Direct oxidation processes, such as ethylene epoxidation to ethylene oxide, have been researched extensively due to the uniqueness of this process and to the commercial significance of the epoxide product. Despite the importance of epoxide production, the mechanistic details of olefin epoxidation remained obscure until recently and the majority of significant advances in Ag catalyst development occurred primarily through empirical methods. Recent surface science studies of ethylene oxide on Ag(111) have identified an oxametallacycle as the active intermediate in ethylene epoxidation; expansion of the epoxide ring to incorporate surface silver atoms forms the oxametallacycle species. Oxametallacycles have been isolated by ring-opening ethylene oxide on Ag(111) and Ag(110), 1-epoxy-3-butene on Ag(111) and Ag(110), styrene oxide on Ag(111) and Ag(110) and isoprene oxide and propylene oxide on Ag(110).
Surface science techniques and Density Functional Theory (DFT) were used in this study to investigate the interactions of styrene oxide and isoprene oxide with the Ag(110) surface, as well as the interactions of ethylene oxide and propylene oxide with the clean and O-covered Ag(110) surfaces.
TPD experiments demonstrate that the styrene oxide ring opens at the substituted carbon, and DFT calculations indicate that the phenyl ring of the resulting oxametallacycle is oriented nearly parallel to the Ag(110) surface. Interaction of the phenyl group with the silver surface stabilizes this intermediate relative to that derived from the mono-olefin epoxide, ethylene oxide. During TPD, the oxametallacycle undergoes ring closure to reform styrene oxide and isomerization to phenylacetaldehyde at 505 K on Ag(110). Styrene oxide-derived oxametallacycles exhibit similar ring-closure behavior on the Ag(111) surface.
Isoprene oxide also forms a strongly bound oxametallacycle intermediate on the Ag(110) surface. The oxametallacycle undergoes ring-closure to reform isoprene oxide in two peaks at 320 and 460 K when synthesized by epoxide adsorption at low temperatures. Epoxide doses at higher surface temperatures (ca. 300 K) lead to isomerization of the oxametallacycle and desorption of the aldehyde isomer, 2-methyl-2-butenal, in a single peak at 460 K. This work represents the first demonstration of a surface oxametallacycle species derived from an allylic epoxide. Similar to oxametallacycles derived from the non-allylic epoxides, isoprene oxide ring opens at the carbon bound to the unsaturated vinyl and methyl substituent groups to form a linear oxametallacycle on the Ag(110) surface. The structure of the isoprene oxide-derived oxametallacycle resembles that formed from ring-opening its non-allylic counterpart, 1-epoxy-3-butene, on Ag(110), according to DFT calculations.
Following adsorption at 250 K on both clean and O-covered Ag(110), ethylene oxide ring-opens to form a stable oxametallacycle. On the clean Ag(110) surface, the oxametallacycle reacts to reform the parent epoxide at 280 K during TPD, while the aldehyde isomer, acetaldehyde, is observed at higher oxametallacycle coverages. In the presence of coadsorbed oxygen atoms, a portion of the oxametallacycles dissociate to release ethylene. However, of those that react to form oxygen-containing products, the fraction forming ethylene oxide is similar to that on the clean surface. The acetaldehyde product of oxametallacycle reactions combusts via formation of acetate species; the acetates react to form CO2 at temperatures as low as 360 K on the O-covered surface. No evidence was observed for other combustion channels. This work provides experimental evidence for the connection of oxametallacycles to combustion via acetaldehyde formation, as well as to ring-closure to form ethylene oxide.
Adsorption of propylene oxide at 120 K with subsequent flash of the surface to 230 K prior to TPD leads to the formation of a stable oxametallacycle on both the clean and O-covered Ag(110) surfaces; the oxametallacycle then undergoes ring-closure to reform propylene oxide at 250 K during TPD. The selectivity to propylene oxide decreases at higher oxametallacycle coverages where acetone and allyl alcohol are formed; carbon dioxide is also formed near 250, 350, and 450 K from reactions of the oxametallacycle and its products on the O-covered surface. At higher oxametallacycle coverages, additional higher temperature desorptions are observed at 300 and 400 K; the product distributions in these peaks are the same as that in the 250 K peak. Similar to reactions of ethylene oxide on the O-covered Ag(110) surface, the same fraction of oxametallacycle species reacts to form propylene oxide in the presence of co-adsorbed oxygen as that on the clean surface, although the coverage of oxametallacycles is 10-20% higher on the O-covered surface. The principal effects of co-adsorbed oxygen are to increase the capacity of the surface and to open an additional combustion pathway to CO2. The oxametallacycle undergoes combustion through a propionaldehyde species. This work represents the first time that an oxametallacycle has been linked to propylene oxide production.