Sabisch Sebastian, Kakiuchi Yuya, Docherty Scott R, Yakimov Alexander V, Copéret Christophe
Department of Chemistry and Applied Biosciences, ETH Zürich, Vladimir Prelog Weg 1-5, 8093 Zurich, Switzerland.
J Am Chem Soc. 2023 Nov 29;145(47):25595-25603. doi: 10.1021/jacs.3c06200. Epub 2023 Nov 14.
Since its emergence over 50 years ago, the structure of surface sites in Ziegler-Natta catalysts, which are responsible for a major fraction of the world's supply of polyethylene (PE) and polypropylene (PP), has remained elusive. This is in part due to the complexity of these systems that involve multiple synthetic steps and components, namely, the MgCl support, a transition-metal chloride, and several organic modifiers, known as donors, that are used prior and in some instances during the activation step with alkyl aluminum. Due to the favorable nuclear magnetic resonance (NMR) properties of V and its use in Ziegler-Natta catalysts, we utilize V solid-state NMR spectroscopy to investigate the structure of VOCl on MgCl(thf). The resulting catalyst shows ethylene polymerization activity similar to that of its Ti analogues. Using carefully benchmarked density functional theory (DFT) calculations, the experimental V NMR signature was analyzed to elucidate the structure of the surface sites. Using this approach, we demonstrate that the V NMR signature contains information about the coordination environment, i.e., the type of ancillary ligand, and the morphology of the MgCl support. Analysis of the NMR signature shows that the adsorption of VOCl on MgCl(thf) generates a well-defined hexacoordinated V-oxo species containing one alkoxy and four chloride ligands, whose local geometry results from the interaction with an amorphous MgCl surface. This study illustrates how NMR spectroscopy, which is highly sensitive to the local environment of the investigated nuclei, here V, enables us to identify the exact coordination sphere and to address the effect of the support morphology on surface site structures.
自50多年前齐格勒-纳塔催化剂出现以来，其表面位点的结构一直难以捉摸，而这种催化剂负责全球大部分聚乙烯（PE）和聚丙烯（PP）的供应。部分原因在于这些体系的复杂性，它们涉及多个合成步骤和组分，即MgCl载体、过渡金属氯化物以及几种有机改性剂（称为给体），这些给体在使用烷基铝进行活化步骤之前以及在某些情况下在活化步骤期间使用。由于V具有良好的核磁共振（NMR）性质且在齐格勒-纳塔催化剂中得到应用，我们利用V固态NMR光谱研究MgCl(thf)上VOCl的结构。所得催化剂显示出与Ti类似物相似的乙烯聚合活性。通过精心校准的密度泛函理论（DFT）计算，对实验得到的V NMR信号进行分析以阐明表面位点的结构。使用这种方法，我们证明V NMR信号包含有关配位环境（即辅助配体的类型）以及MgCl载体形态的信息。对NMR信号的分析表明，VOCl在MgCl(thf)上的吸附产生了一种定义明确的六配位V-氧物种，该物种含有一个烷氧基和四个氯配体，其局部几何结构源于与无定形MgCl表面的相互作用。这项研究说明了对所研究原子核（此处为V）的局部环境高度敏感的NMR光谱如何使我们能够确定确切的配位球，并研究载体形态对表面位点结构的影响。
