Ethene epoxidation over silver catalysts in the presence of carbon monoxide and hydrogen

Abstract

The epoxidation of ethylene over silver catalysts in the presence of added carbon monoxide and hydrogen has been studied as a possible method for directly using ethylene in streams generated by partial oxidation of methane. When tested separately over a silver sponge catalyst the oxidation of both carbon monoxide and hydrogen are more than one order of magnitude faster than that of ethylene. Measured activation energies are 44, 43 and 61 kJ/mol respectively. Under conditions of moderate ethylene conversion (ca. 40% at 239°C) the effect of including carbon monoxide or hydrogen in amounts up to 20% of the ethylene is quite small. The added material is totally oxidised in the initial section of the catalyst bed leaving the remainder available for ethylene epoxidation. Neither ethylene conversion nor ethylene oxide yield is affected significantly. The situation is somewhat different at lower conversions over chlorine moderated catalysts. Added carbon monoxide is then incompletely oxidised and the fraction emerging inhibits ethylene oxidation by direct competition for the silver surface. Conversion is reduced and the selectivity to ethylene oxide is also reduced under ethylene-rich conditions. Under similar conditions hydrogen is also incompletely oxidised but it inhibits less. A particular effect was observed when carbon monoxide was included in the feed for long periods (>10 h). Ethylene conversion fell steadily while the selectivity to ethylene oxide rose. The effect was very similar to that noted when experiments were commenced with 1,2-dichloroethane at the ppm level in the feed. It is attributed to small quantities of an impurity in the carbon monoxide since it disappeared if that stream was purified.