Oxidative dehydrogenation of hydrocarbons

Beside their use in equilibrium-restricted reactions, CMRs have been also proposed for very different applications [6], like selective oxidation and oxidative dehydrogenation of hydrocarbons they may also act as active contactor in gas or gas-liquid reactions. 

Selective oxidations, e.g., propane to acroleine, butane to maleic anhydride, ethylene to ethylene oxide Oxidative dehydrogenations of hydrocarbons Oxidative coupling of methane Methane oxidation to syngas... 

Let us discuss in general gas-phase processes of oxidative dehydrogenation of hydrocarbons involving as reagents substances that easily induce free radical transformations of substrates. Many such substances are known that dissociate to free radicals or induce free radical reactions. However, the most widespread in investigations are compounds that are able to shift dehydrogenation and cracking product ratios toward the first process. 

As follows from a brief consideration of the role of HO radicals in gas-phase oxidation reactions, they are the key active sites in the high temperature range, and studies of homogeneous oxidative dehydrogenation of hydrocarbons allocate the dominating role in unsaturated compound formation to them. 

Rozanska X, Sauer J. Oxidative dehydrogenation of hydrocarbons by V307+ compared to other vanadium oxide species. J Phys Chem A. 2009 113(43) 11586—94. 

Examples of synergistic effects are now very numerous in catalysis. We shall restrict ourselves to metallic oxide-type catalysts for selective (amm)oxidation and oxidative dehydrogenation of hydrocarbons, and to supported metals, in the case of the three-way catalysts for abatement of automotive pollutants. A complementary example can be found with Ziegler-Natta polymerization of ethylene on transition metal chlorides [1]. To our opinion, an actual synergistic effect can be claimed only when the following conditions are filled (i), when the catalytic system is, thermodynamically speaking, biphasic (or multiphasic), (ii), when the catalytic properties are drastically enhanced for a particular composition, while they are (comparatively) poor for each single component. Therefore, neither promotors in solid solution in the main phase nor solid solutions themselves are directly concerned. Multicomponent catalysts, as the well known multimetallic molybdates used in ammoxidation of propene to acrylonitrile [2, 3], and supported oxide-type catalysts [4-10], provide the most numerous cases to be considered. Supported monolayer catalysts now widely used in selective oxidation can be considered as the limit of a two-phase system. 

The quinone-hydroquinone system can be involved in redox mechanisms. So, in addition to acid-base catalysis, carbon materials can promote oxidations, such as the oxidative dehydrogenation of hydrocarbons, which we discuss in detail below. 

The use of carbon as a catalyst was reviewed. It was shown that interesting activity correlations could be obtained by studying the catalytic performance of a series of carbon materials prepared from the same precursor with similar tex-mral properties and different amounts of surface functional groups. The redox couple quinone-hydroquinone was found to be involved in the oxidative dehydrogenation of hydrocarbons, while carboxylic acid groups are the active sites for the dehydration of alcohols. In both cases, thermal treatments at different temperatures were used to identify the nature of the active sites, aud correlations... 

Dehydrogenation of hydrocarbons by cationic metal oxide clusters was studied in several experimental gas-phase and theoretical works. Rozanska and Sauer [132] used DFT methods to establish the mechanism of the oxidative dehydrogenation of hydrocarbons by VjOy (the smallest monocationic polynuclear closed-shell V species). Schwarz et al. [133] studied the gas-phase dehydrogenation of alkanes with Ni202 (see reaction below). 

Distributor The controlled addition of reactant(s) limits side reactions to increase selectivity by optimizing the reactant concentration profile to mitigate the temperature rise in exothermic reactions Partial oxidation oxidative dehydrogenation of hydrocarbons oxidative coupling of methane... 

V-Mo-Nb-W-containing hydrotalcite-like materials as precursors of catalysts for oxidative dehydrogenation of hydrocarbons and alcohols... 

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