Supported Vanadium Oxide Catalysts Research Articles

A DFT study of the catalytic ODH of n-hexane over a cluster model of vanadium oxide

Supported vanadium oxides are one of the most widely reported catalysts for the selective oxidative dehydrogenation (ODH) of alkanes to alkenes. These vanadium oxide catalysts contain at least three distinct O species classified by their coordination to the vanadium cations. The involvement in ODH of the different oxygen species is thought to follow the coordination environment with singly coordinated vanadyl V(V)=O being the most reactive. A quantitative understanding of the relative activity of the different oxygen species in vanadium oxides could inform catalyst design to further enhance productivity and selectivity in the ODH reaction. Here we use computational models based on hybrid density functional theory to study the mechanistic pathway for n-hexane to 1- and 2-hexene. We compare the potential energy surfaces calculated for these reactions initiated by β-H abstraction with H transfer to the vanadyl oxygen, V(V)=O, with that involving H transfer to a two-coordinate bridging O atom. An isolated H4V2O7 cluster is used to represent the supported vanadium oxide catalyst so that the reactivity of oxygen species can be understood in the absence of support effects. Gibbs energy calculations were carried out at temperatures of 573, 673 and 773 K to mirror laboratory experimental conditions. We find that the rate-determining step (RDS) in the conversion of n-hexane to either alkene is associated with n-hexane interaction with H4V2O7 through a secondary CH bond. This leads to β-H abstraction with the calculated reaction barrier for abstraction by a V(V)=O (ΔE‡ = +32.7 kcal mol−1) significantly lower than that for the two-coordinate bridging O (ΔE‡ = +43.9 kcal mol−1). H-abstraction leads to reduction of one of the vanadium cations. Removal of the second H atom to form an alkene can follow several pathways. We have considered the second step leading to 2-hexene (γ-H abstraction) using either an adjacent V(V)=O on the reduced V(IV)-O-V(V) unit, a different active V(V)=O site on a fully oxidised cluster, or gas-phase molecular O2. Our results show that the ODH process is likely to proceed via a Mars-van Krevelen redox mechanism. In a practical catalyst the results imply that catalyst activity will depend on the surface coverage of V(V)=O active sites and the n-hexane to gas-phase molecular oxygen ratio. In addition to the pathways leading to alkene products we have noted the formation of CO bonds by the radical intermediate formed from the initial β-H abstraction. From this observation, we suggest that the low yields of 1- and 2-hexene (< 20%) obtained in our laboratory experiments with V2O5/MgO catalysts may be a result of the chemisorption properties of the radical intermediate (∙C6H13) on bridging or terminal O sites in the V(V)-O-V(V) units, leading to undesired products including oxygenates.

Supported vanadium oxide catalyst over HY-zeolite-alumina composite fabricated by extrusion for oxidative desulfurization of dibenzothiophene

Despite the advantages of zeolite catalysts/carriers, the small pore size of HY zeolite causes diffusion limitations especially for bulky refractory sulfur compounds in oxidative desulfurization (ODS) of fuels. To overcome this restriction, extruded HY-zeolite/Alumina composites containing both micro and meso‑pores were successfully synthesized and used as an attractive carrier for the vanadium dispersion. Insights about the interactions between zeolite and alumina and their effects on the vanadium impregnation were given implementing different analysis approaches; BET, SEM-EDX, TEM, NH3-TPD, and UV-vis. Alumina particle piling created meso‑pores in the composites providing a positive impact on the ODS of bulky dibenzothiophene (DBT). Comparing to physical mixing of the alumina and zeolite, the extrusion approach notably enhanced the weak acid sites due to the removal/distorting tetrahedrally coordinated Al from the zeolite framework. A less increase in the strong acid sites was also resulted from the reaction of Al-OH in the alumina with the free Si-OH over the zeolite. The proposed catalyst (V/HYZ-Al(alumina/zeolite ratio=50%) exhibited an excellent DBT removal so that 1000 ppmw of DBT was eliminated completely under the optimal investigated conditions (T = 50 °C, O/(S+N)=12 mol/mol, time=90 min, Mcatalyst/Vfuel=10 g/L). The catalyst recyclability was proved to be acceptable during four examined successive ODS cycles.

Effect of promoters on o-xylene oxidation pathways reveals nature of selective sites on TiO2 supported vanadia

The o-xylene oxidation pathways on TiO2 supported vanadium oxide catalysts are affected by Cs+ and Sb4+ promoters. Reaction kinetics change remarkably at 360 °C: while at lower temperatures the rate determining step is the C-H bond cleavage in xylene methyl groups, the catalyst reoxidation becomes kinetically limiting at higher temperatures. Promotion by Sb4+ is related to an increase in the availability of oxygen at high temperatures for the reoxidation of V sites. The presence of Cs+ enhances the reoxidation of V sites and increases the ratio of nucleophilic to electrophilic oxygen on the surface. Catalytic tests of benzene oxidation confirm the lack of electrophilic sites on Cs-promoted VOx/TiO2 catalyst. Combined evidence points to a key role of the bridging lattice oxygen in V-O-V species as active site for the activation of o-xylene and its selective transformation to phthalic anhydride.

Cover Feature: Understanding the Synthesis of Supported Vanadium Oxide Catalysts Using Chemical Grafting (Chem. Eur. J. 5/2020)

Cover Feature: Understanding the Synthesis of Supported Vanadium Oxide Catalysts Using Chemical Grafting (Chem. Eur. J. 5/2020)

Understanding the Synthesis of Supported Vanadium Oxide Catalysts Using Chemical Grafting.

The complexity of variables during incipient wetness impregnation synthesis of supported metal oxides precludes an in-depth understanding of the chemical reactions governing the formation of the dispersed oxide sites. This contribution describes the use of vapor phase deposition chemistry (also known as grafting) as a tool to systematically investigate the influence of isopropanol solvent on VO(Oi Pr)3 anchoring during synthesis of vanadium oxide on silica. The availability of anchoring sites on silica was found to depend not only on the pretreatment of the silica but also on the solvent present. H-bond donors can reduce the reactivity of isolated silanols whereas disruption of silanol nests by H-bond acceptors can turn unreactive H-bonded silanols into reactive anchoring sites. The model suggested here can inform improved syntheses with increased dispersion of metal oxides on silica.

Morphology-Oriented ZrO2-Supported Vanadium Oxide for the NH3-SCR Process: Importance of Structural and Textural Properties.

ZrO2 supports, with diverse morphologies (hollow sphere, star, rod, mesoporous), were produced using hydrothermal and evaporation-induced self-assembly methods. Zirconia-supported vanadium oxide catalysts were prepared by wet impregnation and used for the low-temperature selective catalytic reduction (SCR) of NO with ammonia. Characterization of catalysts includes N2 physisorption, elementary analysis, X-ray diffraction, high-resolution transmission electron microscopy, X-ray photoelectron spectroscopy, temperature-programmed reduction by H2, and temperature-programmed desorption of NH3. Significant differences in terms of activity are measured. 3 wt % V2O5 supported on mesoporous ZrO2 (V/MZ) presents excellent N2 yields (>90%, in the 200-400 °C interval), with a wide operating temperature window (NO conversion > 95%, in the 225-425 °C interval), and less interesting performances were obtained when vanadium oxide is supported over stars, hollow spheres, and rods. Surface characterization showed a content of tetravalent vanadium ion, when supported, decreasing in the order of mesoporous > hollow sphere > star > rod. This order is in perfect agreement with the order of performance of the catalyst in the NH3-SCR reaction. The impact of tetravalent ion's presence on the surface is confirmed by diffuse reflectance infrared Fourier transform spectroscopy analysis, Brønsted acid sites generated on the surface, and the V4+-OH species involved in the reaction. The production of more important nitrite species over the tetragonal supported vanadium oxide catalyst could be another reason for the excellent NH3-SCR performance displayed by the V/MZ catalyst. When supported over monoclinic zirconia, like vanadium oxide over star-type morphology, the adsorbed NH3 species (NH4+ and coordinated NH3) reacted with NO x adsorption species (nitrate) to form ammonium nitrate. Ammonium nitrate can be decomposed to N2 and N2O (or NO2). Thus, NO conversion curves and N2 yield curves over tetragonal zirconia (MZ) at lower temperature were ahead of those over V/star ZrO2 because of the higher V4+ surface content and more active B acid sites associated with an easy formation of the nitrito intermediate.

DFT: Kinetics effect with Oxidation of cyclohexenol with air over alumina supported Vanadium oxide catalyst

Cyclohexenon is a chemical of commercial importance. It can be easily converted to Cyclohexenon oxime which isa monomer for production of nylon-6. Attempt have there for made to produce Cyclohexenon at industrial scale. In the present work Cyclohexenol is oxidized with molecular oxygen over vanadium catalyst to Cyclohexenon in a fixed bed reactor. The catalyst was characterized with x-ray diffraction for its phase purity. The reaction parameters were optimizes to get maximum conversion of cyclohexenol and best selectivity of Cyclohexenon. The product was confirmed by producing its derivatives thus reaction of reaction method with, 2-4dinitrophenylhydrazine produced corresponding hydrazones, and reaction with hydroxylamine produced Cyclohexenon oxime.

Vanadium oxide modified H-beta zeolite for the synthesis of polyoxymethylene dimethyl ethers from dimethyl ether direct oxidation

The bifunctional H-beta zeolite supported vanadium oxide catalysts were prepared at different calcination temperatures and investigated for dimethyl ether (DME) direct oxidation to polyoxymethylene dimethyl ethers (DMMx). The impact of the nature of VOx species and the acid properties controlled by the calcination temperature on the DMMx formation has been studied in detail. Vanadium oxide species dispersing well on the H-beta zeolite offer suitable redox sites. Meanwhile, the acid strength and distribution of H-beta zeolite are modulated by the interaction between VOx species and zeolite framework. The acid properties of the catalysts with different calcination temperatures are analyzed by pyridine-Fourier transform infrared spectra and NH3 temperature-programmed desorption measurements, and the selectivity to DMMx shows a linear relationship with the Brønsted/Lewis acid sites ratio, indicating that the catalysts with higher B/L ratio are effective for the DMMx formation. In addition, the NH3-TPD results confirm that more acid sites with strong strength for the catalysts are in favor of the C–O chain growth to synthesize larger DMMx molecules. These results above are helpful for us to well understand the active sites and design the effective catalysts to synthesize DMMx with longer C–O chain.

VOx/γ‐Al2O3 Catalysts for Propane Dehydrogenation Prepared by “Impregnation‐Solid Phase Reaction” Method with Aluminum Hydroxide as Support Precursor

AbstractThe alumina supported vanadium oxide catalysts (VOx/γ‐Al2O3) were prepared by wet impregnation‐solid phase reaction method with aluminum hydroxide as support precursor. The prepared catalysts were characterized by N2 adsorption–desorption, UV–vis, X‐ray photoelectron spectroscopy and Raman spectroscopy. The performances of the catalysts were tested for propane dehydrogenation (PDH) reaction. The results indicate that the loading of vanadium on the aluminum hydroxide can influence the surface morphology of the catalysts and greatly improve the specific surface area of the resulted VOx/γ‐Al2O3 compared to pure γ‐Al2O3 support. The vanadium content markedly influences the polymerization degree of surface VOx species and catalytic performances of VOx/γ‐Al2O3 catalysts for PDH. Among all samples, the 2.9VOX/γ‐Al2O3 catalyst (vanadium content is 2.9 wt.%) possesses the maximum specific surface area and harmonious polymerization degree of VOX species. It exhibits the best catalytic performances during the 8 h of PDH reaction. Meanwhile, the 2.9VOx/γ‐Al2O3 catalyst possesses high regeneration stability and the highest TOF (turnover frequencies) value of 9.0×10−3 s−1, which is evidently higher those of previous works. After 7 reaction–regeneration cycles, the initial activity and propene selectivity can be almost restored.

Efficient oxidative dehydrogenation of ethanol by VOx@MIL-101: On par with VOx/ZrO2 and much better than MIL-47(V)

The possibility to apply metal organic framework (MOF) based catalysts for oxidation reaction in gas phase was explored. Catalytic activity of the vanadium oxide catalyst incorporated in MIL-101(Cr) as support was investigated in gas phase ethanol oxidative dehydrogenation (EtOH-ODH) and compared to that of MIL-47(V) metal organic framework material containing vanadium as central metal in the framework structure and to a classical VOx/ZrO2 supported vanadium oxide catalyst. It was found that vanadium species, incorporated in MIL-101(Cr) support by a variant of vapor deposition method, are stabilized in the form of well dispersed VOx species (no vanadium pentoxide clusters was detected in the catalyst). The obtained VOx@MIL-101(Cr) catalyst exhibited high selectivity towards acetaldehyde (up to 99%) at reaction temperatures not exceeding 200 °C. The catalytic activity of VOx@MIL-101(Cr) catalyst reached an activity level comparable to that of the classical VOx/ZrO2 catalyst, but the specific productivity of acetaldehyde (3 kgAA kgcat−1 h−1) was higher by 75% compared with productivity on VOx/ZrO2 due to much higher content of vanadium species, which could be hosted by the MOF. On the other hand, MIL-47(V) catalyst exhibited negligible activity seemingly due to coordinatively saturated character of the vanadium centers and/or too high stability of the VIV oxidation state. This proof-of-concept study proved that application of MOF materials as host matrices for heterogeneous catalysts aiming oxidation reaction in gas phase could be efficient as demonstrated on the example of oxidative dehydrogenation of ethanol.

Electronic structure changes introduced by nitrogen on the N-doped VOx/TiO2 system: Consequences on partial oxidation catalysis

The catalytic activity for ethanol partial oxidation of supported vanadium oxide catalysts prepared on N doped TiO2 (VOx/N-TiO2) was correlated to the electronic structure of the vanadium clusters present in the catalyst surface. In situ Raman and UV–vis spectra together with ethanol temperature programmed desorption (TPD) profiles indicate that in all materials studied the average VOx cluster size is very similar, irrespective of the presence of nitrogen. However, the distribution of active sites and the reducibility of the VOx species are significantly affected by the presence of nitrogen in the support. The electronic structure of these materials was investigated using periodic density functional theoretical calculations. It was found that, compared to the nitrogen free materials, the electronic structure of reduced VOx is affected by the presence of N, as well as nitrogen position on the TiO2 supports (interstitial vs. substitutional). Our analysis revealed that after vanadia reduction during ethanol oxidation, reduced VOx sites (V+4) form, accompanied by the simultaneous reduction of adjacent Ti cations (Ti+3). Calculated optical absorption spectra are also provided and matched with experimental spectroscopic results, confirming these observations.

Formation of N2O greenhouse gas during SCR of NO with NH3 by supported vanadium oxide catalysts

Selective catalytic reduction of NO by NH3 over supported vanadium oxide catalysts has been studied for decades, but the studies mostly concentrated on the dominant N2 product with much less attention paid to the formation of the undesired N2O product. In the present study, fundamental aspects of the N2O formation reaction were elucidated by a series of temperature-programmed surface reaction studies with isotopic labelled reactants. The surface vanadium oxide species on the TiO2 support are active sites for the N2O formation reaction, while tungsten species mainly function as promoters. Oxygen from NO, gaseous O2 and catalyst oxygen all function as oxygen sources for formation of N2O (∼50%, ∼30% and ∼20%, respectively). The rate-determining-step for N2O formation involves breaking of the ammonia NH bond. These new molecular level insights have the potential to guide the rational design of improved SCR catalysts for diesel engines with reduced N2O produced.

Reaction Pathways and Kinetics for Selective Catalytic Reduction (SCR) of Acidic NOx Emissions from Power Plants with NH3

Selective catalytic reduction (SCR) of NOx with NH3 by supported vanadium oxide catalysts is an important technology for reducing acidic NOx emissions from stationary sources and mobile diesel vehicles. However, rational design of improved catalysts is still hampered by a lack of consensus about reaction pathways and kinetics of this critical technology. The SCR fundamentals were resolved by applying multiple time-resolved in situ spectroscopies (ultraviolet–visible light (UV-vis), Raman and temperature-programmed surface reaction (TPSR)) and isotopically labeled molecules (18O2, H218O, 15N18O, ND3). This series of experiments directly revealed that the SCR reaction occurs at surface V5+O4 sites that are maintained in the oxidized state by O2 and the rate-determining step involves the reduction of V5+O4 sites by NO and NH3, specifically the breaking of N–H bonds during the course of formation or decomposition of the NO–NH3 intermediate.

Supported MgO–V2O5/Al2O3 catalysts for oxidative propane dehydration: Effect of the molar Mg : V ratio on the phase composition and catalytic properties of samples

The physicochemical properties of V2O5/Al2O3 and MgO–V2O5/Al2O3 supported catalysts (Mg : V = 1 : 1, 2 : 1, and 3 : 2) obtained by consecutive impregnation of the support with solutions of vanadium and magnesium precursors are studied using a complex of mutually complementary methods (XRD, Raman spectroscopy, UV–Vis spectrometry, and TPR-H2). The effect of the formation of surface magnesium vanadates of various composition and structure on the catalytic properties of the supported vanadium oxide catalysts in the oxidative dehydrogenation of propane is studied. The introduction of magnesium in the samples and an increase in its content, accompanied by a change in the structure of the surface vanadium oxide phases from polymeric VO6/VO5 species to surface metavanadate species, magnesium metavanadate, and further to magnesium divanadate, significantly affects their catalytic properties in the reaction of the oxidative dehydrogenation of propane to propylene.

Supported, bulk and bulk-supported vanadium oxide catalysts: A short review with an historical perspective

The beginning is a very delicate time for understanding. Concepts are better understood when the student knows the historic perspective. Their minds apprehend the evolution of knowledge and, to some extent, share fundamental discoveries. Students will not understand, for example, the actual model of the structure of an atom if they do not understand first the models proposed by Thomson and Bohr. However, due to the high number of research papers that are nowadays published, sometimes researchers forget to review some of the old papers in which the basis of knowledge were established, and those bases allow to build new achievements, allow to understand where are we, and where are we going. In this context, the main objective of the this article is to present the basis of knowledge regarding supported vanadium oxide catalysts, with an historical point of view. It presents how the main achievements were described in this field, and to do that, the literature century has been reviewed, and especially the progress in the 1980s and 1990s, since the influence of synthesis method and oxide support, the structure of the active phases, and the concept of monolayer were established in those years. The final part of this paper briefly presents the evolution of knowledge that has taken place during the past decade in this field, and some future trends are anticipated.

Ab initio R1 mechanism of photostimulated oxygen isotope exchange reaction on a defect TiO2 surface: The case of terminal oxygen atom exchange

Based on density functional theory we propose R1 mechanism of photostimulated oxygen isotope exchange (POIEx) reaction between 16O18O and terminal oxygen atom of a defect TiO2 surface, which is modeled by amorphous Ti8O16 nanocluster in excited S1 electronic state. The proposed mechanism involves four adsorption intermediates and five transition states. The computed activation energy of the POIEx equals 0.24eV. The computed g-tensors of the predicted ozonide O3− chemisorption species match well EPR data on O2 adsorption on UV-irradiated nanocrystalline TiO2. This match serves a mean of justification of the proposed R1 mechanism of the POIEx reaction. In addition, it is found that the proposed R1 POIEx reaction’s mechanism differs from R1 mechanism of thermo-assisted OIEx reaction on a surface of supported vanadium oxide catalyst VOx/TiO2 reported earlier.

Ethanol gas-phase ammoxidation to acetonitrile: the reactivity of supported vanadium oxide catalysts

The gas-phase ammoxidation of ethanol, a bio-based platform molecule, has been investigated as a possible more sustainable route for the production of acetonitrile, using supported vanadium oxide catalysts. The nature of the interaction between the support and the active species greatly affected the catalytic performance.

Elucidation of Anchoring and Restructuring Steps during Synthesis of Silica-Supported Vanadium Oxide Catalysts

We investigate the chemical reactions involved during the synthesis of supported vanadium oxide catalysts using chemical grafting of vanadium oxytriisopropoxide (VO(OiPr)3) to thermally pretreated silica under solvent-free conditions. VO(OiPr)3 is found to react with both site-isolated silanol (Si−OH) groups and strained siloxane (≡Si−O−Si≡) bridges at the silica surface. Solid-state 51V and 13C MAS NMR confirms the formation of two slightly different vanadium species associated with the two anchoring mechanisms. In situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS), in situ Raman spectroscopy, and thermogravimetric analysis-differential scanning calorimetry-mass spectrometry (TGA-DSC-MS) were used to study the subsequent calcination, revealing the formation of a transient V—OH intermediate upon the release of propene, followed by the formation of isolated VO4 surface species upon elimination of water. X-ray absorption spectroscopy (XAS) and 51V MAS NMR of the calcined material confirm the conversion of the two original vanadium sites to a species with a single isotropic shift, confirming the formation of isolated, tetrahedral VO4 sites.

Molecular mechanism of propane oxidative dehydrogenation on surface oxygen radical sites of VO x /TiO2 catalysts

Minimum energy pathways of propane oxidative dehydrogenation to propene and propanol on supported vanadium oxide catalyst VO x /TiO2 were studied by periodic discrete Fourier transform (DFT) using a surface oxygen radical as the active site. The propene formation pathway was shown to consist of two consecutive hydrogen abstraction steps. The first step includes Cβ–H bond activation of propane followed by the formation of a surface hydroxyl group V–O t H and a propyl radical n-C3H7. This step with the activation energy E* = 0.56 eV (54.1 kJ/mol) appears to be rate-determining. The second step involves the reaction of the bridging O b oxygen atom with the methylene C–H bond of propyl radical n-C3H7 followed by the formation of a hydroxylated surface site HO t –V4+–O b H and propene. The initial steps of the C–H bond activation during propane conversion to propanol and propene by ODH on V5+–(O t O b )− active sites are identical. The obtained results demonstrate that participation of surface oxygen radicals as the active sites of propane ODH makes it possible to explain relatively low activation energies observed for this reaction on the most active catalysts. The presence of very active radical species in low concentration seems to be the key factor for obtaining high selectivity.

XAFS Analysis for Light-induced Excited State of Supported Vanadium Oxide Catalyst for Activation of Molecular Oxygen

XAFS Analysis for Light-induced Excited State of Supported Vanadium Oxide Catalyst for Activation of Molecular Oxygen