MgCl2-Supported Ziegler-Natta Catalysts; An NMR Perspective

Abstract

Ziegler-Natta catalysts (ZNCs) for the production of isotactic polypropylene are one of the most important industrial catalysts, yet they are still not properly understood at the molecular level. This work presents studies of these catalysts using solid state NMR. Different parts of Ziegler-Natta catalyst model systems have been studied in detail, including the donors, the cocatalyst and the TiCl4 adsorption site. The MgCl2-support is readily observed using 25Mg and 35Cl NMR. The surface sites of these nanoparticles are more difficult to observe. A new method, employing 2D nutation NMR of the satellite transitions, to differentiate between central and satellite transitions is introduced. Distinctive patterns originate from the nutation of the satellites that can be clearly set apart from the nutation behaviour of extensively broadened central transitions. 1H NMR showed, for the first time, the presence of Al-methyl groups in MgCl2/Al-alkyl adducts, which also helped to assign the 27Al spectra. The Al-alkyls were found to be very reactive and a variety of resonances was detected, indicating a multitude of coordination modes. This might be one of the reasons why only a fraction of the titanium in the catalyst system is converted into active Ti3+ species. Titanium chlorides are studied as reference compounds using 35Cl and 47,49Ti NMR. Ti signal could also be obtained for varying ZNC samples. Interestingly, the resulting line shapes appear very similar, suggesting that the titanium coordination is not influenced by donors or cocatalyst. Using correlation experiments, interactions between different components are probed. The use of 1H and 13C NMR, in a tandem approach with DFT calculations, proved to be a fruitful method to investigate the preferential adsorption sites of different donors on the MgCl2 surface