge (electron) mediator but it is reasonable to expect that PDDA would interact or attract electrostatically with negatively charged sites of the enzyme (or combination of dehydrogenase enzymes). Our highly porous CNT-based films have presumably acted as three-dimensional network of nanowires around the enzyme molecules and have promoted the efficient electron transfer. On the whole, combination of PDDA-modifed MWCNTs and two distinct dehydrogenase enzymes within the film has produced a catalytic system capable of oxidation of ethanol at fairly low potentials (ca. 0.15 V versus Ag/AgCl). Among important issues are good stability and reproducibility of responses both under voltammetric and amperometric conditions.

Comparative study of photoinduced wettability conversion between [PW12O40]3−/brookite and [SiW12O40]4−/brookite hybrid films

Two tungsten-based Keggin-type heteropolyacids (PW12: ([PW12O40]3−) and SiW12: ([SiW12O40]4−)) were hybridized with brookite-type TiO2. Then photocatalytic decomposition activity, photoinduced hydrophilicity, and sustainability of the hydrophilicity in the dark were evaluated using gaseous 2-propanol (IPA) decomposition and sessile drop method. The obtained films were transparent in the visible wavelength range. Both hybrid films exhibited higher photocatalytic decomposition activity and had higher photoinduced hydrophilicizing rates than pure brookite films under UV illumination. The PW12/TiO2 film exhibited better photocatalytic performance than the SiW12/TiO2 film did. Atmosphere dependence, XPS analysis, and electrochemical experiments indicated the cause of these two films' different levels of sustainability of hydrophilicity to be differences in their electron storage capability. Results show that the electron scavenger capability and reoxidation efficiency of the heteropolyacid are key factors affecting the overall performance of wettability conversion of this hybrid film system before and after UV illumination.

Deep oxidative desulfurization of dibenzothiophene with molybdovanadophosphoric heteropolyacid-based catalysts

Three hydrophobic Keggin-type heteropolyacid catalysts, [C3H3N2(CH3)(C2H4)]5PMo10V2O40 ([C2mim]PMoV), [C3H3N2(CH3)(C4H8)]5PMo10V2O40 ([C4mim]PMoV) and [C3H3N2(CH3)(C6H12)]5PMo10V2O40 ([C6mim]PMoV), were synthesized by reacting molybdovanadophosphoric acid with imidazolium bromides, and characterized by spectroscopic methods. Their use as catalysts in the extractive catalytic oxidative desulfurization process using hydrogen peroxide as the oxidant and acetonitrile as phase transfer agent was studied. The catalytic properties decreased in the order: [C6mim]PMoV > [C4mim]PMoV > [C2mim]PMoV. The main factors influencing the rate of removal of dibenzothiophene (DBT) were investigated, including reaction temperature, the amounts of catalyst, H2O2 and acetonitrile. Nearly 100 % sulfur removal rate was achieved under optimal conditions. The catalyst could be recycled six times with only a slight decrease in activity. A reaction mechanism for DBT oxidation is proposed, in which the Keggin anions first obtain active oxygen from H2O2, then the DBT is oxidized to dibenzothiophene sulfones.

Two proton-conductive hybrids based on 2-(3-pyridyl)benzimidazole molecules and Keggin-type heteropolyacids

Two proton-conductive organic/inorganic complexes were constructed by Keggin-type heteropolyacids and 2-(3-pyridyl)benzimidazole molecules. Single-crystal X-ray diffraction analyses revealed that two complexes crystallized in the monoclinic space group P21/c, exhibited different unit cell parameters, and presented different hydrogen-bonded networks constructed by 2-(3-pyridyl)benzimidazole molecules, [PMo12O40]3− anions and solvent molecules. The results of thermogravimetric analyses suggest that two supramolecular complexes have different thermal stability based on the different hydrogen-bonded networks. Two complexes at 100°C under 35–98% relative humidity showed a good proton conductivity of about 10−3Scm−1. The proton conductivities of two complexes under 98% relative humidity both increase on a logarithmic scale with temperature range from 25 to 100°C. At 100°C, both complexes showed poor proton conductivities of 10−8–10−9S cm−1 under acetonitrile or methanol vapor.

Thermal decomposition of lignocellulosic biomass in the presence of acid catalysts

Transformation of lignocellulosic biomass to biofuels involves multiple processes, in which thermal decomposition, hydrotreatment are the most central steps. Current work focuses on the impact of several solid acids and Keggin-type heteropolyacids on the decomposition temperature (Td) of pine wood and the characterization of the resulted products. It has been observed that a mechanical mixture of solid acids with pine wood has no influence on Td, while the use of heteropolyacids lower the Td by 100°C. Moreover, the treatment of biomass with a catalytic amount of H3PW12O40 leads to formation of three fractions: solid, liquid and gas, which have been investigated by elemental analysis, TGA, FTIR, GC-MS and NMR. The use of heteropolyacid leads, at 300°C, to a selective transformation of more than 50 wt.% of the holocellulose part of the lignocellulosic biomass. Moreover, 60 wt.% of the catalyst H3PW12O40 are recovered.

Synthesis of quinoxalines in the presence of heteropoly acids

Article history: Received October 25, 2012 Received in Revised form December 6, 2012 Accepted 19 January 2013 Available online 19 January 2013 Efficient synthesis of quinoxaline derivatives from the reaction of α-diketones and ophenylenediamines in the presence of Keggin-type heteropolyacids (HPA) such as H3PMo12O40, H4SiW12O40, K7PMo2W9O40, H3PW12O40.SiO2 and H3PW12O40 in high yields and short reaction times, and at room temperature is introduced. © 2013 Growing Science Ltd. All rights reserved.

Controlled immobilization of Keggin-type heteropoly acids on the surface of silica encapsulated γ-Fe2O3 nanoparticles and investigation of catalytic activity in the oxidative esterification of arylaldehydes with methanol

Preparation, leaching and solid acidity measurements of H3PW12O40, H3PMo12O40 and H4SiW12O40 supported on silica-encapsulated γ-Fe2O3 nanoparticles were performed. The impregnating solvent and calcination temperature were optimized using various techniques. Catalytic activity of samples was examined by carrying out oxidative esterification of benzaldehyde with methanol under the same conditions. The acidity of the catalysts that was determined by NH3-temperature-programmed desorption technique and chemisorption of pyridine. The best catalyst was characterized by Fourier transform infrared spectroscopy, X-ray diffraction, scanning electron microscopy and laser particle size analyzer. Finally, a mild, simple and clean procedure was presented for the one-pot oxidative esterification of arylaldehydes with methanol using magnetically recoverable nanocatalyst and hydrogen peroxide as a green oxidant.

Pore morphology-controlled preparation of ZrO2-based hybrid catalysts functionalized by both organosilica moieties and Keggin-type heteropoly acid for the synthesis of levulinate esters

A series of ZrO2-based hybrid catalysts functionalized by both organosilica groups and Keggin type heteropolyacid, H3PW12O40/ZrO2–Si(Et/Ph)Si and H3PW12O40/ZrO2–Si(Ph), with different structural orderings and pore geometries were controllably prepared by a one-pot template-assisted sol–gel co-condensation–hydrothermal treatment route. The pore morphologies, textural properties and acidity of as-prepared hybrid catalysts were characterized by XRD, TEM, nitrogen gas porosimetry, in situ pyridine-FTIR spectroscopy and titration, while the structural integrity of the incorporated functionalities was studied by FT-IR, 31P MAS NMR, 13C CP-MAS NMR and 29Si MAS NMR. As the novel solid acid catalysts, the catalytic performance of as-prepared materials was evaluated by esterification of levulinic acid with various alcohols under mild conditions. Meanwhile, comparative studies were carried out to find out the influences of acidity, porosity and surface hydrophobicity on the catalytic activity of the catalysts. Finally, the recyclability of the catalysts was tested.

Design of a highly ordered mesoporous H3PW12O40/ZrO2–Si(Ph)Si hybrid catalyst for methyl levulinate synthesis

A highly ordered mesoporous ZrO2-based material functionalized by benzene-bridged organosilica groups and the Keggin type heteropolyacid, prepared in a single co-condensation–hydrothermal treatment step, exhibited much higher catalytic activity towards methyl levulinate synthesis compared to alkyl-free H3PW12O40/ZrO2.

Solvent Free One-Pot Synthesis of 1,2,4,5-Tetrasubstituted Imidazoles Catalyzed by Secondary Amine Based Ionic Liquid and Defective Keggin Heteropoly Acid

Secondary amine based ionic liquid and defective Keggin type heteropoly acid (HPA) are separately used for efficient one-pot four-component synthesis of 1,2,4,5-tetrasubstituted imidazoles assisted by microwave (MW). Eco-friendly solvent free procedure, short reaction time, high yield of products and reusability of catalysts are important features of the synthesis. A comparative study on the efficiency of the two catalysts is reported. This work further demonstrates the alternate use of urea, instead of often used ammonium acetate, as source of nitrogen.

Mesoporous catalysts based on Keggin-type heteropolyacids supported on MCM-41 and their application in thiophene hydrodesulfurization

A series of tungsten and molybdenum catalysts supported on MCM-41 were prepared using Keggin heteropolyacids (H3PM12O40, M=Mo or W) as active phase sources and compared with their analogues unsupported. The physicochemical characterization was carried out by, small angle and wide angle X-ray diffraction; thermogravimetric analysis/differential scanning calorimetry; ultraviolet–visible diffuse reflectance and Fourier transform infrared spectroscopies; N2 physisorption and potentiometric titration with N-butylamine, for determination, respectively, of the structure; thermal stabilities; integrity of heteropolyacids; textural properties and superficial acidity. The solids were tested in the thiophene hydrodesulfurization reaction at 400°C, atmospheric pressure and weight hourly space velocity of 0.48h−1. In this study, it has been demonstrated the importance of supporting heteropolyacids to increase the catalytic activity. Furthermore, it was proved a correlation between surface acidity and calcination temperature with the activity and stability of these catalysts in the hydrodesulfurization reaction.

Preparation and Photocatalytic Activity of Heteropolyacid Supported on Titanium Dioxide Photocatalysts

TiO2-based hybrid catalysts functioned by Keggin-type heteropolyacid, H3PW12O40/ TiO2, were prepared via one-pot co-condensation method in the presence of a triblock copolymer surfactant (F127). The materials were well characterized by FT-IR, UV-vis DRS spectroscopy methods, and X-ray diffraction analysis to confirm the structural integrity of the Keggin unit in the hybrid materials, to investigate correlation between TiO2 matrix and H3PW12O40. Phtocatalytic tests show the composite exhibits higher photocatalytic activity to decompose the Rhodamine B ( RhB ) than anatase TiO2.

Two proton-conductive hybrids based on 2,2′-biimidazole molecules and Keggin-type heteropolyacids

Two proton-conductive organic/inorganic complexes were constructed by Keggin-type heteropolyacids and 2,2′-biimidazole molecules. Single-crystal X-ray diffraction analyses revealed that two complexes crystallized in the triclinic space group Pī, exhibited very close unit cell parameters, and presented similar three-dimensional (3D) hydrogen-bonded networks constructed by H2biim, [PM12O40]3− (M=W for 1 and Mo for 2) and solvent molecules. The results of thermogravimetric analyses (TG) suggest that two supramolecular complexes have similar thermal stability based on the same 3D packing. Two complexes at 100°C under 98% relative humidity showed a good proton conductivity of about 10−4Scm−1. A possible mechanism of the proton conduction was proposed according to the experimental results.

Compensation effect in isopropanol dehydration over heteropoly acid catalysts at a gas–solid interface

Kinetics of isopropanol dehydration at a gas–solid interface over a range of bulk and supported Brønsted acid catalysts based on H3PW12O40 (HPW) Keggin-type heteropoly acid was studied in a continuous flow fixed-bed reactor in the temperature range of 55–135°C and isopropanol partial pressure of 0.94–5.52kPa. The bulk catalysts included HPW itself and its Cs acid salts CsnH3−nPW12O40 (CsnH3−nPW). The supported catalysts comprised of 15wt.% HPW supported on SiO2, TiO2, ZrO2 and Nb2O5. Under the conditions studied, the reaction was found to be of zero order in isopropanol. The turnover rate decreased in the order HPW>Cs2.5H0.5PW≈Cs2HPW>HPW/SiO2>HPW/TiO2>HPW/Nb2O5≈HPW/ZrO2, which is in line with the acid strength of these catalysts. The true activation energies E and pre-exponential factors A obtained from zero-order kinetics, were found to exhibit a compensation linear relationship lnA=mE+c. Moreover, the bulk and supported catalysts exhibited different compensation plots. This is suggested to be due to the different chemical structure of Brønsted acid sites in these catalysts, resulting in differing reaction mechanisms. The bulk catalysts possess strong surface H+ sites located on peripheral (bridging) oxygen atoms in the Keggin unit, whereas supported HPW catalysts have weaker H+ sites probably located on oxygen atoms of support. Consequently, on the bulk catalysts, isopropanol dehydration is suggested to occur via E1 elimination pathway, whereas on the supported catalysts it might also involve a contribution of E2 elimination pathway.

Preparation of ethane-bridged organosilica group and keggin type heteropoly acid co-functionalized ZrO2 hybrid catalyst for biodiesel synthesis from eruca sativa gars oil

A series of mesostructured ZrO2-based hybrid catalysts, functionalized by both an ethane-bridged organosilica group and the Keggin type heteropoly acid, H3PW12O40–ZrO2–Si(Et)Si, were prepared by one-step sol-gel-hydrothermal method in the presence of a triblock copolymer surfactant (F127). The materials were well characterized by spectroscopy methods such as FT-IR and MAS NMR, transmission electron microscopy, and nitrogen gas physisorption measurements to confirm the structural integrity of the Keggin units and ethane-bridged organosilica groups in the hybrid materials, and to test the mesostructure, morphology, and porosity of the materials. The materials were subsequently utilized as environmentally-friendly solid acid catalysts in the transesterification of low-cost Eruca Sativa Gars oil with methanol to produce fatty acid methyl esters under atmosphere refluxing. Compared with alkyl-free H3PW12O40–ZrO2, the prepared H3PW12O40–ZrO2–Si(Et)Si exhibited higher catalytic activity toward the target reaction. This enhanced acid-catalytic activity after the introduction of both acidic and hydrophobic functionalities within the ZrO2 matrix was explained in terms of strong Brϕnsted acidity, excellent porosity, and enhanced hydrophobicity of the hybrid materials.

Recovery and reuse of heteropolyacid catalyst in liquid reaction medium through reversible encapsulation in Cu3(BTC)2 metal–organic framework

The solvent dependent solubility of HKUST-1 metal–organic framework (MOF) with encapsulated Keggin 12-tungstophosphoric acid (HPW@Cu3(BTC)2) was exploited to liberate homogeneous heteropolyacid catalyst in a reaction medium and to recover the catalyst by encapsulation in the MOF upon addition of hexane. The application of the new Keggin type heteropolyacid catalyst recovery concept was demonstrated in the esterification of acetic acid with 1-propanol and salicylic acid with ethanol.

Catalytic Hydroxylation of Benzene to Phenol with Hydrogen Peroxide over Cesium Salts of Keggin-type Heteropoly Acids

Catalytic Hydroxylation of Benzene to Phenol with Hydrogen Peroxide over Cesium Salts of Keggin-type Heteropoly Acids

Solid acid catalysts for dehydration of glycerol to acrolein in gas phase

The reaction of glycerol dehydration to acrolein was performed at temperatures 220–300 °C over Keggin-type heteropolyacid (H3PW12O40·xH2O), supported on alumina and W-modified SBA-15 (Si/W = 20). The supports and catalysts were characterized by nitrogen adsorption, XRD, IR, UV–Vis DRS, SEM, and TPD of NH3. The acid sites strength, determined according to data from TPD of ammonia, follows the order: HPW/W-SBA-15 > W-SBA-15 > H3PW12O40 > HPW/γ-Al2O3 > γ-Al2O3. It was found that this order well correlates with the catalytic activity of studied samples. The most active and selective sample, HPW/W-SBA-15, showed acrolein selectivity of about 75% at almost 100% conversion of glycerol in all studied temperature interval.