Sintesi e caratterizzazione di sistemi modello per catalizzatori Ziegler-Natta

This thesis project is focused on the synthesis and characterization of model Ziegler Natta catalysts, which are among the most used and studied catalyst for olefin polymerization, thanks to their propensity to be modified in order to obtain different grades of polyolefins. Despite more than 60 years of success in the industrial production of polyolefins and many decades of academic research, the basic understanding of these catalysts is still poor, mainly due to the complexity of their formulation and to the high heterogeneity of the active sites. Industrially, Ziegler-Natta catalysts are synthetized following chemical routes, where highly crystalline MgCl2 (α form) interacts with Lewis bases (mainly alcohols) during the synthesis, then de-alcoholated and reacted with TiCl4 to form disordered MgCl2 (δ form), and finally reduced with organoaluminium compounds such as triethylaluminium. The number of simultaneous steps involved in the synthesis makes it difficult to understand the processes involved in the construction of the active sites. This research thesis is motivated by the need of finding model systems for Ziegler-Natta catalysts, where reduced titanium sites (which are the active sites in olefin polymerization) and MgCl2 (which is the active support) co-exist, possibly in the absence of the other components. The main target is to obtain a simplified model system to spectroscopically study Ti3+ sites limiting the number of synthetic steps. One of the possibility would be to co-crystallize alcoholic adducts of TiCl3 and of MgCl2, following an azeotropic distillation method. MgCl2 and violet TiCl3 are both soluble in alcohol, and Mg2+ and Ti3+ are both 6-coordinated in the respective alcoholic complexes, suggesting that during the co-crystallization process the formation of a solid solution between the two adducts is possible. Different precursors will be obtained by changing the synthesis parameters (e.g. Ti concentration, nature of the alcohol, temperature, etc.). Finally, these precursors will be subjected to a controlled de-alcoholation procedure in order to remove the alcohol, ending up in different solid solution between MgCl2 and TiCl3, hereafter defined as model catalysts. The obtained model catalysts will be characterized by a series of spectroscopic and analytical techniques. For example, the properties of the titanium sites exposed at the surface will be evaluated by FT-IR measurements of CO adsorption (as a molecular probe), and the results will be compared to those obtained on industrial catalysts. These measurements will allow to understand if there are some sites in MgCl2 which are more favourable to substitution by Ti3+ or if all of them are energetically equivalent. Diffuse Reflectance UV-VIS spectroscopy will allow to determine the electronic properties of the reduced Ti sites (d-d transition of Ti3+). These data will be compared with those collected on other Ti3+-containing samples, either in a solid structure, in a liquid complex and grafted on support from the gas phase. XRPD and N2 physisorption measurements will be used to investigate the structure of the precursors and of the model catalysts obtained after de-alcoholation, as well as the surface area and porosity. Finally, ethylene polymerization tests will be conducted on all the model catalysts, either without or with addition of the aluminium alkyl. Also the effect of successive modifications by different electron donors might be object of investigation.