Vanadium ammoxidation

Catalysts. In industrial practice the composition of catalysts are usuaUy very complex. Tellurium is used in catalysts as a promoter or stmctural component (84). The catalysts are used to promote such diverse reactions as oxidation, ammoxidation, hydrogenation, dehydrogenation, halogenation, dehalogenation, and phenol condensation (85—87). Tellurium is added as a passivation promoter to nickel, iron, and vanadium catalysts. A cerium teUurium molybdate catalyst has successfliUy been used in a commercial operation for the ammoxidation of propylene to acrylonitrile (88). 

Mixed Metal Oxides and Propylene Ammoxidation. The best catalysts for partial oxidation are metal oxides, usually mixed metal oxides. For example, phosphoms—vanadium oxides are used commercially for oxidation of / -butane to give maleic anhydride, and oxides of bismuth and molybdenum with other components are used commercially for oxidation of propylene to give acrolein or acrylonitrile. 

SNAM (2) An ammoxidation process for converting propylene to acrylonitrile. The catalyst is based on molybdenum/vanadium or bismuth, operated in a fluidized bed. Operated in Europe in 1968. 

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... 

V-containing silicalite, for example, has been shown to have different catalytic properties than vanadium supported on silica in the conversion of methanol to hydrocarbons, NOx reduction with ammonia and ammoxidation of substituted aromatics, butadiene oxidation to furan and propane ammoxidation to acrylonitrile (7 and references therein). However, limited information is available about the characteristics of vanadium species in V-containing silicalite samples and especially regarding correlations with the catalytic behavior (7- 6). 

Recently, amorphous high surface area vanadium aluminium oxynitrides have been reported as active catalysts for propane ammoxidation to yield acrylonitrile (AC) at atmospheric pressure. Optimal performance was achieved at 500°C using a C3Hg 02 NH3 molar ratio of 1.25 3 1 (see Tables 4 and 5). The space time yields of these catalysts have been reported to be much higher than for other catalysts reported in the literature. 

The Kinetics of Ammoxidation of Xylenes over Vanadium Catalysts... 

The kinetics of the ammoxidation of xylenes over a vanadium catalyst and mixed vanadium catalysts were studied. The reaction rate data obtained were correlated with the parallel consecutive reaction scheme by the rate equations based upon the Langmuir-Hinshelwood mechanism where the adsorption of xylenes was strong. The reaction rates of each path are remarkably affected by the kind of xylene and catalyst. The results of the physical measurement of catalysts indicated that the activity and the selectivity of reaction were affected by the nature and the distribution of metal ions and oxygen ion on catalyst surface. 

Recently, the kinetics of the ammoxidation of m-xylene (1) and xylene isomers (2) over vanadium catalyst, and of m-xylene over mixed vanadium catalysts (3) were reported. This paper summarizes the results concerning the specific rate constants for each reaction path obtained in the above studies and adds some data on physical properties of each catalyst. 

The fresh catalysts used for the ammoxidation of xylenes consisted essentially of V205, SbV04, and %-phase for a vanadium catalyst respectively. X-ray diffraction patterns for the catalysts showed that a vanadium catalyst consisted essentially of V2O4, while mixed catalysts retained the original composition even after prolonged use. 

On the other hand, a catalyst in which the CrV04 was one of major constituents had little catalytic activity for the ammoxidation of xylene. These observations indicate that the nature and the distribution of metal ions and oxygen ion on the catalyst surface affect the catalytic activity and selectivity. It is difficult to predict the relationship between the adsorptivity of reactants and the physical properties of catalyst, but it may be assumed that adding more electronegative metal ions affects the electronic properties of the vanadium ion, which functions as an adsorption center. Further details on the physical properties of catalysts for the ammoxidation of xylenes will be reported later. 

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 ... 

The multifunctionality is achieved through either the combination of two different compounds (phase-cooperation) or the presence of different elements inside a single crystalline structure. In antimonates-based systems, cooperation between the metal antimonate (having a rutile crystalline structure), employed for propane oxidative dehydrogenation and propene activation, and the dispersed antimony oxide, active in allylic ammoxidation, is made more efficient through the dispersion of the latter compound over the former. In metal molybdates, one single crystalline structure contains both the element active in the oxidative dehydrogenation of the hydrocarbon (vanadium) and those active in the transformation of the olefin and in the allylic insertion of the N H2 species (tellurium and molybdenum). 

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. 

Vanadium phosphate materials have found use as catalysts for a number of reactions beyond the widely practiced partial oxidation of butane. These applications are mainly in selective oxidation 3,73,87,90-94, 96-102,154,195,208,237,251-269), ammoxidation (88-90,270), dehydrogenation (232,271-276), and dehydration (277,278). 

Supporting the vanadium phosphate material on silica 286,293,296-298) or titania 285) has proved beneficial, giving catalysts that provide increased MA yields (20M0%) 297). This observation is in contrast to the results of a number of investigations that indicated the reverse 299,300). Investigations of the ammoxidation of propane to acrylonitrile showed that competitive adsorption of NH3 and O2 could direct the selectivity 88). 

Vanadium phosphorus oxides (VPO) are good catalysts for selective O- and N-insertion reactions of aliphatics and methylaromatics, in particular for the oxidation of -butane to maleic anhydride (MA) and for the ammoxidation of methylaromatics and heteroaromatics to the corresponding benzonitriles (47,48). VOHPO4 O.5H2O is a well-defined crystalline compound used as a precursor of... 

Reddy, B.N., Reddy, B.M. and Subrahmanyam, M. (1991) Dispersion and 3-picoline ammoxidation investigation of vanadium oxide/ aluminum oxide (V205/a-A1203) catalysts. Journal of the Chemical Society, Faraday Transactions, 87 (10), 1649-55. 

Vanadium phosphates have been applied to a number of selective oxidation and ammoxidation reactions, although the partial oxidation of n-butane to maleic anhydride remains the most widely studied reaction for these catalysts. Even though the first patent for this reaction was filed over 40 years ago, there are stQl a large number of papers published on this system every year. 

Aromatic imides are another type of product which can be synthesized by catalytic ammoxidation. o-Xylene is converted over vanadium-titanium oxide catalysts to tolunitrile and then, depending on catalyst composition and reaction conditions, phthalimide or phthalonitrile can be selectively synthesized (Scheme 20.3) [94]. 

In the case of n-butane the yield was lower owing to the lower hydrocarbon conversion, but the selectivity remained close to 50% (46%). In this case, vanadium played the additional role of oxydehydrogenation of butane to butenes. The reactivity of butene was lower than that of butadiene (both were higher than that of n-butane) which indicates that the mechanism requires the oxy-dehydrogenation of butene to yield butadiene, which is the reactive intermediate that undergoes ammoxidation. 

Brazdil, J.F. Cavalcanti, A.P. Padolewski, J.P. Method for Preparing Vanadium Antimony Oxide Based Oxidation and Ammoxidation Catalysts US Patent 5,693,587, Dec 2, 1997 [assigned to The Standard Oil Company of Ohio (Sohio/BP)]. 

Characterization of Vanadium Oxide Catalysts in Relation to Activities and Selectivities for Oxidation and Ammoxidation of Alkylpyridines... 

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