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.
2015
7
9
1432
1437
http://onlinelibrary.wiley.com/journal/10.1002/(ISSN)1867-3899
EXAFS spectroscopy; heterogeneous catalysis; titanium; X-ray absorption spectroscopy; Ziegler-Natta catalysts; Inorganic Chemistry; Organic Chemistry; Physical and Theoretical Chemistry; Catalysis
Groppo, Elena; Gallo, Erik; Seenivasan, Kalaivani; Lomachenko, Kirill A.; Sommazzi, Anna; Bordiga, Silvia; Glatzel, Pieter; Van Silfhout, Roelof; Kachatkou, Anton; Bras, Wim; Lamberti, Carlo
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Utilizza questo identificativo per citare o creare un link a questo documento: https://hdl.handle.net/2318/1530996
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