| 1-Hexene and1-octene are two kinds of high value-added linear alpha olefins, which are used as comonomers in the production of polyethylenes in order to improve product performance. In the past decade, a variety of diphosphinoamine-chromium (PNP-Cr) systems have been synthesized for the trimerization and tetramerization of ethylene. Interestingly, a certain amount of cyclic byproduct with1:1ratio of methylcyclopentane and methylenecyclopentane was reported in most of these PNP-Cr systems.In this work, a PNP-Cr catalyst system was synthesized and applied for ethylene selective oligomerization using methylaluminoxane (MAO) as cocatalyst. It was found that methylenecyclopentane and methylcyclopentane were typically formed in a ratio of1:1. We then conducted a systematic mechanistic study on the formation of these two kinds of cyclic byproducts through density functional theory (DFT) calculations, including the mononuclear mechanism and binuclear mechanism, the oxidation state of chromium center, and three kinds of substituents (linear alkyl, branched alkyl and naphthene base) on the N atom in PNP ligands. Our calculations showed that ⅰ) the formation of methylenecyclopentane and methylcyclopentane follows a mononuclear reaction pathway rather than a binuclear mechanism; ⅱ) the Cr(I)/Cr(Ⅲ) cycle is more convincing than the Cr(Ⅱ)/Cr(Ⅳ) cycle in explaining the proportional formation of C6-cyclics; ⅲ) PNP ligands with linear alkyl substituents showed negligible effect on the ratio of methylenecyclopentane and methylcyclopentane due to the limited steric hindrance, while PNP ligands with branched alkyl and naphthene base substituents tended to produce more methylcyclopentane as larger substituents presented on the N atom in the PNP ligands. These results provide much deeper mechanistic understanding on the formation of cyclic byproducts in the ethylene selective oligomerization and will assist further development of high-efficient catalyst for ethylene selective oligomerization. |
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