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

Abstract

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)/SiO 2 (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 CD 3 CN 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. 15 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.

Tuning the Redox Chemistry of a Cr/SiO2 Phillips Catalyst for Controlling Activity, Induction Period and Polymer Properties

“…This at least excludes the presence of exclusively naked Cr 2 + ions and indicates that reduction by-products remain in the proximity of the Cr active sites, hereby affecting the d-d transition and CT bands. [16,[57][58][59][60][61][62][63] In order to ensure a proper fit, an additional band was required at 24000 cm À 1 , which is expected to be a CT band. This 24000 cm À 1 CT band, which is absent in the CO reduced material, can be explained by reduction by-products remaining in the coordination sphere of the Cr active site: affecting the location the d-d transition bands and the CT bands.…”

The Active Sites in the Phillips Catalysts: Origins of a Lively Debate and a Vision for the Future

“…This is the case, for example, of the Cr­(II) sites modified by AlR 3 and similar activators. We have recently demonstrated that most of the chromium sites modified by diethylaluminoethoxide are not reached by CO, likely because they are shielded by the modifier and/or by some reaction byproducts . In these cases, a possible solution is to make use of stronger probes, which are molecules interacting strongly with the chromium sites (such as acetonitrile).…”

“…Not only hydrosilanes but also many other compounds bearing at least one metal–alkyl bond (such as AlR 3 , BR 3 , and ZnR 2 ) can be employed to affect the distribution of chromium active sites, promote in situ branching, and enhance the sensitivity to hydrogen. , This latter aspect is particularly interesting because the unmodified Phillips catalyst is not sensitive to the presence of hydrogen during the polymerization process, which is among the easiest methods for the regulation of the molecular weight. Although the cocatalysts are commonly employed in industry and their influence on the polymer properties is well assessed, the effect of the cocatalyst on the chromium sites was rarely investigated with spectroscopic methods, − ,,,, likely as a consequence of the difficulty in manipulating metal–alkyls in the proper way. Preliminary spectroscopic data collected in our lab indicate that triethylaluminum and diethylaluminum-ethoxide have an effect similar to that of TES on Cr­(II)/SiO 2 : only a fraction of the Cr­(II) sites are modified (even when an excess of cocatalyst is used), and at least a fraction of the modified sites are more “homogeneous” in nature .…”

Influence of Metal‐Alkyls on Early‐Stage Ethylene Polymerization over a Cr/SiO2 Phillips Catalyst: A Bulk Characterization and X‐ray Chemical Imaging Study

“…The diversity of produced PEs, in terms of PE‐type as well as the broad molecular weight distributions (MWD), is ascribed to the large variety of Cr surface sites. Each surface site produces an average type of PE, [56–58] in which the active sites are not regarded as naked Cr ions: instead, reduction by‐products remain important constituents of the active sites during polymerization [27, 44, 57, 59–61] . For example, PE properties produced from an AlPO 4 ‐supported Cr catalyst can be controlled by the precise addition of tri‐ethyl borane (TEB) to the polymerization process, whereas the SiO 2 analogues were less sensitive to this metal‐alkyl [34, 43, 51, 62–64] .…”

“…For example, PE properties produced from an AlPO 4 ‐supported Cr catalyst can be controlled by the precise addition of tri‐ethyl borane (TEB) to the polymerization process, whereas the SiO 2 analogues were less sensitive to this metal‐alkyl [34, 43, 51, 62–64] . Despite the widespread industrial usage of metal‐alkyl co‐catalysts, such as TEB and tri‐ethyl aluminum (TEAl), owing to their ability to increase control over PE product properties, they have only started to receive increased academic interest in recent years [56, 57, 59, 65–67] …”