Allumina e monoliti di silice drogati con allumina come supporti per catalizzatori di Phillips basati su Cr.

This Master thesis work is devoted to the study of the role of different supports in affecting the catalytic performances of the Cr-based Phillips catalyst, and in particular of Al2O3 compared to SiO2. Phillips catalyst is among the most important heterogeneous catalysts for ethylene polymerization, since it accounts for about 50% of the high density polyethylene (HDPE) world's demand, and it has also a large share of linear low density polyethylene (LLDPE) production. The active phase is constituted by a highly dispersed chromium oxide supported on a high surface area material. Although porous SiO2 is the most employed in the industrial practice, Al2O3 can also serve as a support. In respect to silica, alumina has weakly acidic Al-OH groups and exhibits a higher surface OH groups population in the same activation conditions, both factors influencing the amount and the properties of the grafted chromium sites. According to the specialized literature, Al2O3-supported Phillips catalysts provides only about 10-20% of the polymerization activity of Cr/silica, but exhibits a faster kinetics profile, and produces a polyethylene with an extremely high molecular weight, even reaching the ultrahigh (UHMWPE) classification. In the attempt to understand the origin of the different catalytic behaviour of Cr/alumina vs. Cr/silica catalysts, two classes of systems have been investigated in details by means of FT-IR and UV-Vis spectroscopies, applied also in presence of probe molecules: i) Two Cr/δ-Al2O3 samples in the powder form, with a Cr loading of 0.5 and 1.0 wt%, were prepared by wet-impregnation of a commercial δ-Al2O3 (Halon C, Degussa). ii) Three mesoporous silica-alumina glasses having the same Cr loading (0.5 wt%), but differing in the amount of alumina (1%, 10% and 20% of Al2O3, respectively), were synthesized by one-pot acid catalyzed sol-gel route in form of monoliths. These hybrid materials offer the possibility to study the structure and reactivity of different Cr sites, which can be bonded to only monoanionic [≡SiO-] ligands, only monoanionic [=AlO-] ligands, or a mixture of both. Moreover, the absence of light scattering in the whole IR and UV-Vis region makes them ideal systems to investigate the optical and structural properties of grafted Cr sites by means of transmission spectroscopy. All the samples were investigated in both the oxidized and CO-reduced forms, and for some of them also reduction in H2 was investigated. The reactivity of all catalyst towards ethylene at room temperature was tested in terms of polymerization rate and type of polyethylene produced. This latter was further characterized by DSC thermal analysis.