RESUMEN
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.
RESUMEN
Treatment of Ziegler-Natta (ZN) catalysts with BCl3 improves their activity by increasing the number of active sites. Here we show how 47/49Ti solid-state nuclear magnetic resonance (NMR) spectroscopy enables us to understand the electronic structure of the Ti surface sites present in such treated ZN pre-catalysts, prior to activation with alkyl aluminum. High-field (21.1 T) and low-temperature (â¼100 K) NMR augmented by DFT modeling on the pre-catalyst and corresponding molecular analogues enables the detection of 47/49Ti NMR signatures and a molecular level understanding of the electronic structure of Ti surface sites. The associated Ti surface sites exhibit 49Ti NMR signatures (δiso, exp = -170 ppm; CQ, exp = 9.3 MHz; κ = 0.05) corresponding to well-defined fully chlorinated hexacoordinated Ti sites adsorbed on a distorted surface of the MgCl2 support, formed upon post-treatment with BCl3 and removal of the alkoxo ligands, paralleling the increased polymerization activity.
RESUMEN
Since the inception of the unprecedented rise of halide perovskites for photovoltaic research, ion migration has shadowed this material class with undesirable hysteresis and degradation effects, limiting its practical implementations. Unfortunately, the localized doping and electrochemical reactions triggered by ion migration cause many more undesirable effects that are often unreported or misinterpreted because they deviate from classical semiconductor behavior. In this Perspective, we provide a concise overview of such effects in halide perovskites, such as operational instability in photovoltaics, polarization-induced abnormal external quantum efficiency in light-emitting diodes, and energy channel shift and anomalous sensitivities in hard radiation detection. Finally, we highlight a unique use case of exploiting ion migration as a boon to design emerging memory technologies such as memristors for information storage and computing.