Elucidating the Sectioning Fragmentation Mechanism in Silica-Supported Olefin Polymerization Catalysts with Laboratory-Based X-Ray and Electron Microscopy

Strict morphological control over growing polymer particles is an indispensable requirement in many catalytic olefin polymerization processes. In catalysts with mechanically stronger supports, e. g., polymerization-grade silicas, the emergence of extensive cracks via the sectioning fragmentation mechanism requires severe stress build-up in the polymerizing catalyst particle. Here, we report on three factors that influence the degree of sectioning in silica-supported olefin polymerization catalysts. Laboratory-based X-ray nano-computed tomography (nanoCT) and focused ion beam-scanning electron microscopy (FIB-SEM) were employed to study catalyst particle morphology and crack propagation in two showcase catalyst systems, i.e., a zirconocene-based catalyst (i.e., Zr/MAO/SiO2, with Zr=2,2'-biphenylene-bis-2-indenyl zirconium dichloride and MAO=methylaluminoxane) and a Ziegler-Natta catalyst (i.e., TiCl4/MgCl2/SiO2), during slurry-phase ethylene polymerization. The absence of extensive macropores in some of the catalysts' larger constituent silica granulates, a sufficient accessibility of the catalyst particle interior at reaction onset, and a high initial polymerization rate were found to favor the occurrence of the sectioning pathway at different length scales. While sectioning is beneficial for reducing diffusion limitations, its appearance in mechanically stronger catalyst supports can indicate a suboptimal support structure or unfavourable reaction conditions.

Automated summary

Strict control over the morphology of growing polymer particles is crucial in catalytic olefin polymerization processes. This study investigates the factors that influence the degree of sectioning in silica-supported olefin polymerization catalysts. The accessibility of the catalyst particle interior, initial polymerization rate, and support structure were found to be important factors in determining the occurrence of sectioning.