Ammoxidation of propylene to acrylonitrile

Oxidation Catalysis. The multiple oxidation states available in molybdenum oxide species make these exceUent catalysts in oxidation reactions. The oxidation of methanol (qv) to formaldehyde (qv) is generally carried out commercially on mixed ferric molybdate—molybdenum trioxide catalysts. The oxidation of propylene (qv) to acrolein (77) and the ammoxidation of propylene to acrylonitrile (qv) (78) are each carried out over bismuth—molybdenum oxide catalyst systems. The latter (Sohio) process produces in excess of 3.6 x 10 t/yr of acrylonitrile, which finds use in the production of fibers (qv), elastomers (qv), and water-soluble polymers. 

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). 

Ammoxidation of isobutylene to produce methacrylonitrile is a similar reaction to ammoxidation of propylene to acrylonitrile. However, the yield is low. 

Fig. 2. Mechanism of selective ammoxidation of propylene to acrylonitrile over bismuth molybdate catalyst by Burrington et at. (19).

Figure 12 Targeting curve for ammoxidation of propylene to acrylonitrile.

This article therefore seeks to examine in depth just one mixed oxide catalyst, tin-antimony oxide, which has been commercially developed (2-5) for the oxidation of propylene to acrolein as well as for the ammoxidation of propylene to acrylonitrile and the oxidative dehydrogenation of butenes to 1,3-butadiene. A recent book (6) and a subsequent review (7) have shown how little unanimity has been established about the fundamental properties of the material. In particular there seems to be much confusion as to the phase composition, the nature of the cationic oxidation states, the chemical environment of the cations, the charge compensation mechanism, the nature of the active sites, the distortion of the host tin(IV) oxide lattice by the dopant antimony atoms and whether any changes in the catalyst result from the adsorption and catalytic processes. 

Our recent work on the bismuth-cerium molybdate catalyst system has shown that it can serve as a tractable model for the study of the solid state mechanism of selective olefin oxidation by multicomponent molybdate catalysts. Although compositionally and structurally quite simple compared to other multiphase molybdate catalyst systems, bismuth-cerium molybdate catalysts are extremely effective for the selective ammoxidation of propylene to acrylonitrile (16). In particular, we have found that the addition of cerium to bismuth molybdate significantly enhances its catalytic activity for the selective ammoxidation of propylene to acrylonitrile. Maximum catalytic activity was observed for specific compositions in the single phase and two phase regions of the phase diagram (17). These characteristics of this catalyst system afford the opportunity to understand the physical basis for synergies in multiphase catalysts. In addition to this previously published work, we also include some of our most recent results on the bismuth-cerium molybdate system. As such, the present account represents a summary of our interpretations of the data on this system. 

In accordance with a basic mechanism, the dehydrogenation occurs by abstraction of P-hydrogen to form a jt-adsorbed complex with a scheme of reaction similar to that reported for the ammoxidation of propylene to acrylonitrile [45] ... 

Fluidized beds aie used in both catalytic and noncatalytic systems. Typical examples of catalytic uses aie hydrocarbon cracking and reforming, oxidation of naphthalene to phthalic anhydride, and ammoxidation of propylene to acrylonitrile. Examples of noncatalytic uses are roasting of sulfide ores, coking of petroleum residues, calcination of ores, incineration of sewage sludge, and drying (Peiry and Gi een, 1999). 

The reason for the difference in the effectiveness between each of the crystal face to catalyze the ammoxidation of alkyl aromatics selectively is a result of the specific electronic character of the oxygen atoms associated with the vanadium atoms of the V2 O5 structure. As was learned about the role of lattice oxygen (0 ) in the selective ammoxidation of propylene to acrylonitrile (see above), hydrogen abstraction and oxygen insertion require oxygen atoms with nucleophilic character (79). On the other hand, nonselective oxidation is affected by electrophilic oxygen species, O2 and O . These are the intermediate species in the dissociative chemisorption and reduction of O2 to lattice (80). 

Ammoxidation of propanol is a two-step process. The first reaction step is the dehydration of the propanol to propylene. The second step is the ammoxidation of propylene to acrylonitrile. 

Catalyst Chemistry. Just as in the case of selective ammoxidation of propylene to acrylonitrile, the most effective catalysts for propane ammoxidation are based on the oxides of antimony and molybdenum. In fact, in many instances, modifications in the compositions of highly effective catalysts for propylene ammoxidation result in some of the best reported catalysts for propane ammoxidation. This similarity in catalyst types is easy to rationalize, since, as is discussed in a later section, propylene is the key hydrocarbon intermediate in the mechanism of acrylonitrile formation from propane. It can be further generalized that if... 

The catalyst operates by a redox process in which lattice oxygens, up to 70 layers deep in the Ml phase, palatinate in the activation of propane, conversion to intermediate propylene, and ammoxidation of propylene to acrylonitrile. 

New selective catalytic oxidation processes, such as the oxidation of propylene to acrolein, or the ammoxidation of propylene to acrylonitrile catalyzed by mixed oxides, were developed to produce monomers for a new generation of polymers in the 1960s. 

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