A STUDY ON THE EFFECTS OF CATALYST DISTRIBUTION AND CHANGES TO THE PORE STRUCTURE IN SILICA-SUPPORTED METALLOCENE CATALYST FOR ETHYLENE POLYMERIZATION

Abstract

The disintegration of the catalyst particle during the polymerization of ethylene into successively smaller fragments is necessary for high performance. However, rapid and extensive fragmentation without control may generate fine particles < 100 µm. These fine particles are of poor polymer quality and often lead to reactor fouling and impact process efficiencies and costs. Although methods such as pre-polymerization have been developed to mitigate the effects of fine particles, the root cause of their formation is poorly understood. In this dissertation, the effects of active site and pore size distributions after the immobilization of catalytic compounds such as methylaluminoxane and metallocenes within silica supports on the fragmentation of the catalyst particle during polymerization are systematically investigated. The experimental results indicate a strong correlation between the intraparticle distributions of active sites and the contact time between silica particles and solutions containing catalytic compounds. Non-uniform distributions lead to evidence of extensive fragmentation and higher fractions of fine particles in gas phase polymerization. Next, catalysts with different distributions of active sites and pore diameters were utilized in both gas and slurry phase polymerization of ethylene. The polymer particle morphologies illustrate the effects of the presence and absence of a liquid diluent and the pore diameter on the fragmentation process. A diffusion-adsorption model was developed to generate dynamic radial concentration profiles and total concentrations of catalytic compounds within the particle. The model correlates with the experimental data which assists with the optimization of the preparation condition of the supported catalyst. The effect of pore diameter was further studied by preparing supported catalysts with three different commercially available silicas. The pore diameter was determined to have a significant effect on polymerization activity and polymer properties. Finally, a flat surface silica was developed to directly observe the formation of polymer chains at the active site. These results provide guidance to the preparation and synthesis of metallocene supported catalysts and optimize their performance in ethylene polymerization.