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

Abstract

The Cr/SiO2 Phillips catalyst has taken a central role in ethylene polymerization ever since its discovery in 1953. This catalyst is unique compared to other ethylene polymerization catalysts, since it is active without the addition of a metal‐alkyl co‐catalyst. However, metal‐alkyls can be added for scavenging poisons, enhancing the catalyst activity, reducing the induction period and altering polymer characteristics. Despite extensive research into the working state of the catalyst, still no consensus has been reached. Here, we show that by varying the type of metal‐alkyl co‐catalyst and its amount, the Cr redox chemistry can be tailored, resulting in distinct catalyst activities, induction periods, and polymer characteristics. We have used in‐situ UV‐Vis‐NIR diffuse reflectance spectroscopy (DRS) for studying the Cr oxidation state during the reduction by tri‐ethyl borane (TEB) or tri‐ethyl aluminum (TEAl) and during subsequent ethylene polymerization. The results show that TEB primarily acts as a reductant and reduces Cr6+ with subsequent ethylene polymerization resulting in rapid polyethylene formation. TEAl generated two types of Cr2+ sites, inaccessible Cr3+ sites and active Cr4+ sites. Subsequent addition of ethylene also revealed an increased reducibility of residual Cr6+ sites and resulted in rapid polyethylene formation. Our results demonstrate the possibility of controlling the reduction chemistry by adding the proper amount and type of metal‐alkyl for obtaining desired catalyst activities and tailored polyethylene characteristics.

Mechanistic Insights of Ethylene Polymerization on Phillips Chromium Catalysts

“…Note that a selfalkylation mechanism involving the formation of Cr(IV) bisalkyl sites starting from Cr(II) was also supported by the studies of McDaniel and colleagues [66] (see below). It is noteworthy that in 2020, Weckhuysen and colleagues published the results of similar research of Cr(VI)/SiO2 activation by Et3B and Et3Al [67]. Besides trivial catalytic results (reduction of the induction period, higher catalytic productivity at initial…”

“…It is noteworthy that in 2020, Weckhuysen and colleagues published the results of similar research of Cr(VI)/SiO 2 activation by Et 3 B and Et 3 Al [ 67 ]. Besides trivial catalytic results (reduction of the induction period, higher catalytic productivity at initial polymerization stages), the results of the study of the Cr sites reduced by 1.5 and 10 equivalents of Et 3 M were presented.…”

“…In these works, only the results of polymerization experiments were discussed; the observed synergistic effect of the presence of Cr and V centers remained without mechanistic discussion. In [ 67 ], it was shown that the use of citric acid at the stage of impregnation of Cr and V salts resulted in more uniform distribution and high dispersion of the metal centers, which led to an increase in the activity and broadening of the MWD.…”

NEt3-Triggered Synthesis of UHMWPE Using Chromium Complexes Bearing Non-innocent Iminopyridine Ligands

“…Polymerization of ethylene catalyzed by transition-metal complexes is one of the most industrially relevant synthetic reactions, and the development of high-performance catalysts continues to be a major driving force to access different polyethylene (PE) grades. − Beyond early and late transition-metal catalysts, − chromium catalysts are also of great interest, both homogeneous − and heterogeneous. − The peculiarity of homogeneous organochromium complexes is the ability to give linear α-olefins (selective tri- and tetramerization), high-density PE (HDPE) and even ultrahigh-molecular weight PE (UHMWPE) from ethylene. The selectivity may be obtained either by tuning the steric and electronic properties of the ligand , or by employing neutral donor molecules covalently linked to the chromium complex. , Generally, chromium complexes are renowned for their ability in the oligomerization of ethylene, , while examples of chromium complexes for UHMWPE production are rare.…”

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

“…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] …”