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

Maarten K Jongkind,Nic Friederichs,Bert M Weckhuysen,Benjamin Watts,Theo Van Kessel,Iris C Ten Have,Florian Meirer,Koen W Bossers,Hendrik Ohldag

doi:10.1002/chem.202002632

Abstract

The Cr/SiO2 Phillips catalyst has taken a central role in ethylene polymerization since its invention in 1953. The uniqueness of this catalyst is related to its ability to produce broad molecular weight distribution (MWD) PE materials as well as that no co‐catalysts are required to attain activity. Nonetheless, co‐catalysts in the form of metal‐alkyls can be added for scavenging poisons, enhancing catalyst activity, reducing the induction period, and tailoring polymer characteristics. The activation mechanism and related polymerization mechanism remain elusive, despite extensive industrial and academic research. Here, we show that by varying the type and amount of metal‐alkyl co‐catalyst, we can tailor polymer properties around a single Cr/SiO2 Phillips catalyst formulation. Furthermore, we show that these different polymer properties exist in the early stages of polymerization. We have used conventional polymer characterization techniques, such as size exclusion chromatography (SEC) and 13C NMR, for studying the metal‐alkyl co‐catalyst effect on short‐chain branching (SCB), long‐chain branching (LCB) and molecular weight distribution (MWD) at the bulk scale. In addition, scanning transmission X‐ray microscopy (STXM) was used as a synchrotron technique to study the PE formation in the early stages: allowing us to investigate the produced type of early‐stage PE within one particle cross‐section with high energy resolution and nanometer scale spatial resolution.

Highlights

The production of polyethylene (PE) is estimated to increase to an annual production of over 100 million tons in 2020 and continues to be a ubiquitous material in our society in the decades to come.[1]
The produced catalyst-PE materials were cut into slices of about 100 nm by ultra-microtomy, after embedding in a hard immobilizing epoxy resin, and scanning transmission Xray microscopy (STXM) was used to study the type of PE produced while it was still largely dispersed within the Cr/SiO2 catalyst phase, with polymer yields between 1–2 gPE gcatÀ1
In this work we found that the properties of the polyethylene (PE) produced by a single Cr/SiO2 Phillips catalyst formulation can be carefully tailored by selection of the proper type and amount of tri-ethyl borane (TEB) or tri-ethyl aluminum (TEAl)

Summary

Introduction

The production of polyethylene (PE) is estimated to increase to an annual production of over 100 million tons in 2020 and continues to be a ubiquitous material in our society in the decades to come.[1]. One example of a long-standing discussion is the oxidation state of the Cr active site, which has been investigated with many different spectroscopic techniques, demonstrating that the active valency lies between 2 and 3, with the true value still being questioned by varying insights from different research groups These differences, can in part be ascribed to different reaction set-ups and different catalyst materials.[33, 34, 43,44,45,46, 35,36,37,38,39,40,41,42]. Semi-batch ethylene polymerization reactions at constant polymer yields allowed us to study catalyst activities as well as the resulting polymers, which were studied in terms of molecular weight (Mw), molecular weight distribution (MWD, Mw/Mn), short-chain branching (SCB) and longchain branching (LCB) by bulk characterization techniques, namely size exclusion chromatography (SEC) and 13C NMR.[72,73] These data allowed us to correlate the type of PE produced at the level of a single catalyst particle cross-section with that of bulk PE

Results and Discussion

Conclusions

Conflict of interest