Vanadium based oxides

Furthermore, both bulk and surface properties of the materials can play important roles. For instance the oxidation of alkanes over vanadium-based oxides is beUeved to proceed via a Mars-van Krevelen mechanism [8, 9] with lattice (bulk) oxygen being the active oxygen species. In metal oxide-based cracking catalysts, however, activity is directed by the surface acidity of the material. 

Cho, C. and Hun, S. (2002). Development of New Vanadium-Based Oxide Catalysts for Decomposition of Chlorinated Aromatic Pollutants, Environ. Sci. Technol, 36, pp. 1600-1606. 

Vanadium phosphoms oxide-based catalysts ate unstable in that they tend to lose phosphoms over time at reaction temperatures. Hot spots in fixed-bed reactors tend to accelerate this loss of phosphoms. This loss of phosphoms also produces a decrease in selectivity (70,136). Many steps have been taken, however, to aHeviate these problems and create an environment where the catalyst can operate at lower temperatures. For example, volatile organophosphoms compounds are fed to the reactor to mitigate the problem of phosphoms loss by the catalyst (137). The phosphoms feed also has the effect of controlling catalyst activity and thus improving catalyst selectivity in the reactor. The catalyst pack in the reactor may be stratified with an inert material (138,139). Stratification has the effect of reducing the extent of reaction pet unit volume and thus reducing the observed catalyst temperature (hot... 

Butane-Based Fixed-Bed Process Technology. Maleic anhydride is produced by reaction of butane with oxygen using the vanadium phosphoms oxide heterogeneous catalyst discussed earlier. The butane oxidation reaction to produce maleic anhydride is very exothermic. The main reaction by-products are carbon monoxide and carbon dioxide. Stoichiometries and heats of reaction for the three principal reactions are as follows ... 

A new process for the partial oxidation of n-butane to maleic anhydride was developed by DuPont. The important feature of this process is the use of a circulating fluidized bed-reactor. Solids flux in the rizer-reactor is high and the superficial gas velocities are also high, which encounters short residence times usually in seconds. The developed catalyst for this process is based on vanadium phosphorous oxides... 

Vanadium-based turnover rates for the oxidation of methanol matched those obtained previously using these same samples but a different reactor [12], The oxygen-based turnover rates agreed very well with those obtained by Oyama and Somorjai. 

A wide range of catalytic materials have been investigated for the selective catalytic reduction of NOx. For stationary emissions, NH3-SCR using vanadium-tungsten oxides supported on titania is the most used method however, when there is a simultaneous emission of NO and NOz (in tail gas from nitric acid plants), copper-based zeolites or analogous systems have been proven to be preferable [31b], In fact, there are two main reactions for NH3-SCR ... 

WClg, and vanadium-based initiators (Eq. 53), and the thermal polymerization of hexachlorocyclotriphosphazene (Eq. 54). (Ringopening polymerizations of ethylene and propylene oxides,... 

Dr. Israel Wachs of Lehigh University discusses molecular engineering on oxide catalysts. These materials are especially important for partial oxidation reactions, in which selectivity is difficult to control. This chapter focuses on vanadium-based catalysts, and the approach is applicable to other supported catalysts as well. 

Chronologically, the production of o-xylene from mixed Cg aromatics was the first of these separations. In 1945, the Oronite Chemical Co. produced 85 to 90% purity o-xylene by fractionation from crude xylenes (1). The c-xylene product is oxidized for the production of phthalic anhydride in a vapor phase reaction over a vanadium-base catalyst. By 1947 Oronite provided 5% of the United States production capacity for phthalic anhydride by this process (2). 

In the process based on n-butane feedstock, vanadium phosphorous oxides (V-P-O) catalysts are mainly used.1010-1012 Processes for the oxidation of low-cost C4 fraction from naphtha cracker consisting mainly of butenes have also been developed.1013,1014 In contrast with benzene oxidation where two carbon atoms are lost in the form of ethylene no carbon is lost in the oxidation of C4 hydrocarbons ... 

Finally, it may be noted that vanadium-based catalysts can oxidize butenes to acetic acid. Kaneko et al. [168,169] report a selectivity of 50% at a relatively high conversion of 67%. V2Os is not active but combinations with Sn, W, Ti are, especially at temperatures in the range 200— 300° C. In accordance with the hypothesis of Ai about the effect of the catalyst acidity, a deep oxidation is favoured by the highest acidity. Hence Sn—V—O catalysts with 50% V, which display the maximum acidity per unit of weight, are the most suitable. 

In contrast to the aforementioned binary oxides, V2Os has a stronger oxidation power and is able to attack hydrogen attached to the aromatic nucleus. Sometimes attention is drawn to the importance of a layer structure in the catalyst or to geometric factors (e.g. Sachtler [270]). Unexpectedly, however, very effective vanadium-based catalysts exist which operate in the molten state, indicating that a fixed structure is not important. The catalytic activity of molten oxide phases seems to occur exclusively in the oxidation of aromatic hydrocarbons over V2Os-based catalysts, such systems have not been reported for the selective oxidation of olefins. 

Gaason and co-workers 0 have described a process for the conversion of 2,4,4 tr iniothv 1 - 1-pcntene and 2,4,4-trimothyl -2-pen tone into epoxides with air at 130-140° and 200 lb.y in. over a catalyst of cobalt, manganese, lead or iron oaphthenate in base, or of vanadium pent-oxide (Eqs. J04 and 105). Some uncertainty is coot on this process, however, by (be authors own admission that comparable yields of epoxides were obtainable without catalysts. 

Diacetyl (DA) is used as a flavour enhancer in the food industry and is currently manufactured from methyl ethyl ketone (MEK) in homogeneous systems via an oxime intermediate (ref.1). In principle, DA can also be manufactured by the selective oxidation of MEK and several reports have appeared in the literature which apply heterogeneous catalysts to this task (refs. 2-4). A number of reports have specified the importance of basic or weakly acidic sites on the catalyst surface for a selectively catalysed reaction and high selectivities to DA at moderate conversions of MEK have been reported for catalysts based on C03O4 as a pure oxide and with basic oxides added conversely scission reactions have been associated with acidic oxide additives (refs. 2-4). Other approaches to this problem have included the application of vanadium phosphorus oxide (VPO) catalysts. Ai (ref. 5) has shown that these catalysts also catalyse the selective oxidation of MEK to DA. Indeed this catalyst system, used commercially for the selective oxidation of n-butane to maleic anhydride (ref.6), possesses many of the desired functionalities for DA formation from MEK, namely the ability to selectively activate methylene C-H bonds without excessive C-C bond scission. 

Developing technologies in vanadium science provide the basis for the last two chapters of this book. Vanadium(V) in various forms of polymeric vanadium pen-toxide is showing great promise in nanomaterial research. This area of research is in its infancy, but already potential applications have been identified. Vanadium-based redox batteries have been developed and are finding their way into both large-and small-scale applications. Lithium/silver vanadium oxide batteries for implantable devices have important medical applications. 

Studies have also focused on vanadium-based asymmetric catalysts in addition to these photocatalytic systems. A catalytic achiral version of an oxidative coupling reaction was published in 1999 by Uang and co-workers (see Section 14.4.2) [70]. They developed an air-stable complex (VO(acac)2) that can be used in catalytic quantities in the presence of dioxygen as a re-oxidant. The promising results obtained led to an investigation of chiral versions of this reagent, and the initial reports document that such a reaction was possible with complexes... 

Fig. 8. Amino acid-based vanadium(IV) oxidative coupling pre-catalysts.

Very striking results on the interactions of molecules with a catalyst have been recently reported in zeolite catalysis because of the well ordered structure of these materials it is worth mentioning the subjects of zeolite design [10] and of acidic properties of metallosilicates [11]. In other areas where polycrystallinic or even amorphous materials arc applied, highly interesting results are now numerously emerging (such as hydrocarbon oxidation on vanadium-based catalysts [12] location of transition metal cations on Si(100) [13] CO molecules on MgO surfaces [14] CH4 and O2 interaction with sodium- and zinc-doped CaO surfaces [15] CO and NO on heavy metal surfaces [16]). An illustration of the computerized visualization of molecular dynamics of Pd clusters on MgO(lOO) and on a three-dimensional trajectory of Ar in Na mordenitc, is the recent publication of Miura et al. [17]. 

The few vanadium-based studies present in the literature are summarized briefly here and, as with the manganese systems, they focus on the identification of key Intermediates in vanadium-catalyzed oxidations. These cases readily Illustrate the importance of considering both the positive and negative ion modes of ESl-MS when probing reaction mixtures. 

---