Selective Oxidation of Hydrocarbons

The liquid-phase selective oxidation of hydrocarbons is a wide area of research, and relevant heterogeneously-catalysed industrial processes are mostly carried out with mixed oxides or ordered porous catalytic systems, which escape the scope of this chapter. For a more comprehensive survey of the industrial and technological developments within industrial liquid-phase oxidation reactions, we recommend generic literature, in particular the Handbook of Heterogenous Catalysis and recent reviews on oxidation with with transition-metal complexes by Bregeault et al and Punniyamurthy. However, from a more academic perspective, there has been significant progress in this research area which we describe below, in sections divided on the basis of the substrate of the reaction. 

Epoxidation continues to be an essential part of many chemical syntheses and is often used to functionalise less reactive starting materials to ensure they can be used in downstream processes. In particular, alkene epoxidation is of great relevance for the chemical industry. Molecular oxygen is the preferred oxidant for economic and environmental motivations, however, many molecules and catalysts at present are unreactive with molecular oxygen and consequently more reactive 

Nikolaos Dimitratos, Jose A. Lopez-Sanchez and Graham J. Hutchings 

One of the most important selective oxidation processes in the production of chemicals is the side chain oxidation of alkyl aromatics, which are then further reacted to higher value products that end up in a variety of polymer compositions and specialised chemicals.The largest oxyfunctionalised aromatic products with regard to world production are terephthalic acid, phthalic anhydride and benzoic acid which are produced worldwide with capacities of 30,000, 5,000 and 500 kt a respectively. Worldwide production capacities for benzaldehyde and pyromel-litic dianhydride do not rise above 50 kt a In smaller capacities as specialty chemicals for the pharmaceutical industry, halo-substituted oxyfunctionalized aromatics are also produced. Of the aforementioned products, phthalic anhydride and pyromellitic dianhydride are produced from gas-phase processes using V20s-Ti02 catalysts and a liquid-phase alternative process does not appear immediately desirable. On the other hand, terephtalic and benzoic acids, and benzaldehyde are 

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D. J. HucknaU, Selective Oxidation of Hydrocarbons, Academic Press, Inc., London, 1974. 

This method is especially valid for the preparation of gold NPs mixed with activated carbon, which are active and stable for the selective oxidation of hydrocarbons and alcohols in water. Over activated carbon gold could not be directly deposited as NPs by using the techniques described above, such as DP and even by GG. Gold colloids with mean diameters from 2.5 to lOnm stabilized by poly vinyl alcohol or poly vinyl p5rrolidone are used. 

Attempts to achieve selective oxidations of hydrocarbons or other compounds when the desired site of attack is remote from an activating functional group are faced with several difficulties. With powerful transition-metal oxidants, the initial oxidation products are almost always more susceptible to oxidation than the starting material. When a hydrocarbon is oxidized, it is likely to be oxidized to a carboxylic acid, with chain cleavage by successive oxidation of alcohol and carbonyl intermediates. There are a few circumstances under which oxidations of hydrocarbons can be synthetically useful processes. One group involves catalytic industrial processes. Much effort has been expended on the development of selective catalytic oxidation processes and several have economic importance. We focus on several reactions that are used on a laboratory scale. 

Other successful selective oxidations of hydrocarbons by Cr(VI) have been reported— for example, the oxidation of c/s-decalin to the corresponding alcohol—but careful attention to reaction conditions is required. 

Microporous catalysts such as MAPO-36 (43,44), which are excellent for selective oxidation of hydrocarbons (45), are highly beam-sensitive. Yet HRTEM... 

D. J. Hucknall, "Selective Oxidation of Hydrocarbons", Academic Press, New York, p.153 (1974)... 

Hucknall, D.J., Selective Oxidation of Hydrocarbons Academic Press New York, 1974. 

The results presented above demonstrate that one could obtain fundamental information concerning the structure, oxidation state and reactivities of a model K/NiO/Ni(100) catalyst by using a combination of advanced surface science techniques. The experimentd approaches described here could in principle be applied to other model catalysts which are of importance in the selective oxidation of hydrocarbons. These results also demonstrate that it is very important to use a variety of surface techniques to obtain complementary results. 

The activity data confirm that an IR absorption band at 960 cm" is a necessary condition for titanium silicates to be active for the selective oxidation of hydrocarbons with aqueous H2O2 as suggested by Huybrechts et al. (9). However, this band is not a sufficient condition for predicting the activity of the TS-1 catalyst. Although TS-l(B) and TS-l(C) show intensities for the 960 cm- band similar to TS-1 (A), their activities are different First of all, the reaction data reveal that TS-1 (A) is much more active than TS-l(B) for phenol hydroxylation, while both samples show similar activity for n-octane oxidation and 1-hexene epoxidation. Therefore, the presence of the IR band at 960 cm-i in TS-1 catalysts may correlate with the activities for the oxidation of n-octane and the epoxidation of 1-hexene but not for phenol hydroxylation. However, note that the amorphous Ti02-Si02 also has an IR absorption band at 960 cm- and it does not activate either substrate. 

In the selective oxidation of hydrocarbons or in fuel technologies, catalysed carbon deposition from the gas phase can occur, which can lead to catalyst... 

Oxides are widely exploited as catalysts for the selective oxidation of hydrocarbons. They provide lattice oxygen in selective oxidation reactions and exchange it with oxygen gas (e.g. from air in the reactant stream). The periodic lattice oxygen loss for the hydrocarbon oxidation occurs because of reducing gases, despite the presence of gas phase oxygen in the reactant stream. This results in the formation of anion vacancies, local non-stoichiometry and defect structures as discussed in chapter 1. 

The selective oxidation of hydrocarbons, particularly that of alkanes, remains a challenge. It is not surprising, therefore, that the problems of oxidation processes are addressed in several books,1043-1045 reviews,1046-1057 and a journal special issue,1058 as well as in international conferences1059-1064 devoted to the topic. For the advances in chirally catalyzed oxidation processes, including asymmetric epoxidation and osmylation, Sharpless was one of the recipients of the 2001 Nobel Prize in Chemistry. 

From the different contributions, it may be concluded that, in the oxidation of ammonia, the same type of redox mechanism is operative for metal oxides as in the selective oxidation of hydrocarbons. As a consequence, the hydrogen atoms will be abstracted successively from the NH3 molecule by a stepwise mechanism. 

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