Chapter 7 - Investigation of the Origins of Selectivity in Ethylene Epoxidation on Promoted and Unpromoted Ag/α-Al2O3 Catalysts: A Detailed Kinetic, Mechanistic and Adsorptive Study

Abstract

The reasons for silver being unique in its ability to oxidise ethylene to ethylene oxide (EO) in high selectivity are examined by critically reviewing the literature accumulated over 40 years and by reporting some new, as yet unpublished, data. It is concluded that silver's uniqueness resides in the strength of its bond to adsorbed O atoms. It is shown that the weaker the Ag–O bond is, the more selective it becomes. The role of Cl promoters is shown to weaken the Ag–O bond. In an industrial Ag/a- Al2O3 catalyst containing Cs, the Cs is held in submonolayer quantities on steps on the Ag surface where it prevents the formation of a strong, unselective Ag–O bond. The selective and unselective reactions derive from two nearly equivalently energetic reaction pathways of a common intermediate. It is concluded here that this intermediate is a CH2–CH2–O–Ag species. This undergoes cyclisation to form EO or another form of cyclisation to form an oxametallacycle which produces acetaldehyde and ultimately CO2. Both reactions are concluded to occur on a nearly completely oxidised Ag surface under industrial reaction conditions. Subsurface oxygen is concluded to play no role in promoting the selectivity of the Ag–O surface state and is unselective of itself. It is considered doubtful that subsurface oxygen is present under industrial reaction conditions.