Ethylene Oxidation on Silver Catalysts: Effect of Ethylene Oxide and of External Transfer Limitations

Abstract

Ethylene oxidation to ethylene oxide and to carbon dioxide over silver catalysts was studied in a CSTR. The effects of two factors on the catalyst performance were examined. The first was the presence of excess ethylene oxide in the feed. A kinetic model was introduced which assumed that ethylene and ethylene oxide compete for the same sites on the catalyst surface. This model provided reasonable quantitative agreement with kinetic and potentiometric measurements. The second factor that was studied was the presence of external heat and mass transfer limitations. It was found that such limitations cause a significant decrease of the selectivity to ethylene oxide. This decrease is a result of the temperature difference between the catalyst surface and the bulk of the gas phase and of the fact that the activation energy of ethylene combustion is greater than that of ethylene epoxidation. The contribution of other factors such as inhibition by CO, or possible incomplete mixing in the reactor is shown to be insignificant.

Ethylene Epoxidation on Ag-Cs/α-Al2O3Catalyst: Experimental Results and Strategy for Kinetic Parameter Determination

“…In the data treatment, the oxidation of ethylene oxide was neglected. As established in previous studies, 12,17 the rate of reaction 3 is expected to be much smaller than that of reactions 1 and 2 under our experimental conditions (i.e., low ethylene conversions and no ethylene oxide in feed) because of competitive adsorption of other components on the active surface.…”

“…14,15 The same effect is also reported for oxygen partial pressure. 16 In addition, several studies have shown that all product species can inhibit both reactions, [16][17][18][19] but the reported results are sometimes contradictory.…”

Influence of reaction products on the selective oxidation of ethene

“…In Ref. [2], an inhibition of the epoxidation rate was observed and a slightly higher combustion rate was obtained which was explained by the decomposition of ethene oxide. In Ref.…”

“…In Refs. [2] and [g-16], a mechanism is suggested in which molecular oxygen is the active component for epoxidation. However, on the basis of theoretical considerations [3,17] and studies involving special techniques such as IR spectroscopy, temperature-programmed desorption (TPD) and temporal analysis of products (TAP) [7,, and because the selectivities have been reported to be above 85.7% [16,22], mechanisms based on one single adsorbed state of atomic oxygen in both reactions are favoured.…”

“…[26]. Later [2,27], the epoxidation reaction was observed to be inhibited by C02, but the total combustion reaction was unaffected. In Refs.…”