Ligands Make the Difference! Molecular Insights into CrVI/SiO2 Phillips Catalyst during Ethylene Polymerization

Operando-sensitive spectroscopic techniques were employed for investigating the changes in the molecular structure of the Cr sites in the Cr-VI/SiO2 Phillips catalyst during ethylene polymerization. Practically, the most arduous barrier to be overcome was the separation of the chromates reduction carried out by ethylene from the subsequent polymerization. By carefully tuning the experimental parameters we succeeded in observing these two events separately. We found that the sites involved in ethylene polymerization are mainly divalent Cr ions in a 6-fold coordination, in interaction with the oxygenated byproduct (mostly methylformate, generated from the dis-proportionation of two formaldehyde molecules). Unreduced Cry(VI) species are also present during ethylene polymerization as well as reduced Cr species spectators. Our results challenge the old vision of naked chromium species (i.e., low coordinated) as the active attribute a fundamental role to external (and flexible) oxygenated ligands that resemble the ancillary ligands in homogeneous polymerization catalysis.