Kinetics-based differences
in the early stage fragmentation of
two structurally analogous silica-supported hafnocene- and zirconocene-based
catalysts were observed during gas-phase ethylene polymerization at
low pressures. A combination of focused ion beam-scanning electron
microscopy (FIB-SEM) and nanoscale infrared photoinduced force microscopy
(IR PiFM) revealed notable differences in the distribution of the
support, polymer, and composite phases between the two catalyst materials.
By means of time-resolved probe molecule infrared spectroscopy, correlations
between this divergence in morphology and the kinetic behavior of
the catalysts’ active sites were established. The rate of polymer
formation, a property that is inherently related to a catalyst’s
kinetics and the applied reaction conditions, ultimately governs mass
transfer and thus the degree of homogeneity achieved during support
fragmentation. In the absence of strong mass transfer limitations,
a layer-by-layer mechanism dominates at the level of the individual
catalyst support domains under the given experimental conditions.
In the field of Ziegler–Natta
catalysis for olefin polymerization,
carbon monoxide (CO) is used in the industrial practice to quench
the reaction when it proceeds too fast, approaching critical levels
for the plant safety. The quenching effect is explained as due to
the reversible coordination of CO to the titanium active sites, but
no direct evidence has been ever reported. In this work, we designed
a series of experiments to monitor CO adsorption at variable temperatures
on a model Ziegler–Natta catalyst by means of FT-IR spectroscopy.
For the first time, we have been able to spectroscopically detect
CO coordinated to alkylated Ti
3+
sites and the Ti–acyl
species formed upon the subsequent insertion of CO into the Ti
3+
–alkyl bond, both in the absence and in the presence
of the olefin monomer. In perspective, this has important implications
for the characterization of the active sites in industrial Ziegler–Natta
catalysts, even under working conditions.
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