X-ray nanotomography uncovers morphological heterogeneity in a polymerization catalyst at multiple reaction stages

Abstract

During olefin polymerization on supported catalysts, the controlled morphological evolution of the catalyst particle is vital for ensuring optimal product properties and high catalyst activity. We employed non-destructive hard X-ray holotomography to quantitatively assess the 3D morphology of multiple silica-supported hafnocene-based catalyst particles during the early stages of gas-phase ethylene polymerization. Image processing and pore network modeling revealed clear variations in the dimensions and interconnectivity of pristine particles' macropore networks. This, together with apparent differences in the fragmentation behavior of pre-polymerized particles, suggests that the reactivity and morphological evolution of individual particles are largely dictated by their unique support and pore space architectures. By minimizing the structural heterogeneity among pristine catalyst particles, more uniform particle morphologies may be obtained. Significant polymerization activity, observed in the particles' interiors, further implies that appropriate polymerization conditions and catalyst kinetics can guarantee sufficiently high particle accessibilities and thus more homogeneous support fragmentation. • 3D X-ray nanotomography on metallocene-based polymerization catalyst particles • Inter- and intraparticle heterogeneity observed at an advanced pre-polymerization stage • Pristine catalyst support structure strongly affects morphological evolution • Significant involvement of the layer-by-layer fragmentation mechanism postulated Our society's demand for polyolefins, such as polyethylene, is steadily rising. To improve the physicochemical properties of these high-performance materials, strict morphological control is required during the polymerization of related catalysts, especially at reaction onset. By assessing the 3D morphology of multiple catalyst particles during the early stages of ethylene polymerization, we were able to identify and correlate morphological differences to the initial catalyst particle support structure. Based on this, we believe that the implementation of structurally more uniform catalyst supports, in combination with controlled pre-polymerization procedures, may help optimize the performance of relevant catalysts. Our multi-particle approach also paves the way for other industrial heterogeneous catalysts, where the spatial distribution of catalytically relevant phases and/or carbon-based products is to be investigated in a representative number of catalyst particles. The morphology of an industrial-grade ethylene polymerization catalyst was investigated at multiple reaction stages using X-ray holotomography, a high-resolution 3D imaging technique. Image processing, radial phase distribution analysis, and pore network modeling revealed significant inter- and intraparticle heterogeneity in both pristine and pre-polymerized particles. Differences in the fragmentation and reactivity of individual catalyst particles can be attributed to the particles' unique support architectures. Mild reaction conditions ensured a high accessibility of the catalyst particles' interiors and more homogeneous support fragmentation.