Heterogeneously Catalyzed Ammoxidation: A Valuable Tool for One-Step Synthesis of Nitriles

Ammoxidation is a partial oxidation with selective insertion of nitrogen into a methyl group (aldehyde or alcoholic groups are also able to react) that is in the alpha-position to double bonds of olefinic, aromatic or heteroaromatic hydrocarbons to produce corresponding nitriles. The reaction is accelerated by heterogeneous catalysis and carried out in continuous process, in general. The heterogeneously catalyzed ammoxidation of various hydrocarbons to synthesize a wide range of industrially important nitriles has been the subject of great interest in recent times because nitriles are very useful basic chemicals (e.g. acrylonitrile) and organic intermediates (e.g. nicotinonitrile) used in the manufacture of numerous value-added fine chemicals. The reaction proceeds with H abstraction from the methyl group, leading to an allylic or benzylic intermediate that reacts with catalyst lattice oxygen to form an oxygen-containing intermediate through the Mars-van Krevelen mechanism. Nitrogen insertion seems to be due to adsorbed N species. Finally, nitrile formation occurs via an imine intermediate. Alkanes could be converted in a similar way, albeit with catalysts that possess a dehydrogenation function to give rise to the intermediate allylic species. Furthermore, ammoxidation is a very green reaction because oxygen or air is used as oxidant and water is generally the only byproduct. The ammoxidation reaction is used in the production of large-scale chemicals, such as acrylonitrile, as well as that of intermediates, specialty chemicals, and fine chemicals in the agrochemical, health, and nutrition industries. The reaction is mostly carried out in the gas phase using supported transition metal oxides as catalysts. The most important advantage is that the ammoxidation is a simple, single-step, eco-friendly reaction. Nevertheless, one bottleneck is that only less-functionalized reactants can be converted; that is, higher substituted molecules are often subject to side-reactions. However, at present, ammoxidation is indeed a lucrative field of study, which has recently found much wider application in industry and hence will in years to come continue to offer exceptional commercial rewards for the production of industrially important nitriles.