Figure 1. A bright light prevents oxygen from binding to copper, spurring formation of propylene oxide. Source: Joseph Xu, University of Michigan.
Propylene oxide production traditionally involves a complex chain of reactions that generate unwanted chemicals. A catalyst that enables formation of propylene oxide in a direct reaction, avoiding the byproduct, has been long sought. Metallic copper, considered to be the best potential for a direct reaction, unfortunately binds with oxygen, restricting its ability to act as a catalyst."Copper in metallic form has this unique electronic structure that activates the reaction pathway for propylene oxide more than the undesired pathways," says Marimuthu Andiappan, a graduate student in chemical engineering, who worked on the project.Metallic copper prefers to bind oxygen with two of the propylene's carbon atoms, forming propylene oxide, while copper oxide tends to break the propylene down into CO2
, or creates acrolein. However, the researchers found that if copper is structured in a certain way, light can reverse its oxidation. Copper nanoparticles about 40 nanometers across were peppered with tiny particles of clear silica, and exposed to propylene and oxygen gasses.Under intense white light — five times the intensity of sunlight — the copper remained in a metallic state and converted 50% of the propylene into propylene oxide, compared to only 20% when not exposed to the light. The oxidized surface helped concentrate the light, which freed electrons from copper atoms, thus breaking the bonds between the copper and oxygen. More details appear in an article in Science
.The researchers say the next step is to create a reactor that can illuminate the catalyst for large-scale production. "Theoretically, it is possible to use mirrors to focus sunlight and get this much intensity," notes Andiappan. "This might give us some direction to make a commercial catalyst for the propylene oxide process," he adds. The university is currently seeking commercialization partners to help bring the technology to market."We are just scratching the surface," Linic says. "I can envision many processes that wouldn't be possible with conventional strategies, where changing the oxidation state during the reaction or driving reactions with light could affect the outcome dramatically."