(603f) Selective Epoxidation of Cyclooctene Using Molecular Oxygen over Small Au Clusters

Epoxides are key intermediates in the fine chemical industry and are also important in the resin and pharmaceutical industries.  Currently epoxides are formed using oxidants such as hydrogen peroxide or organic peroxides that are expensive and/or environmentally not friendly.  Molecular O₂ is the most desired oxidant for the formation of epoxides but the only known large scale commercial process that uses this readily available resource is the epoxidation of ethylene over Ag catalyst.   Hutchings and coworkers has shown that a catalytic amount of radical initiator, such as tert-butyl hydroperoxide,  could promote the direct epoxidation of olefin using O₂  over nanoparticles of Au supported on graphite.¹  They proposed that the role of the initiator is to counteract the negative effect of additives found in commercial olefins to deter autoxidation of olefins.  After removal of the additive from cyclooctene, no initiator was needed for the direct utilization of O₂, and 5.6% cyclooctene was converted in 24h.²  However, Donoevea et al.³ observed trace amounts of cyclohexenyl hydroperoxide in the stabilizer free cyclohexene.  Here we report that Au based catalyst that can utilize molecular O₂ to catalyze epoxidation of cyclooctene obtained from a commercial source containing stabilizer additive  and without the addition of radical initiator. The Au catalysts were prepared using formed, stabilized small Au clusters, as Au clusters smaller than a critical size have shown to be active in oxidation reactions.⁴  It was found that the rate of generation of the Au clusters and the type of ligands used to stabilize the clusters have significant effect on the rate of cyclooctene conversion, but not on the selectivity for epoxide formation.  The best catalyst can effect over 85% conversion of cyclooctene with over 80 % selectivity to the epoxide in seven hours.   Fluorescence spectroscopy shows the presence of a species that emit at 427 nm, which according to the jellium model,⁵ corresponds to Au clusters around 6-7 atoms.

References

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