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.
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Within a fluid catalytic cracking (FCC) unit, a mixture of catalyst particles that consist of either zeolite Y (FCC-Y) or ZSM-5 (FCC-ZSM-5) is used in order to boost the propylene yield when processing crude oil fractions. Mixtures of differently aged FCC-Y and FCC-ZSM-5 particles circulating in the FCC unit, the so-called equilibrium catalyst (Ecat), are routinely studied to monitor the overall efficiency of the FCC process. In this study, the age of individual catalyst particles is evaluated based upon photographs after selective staining with substituted styrene molecules. The observed color changes are linked to physical properties, such as the micropore volume and catalytic cracking activity data. Furthermore, it has been possible to determine the relative amount of FCC-Y and FCC-ZSM-5 in an artificial series of physical mixtures as well as in an Ecat sample with unknown composition. As a result, a new practical tool is introduced in the field of zeolite catalysis to evaluate FCC catalyst performances on the basis of photo-spectroscopic measurements with an off-the-shelf digital single lens reflex (DSLR) photo-camera with a macro lens. The results also demonstrate that there is an interesting time and cost trade-off between single catalyst particle studies, as performed with e.g. UV-vis, synchrotron-based IR and fluorescence micro-spectroscopy, and many catalyst particle photo-spectroscopy studies, making use of a relatively simple DSLR photo-camera. The latter approach offers clear prospects for the quality control of e.g. FCC catalyst manufacturing plants.
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