Preactive Site in Ziegler–Natta Catalysts
2019
Blaakmeer, E. S (Merijn) | Wensink, Frank J. | van Eck, Ernst R. H. | de Wijs, Gilles A. | Kentgens, Arno P. M.
Ziegler–Natta catalysts are one of the most important industrial catalysts for the production of isotactic polypropylene, yet they are still not properly understood at the molecular level. We explore the potential of ³⁵,³⁷Cl and ⁴⁷,⁴⁹Ti solid-state NMR to study the formation of catalytically relevant titanium sites. First, a systematic study of titanium chlorides (TiCl₂, TiCl₃, and TiCl₄) is undertaken to gain insight into the spectral characteristics for different titanium coordinations. For these materials, chlorine spectra can be relatively straightforwardly obtained, despite their strongly broadened quadrupolar line shapes. The sensitivity of titanium NMR to its local environment is exemplified by the TiCl₄ spectrum, for which a small quadrupolar interaction is found despite the nearly symmetric Ti coordination. Upon wet impregnation of MgCl₂ with TiCl₄, TiCl₄ is immobilized at the surface, retaining its tetrahedral coordination. For a ball-milled binary MgCl₂/TiCl₂ adduct, a ternary system where donors are added, and a ternary system where Al-alkyl cocatalysts are added, we obtain broad ⁴⁷,⁴⁹Ti spectra after extensive signal averaging, showing that the local environment of Ti is substantially perturbed. The span of the signal is similar for all three samples, suggesting that most of the donor and the cocatalyst do not directly bind to the titanium. Nevertheless, the signal loss from reduction to Ti³⁺ is obvious, indicating that a fraction of the titanium sites is activated.
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