Highly selective ethylene oligomerization to high-carbon α-olefins over PNP chromium complex modified nano-SiO2

PNP chromium complex (PNP-Cr) with high selectivity of high-carbon α-olefins in ethylene oligomerization was synthesized to modify nano-SiO2 to obtain heterogeneous PNP chromium catalyst (PNP-Cr@SiO2) with high dispersibility of catalytic active centers. Upon activation with methylaluminoxane (MAO), PNP-Cr@SiO2 had a high catalytic activity of 1.13 × 105 g/(mol Cr·h) and the selectivity of 79.98% for C8+ α-olefins in ethylene oligomerization. Moreover, PNP-Cr@SiO2 had remarkable recyclability and catalytic stability compared with the corresponding homogeneous catalyst (PNP-Cr), and the catalytic activity decreased to 0.95 × 105 g/(mol Cr·h) and the selectivity for C8+ α-olefins slightly decreased to 78.86% after three cycles. Grey relational analysis confirmed that reaction pressure and reaction temperature were the main factors affecting the catalytic activity of PNP-Cr@SiO2 and the selectivity for high-carbon α-olefins in ethylene oligomerization. PNP-Cr@SiO2 had higher selectivity for C8+ α-olefins because the catalytic active centers were primarily distributed on the surface of nano-SiO2 to facilitate the formation of large metallacycle and the insertion of the oligomer (1-octene) into the metallacycle compared with other heterogeneous catalysts.