Hydrocarbons catalysts, vanadium oxide

Clerici, M.G. and Kholdeeva, O.A. (eds) (2013) Liquid Phase Oxidation via Heterogeneous Catalysis Organic Synthesis and Industrial Applications, John Wiley Sons, Inc., Hoboken. Bruckner, A. and Baems, M. (1997) Selective gas-phase oxidation of polycyclic aromatic hydrocarbons on vanadium oxide-based catalysts. Appt Catal. A- Gen., 157 (1-2), 311-334. Corma, A., Esteve, P., and Martinez, A. (1996) Solvent effects during the oxidation of olefins and alcohols with hydrogen peroxide on Ti-beta catalyst the influence of the hydrophilicity-hydrophobicity of the zeolite. /. Catal, 161 (1), 11-19. 

In this paper selectivity in partial oxidation reactions is related to the manner in which hydrocarbon intermediates (R) are bound to surface metal centers on oxides. When the bonding is through oxygen atoms (M-O-R) selective oxidation products are favored, and when the bonding is directly between metal and hydrocarbon (M-R), total oxidation is preferred. Results are presented for two redox systems ethane oxidation on supported vanadium oxide and propylene oxidation on supported molybdenum oxide. The catalysts and adsorbates are stuped by laser Raman spectroscopy, reaction kinetics, and temperature-programmed reaction. Thermochemical calculations confirm that the M-R intermediates are more stable than the M-O-R intermediates. The longer surface residence time of the M-R complexes, coupled to their lack of ready decomposition pathways, is responsible for their total oxidation. 

Supported vanadium oxides represent one of the technologically most important class of solid catalysts. These catalysts are useful for partial oxidation of various hydrocarbons 0), ammoxidation of alkyl substituted N-heteroaromatic compounds (2) and most recently for NO reduction (3) For a catalyst to be a successful one in industry, it should exhibit high activity with maximum selectivity, thermal and mechanical stability and long life etc. For getting some of these functionalities, the active component has to be dispersed uniformly on a support material. 

Vanadium oxide dispersed on supporting oxides (Si02 Al Oo, Ti02, etc.) are frequently employed as catalysts in reactions like partial oxidation and ammoxidation of hydrocarbons, and NO reduction. The modifications induced on the reactive properties of transition metal oxides like V20 when they are supported on an oxide carrier has been the subject matter of recent study. There is much evidence showing that the properties of a thin layer of a transition metal oxide interacting with the support are strongly modified as compared to the properties of the bulk oxide (1-3). In the recent past, increasing attention has been focussed... 

Ammoxidation of Aromatic Hydrocarbons. - The ammoxidation of toluene with V2O5, both pure and supported on A1203, was studied by Murakami et 0/.27,46,97,98 They conclude that the catalyst is bifunctional toluene is adsorbed oxidatively on V2O5, the oxidized product is stabilized as a benzoate ion on the alumina carrier and subsequently reacts with ammonia giving benzonitrile. It was observed that the oxidation state of the vanadium oxide was close to V204 and that benzaldehyde is probably the product formed in the initial step. 

Fig. 4 shows the TPR profiles of the fresh and spent catalyst. Curve C shows the desorption of hydrocarbons during reduction of the spent catalyst, formed by reduction of carbonaceous deposits on the catalyst surface. The hydrogen consumption profiles of the catalyst (see Curve A and B) show the two peaks, characteristic of palladium sulfate-based catalysts, with a vanadium oxide reduction peak at approximately 400 K and a sulfate reduction peak at 600 K [11,13,16]. The peak position of the sulfate reduction peak is comparable for both catalysts. For the spent catalyst, however, an additional small hydrogen consumption is observed at 700 K, which coincides with the large peak in the FID signal,... 

The oxidation of propane and of propene to acrylic acid has also been investigated 87,106,256-261). Vanadium phosphate catalysts that show good performance for the oxidation of C3 hydrocarbons and vanadium phosphate catalysts that are active and selective for C4 and C5 hydrocarbon oxidation have several differences in their structure and operating conditions. 

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... 

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... 

The V-Ti-0 system has been extensively studied in connection with catalytic oxidation and ammoxidation reactions of aromatic hydrocarbons. Two principally different types of catalysts can be distinguished. One type of catalyst is prepared by impregnation, precipitation or mixing of the vanadium and titanium phases followed by calcination in air below the melting point of V. (1-4). The simultaneous reduction of V 0- and transformation of iiO (anatase) into rutile when heating below the V O melting point has been demonstrated to be due to topotactic reactions ( ). The formation of lower vanadium oxides can be of importance, because it has been found that reduced phases determine the activity and selectivity of catalysts (6,7). 

The nature of supported oxides and of the support plays a critical role in the partial oxidation of hydrocarbons since the support is not only providing a high surface area, but also dispersing the oxide. The interaction between the metal oxide overlayer and the imderlying support similarly determines the performance of the catalyst, which may also be affected by the exposed sites of the support. To fully understand these effects, a series of supported vanadium oxide catalysts at monolayer and submonolayer coverage have been prepared. The monolayer coverage was determined hy Raman spectroscopy and X -ray photoelectron spectroscopy. The activity of the supported vanadium oxide catalysts is determined by the specific support and surface vanadia coverage. 

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. 

Vanadium phosphates (VPO) of different structure are suitable precursors of veiy active and selective catalysts for the oxidation of C4-hydrocarbons to maleic anhydride [e.g. 4] as well as for the above mentioned reaction [5,6]. Normally, VOHPO4 Va H2O is transformed into (V0)2P207 applied as the n-butane oxidation catalyst. Otherwise, if VOHPO4 V2 H2O is heated in the presence of ammonia, air and water vapour a-(NH4)2(V0)3(P207)2 as XRD-detectable phase is formed [7], which is isostructural to a-K2(V0)3(P207)2. Caused by the stoichiometry of the transformation reaction (V/P = 1 V/P = 0.75) (Eq. 2) and the determination of the vanadium oxidation state of the transformation product ( 4.11 [7]) a second, mixed-valent (V 7v ) vanadium-rich phase must be formed. 

Previously we have studied such catalysts in hydrocarbon dehydrogenation and oxidative dehydrogenation reactions [6,7]. Instrumental methods such as XRD, X-ray, photoelectron spectroscopy, DTA, UV-spectroscopy, EM were used. It has been found that activity of the Zn-Cr catalysts is determined by the stoichiometric spinel ZnCr204 [8]. In the case of the vanadium-magnesium system the activity and selectivity depend upon the presence of ions V and V grouped on the catalyst surface into clusters of 2-3 vanadium ions [9]. This was taken as a principal for the purposeful synthesis of the catalytic systems mentioned. In this work an attempt was made to spread the obtained experience on the dehydrogenation of alcohol groups. 

In order to overcome certain difficulties such as the dissipation of heat and the use of inflammable mixtures, certain liquid phase processes have been proposed for the oxidation of aromatic hydrocarbons and compounds. In such a process 100 the aromatic hydrocarbons or their halogenated derivatives are treated with air or gas containing free molecular oxygen in the liquid phase at temperatures above ISO0 C. and under pressure in the presence of a substantial quantity of liquid water. A small quantity of such oxidation catalysts as oxides or hydroxides of copper, nickel, cobalt, iron or oxides of manganese, cerium, osmium, uranium, vanadium, chromium and zinc is used. The formation of benzaldehyde from toluene is claimed for the process. 

While the overall reaction of anthracene oxidation to form anthra-quinone as shown above involves the interaction of three atoms of oxygen per molecule of hydrocarbon, the actual mechanism of the catalysis is more or less obscure. From the observations of Senseman and Nelson86 the vanadium oxide catalysts function by being alternately reduced to a lower oxide by the hydrocarbon and oxidized to the pentoxide by the oxygen of the air used. Thus ... 

It may be seen from comparison of results on ethylene oxidation over silver and vanadium pentoxide that with both catalysts the oxidation of unsaturated hydrocarbons will proceed by the same mechanism. C02 generation is not accelerated in the presence of aldehydes and these cannot be intermediates in ethylene combustion. When aldehydes are introduced into the reactant mixture, the ratio of ethylene oxide to C02 formation rates undergoes a change, due to strong adsorption of aldehydes on the catalyst surface. Ethylene oxide will form on silver and is in fact absent on vanadium oxides. It was shown experimentally that the absence of acetaldehyde and formaldehyde in the products of oxidation over silver, and the low absolute content of these substances for vanadium oxides is due to the fact that they are not formed at all, or formed at a low rate, and not to their oxidation or decomposition. 

Interactions with catalytic metals by the chlorinated hydrocarbon oxidation system have been widely reported. Agazada et al. ° report deactivation of vanadium catalysts in the oxidation of chlorine-containing unsaturated hydrocarbons. Electron spin resonance, ESR, studies indicate a reduction of ions during deactivation. Jin and Abrahaml V report substantial deactivation of a vanadium catalyst during oxidation of dichlorobenzene. MichalowiczlO reports both deactivation and a loss of vanadium from the catalyst during deactivation. 

Since vanadium oxide had been used as an effective catalyst for the dehydrogenation of hydrocarbons, it was expected from purely thermodynamic considerations that conditions could be found for the reverse reaction of hydrogenation to take place. Experiments carried out in our laboratory with coprecipitated vanadia-alumina catalyst showed this to be true. 

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