Hydrocarbon oxidation catalytic, mechanism

A mechanism for the catalytic oxidation of hydrocarbons is due to Mars Van Krevelen (Spec Suppl Chem Eng Sci 3 41, 19541. These assumptions are made ... 

A redox mechanism involving lattice oxygen originally proposed in 1954 by Mars and Van Krevelen (22) for hydrocarbon oxidation over V2O5 can be applied to a variety of catalytic oxidation reactions (23). The following illustrates a lattice redox mechanism for CO oxidation ... 

Detailed microkinetic models are available for CO, H2 and HC oxidation on noble metal(s) (NM)/y-Al203-based catalysts (cf., e.g. Chatterjee et al., 2001 Harmsen et al., 2000, 2001 Nibbelke et al., 1998). The model for CO oxidation on Pt sites includes both Langmuir-Hinshelwood and Eley-Rideal pathways (cf., e.g., Froment and Bischoff, 1990). Microkinetic description of the hydrocarbons oxidation is more complicated, particularly due to a large number of different reaction intermediates formed on the catalytic surface. Simplified mechanisms, using just one or two formal surface reaction steps,... 

The reactivity of the hydrocarbons increases in the order ortho < meta < para in the liquid-phase catalytic oxidation of methyl derivatives of biphenyl into acids by air. The mechanism of the oxidation of hydroxymethylbiphenyls and hydroxymethyl-benzenes involves the formation of an unstable cation radical, which is then stabilized by emitting a proton, giving hydroxybenzyl radical.243... 

In short, catalytic oxidation of hydrocarbons is a structure-sensitive reaction, and its mechanism is strongly dependent on the type of catalyst and on process conditions. This means that the morphology of the active phase will affect the catalyst activity, and hence the preparation procedure will have a strong influence on catalyst performance. 

Further possibilities for catalytic oxidation are realized in the well-known catalysis of the autoxidation of hydrocarbons and other substrates by salts of transition metals such as copper, cobalt, and manganese which exhibit more than one stable oxidation state and which catalyze oxidation through free radical chain mechanisms (16). 

Figure 1. Mechanism of the catalytic oxidation of hydrocarbons. Reproduced with permission from Ref. 31.

As has been stated in the case of ethylene, the catalytic oxidation of unsaturated hydrocarbons is complicated by the fact that such substances are somewhat sensitive to the action of hydrolyzing agents. The presence, therefore, of even small amounts of water in the oxidizing gases makes it difficult to determine the exact mechanism of the process, i.e., whether the primary reaction consists of oxidation, hydration, or simultaneous oxidation and hydration. The same situation is met with again in the catalytic oxidation of acetylene and is further complicated by the fact that the primary products of hydrolysis and oxidation tend to undergo a variety of different secondary reactions. 

A similar mechanism was formulated for the catalytic oxidation of hydrocarbons and for the photo-sensitized oxidation of 2-propanol. (2) A pure dehydrogenation can be depicted in which the platinum cleaves the hydrogen from the substrate alcohol. 

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. 

Isotopic investigations of various reaction mechanisms yielded the scheme for catalytic oxidation of hydrocarbons shown in Fig. 9. This scheme is of great help in the proper choice of catalysts. The first task in improving the results of hydrocarbon oxidation is to find catalysts ensuring the necessary reaction route. Parallel reaction routes are to be suppressed by adjusting the chemical composition of the catalyst through addition of donor and acceptor impurities. 

The various hydrocarbon oxidation schemes discussed above were believed to proceed at the catalyst surface only. The present concepts accept the occurrence of complex heterogeneous-homogeneous reactions proceeding in part at the solid surface and in part in the gas or liquid phase. Many catalytic oxidation processes considered recently as purely heterogeneous appeared to proceed by the heterogeneous-homogeneous mechanism. Such are the oxidations of hydrogen, methane, ethane, ethylene, propene, and ammonia over platinum at elevated temperatures, as studied by Polyakov et al. (131-136). When hydrocarbons are oxidized over platinum the reaction sets in on the catalyst surface and terminates in the gas phase. 

In the oxidation of CH4 and CjHg, no inhibition by the reactant is operative, and initial conversion is higher near the reactor inlet, where favourable conditions for ignition are stabilized. This relationship between dynamic behaviour and adsortion-desorption properties, finds support in the literature. Site competition between hydrocarbons and oxygen has been recognized as a key point in the mechanism of catalytic oxidation [3] in the same study, a substantially different behaviour in catalytic combustion was reported for alkanes and ethylene, and this was related to the strong adsorption properties of the latter. 

The catalytic oxidation of a hydrocarbon A by means of air into the desired product G is assumed to occur according to the mechanism... 

The proposed mechanism for these catalytic oxidations includes two catalytic redox cycles (i) initial oxidation of iodobenzene with Oxone, producing hydroxy(phenyl)iodonium ion and hydrated iodosylbenzene and (ii) the oxidation of iron(III)-porphyrin to the oxoiron(IV)-porphyrin cation-radical complex by the intermediate iodine(III) species (Scheme 4.58) [93]. The oxoiron(IV)-porphyrin cation-radical complex acts as the actual oxygenating agent toward aromatic hydrocarbons. The presence of the [PhI(OH)]+ and PhI(OH)2 species in solutions containing Phi and Oxone has been confirmed by ESI mass spectrometry [93]. 

The mechanism for the catalytic oxidation of methane by Pt(II) was first put forth by Shilov. The first step of the catalytic cycle involves the formation of a methylplatinum(II) intermediate 16 via the C-H activation of hydrocarbon methane with Pt(II) 15. A methylplatinum(IV) species 17 is obtained by the oxidation of methylplatinum(II) intermediate 16 (second step). The formation of the product takes place through reductive elimination from the methylplatinum(IV) 17 either via coordination to water or by the nucleophilic attack at the carbon by an external nucleophile such as water or chloride ion. Considerable amount of experimental data supports has been offered for supporting the given general mechanism (Fig. 6). 

Adsorbed state of oxygen. Research on the adsorbed state of oxygen plays an important role in the reactive mechanism study of catalytic oxidation of hydrocarbon molecules and ammonia oxidation. In general, the adsorptive states of oxygen in the transition metal oxides and metal catalysts are the negative ion type, such as O, O, and 0 etc. Of course, there are also molecular O2, as well as unstable O3 etc. 

Ozone acts as the precursor of the key oxidants in the toluene decomposition. The presence of water vapor is also very important, as has been demonstrated for the catalytic oxidation of benzene with ozone on supported manganese Oxides catalysts [68]. It suppresses the catalyst deactivation by inhibiting the buildup of organic by-products on the catalyst surface, including formic acid and strongly bound surface formates. Scheme 18.2 proposes a general pattern mechanism for the plasma-driven total oxidation of the hydrocarbons. 

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 difficulties. With the powerful transition-metal oxidants, the initial oxidation products are almost always more susceptible to oxidation than the starting material. Once a hydrocarbon is attacked, it is likely to be oxidized to a carboxylic acid, with chain cleavage by successive rapid 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 work has been expended on the development of selective catalytic oxidation processes and several have attained economic importance. Since the mechanisms are often obscured by limited understanding of heterogeneous catalysis, however, we will not devote additional attention to these reactions. 

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