Ligands Make the Difference! Molecular Insights into Cr^VI/SiO₂ Phillips Catalyst during Ethylene Polymerization

Abstract

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 disproportionation of two formaldehyde molecules). Unreduced Cr^VI species are also present during ethylene polymerization as well as reduced Cr species (either Cr^II or Cr^III) acting as spectators. Our results challenge the old vision of "naked" chromium species (i.e., low coordinated) as the active sites and attribute a fundamental role to external (and flexible) oxygenated ligands that resemble the ancillary ligands in homogeneous polymerization catalysis.