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