Phillips catalysts for ethylene polymerization : surface organization, active sites and performances

Cr supported Phillips catalysts used for the polymerization of ethylene have been investigated by combining several characterization techniques, with the aim to study their surface organization, their active sites, and their performances. An original investigation of Phillips catalysts is achieved by the modelization of the internal surface of the pores of a conventional Cr/SiO2 catalyst. The model catalyst is prepared by spin coating of a Cr precursor on a flat silicon wafer. Surface and mass spectrometry techniques provide direct information on the molecular structures of the Cr species and the composition of the catalyst. Dispersed Cr species are anchored to the support as chromate species during activation. The catalyst was active in polymerization. Polyethylene, the chains of which are linked to the support via Cr atoms, is developped as dome structures. This model approach significantly improves the opportunities to study polymerization catalysts and the polymer product. On the other hand, industrial Phillips catalysts, including unpromoted and Al- and Ti-promoted Cr/SiO2 catalysts, are thoroughly characterized. Their catalytic performances, including activity and polymer characteristics, are evaluated under slurry conditions. The Cr molecular structures and the composition of the catalysts are described after different treatments (oxidation, reduction, reoxidation). Various Cr species are identified such as chromate, Cr(V), Cr(III), Cr2O3 and Cr(II) species. The catalysts may be visualized as material walls separated by pores; their grain surface was enriched in Cr species. Potential active sites are identified during reduction as Lewis Cr centres. The Al and Ti promotion affects the nature and strength distribution of the acid sites of the catalysts, their grain composition, the anchoring of the Cr species, the density of the Cr(II) species and stabilizes reduced Cr(III) species. Cr/SiO2 catalysts were much more active by promoter incorporation, Ti-containing catalysts having the highest activity. Decreased or increased melt indexes were obtained for Al- or Ti-promoted catalysts, respectively. Broader polydispersities were observed for promoted systems.Doctorat en sciences appliquées (FSA 3)--UCL, 200