Spectroscopic Signature and Structure of the Active Sites in Ziegler–Natta Polymerization Catalysts Revealed by Electron Paramagnetic Resonance

Despite decades of extensive studies, the atomic-scale structure of the active sites in heterogeneous Ziegler–Natta (ZN) catalysts, one of the most important processes of the chemical industry, remains elusive and a matter of debate. In the present work, the structure of active sites of ZN catalysts in the absence of ethylene, referred to as dormant active sites, is elucidated from magnetic resonance experiments carried out on samples reacted with increasing amounts of BCl3 so as to enhance the concentration of active sites and observe clear spectroscopic signatures. Using electron paramagnetic resonance (EPR) and NMR spectroscopies, in particular 2D HYSCORE experiments complemented by density functional theory (DFT) calculations, we show that the activated ZN catalysts contain bimetallic alkyl-Ti­(III),Al species whose amount is directly linked to the polymerization activity of MgCl2-supported Ziegler–Natta catalysts. This connects those spectroscopic signatures to the active species formed in the presence of ethylene and enables us to propose an ethylene polymerization mechanism on the observed bimetallic alkyl-Ti­(III),Al species based on DFT computations.

尽管历经数十年的深入研究，作为化学工业最重要工艺之一的多相齐格勒-纳塔（Ziegler–Natta, ZN）催化剂，其活性位点的原子级结构仍未明晰，且存在诸多争议。本研究针对无乙烯存在时齐格勒-纳塔催化剂的活性位点（即所谓的休眠活性位点）结构展开解析：通过对与递增剂量三氯化硼（BCl3）反应的样品开展磁共振实验，以提高活性位点浓度并观测到清晰的光谱特征。研究人员借助电子顺磁共振（electron paramagnetic resonance, EPR）与核磁共振（nuclear magnetic resonance, NMR）光谱技术，尤其是辅以密度泛函理论（density functional theory, DFT）计算的二维超精细相关光谱（2D HYSCORE）实验，证实活化后的齐格勒-纳塔催化剂中存在双金属烷基钛(III)-铝物种，其含量与负载氯化镁的齐格勒-纳塔催化剂的聚合活性直接相关。该研究将上述光谱特征与乙烯存在时形成的活性物种建立了关联，并基于密度泛函理论计算，针对观测到的双金属烷基钛(III)-铝物种提出了乙烯聚合机理。