Nb Mixed Oxides Research Articles

Preparation and improved photochemical properties of V-doped Nb2Te4O13

Mixed oxides combining d0-and p-cations with stereoactive lone pair electrons exhibit excellent polarization abilities for potential photoelectric applications. This work reports the photochemical properties of V-doped Te–Nb mixed oxide Nb2Te4O13, which has the typical framework constructed by NbO6 (d0-) octahedra, TeO4, and TeO3 (p-) polyhedra. The samples were prepared using the sol-gel method, which resulted in the final nanoparticles. The luminescence spectra, lifetimes of photoinduced electrons and holes, and conductivity properties of the samples were characterized. Nb2Te4O13 has an indirect transition with a band energy of 2.98 eV V5+-doping shows a narrowing effect on the band gap, enhancing the harvest of the optical absorption in the visible light region. Photocatalysis tests were conducted via the photodegradation of RhB solutions under visible-light irradiation. V5+-doping depresses the intrinsic luminescence efficiency, while it significantly enhances photodegradation. It is suggested that the induced redox couples V5+/4+ and defects are responsible for the improvements of photocatalysis of Nb2Te4O13. This work offers an effective method to modify photochemical properties via redox couples and defects induced by impurity ion doping.

High-Performance Vapor-Phase Selective Oxidation of Ethyl Lactate to Ethyl Pyruvate over SiO2 Supported PMoVNb Oxides

This paper describes the application of P-Mo-V-Nb/SiO2 catalysts for the selective oxidation of ethyl lactate (EL) to ethyl pyruvate (EP). The P-Mo-V-Nb/SiO2 catalysts exhibit superior performance for EP selectivity than the corresponding samples of binary V-Nb/SiO2 ternary P-Mo-V/SiO2 and P-Mo-Nb/SiO2 catalysts at same temperatures. The origin of high EP selectivity of the P-Mo-V-Nb/SiO2 catalysts is explored and attributed to the synergistic effect of P, Mo, V, and Nb mixed oxides presented on the surface of the catalyst. The highly dispersive sites separated active species under the action of phosphorus, suppressing over oxidation and improving the selectivity. The existence of MoO3 to provide higher oxidation for catalyst. The redox cycle of V and Nb oxides could be completed through electron transfer between lattice oxygen and metal cations. Moreover, the weak acidity of catalyst surface is favorable to avoid the decarboxylation reaction to target a high selectivity of EP. Therefore, the P-Mo-V-Nb/SiO2 catalyst obtained the maximum yield of 91.8% with a selectivity of 93.8% and a conversion of 99.0% simultaneously at 280 °C.

A high-throughput reactor system for optimization of Mo–V–Nb mixed oxide catalyst composition in ethane ODH

75 Mo–V–Nb mixed oxide catalysts with a broad range of compositions were tested using a high-throughput reactor in order to optimize the composition for the ethane oxidative dehydrogenation reaction.

Photocatalytic property of strontium–niobium mixed oxide prepared by utilizing organic mediators to the overall splitting of H2O

The effects of the polymer precursor in polymerizable complex (PC) method for the preparation of a Sr–Nb mixed oxide photocatalyst were studied. A polymer made of citric acid and ethylene glycol, which is generally applied in the PC method, as well as that made of succinic acid and 2-(2-aminoethylamino)ethanol (AEEA) were used as polymer chelating ligands. The photocatalytic activity of prepared mixed oxides for the overall H2O splitting depended on the oxide crystal structure, which was affected by the chelating complex polymer precursors and the calcination temperature. In particular, the Sr–Nb mixed oxide prepared from succinic acid and AEEA precursors showed the significantly high photocatalytic activity for the overall splitting of H2O after calcination at 973K. The influences of the origin of polymer precursors to the crystal structure and the composition of the Sr–Nb mixed oxide and its photocatalytic performance to the overall H2O splitting are discussed.

Promotional effect of cerium on Mo–V–Te–Nb mixed oxide catalyst for ammoxidation of propane to acrylonitrile

The MoV0.31Te0.23Nb0.24 mixed oxide (M) catalysts with different cerium amounts for propane ammoxidation to acrylonitrile were synthesized by the co-precipitation method. The effect of cerium content on the physicochemical property of the M catalyst was investigated by low-temperature N2 adsorption, XRD, XPS and H2-TPR. The results show that the presence of cerium in the M catalyst can significantly enhance its catalytic performance for propane ammoxidation. 46% acrylonitrile yield can be obtained over the MCe0.05 catalyst with Ce/Mo=0.05 (mol) at SV of 1200mL/(h.g), in which there are most optimal surface compositions, such as more M1 and M2 active phases, and higher Te4+ concentration and appropriate V content. The presence of Ce3+/Ce4+ pairs promotes the redox capability of the M catalyst and more reasonable atom distribution on the catalyst surface.

Understanding the kinetic behavior of a Mo–V–Te–Nb mixed oxide in the oxydehydrogenation of ethane

Two kinetic models based on Langmuir–Hinshelwood (LH) and Eley–Rideal (ER) mechanisms were developed to describe the oxydehydrogenation of ethane to yield ethylene over a Mo–V–Te–Nb catalyst. Obtained in a lab-scale fixed-bed reactor, experimental data at the steady-state were used to estimate the kinetic models parameters via a nonisothermal regression. Experiments were performed using an ethane, oxygen and nitrogen mixture as feedstock, spanning temperatures from 673 to 753K, inlet partial pressures of oxygen and ethane between 5.0 and 22.0kPa, and space–time from 10 to 70gcath(molethane)−1. Ethylene, CO and CO2 were the only detected products, the selectivity for ethylene ranged from 76% to 96% for an ethane conversion interval 4–85%. A series of tests feeding ethylene instead of ethane were also performed at 713K, varying inlet partial pressures and space–time in the same ranges as was done for ethane. Ethylene conversion was relatively low, 3–14%, the dominant product being CO with CO/CO2 ratios from 0.73 to 0.79. The LH mechanism was found to represent better the experimental data. The oxydehydrogenation of ethane was the reaction with the lowest activation energy, 108–115kJmol−1. Except for the conversion of ethane into CO2, deep oxidations were detected as very energetically demanding steps, 156–193kJmol−1. Competitive adsorption between reagents and products occurred in the two mechanisms particularly at relatively high reaction severity, water re-adsorption being weaker in comparison with COx re-adsorption.

Preparation of NiNbO/AISI 430 Ferritic Stainless Steel Monoliths for Catalytic Applications

AISI 430 ferritic stainless steel was used as a substrate to make catalytic monolithic structures. To increase the surface roughness of the substrate, the steel was oxidized at temperatures in the range of 900–940 °C and times in the range of 30–120 min. The oxide layer formed was characterized. Treatment at 940 °C for 60 min was found to be optimal for obtaining a Cr-rich rough oxide layer, with good adherence and homogeneity. Catalytic monoliths were prepared by dip-coating into four different slurries containing the Ni–Nb mixed oxide catalyst. The samples were characterized and tested in the oxidative dehydrogenation (ODH) of ethane to ethylene. The structured catalysts were active for the ODH of ethane, with very good selectivity to ethylene. The catalytic performance was remarkably constant for 170 h on stream at 400 °C.

Ni–Nb mixed oxides: One-pot synthesis and catalytic activity for oxidative dehydrogenation of ethane

Abstract Ni–Nb mixed oxides were prepared by two methods, conventional evaporation method and combustion synthesis. With the latter technique, the mixed oxide can be quickly prepared from an aqueous solution of metal precursors (oxidizers) and sucrose (fuel). The solids with a 0.15/0.85 Nb/Ni atomic ratio were characterized by different techniques including X-ray diffraction, scanning electron microscopy, XPS and Raman spectroscopy, and N2 adsorption–desorption measurements. Ni–Nb mixed oxides were tested in the oxidative dehydrogenation (ODH) of ethane to ethylene. The catalyst prepared by combustion synthesis (a technique that is simpler than evaporation method) showed a good performance in terms of activity and stability levels when compared to the catalyst synthesized by the evaporation method, even for long-term runs. Regarding the effect of the metal precursor, nickel nitrate and ammonium niobate oxalate generated the better catalyst in both synthesis routes. Surface activity for ethane consumption and ethylene formation was higher than that observed for the sample prepared by the evaporation method with the same metal precursors. Both prepared catalysts presented almost constant ethylene yields for 120 h in stream at 400 °C and W/F = 0.06 g s/cm3 (8% and 15% for combustion and evaporation synthesis, respectively) with a slight decrease over time on stream (13% and 11%).

One-pot glycerol oxidehydration to acrylic acid on multifunctional catalysts: Focus on the influence of the reaction parameters in respect to the catalytic performance

We report on a study of the key reaction parameters in glycerol oxidehydration into acrylic acid over W–V–Nb mixed oxides with a hexagonal tungsten bronze structure. This investigation demonstrated that the optimal control of the two consecutive steps of acid-catalysed glycerol dehydration into acrolein and aldehyde oxidation into acrylic acid, and of the parallel reaction of acrolein transformation into by-products (ketals and oligomers), was achieved in the presence of a defined glycerol-to-oxygen inlet ratio. Indeed, oxygen played the fundamental role of accelerating the oxidation of the intermediately formed acrolein into acrylic acid, by allowing a greater concentration of the oxidizing V5+ sites. In fact, an unprecedented higher yield to acrylic acid and acrolein compared to W–V bronzes was registered (maximum acrylic acid yield 50.5%) together with an increase of more than one order of magnitude in productivity, because of both the greater concentration of glycerol used in the inlet feed and the lower contact time needed. Further experiments were carried out by reacting acrolein and methanol in oxidative conditions, the latter as a model molecule for the determination of surface acid and redox properties.

Influence of the drying treatment on the performance of V–Nb mixed oxides catalysts synthesised via sol–gel

Mixed oxides NbVO5 amorphous catalysts have been synthesised by means of the sol–gel technique using different vanadium oxide to water ratios and gel drying treatments: in particular air-drying at room temperature and microwave heating have been adopted. Successively, the dried materials were calcinated up to 550°C in flowing air. Brunauer–Emmett–Teller (BET) surface area analysis showed that this parameter depends on drying method and on the thermal treatment. Differential thermal analysis (DTA) proved that the crystallisation process of the NbVO5 powders occurred at lower temperature when microwave drying was used. An increase of crystallinity was also recorded by X-ray diffraction (XRD) analysis for samples prepared using a high V/H2O ratio and dried by microwave heating. These samples also show high reducibility and surface acidity, as demonstrated by temperature programmed reduction (TPR) and temperature programmed desorption (TPD) techniques respectively.An improvement of the catalytic performances for the gel dried by microwave heating in the oxidative dehydrogenation of ethane at 550°C has been observed.

Economics of Acetic Acid Production by Partial Oxidation of Ethane

Catalysts based on Mo, V, Nb mixed oxides can effect the partial oxidation of ethane to acetic acid. The process requires a pressure of about 20 atm and acetic acid selectivity is about 20%, while ethylene selectivity is about 70%. The addition of Pd to the MoVNb catalyst greatly increases the selectivity to acetic acid to about 80% and completely oxidizes CO to CO2. In this paper, the economics of acetic acid production by partial oxidation of ethane will be evaluated and compared with the carbonylation of methanol. In particular we study operating fixed bed reactors in series and in two steps. In the first step ethylene is produced and co-produced water is separated. In the second step ethylene is oxidized to acetic acid. This mode of operation increases the concentration of acetic acid leading to great saving in the product cost.

Epoxidation of cyclooctene over mesoporous Ga, Ga–Nb, and Ga–Mo oxide catalysts

We demonstrate the epoxidation of cyclooctene to epoxycyclooctane over mesoporous Ga and mixed Ga–Nb and Ga–Mo oxides synthesized via self-assembly hydrothermal-assisted approach. These mesoporous catalysts displayed high epoxide selectivities at moderate cyclooctene conversions. Mesoporous Ga oxides with average particle sizes between 2 and 3 μm exhibited higher cyclooctene conversions, while larger particle sizes between ~ 4.5–6.5 μm showed lower conversions. The incorporation of Nb led to an increase in the acidity of the resultant Ga–Nb mixed oxides. These mesoporous Ga–Nb mixed oxides displayed 80–100% selectivity for the epoxide at conversion levels of cyclooctene in the 17 to 30% range. Finally, a mesoporous Ga–Mo oxide with molar composition of 35% Mo displayed the highest cyclooctene conversion of all mesoporous samples at 60 °C. The conversion of this sample was 41% with 100% selectivity.

Synthesis of Acetoaldehyde, Acetic Acid, and Others by the Dehydrogenation and Oxidation of Ethanol

Following gradual shift of primary resources from fossil towards renewable ones in chemical industry, biomass based ethanol has been attracting growing interests as a fuel to replace gasoline and as a chemical feed stock to replace ethylene. This paper reviews major work reported in the last 10 years for the production of acetaldehyde, acetic acid, and other related compounds from ethanol. At present acetic acid can be industrially produced more economically from methanol than from ethylene, the production of acetic acid from ethanol is not profitable. Acetaldehyde, which is more expensive than ethanol, can be selectively produced in gas phase by dehydrogenation over supported Cu catalysts and by oxidation with O2 over V and Mo based oxides. It is noteworthy that gold nanoparticles deposited on basic and acidic metal oxides are highly selective to acetaldehyde by oxidation with O2. Acetic acid can be produced in water solvent over Au catalysts supported on MgAl2O4 or on Cu doped NiO, while in gas phase over Mo–V–Nb mixed oxides combined with TiO2 colloids.

Catalytic transformations of methyl oleate and biodiesel over mesoporous gallium–niobium oxides

Abstract The catalytic transformations of methyl oleate and a commercial sample of biodiesel over novel, mesoporous, gallium–niobium mixed oxides possessing both acidic and redox activities are compared with the corresponding results from acidic zeolites (H–Y, H–beta and H–ZSM-5) and SAPO-11. The products from the reaction included branched alkyl esters of FAMEs, and their double bond and cis/trans isomers. In the case of products from reactions over Ga–Nb oxides, dehydrogenated, diolefinic products (mainly, linoleates, and linolinates) were also observed. When traces of water were present (> 100 ppm) in the biodiesel, hydrolysis of the FAME to fatty acids was also observed. Dimerisation of the FAME molecules also occurred to some extent. Both the niobium content and the surface acidity of the mesoporous Ga–Nb mixed oxides influenced their catalytic activity and selectivity. The selectivity for skeletal isomerisation of the linear FAMEs over the mesoporous Ga–Nb oxides is comparable, if not better, than those observed over the aluminosilicate and silicoaluminophosphate molecular sieves.

Selective oxidation of ethane: Developing an orthorhombic phase in Mo–V–X (X = Nb, Sb, Te) mixed oxides

Mo–V–X (X = Nb, Sb and/or Te) mixed oxides have been prepared by hydrothermal synthesis and heat-treated in N 2 at 450 °C or 600 °C for 2 h. The calcination temperature and the presence or absence of Nb determines the nature of crystalline phases in the catalyst. Nb-containing catalysts heat-treated at 450 °C are mostly amorphous solids, while Nb-free catalysts heat-treated at 450 °C and samples treated at 600 °C clearly contain crystalline phases. TPR-H 2 experiments show higher H 2-consumption on catalysts with amorphous phases. Catalytic results in the oxidative dehydrogenation of ethane indicate that the selective production of the olefin is strongly related to the development of the orthorhombic Te 2M 20O 57 or (SbO) 2M 20O 56 (M = Mo, V, Nb) phase (the so-called M1 phase), which is mainly formed at 600 °C. This active and selective crystalline phase is characterized to show moderate reducibility and active centers enough for the selective oxidative activation of ethane with the minimum quantity possible of active centers for ethylene activation. In this sense, the best yield to ethylene has been achieved on a Mo–V–Te–Nb mixed oxide.

Mesostructured mixed Mo–V–Nb oxides for propane ammoxidation

Abstract Mesostructured Mo–V–Nb mixed oxide phases were prepared by reacting inorganic precursors in the presence of cationic, anionic and alkylamine surfactants. The occurrence of these mesostructured phases was explained in part by charge-matching considerations at the inorganic–organic interface. Other interactions, such as covalent bonding between the surfactant headgroup and metal atoms, could direct the self-assembly process. Despite of the limited thermal stability of the mesophases, the air-calcined phases were catalytically active and selective for propane ammoxidation to acrylonitrile and acetonitrile. The selectivities to acrylonitrile and acetonitrile were as high as 26 and 49 mol%, respectively, at 66% propane conversion.

Effect of preparation and reaction condition on the catalytic performance of Mo–V–Te–Nb catalysts for selective oxidation of propane to acrylic acid by high-throughput methodology

Combinatorial screening technique has been applied to investigate the catalytic activity and selectivity of quaternary Mo–V–Te–Nb mixed oxide catalysts treated with various chemicals during preparation for selective oxidation of propane to acrylic acid. The catalyst libraries were prepared by the slurry method and catalytic activities were examined in 32-channel high-throughput screening reactor system coupled with a mass spectrometer and/or gas chromatograph. The obtained results provided substantial evidence that the sample preparation condition would have strong effect on the catalytic performance for propane selective oxidation. Among screened samples, Mo–V–Te–Nb treated with HIO 3 solution presented a better performance. The reaction results of promising catalysts selected from the libraries were applied to further scale-up of the system and confirmed catalytic performance. Quantification of the result of Mo–V–Te–Nb treated with HIO 3 solution was realized by combination of GC and MS and relationship between the MS data and the GC results can be established.

Preparation of mesoporous In–Nb mixed oxides and its application in photocatalytic water splitting for hydrogen production

A series of mesoporous In–Nb mixed oxides was synthesized using NbCl 5 and In 2O 3 as the starting material and triblock copolymer P123 as template. We investigated the influence of indium content on the synthesis and characteristics of the mesoporous In–Nb mixed oxides, and their photocatalytic activities for water splitting. The materials were characterized by small angle X-ray scattering, powder X-ray diffraction, extended X-ray absorption fine structure, transmission electron microscopy, scanning electron microscopy, energy dispersive spectrometer, N 2 sorption and UV–vis spectroscopy. The surface area of mesoporous In–Nb mixed oxides was greater than 90 m 2/g with a wormhole framework. The optimization of synthesis condition of the mesoporus In–Nb oxides catalyst contained a small fraction of highly dispersed indium (In/Nb = 0.13) species intercalated into the framework of mesoporous niobium oxides and exhibited a high photocatalytic activity for water splitting reaction which was about 2.7 times as compared to mesoporous Nb 2O 5 and was about 19 times higher than commercial bulk Nb 2O 5.

Characterization and evaluation of Mo [sbnd]V [sbnd]Te [sbnd]Nb mixed metal oxide catalysts fabricated via hydrothermal process with ultrasonic pretreatment for propane partial oxidation

Mo 1.00V x Te 0.20Nb 0.16O n ( x = 0.35 – 0.50 ) mixed-metal oxide catalysts were synthesized through ultrasonic and hydrothermal treatments. Both TeO 2 and H 6TeO 6 were used as tellurium sources. The enhanced dispersion of TeO 2 by ultrasonic treatment is crucial for obtaining an active and selective Mo V Te Nb O catalyst for acrylic acid (AA) formation from propane oxidation. The TeO 2-derived Mo 1.00V x Te 0.20Nb 0.16O n ( x = 0.35 – 0.41 ) are superior to their H 6TeO 6-derived counterparts; propane conversion and AA selectivity over the Mo 1.00V 0.41Te 0.20Nb 0.16O n is 55% and 60 mol% at 380 °C, respectively, giving an AA formation rate of 22.3 μmol g −1 min −1. Based on the physicochemical properties of the catalysts, we propose that the ultrasonic treatment can give rise to (i) enhanced presence of the orthorhombic Te 2 M 20 O 57 ( M = Mo , V, Nb) and hexagonal Te 0.33 MO 3.33 ( M = Mo , V, Nb) phases, (ii) surface enrichment of Te, (iii) enhanced reactivity of lattice oxygen, (iv) an increase in Mo O Te and V O Te entities, and (v) better isolation of active sites.

Selective oxidation of propane and ethane on diluted Mo–V–Nb–Te mixed-oxide catalysts

Te-free and Te-containing Mo–V–Nb mixed oxide catalysts were diluted with several metal oxides (SiO 2, γ-Al 2O 3, α-Al 2O 3, Nb 2O 5, or ZrO 2), characterized, and tested in the oxidation of ethane and propane. Bulk and diluted Mo–V–Nb–Te catalysts exhibited high selectivity to ethylene (up to 96%) at ethane conversions < 10 % , whereas the corresponding Te-free catalysts exhibited lower selectivity to ethylene. The selectivity to ethylene decreased with the ethane conversion, with this effect depending strongly on the diluter and the catalyst composition. For propane oxidation, the presence of diluter exerted a negative effect on catalytic performance (decreasing the formation of acrylic acid), and α-Al 2O 3 can be considered only a relatively efficient diluter. The higher or lower interaction between diluter and active-phase precursors, promoting or hindering an unfavorable formation of the active and selective crystalline phase [i.e., Te 2M 20O 57 (M = Mo, V, and Nb)], determines the catalytic performance of these materials.