Further reactions of ethylene oxide over oxide supports have been investigated in the context of the oxidation of ethylene to ethylene oxide using supported silver. As noted by others, high area aerosil type silicas have low activity for the isomerisation of ethylene oxide to acetaldehyde and silver supported on such materials exhibits good selectivity for ethylene epoxidation. By contrast silver supported on TiO2 and γ-Al2O3 shows zero selectivity and this correlates with the high activity of these supports for further reactions of ethylene oxide. In the case of γ-Al2O3 there is complete conversion of ethylene oxide at 239°C in the presence or absence of oxygen. Acetaldehyde is the major organic product in both cases but dioxane and small amounts of 2,3-butanedione, ethyl acetate and an unidentified compound, probably an unstable C4, are also observed. With oxygen present some carbon oxides are seen. In the absence of oxygen the amount of the unknown is greater and the gaseous mass balance is incomplete. With TiO2 the distribution amongst the major products is similar to that for γ-Al2O3 but the conversion of ethylene oxide is much less. In situ infrared measurements with TiO2 show that ethylene oxide reacts instantaneously with surface hydroxyl groups to form a glycol like species. Reaction of acetaldehyde over TiO2 and γ-Al2O3occurs at a much slower rate than that of ethylene oxide under the same conditions. The dominant product is 2-butenal. Thus the C4 products formed when using ethylene oxide do not arise by further reaction of acetaldehyde. Formation of the major organics from both ethylene oxide and acetaldehyde can be accounted for in terms of catalysis by weak Brønsted acid sites. Silicas have little activity for the further reaction of acetaldehyde but silver supported on them induce oxidation of it. It is concluded that the zero selectivity of silver supported on TiO2 and γ-Al2O3 for ethylene epoxidation is due to isomerisation of ethylene oxide to acetaldehyde on the support followed by complete oxidation on the silver.
- acid sites
- catalyst characterization (in situ IR)
- ethylene oxidation
- ethylene oxide