Alkanes vanadium oxides

In 1992, Hari Prasad Rao and Ramaswamy reported on the oxyfunctionalization of alkanes with H2O2 using a vanadium silicate molecular sieve s . With this catalyst acyclic and cyclic alkanes were oxidized to a mixture of the corresponding alcohols (primary and secondary ones), aldehydes and ketones. Unfortunately, most of the early attempts were of rather limited success due to low turnover frequencies and radical producing side reactions as observed, for example, by Mansuy and coworkers in 1988. ... 

This paper summarized our current understanding of the factors that determine selectivity for dehydrogenation versus formation of oxygen-containing products in the oxidation of light alkanes. From the patterns of product distribution in the oxidation of C2 to C6 alkanes obtained with supported vanadium oxide, orthovanadates of cations of different reduction potentials, and vanadates of different bonding units of VO in the active sites, it was shown that the selectivities can be explained by the probability of the surface alkyl species (or the... 

Oxidative Dehydrogenation. Specific reviews deal with oxidative dehydro-genation of lower alkanes, particularly over vanadium oxide-based cata-... 

The catalysts based on vanadium oxide are one of the better studied systems. A V-Mg oxide in which Mg orthovanadate (Mg2(V04)2) and MgO were the only identifiable phases was a rather selective catalyst (27). Since MgO was relatively inactive in alkane activation, Mg orthovanadate was assumed to be the active component. Indeed, Mg orthovanadate prepared as a stoichiometric compound showed high selectivities for the oxidative dehydrogenation of propane (29). In this latter study, it was shown interestingly that Mg orthovanadate was the only alkali or alkali earth orthovanadate that... 

In summary, catalytic C-H transformations in small unfunctionalized alkanes is a technically very important family of reactions and processes leading to small olefins or to aromatic compounds. The prototypical catalysts are chromia on alumina or vanadium oxides on basic oxide supports and platinum on alumina. Reaction conditions are harsh with a typical minimum temperature of 673 K at atmospheric pressure and often the presence of excess steam. A consistent view of the reaction pathway in the literature is the assumption that the first C-H abstraction should be the most difficult reaction step. It is noted that other than intuitive plausibility there is little direct evidence in heterogeneous reactions that this assumption is correct. From the fact that many of these reactions are highly selective toward aromatic compounds or olefins it must be concluded that later events in the sequence of elementary steps are possibly more likely candidates for the rate-determining step that controls the overall selectivity. A detailed description of the individual reactions of C2-C4 alkanes can be found in a comprehensive review [59]. 

Air oxidation of n-butane to maleic anhydride is possible over vanadium phosphate and, remarkably, a 60% selectivity is obtained at 85% conversion. In the gas phase oxidation, in contrast to the situation found in the liquid, n-alkanes are oxidized more rapidly than branched chain alkanes. This is because secondary radicals are more readily able to sustain a chain for branched alkanes the relatively stable tertiary radical is preferentially formed but fails to continue the chain process. Vanadium(V)/ manganese(II)/AcOH has been used as a catalyst for the autoxidation of cyclohexane to adipic acid, giving 25-30% yields after only 4 h. ... 

Argyle, M.D., Chen, K.D., Iglesia, E. and Bell, A.T. (2005) In situ UV-visible spectroscopic measurements of kinetic parameters and active sites for catalytic oxidation of alkanes on vanadium oxides. Journal of Physical Chemistry B, 109 (6), 2414-20. 

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 ... 

Because oxidations with oxygen are free-radical reactions, free radicals should be good initiators. Indeed, in the presence of hydrogen bromide at high enough temperatures, lower molecular weight alkanes are oxidized to alcohols, ketones, or acids [5 7]. Much more practical are oxidations catalyzed by transition metals, such as platinum [5, 6, 55, 56], or, more often, metal oxides and salts, especially salts soluble in organic solvents (acetates, acetylacetonates, etc.). The favored catalysts are vanadium pent-oxide [3] and chlorides or acetates of copper [2, 57, 58, 59, 60, 61, 62, 63, 64, 65, 66], iron [67], cobalt [68, 69], palladium [60, 70], rhodium [10], iridium [10], and platinum [5, 6, 56, 57]. 

It is well established [1-3] that vanadyl pyrophosphate (VO)2P207 is an essential component of the most selective VPO catalysts. For example, structural and chemical characterization studies of "reactor equilibrated" VPO catalysts indicate that the predominate crystalline phase is vanadyl pyrophosphate (VO)2P207 [1-3], that the bulk P/V ratio is close to 1.0, and that the average vanadium oxidation state is close to -1-4.0 [3-5]. A number of studies [2,5] have indicated that alkane oxidation primarily involves oxygen adspecies adsorbed at vanadium surface sites, and relatively little bulk lattice oxygen. 

Supported metal oxides are currently being used in a large number of industrial applications. The oxidation of alkanes is a very interesting field, however, only until recently very little attention has been paid to the oxidation of ethane, the second most abundant paraffin (1). The production of ethylene or acetaldehyde from this feed stock is a challenging option. Vanadium oxide is an important element in the formulation of catalysts for selective cataljdic reactions (e. g. oxidation of o-xylene, 1-3, butadiene, methanol, CO, ammoxidation of hydrocarbons, selective catalytic reduction of NO and the partial oxidation of methane) (2-4). Many of the reactions involving vanadium oxide focus on the selective oxidation of hydrocarbons, and some studies have also examined the oxidation of ethane over vanadium oxide based catalysts (5-7) or reviewed the activity of vanadium oxide for the oxidation of lower alkanes (1). Our work focuses on determining the relevance of the specific oxide support and of the surface vanadia coverage on the nature and activity of the supported vanadia species for the oxidation of ethane. 

Supported vanadium oxides have been proposed as selective catalysts in partial oxidation reactions [1] and more specifically in the oxidative dehydrogenation (ODH) of short chain alkanes [2, 3]. However, it has been observed that the catalytic behavior of these catalysts during the oxidation of alkanes depends on the vanadium loading and the acid-base character of metal oxide support. In this way, alumina-supported vanadia catalysts with low V-loading are highly active and selective during the ODH of ethane [4-7] and propane [8] but they show a low selectivity in the ODH of n-butane [4, 5, 9, 10]. 

Mamedov, E. and Cortes Corberan, V. (1995). Oxidative Dehydrogenation of Lower Alkanes on Vanadium Oxide-based Catalysts. The Present State of the Art and Outlooks, Appl. Catal. A Gen., 127, pp. 1 10. 

For the oxidative dehydrogenation of alkanes, vanadium supported on metal oxides or incorporated in molecular sieves have been the most studied catalyst... 

One of the key aspects in the catal5dic behavior for alkane partial-oxidation catalysts seems to be related to the coordination number (Fig. 24.3), the aggregation degree, and the oxidation state of the vanadium species as active sites. It fact, it has been shown that the V-environment plays an important role in catalytic behavior. 

Blasco, T. and Lopez Nieto, J. (1997). Oxidative Dehydrogenation of Short Chain Alkanes on Supported Vanadium Oxide Catalysts, Appl Catal A Gen., 157, pp. 117-142. 

Solsona, B., Blasco, T., Lopez Nieto, J., et al. (2001). Vanadium Oxide Supported on Mesoporous MCM-41 as Selective Catalysts in the Oxidative Dehydrogenation of Alkanes, J. Catal, 203, pp. 443-452. 

Solsona B, Blasco T, Nieto JML, Pena ML, Rey F, Vidal-Moya A (2001) Vanadium oxide supported on mesoporous MCM41 as selective catalysts in the oxidative dehydrogenation of alkanes. J Catal 203 443 52... 

Blasco T, Nieto JML (1997) Oxidative dehydrogenation of short chain alkanes on supported vanadium oxide catalysts. Appl Catal Gen 157 117-142... 

The oxidation of CH3OH to HCHO is considered as a probe reaction for other selective oxidation reactions such as butane to maleic anhydride, o-xylene to phthalic anhydride, and ODH of alkanes to alkenes. Consequently, the concepts developed for the selective oxidation of methanol over vanadium oxide catalysts can be easily transferred to other catalytic reactions. Weckhuysen and Keller [82] carried out methanol oxidation as a probe reaction over various V2O5/S oxides (S = HO2, Zr02, Nb205, Ce02, and AI2O3). The relative independence of turnover frequency (TOP) to vanadia loading on amorphous oxide supports indicated that the reaction was first order with respect to surface vanadium oxide site. 

Mamedov, E.A. and Cortes Corberan, V. Oxidative dehydrogenation of lower alkanes on vanadium oxide-based cattalysts the present state of art and outlooks. AppL Catal A Gen. 1995, 127, 1-40. 

Argyle, M. D., Chen, K., Resini, C., Krebs, C., Bell, A. T., and Iglesia, E. Extent of reduction of vanadium oxides during catalytic oxidation of alkanes measured by in-situ UV-visible spectroscopy. J. Phys. Chem. B 108, 2345-2353 (2004). 

Solsona, B., Blasco, T, L6pez Nieto, J. M., Pena, M. L., Rey, R, and Vidal-Moya, A. Vanadium oxide supported on mesoporous MCM-41 as selective catalysts in the oxidative dehydrogenation of alkanes. J. Catal. 203,443 52 (2001). Martinez-Huerta, M. V, Fierro, J. L. G., and Banares, M. A. Monitoring the states of vanadium oxide during the transformation of TiO anatase-to-rutile under reactive environments reduction and oxidative dehydrogenation of ethane. Catal. Commun. 

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