Abstract
Ph.D. (Chemistry)
Ethylene oligomerisation remains an industrially relevant and fundamentally intriguing research area. Researchers, in industry and academia, strive to understand the process and to improve the selectivity and productivity towards 1-hexene and 1-octene which are sought-after co-monomers for the production of linear low density polyethylene. The primary aim of this study was to add to the understanding of tetramerisation by critically reviewing results that have been obtained in this area and exploring new catalyst systems for this transformation.
The discovery and development of chromium-catalysed ethylene trimerisation and tetramerisation is summarised in Chapter 1, with a particular focus on the ligand development endeavours. Sasol was the pioneer of ethylene tetramerisation. A substantial amount of work was devoted to improving the Sasol diphosphinoamine (PNP) ligand. In Chapter 2, results pertaining to this ligand class which were generated in one lab, under similar conditions, are analysed. Recently published mechanistic models are used to interpret the data.
The direct correlation between product selectivity and ligand structural features, ligand sterics in particular, was demonstrated in Chapter 2. A mechanistic proposal was suggested to explain why the PNP-based catalysts do not form substantial amounts of 1-decene and higher olefins via the metallacycle mechanism. A proposal for the formation of catalytic species involved in competitive polyethylene formation was also put forward. These postulations were supported by published theoretical studies and experimental data.
The active oxidation states have been a topic of much debate. Both Cr(I)/Cr(III) as well as Cr(II)/Cr(IV) redox cycles have been proposed. In an attempt to unravel this mystery, Chapter 3 begins with a brief account of the EPR and X-ray spectroscopy experiments which have been conducted previously. Upon reviewing all the results, it is proposed that the Cr(I)/Cr(III) redox pair is responsible for active oligomerisation. Cr(III) is reduced to Cr(II) in the presence of MMAO. However, further reduction to Cr(I) is possible in the presence of ethylene, as seen in EPR spectroscopy studies. The latter part of this chapter investigates the role that the catalyst activation conditions play in formation of the active catalyst species. The Cr precursors, the mode of complexation (either in situ or pre-complexation) as well as the solvent choice were found to affect catalyst activity for different diphosphine ligand classes...