Adsorbed O promotes alternative, nonselective oxametallacycle reaction pathways in Ag-catalyzed epoxidation

Ethylene oxide (EO) is a vital compound used as an intermediate in the production of other important compounds, such as ethylene glycol and glycol ether. EO is produced by selective ethylene oxidation (epoxidation) over supported Ag catalysts. Achieving high selectivity is the primary goal of research in this area, and understanding the factors that influence selectivity is thus critical for improving performance. The most widely accepted intermediate in EO production is the oxametallacycle (OMC). However, possible reactions between surface O and the OMC have not been comprehensively studied. In this work, density functional theory was used to systematically study the possible reaction pathways from the OMC in the presence of surface O. We find that surface O opens up two kinetically and thermodynamically favorable pathways that have received little or no attention in previous studies, neither of which form EO. Specifically, O-assisted C-H bond scission and the formation of ethylenedioxy are quite facile and predicted to be more favorable than the traditional EO (ring-closure) and acetaldehyde (H transfer) pathways. Thus, the predicted selectivity in the presence of coadsorbed O is very low, less than 0.1% at typical reaction temperatures. Furthermore, surface O has a similar effect on the propylene-derived OMC, which may have relevance to propylene oxidation. These results show the potential importance of surface O in influencing selectivity, as surface O greatly promotes these non-selective reactions and should therefore be minimized. These O-promoted reaction pathways should be considered in both design and kinetic modelling of EO catalysts.