Geometry and local environment of surface sites in Vanadium-based Ziegler-Natta catalysts from 51V solid-state NMR Spectroscopy

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 the systems that involve multiple synthetic steps and components, namely the MgCl2 support, a transition-metal chloride (commonly TiCl4), 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 NMR properties of V and its use in ZieglerNatta catalysts, we utilize 51V solid-state NMR spectroscopy to investigate the structure of V surface species resulting from the adsorption of VOCl3 on MgCl2(thf)1.5 as support. The resulting V-based ZieglerNatta catalyst shows similar ethylene polymerization activity upon activation with alkyl aluminum as its Ti homologs. Using DFT calculations, benchmarked on a library of molecular structural analogs, the experimentally obtained 51V NMR signature was analyzed to elucidate the structure of the surface sites. Using this approach, we demonstrate that the 51V NMR signature contains detailed information about the coordination environment around V, i.e. the type of ancillary and ligands as well as the effect of the morphology of the MgCl2 support on the geometry of the V surface sites, and corresponds to one species. Analysis of the NMR signatures shows that the adsorption of VOCl3 on MgCl2(thf)1.5 generates a welldefined hexacoordinated V-oxo species containing one alkoxy and four chloride ligands, whose local geometry results from the interaction of the V fragment 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 support morphology on surface site structures.