Reaction propene ammoxidation

HOMOGENEOUS MODELS FOR MECHANISMS OF SURFACE REACTIONS PROPENE AMMOXIDATION... 

Compared with propene, the oxidation of isobutene is more rapid but less selective, yet selectivities of over 75% appear feasible. Combustion is the main side reaction. One would expect that some considerable attention would be shown in the literature to isobutene oxidation as a route to the industrially important methacrylic acid, but this is not the case. Nor is it with the production of methacrylonitrile, analogous to the propene ammoxidation. Only in the patent literature is a high activity noticeable. 

The ammoxidation of propene to acrylonitrile described by the global equation (11.1) actually involves a very complex reaction mechanism. More generally, the reaction of ammoxidation refers to the interaction of ammonia with a hydrocarbon... 

Table 11.5 Chemical reactions at ammoxidation of propene in a fluid-bed reactor.

The considerations reported above suggest that the mechanism of reaction might not be the same as the well known allylic insertion of a nucleophilic NH " species onto the activated hydrocarbon, to generate the precursor of the cyano group. Indeed, none of the elements included in catalyst formulation is able to produce the M=NH species (which is generated by Mo and Sb in propene ammoxidation catalysts). 

This coherent reaction network clearly demonstrates the in ortance of the 30-40 kJ mole selectivity limit. When it is exceeded, as is the case with propane oxidation to acrolein, selectivity declines drastically. Similarly the accnmmulated data for propane and propene ammoxidation [27,28] to acrylonitrile indicate selectivities at 30% conversion of 50% and 85% respectively. These data are consistent with the 41 kJ mole difference in bond enthalpies shown in scheme 2 for propane and propene. 

Heterogeneous catalytic oxidation is a well studied and industrially useful process. Industrial catalytic oxidation of vapors and gases is a very broad field and is dealt with in several texts and review articles. Catalytic oxidation, both partial and complete, is an important process for such reactions as the partial oxidation of ethene and propene, ammoxidation of propene to acrylonitrile, maleic anhydride production, production of sulfuric acid, and oxidation of hydrocarbons in automotive exhaust catalysts. By far, the majority of oxidation catalysts and catalytic oxidation processes have been developed for these industrially important partially oxidized products. However, there are important differences between the commercial processes and the complete catalytic oxidation of VOCs at trace concentrations in air. For instance, in partial oxidation, complete oxidation to CO2 and H2O is an undesirable reaction occurring in parallel or in series to the one of interest. Other differences include the reactant concentration and temperature, the type of catalyst used, and the chemical nature of the oxidizable compound. Approximate ranges of the major independent variables of interest in this review are shown in Table 1. 

In the patent literature, numerous catalyst formulations for propene ammoxidation can be found, and the majority of these are formulated around molybdenum and/or antimony base oxides. The most effective catalysts for these reactions are complex metal oxide mixtures containing three to six... 

Although the aim of this chapter is to show how a thermodynamic relationship between A and AI allows to predict the type of catalysts needed for a reaction, it is worth recalling that A can be correlated with experimental parameters related to catalysis, or values of selectivity, provided the same reaction is studied [33]. For example, by using data proposed by Matsuura [60], the heat of adsorption A//ads for a series of catalysts of oxidation of 1-butene to butadiene, or the Mossbauer quadruple shift values for Fe +-containing catalysts of propene ammoxidation, could be related to the A value of the respective catalysts [33]. In a study of the ODH... 

Most industrially desirahle oxidation processes target products of partial, not total oxidation. Well-investigated examples are the oxidation of propane or propene to acrolein, hutane to maleic acid anhydride, benzene to phenol, or the ammoxidation of propene to acrylonitrile. The mechanism of many reactions of this type is adequately described in terms of the Mars and van Krevelen modeE A molecule is chemisorbed at the surface of the oxide and reacts with one or more oxygen ions, lowering the electrochemical oxidation state of the metal ions in the process. After desorption of the product, the oxide reacts with O2, re-oxidizing the metal ions to their original oxidation state. The selectivity of the process is determined by the relative chances of... 

Hur et al. (252,277,278) reported the use of alkali metal-doped MgO to catalyze the synthesis of acrylonitrile and propionitrile (278). Acrylonitrile is an important chemical, especially in the polymer industry it is generally synthesized by the ammoxidation of propene catalyzed by multicomponent bismuth molybdates (279). An alternative method of synthesis of acrylonitrile is the reaction between methanol and acetonitrile (Scheme 42). 

The oxidation of propene to acrolein has received much attention for several reasons. Firstly, the process is of industrial importance in itself, and it is also a suitable model reaction for the even more important, but at the same time more complicated, ammoxidation. Secondly, propene oxidation is, in many aspects, representative of that of a class of olefins which possesses allylic methyl groups. Last, but not least, the allylic oxidation is a very successful example of selective catalysis, for which several effective metal oxide systems have been discovered. The subject has therefore attracted much interest from the fundamental point of view. 

The ammoxidation of propene to acrylonitrile is of great industrial importance and accordingly the literature is abundant. The reaction is very similar to the oxidation of propene to acrylonitrile and carried out at the same conditions and over the same kind of catalysts. The famous bismuth phosphomolybdate catalyst developed by Sohio was the first of a series of highly effective mixed-oxide catalysts. The optimum yields are generally obtained at temperatures of 400—500°C. Initial selectivities over 95% and yields up to 80% are feasible. The superior selectivity of the ammoxida-... 

The strong parallel with the acrolein formation initially suggested the idea that acrolein is a reaction intermediate in the ammoxidation, and can further react with ammonia and oxygen to form acrylonitrile. Although the ammoxidation of acrolein is indeed a very rapid reaction, it is generally accepted today that a direct reaction path to acrylonitrile predominates. The differences between both theories are very small, however, when one assumes that the ammoxidation of acrolein and propene involves the same reaction intermediate. Thus the various kinetic schemes proposed in the literature can be derived from the general scheme below by omitting the reaction steps (3), (4) and/or (5) and variation of the ratio between (2) and (3). 

Iron oxide is an important component in catalysts used in a number of industrially important processes. Table I shows some notable examples which include iron molybdate catalysts in selective oxidation of methanol to formaldehyde, ferrite catalysts in selective oxidative dehyrogenation of butene to butadiene and of ethylbenzene to styrene, iron antimony oxide in ammoxidation of propene to acrylonitrile, and iron chromium oxide in the high temperature water-gas shift reaction. In some other reactions, iron oxide is added as a promoter to improve the performance of the catalyst. 

A somewhat similar mechanism involving Movl-imino species (Mo=NH) resulting from the reaction of ammonia with Mo—O bonds has been suggested for the industrially important ammoxidation of propene to acrylonitrile (equation 109).308... 

About 90% of the worldwide acrylonitrile (AN) is manufactured today by the ammoxidation of propene, as described by the reaction ... 

Bismuth molybdates having a Bi/Mo ratio in the range of 0.67 2.0 catalyze the selective oxidation of propene to acrolein, and the ammoxidation of propene to acrylonitrile (equations 5 and 6). Both reactions proceed through an aUyhc intermediate. Three typical active phases o -Bi2Mo30i2,... 

In the propane ammoxidation a lower selectivity for acrolein plus acrylonitrile is observed. The formation of partial (amm)oxidation products from propane requires more elemental steps than their formation from propene. All these intermediates can undergo a side reaction with electrophilic oxygen species yielding degradation products. 

Another example which is directly related to industrial catalysis is the adsorption and decomposition of propene from a mixed oxide, namely FeSbOj- This material is used for the industrial production of acrolein and acrylonitrile (Yoshino et al., 1971). If the surface is dosed with both propene and ammonia, then all the reaction products in the industrial process are seen to evolve as shown in fig. 24 (Hutchings et al., 1991). Some intact propene desorbs at low temperatures, while the selective ammoxidation... 

It was once widely used to make acrylonitrile by reaction with acetylene in presence of an aqueous cuprous chloride-ammonium chloride catalyst, but this process is obsolescent and is being displaced by ammoxidation of propene with bismuth molybdate catalysts ... 

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