New perspectives on direct heterogeneous olefin epoxidation

Important commercial direct oxidation processes include the epoxidation of ethylene to ethylene oxide and the newer epoxidation of butadiene to epoxybutene ( EpB), both carried out with silver catalysts. However, detailed reaction mechanisms for these processes are still matters of debate. The guiding hypothesis of our research is that surface oxametallacycles are key intermediates in selective olefin epoxidation. By a combination of surface science experiments and density functional theory (DFT) calculations, we have synthesized the first stable surface oxametallacycles and have verified their identities and structures. In the case of EpB chemistry, we have been able to demonstrate direct connections between surface oxametallacycles and epoxide products. The EpB ring opens with an activation energy of 8.4 kcal/mol on Ag(110) to form a stable surface oxametallacycle. Spectroscopic results for this species are in excellent agreement with DFT calculations for an oxametallacycle bound to three silver atoms of a seven-atom cluster. This oxametallacycle undergoes 1,2 and 1,4 ring-closure reactions during temperature programmed desorption to form EpB and 2,5-dihydrofuran, respectively. This reaction represents the first demonstration of surface oxametallacycle ring closure to form an epoxide, and we suggest that surface oxametallacycles are of general importance in silver-catalyzed olefin epoxidation.