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Gas-phase continuous synthesis of butadiene from propene and formaldehyde has been studied over heteropolyacids (HPA) supported on silica. Silicotungstic acid (SiW) is found to be the most active a...

In addition to conductivity, recent evidence suggests that adhesive, self-healing, and antibacterial properties are also important aspects of wearable force sensors. However, preparation of hydroge...

In this work, efficient solid acid catalysts were obtained from the stoichiometric reactions of Keggin heteropolyacids with SnSO4 and evaluated in the etherification reactions of glycerol with tert...

Keggin-type polyoxometalates are effective catalytic materials for acid-catalyzed reactions and oxidation reactions. In this paper, we describe the fabrication of a porous membrane of Keggin-type polyoxometalates at the gas-liquid interface and the structural characterization of the membrane via aberration-corrected scanning transmission electron microscopy. The membrane is formed by the introduction of n-octylamine vapor into aqueous solution of Keggin-type silicotungstic acid (H4SiW12O40) and cesium chloride. The membrane obtained has the cubic structure of cesium salts of silicotungstic acid (CsxH4-xSiW12O40), exposing the (110) crystal plane preferentially. The single-unit cell layer of the upper surface of the membrane is densely packed with Keggin anions, while the lower layers are rich in vacancies at both the anion and cation sites. This structure results from the fast formation of the single-unit cell layer at the surface of the solution and slow stacking of Keggin anions under the layer.

In the present work, the hybridization of subnanometer-scale metal oxide cluster SiW₁₂O₄₀⁴– (STA) with poly(ethylene oxide) (PEO) and PEO-block-poly(propylene oxide)-block-PEO was investigated. A new crystalline phase was formed in the hybrid nanocomposites, which is attributed to the inclusion of PEO in the lattices of STA, that is, inclusion crystallization. The crystal lattice and dimensions of STA/PEO inclusion crystals are determined to be an orthorhombic unit cell with a = 1.52 nm, b = 2.74 nm, c = 2.52 nm, and α = β = γ = 90°. Remarkably, hybrid nanosheets with large size and high shape asymmetries can be formed by lamellar inclusion crystals. The dependence of proton conductivities of STA nanocomposites on crystalline structures was examined, and it was found that the formation of hybrid nanosheets could enhance proton conductivities of STA nanocomposites. This work provides new insight into the development of cluster-based nanocomposites with controlled morphology and optimized performance.

A Keggin silicotungstic acid (HPS) catalyst was heterogenized by loading (10–40 wt %) over zirconia support, and the resulting catalysts were named ESZN-1 (10 wt %), ESZN-2 (20 wt %), ESZN-3 (30 wt...

The effects of microwave and nonmicrowave heating methods on the synthesis of ethyl levulinate (EL) from levulinic acid (LA) have been investigated in the present study. The levulinic acid esterification experiments were performed in the presence of silicotungstic acid catalyst and ethanol in a microwave and nonmicrowave instant heating reactor. An experimental fit of experimental data in a kinetic model suggested that LA esterification follows a pseudo-first-order reaction mechanism. Consequently, activation barriers were calculated (44–45 kJ/mol) and a negligible difference was found for LA esterification reaction performed in both the reactors. Nevertheless, slightly higher LA conversions were measured under microwave irradiations as compared to experiments performed in a nonmicrowave instant heating reactor. Thus, series of experiments were performed to study the (i) nonthermal and (ii) thermal effects of microwave heating irradiations. Eventually, it was found that the enhanced LA conversion in micro...

A highly efficient and recyclable catalyst, cerium-immobilized silicotungstic acid nanoparticle-impregnated zirconia (Ce@STANPs/ZrO2), has been synthesized. The catalytic activity of Ce@STANPs/ZrO2 was investigated for the first time in multicomponent synthesis of isatin-based imidazoles under microwave irradiation in water. Ce@STANPs/ZrO2 was used for C=O bond activation in overall reaction to synthesize isatin-based imidazoles. The structure of catalyst was confirmed by characterization techniques, such as Fourier transform infrared (FTIR), scanning electron microscopy (SEM)/energy dispersive X-ray, elemental mapping, transmission electron microscopy, ζ-potential and diffraction light scattering, X-ray diffraction (XRD), thermogravimetric analysis, temperature-programmed desorption-NH3, electron paramagnetic resonance (EPR), and inductively coupled plasma atomic emission spectroscopy (ICP-AES) analyses. The recovered catalyst was found to be efficient up to seventh cycle and was confirmed by FTIR, SEM, XRD, EPR, and ICP-AES analyses. The advantages of the present protocol are recyclability of catalyst, green reaction conditions, excellent yield (94%) of the products, shorter reaction time period (5–7 min), and clean reaction profile.

Dimethyl ether (DME) is a promising nonpetroleum diesel fuel alternate. Synthesis of DME was achieved directly from synthesis gas having different compositions, over bifunctional hybrid catalysts. Silicotungstic acid impregnated mesoporous alumina (STA@MA) was shown to be an excellent catalyst to be used as the dehydration component of the hybrid catalyst combination. Results proved that feed composition, as well as reaction temperature had important influence on product distributions, as well as on DME yield. Results obtained with different CO/CO2 ratios in the feed stream proved the positive effect of CO2 on both overall conversion of CO + CO2 and DME yield. Overall fractional conversion of CO + CO2 and DME yield values were shown to increase with an increase in CO2/CO ratio and also with an increase in reaction temperature, reaching to the values of 0.70 and 0.55, respectively, at 275 °C, with the feed stream composition of H2/CO/CO2 = 50/10/40. However, from the DME selectivity point of view, a DME se...

Pt, Pd, Ni, and Cu supported on HSiW/Al2O3 catalysts were studied for the hydrogenolysis of glycerol. It was found that Pt is the best promoter for the production of 1,3-propanediol (1,3-PD) and 1-propanol (1-PO). Ni, a much cheaper metal, has fairly comparable reactivity to Pt, while Cu does not show any activity for the production of 1,3-PD. The catalysts were characterized by XRD and NH3-TPD. The strength of the acid sites affects the distribution of products. A reaction mechanism for a NiHSiW/Al2O3 catalyst involving rate-determining parallel dehydration of primary alcohol to produce acetal and of secondary alcohol to produce 3-hydroxypropylaldehdye (3-HPA) was proposed. Hydrogenolysis of 1,3-PD is 15 times slower than that of 1,2-PD. Most of the 1-PO is derived from 1,2-PD. An optimal balance of acid sites of appropriate acid strength and hydrogenation sites will lead to a highly selective catalyst for the production of higher value sustainable chemicals from glycerol.

The kinetics of gas-phase glycerol dehydration in a packed-bed reactor over a highly active and stable supported silicotungstic acid catalyst with zirconium oxide and nanosized aluminum oxide (30HZ-20A) was investigated. The kinetic study is based on the optimal reaction conditions determined by response surface methodology. The reaction rate followed first-order kinetics with the activation energy and frequency factor, E = 27.5 kJ/mol and A = 5.35 × 105 s–1, respectively. Based on thermodynamic analysis, the values of ΔH° and ΔS° of the endothermic reaction were 14.70 and 0.09 kJ/(mol K), respectively, and ΔG° = −12.12 kJ/mol. The mass-transfer analysis revealed the pellet sizes of dp < 1 μm proceeded under reaction-limiting conditions. Experimental results confirmed high efficacy of the tested catalyst due to unity effectiveness factor and very low Thiele modulus.

Nanospherical cellulose containers (SCC) (average size, ∼50 nm) are prepared by the sonochemical method. Organic and water-soluble materials are encapsulated within the SCC using only microcrystalline cellulose (MCC), an organic solvent, and water. Using silicotungstic acid (H4SiW12O40), the prepared SCCs can be catalytically converted (78 wt % conversion) to glucose (30 wt % glucose yield) by applying microwave irradiation for only 3 min. These results are attributed to the large increase of surface area of the cellulose sphere relative to MCC, and a better contact with the solid catalyst. Sealed in a vessel, SCCs were found stable for more than 6 months when stored at 4 °C or at room temperature. The encapsulation efficiency of the organic phase was measured and found to be approximately 90%. The creation of the sphere involves the degradation of the MCC to smaller fragments by high-intensity ultrasound irradiation, and the organization of these fragments in nanospheres involves the formation of bonds o...

The aim of this work was to study the production, characterization, and performance of ionic liquid (IL) on silicotungstic acid (STA-IL) via the sol–gel method as catalysts in the esterification of oleic acid (OA) and purified glycerol, as well as to examine the effects of IL on heterogeneous catalysts. The STA-IL catalyst showed a relatively high surface area of 88.36 m2/g due to the presence of nanoporous silica. XPS analyses showed the significant formation of W–O–Si, W–O–W, and Si–O–Si bonding. The STA-IL catalyst enabled a conversion of 96% of OA with a significantly higher monoolein selectivity and yields of 96% and 92.2%, indicating that it is the more efficient solid acid catalyst. The presence of IL serves as PTC or medium, enhancing reaction synergistic effect. In addition, the ease of separation of the STA-IL catalyst due to its insolubility in the product phase can improve the recyclability.

Reduced graphene oxide (RGO) supported silicotungstic acid (STA) as a redox catalyst (STA-RGO) for efficient conversion of thiols to disulfide was prepared with an impregnation method. The catalyst was characterized by XRD, FTIR, UV–vis, TEM techniques, and zeta potential measurements. The uniform distribution of the STA on RGO was indicated by broadening of the XRD peaks of RGO. The Keggin unit of STA on the prepared catalyst was found to remain intact as noticed by FTIR. The increase in negative zeta potential and shifts in the characteristic UV–vis absorption bands of STA-RGO as compared with pure STA and RGO indicate the interaction between the two. The prepared catalyst was found to facilitate the oxidation of various thiols to their corresponding disulfides in the presence of hydrogen peroxide as an oxidant at room temperature. A probable mechanism involving sufenic acid as an intermediate is proposed.

The effects of nano-SiO2 and nano-TiO2 fillers on a thin film silicotungstic acid (SiWA)-H3PO4-poly(vinyl alcohol) (PVA) proton conducting polymer electrolyte were studied and compared with respect to their proton conductivity, environmental stability, and dielectric properties, across a temperature range from 243 to 323 K. Three major effects of these fillers have been identified: (a) barrier effect; (b) intrinsic dielectric constant effect; and (c) water retention effect. Dielectric analyses were used to differentiate these effects on polymer electrolyte-enabled capacitors. Capacitor performance was correlated to electrolyte properties through dielectric constant and dielectric loss spectra. Using a single-ion approach, proton density and proton mobility of each polymer electrolyte were derived as a function of temperature. The results allow us to deconvolute the different contributions to proton conductivity in SiWA-H3PO4-PVA-based electrolytes, especially in terms of the effects of fillers on the dynamic equilibrium of free protons and protonated water in the electrolytes.

Microencapsulation is used as a means to retain the activity of silicotungstic acid (STA), an industrially important catalyst, during repeated use in ethyl acetate synthesis reaction. Standard sol-gel microencapsulation procedures were modified to microencapsulate STA inside a silica shell. The microcapsules were characterized to determine their morphology, the presence and amount of the active material, and the pore structure. The activity of the catalyst was tested in an esterification reaction between acetic acid and ethanol. The catalyst was used multiple times after being washed and dried after each run. Results indicate no leaching of the active material and no significant loss in catalyst activity after repeated runs.

The present study deals with the development and application of a proton-exchange polymer membrane separator consisting of graphene oxide (GO), poly(vinyl alcohol) (PVA), and silicotungstic acid (STA) in a single-chambered microbial fuel cell (sMFC). GO and the prepared membranes were characterized by FT-IR spectroscopy, XRD, SEM, TEM, and AC impedance analysis. Higher power was achieved with a 0.5 wt % GO-incorporated PVA–STA–GO membrane compared to a Nafion 117 membrane. The effects of oxygen crossover and membrane-cathode-assembly (MCA) area were evaluated in terms of current density and Coulombic efficiency. The electrochemical behavior of the membrane in an MFC was improved by adding different amounts of GO to the membrane to reduce biofouling and also to enhance proton conductivity. A maximum power density of 1.9 W/m3 was obtained when acetate wastewater was treated in an sMFC equipped with a PVA–STA–GO-based MCA. Therefore, PVA–STA–GO could be utilized as an efficient and inexpensive separator for ...

In this paper, carbon supported noble metal nanoparticles synthesized by using Keggin type polyoxometalate (POM), silicotungstic acid (STA), were effectively utilized as electrodes in an effort to improve the efficiency of electrochemical energy conversion processes related to direct methanol fuel cells. The excellent electron transfer and strong adsorption abilities of STA have been well exploited for the reduction of metal ions as well as the modification of carbon support to form Pt and Pt-Ru metal nanoparticles supported on carbon nanostructure composites (Pt/STA-C and Pt-Ru/STA-C) under microwave irradiation. Transmission electron microscopy results indicated that well-dispersed metal nanoparticles were formed on the carbon support. The electrocatalytic properties of Pt/STA-C and Pt-Ru/STA-C were analyzed by cyclic voltammetry and chronoamperometry through the methanol oxidation reaction for fuel cell applications. As compared with the commercial Pt-Ru/C (Johnson Matthey) and Pt/C and Pt-Ru/C electrocatalysts prepared by using the common hydrogen reduction method, the STA containing Pt/STA-C and Pt-Ru/STA-C synthesized by the method developed in this work exhibited lower onset potential, high activity with enhanced stability, and better tolerance toward poisoning by CO for the methanol oxidation reaction. �2009 American Chemical Society.

Hexagonally mesostructured polyoxometalate-based hybrid was prepared by a mild-solution method through the self-assembly of an amphiphilic poly(ethylene oxide) octadecyldimethylammonium (C18NEO12) and a polyanionic silicotungstic acid (HSiW). The composition of the hybrid was characterized through 1H NMR, infrared spectroscopy, mass spectroscopy, elemental analysis, and thermogravimetric analysis, indicating an approximate formula (C18NEO12)1.6H2.4(SiW12O40). The structure of the hybrid was investigated by transmission electron microscopy (TEM), scanning electron microscopy, X-ray powder diffraction, and contact angle measurement. The hybrid is a rodlike grain with the hexagonal mesostructure, which consists of one-dimensional column micelles in which alkyl chains of C18NEO12 locate at the center and poly(ethylene oxide) (PEO) part at the outside, with HSiWs anchoring at the interface, but more close to the PEO part. Furthermore, the observation of the formation process of hybrids by TEM shows that ethanol plays an important role in the self-assembly. More importantly, the hexagonal mesostructure can transform into nanofibers comprising nanofibrils in water through the disassembly.

Solid acid catalysts are extensively used in a number of industrially important catalytic reactions. Here, waterinsoluble novel mesoporous solid acid catalysts, with a unique structure composed of WOx nanorods and W nanoballs within an amorphous silicate lattice, were synthesized following a one-pot hydrothermal route, using silicotungstic acid (STA) as the active component. Surface area of these catalysts are more than 2 orders of magnitude higher than that of pure STA, and they showed very high activity for the simultaneous production of ethylene and diethylether (DEE), which is an attractive transportation fuel alternate, from a nonpetroleum feedstock, namely ethanol. Depending upon the operating conditions, ethylene or DEE selectivities approaching 1.0 or 0.8 were obtained, respectively. © 2009 American Chemical Society.

[Tetrakis (N-methylpyridyl) porphyrinato] cobalt (CoTMPyP) and 1:12 silicotungstic acid (SiW12) were alternately deposited on a 4-aminobenzoic acid (4-ABA) -modified glassy carbon electrode through a layer-by-layer method. The resulting organic−inorganic hybrid films were characterized by cyclic voltammetry (CV) and UV/vis absorption spectroscopy. We proved that the prepared multilayer films are uniform and stable. SiW12-containing multilayer films (SiW12 as the outermost layer) exhibit remarkable electrocatalytic activity for the hydrogen evolution reaction (HER). The kinetic constants for HER were comparatively investigated at different layers of SiW12/CoTMPyP multilayer film-modified electrodes by hydrogen evolution voltammetry. In addition, rotating disk electrode (RDE) and rotating ring−disk electrode (RRDE) voltammetric methods confirm that SiW12/CoTMPyP (CoTMPyP as the outermost layer) multilayer films catalyze almost a two-electron reduction of O2 to H2O2 in pH 1−6 buffer solutions. Furthermore, P...

Hybrid organic−inorganic proton conducting gels (PWA/ICS−PPG or SiWA/ICS−PPG) were made from 3-isocyanatopropyltriethoxysilane and poly(propylene glycol)bis(2-aminopropyl ether), resulting in a hybrid silicon precursor (ICS−PPG) in which was dispersed phosphotungstic acid (PWA) or silicotungstic acid (SiWA) in ethanol. In this system, heteropoly acid takes on the additional role of catalyst, initiating the hydrolysis/condensation reactions of the sol−gel composite. The existence of amidonium ions [C(OH)NH+] was established from the IR and Raman spectra of PWA/ and SiWA/ICS−PPG composites below and close to the crystallization limits (70−75%). The crystallization of the Keggin ions inside the ICS−PPG host was determined from the XRD spectra of the composites and confirmed also using vibrational spectroscopy, which shows the appearance of H3O+ modes at 1700−1710 cm-1. The amidonium ions help in the immobilization of the negatively charged Keggin ions in the membranes, ensuring their uniform distribution in ...

The acidity and its effects on reactivity of Keggin-type heteropolycompounds were examined by catalytic probe reactions, microcalorimetry of ammonia sorption, and density functional quantum chemical calculations. Phosphotungstic, phosphomolybdic, silicotungstic, and silicomolybdic acids were used as model compounds. The specific rates of double-bond isomerization of both 1-butene and cis-2-butene were orders of magnitude greater on the tungsten heteropolyacids than on molybdenum heteropolyacids, which suggests the tungsten-containing solids are stronger acids. The rate of double-bond isomerization over silicotungstic acid was similar to that over phosphotungstic acid, indicating the minor role of the heteroatom. Results from ammonia sorption microcalorimetry showed ΔHsorp on tungsten-based heteropolyacids was approximately 40 kJ mol-1 higher than the corresponding enthalpy obtained on molybdenum-based heteropolyacids. Residual waters of hydration significantly affected both reaction rates and sorption ent...

An electroanalytical approach for the quantitative and qualitative charactorization of certain fast redox-conducting molecular solids is proposed here. The method utilizes ultramicroelectrodes within a three-electrode solid-state electrochemical cell and is based on a set of potential step and cyclic voltammetry experiments. Such measurements permit the determination of the following parameters: the concentration of mixed-valence redox sites (C o ) that are electrochemically accessible in the system, the effective diffusion coefficient for charge propagation (D eff ), the standard heterogeneous rate constant for electron transfer between the working ultramicroelectrode and the electroactive sites (K o ), and the formal potentials (E o' ) for solid-state redox reactions

The authors have shown that electrochemical methodology can be extended to the investigation of the protonically conducting, crystalline mixed-valence molecular materials. Such studies provide information about thermodynamics and kinetics of redox processes in these materials. Additional results, with thin films and powders of silicotungstic acid and related oxide systems, clearly suggest that solid-state electrochemical responses are very sensitive to the degree of hydration, which probably controls the protonic mobility. They have been able to perform useful solid-state electrochemical measurements on other powdered materials, including Prussian blue analogues and metalloproteins, by essentially the same methodology.