Ammoxidation of Aromatic Side-chains

Whereas the ammoxidation of lower hydrocarbons leads to more large-scale products such as acrylonitrile [7] or methacrylonitrile [8], the ammoxidation of substituted aromatics or heteroaromatics opens the way for the synthesis of fine chemicals or intermediates for fine-chemical syntheses. A variety of pharmaceuticals, pesticides, dyestuffs, and other speciality products are produced from substituted aromatic or heteroaromatic nitriles [e.g. 9,10]. 

Conventional organic synthesis for aromatic nitriles [e. g. 10], for example the Rosenmund-von Braun reaction, the Sandmeyer reaction, or the oximation of aldehydes in the presence of dehydrating agents, performed in batch processes, are cumbersome and result in different amounts of by- and waste products. The ineffectiveness of such processes with regard to the amount of feedstock necessary per formed nitrile and the waste produced is rather high (in terms of the effectiveness factor defined by Sheldon [11]). 

The ammoxidation reaction can, on the other hand, be performed continuously in fixed-bed and fluid-bed reactors, and by-products (particularly CO2) can be easily removed. The fluidized bed has some advantages in terms of heat transfer but demands are made on the mechanical durability of the catalyst and so catalyst choice is limited. The long-term stability of the catalysts is also important and so multicomponent systems are recommended [e.g. 1,12]. The separation of the nitrile formed can be achieved by condensation, centrifugation, filtration, or rectification. Sometimes the formation of hazardous by-products (HCN, CO) must be considered. 

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Methyl side chains of aromatic hydrocarbons can be selectively ammox-idized to nitrile groups. The process is very similar to the ammoxidation of propene and the same catalysts are found to be effective. Identical mechanisms have been proposed, and will not be discussed here. 

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