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

Abstract

AbstractThe 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.