Optimization of a silica supported bis(butylcyclopentadienyl)-zirconium dichloride catalyst for ethylene polymerization

Abstract

A series of heterogeneous α-olefin polymerization catalysts were prepared by the immobilization of bis(butylcyclopentadienyl)zirconium dichloride, (nBuCp)2ZrCl2, on a commercial silica support (Grace 948) using different procedures. The preparation parameters, namely, silica activation temperature, grafting temperature, grafting time, and solvent, were evaluated in terms of metal content on silica and ethylene homopolymerization activity. Metal contents were determined by Rutherford back-scattering spectrometry (RBS). In the temperature activation range between 373 and 723 K, silica surface saturation in Zr was found to be around 0.34 wt.-% Zr/SiO2. However, polymer polydispersity is shown to decrease with increasing support activation temperature. A better control in the generation of the active surface species was achieved with thermal pretreatment temperatures close to 723 K. The grafting reaction was seen to be immediate. Longer grafting times or higher temperatures bore deactivated species. Practically all the systems were active in ethylene polymerization in the presence of MAO, but the highest yield was obtained after grafting at 353 K for 1 h in toluene solution, employing silica pretreated at 723 K.