Effect of Metal-Alkyl Components on the Formation of Active Sites in Chromium-Based Ethylene Polymerization

The annual production of plastics is soon expected to exceed the 400 million metric tons, of which polyethylene has the largest market volume share. The polyolefin industry currently stands as one of the most mature, sustainable, and efficient technologies relying on fossil, and more recently also renewable, feedstocks. These materials find applications everywhere in our daily life, mainly because their properties, such as rigidity and flexibility, are tuned with relative ease. Despite these benefits, recent years have seen increased attention to the environmental issues that plastics cause. These factors continue to drive research towards the improved production of polyethylene. This research revolved around ethylene polymerization, in which Cr/SiO2 Phillips-type catalysts are one of the grand workhorses. The Phillips Catalyst is unique since it does not require activation by a co-catalyst. However, these components are often added on an industrial scale because they significantly improve catalytic performances and allow tailoring of the final polyethylene properties. It is critical to understand this activation procedure for improving existing catalysts and for developing new ethylene polymerization catalysts. To that end, we used advanced spectroscopic and microscopic techniques to investigate the activation procedure of Cr/SiO2 Phillips-type catalysts as well as the influence of metal-alkyl components on the properties of the polyethylene products. In the first part we studied the bulk catalyst performances with bulk reactions, finding that the unique co-catalysts result in distinct performances. We opted to answer this question by studying the effect of metal-alkyl co-catalysts on the redox chemistry of this catalyst with in-situ UV-Vis-NIR Diffuse Reflectance Spectroscopy experiments, finding that indeed different redox chemistries are enabled. Hereafter we opted to study the effect of these different metal-alkyl co-catalysts on bulk polyethylene performances, finding that defining properties such as Molecular Weight Distribution, Short-Chain Branching and Long-Chain Branching can be carefully tailored by applying the right co-catalyst and the right amount. We used Scanning Transmission X-ray Microscopy to investigate whether these differences already existed in the earliest stages of polymerization, finding that the polyethylene materials did not resemble their bulk counterparts, but instead demonstrated significantly lower densities, likely caused by the confinement of the carrier material in these early stages. Analysis of the polyethylene materials revealed that a multitude of Cr surface structures exist, the next part focused on investigating these surface structures with Raman Microscopy, finding that two major types of surface structures exist and that these structures react differently with a variety of reduction gases: one type reducing more easily than the other. In a last part we combined our obtained knowledge to investigate several renowned Metal-Organic Frameworks (MOFs) as ethylene polymerization catalysts. This work combined 10-bar ethylene polymerization reactions and spectroscopic investigations to disentangle prerequisites for attaining catalytic activity