Methyl radical oxidation recombination

In some rare cases, heavier hydrocarbons are formed, for example, benzene. The process was discovered in the early 1980s [5,335]. Due to a large number of works, the OCM has become one of the most thoroughly studied reactions of the oxidative conversion of methane, and in particular, its essentially homogeneous—heterogeneous nature has been experimentally demonstrated. Methyl radicals CH3 formed by e catalytic reaction escape into the bulk of the reactor. In the temperature range optimal for the OCM, 600—950 °C (Fig. 12.2), at an oxygen concentration below 20% and atmospheric pressure, the main reaction of methyl radical is recombination to form ethane (reaction (12.4)). 

The EPR intensity of the ethyl radicals is irreversibly attenuated above 50 K and falls below the detection limit above 80 K. This can be explained by assuming the ethyl radicals to diffuse and recombine at these temperatures, as has been observed for methyl radicals above 45 K [ 124] and for NO2 radicals on an oxide surface above 75 K [125]. 

The fact that reaction (3.82) may not proceed as written at high temperatures may explain why methane oxidation is slow relative to that of other hydrocarbon fuels and why substantial concentrations of ethane are found [4] during the methane oxidation process. The processes consuming methyl radicals are apparently slow, so the methyl concentration builds up and ethane forms through simple recombination ... 

Of course, all the appropriate higher-temperature reaction paths for H2 and CO discussed in the previous sections must be included. Again, note that when X is an H atom or OH radical, molecular hydrogen (H2) or water forms from reaction (3.84). As previously stated, the system is not complete because sufficient ethane forms so that its oxidation path must be a consideration. For example, in atmospheric-pressure methane-air flames, Wamatz [24, 25] has estimated that for lean stoichiometric systems about 30% of methyl radicals recombine to form ethane, and for fuel-rich systems the percentage can rise as high as 80%. Essentially, then, there are two parallel oxidation paths in the methane system one via the oxidation of methyl radicals and the other via the oxidation of ethane. Again, it is worthy of note that reaction (3.84) with hydroxyl is faster than reaction (3.44), so that early in the methane system CO accumulates later, when the CO concentration rises, it effectively competes with methane for hydroxyl radicals and the fuel consumption rate is slowed. 

Recombination of the intermediate methyl radical (at position 17) with 52 provides benzoate ester 56 and regeneration of the cop-peril) bromide. This reaction is also known as a Kfuimsch oxidation.19... 

In order to explain the data of Aronowitz et al (12) and previous shock—tube and flame data, Westbrook and Dryer (12) proposed a detailed kinetic mechanism involving 26 chemical species and 84 elementary reactions. Calculations using tnis mechanism were able to accurately reproduce experimental results over a temperature range of 1000—2180 K, for fuel—air equivalence ratios between 0.05 and 3.0 and for pressures between 1 and 5 atmospheres. We have adapted this model to conditions in supercritical water and have used only the first 56 reversible reactions, omitting methyl radical recombinations and subsequent ethane oxidation reactions. These reactions were omitted since reactants in our system are extremely dilute and therefore methyl radical recombination rates, dependent on the methyl radical concentration squared, would be very low. This omission was justified for our model by computing concentrations of all species in the reaction system with the full model and computing all reaction rates. In addition, no ethane was detected in our reaction system and hence its inclusion in the reaction scheme is not warranted. We have made four major modifications to the rate constants for the elementary reactions as reported by Westbrook and Dryer (19) ... 

Since the metal filament is inert in both methane and ethane activation, but active in the binary catalyst, this effect is likely due to reactions involving some intermediates. In the absence of the metal filament, the oxide component is a very efficient catalyst for the OCM process, which is well-known to proceed via the formation and recombination of free methyl radicals [6] ... 

As we emphasized above, although the oxidation of methane is accompanied by the formation of a variety of compounds, C2-hydrocarbons play among them a special role. First of all, recombination of methyl radicals, which are primary and in some cases the most abundant radicals in the system, leads to a quadratic... 

The oxidative dehydrogenation reactions over these catalysts are similar to the gas phase result of shock tube experiments determined by Skinner et al. (ref. 6). This observation supports the fact that the recombination reactions of methyl radicals in the gaseous phase are an important source of ethane and that the ethene is a secondary product derived from ethane. This secondary reaction proceeds in the gaseous phase as well as the catalyst surface. The major role of the MgO surface is to produce the methyl radical efficiently. The active sites for cleaving the H-CH3 bond should be moderated by Li to enforce C2 selectivity. In addition to gas phase oxidation, the direct surface oxidation of the hydrocarbon adsorbate is very significant especially for acidic materials. 

The difference in product spectrum obtained from a system operating in the SCF phase compared with the liquid phase is probably a function of the types of free radicals that are formed in each phase. In the SCF phase, the butane-derived free radicals have a higher probability of further decomposing into methyl radicals instead of terminating the reaction by recombining. This is because the reaction temperature is greater in the SCF phase than in the liquid phase. If the methyl radicals undergo further oxidation, a broad spectrum of products will be obtained (Winkler and Hearne, 1961). 

---