Zirconocene immobilization into organic-inorganic dual-shell silicas prepared by the nonhydrolytic sol-gel method for polyethylene production

摘要

Cp2ZrCl2 was entrapped in dual-layered silica by two iterated nonhydrolytic sol-gel steps with organosilanes (methyltriethoxysilane, isopropyltriethoxysilane, octyltriethoxysilane, octadecyltrimethoxysilane) and WCl6 as modifiers. For comparative reasons, organic-inorganic hybrid one-layer systems were also synthesized. The hybrid systems were evaluated for ethylene polymerization, with methylaluminoxane (MAO) as the cocatalyst. Increasing catalytic activity was observed in the case of the dual-layered silica systems. Modification of the silica surface with octadecylsilane (C8-Si) in combination with the presence of added Lewis acid centers (W) afforded the highest catalytic activity among the systems tested. The presence of organosilanes on the silica surface may affect polyethylene polydispersity. In the case of dual systems, the presence of two distinct hybrid silica layer effects, an internal silica layer modified with C18-Si and an external layer with C8-Si moieties, impinged on the broadening of polydispersity. Inverse moieties have distinct polydispersity indexes that indicate the surface modification at internal/external silica layers affects polymer characteristics. Microstructural features of hybrid silicas determined by smallangle X-ray scattering indicate that a low number of neighbors and larger correlation distances could explain higher catalytic activities. Similarly, long carbon chains in organoalkoxysilanes tend to reduce the radii of second level particles in a hierarchical structure of dual-shell systems, which in turn leads to higher catalytic activity. (C) 2020 Elsevier Inc. All rights reserved.