Photoinduced Ethylene Polymerization on the CrVI/SiO2 Phillips Catalyst

Herein we report on two photo-induced reactions occurring at the surface of the CrVI/SiO2 Phillips catalyst. The first is the photo-reduction of CrVI/SiO2 in CO at room temperature, that leads to the reduction of a fraction of the CrVI sites to CrIV(=O) intermediates and then to CrII. With respect to the thermal reduction of CrVI/SiO2 in CO, the relative amount of highly uncoordinated CrII sites is larger, suggesting that the relaxation of the CrII sites at the silica surface strongly depends on the temperature at which they are formed. The second is the photo-induced ethylene polymerization on CrVI/SiO2 at room temperature that is reported here for the first time. We demonstrate that UV-Vis light has the potential to trigger the reduction of chromates by ethylene and the successive ethylene polymerization. DR UV-Vis and FT-IR spectroscopies allowed us to identify the nature of the Cr sites involved in the polymerization (i.e. 6-fold coordinated CrII) and of the oxidized by-products remaining in their coordination sphere, which are methylformate and ethylene oxide. While the former was previously observed during the thermally-induced ethylene polymerization on CrVI/SiO2, the latter was never observed.