Heterogeneous epoxidation on silver catalysts

The silver-catalyzed oxidation of ethylene was first reported in patents by Lefort in 1931 and 1935 [320,321] which, interestingly, also described the effectiveness of gold, iron, and copper promoters. These are elements that have been rediscovered in recent studies [322,323] (see Chapters 8 and 9). Due to the importance of the commercial process, which has seen phenomenal growth since 1940, the mechanism of the reaction has been investigated extensively. 

Molecularly adsorbed oxygen results from nondissociative adsorption and is more weakly held. 

Thus for every six EO molecules formed, one C2H4 molecule will be combusted and the maximum selectivity to EO should be 6/7 or 85.7% [324]. This so-called 6/7 rule has been violated in a number of cases [334] leading to the conclusion that molecular oxygen is not the active species. Moreover, there have been considerable studies indicating that atomic oxygen can form EO [335]. 

In the steps above, the symbol indicates empty sites or adsorbed intermediates, the symbol indicates a rate-determining step, and the symbol reversible steps. The mechanism involves dissociative adsorption of oxygen, adsorption of ethylene, a surface reaction between adsorbed ethylene and adsorbed oxygen to form the species CH2CH2O, which denotes the oxametallacycle, and finally a reaction/desorption step where the oxametallacycle forms ethylene oxide which is released to the gas phase. 

The studies on the oxametallacycle intermediate have led to theoretical work with DFT that predicts that Cu-Ag alloys will have enhanced activity [342] (see Chapter 8), and this has been experimentally verified [323,343], validating the initial reports by Lefort [320,321]. 

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