Effect of Intraparticle Mass Transfer on the Catalytic Site Formation in the Preparation of Silica-Supported Metallocene Catalysts

The preparation conditions of silica-supported metallocene catalysts impact the catalytic performance and consistency in ethylene polymerization. In a commonly used catalyst impregnation technique, the concentrations of catalyst solutions and the contact time between silica microparticles and catalyst solutions are used to ensure uniform and stable immobilization of coactivator methyl aluminoxane (MAO) and metallocenes. The overall catalytic activities for olefin polymerization, catalyst particle fragmentation, and the resulting polymer particle morphology depend upon the distribution of active catalyst sites within the silica catalyst particle. This work presents both experimental and theoretical modeling studies on the intraparticle spatial distribution of MAO and metallocene in highly porous silica microparticles for ethylene polymerization in a liquid-slurry phase. The silica-catalyst solution contact times and the catalyst solution concentrations are used as two major variables to vary the intraparticle catalyst site distributions. The overall aluminum and zirconium contents in the catalyst and their radial distributions are observed to be strongly affected by the impregnation conditions. A mathematical model is also derived for the catalyst impregnation process and it provides adequate predictions of the impregnation process characteristics.