0000000000011334
AUTHOR
Patricia Concepción
Mo-W-containing tetragonal tungsten bronzes through isomorphic substitution of molybdenum by tungsten
Mixed metal oxides based in Mo(W)–Nb–V–Te with tetragonal tungsten bronze (TTB) structure have been synthesized by a hydrothermal method from aqueous solutions of the corresponding Keggin-type heteropolyacids and further heat-treatment in N2 at 700 ◦ C. The materials have been characterized by several physico-chemical techniques, i.e. XRD, Raman, FTIR, SEM-EDS, and TEM. This procedure allows controlling the chemical species to be distributed in the different interstices of the TTB skeleton, which is a key factor to regulate the catalytic properties of the final solid. In this sense, the isomorphic replacement of Mo by W results in lattice parameter and crystal morphology variation, although…
Evolution of the optimal catalytic systems for the oxidative dehydrogenation of ethane: The role of adsorption in the catalytic performance
Three samples that correspond to the evolution of optimal catalytic systems for the oxidative dehydrogenation of ethane have been synthesized and compared in terms of catalytic behavior and adsorption properties: (i) vanadium oxide supported on alumina, (ii) Sn-promoted NiO, and (iii) multicomponent MoVTeNbO with the M1 structure. The main difference in catalytic performance lies in the extent of the overoxidation of the ethylene formed, following the order VOx/Al2O3 > NiSnOx > MoVTeNb-M1. Accordingly, the selectivity to ethylene at medium and high ethane conversion follows the order MoVTeNb-M1 > NiSnOx > VOx/Al2O3. These results are confirmed by the relative reaction rates observed for the…
Oxidative dehydrogenation of ethane over NiO–CeO2 mixed oxides catalysts
[EN] In this paper we present the synthesis, characterization and catalytic behaviour in the oxidative dehydrogenation of ethane of NiO-CeO2 mixed oxides. The addition of cerium oxide to NiO strongly modifies its catalytic performance, although the changes in the catalytic behaviour depend on the catalyst composition. The incorporation of low amounts of cerium oxide to NiO multiplies the productivity to ethylene in the oxidative dehydrogenation of ethane by a factor of ca. 7, which is the result of an increase in both catalytic activity and selectivity to ethylene. The enhanced activity has been related to the remarkable increase of the surface area of catalyst and the low crystal sizes of …
Co3O4 particles grown over nanocrystalline CeO2: influence of precipitation agents and calcination temperature on the catalytic activity for methane oxidation
Crystalline cobalt oxides were prepared by a precipitation method using three different precipitation agents, (NH4)(2)CO3, Na2CO3 and CO(NH2)(2). Cobalt oxide nanoparticles corresponding to a Co3O4 loading of 30 wt% were also deposited over high-surface area nanocrystalline ceria by the same precipitation agents. The effect of calcination temperature, 350 or 650 degrees C, on the morphological and structural properties was evaluated. Characterization by BET, XRD, SEM, TEM, Raman spectroscopy, H-2-TPR, XPS and NH3-TPD was performed and the catalytic properties were explored in the methane oxidation reaction. The nature of the precipitation agent strongly influenced the textural properties of…
On the influence of the acid-base character of catalysts on the oxidative dehydrogenation of alkanes
Vanadium oxides supported on metal oxide, i.e. Al2O3, MgO and Mg-Al mixed oxide, and V-containing microporous materials (VAPO-5 and MgVAPO-5) have been tested in the oxidative dehydrogenation of C2-C4 alkanes. In all cases, tetrahedral vanadium species (isolated and/or associated) were mainly observed from51V-NMR and diffuse reflectance spectroscopies. The reducibility of V5+-species, determined from the onset-reduction temperature, decreases as follows: VOx/AL > VAPO-5 > MgVAPO-5 =VOx/MG > VOx/MG + AL. The acid character of catalysts, determined from the FTIR spectra of pyridine adsorbed, decreases as: MgVAPO-5 > VOx/AL > VAPO-5 > VOx/MG + AL > VOx/MG. A similar trend between V-reducibilit…
Reaction products and pathways in the selective oxidation of C-2-C-4 alkanes on MoVTeNb mixed oxide catalysts
[EN] The catalytic properties of MoVTeNbO catalysts during the selective oxidation of short chain alkanes and olefins (C-2-C-4) have been comparatively studied The main reaction products have been ethylene from ethane acrylic acid from propane maim anhydride from n-butane and methacrolein from isobutane FTIR studies of the adsorption of the main reaction products i e olefins and aldehydes over MoVTeNbO catalyst has been carried out Accordingly the reaction pathway is explained on the basis of the characteristics of the alkane fed the stability and reactivity of both the intermediates and the reaction products and the nature of the catalytic sites involved in each reaction (C) 2010 Elsevier …
Selective oxidation of propane and ethane on diluted Mo–V–Nb–Te mixed-oxide catalysts
Abstract Te-free and Te-containing Mo–V–Nb mixed oxide catalysts were diluted with several metal oxides (SiO 2 , γ -Al 2 O 3 , α -Al 2 O 3 , Nb 2 O 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 forma…
Oxidative dehydrogenation of ethane on Te-containing MoVNbO catalysts
Abstract Ethylene has selectively been obtained during the oxidative dehydrogenation of ethane on a Mo–V–Te–Nb–O mixed oxide catalyst. The catalyst was prepared hydrothermally and heat-treated at 600 °C for 2 h in a N2-stream. For comparison, undoped and Te-doped Mo–V–Nb–O catalysts and Mo–V–Te–O and Mo–Te–Nb–O mixed oxides have also been prepared. The catalytic performance of the Mo–V–Nb–Te–O mixed oxide catalyst cannot be explained by the incorporation of Te on the surface of a Mo–V–Nb–O mixed oxide but by the presence of an orthorhombic Te2M20O57 (M=Mo, V, Nb) crystalline phase. The role of tellurium in these catalysts is also discussed.
Selective oxidative dehydrogenation of ethane over MoVSbO mixed oxide catalysts
Abstract Mo–V–Sb–O mixed metal oxides were prepared, characterized and tested for the oxidative dehydrogenation of ethane (ODH). These catalysts are active and selective in the oxidative dehydrogenation of ethane to ethylene, although their catalytic performance depends strongly on the calcination temperature. The best catalytic results were obtained on samples heat-treated at 600 °C in N 2 (with selectivities to ethylene higher than 80% at ethane conversions about 65%). Moreover, a loss in the catalyst activity is initially observed on these catalysts as a consequence of modifications in the nature of the crystalline phases. Changes on the oxidation state of antimony ions on the surface of…
Anionic Organic Guests Incorporated in Zeolites: Adsorption and Reactivity of a Meisenheimer Complex in Faujasites
Zeolites are suitable microporous hosts for positively charged organic species, but it is believed that they cannot adsorb organic anions. Pure Meisenheimer complex, derived from reduction of 2,4-dinitroaniline with NaBH4, was adsorbed inside faujasite cavities. Evidence for the internal incorporation of this negatively charged reaction intermediate comes from 1) XPS elemental analysis as a function of the depth of penetration into the particle, 2) the remarkable blue shift in lambda(max) of the Meisenheimer complex adsorbed on zeolite (ca. 470 nm) as compared to that in acetonitrile (580 nm) and 3) from the lack of reactivity with size-excluded hydride-acceptor reagents. Evidence is provid…
Influence of the preparative route on the properties of WOx–ZrO2 catalysts: A detailed structural, spectroscopic, and catalytic study
Abstract Two series of tungstated zirconia (WZ) solid acids covering a wide range of tungsten surface densities ( δ, W at/nm2 ) were prepared by nonconventional impregnation and coprecipitation routes, leading to samples with enhanced surface area ( ∼ 70 – 120 m 2 / g ) on annealing at 973–1073 K. The materials were thoroughly characterized by N2 physisorption , XRD, Raman, XPS, H 2-TPR, and DR UV–vis spectroscopy. The catalytic behavior of the Pt-promoted WZ catalysts (1 wt% Pt) was evaluated for the hydroconversion of n-hexadecane used as model feed representative of Fischer–Tropsch waxes. Both series of catalysts displayed a pronounced maximum in the reaction rate and a minimum in the se…
Selective oxidative dehydrogenation of ethane over SnO2-promoted NiO catalysts
[EN] Ni-Sn-O mixed oxides catalysts have been investigated for the oxidative dehydrogenation of ethane. The catalysts were prepared through the evaporation of aqueous solutions of nickel nitrate and tin oxalate and finally calcined in air at 500 degrees C for 2 h. These materials have been characterized by several techniques (N-2-adsorption, X-ray diffraction, High-Resolution Electron Microscopy, temperature programmed reduction, X-Ray Photoelectron Spectroscopy, Fourier Transformed Infrared Spectroscopy of adsorbed CO and O-18/O-16 isotope exchange). The addition of just a tiny amount of tin highly increases the selectivity to ethylene (from ca. 40% to 80-90%). Thus, high selectivity to et…
Tungsten-titanium mixed oxide bronzes: Synthesis, characterization and catalytic behavior in methanol transformation
[EN] Tungsten oxide bronze-based materials show extremely adaptive structural and compositional features that make them suitable for functional properties modulation. Herein we report the preparation of a series of Ti-containing tungsten oxide catalysts presenting a hexagonal tungsten bronze-type structure. The insertion of Ti4+ within the structure (likely in the octahedral framework of the hexagonal tungsten bronze) leads to an increase in the number of strong acid sites, and the disappearance of W5+ surface species found in the undoped tungsten oxide. With the aim of studying the acid-redox properties of the titled catalysts, the catalytic transformation of methanol has been carried out …
Nickel oxide supported on porous clay heterostructures as selective catalysts for the oxidative dehydrogenation of ethane
[EN] Porous clay heterostructures (PCH) have shown to be highly efficient supports for nickel oxide in the oxidative dehydrogenation of ethane. Thus NiO supported on silica with a PCH structure shows productivity towards ethylene three times higher than if NiO is supported on a conventional silica. This enhanced productivity is due to the increase in the catalytic activity and especially to the drastic increase in the selectivity to ethylene. Additionally, PCH silica partially modified with titanium in the columns (PCH-Ti) have also been synthesized and used as supports for NiO. An enhanced activity and selectivity to ethylene was found over NiO supported over PCH-Ti compared to the corresp…
Molybdenum–vanadium supported on mesoporous alumina catalysts for the oxidative dehydrogenation of ethane
Abstract Vanadium and/or molybdenum oxides supported on mesoporous alumina have been prepared by a “wet” impregnation method, characterized (by using several physico-chemical techniques TPR, DR-UV–vis, Raman spectroscopies) and tested in the oxidative dehydrogenation (ODH) of ethane. For comparison Mo–V-mixed oxides supported on γ-Al2O3 have also been studied. The characterization results indicate that the main vanadium species in both Mo-free and Mo-containing V-based samples present low aggregation and coordination, whereas molybdenum is present as monomeric or polymeric tetrahedral molybdenum species. Interestingly, no apparent interaction between molybdenum and vanadia takes place. The …