The Effect of Al-Alkyls on the Phillips Catalyst for Ethylene Polymerization: The Case of Diethylaluminum Ethoxide (DEALE)

Al-alkyls are often used in the industrial practice for modifying the Phillips catalysts for polyethylene production: they are not necessary to develop the activity, but they have relevant effects on the catalysis, decreasing the induction time, promoting the in situ branching, and enhancing the H-2 sensitivity for the molecular weight regulation. Herein we investigate the effect of diethylaluminum ethoxide (DEALE) on Cr(II)/SiO2 (in a stoichiometric amount of Al:Cr=2:1), focusing the attention on the modification of the Cr(II) sites at a molecular level. Diffuse reflectance UV-Vis and FT-IR spectroscopies, applied in the presence of CO and CD3CN as molecular probes, unequivocally demonstrate that: (1) DEALE modifies only a fraction of the Cr(II) sites (ca. 30%) even if dosed in excess with respect to the Cr sites; (2) DEALE reacts with the silica surface, forming Al-grafted species which are at least partially in interaction with the Cr(II) sites and hence act as ancillary ligands; (3) the modified Cr(II) sites are more acidic and likely mono-grafted to the silica surface; the presence of Al-grafted species nearby is essential for their stabilization. Finally, kinetic experiments indicate that the modified Cr(II) sites are ca. 60 times faster in inserting ethylene than the unmodified Cr(II) sites. The intrinsic higher activity of the modified Cr(II) sites is a consequence not only of their molecular structure, but of the whole series of effects listed above.