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Well-Defined Ti Surface Sites in Ziegler–Natta Pre-Catalysts from 47/49Ti Solid-State Nuclear Magnetic Resonance Spectroscopy

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posted on 2024-03-13, 22:15 authored by Alexander V. Yakimov, Christoph J. Kaul, Yuya Kakiuchi, Sebastian Sabisch, Felipe Morais Bolner, Jean Raynaud, Vincent Monteil, Pierrick Berruyer, Christophe Copéret
Treatment of Ziegler–Natta (ZN) catalysts with BCl3 improves their activity by increasing the number of active sites. Here we show how 47/49Ti solid-state nuclear magnetic resonance (NMR) spectroscopy enables us to understand the electronic structure of the Ti surface sites present in such treated ZN pre-catalysts, prior to activation with alkyl aluminum. High-field (21.1 T) and low-temperature (∼100 K) NMR augmented by DFT modeling on the pre-catalyst and corresponding molecular analogues enables the detection of 47/49Ti NMR signatures and a molecular level understanding of the electronic structure of Ti surface sites. The associated Ti surface sites exhibit 49Ti NMR signatures (δiso, exp = −170 ppm; CQ, exp = 9.3 MHz; κ = 0.05) corresponding to well-defined fully chlorinated hexacoordinated Ti sites adsorbed on a distorted surface of the MgCl2 support, formed upon post-treatment with BCl3 and removal of the alkoxo ligands, paralleling the increased polymerization activity.

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