Molecular Design And Mechanistic Study Over Chromium Based Catalyst Systems For Ethylene Trimerization/Tetramerization To1-Hexene/1-Octene

1-Hexene and1-octene are important comonomers for the synthesis of high performance polyolefins. Ethylene selective oligomerization with advantages of high atomic efficiency and a simple reaction procedure has attracted comprehensive interests to match the increasing demand of highly valuable linear a-olefins (LAOs) including1-hexene and1-octene in the last decades. Many catalytic systems were carried out for ethylene selective trimerization, and ethylene trimerization was first commercialized over Cr-pyrrole catalyst system by Chevron-Phillips Company in2003at Qatar. Since the first ethylene tetramerization report in2004, several chromium based catalytic system were reported to be effective to catalyze ethylene tetramerizaiton. However, no commercialization of ethylene tetramerization is reported yet. Although the metallacyclic mechanism is well accepted for ethylene selective oligomerization, the active site structures and the redox valence are still controversial. In this work, two ethylene selective tetramerization systems were studied using the quantitative structure activity/property relationship (QSAR/QSPR) and the density functional theory (DFT) methods, respectively. The alkyl and aryl substituted Cr-bis(diphenylphosphino)amine (PNP-Cr) catalysts which show the potential as excellent candidates for highly selective ethylene trimerization/tetramerization were studied with QSPR method based on DFT calculations. The linear regression models of1-hexene selectivity and1-octene selectivity were established over the alkyl and aryl substituted PNP-Cr systems, respectively. Several new kinds PNP ligand proposed by molecular design were predicted to be with a better ethylene trimerization/tetramerization performance. A switching mechanism between ethylene tetramerization and trimerization on the Cr-2,2-dipyridylamine catalyst was studied using DFT calculation. And a Cr(II)/Cr(IV) metallacycle reaction pathway on the mononuclear active site was found to be most plausibly responsible for ethylene tetramerization.A series of alkyl-substituting PNP-Cr catalysts were studied by QSPR method based on DFT calculations in Chapter2. The heuristic method (HM) and best multi-linear regression (BMLR) were used for establishing the best linear regression models to describe the relationship between catalyst selectivity and its structure. Both Cr(I) and Cr(II) active site models for ethylene trimerization/tetramerization were considered. It was found that:(1) Using self-defined descriptors from DFT calculations could increase the relativity and stability of the models.(2) Monovalent Cr(I) center was the most plausible active site for ethylene trimerization, while ethylene tetramerization was most possibly proceeded over divalent Cr(II) active site.(3) The skeleton structures of the PNP-Cr system especially a small PNP angle were crucial for achieving excellent catalytic selectivity. Nine new PNP ligands with high selectivity towards ethylene trimerization/tetramerization were predicted based on the best linear regression models providing a good basis for further development of novel catalyst systems with better performance. A series of aryl-substituted PNP-Cr catalysts were studied by QSPR method based on DFT calculations in Chapter3. The effects of the sample set, data set, linear regression method and the active site oxidation state on the linear regression models were investigated separately. The best1-hexene selectivity linear model was built with the Cr(Ⅰ) active site, while the best1-octene selectivity linear model was built with the Cr(Ⅱ) sites. It was also found that the skeleton structures of the PNP-Cr system with good complanation and symmetry were crucial for achieving excellent catalytic selectivity of1-octene, while the PNP-Cr backbone with a large steric effect on N atom would benefit ethylene trimerization. Six new PNP ligands with high selectivity towards ethylene trimerization/tetramerization were predicted based on descriptor analysis and the best linear regression models. By comparing the QSPR results of Chapter2and Chapter3, it is found that QSPR method is effective to study the relationship between the chromium based catalyst systems and its ethylene selective oligomerizaion properties. The QSPR study can also provide a good basis for further development of novel catalyst systems with better performance.The switching mechanism between ethylene tetramerization and trimerization using a different substituting Cr-2,2-dipyridylamine catalysts were disclosed with DFT calculation and turnover of frequency (TOF) calculation in Chapter4. This catalyst system was reported to be able to catalyze ethylene tetramerization with a very high1-octene selectivity (>99%). Both the binuclear site and the mononuclear site were proposed to be possible active site model for the ethylene tetramerization. And different oxidation states were considered for the calculations. The binuclear models were predicted to be unstable in the experimental conditions. The metalacyclic mechanism over Cr(Ⅱ) mononuclear active site was confirmed to be responsible for ethylene tetramerization, while one spin flipping from the quintet potential energy surfaces to triplet potential energy surfaces was found in the calculations. Consequently, the divalent mononuclear active site which involves a mixing of quintet and triplet potential energy surfaces was found to be responsible for the switching between ethylene tetramerization and trimerization.