(720f) the Role of Au in Au-Catalyzed, Solvent-Free, Selective Epoxidation of Cyclooctene with Molecular Oxygen
Au catalyzes solvent-free epoxidation of cyclooctene with molecular oxygen with a high selectivity (>85%) for cyclooctene oxide. The catalytic effect is found to be independent of the Au precursor tested, which include Au/SiO2, AuCl, and AuCl3. There is an induction period before the reaction proceeds with a significant rate, and the catalytic significant species is solubilized Au that is formed under reaction conditions. The solubilized Au exhibits strong fluorescence in the UV-vis region, and is present in concentrations around 10-100 ng/mL. The fluorescence peak maxima are in the range around 400 nm, and shift depending on the concentrations of Au, reaction products, and other intentionally added chelating agents. The solubilized Au is present as atomic clusters mostly less than 10 atoms in size. These clusters are in a dynamic equilibrium with solubilized Au atoms, and the size distribution depends on the concentration of various species in solution. The solubilized Au is essential in initiating the reaction. After initiation, the reaction is dominated by the chain reaction of homogeneous hydrocarbon autooxidation, resulting in an apparent turnover frequency based on solubilized Au as high as ~3000 s-1.