Electrophilic property of image photoformed on isolated Ti species in silica promoting alkene epoxidation

We previously reported that photoformed image species on highly dispersed titanium oxide species are the active oxygen species for the epoxidation of propene. In the present study, it was found that this epoxidation system could be extended to other light alkenes such as ethene and butene, and the electronic structure of the image species and reaction mechanism of the insertion of atomic oxygen into the Cdouble bond; length as m-dashC bond in olefin were revealed. An isotope-labeled reaction test with 18O2 and ESR measurement with 17O2 clarified the following mechanism: the image species is generated by the reaction between an O2 molecule and a photoformed hole center on lattice oxygen (image), and the photoformed hole in the image moves from the lattice oxygen (image) to the O2 moiety derived from molecular oxygen. The image species can be also described as a surface complex of image (electronic neutral) and adsorbed image. This electron-deficient oxygen moiety derived from molecular oxygen could preferably react with the electron rich part of alkene, the Cdouble bond; length as m-dashC bond, to produce epoxide.