A Well-Defined Ti Surface Site in Ziegler-Natta Pre-Catalysts from High-field Low Temperature 47/49Ti solid-state NMR Spectroscopy

Ziegler-Natta (ZN) catalysts – typically formulated as TiCl4/MgCl2/AlR3 and possibly containing additional organic ligands – are essential to the production of polyethylene and polypropylene. Despite their industrial relevance and years of research on these materials, the role of each constituent (support, organic ligands, post-treatment with organic or inorganic modifiers…) on the structure of Ti surface sites responsible for polymerization remains poorly understood, partly because of the high complexity of such materials. Herein, we show how high-field 47/49Ti NMR can bring about new lights on the structures of the Ti surface sites in ZN pre-catalysts (prior to activation with alkyl aluminium) resulting from adsorption of TiCl4 on MgCl2 followed by a post-treatment with BCl3, an additive used to improve catalytic activity by increasing the amounts of active sites. The implementation of high-field NMR (900 MHz), low temperature (~100 K), magic angle spinning (10 kHz), CPMG echo train acquisition and DFT modelling, to study this material (TiCl4/MgCl2/BCl3) and molecular analogs, allows the detection of a 47/49Ti NMR signature and the development of a molecular level understanding of the NMR signature of Ti surface sites. The extracted 49Ti NMR parameters (δiso, exp = –170 ppm and CQ, exp = 9.3 MHz) from this signature analyzed by DFT modeling indicate the presence of one specific coordination sphere for Ti, namely a fully chlorinated hexacoordinated Ti site with a symmetric charge distribution, due to the post-treatment with BCl3 (that removes the alkoxide ligands) and the coordination environment provided by surface of an amorphous MgCl2.