Heterogeneous catalytic oxidation of hydrocarbons

Ionization from the Heterogeneous Catalytic Oxidation of Hydrocarbons in the Vapor Phase... 

The oxidation of gaseous hydrocarbons over solid catalysts has been the subject of extensive industrial and academic research for many years. The mutual interest in these reactions reflects both their commercial importance and their value in studies of the fundamental features of catalytic processes. Indeed, the source of several theoretical investigations of catalytic reactions may readily be traced to developments in the modern chemical industry. Moreover, the numerous studies that have been conducted are a further reflection of the diversity of scientific interests embodied in this subject and the wide range of techniques currently available for their investigation. It is not surprising, therefore, that the heterogeneous catalytic oxidation of hydrocarbons is a subject in which a wealth of data has been accumulated and a diversity of opinions recorded. Hence, it is inevitable that in an article of this type any worthwhile inquiry into such a large field of scientific interest requires the consideration of a restricted aspect of the subject. 

Margolis, L. J., Heterogeneous Catalytic Oxidation of Hydrocarbons. Gostop-tehizdat, Moskwa, 1962. 

A theory of heterogeneous catalytic oxidation of hydrocarbons would be impossible without knowledge of the elementary mechanism of oxidation, of the fundamental laws governing this process, and of its rate-determining steps. Insufficient theoretical treatment of the wide amount of experimental information on the proper choice of catalysts for hydrocarbon oxidations also hampers advances in this field. 

Heterogeneous catalytic oxidation of hydrocarbons has been extensively studied for the last century, since a large fraction of high value chemical feed stocks, such as organic aldehydes, acids, ketones, alcohols, etc., are produced by the catalytic partial oxidation of light alkanes and alkenes. Historically, oxidation reactions have always been carried out over either noble metals or transition metal oxides. Noble metals have been almost exclusively applied to complete oxidations, whereas transition metal oxides have been used for both selective and complete oxidations. 

Figure 2 The transfer of electrons across the adsorbate/oxide interface in the course of the heterogeneous catalytic oxidation of a hydrocarbon molecule by gas phase oxygen.

Ioffe (108) and later on d Alessandro and Farkas (42) have found from kinetic studies that heterogeneous catalytic oxidation of aromatic hydrocarbons involves a number of parallel and parallel-consecutive reactions, and that high conversion does not necessarily include the formation of low conversion products. For example... 

Formation of products containing less than four carbon atoms is not related to the catalytic activity of the metal on the decomposition of hydroperoxides. Hence, the liquid-phase oxidation of hydrocarbons involves heterogeneous catalytic reactions of isomerization and decomposition of peroxide radicals, proceeding on the reactor surface. 

The catalytic oxidation of alkanes on heterogeneous catalysts is potentially a very important field, but even now it is still at the beginning of its development, particularly in comparison with the oxidation of olefins and aromatic hydrocarbons, which is already widely used in chemical industry (see Table III.l). 

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. 

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