1-Hexene and 1-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 α-olefins (LAOs) including 1-hexene and 1-octene in the last decades.1 Many catalytic systems were carried out for ethylene selective oligomerization. However, ethylene tetramerization to 1-octene always accompanies by ethylene trimerization.2 Although the metallacyclic mechanism is well accepted for ethylene selective oligomerization, the active site structures and the redox valence are still controversial. To declose the switching mechanism between ethylene tetramerization and trimerization, an extensive experimental kinetic study of ethylene trimerization was carried out over the Chevron-Phillips homogeneous catalyst system.3 Two PNP-Cr catalysts systems for ethylene selective tetramerization were studied using the quantitative structure activity/property relationship (QSAR/QSPR) and the density functional theory (DFT) methods, respectively.4 The switching mechanism between ethylene tetramerization and trimerization using a different substituting Cr-2,2-dipyridylamine catalysts5 were also disclosed with DFT calculation and turnover of frequency (TOF) calculation. Both the binuclear site and the mononuclear site were proposed to be possible active site model for the ethylene tetramerization.6 The metalacyclic mechanism over Cr(II) mononuclear active site was confirmed to be responsible for ethylene tetramerization, while one spin flipping7 from the quintet potential energy surfaces to triplet potential energy surfaces was found in the calculations.8 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.
Keywords: Cr-based catalyst; ethylene trimerization; ethylene tetramerization; quantitative structure activity/property relationship; density functional theory
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