XAS and XES Techniques Shed Light on the Dark Side of Ziegler–Natta Catalysts: Active‐Site Generation

Abstract

The local structure and the electronic properties of the active Ti sites in heterogeneous Ziegler–Natta catalysts, generated in situ by interaction of the precatalyst with different Al‐alkyl activators, were investigated by combining X‐ray absorption and valence‐to‐core X‐ray emission spectroscopy (XAS and vtc‐XES), coupled with UV/Vis, FTIR, and DFT theoretical calculations. Irrespective of the activator used, the active system was found to be a highly dispersed TiCl3‐like phase in which the Ti sites are surrounded, not only by bridged chlorine ligands (with the same bond length of bulk TiCl3), but also by terminal chlorine ligands, at a much shorter distance. These results set Ziegler–Natta catalysts in the category of complex nanomaterials. Despite the observation that the investigated catalysts polymerize ethylene, cutting‐edge XAS and XES techniques do not yet offer unequivocal proof for the presence of any alkyl chain attached to the Ti sites, as a consequence of the small fraction of the active sites.

Of Poisons and Antidotes in Polypropylene Catalysis

“…Redox titrations concluded that the process is complex, and yields a distribution of titanium oxidation states, with residual Ti IV , and comparable amounts of Ti III and Ti II . The majority of the Ti III species are EPR silent; this behavior has been traced to the formation of aggregates . In this respect, one should recall that the reaction of TiCl 4 with alkyl–aluminum compounds in hydrocarbon solution ultimately leads to the precipitation of crystalline TiCl 3 , which is a moderately active and largely EPR‐silent heterogeneous propene polymerization catalyst .…”

Activation and In Situ Ethylene Polymerization on Silica-Supported Ziegler–Natta Catalysts

“…5,7,8,[29][30][31] In our previous works we have demonstrated the potential of a multi-technique approach in unraveling the nature of the active TiCl x phase in catalysts obtained by reacting together titanium and magnesium chloride tetrahydrofuranate precursors and the aluminum-alkyl activator. 7,8 The investigation is more complex when the TiCl x and MgCl x phases are simultaneously supported on dehydroxylated silica in presence of tetrahydrofuran. This catalyst was developed in the 1980s by a group of scientists from Union Carbide Corporation in order to be used in the UNIPOL process to produce high density polyethylene.…”

Médaille Lavoisier: J. Livage / Neue Mitglieder der Academia Europaea Science Award Electrochemistry: B. D. McCloskey / Ernst‐Haage‐Preis: I. Siewert / Einstein‐Stipendium: D. W. Stephan

“…Zu ihren Forschungsinteressen gehören die Eigenschaften von Oxiden, Zeolithen und Metall‐organischen Gerüstverbindungen. In ChemSusChem hat sie aminfunktionalisierte Metall‐organische Gerüstverbindungen mit gemischten Liganden und in ChemCatChem aktive Ti‐Zentren in heterogenen Ziegler‐Natta‐Katalysatoren beschrieben …”