Oxidation heterogeneous, organic chemicals, production

Allylic oxidation, that is, the selective oxidation of olefins at the allylic position, represents a substantial portion of the production of important organic chemicals by heterogeneous oxidation. Furthermore, the development and study of selective catalysts for allylic oxidation has led not only to successful commercial processes, but also to important concepts concerning selective oxidation and the phenomena of catalysis in general. 

In fine-chemicals production three-phase reaction systems are common for the hydrogenation and hydrogenolysis of different organic functional groups. Other reactions, such as heterogeneously catalyzed catalytic oxidations, hydrodesulfurizations, and reductive aminations are encountered less frequently. 

In this review we shall focus on the use of heterogeneous catalysts for the liquid phase epoxidation of olefins with alkyl hydroperoxides or hydrogen peroxide. The latter is generally the oxidant of choice for fine-chemicals production owing to a better availability and lower price. Emphasis is placed on methods with a broad scope in organic synthesis. 

One factor which influences the preference for heterogeneous catalysts in industry is the relative ease of separation of products from catalyst. Homogeneous catalysts often require a solvent. If this is an organic chemical, additional cost is incurred. There are also the problems of product separation and solvent recovery. The homogeneously catalysed Wacker process for oxidation of ethene (p. 380) does not suffer from these disadvantages because the product, acetaldehyde, is very volatile and water (containing some acetic acid) can be used as the solvent. 

Fleischmann et al. [549] studied the electro-oxidation of a series of amines and alcohols at Cu, Co, and Ag anodes in conjunction with the previously described work for Ni anodes in base. In cyclic voltammetry experiments, conducted at low to moderate sweep rates, organic oxidation waves were observed superimposed on the peaks associated with the surface transitions, Ni(II) - Ni(III), Co(II) -> Co(III), Ag(I) - Ag(II), and Cu(II) - Cu(III). These observations are in accord with an electrogenerated higher oxide species chemically oxidizing the organic compound in a manner similar to eqns. (112) (114). For alcohol oxidation, the rate constants decreased in the order kCn > km > kAg > kCo. Fleischmann et al. [549] observed that the rate of anodic oxidations increases across the first row of the transition metals series. These authors observed that the products of their electrolysis experiments were essentially identical to those obtained in heterogeneous reactions with the corresponding bulk oxides. 

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