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.
Structural dynamics of a Mn‐Na2WO4/SiO2 catalyst were detected directly under reaction conditions during the oxidative coupling of methane via in situ XRD and operando Raman spectroscopy. A new concept of fluctuating storage and release of an active phase in heterogeneous catalysis is proposed that involves the transient generation of active sodium oxide species via a reversible reaction of Na2WO4 with Mn7SiO12. The process is enabled by phase transitions and melting at the high reaction temperatures that are typically applied.
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