Science 328, 224–228 (2010)

The epoxidation of ethene — which involves the conversion of a carbon–carbon double bond into a ring that contains two carbon atoms and an oxygen atom — is an important industrial reaction that, since the 1930s, has been efficiently carried out using a silver catalyst. However, when the same catalyst is used to epoxidize propene — an industrial reaction of even greater significance — the process is very unselective, despite the fact that the two reactants differ only by a single methyl group (–CH3). As a result, the large-scale production of propene oxide currently involves an inefficient, two-stage process that can create undesirable by-products. Larry Curtiss, Stefan Vajda and colleagues at Argonne National Laboratory, the University of Illinois in Chicago, the Fritz Haber Institute in Berlin and Yale University, have now developed a catalyst containing three-atom clusters of silver that is active towards the epoxidation of propene.

The catalysts were prepared by depositing mass-selected clusters from a molecular beam onto an alumina support. The supported silver trimers are stable up to temperatures of about 110 °C, above which they start to agglomerate, and the researchers used this effect to prepare nanoparticles with sizes of around 3.5 nm. Both the clusters and the nanoparticles catalyse the direct epoxidation of propene by molecular oxygen, returning high selectivities towards propene oxide at low temperatures.

Density functional calculations suggest that the superior catalytic capabilities of these silver catalysts are due to the open-shell nature of their electronic structure.