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

Abstract

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/SiO 2 , with Zr = 2,2'biphenylene-bis-2-indenyl zirconium dichloride and MAO = methylaluminoxane) and a Ziegler-Natta catalyst (i.e., TiCl 4 / MgCl 2 /SiO 2 ), 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.

Transport limitations in polyolefin cracking at the single catalyst particle level

“…Particles of the composite including completely embedded FCC-cat particles were cut out using a scalpel and transferred to a scanning electron microscope (SEM) equipped with a focused ion beam (FIB). Following a method adopted by our group for similar composite systems, 54,55 the composite particles were cut open to reveal cross sections of FCC particles embedded in polymer (Fig. 4).…”

Visualizing the Structure, Composition and Activity of Single Catalyst Particles for Olefin Polymerization and Polyolefin Decomposition

“…Extensive crack formation-associated with the fast kinetics of the Ziegler-Natta catalyst, a rapid formation of polyethylene and significant pressure build-up-led to the break-up of entire catalyst particle-particles in the early stages of slurry-phase ethylene polymerization. [48] SEM instruments are usually equipped to perform energy-dispersive X-ray spectroscopy (EDX). This can be a useful complementary tool for determining the distribution of different catalyst components and reaction products (Figure 2C).…”

“…Last but not least, laboratory-based computed tomography (CT), an absorption contrast-based technique, represents an accessible alternative to synchrotron-based methods for characterizing pristine and polymerized catalyst particles. [28,48,[64][65][66] Nano-computed tomography (nanoCT) has been reported to deliver sub-180 nm resolutions for different silica-supported olefin polymerization catalysts (Figure 8), as determined via Fourier shell correlation analysis (FSC). [48] In general, laboratory-based CT instruments offer experimental and operational flexibility to researchers who do not have regular access to synchrotron facilities.…”

“…[38,46] In recent studies, SEM has been used to image and investigate the early stages of pre-polymerization and fragmentation in silica-supported metallocenes, both at low and high ethylene pressures. [38,[47][48][49] Here, the particle crosssections were imaged in backscattered electron (BSE) mode, thus yielding clear contrast between the polymer and the more electron dense silica fragments (Figure 2A). From a mechanistic point of view, the layer-by-layer mechanism was found to play a prominent role in the fragmentation of these catalyst systems.…”

EIGER2 hybrid-photon-counting X-ray detectors for advanced synchrotron diffraction experiments

“…EIGER2 detectors were also used in laboratory settings, e.g. to facilitate the transition of X-ray absorption spectroscopy (XAS) to laboratory spectrometers (Malzer et al, 2021;Zimmermann et al, 2020;Schlesiger et al, 2020), operando SAXS on batteries (Prehal et al, 2022), nanocomputed tomography (Werny et al, 2022), micro-computed tomography (Solem et al, 2021;Mu ¨ller et al, 2021) and plenoptic X-ray microscopy (Sowa et al, 2020).…”