SILICA-SUPPORTED ZIRCONOCENES AND AL-BASED COCATALYSTS - SURFACE METAL LOADING AND CATALYTIC ACTIVITY

The Zr surface loading from Cp2ZrCl2 toluene solutions on different ki nds of commercial silicas (Grace 948, 952 and 956) was determined by R utherford backscattering spectrometry. In all silicas, a saturation tr end was observed with increasing amount of metal in the initial soluti on, the largest Zr loading value (1 wt.-%) being observed for silica G race 948. Solid surface areas were determined by the BET method before and after metallocene immobilization, revealing that the values do no t change after zirconocene impregnation. Two different kinds of cocata lyst were used, methylaluminoxane and trimethylaluminium. The aluminiu m loading on silica Grace 948 was determined using the Al-27(p,gamma)S i-28 nuclear reaction and titrometry, both showing that methylaluminox ane leads to an 'Al grafting' four times more efficient than does trim ethylaluminium. Finally, the catalytic activity of these systems was t ested in the polymerization of ethylene. The supported catalysts were shown to be active, specially using the low cost trimethylaluminium as a cocatalyst. The polymers obtained with the supported systems exhibi ted a higher molecular weight than those obtained with the homogeneous system.