The effect of silica dehydroxylation temperature on the activity of SiO2-supported zirconocene catalysts

A series of heterogeneous catalyst systems was prepared by the immobilizati on of bis( n-butylcyclopentadienyl) zirconium dichloride, (nBuCp)(2)ZrCl2, on silica supports activated at different temperatures. Silica dehydroxylat ion was evaluated in terms of retained metal loading and activity in ethyle ne homopolymerization. Characterization of the catalyst systems was accompl ished by Rutherford back-scattering spectrometry and infrared spectroscopy. The highest metal loading (0.48 wt.% Zr/SiO2) was achieved with silica tre ated under vacuum at room temperature (298 K), but the catalyst showed only a minor polymerization activity which may be attributed to a large number of inactive Zr-support bidentate species formed at a high surface density o f OH in silica. IR spectroscopic data show that, regardless of support acti vation temperature, a significant number of isolated OH groups remains afte r zirconocene fixation. The presence of bulky ligands in the catalyst molec ule seems to prevent the remaining OH groups from reacting with additional metallocene complexes, keeping the metal loading around 0.35 wt.% Zr/SiO2 f or silica activated between 373 and 723 K. High polymerization activity obs erved for the system based on 373 K-activated silica suggests a role for th ese OH groups in the generation of active alkylated species when methylalum inoxane and trimethylaluminum (contained in MAO itself) are added at the be ginning of the polymerization reaction. IR analysis shows that TMA, which i s a less sterically demanding compound than MAO, can effectively access the remaining OH groups consuming them thoroughly. Practically all the prepare d systems presented activity in ethylene polymerization with MAO as cocatal yst, the highest activity (5.1 x 10(5) PE g mol(-1) Zr h(-1)) having been o btained with silica dehydroxylated at 723 K. (C) 1999 Elsevier Science B.V. All rights reserved.