Understanding the role of paramagnetic organometallic redox centres in oligomerisation catalysis

Lead Research Organisation: CARDIFF UNIVERSITY
Department Name: Chemistry


Chain growth reactions involving alkenes are very important in Industry. One example of this process, ethylene oligomerization, leads to simple chemicals known as alpha-olefins. In turn, these alpha-olefins can be transformed into a range of commodity chemicals, e.g. surfactants, lubricants, plasticizers, LDPE and polymer additives. The oligomerization process requires homogeneous catalysts, commonly based on Nickel or Chromium compounds. A major problem associated with these catalysts is the formation of an undesirable mathematical distribution of alpha-olefins. One development that solves this problem, which operates via a uniquely different mechanism, is selective ethylene trimerization and tetramerization giving 1-hexene or 1-octene respectively. Recent developments have focussed on designing highly selective catalysts. To date, chromium catalysts account for over 90% of the patent and scientific literature for ethylene oligomerization. The mechanism is generally thought to follow a route involving concerted addition of two ethylene molecules to the metal centre followed by insertion of another ethylene molecule to yield a cyclic metal centred species. With regard to the formal oxidation state of Cr, there is evidence for both Cr(I/III) and Cr(II/IV) couples, and oxidation states of Cr in the catalytic cycle could even be ligand dependent. Several attempts have been made to determine the oxidation states during the catalysis by experimental and computational studies. But the debate still continues, and the precise role of the redox couple in these reactions is still not settled.The current programme of research will attempt to settle this debate by providing a comprehensive insight into the oxidation and spin states involved in the catalytic reaction using EPR spectroscopy. Our goal is less about generating better oligomerisation catalysts, and more about using the N-heterocyclic carben (NHC) ligands to follow the reaction mechanism in detail by EPR. More generally, it will provide a unique opportunity to probe the precise role played by paramagnetic organometallic redox/spin centres in catalysis, an area which is surprisingly poorly researched. The novelty of this work will be the examination of the role played by the paramagnetic spin states in the mechanism. Novel ligands based on NHC's will be synthesised for this purpose in order to stabilise the various oxidation states of the transition metal ions thought to be involved in these reactions. Through precise experimental control of reaction conditions and with advanced EPR spectroscopic techniques, we will peer into the heart of the reaction cycle, and tease out the structure of the paramagnetic reaction intermediates crucial for the successful catalytic cycle. By performing spectroscopic measurements in situ, we can probe the dynamics of the electron spins as the catalyst is activated before and during the reaction. These aims will be achieved through a combination of synthetic chemistry, mechanistic chemistry and advanced spectroscopy (specifically the family of EPR techniques). Ligand design is a key component of the project, focussing on NHC complexes of Cr and group 4 & 5 elements to monitor the reaction. The electronic structure of these new organometallic complexes and intermediates will be thoroughly probed by cw- & pulsed EPR methodologies. While this study is primarily fundamental in scope, we will explore the use of the complexes for ethylene oligomerization. By adopting this unique approach of using pulsed EPR, an unsurpassed glimpse into the redox/spin state, ligand structure and intermediates involved in the catalytic reaction for ethylene oligomerization will be achieved. For the first time pulsed EPR methods will be uniquely used to examine the mechanism of these important industrial reactions.

Planned Impact

Several beneficiaries of this project, from short to medium to long term, include: PDRA/Students; Both the PDRA and the students will acquire advanced skills and training during this project, and will ultimately be exposed to diverse themes of research in this multi-disciplinary research programme. They will be supported directly by the guidance of the PI and Co-I's, and indirectly through the numerous training courses offered by the University, to help young scientists advance in their career through courses in skills training and careers advice. Providing training to the next generation of scientists will help foster the competitiveness of the UK in a global market. Industrial Sector; The fundamental insights gained from this project will be of benefit in the medium and long term to the private/industrial sector. Chain-growth processes represent an enormous and well established part of the chemical industry, so a detailed understanding of reaction mechanisms involved in developing high specificity reactions will lead to improved design, and the next generation of efficient catalysts. The investigators already have close collaborations with SASOL, where commercial exploitation could be quickly realised. SASOL are a leading market player in olefin oligomerization and recently commissioned a 100K tons per annual 1-octene plant. A number of pathways will be pursued to ensure these potential benefits are fully realised. Firstly, the Research & Consultancy Division (RACD) at Cardiff University support and facilitate the process of University-wide research and commercialisation of all research outputs. This includes advice on technology licensing, business development and other commercial opportunities. Secondly, the Cardiff University Innovation Network (CUIN) supports the process of dissemination, transfer, application and exploitation of knowledge outside the academic environment, by establishing contacts with business. Part of this service includes the Technology Transfer Group to promote, raise awareness and publicise the importance of knowledge/technology transfer and commercialisation of research outputs. Thirdly, the Welsh Assembly also provides start-up funds and guidance for SME's through the Wales Spin Out programme. Therefore, appropriate procedures are in place to ensure private sector beneficiaries and possible commercial exploitation can be achieved when the opportunity arises. Society; Linear alpha olefins are used as an important feedstock in commodity chemicals such as surfactants, lubricants, plasticizers, polymer additives. Although demand peaked at 5.8% increase between 1995-2000, the current demand is still growing at 4.2% per year. Society depends on the ready and cheap availability of these important feedstocks. Therefore, in the long term benefits may be manifested to society via cheaper feedstocks, realised through more efficient and better catalysts, or through a greater understanding of existing ones.


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Alberola A (2011) Crystal structures, EPR and magnetic properties of 2-ClC6H4CNSSN? and 2,5-Cl2C6H3CNSSN?. in Chemical communications (Cambridge, England)

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Bin Saiman MI (2012) Involvement of surface-bound radicals in the oxidation of toluene using supported Au-Pd nanoparticles. in Angewandte Chemie (International ed. in English)

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Conte M (2012) Cyclohexane oxidation using Au/MgO: an investigation of the reaction mechanism. in Physical chemistry chemical physics : PCCP

Description Electron Paramagnetic Resonance (EPR) and Electron Nuclear Double Resonance (ENDOR) spectroscopies were used to study the fundamental nature of chromium-based selective oligomerisation catalysts. Owing to the paramagnetic states of the intermediates involved, EPR spectroscopy was crucial to understand the nature and electronic structure of the centres involved in the reaction. A series of typical pre-catalyst complexes were initially fully characterised and CW-EPR revealed that each complex possessed an axial g matrix (g- > ge > g¦) with superhyperfine coupling to two equivalent 31P nuclei, consistent with a low-spin d5 species of approximate C2v symmetry, where the metal contribution the SOMO was primarily dxy. The isotropic component to the 31P coupling was of a larger magnitude in those Cr(I) complexes bearing PNP ligands than those bearing PCP, indicating that the phosphorus 3s character in the SOMO was higher for the former. CW-ENDOR demonstrated that subtle structural differences in the complexes, namely in the phenyl ring conformations, occurred as a function of ligand type and pulsed experiments proved that the technique remains valid and viable for further work on the activated system.
Upon activation of the pre-catalyst with an alkylaluminium, four distinct paramagnetic centres were identified. A Cr(I) bis-arene complex was firstly detected; it was found to form either via intramolecular co-ordination of the ligand phenyl groups, or preferentially via solvent-based arene co-ordination, if such groups were available. Two further species were subsequently observed at low temperatures; the spin Hamiltonian parameters extracted for both showed that a significant modification to the structure of the pre-catalyst had occurred. Half-field transitions indicated the possibility of a dimeric nature to some of these species. ENDOR measurements detected an exceptionally large proton coupling in the activated system, possibly due to the co-ordination of alkyl fragments to the metal centre. A final, fourth paramagnetic centre, was detected and classed as an intermediate species, due to the greater similarity between its g and A matrices with those of the parent complex, than the other activated species. Finally, a preliminary investigation into analogous pre-catalyst complexes bearing N-heterocyclic carbene ligands was also performed, due to their similar employment in oligomerisation catalysis. The CW-EPR spectra revealed information on both their electronic and structural natures. What our research has shown, is that a series of homogeneous Cr based catalysts undergo complex structural transformations during activation, and these changes may explain the high selectivity of the catalysts for ethylene oligomerisation. Because of the paramagnetic species involved, only EPR spectroscopy can shed light on these species.
Exploitation Route We hope that our findings will enable synthetic chemists to develop new ligands which help to restrict the number of possible isomers formed during the activation step and also help to stabilise the low valent chromium centres involved in the catalytic reactions.
Sectors Chemicals

Description In the original research application, the areas highlighted for Impact were opportunities for commercial exploitation, efficient dissemination, engaging with the private sector, and preparing scientists of the future. In the former case, the findings from the research are still at too early a stage to assess whether they can be exploited by the commercial sector to test if improved catalysts performance can be achieved. In the second case, a good series of publications (14) and numerous presentations at conferences resulted from the work. In the third case, we continued to work with Industry both directly (through collaborative links with Sasol, a leading commercial entity with a board interest in oligomerisation catalysis) and indirectly (we were subsequently approached by a number of large companies who started collaborative research projects with us based on the recognition of how EPR spectroscopy can yield important information on paramagnetic species). Finally, four young research scientists were involved int he project, three of whom now have positions in industry, whilst a third holds an independent fellowship position at University. Since all of these young scientists have received training in advanced spectroscopic methods (EPR), this project has facilitated the training of a highly skilled set research scientists.
First Year Of Impact 2014
Sector Chemicals,Education
Impact Types Societal