Supported Vanadium Oxide Catalysts Research Articles

The Oxygen Isotopic Exchange Reaction on Vanadium Oxide Catalysts

The reactivity of lattice oxygen of vanadium oxide catalysts was studied with the oxygen isotopic exchange reaction. The reactivity of pure V2O5 is compared with the reactivity of Li0.33V2O5, V2O5/TiO2, V2O5/Al2O3, V2O5/SiO2, δ-VOPO4, and (VO)2P2O7. According to their behaviour in the oxygen exchange reaction, two types of vanadium oxide catalysts could be distinguished. The first type of catalysts only showed exchange activity in the R2 exchange mechanism and the second type showed activity in both the R1 and R2 exchange mechanisms (mechanisms in which, respectively, one or two oxygen atoms of the gas phase molecule are exchanged with oxygen atoms of the metal oxide). The catalysts which belong to the first group are bulk V2O5 and δ-VOPO4 and the catalysts which belong to the second group are Li0.33V2O5, V2O5/TiO2, V2O5/Al2O3, V2O5/SiO2, and (VO)2P2O7. If only the R2 mechanism is observed then diffusion of lattice oxygen is probably faster than when both the R1 and R2 mechanisms are observed. The activity of the supported vanadium oxide catalysts in the oxygen exchange reaction is dependent on the support. The reactivity order is V2O5/TiO2>V2O5/Al2O3∼V2O5/SiO2.

Oxidation of sulfur dioxide over supported vanadia catalysts: molecular structure – reactivity relationships and reaction kinetics

The oxidation of sulfur dioxide to sulfur trioxide over supported vanadium oxide catalysts occurs as both a primary and secondary reaction in many industrial processes, e.g., the manufacture of sulfuric acid, the selective catalytic reduction of NO x with ammonia and the regeneration of petroleum refinery cracking catalysts. This paper discusses the fundamental information currently available concerning the molecular structure and sulfur dioxide oxidation reactivity of surface vanadia species on oxide supports. Comparison of the molecular structure and reactivity information provides new fundamental insights on the following topics related to the catalytic properties of surface vanadia species during the sulfur dioxide oxidation reaction: 1. role of terminal VO, bridging V–O–V and bridging V–O-support bonds, 2. number of surface vanadia sites required to perform SO 2 oxidation, 3. influence of metal oxide additives, 4. generation and influence of the surface sulfate overlayer, 5. effect of surface acidity on the reaction turnover frequency, 6. competitive adsorption between SO 2 and SO 3 and 7. reaction kinetics.

Supported Vanadium Oxide Catalysts: Quantitative Spectroscopy, Preferential Adsorption of V4+/5+, and Al2O3 Coating of Zeolite Y

A series of supported vanadium oxide catalysts were prepared by the incipient wetness method as a function of the support composition (Al2O3, SiO2, and USY), the metal oxide loading (0−1 wt %), and the impregnation salt (vanadyl sulfate and ammonium vanadate). These catalysts have been studied by combined DRS-ESR spectroscopies in order to quantify the amount of V4+ and V5+ and to unravel their coordination geometries. These spectroscopic fingerprints have been used to study the preferential adsorption of V4+/5+ ions on SiO2, Al2O3, and USY. Both V4+ and V5+ were preferentially adsorbed on Al2O3 and showed a much smaller preference for USY and SiO2. The observed preference orders are discussed in relation with the properties of the support. In addition, a novel method is proposed to coat the external surface of USY with a thin film of Al2O3. The method is based on the deposition of USY with the so-called Keggin ion, [Al13O4(OH)24(H2O)12]7+, which is too big to enter the USY channels or pores. The obtained...

Selective photo-assisted catalytic oxidation of methane and ethane to oxygenates using supported vanadium oxide catalysts

Selective photooxidation of light alkanes, mainly methane and ethane, into the corresponding aldehydes was achieved using silica-supported vanadium oxide catalysts under UV irradiation at elevated temperature. Photooxidation of methane using the V2O5/SiO2-IW (incipient wetness) (0.6 mol% V) catalyst at 493 K for 2 h gave 68 µmol of methanal, which corresponds to 76 mol% selectivity and 0.48 mol% one-pass yield. Photooxidation of ethane using V2O5/SiO2 (calcined at 1023 K) -IW (0.6 mol% V) catalyst for 1 h gave 85 µmol of ethanal, which corresponds to 90% selectivity and 1.1% one-pass yield. The catalysts prepared by the sol–gel method also showed activity, especially for the reaction of ethane. Both UV irradiation and a reaction temperature as high as 500 K were essential. The photo-assisted catalytic reactions were very sensitive to the reaction temperature, method of preparation of the catalyst, and addition of water vapour. While the reaction of methane was inhibited by the addition of water vapour, the photooxidation of ethane and propane was promoted in the presence of a controlled amount of water vapour. In addition, the reaction with methane required UV irradiation at a wavelength <310 nm, whereas the reaction with ethane or propane proceeded by irradiation at longer wavelength. According to Raman and UV diffuse reflectance spectroscopic studies and XRD studies, only isolated four-coordinated vanadium oxide surface species were considered to show catalytic activity toward the photooxidation of methane, while accumulated vanadium surface species, not crystalline V2O5, were shown to be active for the photooxidation of ethane and propane.

The adsorption of VO(acac)(2) on a mesoporous silica support by liquid phase and gas phase modification to prepare supported vanadium oxide catalysts

Supported vanadium oxide catalysts are prepared by adsorption and subsequent calcination of the vanadyl acetylacetonate complex on silica by liquid phase and gas phase modification. The influence of the pretreatment temperature and the effect of the solvent in the liquid phase are discussed. Two types of gas phase deposition processes are used: flow-type reactions and vacuum deposition. The bonding mechanism, the influence of pretreatment temperature of the support and the influence of the reaction temperature are investigated by FTIR, XRD, TGA and chemical analysis. After calcination the obtained vanadium oxide layer is characterized by XRD and UV-Vis diffuse reflectance spectroscopy. The gas phase modification enables the creation of well dispersed supported VOx catalysts. Loadings up to 1.4 mmol g-1 (7 wt% V) without the formation of a crystalline fraction can be achieved. The selective oxidation of methanol to formaldehyde is used as a probe reaction to assess the catalytic activity and selectivity of the catalysts. It is shown that not the concentration of vanadium species, but their surface configuration is the determining factor in catalytic reactions.

V-BaCO 3 catalysts for the oxidative dehydrogenation of ethane

A new type of supported vanadium oxide catalyst V–BaCO3, which consists of barium orthovanadate Ba3(VO4)2 and BaCO3 phases, has been used in the oxidative dehydrogenation of ethane. The catalyst with the ratio of V/Ba from 0.1 to 0.3 exhibited high catalytic activity for oxidative dehydrogenation of ethane, with particularly high activity for ethene production.

Vapour-Phase Selective Oxidation of 4-Methylanisole to Anisaldehyde over V2O5/Ga2O3-TiO2 Catalyst

Abstract The vapour phase partial oxidation of 4-methylanisole to p-anisaldehyde was investigated over binary oxide supported vanadium oxide catalysts at 623-748 K under normal atmospheric pressure. Among various catalysts the V2O5/Ga2O3-TiO2 effectively performed this reaction with high activity and selectivity.

Oxidative dyhydrogenation of short chain alkanes on supported vanadium oxide catalysts

This paper summarizes the main data appeared in the last years on the oxidative dehydrogenation (ODH) of short chain alkanes on supported vanadium oxide catalysts. The acid-base character of metal oxide support influences the dispersion of vanadium on the surface of the support, as well as the nature of the vanadium species. The reducibility and structure of surface vanadium oxide species and the acid-base character of catalysts, in addition to their catalytic properties in the ODH of C 2–C 4 alkanes, strongly depend on the metal oxide used as support and the vanadium loading. In this way, it appears that tetrahedral V 5+-species are active and selective sites in the ODH of C 2–C 4 alkanes. The effect of the coordination number and aggregation state of surface vanadium oxide species, and the presence of acid/base sites on the catalytic behavior of supported vanadium oxide catalysts are discussed. It is concluded that these are important factors that must be considered to develop selective catalysts in ODH reactions.

Structure and reactivity of surface vanadium oxide species on oxide supports

Supported vanadium oxide catalysts, containing surface vanadia species on oxide supports, are extensively employed as catalysts for many hydrocarbon oxidation reactions. This paper discusses the current fundamental information available about the structure and reactivity of surface vanadia species on oxide supports: monolayer surface coverage, stability of the surface vanadia monolayer, oxidation state of the surface vanadia species, molecular structures of the surface vanadia species (as a function of environment and catalyst composition), acidity of the surface vanadia species and reactivity of the surface vanadia species. Comparison of the molecular structure and reactivity information provides new fundamental insights into the catalytic properties of surface vanadia species during hydrocarbon oxidation reactions: (1) the role of terminal VO, bridging VOV and bridging VO-support bonds, (2) the number of surface vanadia sites required, (3) the influence of metal oxide additives, (4) the influence of surface acidic and basic sites, (5) the influence of preparation methods and (6) the influence of the specific oxide support phase. The unique physical and chemical characteristics of supported vanadia catalysts, compared to other supported metal oxide catalysts, for hydrocarbon oxidation reactions are also discussed.

Vapour phase synthesis of isobutyraldehyde from methanol and ethanol over mixed oxide supported vanadium oxide catalysts

The vapour phase synthesis of isobutyraldehyde from methanol and ethanol in one step was investigated over titania-silica, titania-alumina, titania-zirconia, titania-silica-zirconia, and magnesia supported vanadium oxide catalysts at 623 K and under normal atmospheric pressure. Among various catalysts the titania-silica binary oxide supported vanadia provided higher yields than the other single or mixed oxide supported catalysts. The high conversion and product selectivity of V2O5/TiO2-SiO2 catalyst (20 wt% V2O5) was related to the better dispersion of vanadium oxide over titania-silica mixed oxide support in addition to other acid-base and redox characteristics. A reaction path for the formation of isobutyraldehyde from methanol and ethanol mixtures over these catalysts was described.

In SituRaman Spectroscopy during the Partial Oxidation of Methane to Formaldehyde over Supported Vanadium Oxide Catalysts

Vanadia was found to be well dispersed and present as a two-dimensional overlayer when supported on SiO2, TiO2, SnO2, 3 wt% TiO2/SiO2, 3 wt% MoO3/SiO2, and 3 wt% SnO2/SiO2. Partial oxidation of methane by oxygen formed formaldehyde most selectively over the V2O5/SiO2catalyst, but catalytic performance was strongly dependent on vanadia coverage and autocatalytic behavior was observed. At very low conversions, the formaldehyde activity increased linearly with vanadia coverage, indicating that isolated V5+species were responsible for the active sites. No significant structural changes were revealed byin situRaman spectroscopy for the V2O5/SiO2catalyst, which indicated that the fully oxidized surface sites were related to the high formaldehyde selectivity. This selectivity exhibited a maximum at 1 wt% V2O5content, and the lower selectivities at higher loadings appeared to be due to the increasing Lewis acidity of the catalysts. Space–time yields of 0.1–1.4 kg CH2O/kg cat/hr and selectivities of 2–78% are reported herein for the V2O5/SiO2catalysts. Deep oxidation products, CO and CO2, were principally produced over the V2O5/TiO2and V2O5/SnO2catalysts. For the first time,in situRaman analysis clearly showed that for these latter catalysts the surface vanadium(V) oxide species were partially reduced under the steady-state reaction conditions. The performance of the V2O5/TiO2/SiO2catalyst was similar to that of the V2O5/TiO2catalyst, consistent with the earlier observation that vanadia was largely bound to the titania overlayer. It appears that formaldehyde selectivity decreased with increasing catalyst reducibility, but no direct correlation of catalyst activity with reducibility was observed.

Nuclear magnetic resonance studies on supported vanadium oxide catalysts

51V wideline and magic angle spinning (MAS) NMR techniques are applied to the characterization of the V 5+ species formed on MgO, calcined hydortalcite, sepiolite and Al 2O 3 after calcination at 600°C. Except in VO x Al 2O 3 catalysts, only tetrahedral V 5+ species have been observed. On MgO, highly dispersed [VO 4] 3− tetrahedra and Mg 3(VO 4) 2 are formed. In catalysts supported on calcined hydrotalcite, both Mg 3(VO 4) 2 and α-Mg 2V 2O 7 like phases are present. A signal tentatively attributed to [V 2O 7] 4− dimers is detected on sepiolite-supported catalysts, leading to the formation of bulk α- and β-Mg 2V 2O 7 at higher V coverages. A distorted V 5+ octahedral environment is present only in VO x Al 2O 3 , which is the major component at a monolayer coverage of 0.6. At lower vanadium contents, tetrahedral V 5+ species, of Q (2) type, are also observed. These results can be explained on the basis of the acid-base character of the metal oxide support. When increasing the acidity of the support, a weaker interaction with the vanadium atoms favours the association of the V 5+ tetrahedra.

Effect of water vapor on the molecular structures of supported vanadium oxide catalysts at elevated temperatures

The effect of water vapor on the molecular structures of V 2O 5-supported catalysts (SiO 2, Al 2O 3, TiO 2, and CeO 2) was investigated by in situ Raman spectroscopy as a function of temperature (from 500°;C to 120°;C). Under dry conditions, only isolated surface VO 4 species are present on the dehydrated SiO 2 surface, and multiple surface vanadium oxide species (isolated VO 4 species and polymeric vanadate species) are present on the dehydrated Al 2O 3, TiO 2, and CeO 2 surfaces. The Raman features of the surface vanadium oxide species on the SiO 2 support are not affected by the introduction of water vapor due to the hydrophobic nature of the SiO 2 support employed in this investigation. However, the presence of water has a pronounced effect on the molecular structures of the surface vanadium oxide species on the Al 2O 3, TiO 2, and CeO 2 supports. The Raman band of the terminal V = O bond of the surface vanadia species on these oxide supports shifts to lower wavenumbers by 5–30 cm −1 and becomes broad upon exposure to moisture. Above 230°C, the small Raman shift of the surface vanadium oxide species in the presence of water suggests that the dehydrated surface VO x species form a hydrogen bond with some of the adsorbed moisture. Upon further decreasing the temperature below 230°C, the hydrogen-bonded surface VO x species are extensively solvated by water molecules and form a hydrated surface vanadate structure (e.g. decavanadate). The broad Raman band at ≈ 900 cm −1, which is characteristic of the polymeric VOV functionality, appears to be minimally influenced by the presence of water vapor and is a consequence of the broadness of this band. Oxygen-18 isotopic labeling studies revealed that both the terminal V=O and bridging VOV bonds readily undergo oxygen exchange with water vapor. The current observations account for the inhibiting effect of moisture upon oxidation reactions over supported metal oxide catalysts and are critical for interpreting in situ Raman data during hydrocarbon oxidation reactions where H 2O is a reaction product.

The Role of Vanadium Oxide on the Titania Transformation under Thermal Treatments and Surface Vanadium States

High surface area titania-supported materials prepared from V(IV) precursors and calcined at high temperatures have been characterized by Vis–UV diffuse reflectance, FT Raman, electron spin resonance, and X-ray photoelectron spectroscopies and tested in the partial oxidation of methane. Vanadium oxide loading and calcination temperature determine the structure of V2O5/TiO2materials. Below theoretical surface monolayer coverage, V(IV) species closely interacting with the support are observed. Vanadiam oxide species anchor by reaction with titanium oxide surface hydroxyl groups. The V(IV) species are stabilized by interaction with titania support and further stabilization occurs at high calcination temperatures by their location in titania (rutile) lattice. Larger loadings of vanadium decrease the temperatures required for conversion of titania (anatase) to titania (rutile). At higher vanadium loading segregation into bulk V2O5oxide takes place, thus decreasing interaction with titania support. This enables a larger population of V(V) species than samples with surface dispersed vanadium oxide species. Although partial oxidation of methane is nonselective on titania (anatase), partial oxidation products are observed on titania (rutile)-supported vanadium oxide catalysts. The higher selectivity to partial oxidation product formaldehyde appears to be related to the high stability of V(IV) cations located on rutile lattice and the absence of V(V) sites.

Oxidative dehydrogenation of propane over niobia supported vanadium oxide catalysts

Oxidative dehydrogenation (ODH) of propane is examined over a series of catalysts, which include Nb 2O 5 supported monolayer V 2O 5 catalysts, bulk vanadia-niobia with different vanadium oxide loadings and prepared by four different methods, V 2O 5and Nb 2O 5. The intrinsic activity (TOF) of the samples studied indicates that vanadium containing active sites are indispensable for catalytic oxidative dehydrogenation of propane. Variations in the chemical environment of the vanadium ion do not cause significant changes in activity per site and, hence, all samples show similar TOF when the rates are normalised to the concentration of V on the surface. Selectivity to propene on the other hand strongly depends on the nature of the catalyst because readsorption and interaction of propene with the acid sites leads to total oxidation. Optimization of the weak sorption of propene is, therefore, concluded to be the key factor for the design of selective oxidative dehydrogenation catalysts.

ChemInform Abstract: Supported Vanadium Oxide Catalysts. Molecular Structural Characterization and Reactivity Properties

AbstractChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.

Enhancement of the catalytic performance of V2O5/?-Al2O3 catalysts in the oxidehydrogenation of propane to propylene by the use of a monolith-type reactor

The catalytic performance of aγ-Al2O3 -supported vanadium oxide catalyst in the oxidehydrogenation of propane to propylene was compared when the catalyst was used in the form of granules (packed fixed-bed reactor), and when the V2O5/γ-Al2O3 was coated in the form of a thin layer over a cylindrical ceramic support (monolith-type reactor configuration). A considerable enhancement of the catalytic performance was achieved in the latter case, because the consecutive reaction of propylene overoxidation, responsible for a remarkable decrease in selectivity at high conversions in packed-bed reactors, was minimized.

Synthesis and characterization of supported vanadium oxides by adsorption of the acetylacetonate complex

Supported vanadium oxide catalysts are prepared by adsorption and calcination of vanadium acetylacetonate complexes on the surfaces of silica and alumina. The interaction of the complex with the support is studied by FTIR, XRD, TGA and chemical analysis. The synthesis parameters are optimized in order to obtain a well dispersed layer on the surface. After calcination, the acetylacetonate complexes decompose and form a surface layer of VOx species. These surfaces have been studied with 51V NMR, UV–VIS diffuse reflectance and XRD spectroscopies. The method of acetylacetonate adsorption on silica enables the creation of supported VOx catalysts with a V loading of 0.5 mmol g–1(2.5 wt.% V), with all species in a strictly tetrahedral configuration and no formation of microcrystallites. On the alumina surface, vanadium loadings that are 5 times higher can be achieved without induced crystallinity.

Promoted TiO2 (Anatase)-Supported Vanadium Oxide Catalysts. TPR Study and Activity in Oxidation of Toluene

Vanadium oxide doped with K, Li, Bi, Sb, Te, U or Mo oxide, supported on TiO2 - anatase, was studied by temperature programmed reduction (TPR). The influence of the addition of promoters (up to molar ratio M : V = 0.5) to 5 wt.% V2O5/TiO2 catalyst on the TPR profile is presented in correlation with their catalytic activity in the vapor phase oxidation of toluene. All promoters, except Bi2O3, decrease the catalyst reducibility and decrease the rate of the toluene oxidation. A strong negative influence on the activity of the toluene oxidation have K, Li, and Te oxides. However, the presence of all tested promoters in the molar ratio M : V = 0.05 has a positive effect on the selectivity of benzoic acid formation. A further increase of this ratio leads to a decrease of the selectivity in the case of U, Mo and mainly K oxides, while with Li, Bi, Sb, and Te oxides, the selectivity remains almost unchanged. No correlation between TPR profiles of doped catalysts and their selectivity was found. The most effective promoter of vanadia catalysts for the benzoic acid production is Sb oxide, possessing a very high selectivity at high conversion of toluene.

Preparation of supported vanadium oxide catalysts. Adsorption and thermolysis of vanadyl acetylacetonate on a silica support

Adsorption and subsequent thermolysis of vanadylacetylacetonate [VO(acac)2] on a silica support yields supported V—O structures. The initial concentrations of the complex, the drying temperature and the calcination are the factors with the highest impact on the nature of the vanadium oxide coating. Too high initial concentrations of the complex induce coalescence and clustering of the adsorbed product. During the drying or curing step, important rearrangements occur in the adsorbed layer, creating additional covalent bonds between the complex and the substrate. Calcination is accompanied by thermolysis: a conversion of the supported VO(acac)2 towards supported vanadium oxide. The evolved products were identified and a conversion mechanism is suggested. The thermolysis of adsorbed VO(acac)2 is a fast and effective way to create supported vanadium oxide catalysts with a high surface area.