Olefin epoxidation is an important reaction for making a range of industrial intermediates in processes for epoxy coatings, polyurethanes, polyester textiles, surfactants, specialty chemicals, pharmaceuticals and others. Conventional olefin epoxidation involves organo-peroxides or organo-halogen oxidants, which present safety and environmental hazards. Water would be a safer, more environmentally attractive oxidant, but activating water molecules to donate their oxygen is chemically difficult.
A new approach to olefin epoxidation, involving visible-light irradiation and electrochemistry was described by a research team from the University of Illinois Urbana-Champaign (UIUC; www.illinois.edu) in a study published in a recent issue of the Journal of the American Chemical Society. The study identifies a new route to activating water molecules to allow them to serve as the oxidant in epoxidation reactions. The UIUC team’s approach would dramatically reduce greenhouse-gas emissions and eliminate the need for hazardous oxidants in olefin epoxidation.
Source: UIUC
To effectively activate water molecules, the team designed light-absorbing, nanoscale structures made from gold nanoparticles, combined with nanowires of the water-oxidation electrocatalyst manganese dioxide. These hybrid nanostructures harness the phenomenon of localized surface plasmon resonance (LSPR), induced by visible-wavelength photons, to produce energetic electrons that facilitate breakage of H–O–H bonds in water. UIUC chemistry professor and laboratory leader Prashant Jain explains: “Gold has a large free-electron density. When a gold nanoparticle is irradiated by visible light, the electromagnetic field of light sets in motion a collective oscillation of this sea of free electrons. This is known as LSPR.”
LSPR concentrates absorbed light energy and electric fields into a nanoscale volume, and this concentrated energy and electric field extracts electrons from water molecules, effectively inducing water oxidation without requiring high temperatures, Jain adds. The oxygen atoms that are produced by the plasmon-assisted excitation then attack carbon-carbon double bonds of an olefin (styrene, in this case) to form an epoxide.
The UIUC research team reported that visible-light irradiation of the electrocatalyst enhanced the epoxidation of the styrene molecule they used in their study as a model, as compared to that of non-irradiated conditions at the same temperature.
“This work demonstrates a proof of concept and establishes the mechanistic basis for plasmon-assisted activation of water as an oxygen atom source for electrochemical epoxidations,” the researchers write.