Liquid-phase oxidation of benzene to phenol by molecular oxygen over transition metal substituted polyoxometalate compounds

Transition metal substituted polyoxometalate (TMSP) compounds were used as catalysts for the liquid-phase oxidation of benzene to phenol by molecular oxygen with ascorbic acid as a reducing agent in an acetone/sulfolane/water mixed solvent. [(C₄H₉)₄N]₅[PW₁₁CuO₃₉(H₂O)] is the best catalyst tested in this study. It showed 9.2% benzene conversion (TON = 25.8) and 91.8% selectivity to phenol for the oxidation of benzene at 323 K for 12 h. The effect of the substituted transition metal in the TMSP compounds on the benzene conversion is in the order: Cu > V > Fe ≫ Mn > Ti > Cr > Co > Ni > Zn. The effect of the central atom in the TMSP compounds on the benzene conversion is in the order: P ≈ Si > Ge > B. [SiW₁₁O₃₉]⁸⁻ is a good polyoxometalate ligand for transition metal ions as [PW₁₁O₃₉]⁷⁻ for the oxidation of benzene. The selectivity to phenol was dramatically improved by adding the sulfolane solvent, but the benzene conversion decreased when a large amount of sulfolane was used in the mixed solvent. Ascorbic acid is indispensable for forming phenol from benzene oxidation by O₂ over TMSP compounds. Higher O₂ pressure in the catalytic system ensured more oxygen molecules solving in the solvent, and promoted the benzene conversion.     

The effect of sintering temperature and time on microwave properties of Sm(Zn1/2Ti1/2)O3 ceramics for resonators

The microwave dielectric properties of Sm(Zn_1/2Ti_1/2)O₃ ceramics have been investigated. Sm(Zn_1/2Ti_1/2)O₃ ceramics were prepared by conventional solid-state route with various sintering temperatures and times. The prepared Sm(Zn_1/2Ti_1/2)O₃ exhibited a mixture of Zn and Ti showing 1:1 order in the B-site. Higher sintered density of 7.01 g/cm³ can be produced at 1310 °C for 2 h. The dielectric constant values (ɛ_r) of 22–31 and the Q × f values of 4700–37,000 (at 8 GHz) can be obtained when the sintering temperatures are in the range of 1250–1370 °C for 2 h. The temperature coefficient of resonant frequency τ_f was a function of sintering temperature. The ɛ_r value of 31, Q  ×  f value of 37,000 (at 8 GHz) and τ_f value of −19 ppm/°C were obtained for Sm(Zn_1/2Ti_1/2)O₃ ceramics sintered at 1310 °C for 2 h. For applications of high selective microwave ceramic resonator, filter and antenna, Sm(Zn_1/2Ti_1/2)O₃ is proposed as a suitable material candidate.     

Precursor phenomenon on ferroelectric transition in multiferroic YMn2O5

Series of RMn₂O₅ (R = Sm-Lu, Bi, Y) are simultaneously ferroelectric and antiferromagnetic at low temperatures. The ferroelectric Curie temperature (T_CE = 25–40 K) is slightly lower than the antiferromagnetic Néel temperature (T_N = ～45 K), implying that the ferroelectricity is induced with the antiferromagnetic long-range order in RMn₂O₅. Previously, we have investigated the thermal expansion anomaly in YMn₂O₅ at higher temperatures than T_N = 45 K, which would be the precursor phenomenon relating to the ferroelectric transition. This study tries to investigate a further detailed precursor phenomenon to make clear the origin of the ferroelectricity in YMn₂O₅. Particularly, we focus on the temperature factors of each cation in the temperature range from 50 K to 170 K. The single crystal X-ray diffraction of YMn₂O₅ was integrated at 298 K and 112 K by using a single crystal diffractometer with an imaging plate. The structural parameters were well refined as Pbam at both temperatures. In the refined result, it was found that the thermal ellipsoid became more anisotropic for the Mn or the Y ions with decreasing temperature from 298 K to 112 K. In addition, the long principal axis of the thermal ellipsoids rotated in the a–b plane with decrease of temperature. At 112 K, the long principal axis of Mn³⁺ thermal ellipsoid in a–b plane is almost parallel to the direction from the apex to the base of the pyramid square. The direction agrees with a possible ionic displacement speculated from the irreducible representation analysis.     

Polymorphism in the Lu2 − xYxSi2O7 system at high temperatures

Samples in the system Lu_2 − xY_xSi₂O₇ (1.25 ≤ × ≤ 2) have been synthesised following a sol–gel method and calcined to high temperatures (≥1400 °C). X-ray diffraction (XRD) has shown that all compositions crystallize as β-Lu_2 − xY_xSi₂O₇ at the low temperatures, while increasing calcination temperature produces the formation of the γ- and δ-polymorphs, the temperatures of formation of each polymorph depending on the Y/Lu ratio. Unit cell parameters of the samples crystallizing as γ-Lu_2 − xY_xSi₂O₇ have been calculated and plotted as a function of composition. They show a linear change with increasing Y content, indicating a degree of solid solubility of Lu₂Si₂O₇ in γ-Y₂Si₂O₇. Based on these data and on those reported in our previous studies [Becerro, A.I. and Escudero, A., XRD and ²⁹Si MAS NMR spectroscopy across the β-Lu₂Si₂O₇–β-Y₂Si₂O₇ solid solution. J. Solid State Chem., 2005, 178; Becerro, A.I. and Escudero, A., Phase transitions in Lu-doped Y₂Si₂O₇ at high temperatures. Chem. Mater., 2005, 17, 112] a temperature–composition diagram of the Lu₂Si₂O₇–Y₂Si₂O₇ system is given. Finally, the influence of Lu on the reversibility of the γ-Y₂Si₂O₇ → β-Y₂Si₂O₇ transition is studied by means of XRD and ²⁹Si MAS NMR spectroscopy.     

Recyclable,green and efficient epoxidation of olefins in water with hydrogen peroxide catalyzed by polyoxometalate nanocapsule

A practical method for the selective epoxidation of alkenes was discovered using H_xPMo₁₂O₄₀ ⊂ H₄Mo₇₂Fe₃₀(CH₃COO)₁₅O₂₅₄ as a catalyst in the presence of H₂O₂ as a green oxidant. However, the simple catalyst system involving polyoxometalates and H₂O₂ exercised the most successful system in obtaining high to excellent yields of epoxide products for different alkenes, including aromatic and aliphatic alkenes at room temperature in water. The effectiveness of this catalyst is evidenced by 99% selectivity to epoxide and 97–99% efficiency of H₂O₂ utilization. The stability of PMo ⊂ Mo₇₂Fe₃₀ under a catalytic reaction has been confirmed by XRD, FT-IR and Raman spectroscopies.     

Effects of mechanical milling on preparation and properties of CuAl1−xFexO2 thermoelectric ceramics

CuAl_1?xFe_xO₂ (x = 0, 0.1, and 0.2) thermoelectric ceramics produced by a reaction-sintering process were investigated. Pure CuAlO₂ and CuAl_0.9Fe_0.1O₂ were obtained. Minor CuAl₂O₄ phase formed in CuAl_0.8Fe_0.2O₂. Addition of 10 mol% Fe lowered the sintering temperature obviously and enhanced the grain growth. At x = 0.1, electrical conductivity = 3.143 Ω^?1 cm^?1, Seebeck coefficient = 418 μV K^?1, and power factor = 5.49 × 10^?5 W m^?1 K^?2 at 600 °C were obtained. The reaction-sintering process is simple and effective in preparing CuAlO₂ and CuAl_0.9Fe_0.1O₂ thermoelectric ceramics for applications at high temperatures.     

Effect of substrate temperature on microstructure and optical properties of nanocrystalline alumina thin films

Aluminum oxide (Al₂O₃) thin films were deposited on silicon (100) and quartz substrates by pulsed laser deposition (PLD) at an optimized oxygen partial pressure of 3.0×10^?3 mbar in the substrate temperatures range 300–973 K. The films were characterized by X-ray diffraction, transmission electron microscopy, atomic force microscopy, spectroscopic ellipsometry, UV–visible spectroscopy and nanoindentation. The X-ray diffraction studies showed that the films deposited at low substrate temperatures (300–673 K) were amorphous Al₂O₃, whereas those deposited at higher temperatures (≥773 K) were polycrystalline cubic γ-Al₂O₃. The transmission electron microscopy studies of the film prepared at 673 K, showed diffuse ring pattern indicating the amorphous nature of Al₂O₃. The surface morphology of the films was examined by atomic force microscopy showing dense and uniform nanostructures with increased surface roughness from 0.3 to 2.3 nm with increasing substrate temperature. The optical studies were carried out by ellipsometry in the energy range 1.5–5.5 eV and revealed that the refractive index increased from 1.69 to 1.75 (λ=632.8 nm) with increasing substrate temperature. The UV–visible spectroscopy analysis indicated higher transmittance (>80%) for all the films. Nanoindentation studies revealed the hardness values of 20.8 and 24.7 GPa for the films prepared at 300 K and 973 K respectively.     

Solubility and microstructural development of TiO2-containing 3Al2O3·2SiO2 and 2Al2O3·SiO2 mullites obtained from single-phase gels

The interdependence of the titanium oxide amount and the anisotropic growth of mullites prepared from single-phase gels were investigated. Gels with stoichiometries 3(Al_2−xTi_xO₃)·2(SiO₂) and 2(Al_2−xTi_xO₃)·(SiO₂), with 0 ≤ x ≤ 0.15 were prepared by the semialkoxide method. Gels and specimens heated at temperatures between 1200 and 1600 °C were characterized by using infrared spectroscopy (IR), X-ray diffraction (XRD) and transmission and field emission scanning electron microscopies (TEM and FESEM). Al₂TiO₅ as minor impurity was detected in both series of mullites for gel precursor compositions x = 0.10 and x = 0.15, obtained at temperatures between 1200 and 1600 °C. Variations of lattice parameters of mullite, processed at temperatures from the range between 1400 and 1600 °C, with the starting nominal amount of titanium oxide indicated that the solubility limit of titanium oxide was in ranges 3.8–4.1 and 4.1–4.4 wt% TiO₂ for 3:2 and 2:1 mullites series, respectively. The anisotropic growth of titanium-doped mullite crystalline grains was significant only when the nominal amount of titanium oxide exceeded the limit of solubility into the mullite structure (for both mullite series). Stronger anisotropy occurred for the 3:2 series specimens, i.e. for the SiO₂-richer mullites. In both series of mullites, the anisotropic grain growth was observed for the process temperatures higher than 1400 °C; the crystalline grains of mullites processed at lower temperatures were equiaxials and of almost the same size.     

Preparation and catalytic activity of K4Zr5O12 for the oxidation of soot from vehicle engine emissions

Pure phase K₄Zr₅O₁₂ is synthesized via solid state method in the present work. Various K/Zr ratios and temperatures are applied, and the synthesis process is investigated in detail by means of X-ray diffraction, Fourier transform infrared spectroscopy, scanning electron microscopy, and X-ray photoelectron spectroscopy. Its catalytic activity for soot oxidation is studied by temperature programmed oxidation with different types of soot/catalyst contacts. It is revealed that K₄Zr₅O₁₂ is very active in the presence of 2–10% O₂ for both tight and loose contacts (T_p(tight) = 335 °C, T_p(ethanol) = 355 °C and T_p(shaking) = 370 °C). Thermal stability study shows that K₄Zr₅O₁₂ is highly stable up to at least 900 °C.     

Low-temperature synthesis and microwave dielectric properties of trirutile-structure MgTa2O6 ceramics by aqueous sol–gel process

Trirutile-structure MgTa₂O₆ ceramics were prepared by aqueous sol–gel method and microwave dielectric properties were investigated. Highly reactive nanosized MgTa₂O₆ powders were successfully synthesized at 500 °C in oxygen atmosphere with particle sizes of 20–40 nm. The evolution of phase formation was detected by DTA–TG and XRD. Sintering characteristic and microwave dielectric properties of MgTa₂O₆ ceramics were studied at different temperatures ranging from 1100 to 1300 °C. With the increase of sintering temperature, density, ?_r and Q · f values increased and saturated at 1200 °C with excellent microwave properties of ?_r ～ 30.1, Q · f ～ 57,300 GHz and τ_f ～ 29 ppm/°C. The sintering temperature of MgTa₂O₆ ceramics was significantly reduced by aqueous sol–gel process compared to conventional solid-state method.     

Influence of the support nature and morphology on the performance of ruthenium catalysts for partial hydrogenation of benzene in liquid phase

This paper is aimed at studying the influence of the support nature and morphology on the performance of ruthenium catalysts for partial hydrogenation of benzene in liquid phase. Therefore, Al₂O₃ and Nb₂O₅ supports were employed with different values of particle diameter and superficial specific area. The catalysts were prepared through incipient impregnation from an aqueous solution of the RuCl₃·xH₂O precursor. After impregnation, the solids underwent a reduction treatment under H₂ flow at the temperature of 573 K. The solids were characterized through the techniques of particle size distribution, XRD, BET, SEM + EDX and TPR. The catalytic performances were evaluated within the hydrogenation of benzene in liquid phase, conducted at 373 K under constant pressure of 5.0 MPa of H₂. For the conditions employed, the results show that the support nature practically exerts no influence upon the selectivity of the intermediate product to the benzene → cyclohexene → cyclohexane reaction. However, the increase in the particle diameter or in the superficial specific area of the support decreases the yield of cyclohexene.     

The low-temperature sintering of M-type hexaferrites

The sintering of the M-type hexaferrites, AFe₁₂O₁₉ (A = Ba, Sr), was studied. The effects of the A cation type, the partial CuO substitution for Fe₂O₃ and the Fe deficiency on the sintering were examined. Samples with nominal compositions of Ba_1?zSr_zCu_xFe_y?xO_19?δ (z = 0 or 0.5, x = 0–1.0 and y = 11–12) were prepared with a solid-state reaction at 1000 °C. The CuO addition enhanced significantly the sintering rate of the hexaferrites at temperatures below 1100 °C, while a moderate effect on the sintering was caused by a partial substitution of Sr for Ba. The diffusion studies showed that the CuO addition enabled the reactive liquid-phase sintering of hexaferrites at 1000 °C. The relative sintered densities of the Cu-containing hexaferrite ceramics sintered at 1000 °C for 3 h were around 90%. The addition of CuO had a strong effect on the coercivity, but only a moderate effect on the magnetization of the studied ceramics.     

Modeling thermal and catalytic conversion of decalin under industrial FCC operating conditions

The processability of decalin, a two-fused ring cycloparaffin, in the absence and presence of USY catalysts of different zeolite crystallite sizes is investigated under actual FCC conditions. Thermal and catalytic cracking experiments using decalin are carried out in the mini-fluidized CREC riser simulator. This novel unit operates under relevant FCC process conditions in terms of partial pressures of decalin, temperatures (450–550 °C), contact times (3–7 s), catalyst–decalin mass ratios (5) and using well-fluidized catalysts. Decalin overall conversions ranged between 8–19wt% at low reaction temperatures and 14–27 wt% at high temperatures. It is found that decalin undergoes reactions such as ring opening, protolytic cracking, isomerization, hydrogen transfer and transalkylation. A heterogeneous kinetic model for decalin conversion including thermal effects, adsorption and intrinsic catalytic reaction phenomena is established. Adsorption and kinetic parameters are determined, including the heat of adsorption (?61 kJ/mol) as well as thermal and primary catalytic intrinsic activation energies, which are in the range of 56–59 kJ/mol and 74–91 kJ/mol, respectively. It is determined that hydrogen transfer reactions are more pronounced and selectively favored against other reactions at lower reaction temperatures, while ring-opening and cracking reactions predominate at higher reaction temperatures.     

High selective and stable performance of catalytic aromatization of alcohols and ethers over La/Zn/HZSM-5 catalysts

The catalytic conversions of methanol, ethanol, dimethyl ether and diethyl ether to aromatic hydrocarbons (especially benzene, toluene and xylene) were achieved over 0.8%Zn/0.6%La/HZSM-5 catalyst with a BTX selectivity as high as over 50% at reaction conditions of 710 K, WHSV 0.8 h^?1. The selectivity of BTX hydrocarbons in methanol aromatization reaction could remain 35% in 40 h.     

Yttria nettings by colloidal processing

Porous ceramic burners have been shown as a promising technology to produce heat and lighting by burning low calorific fuels like modern biomass. Among ceramics, yttria (Y₂O₃) presents considerable luminescent proprieties for gas burner technology. By colloidal processing of yttria, this work aims to produce luminescence ceramic nettings with potential to be used as gas burners. Processing parameters such as mean particle size, zeta potential and flow behavior were evaluated in order to prepare suitable suspensions for replica method. Yttria nanoparticles presented light emission with λ = 550 nm when being thermal stimulated at 150 °C. Besides, the nano sized powders d₅₀ = 113.8 nm and specific surface area of 13.6 m² g⁻¹ could be highly stabilized at pH 10.5. Suspensions with 30 vol% of solids, pH 10.5, 1 wt% of dispersant and 0.3 wt% of binder presented shear thinning behavior and thixotropy suitable for replica method. As a result, samples sintered at 1600 °C/1 h showed homogeneous morphology of struts and porous microstructure desirable for gas recirculation and burning process.     

Generation of nitric oxide by photolysis of silver hyponitrite in suspension induced by LMCT excitation

The yellow colour of silver hyponitrite is attributed to an LMCT band at λ_max = 419 nm. Solid Ag₂N₂O₂ and its suspensions in water or acetonitrile are light sensitive. LMCT excitation leads to a photolysis according to the equation Ag₂N₂O₂ → 2Ag + 2NO. In the presence of air and water, NO is finally converted to HNO₂.     

Using Al2O3 defect levels to enhance the photoelectrocatalytic activity of SnS2 nanosheets

Hexagonal SnS₂ nanosheets and Al₂O₃/SnS₂ composites were fabricated via a one-step hydrothermal synthesis method. The investigation indicates that hexagonal SnS₂ with diameters of 100–200 nm are well dispersed on the surface of Al₂O₃. The band gap of SnS₂ after coupling with 11 wt%-Al₂O₃ is reduced by 0.042 eV compared with the pure SnS_2. The composite with 11 wt%-Al₂O₃ shows the highest photocurrent density of 37 μA/cm² at 0.49 V (vs. Ag/AgCl) under visible light (λ>420 nm), which is approximately 1.2 times that of the pure SnS₂ nanosheets. Photoelectrocatalytic measurements demonstrate that an appropriate amount of Al₂O₃ can enhance the photoelectrocatalytic efficiency of SnS₂. The 11 wt%-Al₂O₃/SnS₂ composite (AOSS-11) can degrade 85.9% MB after 3 h under visible light illumination at an applied potential of 0.49 V (vs. Ag/AgCl). The highly effective photoelectrocatalytic activity of the Al₂O₃/SnS₂ composite is attributed to the efficient separation of photoinduced electron-hole pairs based on the defect levels. This work may provide a new design idea for constructing the effective SnS₂-based photocatalysts with other defective semiconductors.     

Microwave dielectric properties and microstructures of Nd(Mg0.5Sn0.5−xTix)O3 ceramics

This study elucidates the microwave dielectric properties and microstructures of Nd(Mg_0.5Sn_0.5?xTi_x)O₃ ceramics with a view to their potential for microwave devices. The Nd(Mg_0.5Sn_0.5?xTi_x)O₃ ceramics were prepared by the conventional solid-state method with various sintering temperatures. The X-ray diffraction patterns of the Nd(Mg_0.5Sn_0.4Ti_0.1)O₃ ceramics revealed no significant variation of phase with sintering temperatures. A dielectric constant (?_r) of 21.1, a quality factor (Q × f) of 50,000 GHz, and a temperature coefficient of resonant frequency (τ_f) of ?60 ppm/°C were obtained for Nd(Mg_0.5Sn_0.4Ti_0.1)O₃ ceramics that were sintered at 1550 °C for 4 h.     

SrCo1−xSbxO3−δ cathode materials prepared by Pechini method for solid oxide fuel cell applications

In this study, SrCo_1?ySb_yO_3?δ powders were prepared by a modified Pechini method. According to the study results, the cubic Pm3m phase of the SrCo_1?ySb_yO_3?δ ceramics was obtained as 10% of cobalt ions were substituted by antimony ions. Doping of Sb³⁺ ions appeared both to stabilize the Pm3m phase of the SrCo_1?ySb_yO_3?δ ceramics and to enhance densification and retard grain growth. The coefficient of thermal expansion of the SrCo_1?xSb_xO_3?δ ceramics increased with the content of the antimony ions, ranging from 10.17 to 15.37 ppm/°C at temperatures lower than the inflection point (ranging from 450 °C to 550 °C) and from 22.16 to 29.29 ppm/°C at higher temperatures. For the SrCo_0.98Sb_0.02O_3?δ ceramic, electrical conductivity reached a maximum of 507 S/cm at 450 °C. The ohmic and polarization resistances of the single cell with the pure SrCo_0.98Sb_0.02O_3?δ cathode at 700 °C read respectively 0.298 Ω cm² and 0.560 Ω cm². The single cell with the SrCo_0.98Sb_0.02O_3?δ-SDC composite cathode appeared to reduce the impedances with the R₀ and R_P at 700 °C reading respectively 0.109 Ω cm² and 0.127 Ω cm². Without microstructure optimization and measured at 700 °C, the single cells with the pure SrCo_0.98Sb_0.02O_3?δ cathode and the SrCo_0.98Sb_0.02O_3?δ-SDC composite cathode, demonstrated maximum power densities of 0.100 W/cm² and 0.487 W/cm². Apparently, SrCo_1?ySb_yO_3?δ is a potential cathode for use in IT-SOFCs.     

Investigation of reactions between vanadium oxide and plasma-sprayed yttria-stabilized zirconia coatings

The phase evolution occurring during the reaction between corrosive V₂O₅ (T_m = 690 °C) and a plasma-sprayed 7 wt.% Y₂O₃–ZrO₂ (YSZ) coating from 700 to 900 °C has been investigated in situ by X-ray diffraction. The temperature and time of interaction between the V₂O₅ and YSZ coating determines the phases observed. Between 700 and 750 °C, reaction products of ZrV₂O₇ and YVO₄ were observed within minutes of reaching the test temperature. m-ZrO₂ was observed after 220 and 60 min at 700 and 750 °C, respectively. The simultaneous formation of both ZrV₂O₇ and YVO₄ at the beginning of the reaction along with the delay of the m-ZrO₂ formation suggests similar reactivity between both Zr and Y with V₂O₅. The weight percent of the ZrV₂O₇ phase began to diminish after 150 and 60 min at 700 and 750 °C, respectively. For reaction temperatures of 800 and 900 °C, there is a rapid decrease in the amount of t′-ZrO₂ and a rapid increase in the amount of m-ZrO₂ with reaction time. YVO₄ was also observed at these reaction temperatures. SEM and TEM microstructural observations confirmed the phases detected from the in situ XRD experiments. Reactions between YSZ and V₂O₅ suggest that the formation of a liquid phase due to the high solubility of both zirconia and yttria in vanadia is the dominate mechanism that damages the coating. The thermal conductivity of a plasma-sprayed YSZ coating reacted with up to 1 wt.% V₂O₅ did not significantly change due to the small volume affected.