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

• DOI: 10.1021/acs.jpclett.3c03119
• Publication Date: 2024-03-13T22:14:50Z
• Authors: Alexander V. Yakimov; Christoph J. Kaul; Yuya Kakiuchi; Sebastian Sabisch; Felipe Morais Bolner; Jean Raynaud; Vincent Monteil; Pierrick Berruyer; Christophe Copéret
• Keywords: Chemical-Shifts; Electron-Donors; Olefin Polymerization; Polymers; Crystallites; Active-Sites; Elements; Mgcl2; Nmr; Structural-Characterization

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 to understand the electronic structure of the Ti surface sites present in such ZN pre-catalysts, prior to activation with alkyl aluminium. High-field (21.1 T) and low temperature (~100 K) NMR augmented by DFT modelling 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 associate Ti surface sites exhibit 49Ti NMR signatures (δiso, exp = –170 ppm / CQ, exp = 9.3 MHz / κ = 0.05) corresponding to well-defined fully chlorinat-ed hexacoordinated Ti sites adsorbed on a distorted surface of the MgCl2 support, formed upon post-treatment with BCl3 and re-moval of the alkoxo ligands, paralleling with the increased polymerization activity.