In Situ X- and Q-Band EPR Investigation of Ethylene Polymerization on Cr/SiO2 Phillips Catalyst

We present an X- and Q-band electron paramagnetic resonance (EPR) investigation of a 0.5 wt % Cr/SiO2 Phillips catalyst, aimed at establishing the Cr oxidation and spin state under different ethylene polymerization conditions. Our data indicate the formation of different Cr(III) species upon reaction of Cr(VI)/SiO2 with ethylene at 150 degrees C, which is correlated to the disappearance of Cr(V) sites originally present in the catalyst. These species are characterized by unusually large zero field splitting (ZFS). No EPR active species are formed when ethylene is reacted at room temperature with Cr(II)/SiO2, despite clear evidence of a consistent ethylene polymerization activity.