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

We present a X- and Q-band 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 °C, which is correlated to the disappearance of Cr(V) sites originally present in the catalyst. These species are characterized by unusually large ZFS. No EPR active species are formed when ethylene is reacted at room temperature with Cr(II)/SiO2, despite clear evidences of a consistent ethylene polymerization activity.