US chemists have developed a greener, higher yielding and more selective route to propylene oxide, an important industrial chemical that is manufactured worldwide on a million tonne scale.

Propylene oxide is a vital building block for making many other compounds and materials, such as surfactants and foams. However, the two routes used to manufacture it in bulk involve environmentally unfriendly peroxides and chlorinated materials to oxidise propylene, and produce undesirable waste products.

Qunlai Chen and Eric Beckman at the University of Pittsburgh have devised a one-pot route that uses a titanium-modified zeolite catalyst to prepare the oxidant hydrogen peroxide in situ from hydrogen and oxygen. Additionally the reaction solvent is environmentally benign liquid or supercritical carbon dioxide containing small quantities of methanol and water.

According to Beckman, many of the common side reactions of the process (such as propylene hydrogenation or propylene oxide hydrolysis) can be effectively suppressed by using an ammonium acetate additive to neutralise the surface acidity of the catalyst. 

Propylene conversion rates are normally limited to less than 10% in order to maintain the required product selectivity. Using the one-pot route, Beckman obtains yields of over 21% with an 82% selectivity for the first time.

Yasutaka Ishii, an expert in hydrocarbon oxidation reactions using hydrogen peroxide, based at Kansai University, Osaka, Japan, considers this method to be a very interesting green route for making propylene oxide, pointing out that no harmful waste is produced.

David Parker