Why silver is the unique catalyst for ethylene epoxidation

The activities of Cu2O(0 0 1), Ag2O(0 0 1), and Au2O(0 0 1) surfaces for direct ethylene epoxidation and alternative paths for EO isomerization are studied. Among these three oxide surfaces, only Ag2O(0 0 1) surface enables direct path without a barrier. Au2O cannot regenerate surface oxygen, and overall reaction on Cu2O is endothermic. Furthermore, ring opening of ethylene oxide (EO) and subsequent acetaldehyde (AA) formation on Cu2O is more favorable than EO desorption. Ethylene adsorption on an oxygen vacancy results in the oxametallacycle (OMC) formation, which causes AA formation and reduces EO selectivity. Cl adsorption removes these surface vacant sites and hence prevents the formation of the OMC intermediate.

The difference in selectivity of ethylene epoxidation of Ag, Cu, and Au relates to the different reactivity of their oxidized surfaces. Au2O is predicted to have the highest selectivity for ethylene oxide (EO) formation; however, catalytically it cannot be applied because O2 will not regenerate the oxide. Cu2O is found to activate ring opening EO with subsequent acetaldehyde formation. The latter barrier is higher than the desorption energy of EO on Ag2O.Figure optionsDownload high-quality image (40 K)Download as PowerPoint slideHighlights
► Direct ethylene epoxidation is possible through 2-fold oxygen in .bridge positions.
► Non-selective reaction proceeds through OMC intermediate that forms on oxygen vacant sites.
► Cl promoter effect blocks vacant sites and therefore has a steric effect.