Introductory Remarks

The global production of ethylene oxide (EO) currently exceeds 60B lbs/yr and generates approximately 50% of all economic value for catalytic oxidation reactions. Until the mid-1980’s the preferred catalyst was Cs-promoted Ag with 75 - 80% selectivity. Shell then introduced a new generation of catalysts with higher selectivity that contained high-valent oxyanions of Re (ReO₄) and other high-valent co-promoters such as MoO₄, WO₄, and even SO₄, to increase EO selectivities to the mid 80’s. Current improvements have raised EO selectivity to 90%. An important theme that has emerged from the study of Cs-promoted, Ag catalysts is the importance of atomic oxygen adsorbed on the Ag surface during reaction conditions that is considered to be electron deficient or “electrophilic-type” for the selective addition to the e-rich C=C moiety to form EO. Alternatively, an electron rich or “nucleophilic-type” Ag-O species preferentially abstracts a hydrogen atom from adsorbed ethylene in a non-selective reaction pathway to form CO₂ and H₂O.

In this presentation we describe characterization and performance of several series of Cs-Ag, Re-Ag, Cs-Re-Ag and Cs-Re-Mo-Ag catalysts, all supported on low surface area α-Al₂O₃, for ethylene epoxidation to determine the mechanism of selectivity enhancement for the current generation of high selectivity ethylene oxide (EO) catalysts. A key question is how Cs and Re, two very different classes of promoters, work in concert to markedly increase selectivity to EO. Analyses by XPS and SEM suggest that both Re and Cs promotions are electronic in nature. Finally, a modified reaction scheme detailing the mechanism of EO formation over Re-modified, Cs-promoted Ag catalysts is also presented.