Geometry and Local Environment of Surface Sites in Vanadium-Based Ziegler-Natta Catalysts from ⁵¹V Solid-State NMR...

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 MgCl₂ 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 ⁵¹V solid-state NMR spectroscopy to investigate the structure of VOCl₃ on MgCl₂(thf)_1.5. The resulting catalyst shows ethylene polymerization activity similar to that of its Ti analogues. Using carefully benchmarked density functional theory (DFT) calculations, the experimental ⁵¹V NMR signature was analyzed to elucidate the structure of the surface sites. Using this approach, we demonstrate that the ⁵¹V NMR signature contains information about the coordination environment, i.e., the type of ancillary ligand, and the morphology of the MgCl₂ support. Analysis of the NMR signature shows that the adsorption of VOCl₃ on MgCl₂(thf)_1.5 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 MgCl₂ 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.