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This PhD Thesis focuses mainly on the characterization of complex porous materials at the low micrometer and sub-micron length scale. The characterization techniques used in this PhD Thesis can be classified into two synchrotron-based X-ray microscopy (XRM) techniques: i) X-ray holotomography and ii) correlative ptychographic X-ray computed tomography (PXCT) and X-ray fluorescence (XRF) tomography. X-ray holotomography combined with pore network (PN) modeling was used to study deactivation mechanisms caused by coke and metal deposition within a single fluid catalytic cracking (FCC) catalyst particle. Furthermore, the same combination was used for studying the heterogeneity in the fragmentation behavior of a set of metallocene-type catalyst particles at different stages of ethylene polymerization. Correlative PXCT and XRF tomography was used to perform statistical analysis and to quantify the degree of fragmentation of an ensemble of Ziegler-type ethylene polymerization catalyst particles by mapping support, polymer, and mixed phases at high spatial resolution. Moreover, the pore structure as well as the element distribution within a single FCC catalyst particle was mapped using correlative PXCT and XRF tomography. The obtained morphological and elemental information allowed performing a mass transport simulation based on pore network modeling, which revealed invaluable insights into the diffusion behavior of reactant and product molecules as well as active site accessibility within a single FCC catalyst particle.

Metal–Organic Frameworks (MOFs) have the potential to change the landscape of molecular separations in chemical processes owing to their ability of selectively binding molecules. Their molecular sorting properties generally rely on the micro- and meso-pore structure, as well as on the presence of coordinatively unsaturated sites that interact with the different chemical species present in the feed. In this work, we show a first-of-its-kind tomographic imaging of the crystal morphology of a metal–organic framework by means of transmission X-ray microscopy (TXM), including a detailed data reconstruction and processing approach. Corroboration with Focused Ion Beam-Scanning Electron Microscopy (FIB-SEM) images shows the potential of this strategy for further (non-destructively) assessing the inner architecture of MOF crystals. By doing this, we have unraveled the presence of large voids in the internal structure of a MIL-47(V) crystal, which are typically thought of as rather homogeneous lattices. This challenges the established opinion that hydrothermal syntheses yield relatively defect-free material and sheds further light on the internal morphology of crystals., TXM-tomography unraveled large macropore defects within a MIL-47(V) MOF crystal. These pores do not seem to be well connected and they show a preferential orientation.

Catalyst deactivation involves a complex interplay of processes taking place at different length and time scales. Understanding this phenomenon is one of the grand challenges in solid catalyst characterization. A process contributing to deactivation is carbon deposition (i. e., coking), which reduces catalyst activity by limiting diffusion and blocking active sites. However, characterizing coke formation and its effects remains challenging as it involves both the organic and inorganic phase of the catalytic process and length scales from the atomic scale to the scale of the catalyst body. Here we present a combination of hard X-ray imaging techniques able to visualize in 3-D the distribution, effect and nature of carbon deposits in the macro-pore space of an entire industrially used catalyst particle. Our findings provide direct evidence for coke promoting effects of metal poisons, pore clogging by coke, and a correlation between carbon nature and its location. These results provide a better understanding of the coking process, its relation to catalyst deactivation and new insights into the efficiency of the industrial scale process of fluid catalytic cracking.

JACS Au 1(6), 852 - 864 (2021). doi:10.1021/jacsau.1c00130, Ziegler-type catalysts are the grand old workhorse of the polyolefin industry, yet their hierarchically complex nature complicates polymerization activity–catalyst structure relationships. In this work, the degree of catalyst framework fragmentation of a high-density polyethylene (HDPE) Ziegler-type catalyst was studied using ptychography X-ray-computed nanotomography (PXCT) in the early stages of ethylene polymerization under mild reaction conditions. An ensemble consisting of 434 fully reconstructed ethylene prepolymerized Ziegler catalyst particles prepared at a polymer yield of 3.4 g HDPE/g catalyst was imaged. This enabled a statistical route to study the heterogeneity in the degree of particle fragmentation and therefore local polymerization activity at an achieved 3-D spatial resolution of 74 nm without requiring invasive imaging tools. To study the degree of catalyst fragmentation within the ensemble, a fragmentation parameter was constructed based on a k-means clustering algorithm that relates the quantity of polyethylene formed to the average size of the spatially resolved catalyst fragments. With this classification method, we have identified particles that exhibit weak, moderate, and strong degrees of catalyst fragmentation, showing that there is a strong heterogeneity in the overall catalyst particle fragmentation and thus polymerization activity within the entire ensemble. This hints toward local mass transfer limitations or other deactivation phenomena. The methodology used here can be applied to all polyolefin catalysts, including metallocene and the Phillips catalysts to gain statistically relevant fundamental insights in the fragmentation behavior of an ensemble of catalyst particles., Published by ACS Publications, Washington, DC

Journal of the American Chemical Society 142(8), 3691 - 3695 (2020). doi:10.1021/jacs.9b13485, A combination of X-ray ptychography and X-ray fluorescence tomography (XRF) has been used to study the fragmentation behavior of an individual Ziegler–Natta catalyst particle, ∼40 μm in diameter, in the early stages of propylene polymerization with submicron spatial resolution. The electron density signal obtained from X-ray ptychography gives the composite phases of the Ziegler–Natta catalyst particle fragments and isotactic polypropylene, while 3-D XRF visualizes multiple isolated clusters, rich in Ti, of several microns in size. The radial distribution of Ti species throughout the polymer–catalyst composite particle shows that the continuous bisection fragmentation model is the main contributor to the fragmentation pathway of the catalyst particle as a whole. Furthermore, within the largest Ti clusters the fragmentation pathway was found to occur through both the continuous bisection and layer-by-layer models. The fragmentation behavior of polyolefin catalysts was for the first time visualized in 3-D by directly imaging and correlating the distribution of the Ti species to the polymer–catalyst composite phase., Published by American Chemical Society, Washington, DC

ChemCatChem 13(10), 2494 - 2507 (2021). doi:10.1002/cctc.202100276, Catalyst deactivation involves a complex interplay of processes taking place at different length and time scales. Understanding this phenomenon is one of the grand challenges in solid catalyst characterization. A process contributing to deactivation is carbon deposition (i. e., coking), which reduces catalyst activity by limiting diffusion and blocking active sites. However, characterizing coke formation and its effects remains challenging as it involves both the organic and inorganic phase of the catalytic process and length scales from the atomic scale to the scale of the catalyst body. Here we present a combination of hard X-ray imaging techniques able to visualize in 3-D the distribution, effect and nature of carbon deposits in the macro-pore space of an entire industrially used catalyst particle. Our findings provide direct evidence for coke promoting effects of metal poisons, pore clogging by coke, and a correlation between carbon nature and its location. These results provide a better understanding of the coking process, its relation to catalyst deactivation and new insights into the efficiency of the industrial scale process of fluid catalytic cracking., Published by WILEY-VCH Verlag, Weinheim