Supported catalysts vanadium oxide

Supported vanadium catalysts, whereby vanadium oxide is dispersed on a support such as alumina or titania are of particular importance in, for instance, the oxidative dehydrogenation of alkanes [58-64]. Such materials have attracted considerable interest in the direct dehydrogenation of butane, where a key driver is to identify the relationship between catalytic activity and structural properties [5, 6, 65-68]. In the pure (solid) metal oxides the coordination of vanadium is well defined. However, this is not necessarily true in the case of supported catalysts. Vanadium may be present on the support surface as isolated vanadium ions dimeric or polymeric species one- and two-dimensional chains of vanadium ions  

The complex nature of vanadium species in catalytic materials means that no single technique is ideally suited to their study. NMR techniques can, however, provide information on the local environment of vanadium nuclei, including the geometry and coordination number of species, the number of non-equivalent 

Recent developments have allowed for an improved understanding of the local environment of vanadium in supported catalysts. Lapina and coworkers [27] have studied phosphorus-modified VO /TiOj catalysts by SATRAS and MQMAS techniques. A key question in such materials is what influence the phosphorus has on the structure of the catalyst. This is not directly answerable by conventional 

MQMAS studies are at present relatively rare. However, MQMAS techniques have been applied in studies of other nuclei such as Al and Mo (see Section 5.3.2). The success of these studies suggests that MQMAS NMR may play a role in future investigations of vanadium oxide catalysts. 

This problem is highlighted in the case of an alumina-supported vanadium catalyst employed for the dehydrogenation of n-butane. The hydrocarbon creates a reducing environment, which results in a reduction in the oxidation state of 

In the preceding sections the use of catalysts in which vanadium oxides are supported on a more or less inert carrier has been mentioned quite often. Because of the importance of this type of catalyst they are discussed more extensively in this section. Often a distinction is made between the normal supported catalysts and so called monolayer catalysts. In the latter the vanadium oxide is supposed to be dispersed in a monomolecular layer on the support, which may be covered completely or only partly. The normal supported catalysts are usually made by impregnation, either wet or dry, of the porous carrier with an aqueous solution, often of NH4V03, sometimes with oxalate added.12 14,75,95,139,140 

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The anhydride of 1,8-naphthalenedicarboxyHc acid is obtained in ca 95—116 wt % yield by the vapor-phase air-oxidation of acenaphthene at ca 330—450°C, using unsupported or supported vanadium oxide catalysts, with or without modifiers (96). 

The pyromellitic dianhydride is itself obtained by vapour phase oxidation of durene (1,2,4,5-tetramethylbenzene), using a supported vanadium oxide catalyst. A number of amines have been investigated and it has been found that certain aromatic amines give polymers with a high degree of oxidative and thermal stability. Such amines include m-phenylenediamine, benzidine and di-(4-amino-phenyl) ether, the last of these being employed in the manufacture of Kapton (Du Pont). The structure of this material is shown in Figure 18.36. 

Watling, T.C. Deo, G. Seshan, K. Wachs, I.E. Lercher, J.A. Oxidative dehydrogenation of propane over niobia supported vanadium oxide catalysts. Catal. Today 1996, 28, 139-145. 

Gao, X. Banares, M.A. Wachs, EE. Ethane and w-butane oxidation over supported vanadium oxide catalysts An in situ UV-visible diffuse reflectance spectroscopic investigation. J. Catal. 1999,188, 325-331. 

Ethane Oxidation on Supported Vanadium Oxide. Figure 1 shows the rates of production of the major products of ethane oxidation over a series of silica-supported vanadium oxide catalysts. As was described earlier, the structure of the catalyst changed considerably with the active-phase loading (77). The low loading samples (0.3 -1.4%) were shown to consist primarily of 0=V03 monomeric units, while the high loading catalysts (3.5 - 9.8%) were composed of V2O5 crystallites. 

Niobium Products Co., 50 m /g). Many different synthesis methods have been used to prepare supported metal oxide catalysts. In the case of supported vanadium oxide catalysts, the catalysts were prepared by vapor phase grafting with VOCI3, nonaqueous impregnation (vanadium alkoxides), aqueous impregnation (vanadium oxalate), as well as spontaneous dispersion with crystalline V2O5 [4]. No drastic reduction of surface area of the catalysts was observed. 

The vanadium oxide species is formed on the surface of the oxide support during the preparation of supported vanadium oxide catalysts. This is evident by the consumption of surface hydroxyls (OH) [5] and the structural transformation of the supported metal oxide phase that takes place during hydration-dehydration studies and chemisorption of reactant gas molecules [6]. Recently, a number of studies have shown that the structure of the surface vanadium oxide species depends on the specific conditions that they are observed under. For example, under ambient conditions the surface of the oxide supports possesses a thin layer of moisture which provides an aqueous environment of a certain pH at point of zero charge (pH at pzc) for the surface vanadium oxide species and controls the structure of the vanadium oxide phase [7]. Under reaction conditions (300-500 C), moisture desorbs from the surface of the oxide support and the vanadium oxide species is forced to directly interact with the oxide support which results in a different structure [8]. These structural... 

Table II. The TON and selectivity to formaldehyde for the methanol oxidation reaction on various 1% supported vanadium oxide catalysts...

The reactivity of the supported vanadium oxide catalysts for other oxidation reactions also show similar trends as the oxide support is varied from titania to silica [13]. The activity and selectivity for partial oxidation products of vanadium oxide supported on titania being higher than vanadium oxide supported on silica. The oxidation activity of the supported vanadium oxide catalysts is related to the ability to donate oxygen to form the required oxidation products. The... 

The reactivity ot the titania supported vanadium oxide catalysts was probed by the methanol oxidation reaction. The oxidation ot methanol over the titania supported vanadia catalysts exclusively yielded tormaldehyde, 95%+, as the reaction product. The titania support in the absence ot surtace vanadia yielded dimethyl ether and trace amounts ot CO2 The almost... 

Structure and Reactivity of Tin Oxide-Supported Vanadium Oxide Catalysts... 

The surface structure and reactivity of vanadium oxide monolayer catalysts supported on tin oxide were investigated by various physico-chemical characterization techniques. In this study a series of tin oxide supported vanadium oxide catalysts with various vanadia loadings ranging from 0.5 to 6. wt.% have been prepared and were characterized by means of X-ray diffraction, oxygen chemisorption at -78°C, solid state and nuclear magnetic resonance... 

The technique of solid-state NMR used to characterize supported vanadium oxide catalysts has been recently identified as a powerful tool (22, 23). NMR is well suited for the structural analysis of disordered systems, such as the two-dimensional surface vanadium-oxygen complexes to be present on the surfaces, since only the local environment of the nucleus under study is probed by this method. The nucleus is very amenable to solid-state NMR investigations, because of its natural abundance (99.76%) and favourable relaxation characteristics. A good amount of work has already been reported on this technique (19, 20, 22, 23). Similarly, the development of MAS technique has made H NMR an another powerful tool for characterizing Br 6nsted acidity of zeolites and related catalysts. In addition to the structural information provided by this method direct proportionality of the signal intensity to the number of contributing nuclei makes it a very useful technique for quantitative studies. 

In experiments run over a number of cycles, the activity was observed to increase after the first cycle, unlike the y-A Os counterpart which deactivated. Using BN, no Pt sintering occurred and this was ascribed to the high thermal conductivity of BN, ensuring that no local hot-spots were formed. On the basis of XPS, the locus of Pt particle attachment was proposed to be surface boron oxide impurities. Taylor and Pollard have compared the activities of silica (194 m g ) and boron nitride (7 m g ) supported vanadium oxide catalysts for propane oxidation. The use of boron nitride was reported to significantly... 

Analysis of structure-activity relationships shows that various species characterized by different reactivities exist on the surface of vanadium oxide-based catalysts.339 The redox cycle between V5+ and V4+ is generally accepted to play a key role in the reaction mechanism, although opposite relationships between activity and selectivity, and reducibility were established. More recent studies with zirconia-supported vanadium oxide catalysts showed that vanadium is present in the form of isolated vanadyl species or oligomeric vanadates depending on the loading.345,346 The maximum catalytic activity was observed for catalysts with vanadia content of 3-5 mol% for which highly dispersed polyvanadate species are dominant. 

Figure 9.15 Comparison of selectivity to propene obtained by cyclic feed (redox mode) (open symbols) and co-feed (filled symbols), for supported vanadium oxide catalysts [76m]. V205-Al203 (0, ), V205-Ti02 (O, ), V205-Si02 ( , ), V205-Ti02/Al203 (A, A).

Cortez, G.G. Banares, M.A. A Raman Spectroscopy Study of Alumina-Supported Vanadium Oxide Catalyst During Propane Oxidative Dehydrogenation with Online Activity Measurement /. Catal. 2002, 209, 197-201. 

Dobler J, Pritzsche M, Sauer J. Oxidation of methanol to formaldehyde on supported vanadium oxide catalysts compared to gas phase molecules. J Am Chem Soc. 2005 127 (31) 10861-8. 

The structure-activity relationships characteristic of supported vanadium oxide catalysts have been evaluated for S02 oxidation to S03 by Wachs and coworkers (Dunn et al., 1998, Dunn et al., 1999). It was demonstrated for individual supported vanadium oxide catalysts that the oxygen of the bridging V-0-Msupport bond was involved in the ratedetermining step of S02 oxidation (Dunn et al., 1998). The specific reaction rate (TOF) was found to be the same for both isolated and polymeric surface vanadia species, and depended only on the oxide support changes in the support led to dramatic changes in the TOF (Dunn et al., 1998). 

Recent studies of supported vanadium oxide catalysts have revealed that the vanadium oxide component is present as a two-dimensional metal oxide overlayer on oxide supports (1). These surface vanadium oxide species are more selective than bulk, crystalline V2O5 for the partial oxidation of hydrocarbons (2). The molecular structures of the surface vanadium oxide species, however, have not been resolved (1,3,4). A characterization technique that has provided important information and insight into the molecular structures of surface metal oxide species is Raman spectroscopy (2,5). The molecular structures of metal oxides can be determined from Raman spectroscopy through the use of group theory, polarization data, and comparison of the... 

In the present investigation, the interaction ot vanadium oxide with dif-ferent alumina phases (7, 6-6, and a) is examined with Raman spectroscopy. Comparison ot the Raman spectra ot the supported vanadium oxide catalysts with those obtained -from vanadium oxide reference compounds allows for the structural assignment of these supported species. The present Raman data demonstrate that the molecular structures of the surface vanadium oxide phases are significantly influenced by the presence of surface impurities on the alumina supports and this overshadows the influence, if any, of the alumina substrate phase. 

The predominant commercial synthesis of MA is by vapor-phase oxidation of hydrocarbons, e.g. benzene, n-butane, or a C-4 hydrocarbon mixture, over a solid catalyst [67]. The oxidation of benzene over a supported vanadium oxide catalyst is the preferred procedure. In a typical process, the reactor gas containing low concentrations of MA is passed through a heat exchanger and... 

Resonance Raman Spectroscopy - 0-Al2O3-Supported Vanadium Oxide Catalysts as an Illustrative Example... 

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