Gas-phase propene epoxidation over coinage metal catalysts

Propene oxide (PO) is a very important bulk chemical and is produced on a scale of about 7.5 million tons per year. In industry, PO is produced via multiple reaction steps in the liquid phase, using hazardous chlorine or costly organic hydroperoxides as oxidants. Accordingly, development of a simple and green process to produce PO has been desired. This paper presents an overview of one-step propene epoxidation in the gas phase over coinage metal catalysts with a mixture of O-2 and H-2 or with molecular O-2 alone as oxidant. Silver (Ag) and gold (Au) catalysts can catalyze propene epoxidation with a mixture of O-2 and H-2, with high selectivity, whereas copper (Cu) catalysts cannot. In this reaction Au catalysts are much more active than Ag catalysts. All the coinage metals can catalyze propene epoxidation by molecular O-2, but with selectivity usually below 60%. The valence states of Cu species and the sizes of Ag particles and Au particles are of crucial importance in PO synthesis.