Illuminating Copper Promotes Propylene Oxide Production

Propylene oxide is a commodity chemical used to make polyurethane plastics. Commercial processes for epoxidation of propylene to propylene oxide use hazardous reagents that chemists would like to avoid. One prospect for a catalyst is metallic copper, which shows high selectivity for forming propylene oxide in a vacuum. However, the copper tends to oxidize and lose selectivity under commercial reaction conditions. Looking for a way to preserve the metallic copper surface, Andiappan Marimuthu, Jianwen Zhang, and Suljo Linic of the University of Michigan demonstrated that exposing the catalyst to visible light can solve the problem (Science, DOI: 10.1126/science.1231631). Working with copper nanoparticles deposited on silica in a packed-bed reactor, they found that illuminating the catalyst with a broadband visible light source increases selectivity for propylene oxide from 20% to 50%, although current processes that use acid catalysts or more expensive gold catalysts may approach 90% selectivity. Marimuthu and colleagues believe the copper atoms still oxidize on the nanoparticle surfaces. But light-induced localized surface plasmon resonance of the metallic copper core causes electron transfer to Cu–O bonds, weakening them and promoting reduction back to metallic copper. The researchers suggest the approach could be used to tune the oxidation state of other nanoparticle catalysts with a plasmonic core, such as gold or silver.