Preparation and photocatalytic activity of Keggin-ion tungstate and TiO2 hybrid layer-by-layer film composites

Keggin ions (PW₁₂O₄₀³⁻ (PW₁₂), SiW₁₂O₄₀⁴⁻ (SiW₁₂), H₂W₁₂O₄₀⁶⁻ (H₂W₁₂)) and TiO₂ hybrid thin films were prepared using the layer-by-layer method. Their photocatalytic activities were investigated using gaseous 2-propanol decomposition. All films were transparent in the visible wavelength range. For 2-propanol decomposition, H₂W₁₂ was the most effective for the combination with TiO₂ despite having the smallest TiO₂ deposition amount. The photocatalytic activity of the PW₁₂–TiO₂ hybrid film was increased 2.3 times by visible light with UV illumination. This increase was less remarkable for hybrid films of other Keggin ions, suggesting that the visible light excitation of reduced PW₁₂ plays an important role in the enhancement of 2-propanol decomposition.     

Biodiesel production from soybean oil catalyzed by multifunctionalized Ta2O5/SiO2-[H3PW12O40/R] (R = Me or Ph) hybrid catalyst

Mesoporous Ta₂O₅ materials functionalized with both alkyl group and a Keggin-type heteropoly acid, Ta₂O₅/SiO₂-[H₃PW₁₂O₄₀/R] (R = Me or Ph), was prepared by a single step sol–gel co-condensation method followed by a hydrothermal treatment in the presence of a triblock copolymer surfactant. The catalytic performance of the resulting multifunctionalized organic–inorganic hybrid materials was evaluated by a direct use of soybean oil for biodiesel production in the presence of 20 wt% myristic acid under atmosphere refluxing, and the influences of the catalyst preparation approaches, functional component loadings, and molar ratios of oil to methanol on the catalytic activity of the Ta₂O₅/SiO₂-[H₃PW₁₂O₄₀/R] were studied. In addition, the recyclability of the hybrid materials was evaluated via four catalytic runs. Finally, the network structures of the hybrid materials and the functions of the incorporated alkyl groups on the catalytic activity of the materials were put forward.     

17O MAS NMR study of 12-molybdophosphoric acid

¹⁷O MAS NMR spectra for¹⁷O enriched solid heteropoly acid H₃PMo₁₂O₄₀ are reported. The oxygen exchange between solid H₃PMo₁₂ ¹⁷O₄₀ and H₂ ¹⁶O vapor at 200–250°C is shown to be accompanied by fast mixing of terminal Mo=O and bridging Mo-O-Mo oxygens in the Keggin unit. The oxygen exchange in H₃PMo₁₂ ¹⁷O₄₀ is much faster than in H₃PW₁₂ ¹⁷O₄₀ in solution as well as in the gas phase.     

Influence of the nature and porosity of different supports on the acidic and catalytic properties of H3PW12O40

In order to obtain highly dispersed heteropolyacid (HPA) species, H₃PW₁₂O₄₀ was supported on various supports exhibiting different porosities and surface chemical properties. Amorphous and monoclinic amphoteric zirconias, activated montmorillonite (AC) and hexagonal silica (HMS) were chosen as supports. It was observed that the zirconia support partly decomposed HPA at low coverage, giving the lacunary anion PW₁₁O₃₉ ^7–, due to the reaction with basic hydroxyl groups, whereas montmorillonite and HMS did not. Catalytic properties for n-butane to isobutane isomerisation at 473 K and propan-2-ol decomposition at 353 K were compared for all samples as a function of HPA loadings and compared to data already published on bulk H₃PW₁₂O₄₀ and Cs_1.9H_1.1PW₁₂O₄₀ samples. It was found for HPA/AC samples that, although their activity for propan-2-ol decomposition varied linearly with HPA loading, their activity for n-butane isomerisation was very weak, which indicates a weaker acid strength by supporting the HPA. This is probably due to the exchange of protons from the HPA by the exchangeable cations of the montmorillonite, the new protons associated with the clay being much less acidic. It also appeared, when comparing with catalytic data already published on for HPA/HMS, that HMS was the best support for HPA without modifying appreciably its catalytic and thus their acid properties, the HPA being certainly bonded to the HMS walls by hydrogen bonding. Assuming a diameter of 1.2 nm for each Keggin anion, the turnover frequency (TOF) values for n-butane isomerisation at 473 K were calculated per surface Keggin species for HPA/HMS, bulk H₃PW₁₂O₄₀ and Cs_1.9H_1.1PW₁₂O₄₀, and were found to be equal to 155, 113 and 100×10^–5 s^–1, respectively, and thus to be very close, showing that the acid strength of the three samples was comparable. It was found to equal only 3.4×10^–5 s^–1 for the HPA/AC sample, in agreement with a weaker acid strength. At last, mesoporosity of the support was found to favour n-butane isomerisation reaction.     

Cs exchanged phosphotungstic acid as an efficient catalyst for liquid-phase Beckmann rearrangement of oximes

Cs exchanged phosphotungstic acid is a highly efficient and environmentally benign solid acid catalyst for the liquid-phase Beckmann rearrangement of ketoximes to the corresponding amides. The catalysts Cs_xH_3−xPW₁₂O₄₀ (x = 1.5, 2, 2.5 and 3) were prepared by a titration method. The characterization results indicated that the primary Keggin structure remained intact after exchanging the protons with Cs ions. Moreover, the Cs exchanged catalysts were insoluble and exhibited larger BET surface area than the parent acid. The catalysts exhibited high reactivity and selectivity for the formation of -caprolactam, the precursor of Nylon 6, from cyclohexanone oxime. The catalyst can be recovered after reaction without any structural transformation.     

磷钨酸盐催化转化葡萄糖合成乙酰丙酸

制备了一系列金属离子修饰的磷钨酸盐(M_XH_3-2XPW₁₂O₄₀,M=Zn,Cu,Cs,Ag)催化剂,并将磷钨酸银盐(Ag₃PW₁₂O₄₀)用于水解葡萄糖制备乙酰丙酸的实验中。采用FTIR、XRD、SEM和EPMA等技术对磷钨酸盐性能进行了表征,并分析了磷钨酸银盐催化剂在反应前后结构和表面元素相对含量的变化。考察了反应温度、反应时间、催化剂用量和葡萄糖浓度等对乙酰丙酸得率的影响。结果表明:合成的磷钨酸盐具有磷钨酸的Keggin结构,并且Ag₃PW₁₂O₄₀催化剂在多次使用后Keggin结构没有被破坏。在催化合成反应中,在反应温度200℃、反应时间2 h、Ag₃PW₁₂O₄₀催化剂用量0.7 g和葡萄糖浓度40 g·L^-1的条件下,乙酰丙酸的最大得率可达到81.61%,催化剂可重复利用。     

17O NMR determination of proton sites in solid heteropoly acid H3PW12O40.31P,29Si and17O NMR,FT-IR and XRD study of H3PW12O40 and H4SiW12O40 supported on carbon

¹⁷O MAS NMR spectra for solid heteropoly acid (HPA) H₃PW₁₂O₄₀ are reported. Comparison of solid-state and solution¹⁷O resonances shows that in the solid dehydrated H₃PW₁₂O₄₀ terminal W=O oxygen atoms are the predominant protonation sites. H₃PW₁₂O₄₀ and H₄SiW₁₂O₄₀ supported on chemically activated carbon have been studied by means of NMR, FT-IR and XRD. The carbon-supported HPA's retain their Keggin structure and form finely dispersed HPA species. No HPA crystal phase is developed even at an HPA loading as high as 45 wt%.³¹P,²⁹Si and¹⁷O MAS NMR spectra for bulk and carbon-supported HPA's indicate interaction of the HPA Keggin units with the carbon surface, causing large line broadening in the NMR spectra.     

Single-walled carbon nanotubes functionalized with polydiphenylamine as active materials for applications in the supercapacitors field

Composites based on polydiphenylamine (PDPA) doped with heteropolyanions of H₃PW₁₂O₄₀ and single-walled carbon nanotubes (SWNTs) were prepared by electrochemical polymerization of diphenylamine (DPA) on carbon nanotube films deposited onto Pt electrodes. HRTEM studies reveal that the electrochemical polymerization leads to the filling the spaces between tubes which compose the bundles, creating a monolithic film on the Pt electrode. The resulting composites were tested as active materials in supercapacitors. Resonant Raman scattering studies showed that the electropolymerization of DPA in the presence of H₃PW₁₂O₄₀ and SWNTs leads to the covalent functionalization of SWNTs with doped PDPA. The covalent functionalization of SWNTs with PDPA doped with H₃PW₁₂O₄₀ heteropolyanions was revealed by FTIR spectroscopy, based on the changes in the vibrational features of PDPA and H₃PW₁₂O₄₀. These changes included i) a down-shift of the PDPA IR bands, which was attributed to the C–H bending vibrational mode of benzene (B), C_aromatic–N, C–C stretching (B) + C–H bending (B) and C–C stretching vibrations of the B ring, from 1174, 1321, 1495 and 1603 cm^− 1 to 1165, 1313, 1487 and 1599 cm^− 1, respectively; ii) a change in the peak positions of IR bands associated with the W = O and P-O-W vibration modes of H₃PW₁₂O₄₀; and iii) a down-shift of the IR band situated in the spectral range 650–725 cm^− 1, which was assigned to the inter-ring deformation vibration mode.The characterization of symmetric solid-state supercapacitors was performed for electrodes prepared from i) SWNTs functionalized with PDPA doped with H₃PW₁₂O₄₀ heteropolyanions, ii) SWNTs electrochemically decorated with H₃PW₁₂O₄₀ heteropolyanions, and iii) PDPA doped with H₃PW₁₂O₄₀ heteropolyanions. Preliminary results indicate high discharge capacitance values of up to 157.2 mF/cm² for SWNTs functionalized with PDPA doped with H₃PW₁₂O₄₀ heteropolyanions. The discharge capacitance of this material is superior to those recorded for SWNTs electrochemically decorated with H₃PW₁₂O₄₀ heteropolyanions (~ 18.2 mF/cm²) and PDPA doped with H₃PW₁₂O₄₀ heteropolyanions (~ 62.1 mF/cm²).     

Micropore size distribution by argon porosimetry for cesium hydrogen salts of 12-tungstophosphoric acid

Micropore size distributions of Cs_2.1H_0.9PW₁₂O₄₀ and 0.5 wt% Pt-Cs_2.1H_0.9PW₁₂O₄₀ as well as H-ZSM-5, H-Y and AlPO₄-5 as standard porous materials were analyzed by Ar porosimetry, assuming cylindrical pores consisting of oxide ions. The calculated pore-widths of H-ZSM-5, HY and AlPO₄-5 were in agreement with the pore-opening sizes determined by XRD. The Ar porosimetry demonstrated that Cs_2.1H_0.9PW₁₂O₄₀ and 0.5 wt% Pt-Cs_2.1H_0.9PW₁₂O₄₀ possess only ultramicropores having widths of about 0.5 nm. Adsorption of various molecules revealed that the pore widths of these heteropoly compounds were in the range 0.43–0.59 nm, which is consistent with the results of the Ar porosimetry. In conclusion, nearly uniformly sized micropores are formed on both Cs_2.1H_0.9PW₁₂O₄₀ and 0.5 wt% Pt-Cs_2.1H_0.9PW₁₂O₄₀.     

Fabrication of zero-thermal-expansion ZrSiO4/Y2W3O12 sintered body

We focused on the linear negative thermal expansion of Y₂W₃O₁₂ in a wide-temperature range and on the chemical stability of ZrSiO₄ in the fabrication of the composite material ZrSiO₄/Y₂W₃O₁₂ with a zero-thermal-expansion. The compact composed of Y₂W₃O₁₂ and ZrSiO₄ had a thermal shrinkage rate smaller than that of Y₂W₃O₁₂ and higher than that of ZrSiO₄. SEM–EDX observation clarified that the ZrSiO₄/Y₂W₃O₁₂ sintered body fabricated at 1400 °C for 10 h had a microstructure composed of ZrSiO₄ and Y₂W₃O₁₂ grains, and XRD indicated that only ZrSiO₄ and Y₂W₃O₁₂ phases existed in the sintered body. The relative density of the ZrSiO₄/Y₂W₃O₁₂ sintered body reached 92%, which was larger than that of the ZrSiO₄ sintered body because Y₂W₃O₁₂ grains could be sintered at lower temperatures. The average linear thermal expansion coefficients of the ZrSiO₄/Y₂W₃O₁₂ sintered body were −0.4 × 10⁻⁶ and −0.08 × 10⁻⁶ °C⁻¹ in the temperature ranges from 25 to 500 °C and from 25 to 1000 °C, respectively, which showed an almost zero-thermal-expansion.     

Effect of CsxH3?xPW12O40 addition on the catalytic performance of ZnFe2O4 in the oxidative dehydrogenation of n-butene to 1,3-butadiene

Oxidative dehydrogenation of n-butene to 1,3-butadiene over ZnFe₂O₄ catalyst mixed with Cs _x H_3−x PW₁₂O₄₀ heteropolyacid (HPA) was performed in a continuous flow fixed-bed reactor. The effect of Cs _x H_3−x PW₁₂O₄₀ addition on the catalytic performance of ZnFe₂O₄ was investigated. Cs _x H_3−x PW₁₂O₄₀ itself showed very low catalytic performance in the oxidative dehydrogenation of n-butene. However, addition of small amount of Cs _x H_3−x PW₁₂O₄₀ into ZnFe₂O₄ enhanced the catalytic performance of ZnFe₂O₄ catalyst. The catalytic performance of ZnFe₂O₄-Cs _x H_3−x PW₁₂O₄₀ mixed catalysts was closely related to the surface acidity of Cs _x H_3−x PW₁₂O₄₀. Among the catalysts tested, ZnFe₂O₄-Cs_2.5H_0.5 PW₁₂O₄₀ mixed catalyst showed the best catalytic performance. Strong acid strength and large surface acidity of Cs_2.5H_0.5PW₁₂O₄₀ was responsible for high catalytic performance of ZnFe₂O₄-Cs_2.5H_0.5PW₁₂O₄₀ mixed catalyst. Thus, Cs_2.5H_0.5PW₁₂O₄₀ could be utilized as an efficient promoter and diluent in formulating ZnFe₂O₄ catalyst for the oxidative dehydrogenation of n-butene.     

H3PW12O40 acid dispersed on its Cs salt: improvement of its catalytic properties by mechanical mixture and grinding

Acidic cesium salts Cs_x H_3-xPW₁₂O₄₀ have been prepared by grinding together amounts of H₃PW₁₂O₄₀ and porous Cs₃PW₁₂O₄₀ compounds in varying stoichiometries. It is shown that this procedure leads to a dispersion of the acid on top of the high surface area Cs₃PW₁₂O₄₀ salt (160 m² g^-1), and, subsequently, yields high surface area materials which exhibit a much higher catalytic activity for n-butane isomerisation at 473 K when compared with samples prepared directly by chemical precipitation. This improvement holds particularly true with low Cs content (x < 2). This revised version was published online in July 2006 with corrections to the Cover Date.     

On the active site in H3PW12O40/SiO2 catalysts for fine chemical synthesis

The surface environment and structural evolution of silica supported phosphotungstic acid (H₃PW₁₂O₄₀) catalysts have been investigated as a function of acid loading. H₃PW₁₂O₄₀ clusters are deposited intact upon the silica surface, adopting a Stranksi-Krastanov growth mode forming a two-dimensional adlayer which saturates at 45wt% acid. Intimate contact with the silica support perturbs the local chemical environment of three tungstate centres, which become inequivalent with those in the remaining cluster, suggesting an adsorption mode involving three terminal W==O groups. Above the monolayer, H₃PW₁₂O₄₀ clusters form three-dimensional crystallites with physico-chemical properties indistinguishable from those in the bulk heteropoly acid. These H₃PW₁₂O₄₀/SiO₂ materials are efficient for the solventless isomerisation of α-pinene under mild reaction conditions. Activity scales directly with the number of accessible perturbed tungstate sites at the silica interface; these are the active species.     

Highly efficient synthesis of coumarin derivatives in the presence of H14[NaP5W30O110] as a green and reusable catalyst

A variety of heteropolyanions including: Keggin, Dawson, Preyssler, mixed addenda and sandwich types, catalyzed the formation of 4-methylnaphtho-(1,2-b)-pyran-2-one (coumarin) from the condensation of α-naphthol and ethylacetoacetate in a solvent free system and under heating conditions. Our data vividly indicate that sodium30–tungsto pentaphosphate, [NaP₅W₃₀O₁₁₀]¹⁴⁻, which so-called Preyssler’s anion, with high hydrolytic (pH 0–12) and thermal stability is the catalyst of choice. This catalyst catalyzed the synthesis of other coumarin derivatives in high yields and good selectivity.     

Synthesis, crystal structure and NMR of [Na(DB18C6)(CH3CN)]3[α-AsM12O40] (M = Mo/W)

Two super-molecular complexes [Na(DB18C6)(CH₃CN)]₃[α-AsM₁₂O₄₀] (M = Mo/W) were obtained by solvothermal reaction and characterized by IR,¹H ¹³C and gUMBC NMR, X-ray. The result reveals that the complex consists of a [α-AsM₁₂O₄₀]³⁻ (M = Mo/W) anion with α-Keggin structure, and three complex [Na (DB18C6)(CH₃CN)]⁺ cations in which every sodium ion is located in the cavity of dibenzo-18-crown-6 with 6 Na–O bonds and coordinated with one of the terminal O atom of [α-AsM₁₂O₄₀]³⁻ (M = Mo/W) and the N atom of CH₃CN from two sides of the distorted DB18C6 plane, respectively. The three terminal O atoms linked with sodium ion are from a single M₃O₁₃ (M = Mo/W) triplet of the α-Keggin metalatoarsenate anion, and M–O_b (M = Mo/W) bonds exhibit alternating single–double bond character.     

V-, Mo- and W-containing layered double hydroxides as effective catalysts for mild oxidation of thioethers and thiophenes with H2O2

Catalytic oxidation of thioethers and thiophene derivatives with H₂O₂ was performed at 40 °C and atmospheric pressure, in presence of W-, V- and Mo-containing layered double hydroxides (LDH). The catalysts were prepared by direct ion exchange with metal-oxoanions, i.e. WO₄²⁻, W₇O₂₄⁶⁻, V₂O₇⁴⁻, V₁₀O₂₈⁶⁻, MoO₄²⁻ and Mo₇O₂₄⁶⁻, of the LDH containing aluminum and magnesium atoms in the brucite layer. All the catalysts showed good activity and selectivity in the sulfoxidation reaction, but the catalyst performances strongly depended on the nature of the anion species intercalated in the interlayer gallery. Thus, the W-based LDH was more active and more stable than the V-LDH and Mo-LDH catalysts. The conversion of sulfur-containing compounds is also dependent on the nucleophilicity of substrates and the following order of reactivity was observed benzothiophene < dibenzothiophene < diphenyl-sulfide < benzyl-phenyl-sulfide < methyl-phenyl-sulfide.     

One-pot generation of mesoporous carbon supported nanocrystalline H3PW12O40 heteropoly acid with high performance in microwave esterification of acetic acid and isoamyl alcohol

Ordered mesoporous carbon-supported H₃PW₁₂O₄₀ heteropoly acid materials (HPW/OMCs) have been rationally synthesized for the first time. The method is based on the evaporation-induced triconstituent co-assembly effect using the sol–gel process, wherein soluble resol polymer is used as an organic precursor, and triblock copolymer F127 is used as a template. The ordered mesoporous carbon-supported H₃PW₁₂O₄₀ heteropoly acid materials were analyzed and characterized by X-ray diffraction, N₂ adsorption and desorption (BET), and transmission electron microscope. The mesoporous carbon-supported H₃PW₁₂O₄₀ materials possess an ordered mesostructure, narrow pore size distributions (around 2.8–3.6 nm), high pore volumes (up to 0.49 cm³ g^?1), high specific BET surface areas (up to 590 m² g^?1), tailorable HPW content (up to 30 wt%), and well dispersion of HPW particles. Moreover, the resultant mesoporous ordered mesoporous carbon-supported H₃PW₁₂O₄₀ materials exhibit high catalytic activity in microwave esterification of acetic acid and isoamyl alcohol. The obtained 20 % HPW/OMC catalyst exhibits high catalytic performance with 96.7 % of isoamyl acetate yield at temperature of 120 °C, alcohol/acid molar ratio of 2, catalyst amount of 0.2 g, microwave irradiation power of 800 W, and reaction time of 18 min. It was believed that the concentration of H₃PW₁₂O₄₀ have a crucial effect on the HPW/OMCs’ porosity, mesostructure and catalytic performance.     

An intertwined double chain constructed by [SiMo12O40] clusters and unusual [Cu2(DF)2H2O] dimers

A new compound, {[Cu₂(DF)₂H₂O]₂SiMo₁₂O₄₀} (1) (DF = 4,5-diazafluoren-9-one), has been hydrothermally synthesized and characterized by routine methods and single crystal X-ray diffraction. In 1, all the DF only adopt the unusual bidentate coordination mode for the first time coordinating to Cu atoms to form copper-dimers which link [SiMo₁₂O₄₀]⁴⁻ clusters obtaining an intertwined double chain. Furthermore, the luminescent and electrochemical properties of the title compound have been studied.     

Synthesis and properties of a zeolite LEV analogue from the system—Na2O–Al2O3–SiO2–N,N-dimethylpiperidine chloride–H2O

A new structure-directing agent (SDA) was firstly reported for the synthesis of a zeolite LEV analogue. N,N-dimethyl piperidine performed the SDA function, and induced the synthesis of products from a zeolite MOR with 12-ring channels to a zeolite LEV analogue with only 8-ring channels. The zeolite LEV analogue was synthesized from gels with initial compositions (5.0–6.0)Na₂O–Al₂O₃–(10–200)SiO₂–(4.0–8.0)N,N-dimethyl piperidine–400H₂O at 150 °C. The ²⁹Si NMR spectra showed that the relative intensities of the first line at −115 ppm for low Si/Al ratios were lower than that at high Si/Al ratios. Varying ion exchanges led to different acidities in the zeolite LEV analogue, with the acidity of H-LEV-HCl higher than that of H-LEV-NH₃·H₂O. Zeolite H-LEV in hydration of propene showed a higher selectivity of 1-propanol.     

Tetrahydrofuran polymerization initiated by heteropolyacid in the presence of ethylene oxide: Reaction behavior of water and glycol

The reaction behavior of water and low molecular weight glycol in tetrahydrofuran polymerization initiated by H₃PW₁₂O₄₀ in the presence of ethylene oxide has been studied. A lot of water was used in the hydrolysis reaction of ethylene oxide at the early stage of the polymerization and transformed into ethylene glycol (EG), which was consumed subsequently through a chain transfer reaction. EG was more reactive both than 1,4‐butylene glycol and hexamethylene glycol toward propagating species, and the reaction rate constants at 0°C were determined by GC to be 0.142, 8.83 × 10⁻², and 5.53 × 10⁻² L · mol⁻¹s⁻¹, respectively. The molecular weight of the product can be predicted by an equation based upon conversion of polymerization and the concentrations of molecular weight controller and H₃PW₁₂O₄₀. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 74: 1821–1826, 1999