Il ruolo del supporto nei catalizzatori Ziegler-Natta: una indagine spettroscopica in situ

In the Master Thesis work we investigated the role of the support in Ti-based heterogeneous Ziegler-Natta catalysts. Ziegler-Natta catalysts are a wide class of polymerization catalysts for the production of high density polyethylene (HDPE), linear low density polyethylene (LLDPE) and polypropylene (PP); besides the advantage of operating in much less drastic conditions of the processes of radical polymerization, the main benefit of Ziegler-Natta catalysts is to selectively convert the propylene polypropylene, thus obtaining a polymer with a specific configuration (isotactic, syndiotactic or atactic). The investigated catalysts were all obtained in a double step: i) reaction of TiCl4 from the gas phase with a pre-activated support; and ii) successive reduction of the supported Ti phase by means of an activator (aluminum-alkyl or hydrogen). Three different supports were tested: ¿ MgCl2, which is the traditionally employed support in Ziegler-Natta catalysts for propylene polymerization. High surface area MgCl2 supports were obtained through a controlled dealcoholation of different MgCl2-alcoholate adducts; ¿ SiO2, which is widely employed in Ziegler-Natta catalysts for ethylene polymerization. The silica-supported catalyst were used as a reference in spectroscopic analysis; ¿ Al2O3, which is widely employed in acid catalysis. In this case the aim was to match the titanium activity towards ethylene with the reactivity of the support. The properties of the grafted Ti sites before and after activation were investigated by means of different spectroscopic and structural techniques, employed in a synergic way, among which in situ FT-IR and diffuse reflectance UV-Vis spectroscopies and XRPD. In situ FT-IR spectroscopy was also used to evaluate the catalysts activity in ethylene polymerization/oligomerization. Attempts to correlate the structure of the active sites with the catalytic activity were done.