Oxidation of ethane
Wells, Clifford H.J.
The oxidation of ethane like that of other aliphatic hydrocarbons, apart from methane, can proceed by two distinct mechanisms. One operates above and the other below ca. 4OO°C. Analytical work on ethane oxidation has shown that in the later stages of the high temperature oxidation ethylene is the major product whereas in the low temperature region oxygenated compounds are the major products. Through the development of gas chromatography it is now possible to analyse the products in the early stages of reaction. The aim of the present work was to apply this technique to the analysis of the products in the initial stages of ethane oxidation and to determine the relative importance of ethylene and of oxygenated compounds in the early stages. At the same time analysis of the products formed throughout the course of the whole reaction would provide valuable analytical data for the elucidation of the oxidation mechanism. The oxidations were carried out in a static system between 318-386°C using mixtures of different composition. The relative yields of the initial products did not change appreciably when the ethanesoxygen ratio was altered by a factor of six. However the initial yields changed with temperature, and at the higher temperature ethylene was the main initial product whereas at the lower temperatures formaldehyde predominated. A value is given for the difference in activation energy between the reactions producing ethylene and formaldehyde. The kinetics of the oxidation at 362°C were investigated and the variation of the acceleration constant with oxygen, ethane, "inert" gas, and ethylene oxide pressure was determined. Ethylene oxidations were carried out at 318 and 362°C with mixtures of different composition and the products from "both the early and later stages of reaction were examined. The oxidation of acetaldehyde at 362°C was also investigated and a mechanism for the oxidation has been put forward. In the latter part of this thesis the experimental results were discussed. It appears that the degradation of the ethane molecule is a stepwise process involving the intermediate formation of ethylene and formaldehyde. It was concluded that the ethylene was formed by oxygen abstracting hydrogen directly from ethyl radicals, and that the formaldehyde was formed by isomerisation and decomposition of ethyl peroxy radicals. In the later stages of reaction the ethylene and formaldehyde are themselves oxidised, ethylene mainly to formaldehyde, and formaldehyde to the final oxidation products carbon monoxide, carbon dioxide, and water. A mechanism involving the intermediate formation of a cyclic peroxide has been proposed for the oxidation of ethylene to formaldehyde. As no experiments were carried out on the oxidation of formaldehyde the scheme proposed by Lewis and von Elbe for formaldehyde oxidation has been adopted. A reaction scheme, which can explain the formation of the products of ethane oxidation and which embodies the basic schemes of ethylene and formaldehyde oxidations, has been put forward for the oxidation of ethane.