Primary Keggin Structure Research Articles

Self-Supported Smart Bacterial Nanocellulose–Phosphotungstic Acid Nanocomposites for Photochromic Applications

Bacterial nanocellulose (BNC) is a natural biopolymer obtained by gram-negative bacteria by means of a green and inexhaustible biotechnological process using glucose as producing source. BCN hydrogels is formed by cellulose nanofibrils that maintain an open network structure, an ideal matrix to produce new class of organic-inorganic nanocomposites (OIN) for multifunctional applications. The polyoxometalates (POMs) are complex molecules with several metallic ions sharing oxide ions, forming a highly symmetrical metal oxide cluster. Phosphotungstic acid (PWA), H3PW12O40 photoreduction process activated under ultraviolet irradiation, promoting color change. In this work, photochromic organic-inorganic nanocomposites were prepared by soaking phosphotungstic acid (H3PW12O40) in wet BNC membranes mats at room temperature. Semi-transparent and free-standing BNC/PWA nanocomposite with paper-like aspect were obtained. BNC network was able to control, stabilize and disperse PWA particles in a narrow nanometric distribution, and FTIR spectra indicated that the primary Keggin structure was also preserved in the nanocomposites, independently on the PWA content. The nanoparticles present a narrow distribution of around 16 nm, independently on the PWA concentration. BNC/PWA nanocomposites showed reversible photochromic behavior characteristic of the equilibrium between different tungsten oxidation states. PWA reduction (W6+→ W5+) and organic matrix oxidation is proposed to occur through a radical process involving the interaction of one electron from the oxygen atom of the PWA and one hydrogen from BNC matrix. The photochromic effect vanishes almost completely after 5 h. This mechanism is real in the presence of oxygen, however, if the membranes are left in nitrogen or under vacuum the blue color remains longer than 45 days. Photo-electrochemical behavior was studied by spectroelectrochemistry measurements. It is worth noting that all processes were still reversible in the timescale of the experiment and color changes were observed in several cycles.

A novel heterogeneous photocatalyst for Cr (VI) reduction via planting silicotungstic acid on the surface of calcium tantalate

A novel heterogeneous photocatalyst CaxH4-xSi4W12O40/Ca2Ta2O7 was prepared by planting silicotungstic acid on the surface of calcium tantalate. The materials are characterized by X-ray diffraction (XRD), scanning electron microspectroscopy (SEM), X-ray photoelectron spectroscopy (XPS), Fourier transformation infrared spectroscopy (FTIR), UV–vis diffuse reflectance spectroscopy (UV–vis), Energy-dispersive X-ray spectroscopy (EDS), BET surface area, and Malvern ZEN3690 Particle Analyzer (ZP). The results recovered Ca2Ta2O7 presented regular octahedron morphology with the cubic phase structure, and H4Si4W12O40 uniformly grew on the surface of Ca2Ta2O7 with the form of CaxH4-xSi4W12O40. The primary Keggin structures of H4Si4W12O40 was found to remain intact as noticed by XRD and FTIR. The modification of H4Si4W12O40 on Ca2Ta2O7 surface contributed the increase in negative zeta potential and red shifts. The as-prepared catalyst exhibited greatly enhanced UV photocatalytic efficiency in the reduction of Cr(VI) ions as high as 94.6% under the optimum conditions (Ca2Ta2O7 to H4Si4W12O40 was 1:2). Moreover, the composite presented an excellent reusability, which was mainly ascribed to the locally formation of water-insoluble CaxH4-xSi4W12O40. A probable mechanism involved in the enhanced reducible activity and well recycled performance via CaO binding linkage of H4Si4W12O40 and Ca2Ta2O7 is proposed by DFT simulation. This work will be significant in the design of heteropoly acid as stable heterogeneous photocatalyst applied to heavy metal ions reduction.

Production of Structured Lipids Containing Medium-Chain Fatty Acids by Soybean Oil Acidolysis Using SBA-15-pr-NH2–HPW Catalyst in a Heterogeneous Manner

The Keggin-type H3PW12O40 (HPW) was successfully anchored onto the surface of amino-functionalized SBA-15 silica by means of protonating of amino-functionalized silica with the phosphotungstic acid. The as-prepared heterogeneous catalysts were fully characterized at the current research by using small-angle XRD, FT-IR, TEM, SEM, solid-state NMR, and nitrogen porosimetry measurement. The characterization results showed that the functionalized SBA-15 materials still retained the ordered mesoporous structure of SBA-15 silica with high surface area, and meanwhile the primary Keggin structure of heteropolyanions could be also remained intact upon the immobilization of the HPW. By using this solid catalyst, the acidolysis of soybean oil with caprylic or capric acid was carried out in a batch reactor to produce structured lipids containing medium-chain fatty acids. The influence of reaction parameters, such as the substrate ratio, reaction temperature, catalyst loading, and reaction time, on the acidolysis react...

Cesium exchanged tungstophosphoric acid supported on tin oxide: An efficient solid acid catalyst for etherification of glycerol with tert-butanol to synthesize biofuel additives

Cesium exchanged tungstophosphoric acid (CsTPA) supported on tin oxide catalysts were prepared and their physio-chemical properties were derived from X-ray diffraction, FT-IR, laser Raman spectroscopy and temperature programmed desorption of NH3. The catalysts activity was evaluated for etherification of glycerol with tert-butanol. The characterization results of the catalysts revealed that the primary Keggin structure remained intact during the exchange of TPA protons with Cs+ ions. The activity results showed that etherification activity depended on the amount of the CsTPA over SnO2 and the catalyst with 20wt% CsTPA supported on SnO2 showed high catalytic activity with 90% glycerol conversion with 44% selectivity toward higher ethers. The activity of the catalyst depends on amount of surface acidic sites and dispersion amount of CsTPA over SnO2. The etherification reaction was carried at different reaction parameters and optimum reaction conditions were established. The catalysts were recyclable and showed constant activity up on reuse.

Gallium exchanged phosphotungstic acid supported on zirconia: Efficient catalyst for Friedel–Crafts benzylation

Gallium exchanged phosphotungstic acid (GaTPA) supported on zirconia catalysts were prepared and their physico-chemical properties were derived from FT-infrared spectra, X-ray diffraction, X-ray photoelectron spectroscopy, temperature programmed desorption of ammonia, pyridine adsorbed FT-IR spectra and solid state 31P NMR spectroscopy. The catalysts activity was studied for Friedel–Crafts benzylation of anisole with benzyl alcohol. The characterization of catalysts revealed that the primary Keggin structure remained intact during exchange of TPA protons with Ga3+. The exchange of Ga3+ with the protons of TPA led to increase in Lewis acidity. Benzylation activity depended on the amount of the GaTPA and the catalyst with 20wt% GaTPA supported on zirconia showed highest activity. The activity profiles of the catalysts were well correlated with the characteristics of the catalysts. Reaction parameters such as effect of reaction temperature, catalyst weight, anisole to benzyl alcohol molar ratio were also optimized. Under optimized conditions complete conversion of benzyl alcohol was obtained. The catalyst is reusable with consistent activity.

Preparation, characterization, and photocatalytic activity of three-dimensionally ordered macroporous hybrid monosubstituted polyoxometalate K5[Co(H2O)PW11O39] amine functionalized titanium catalysts

A novel and efficient, amine-functionalized three-dimensionally ordered macroporous (3DOM) titanium (Ti) catalyst modified using transition metal-monosubstituted polyoxometalate (3DOM K5[Co(H2O)PW11O39]–(EtO)3SiCH2CH2CH2NH2–TiO2 or 3DOM PW11Co–APS–TiO2) was fabricated by self-assembly method. First, methanol bath was used to soak polystyrene spheres to obtain large size 3DOM TiO2, which was further functionalized using 3-aminopropyltriethoxysilane (APS). Amine-functionalized 3DOM TiO2 materials impregnated with monosubstituted polyoxometalate (POM, K5[Co(H2O)PW11O39]) were prepared by coordination of cobalt (Co) present in the units of POM, with the amine groups present on the surface of 3DOM TiO2. The materials so obtained were characterized by Fourier transform infrared spectroscopy, X-ray diffraction, Inductively coupled plasma atomic emission spectrometry, Ultraviolet–visible (UV–vis) diffuse reflectance spectroscopy, Raman spectroscopy, X-ray photoelectron spectroscopy, Scanning electron microscopy, Transmission electron microscopy, and nitrogen adsorption–desorption tests. The results showed that the primary Keggin structures of the composites remained intact, simultaneously, 3DOM PW11Co–APS–TiO2 had anatase crystalline structure, and PW11Co was successfully bonded to 3DOM TiO2 by CoN bond with APS as the coupling agent. Broadening of absorption band was observed for the product compared to the monomers, 3DOM TiO2 and PW11Co. However, the 3DOM structure was retained in the product; therefore, the accessibility of the diffusing species was significantly improved because of the permeability offered by the macroporous structure. The UV photocatalytic degradation of organic pollutants revealed better activities of 3DOM PW11Co–APS–TiO2 compared to PW11Co, 3DOM TiO2, and PW11Co–APS–TiO2. Moreover, 3DOM PW11Co–APS–TiO2 exhibited superior photocatalytic degradation of Rhodamine B (RB), Salicylic acid (SA), Congo red (CR), and Malachite green oxalate (MG) under microwave-assisted irradiation. Leakage of PW11Co from the hybrid photocatalyst was barely observed during the photocatalytic tests, attributed to strong coordination interactions between the Keggin units and the amine-functionalized Ti surface. The capture experimental results of photocatalysis revealed that the dominant active species were OH• and O2−•. Moreover, in this study, the procedure and possible mechanism of the UV photodegradation of organic pollutants in the aqueous system was proposed and discussed.

Synthesis, characterization and catalytic performance of a Fe polyoxometalate/silica composite in the oxidation of alcohols with hydrogen peroxide

Fe(III) substituted Keggin type polyoxometalate in amorphous silica matrix, (n-C4H9)4N)4[PW11FeO39]/SiO2 (PWFe/SiO2), has been synthesized by sol–gel method. The synthesized PWFe/SiO2 composite was characterized by various analytical and spectroscopic techniques. Results indicated that the primary Keggin structure remain after incorporation in silica matrix. As a heterogeneous catalyst, PWFe/SiO2 composite showed an elevated catalytic activity and selectivity with high to excellent yields for oxidation of alcohols into aldehydes by H2O2. The stability and reusability of the catalyst was very high, so these composites are suitable for the heterogeneous catalysis by hydrogen peroxide.

Photocatalysis of dinitrotoluene decomposition by H3PW12O40/TiO2 and H4SiW12O40/TiO2 prepared by a modified sol-gel synthesis and solvothermal treatment method

Mesoporous H 3 PW 12 O 40 /TiO 2 and H 4 SiW 12 O 40 /TiO 2 were synthesized by combining sol-gel technology using a nonionic surfactant P123 as a structure directing agent with solvothermal treatment. X-ray diffraction and Raman spectroscopy results indicated that the TiO 2 particles had the anatase phase, and the primary Keggin structures of H 3 PW 12 O 40 and H 4 SiW 12 O 40 remained intact after calcination at 400 °C. N 2 adsorption-desorption analysis and scanning electron microscopy showed that the specific surface area and pore volume were increased by the addition of P123. The specific surface area of H 3 PW 12 O 40 /TiO 2 and H 4 SiW 12 O 40 /TiO 2 increased to 252.2 and 250.0 m 2 /g, respectively. Ultraviolet-visible absorption spectroscopy showed that compared with pure TiO 2 , an obvious red shift occurred and the absorption intensity was increased for the composite catalysts. The catalysts were tested and the degradation rate of dinitrotoluene was as high as 95% under the optimum conditions. This article focuses on the preparation of new materials (POMs/TiO 2 ) using P123 as a structure directing agent with solvothermal treatment. Photocatalytic tests show that new materials exhibit high photocatalytic activity for DNT degradation.

Preparation of Y-H3PW12O40/TiO2 and La-H3PW12O40/TiO2 by hydrothermal method and its photocatalytic activity for dinitrotoluene decomposition

A series of mesoporous materials (H3PW12O40/TiO2, Y-H3PW12O40/TiO2 and La-H3PW12O40/TiO2) were prepared by a modified sol-gel-hydrothermal route, which realized the load and modification of H3PW12O40 at the same time. The prepared samples were characterized by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), nitrogen adsorption-desorption analysis, ultraviolet-visible absorption spectrum (UV-Vis) and scanning electron microscopy (SEM). The XRD and FT-IR results displayed that the catalysts had uniform anatase phase and the primary Keggin structure of H3PW12O40 remain intact. Nitrogen adsorption-desorption analysis suggested that suitable doping of rare earth elements could increase the specific surface area from 177.9 m2/g (H3PW12O40/TiO2) to 229.5 (1 wt.%Y-H3PW12O40/TiO2) or 236.1 m2/g (1 wt.%La-H3PW12O40/TiO2). Results of UV-Vis spectra showed that the band of the 1 wt.%Y-H3PW12O40/TiO2 and 1 wt.%La-H3PW12O40/TiO2 have an obvious redshift compared with the H3PW12O40/TiO2. Additionally, the composites were used as heterogeneous photocatalysts to the degradation of dinitrotoluene (DNT). It is the first time that polyoxometalate (POM) is applied in the degradation of explosive wastewater.

Preparation, characterization and photochemical degradation of dyes under UV light irradiation by inorganic–organic nanocomposite

An inorganic–organic nanocomposite consisting of 12-tungstophosphoric acid (H3PW12O40) supported on Polyaniline (PANI) has been successfully synthesized by one-step and two-step methods. X-ray diffraction, Fourier-transformed infrared spectroscopy, scanning electron microscopy and electronic absorption spectroscopy indicated that the Polyphosphotungstate (PPT) was chemically attached to the PANI support; the primary Keggin structure remained intact. The photocatalytic and sonocatalytic activity of PPT/PANI was tested by degradation of organic dyes such as Methylene Blue, Rhodamin B, Bromothymol Blue, Methyl Orange and Nylosan Black 2-BC in aqueous solution; the nanocomposite showed higher photocatalytic and sonocatalytic activity than pure polyphosphotungstate or pure PANI.

Sulfonated poly(arylene ether)/heteropolyacids nanocomposite membranes for proton exchange membrane fuel cells

Heteropolyacids (HPAs) are nano-size metal–oxygen clusters with anion acids stronger than the conventional solid acids. Insoluble and high surface area solid acids, i.e. Cs 2.5H 0.5PW 12O 40 (CsPW) and H 3PW 12O 40/SiO 2 (PWS) are dispersed throughout End-group cross-linkable, Sulfonated Fluorinated Bi- Phenol (ESF-BP) copolymer to produce organic–inorganic nanocomposite proton exchange membranes (PEMs). The molecular structure of ESF-BP copolymer is identified by 1H NMR spectra. The FT-IR of prepared CsPW and PWS inorganic powders show that the primary Keggin structure remains almost unaltered. The PEMs properties including density, water uptake, water diffusivity, swelling, ion exchange capacity (IEC), oxidative stability and proton conductivity are investigated to characterize the influence of added HPAs on the composite PEMs. The intermolecular interaction between HPAs and the sulfonic group of the copolymer affects the physico-chemical characteristics of membranes. The addition of HPAs to the polymer matrix increases oxidative stability, water uptake and conductivity but decreases the IEC, the water diffusivity and the density of membrane. The cells with the nanocomposite membranes have better performance than the cell with the plain polymer membrane. The prepared composite membrane using dispersion of PWS through the ESF-BP has superior characteristics and voltage–current responses in the PEMFC rather than using CsPW.

Dynamic Surface Structures and Reactivity of Vanadium-Containing Molybdophosphoric Acid (H3+xPMo12–xVxO40) Keggin Catalysts during Methanol Oxidation and Dehydration

The influence of introducing VOx sites into the primary, bulk structure of molybdenum-containing polyoxometalate H3+xPMo12–xVxO40 (x = 0, 1, 2, 3) and secondary, surface structure of supported VOx/H3PMo12O40 Keggins was investigated to establish their structure–reactivity/selectivity relationships. The resulting Keggins were physically characterized (solid-state 51V NMR and in situ FT-IR, Raman and UV–vis spectroscopy) and chemically probed with CH3OH (CH3OH-TPSR spectroscopy and steady-state methanol oxidation/dehydration). The introduction of the VOx sites into the primary Keggin structure resulted in structural disorder that facilitated decomposition of the Keggins at elevated temperatures and under the corrosive methanol reaction environment. The decomposition was reflected in the expelling of the VOx units from the primary Keggin structure into the secondary surface VOx sites where they reacted with surface acidic hydroxyls. For the first time, it was demonstrated that methanol is present as both sorbed species within the Keggin structure as well as surface reactive species during methanol reaction conditions at elevated temperatures (T < 400 °C). Introduction of the VOx sites increased the formaldehyde selectivity and decreased the dimethyl ether selectivity. The VOx units in the primary Keggin structure were slightly more active than the surface VOx species, and both VOx sites were significantly more active than the MoOx sites in the primary Keggin structure. The relatively constant TOFredox values with number of VOx sites in the Keggin reflect that only one VOx site is involved in methanol oxidation to formaldehyde. Although introducing VOx into the Keggin decreased the UV–vis edge energy (Eg) and increased the TOFredox relative to the V-free H3PMo12O40 Keggin, a direct relationship between TOFredox and Eg was not found to be present for the H3PMo12–xVxO40 Keggins. These new fundamental insights demonstrate the importance of performing in situ molecular spectroscopy characterization studies under reaction conditions and reveal that the Keggin structures are dynamic and should not be assumed to be ideal, static model catalytic structures.

Preparation and characterization of polyphosphotungstate/ZrO 2 nanocomposite and their sonocatalytic and photocatalytic activity under UV light illumination

Polyoxometalates (POM) supported on zirconia, H 3PW 12O 40/ZrO 2, were prepared by incorporating polyphosphotungstate into a zirconia matrix via sol–gel technique that involving the hydrolysis of zirconium (IV) n-butoxide, Zr ( n-OBu) 4, as the ZrO 2 source. This insoluble and readily separable catalyst was characterized by using XRD, FT-IR, SEM, and UV diffuse reflectance spectroscopy (UV-DRS), indicating that the polyphosphotungstate was chemically attached to the zirconia supports, and primary Keggin structure remained intact. The photocatalytic and sonocatalytic activity of the supported polyphosphotungstate was tested via degradation of different dyes in aqueous solutions. The POM–ZrO 2 nanocomposite showed higher photocatalytic and sonocatalytic activity than pure polyoxometalate or pure ZrO 2.

The Preparation of a Novel Composite Film Based on Polyoxometalate for Visible Light Photocatalysis

The composite film, FeW11O399-(FeW11)/TiO2/ SiO2, was prepared by liquid phase deposition method (LPD) at low processing temperature for the first time. Formation of the stable composite film is due to strong chemical reaction of ≡Ti-OH from TiO2/SiO2 with the surface oxygen atoms at vacant sites of FeW11. The structural properties of the film were characterized using FT-IR、UV-vis spectra, Elemental Analysis. The experimental results confirmed the retention of the primary Keggin structure and the saturation of the surface of the lacunary FeW11; Formation of Ti–O–Si bond implies that the TiO2 crystallites highly dispersed on the silica surface via oxygen as a bridge. The catalytic activity was tested by the photo-degradation of aqueous azo dye congo red (CR) (0-20 mmol•L-1). The resulting film exhibited efficient visible-light photocatalytic activity to decompose aqueous azo dye CR，which is mainly attributed to long time existence of Fenton-assisted FeW11/TiO2/SiO2 excitation state. According to the experimental results, a synergistic effect mechanism of the photodegradation process is proposed.

Zirconia-supported Keggin phosphomolybdovanadate nanocomposite: A heterogeneous and reusable catalyst for alkene epoxidation under thermal and ultrasonic irradiation conditions

Abstract Keggin phosphomolybdovanadate, PVMo, was supported on a zirconia matrix by the wet impregnation method. The phase and chemical structure, optical absorption, surface physicochemical properties and morphology of PVMo–ZrO 2 composite were studied by X-ray diffraction (XRD) technique, FT-IR and diffuse reflectance UV–vis spectroscopic (DR UV–vis) methods, and scanning electron microscopy (SEM), which indicated that the primary Keggin structure has remained intact after formation of the composite. Moreover, the obtained nanocomposite was used as an efficient catalyst of olefins under reflux and ultrasonic irradiation conditions. The catalyst was reused several times, without observable loss of activity and selectivity. Indeed, the catalytic activity of the PVMo–ZrO 2 was compared with pure Keggin phosphomolybdovanadate.

H3PW12O40/SiO2 for sorbitol dehydration to isosorbide: High efficient and reusable solid acid catalyst

Tungstophosphoric acids (PW) supported on various metal oxides (SiO2, γ-Al2O3, TiO2, ZrO2 and CeO2) were synthesized and used as catalysts for sorbitol dehydration to isosorbide for the first time. 30% PW/SiO2 exhibited the best catalytic performance for sorbitol dehydration, over which 56% isosorbide selectivity could be gained at a 95% sorbitol conversion at 250 °C. The catalytic performance of regenerated 30% PW/SiO2 catalysts by dichloromethane elution showed no loss after recycling five runs. Characterizations with UV-Vis, XRD, NH3-TPD and thermal analysis techniques revealed that PW had a good dispersion, and the primary Keggin structure was preserved after supporting PW on different supports. The interaction between PW and oxides resulted in different acidity of catalysts, which affected conversion of sorbitol and selectivity for isosorbide. The final acidity order of supported PW catalysts was the following: SiO2>γ-Al2O3>TiO2 ≈ ZrO2>CeO2.

Mesoporous titania-based H 3PW 12O 40 composite by a block copolymer surfactant-assisted templating route: Preparation, characterization, and heterogeneous photocatalytic properties

Mesoporous H 3PW 12O 40/TiO 2 composite with H 3PW 12O 40 loading from 3.2 to 16.6% was prepared by the combination of sol–gel chemistry, templated self-assembly, and solvothermal treatment in the presence of a triblock copolymer surfactant. At relatively low solvothermal treatment temperature (80 to 120 °C), as-prepared composite exhibited an anatase phase structure. Bulk and surface sensitive probe testing results indicated that the primary Keggin structure remained intact after formation of the composite, and strong interaction between the Keggin unit and TiO 2 network existed in the composite. Nitrogen gas porosimetry, low-angle XRD, and TEM testing results suggested the composite exhibited well-distributed three-dimensionally interconnected pore structure with pore diameter of 3.3–4.0 nm, BET surface area of 151.5–195.1 m 2 g − 1 , and pore volume of 0.17–0.26 cm 3 g − 1 . The composite was used as a heterogeneous photocatalyst to degradation of dyes with azoic (methyl orange), heteropolyaromatic (methylene blue), triarylmethane (crystal violet), and fluorescent (rhodamine B) structures; at the same time, adsorption behaviors of dyes on mesoporous H 3PW 12O 40/TiO 2 composite are tested. Finally, photodegradation pathway of dye crystal violet over mesoporous H 3PW 12O 40/TiO 2 was tentatively put forward based on the identified intermediates generated during the photocatalytic degradation process. The results shown in Fig. 9 demonstrate that PW 12/TiO 2 with higher H 3PW 12O 40 loading has higher photocatalytic activity.

Preparation and characterization of HMS supported 11-molybdo-vanado-phosphoric acid for selective oxidation of propylene

The preparation of Keggin-type phosphovanadomolybdic acid H 4PMo 11VO 40 (PMo 11V) supported on a hexagonal mesoporous silicate (HMS) material was performed via three different methods: dry impregnation, wet impregnation and mechanical mixing. The state of the heteropolyacid (HPA) on the siliceous surface and the integrity of the support were characterized by several physico-chemical techniques: elemental analysis, X-ray diffraction, transmission and diffuse reflectance (DR) FT–IR study of the framework vibrations, Raman and X-ray photoelectron spectroscopies, nitrogen physisorption, thermal analysis (TG–DTA) and scanning electron microscopy (SEM). The catalytic performance of supported materials was compared to that of the bulk H 4PMo 11VO 40 in the selective oxidation of propylene by molecular oxygen in the temperature range of 200–400 °C. The physico-chemical characterization of the supported HPA showed mainly that the primary Keggin structure is intact and that a good dispersion of the heteropolyacid on the HMS surface is obtained when the impregnation methods are used. In the case of mechanical mixture, the dispersion of heteropolyacid clusters is not homogeneous. The surface area of HMS decreased when the HPA was added. The impregnation methods lead to mesopores with uniform size of approximatively 3.2 nm in diameter. The TG–DTA showed that the supported HPAs obtained by the impregnation methods have a higher thermal stability than that of bulk HPA. The catalytic results showed that H 4PMo 11VO 40/HMS are more active than the bulk H 4PMo 11VO 40 and lead to oxygenated compounds formation (acrolein, acetaldehyde and acetic acid), whereas the bulk HPA only leads to the formation of CO x .

Surface and Bulk Aspects of Mixed Oxide Catalytic Nanoparticles: Oxidation and Dehydration of CH3OH by Polyoxometallates

The molecular structures and surface chemistry of mixed metal oxide heteropolyoxo vanadium tungstate (H(3+x)PW(12-x)V(x)O(40) with x = 0, 1, 2, and 3) Keggin nanoparticles (NPs), where vanadium is incorporated into the primary Keggin structure, and supported VO(x) on tungstophosphoric acid (TPA, H(3)PW(12)O(40)), where vanadium is present on the surface of the Keggin unit, were investigated with solid-state magic angle spinning (51)V NMR, FT-IR, in situ Raman, in situ UV-vis, CH(3)OH temperature-programmed surface reaction (TPSR), and steady-state methanol oxidation. The incorporated VO(x) unit possesses one terminal V horizontal lineO bond, four bridging V-O-W/V bonds, and one long V-O-P bond in the primary Keggin structure, and the supported VO(x) unit possesses a similar coordination in the secondary structure under ambient conditions. The specific redox reaction rate for VO(x) in the Keggin primary structure is comparable to that of bulk V(2)O(5) and the more active supported vanadium oxide catalysts. The specific acidic reaction rate for the WO(x) in the TPA Keggin, however, is orders of magnitude greater than found for bulk WO(3), supported tungsten oxide catalysts, and even the highly acidic WO(3)-ZrO(2) catalyst synthesized by coprecipitation of ammonium metatungstate and ZrO(OH)(2). From CH(3)OH-TPSR and in situ Raman spectroscopy it was found that incorporation of vanadium oxide into the primary Keggin structure is also accompanied by the formation of surface VO(x) species at secondary sites on the Keggin outer surface. Both CH(3)OH-TPSR and steady-state methanol oxidation studies demonstrated that the surface VO(x) species on the Keggin outer surface are significantly less active than the VO(x) species incorporated into the primary Keggin structure. The presence of the less active surface VO(x) sites in the Keggins, thus, decreases the specific reaction rates for both methanol oxidation and methanol dehydration. During methanol oxidation/dehydration (O(2)/CH(3)OH = 2.17, T = 225 degrees C), in situ UV-vis diffuse reflectance spectroscopy revealed that vanadium oxide is primarily present as the V(+5) cation, which reflects the Mars-van Krevelen redox mechanism and rapid reoxidation by molecular O(2). The bulk TPA Keggin structure becomes more disordered and less thermally stable as the vanadium content increases. Although surface polyaromatic carbon forms during methanol oxidation on the Keggin surfaces, its influence on the reaction kinetics seems minimal as the carbon content diminishes as the vanadium oxide content increases and the reaction temperature is raised. No relationships were found between the electronic structure (UV-vis E(g) values) and TOF(redox) and TOF(acid) (TOF = turnover frequency) kinetics, which reflect the complexity of H(3+x)PW(12-x)V(x)O(40) Keggins. The overall catalytic performance of the H(3+x)PW(12-x)V(x)O(40) Keggin materials results from a complex interplay among the presence of redox vanadium (as secondary surface VO(x) species and substituted VO(x) sites in the primary Keggin NP structure), structural disorder of the Keggin NPs, exposed surface acid and redox sites, and coke deposition. These new insights reveal that the Keggin heteropolyoxometallates are much more complex than originally thought and that care must be taken in using Keggins as model mixed metal oxide NPs in catalytic kinetic and theoretical studies because their surface and bulk structures are dynamic under the reaction conditions.

Heteropolyacid Nanoreactor with Double Acid Sites as a Highly Efficient and Reusable Catalyst for the Transesterification of Waste Cooking Oil

A solid zirconiumdodecatungstophosphate Zr0.7H0.2PW12O40 (abbreviated as ZrHPW) with nanotube structure has been synthesized using natural cellulose fiber as a template. The structure and morphology were characterized to show that the primary Keggin structure remained intact in a nanotube with large surface area (256 m2/g). On the basis of the research of acidity, ZrHPW exhibited higher acid strength, both Brønsted acidity and Lewis acidity in one to show higher efficiency, and reusable catalytic activity in biodiesel production from low-quality feedstocks. A synergistic effect results from the channels in the nanotube and doubly catalytic sites to provide an efficient approach to acidic sites for reactants, as well as enough space for subsequent recycling.