Operando-sensitive spectroscopic techniques were employed for investigating the changes in the molecular structure of the Cr sites in the Cr/SiO 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 species are also present during ethylene polymerization as well as reduced Cr species (either Cr or Cr) 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.

In this work, we summarize and critically compare some of the experimental results recently published on the Phillips catalyst, in the attempt to make the point on a few particularly debated questions that have recently animated the specialized literature; in particular, we discuss the structure of the active chromium sites and how ethylene polymerization initiates on them. The data collected in this article unequivocally demonstrate that the structural and electronic properties of the chromium sites strongly depend on the strain of the silica surface, which in turns is affected by both the activation treatment and the chromium loading. This explains, at least partially, the differences of results obtained in different research groups. Another fundamental message is the need of applying the largest possible set of characterization methods, including theoretical calculation on large and flexible models. Our final purpose (and hope) is to promote a positive and constructing discussion on this catalyst, as a premise to create a solid scientific base useful to both the young researchers approaching this field and the industrial researchers who daily work with it.

We present an X-and Q-band electron paramagnetic resonance (EPR) investigation of a 0.5 wt % Cr/SiO 2 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)/SiO 2 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 zero field splitting (ZFS). No EPR active species are formed when ethylene is reacted at room temperature with Cr(II)/SiO 2 , despite clear evidence of a consistent ethylene polymerization activity.