Air- and Oxygen-based Industrial Processes

The air-based process was the first to be commercialized in 1937, some years after the discovery of the reaction by Lefort in 1931. The oxygen-based process introduced in 1958, was however more competitive and has become increasingly preferred for new installations and for the revamping of earlier ones. It is now estimated that the use of air is only competitive for production capacities of 20 kt/a or less, for which an oxygen separation unit would be too expensive. 

The oxidation mechanism is still being debated, with the discussion centred on the nature of surface species responsible for the different reaction paths. It is generally agreed however that at least three sorts of oxygen species are 

In early proposals the species responsible for epoxidation was identified as the adsorbed molecular oxygen, Ag 02(ads) while combustion was attributed to monoatomic Ag O(ads) (Equations 14-16). The oxidation step envisages the transfer of one atom of molecularly adsorbed oxygen to the double bond, while the other remains adsorbed on silver. The consumption of the latter by the total oxidation of ethylene restores the site vacancies necessary for the continuation of catalysis. Up to a maximum of six oxygen atoms are required for the combustion of one ethylene molecule. Thus, the combination of the reactions (Equation 14) and (Equation 15) predicts that the maximum attainable selectivity in the epoxidation of ethylene is 6/7, i.e., 85.7% (Equation 16). A lower selectivity should normally be expected because some monoatomic oxygen independently formed by dissociative adsorption (Equation 13) raises the level of ethylene combustion above that predicted by Equation 16. 

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