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.
XAS and XES techniques shed light on the dark side of Ziegler-Natta catalysts: Active-site generation
GROPPO, Elena Clara;BORDIGA, Silvia;LAMBERTI, Carlo
2015-01-01
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.File | Dimensione | Formato | |
---|---|---|---|
GROPPO_ChemCatChem2015_POSTPRINT.pdf
Open Access dal 02/12/2015
Descrizione: Articolo Post-Print
Tipo di file:
POSTPRINT (VERSIONE FINALE DELL’AUTORE)
Dimensione
544.51 kB
Formato
Adobe PDF
|
544.51 kB | Adobe PDF | Visualizza/Apri |
15_ZN_ActiveSiteGeneration.pdf
Accesso riservato
Descrizione: Printed version
Tipo di file:
PDF EDITORIALE
Dimensione
2.31 MB
Formato
Adobe PDF
|
2.31 MB | Adobe PDF | Visualizza/Apri Richiedi una copia |
I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.