Vanadium complex, oxide-supported

Early catalysts for acrolein synthesis were based on cuprous oxide and other heavy metal oxides deposited on inert siHca or alumina supports (39). Later, catalysts more selective for the oxidation of propylene to acrolein and acrolein to acryHc acid were prepared from bismuth, cobalt, kon, nickel, tin salts, and molybdic, molybdic phosphoric, and molybdic siHcic acids. Preferred second-stage catalysts generally are complex oxides containing molybdenum and vanadium. Other components, such as tungsten, copper, tellurium, and arsenic oxides, have been incorporated to increase low temperature activity and productivity (39,45,46). 

Benzene-Based Catalyst Technology. The catalyst used for the conversion of ben2ene to maleic anhydride consists of supported vanadium oxide [11099-11-9]. The support is an inert oxide such as kieselguhr, alumina [1344-28-17, or sUica, and is of low surface area (142). Supports with higher surface area adversely affect conversion of benzene to maleic anhydride. The conversion of benzene to maleic anhydride is a less complex oxidation than the conversion of butane, so higher catalyst selectivities are obtained. The vanadium oxide on the surface of the support is often modified with molybdenum oxides. There is approximately 70% vanadium oxide and 30% molybdenum oxide [11098-99-0] in the active phase for these fixed-bed catalysts (143). The molybdenum oxide is thought to form either a soUd solution or compound oxide with the vanadium oxide and result in a more active catalyst (142). 

Components of fluidized cracking catalysts (FCC), such as an aluminosilicate gel and a rare-earth (RE) exchanged zeolite Y, have been contaminated with vanadyl naphthenate and the V thus deposited passivated with organotin complexes. Luminescence, electron paramagnetic resonance (EPR) and Mossbauer spectroscopy have been used to monitor V-support interactions. Luminescence results have indicated that the naphthenate decomposes during calcination in air with generation of (V 0)+i ions. After steam-aging, V Og and REVO- formation occurred. In the presence of Sn, Tormation Of vanadium-tin oxide species enhance the zeolite stability in the presence of V-contaminants. 

However, there are one or two instances where enough is known about catalysts of these types to justify some attention at this point. Some of the most thoroughly investigated cases constitute the group of catalysts used in the ring-closing reactions which lead to the production of aromatic hydrocarbons from paraffins or olefins. These consist of oxides of vanadium, chromium, and molybdenum, or of complex and supported catalysts containing one of these oxides. 

The first supported molten salt catalyst systems date from 1914, where BASF filed a patent on a silica-supported V20s-alkali pyrosul te sulfur dioxide oxidation catalyst [48], which even today - as a slightly modified catalyst system - is still the preferred catalyst for sulfuric acid production [49]. However, it took many years to realize in the 1940s [50,51], that the catalyst system actually was a molten salt SLP-type system which is best described by a mixture of vanadium alkah sulfate/hydrogensulfate/pyrosulfate complexes at reaction conditions in the temperature range 400-600 °C with the vanadium complexes playing a key role in the catalytic reaction [49]. 

V2O5 Oxides. - Supported V2O5 oxides are extremely important industrial catalysts for environmental pollution control, and are used in catalytic scrubbers for SO2 oxidation and NO reduction. During the operation of the catalyst (usually at 400-600 °C) in the SO2 oxidation reactions, pyrosulfate melts are formed in the pores of the catalysts, and V2O5 can dissolve in these melts forming vanadium oxo-sulfate complexes ... 

Van der Voort et al. [261-263] prepared vanadium oxide species in the meso-porous material MCM-48 by reacting the support with gaseous vanadyl acetyla-cetonate [VO(acac)2]. The vapor deposition was carried out in a vacuum reactor (see Fig. 9). VO(acac)2 is sublimed and reacts with the heated substrate at 150°C until a saturation loading is achieved. This takes approximately 16 h, visible by the formation of crystals of the complex on colder parts of the reactor [261]. Subsequently, the sample is purged with dry nitrogen at reaction temperature and calcined in ambient air at 500 °C. The uncalcined zeolite-supported vanadium complex and the calcined catalyst were characterized by X-ray diffraction, nitrogen absorption, IR and UV-Vis spectroscopy. 

However, oxidation with H2O2 in acetone resulted in a high diol selectivity with an equilibrium mixture of the cis- and trans-diols, illustrating the role of the residual acidity of the support The reaction is suggested to occur via heterolyhc cleavage of the vanadium peroxo species. Less than 0.5% leaching of the bipy complex was observed over 50 h of operation. 

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. 

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. 

Oxidation, Ammoxidation, and Oxychlorination Numerous catalysts have been developed for a number of processes in this category. Examples arc supported vanadium oxide, complex muitimetallic oxides, and supported cupric chloride, used respectively for the following reactions ... 

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