Silica-supported metallocene catalyst poisoning: The effect of surface modification on the efficiency of the catalytic system

Cp2ZrCl2 was immobilized on silica by two methods: direct grafting (GS) and via entrapment within silica matrix, produced by the non-hydrolytic sol-gel method (NH). In the latter case, the surface was further modified by metal salts (Cr, Mo and W). The supported catalysts (three GS and four NH systems) were evaluated in ethylene polymerization, having methylaluminoxane (MAO) as the cocatalyst. Almost all of them resisted to at least 0.2 mu g g(-1) acetone (used as probe molecule for oxygenated poisons). NH systems bearing W or Mo (3 wt.%) on its structure were shown to be more productively efficient against poisoning. NH containing W in its structure tended to keep its catalyst activity even after the addition of the poison. Therefore, metal doping may be a suitable strategy to keep or increase the efficiency of entrapped zirconocene systems against oxygenated poisons. SAXS analyses show likewise that the smaller the primary spheres, the more protected will be the active site. Texture and microstructure of the catalytic systems and properties of the yielded polyethylenes reveal that the nature of the immobilization route affects the polymer properties (Mw and T-m).