Rationalizing the Effect of Triethylaluminum on the Cr/SiO2 Phillips Catalysts

In contrast to most of the other olefin polymerization catalysts, the Cr/SiO2 Phillips catalyst does not need any activator to develop its activity. However, it is known that the addition of a small amount of metal-alkyls has drastic effects on the catalyst performance, and it is one of the strategies adopted in industrial practice to tailor the properties of the polyethylene produced. In this work, we conducted a systematic investigation on the effect of triethylaluminum on the Cr(VI)/SiO2 catalyst, with the ultimate goal to determine the properties of the Cr sites at a molecular level and to correlate them to the properties of the obtained polyethylene. To this aim, we coupled kinetic tests, spectroscopic methods (diffuse reflectance UV-vis-near-infrared, electron paramagnetic resonance, Fourier-transform infrared spectroscopy of probe molecules), and different polymer characterization techniques. We found that, at an Al/Cr ratio of 2:1, only ca. 50% of the original Cr(VI) sites are reduced to a variety of species comprising: (1) Cr(IV) bisalkyl sites, which are probed by carbon monoxide and are the major actors in the polymerization of ethylene, explaining the faster polymerization initiation rate; (2) 6-fold coordinated Cr(III) sites, which are not accessible by probes and whose amount does not correlate with the catalyst activity; and (3) two types of monografted Cr(II) sites, all deriving from the over-reduction of the Cr(IV) bisalkyl sites, having, respectively, weak and strong Lewis acid characters, the former responsible for in situ α-olefin generation (and hence contributing to the enchainment of short polymer branching) and the latter accounting for the formation of the high-molecular-weight polymer fraction. Overall, our results demonstrate that the catalytic performance of the Cr sites depends not only on the oxidation state but rather on a combination of molecular structure, acidic character, and nature of the ancillary ligands. Hence, by tuning the structure and acidity of the Cr sites, it is possible to move from a system that essentially affords oligomers from ethylene to one that affords high-molecular-weight polymers.