Polynuclear (α-diimine) nickel(II) complex as catalyst for ethylene oligomerization
Late transition metal catalysts with single or multiple active centers have great attention in the ethylene oligomerization application owing to their unique properties. A new polynuclear (α-diimine) nickel(II) complex (PS-Ni complex) bearing the conjugated poly(Schiff-base) ligand (PS-ligand) was synthesized. The structures and the composition of the PS-ligand and the PS-Ni complex were characterized by Fourier transform infrared (FTIR) analysis, X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), thermogravimetric (TG) analysis, nuclear magnetic resonance (NMR) and gel permeation chromatography (GPC). The PS-Ni complex was used as precatalyst in ethylene oligomerization combined with methylaluminoxane (MAO) and has good catalytic activity for ethylene oligomerization under mild conditions. Compared with the corresponding mononuclear (α-diimine) nickel(II) complex, the PS-Ni complex showed higher activity of 17.62 × 10 5 g/(mol Ni•h), and the main products were C 4 and C 6 olefins in cyclohexane when the dosage of precatalyst was 2 μmol, the Al/Ni molar ratio was 500, the ethylene pressure was 0.5 MPa, the reaction time was 60 min, and the reaction temperature was 25°C. The main oligomers were α-olefins, and the selectivity was up to 98.68%, due to the more electron-deficient catalytic centers of the conjugated structure of poly(Schiff-base) ligand.
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