Linear alpha-olefins (LAOs) possessing a reactive C-C-double bond at the α- position serve as the crucial organic raw material and chemical intermediates, which are usually used for the production of plasticizers, surfactants, lubricants, long chain carboxylic acids, epoxides, alcohols, and comonomers. In an endeavor to explore promising ethylene tetramerization catalysts based on chromium, a series of novel PNP (bisphosphineamine) ligands bearing bulky triptycene moiety as metal-supporting organic ligands N-atom were developed and evaluated for selective ethylene oligomerization. This study was based upon our anticipation that the triptycene’s unique paddle-wheel configuration with D3h symmetry coupled with an inherent β-branching with respect to the PNP moiety at 1- and 4- position of the triptycene backbone may provide favorable steric bulkiness required for a selective oligomerization reaction. Indeed catalytic studies reveal that upon activation with MMAO-3A, the pre-catalyst mixture containing Cr(acac)₃/ligand can efficiently promotes ethylene tetramerization with remarkably high productivities (up to 1733 kg.gCr^-1.h^-1) and C8 olefins selectivities (up to 74.1 wt.%). Most significantly, ligands with PNP moiety connecting at 1- or 1,4-position of the triptycene molecule could achieve exceptionally high alpha (1-C6+1-C8) selectivities, exceeding 90 wt.%, as a result of high 1-C6 purity (> 90 wt.%) in C6 fraction. Based on a comparative catalytic study employing various PNP ligands with or without N-triptycene backbone, we were able to establish the pre-anticipated fact that a rational designing of PNP ligands with an optimum degree of steric profile around N-center could provide C6 cyclics controlled high a-selective ethylene oligomerization.

Motaz Khawaji

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