Methan oxidation, formation condensation products

Beside methanol and formaldehyde, the oxidation of methane may be directed to another route, leading to the formation of its condensation products, for example, ethane, ethylene and benzene. This route may provide an alternative way for the chemical use of natural sources of methane. Here, various catalysts were also tested using both 02 and N20 as the oxidants [22], The general picture observed by most authors was similar to that with methane oxidation to oxygenates. The conversion of methane was always higher with 02 than with N20. However, the selectivity to the coupling products showed an opposite trend. 

More recently, direct catalytic oxidative condensation of methane to ethane (with metal oxides),57,82-84 as well as to ethylene and acetylene (via high-temperature chlorinative conversion) was explored.76 In all these processes, however, a significant portion of methane is lost by further oxidation and soot formation. The selectivity in obtaining ethane and ethylene (or acetylene), respectively, the first C2 products, is low. There has, however, been much progress in metal-oxide-catalyzed oxidative condensation to ethane. 

Hydrocarbon formation from methyl chloride can be catalyzed by ZSM-5482 483 or bifunctional acid-base catalysts such as W03 on alumina.420,447 The reaction on ZSM-5 gives a product distribution (43.1% aliphatics and 57.1% aromatics at 369°C) that is very similar to that in the transformation of methanol, suggesting a similar reaction pathway in both reactions.483 W03 on A1203 gives 42.8% C2-C5 hydrocarbons at 327°C at 36% conversion.447 When using methyl bromide as the feed, conversions are comparable. However, in this case, HBr can be very readily air-oxidized to Br2 allowing a catalytic cycle to be operated. Since bromine is the oxidant, the reaction is economical. The one step oxidative condensation of methane to higher hydrocarbons was also achieved in the presence of chlorine or bromine over superacidic catalysts.357... 

Figure 7. Time dependent product formation for Pd(ll)-catalyzed oxidative condensation of methane directly to acetic acid.

As can be seen from the above equation, formation of HCN is in reality a hetero-bimolecular oxidative coupling reaction of methane with ammonia. The ammoxidation reactor construction is a simple fixed-bed multi-tube and the catalyst is usually a platinum or sometimes a Group V or VI metal oxide on a silica or alumina support. The HCN product is recovered by condensation and fractionation. With the reaction simplicity and yield, and widespread availability of starting materials, in-situ HCN generation is an ideal industry solution to HCN supply. (See Chapter 29 for more details.)... 

Various explanations have been offered for the mechanism of the formation of the carbon dioxide and of the ethane which has also been obtained in certain cases. None of these are entirely free from objections. Aldehydes are known to condense to esters under certain conditions and the decarboxylation of such has been offered as one explanation. However, the presence of carbon dioxide by this mechanism has not been supported by the evidence of other products of ester decomposition. Methane formation has not been reported in all cases where carbon dioxide has been found and this, together with the fact that entirely inadequate amounts of carbon have been found, seems to point that the rupture of acetaldehyde to C -+- C02 instead of carbon monoxide does not occur. The decomposition of aldehyde alone in the presence of precipitated iron oxide at 400° C. gave 40 per cent carbon dioxide and a large quantity of resinous matter.70 In the presence of reduced nickel, however, no carbon dioxide was formed and no resinous matter or oil resulted although nickel is an active catalyst for aldehyde decomposition. 

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