Golden Propene Epoxidations

A potential environmentally friendlier propylene oxide synthesis catalyzed by gold clusters is being reported by Masa­take Haruta of Tokyo Metropolitan University and colleagues (Angew. Chem. Int. Ed. 2009, 48, 7862). The synthesis produces the epoxide by using O₂ as the oxidizing agent rather than Cl₂, which leads to chlorinated waste and by-products, or peroxides, which increase costs. The new approach could be a boon for industrial production of the feedstock chemical that is used for making polyurethane plastics and propylene glycol. Key to the reaction is the uniform size and shape of 1.8-nm gold clusters on a special titanium-silica support, as opposed to nonuniform gold nanoparticles, says Haruta, who first discovered that the secret to gold catalysis is particle size. In the reaction, gold clusters merge water and O₂ into a hydroperoxide species taken up by titanium. The resulting TiOOH group is the reaction partner with propene, inserting an oxygen atom into propene’s double bond to form the epoxide ring. Current performance of the reaction isn’t good enough for an industrial application, Haruta notes, but it’s better than copper and silver catalysts and should improve by refining the catalyst and controlling the reactivity of oxygen radicals on gold clusters.