Mechanistic Studies And Development Of Novel Catalysts For Cr-based Selective Ethylene Oligomerization Systems

Since the discovery of ethylene trimerization in1967, selective ethylene oligomerization has stimulated both industrial and academic research for its ability to selectively produce a-olefins for linear low density polyethylene (LLDPE) manufacture. Compared to the conventional process for1-hexene and1-octene production via non-selective ethylene oligomerization, ethylene trimerization and tetramerization are favored for high atom efficiency and reduction of separation costs. Over the last decade, numerous selective ethylene tri-and tetramerization systems have been developed, including the successfully commercialized Chevron-Phillips ethylene trimerization system. However, lack of theoretical understanding of the factors influencing the catalytic performance makes trial and error the main approach in the catalyst development. In this thesis, the intriguing but challenging topic of selective ethylene oligomerization has been studied with both experimental and theoretical approaches.The first part of the thesis provides some mechanistic insights into two typical ethylene trimerization systems, investigating experimentally unsolved issues exclusively with theoretical methods. For the landmark Chevron-Phillips ethylene trimerization catalyst, a detailed mechanistic study has been carried out by density functional theory (DFT) calculations on a Cr/pyrrole-based model system. Possible reaction pathways are located on the basis of the metallacycle mechanism. Consistent with experimental results, the trimerization route is proven to be energetically preferred compared to ethylene dimerization or further ring expansion of the chromacycloheptane intermediate. From detailed analyses of oxidation states and electronic configurations, the Cr(I)/Cr(III) redox couple is found to be responsible for1-hexene selectivity and all active species involved in the catalytic cycle are constantly under quartet spin state. The importance of the coordination of a pendant chlorine, as was observed in various crystal structures, has been investigated for different intermediates. Bond distances and angles and charge analyses clearly prove a hemilabile behavior of the chlorine, which is considered a key factor for1-hexene selectivity.In the Cr-SNS ethylene trimerization system, long-term existing debates from experimental evidence, including the oxidation states of the active species and the occurrence of ligand deprotonation, have been examined by DFT methods. Gibbs free energy surfaces of full reaction cycles have been completely located, and formation of chromacycloheptane is identified as the rate-determining step. A detailed spin state analysis reveals that the ground states of the intermediates change along the redox cycle. A spin crossover occurs at the minimum energy crossing point before chromacyclopentane formation, which opens up a much lower energy pathway by spin acceleration. By comparison of the activation energies of the rate-determining step, Cr(I)/Cr(III) complexes bearing non-deprotonated ligands are proposed to be the most plausibly active species, which has been supported by experimental proof. Frontier orbital and natural population analyses have also been carried out to further elucidate the reason for high1-hexene selectivity in this system.In the second part of the thesis, two novel ethylene tri-and tetramerization systems have been developed and explored. The first system consists of Cr(acac)3, N-pyrrolyldiphenylphosphine ligand and aluminum alkyls as co-catalysts. Upon activation with a co-catalyst, the system is capable of selectively producing1-hexene and1-octene with moderate catalytic activity. The1-hexene/1-octene ratio can be tuned from0.3to20by adjusting the reaction conditions (solvent, co-catalyst, catalyst loading, ligand/Cr ratio, temperature and pressure). And the highest1-hexene and1-octene selectivities can be achieved up to91%and74%, respectively. Furthermore, addition of a small volume percentage of toluene to a system running in aliphatic hydrocarbons has been found crucial to generate1-octene as the dominant liquid fraction, whereas using higher concentrations of toluene gradually switches the system non-selective, producing oligomers with a statistic distribution.In the second catalytic system, a series of chromium catalysts containing pyridine-phosphine ligands with similar backbones have been synthesized and characterized. These catalysts show varying catalytic activities and selectivities towards the formation of a-olefins and polyethylene (PE). The ligand structure dramatically influences the catalytic behavior. Selectivities towards C6or C8as well as the purity of the1-hexene and1-octene can be controlled by subtle modifications of the substituents on phosphorous or the P-Cr-N bite angles. Ligand PyCH2N(Me)P’Pr2in combination with CrCl3(THF)3has been found to enable selective ethylene tri-and tetramerization, affording1-hexene and1-octene with good overall selectivity and high purity, albeit with the presence of small amounts of PE.Combining experiments and theoretical calculations, this thesis provides an example of effective teamwork in exploring and resolving chemical problems. Although both experimental and computational studies can give helpful suggestions on their own, the results presented in this thesis show that a more comprehensive understanding regarding selective ethylene oligomerization can be obtained by merging these two tools.