Vanadium Oxide-based Catalysts Research Articles

Enhancing propane direct dehydrogenation performances through temperature induced VOx dispersion and alumina support phase transformation

Vanadium oxide-based catalysts are promising candidates for propane direct dehydrogenation. A strategy was developed in the present study to enhance the intrinsic activity and coking-resistance of supported VOx catalysts at the same time. The annealing temperature and VOx surface density were identified as regulation parameters to tailor the dispersion of VOx and micro-environment of V-O-Al active sites. The surface acidic VOx was found capable of inducing local structure rearrangement of transition alumina and the transformation of amorphous δ- to more crystalline θ-alumina at relatively lower temperatures, contributing to lowering VOx-support interactions and enhancing VOx reducibility. The DFT calculations verify that the chemical states of V-O active sites could be tuned by the ligand Al3+ coordination micro-environment, and the octahedral Al3+ serving as ligands contributes to enhancing the reducibility and C–H bond rupture capability of V-O site. The annealing temperature was also found capable of dispersing VOx species over alumina surface with the optimal temperature dependent of specific VOx loadings. As a consequence, the intrinsic activity of 3.1 V/Al2O3-900 and the deactivation rate of 3.1 V/Al2O3-700 with dominantly isolated VOx is 1.4 times and 0.65 times that of 3.1 V/Al2O3-550, respectively.

Effects of Support and CO2 on the Performances of Vanadium Oxide-Based Catalysts in Propane Dehydrogenation

Effects of Support and CO₂ on the Performances of Vanadium Oxide-Based Catalysts in Propane Dehydrogenation

Enhanced activity of vanadia supported on microporous titania for the selective catalytic reduction of NO with NH3: Effect of promoters

Vanadium oxide-based catalysts are considered a promising catalyst for selective catalytic reduction (SCR) of NO with NH3, which is an effective NOx removal technology. As environmental issues have garnered more attention, however, improvements to vanadium-based SCR catalysts are strongly required. In a previous study, we found that vanadium oxide on microporous titania as a support (V/MPTiO2) has certain advantages, such as improved thermal stability and more suppressed N2O formation, over the use of conventional nanoparticle titania (DT-51) as a support. In this study, widely used promoters, such as W, Sb, and Mo, were added to V/MPTiO2 to investigate whether they have promoting effects on V/MPTiO2 as well. Among these promoters added catalysts, the W and Mo were found to have significant promoting effects on the enhancement of deNOx activities at low temperatures, while the addition of Sb to V/MPTiO2 tended to have a negative effect on the SCR activity. Based on the characterizations, including laser Raman, H2-temperature programmed reduction (H2-TPR), and in situ diffuse reflectance infrared Fourier transform (in situ DRIFT) analysis, we found that the addition of W and Mo increased the degree of polymerization in V/MPTiO2, which generated more reactive vanadia species. Hence, such changes, resulting from the addition of W and Mo promoters to V/MPTiO2, yielded enhanced catalytic activity at low temperatures.

Serendipity in Catalysis Research: Boron-Based Materials for Alkane Oxidative Dehydrogenation.

Light olefins such as ethylene and propylene form the foundation of the modern chemical industry, with yearly production volumes well into the hundreds of millions of metric tons. Currently, these light olefins are mainly produced via energy-intensive steam cracking. Alternatively, oxidative dehydrogenation (ODH) of light alkanes to produce olefins allows for lower operation temperatures and extended catalyst lifetimes, potentially leading to valuable process efficiencies. The potential benefits ofthis route have led to significant research interest due to the wide availability of natural gas from shale deposits. Advances in this area have still not yielded catalysts that are sufficiently selective to olefins for industrial implementation, and ODHstill remains a holy grail of selective alkane oxidation research. The main challenge in selective oxidation lies in preventing the overoxidation of the desired product, such as propylene during propane oxidation, to CO and CO2. Research into selective heterogeneous catalysts for the oxidative dehydrogenation of propane has led to the extensive use of vanadium oxide-based catalysts, and studies on the surface mechanism involved have been used to improve the catalytic activity of the material. Despite decades of research, however, selectivity toward propylene has not proven satisfactory at industrially relevant conversions. It is imperative for new catalytic systems that minimize product overoxidation to be developed for future applications of oxidative dehydrogenation processes. While rational catalyst design has been successful in developing homogeneous catalyst systems, its practical use in heterogeneous catalyst development remains modest. The complexity of surfaces with a variety of terminations and bulk structures, let alone their modification by the chemical potential of a reaction mixture, makes heterogeneous catalyst discovery serendipitous in many cases. The catalyst family presented in this Account is no exception. The importance of catalysis research lies in exploring the science behind serendipity. In this Account, we will first present our initial discovery of boron nitride (BN) as an unexpected catalyst for the oxidative dehydrogenation of light alkanes. Beyond its surprising activity, BN also drew interest due to its low selectivity to carbon oxides. This observation made BN distinct from previously studied metal oxide catalysts for selective alkane oxidation. We narrowed down its unique reactivity to the oxygen functionalization of the catalyst surface, particularly the formation of B-O species as probed by various spectroscopic techniques. In investigating the critical role of each of the structural elements during ODH, we discovered that not only BN but an entire class of boron-containing compounds are active and selective for the formation of propylene from propane. All these materials form a complex oxidized surface with a distribution of BO x surface sites. This discovery opens the doors to a new field of boron-based oxidation chemistry that currently has more questions than answers. We aim to make this Account a starting point for the research community to explore these new materials to understand their surface mechanisms and the surface species that offer a unique selectivity toward olefinic products. Effective use of these materials may lead to novel processes for efficient use of abundant light alkane resources by oxidation chemistry.

Visualizing atomic-scale redox dynamics in vanadium oxide-based catalysts

Surface redox processes involving oxygen atom exchange are fundamental in catalytic reactions mediated by metal oxides. These processes are often difficult to uncover due to changes in the surface stoichiometry and atomic arrangement. Here we employ high-resolution transmission electron microscopy to study vanadium oxide supported on titanium dioxide, which is of relevance as a catalyst in, e.g., nitrogen oxide emission abatement for environmental protection. The observations reveal a reversible transformation of the vanadium oxide surface between an ordered and disordered state, concomitant with a reversible change in the vanadium oxidation state, when alternating between oxidizing and reducing conditions. The transformation depends on the anatase titanium dioxide surface termination and the vanadium oxide layer thickness, suggesting that the properties of vanadium oxide are sensitive to the supporting oxide. These atomic-resolution observations offer a basis for rationalizing previous reports on shape-sensitive catalytic properties.

Catalytic destruction of PCDD/Fs over vanadium oxide-based catalysts.

Vanadium oxide-based catalysts were developed for the destruction of vapour phase PCDD/Fs (polychlorinated dibenzo-p-dioxins and polychlorinated dibenzofurans). A vapour phase PCDD/Fs generating system was designed to supply stable PCDD/Fs steam with initial concentration of 3.2ngI-TEQNm(-3). Two kinds of titania (nano-TiO2 and conventional TiO2) and alumina were used as catalyst supports. For vanadium-based catalysts supported on nano-TiO2, catalyst activity is enhanced with operating temperature increasing from 160 to 300°C and then reduces with temperature rising further to 350°C. It is mainly due to the fact that high volatility of organic compounds at 350°C suppresses adsorption of PCDD/Fs on catalysts surface and then further inhibits the reaction between catalyst and PCDD/Fs. The optimum loading of vanadium on nano-TiO2 support is 5wt.% where vanadium oxide presents highly dispersed amorphous state according to the Raman spectra and XRD patterns. Excessive vanadium will block the pore space and form microcrystalline V2O5 on the support surface. At the vanadium loading of 5wt.%, nano-TiO2-supported catalyst performs best on PCDD/Fs destruction compared to Al2O3 and conventional TiO2. Chemical states of vanadium in the fresh, used and reoxidized VOx(5%)/TiO2 catalysts at different operating temperature are also analysed by XPS.

Vanadium Oxide-Based Composite Catalysts for the Oxidation of Styrene: A Comparative Study

Vanadium oxide-based composite catalysts were prepared, and their activities in a styrene oxidation reaction were tested. These catalysts included (i) vanadium oxide (VxOy) supported on multi-walled carbon nanotubes (CNTs) (CNTs-VxOy), (ii) a carbon nanotube CNTs-VxOy template coated with γ-Al2O3 (CNTs-VxOy/Al2O3), (iii) γ-Al2O3/VxOy nanoparticles and (iv) vanadium oxide nanotubes (VxOy-NTs). The structure of these catalysts was determined by TEM, SEM, nitrogen adsorption (BET surface area), TGA and FTIR. The oxidation process was examined between styrene and anhydrous hydrogen peroxide which was catalyzed using the above VxOy-based composite catalyst. The vanadium particles embedded on the wall of CNTs have a satisfactory catalytic activity for oxidation. The effect of the operating parameters, including the reaction temperature, solvent type, reaction time, oxidant concentration and catalyst loading, on styrene conversion and benzaldehyde selectivity was studied. Keywords: Carbon nanotubes (CNTs), Composite materials, Nanostructure materials, Transition metal alloys and compounds, Sol-gel processes, Vanadium oxide, Styrene oxidation, Benzaldehyde, Anhydrous hydrogen peroxide, Nano catalyst

Mono- and Dinuclear Oxovanadium(V)calixarene Complexes and Their Activity as Oxidation Catalysts

The background of the investigation is constituted by reactive moieties and intermediates playing relevant roles on the surfaces of vanadiumoxide-based catalysts during the oxygenation/dehydrogenation of organic substrates. With the aim of modeling such species, a series of mono- and dinuclear charged and uncharged vanadium oxo complexes containing p-tert-butylated calix[4]arene and calix[8]arene ligands (denoted H4B and H8B' ', respectively, in the protonated forms) has been synthesized and characterized: PPh4[O=VB] ((PPh41), O=VB(OAc) (2), PPh4[O2V2HB' '] (3), and [mu-O(O=V(OMe))2B(Me2)] (4), where superscripts OAc and Me2 indicate that one or two protons of H4B are substituted by these residues, respectively. These compounds were analyzed both in solution and by means of single-crystal X-ray crystallography; it turned out that the crystal structures are retained on dissolution (2 changed only from the paco to the cone structure). In the case of 4, it could be shown that the bulk product consists of a mixture of two isomers (4t and 4c) differing in the relative positions of the vanadium-bound methoxy groups. Subsequently, all compounds were tested as catalysts for the oxidation of alcohols with O2. It turned out that the two dinuclear complexes efficiently catalyze the oxidation of 1-phenyl-1-propargyl alcohol and fluorenol; in addition, they even show some activity with respect to the oxidation of dihydroanthracene. This may hint to a higher activity of dinuclear sites on the surfaces of heterogeneous catalysts as well.

The oxygen-assisted transformation of propane to CO x/H 2 through combined oxidation and WGS reactions catalyzed by vanadium oxide-based catalysts

This paper reports about the gas-phase oxidation of propane catalyzed by bulk vanadium oxide and by alumina- and silica-supported vanadium oxide. The reaction was studied with the aim of finding conditions at which the formation of H 2 and CO 2 is preferred over that of CO, H 2O and of products of alkane partial oxidation. It was found that with bulk V 2O 5 considerable amounts of H 2 are produced above 400 °C, the temperature at which the limiting reactant, oxygen, is totally consumed. The formation of H 2 derived from the combination of: (i) oxidation reactions, with generation of CO, CO 2, oxygenates (mainly acetic acid), propylene and H 2O, all occurring in the fraction of catalytic bed that operated in the presence of gas-phase oxygen, and (ii) WGS reaction, propane dehydrogenation and coke formation, that instead occurred in the fraction of bed operating under anaerobic conditions. This combination of different reactions in a single catalytic bed was possible because of the reduction of V 2O 5 to V 2O 3 at high temperature, in the absence of gas-phase oxygen. In fact, vanadium sesquioxide was found to be an effective catalyst for the WGS, while V 2O 5 was inactive in this reaction. The same combination of reactions was not possible when vanadium oxide was supported over high-surface area silica or alumina; this was attributed to the fact that in these catalysts vanadium was not reduced below the oxidation state V 4+, even under reaction conditions leading to total oxygen conversion. In consequence, these catalysts produced less H 2 than bulk vanadium oxide.

Supported vanadium oxide-based catalysts for the oxidehydrogenation of propane under cyclic conditions

Heterogeneous catalysts were prepared by deposition of vanadium oxide on different supports, namely alumina, titania, titania–alumina cogel and silica. Catalysts were tested in the reaction of propane oxidehydrogenation to propylene under co-feed and under redox-decoupling conditions (i.e., by alternating the reducing and the re-oxidation steps). Under cyclic conditions, the selectivity to propylene was higher than under co-feed conditions when the catalyst was treated for longer reduction times, but this was due to the fact that in the former case the mechanism was dehydrogenative rather than oxidative. On the contrary, on oxidized catalysts the mechanism was an oxidative one even in the absence of molecular oxygen, and conversions were higher than 20%; however, in this case the improvement in selectivity to propylene with respect to the co-feed operation was low. Catalysts made of vanadium oxide either dispersed inside a high-surface-area silica gel, or supported over silica, allowed higher instantaneous propane conversions than catalysts in which vanadium oxide was deposited over alumina or titania, and also gave the greater improvement of selectivity to propylene with respect to the co-feed conditions. These differences were attributed to the nature of the vanadium species which develop when silica is the support.

Chemistry, Spectroscopy and the Role of Supported Vanadium Oxides in Heterogeneous Catalysis

AbstractFor Abstract see ChemInform Abstract in Full Text.

Chemistry, spectroscopy and the role of supported vanadium oxides in heterogeneous catalysis

Supported vanadium oxide catalysts are active in a wide range of applications. In this review, an overview is given of the current knowledge available about vanadium oxide-based catalysts. The review starts with the importance of vanadium in heterogeneous catalysis, a discussion of the molecular structure of vanadium in water and in the solid state and an overview of the spectroscopic techniques enabling to study the chemistry of supported vanadium oxides. In the second part, it will be shown that advanced spectroscopic tools can be used to obtain detailed information about the coordination environment and oxidation state of vanadium oxides during each stage of the life-span of a heterogeneous catalyst. Three topics will be discussed: (1) the molecular structure of supported vanadium oxide catalysts under hydrated, dehydrated and reduced conditions, including the parameters, which influence the molecular structures formed at the surface of the support oxide; (2) elucidation of the active surface vanadium oxide during the oxidation of methanol to formaldehyde, the reaction mechanism and the vanadium oxidesupport effect; and (3) deactivation of fluid catalytic cracking (FCC) catalysts by migration of vanadium oxides and the development of a method preventing the structural breakdown of zeolites by trapping the mobile vanadium oxides in an aluminum oxide coating.

Phase cooperation of V2O5 and Bi2O3 in the selective oxidation of H2S containing ammonia and water

The selective oxidation of hydrogen sulfide containing excess water and ammonia was studied over vanadium oxide-based catalysts. The investigation was focused on the role of V2O5, and phase cooperation between V2O5 and Bi2O3 in this reaction. The conversion of H2S continued to decrease since V2O5 was gradually reduced by treatment with H2S. The activity of V2O5 was recovered by contact with oxygen. A strong synergistic phenomenon in catalytic activity was observed for the mechanically mixed catalysts of V2O5 and Bi2O3. Temperature-programmed reduction (TPR) and oxidation (TPO) and two bed reaction tests were performed to explain this synergistic effect by the reoxidation ability of Bi2O3.

Thermal analysis characterization of promoted vanadium oxide-based catalysts

Vanadium oxide-based catalysts promoted with KCl and a transition metal chloride (M=Cu, Ce, Fe, Co, Zn) have been investigated in the oxidation of diesel soot and characterized by thermal analysis techniques, such as temperature-programmed reduction (TPR) and differential scanning calorimetry (DSC). Aim of the study was to acquire information on the reaction mechanism of the catalytic soot combustion over these catalytic systems. Data presented suggest that the activity of investigated catalysts is correlated to their reducibility which favor the redox activity of the active vanadium species. The formation of eutectics between components of the catalyst also occur improving the catalyst–carbon contact.

Selective gas-phase oxidation of polycyclic aromatic hydrocarbons on vanadium oxide-based catalysts

The selective gas-phase oxidation of fluorene, anthracene, phenanthrene and naphthalene over supported and unsupported vanadium oxide-based catalysts frequently modified with other oxides such as Fe 2O 3 or MoO 3 is reviewed and the effect of doping with alkali metal compounds is discussed. In particular, the oxidation of fluorene to 9-fluorenone is used as a key reaction for the discussion of the results. Reaction schemes derived from catalytic and kinetic measurements show strong similarities for all polycyclic aromatic hydrocarbons investigated indicating that selective products of inner and outer-ring oxidation are formed in primary steps while their decomposition occurs consecutively particularly at high degrees conversion. In addition to the catalytic studies reaction engineering simulations with respect to fixed-bed and fluidized-bed reactor performance are discussed. By comparing textural and structural peculiarities of the catalysts (e.g. surface acidity, phase composition, redox properties, active sites and role of the support) with their catalytic performance it becomes evident that high oxygenate selectivities are favoured by moderate surface acidity and oxidation potentials which can be controlled by admixing alkali compounds or less acidic metal oxides like Fe 2O 3 to V 2O 5. In cesium-doped vanadium/iron oxides an amorphous phase was identified containing lattice-oxide vacancies occupied by electrons which, presumably, act as catalytically active sites. On the basis of these considerations essential common features for the different oxidation processes are pointed out and some rules for catalysts design are derived.