Surface Investigation and Morphological Analysis of Structurally Disordered MgCl2 and MgCl2/TiCl4 Ziegler-Natta Catalysts

Activated MgCl2 nanocrystals were prepared by controlled dealcoholation of the MgCl2·6CH3OH adduct, mimicking the routinely adopted methods to synthesize industrial heterogeneous Ziegler-Natta catalysts. The effect of the alcohol in driving the morphology of MgCl2 crystals, i.e., the type and extension of the exposed surfaces, was investigated by integrating a detailed structural, morphological, and surface characterization with a state-of-the-art computational modeling. FT-IR spectroscopy of CO adsorbed at 100 K emerged as a feasible, simple, and powerful method to characterize the surface of structurally disordered MgCl2 and MgCl2-based Ziegler-Natta catalysts. Our computational morphological analysis revealed that the (012), (015), and (110) surfaces are highly stabilized by methanol as an electron donor, especially at the temperature typically adopted in the preparation of the precatalysts. FT-IR spectroscopy of adsorbed CO allows distinguishing these surfaces from the other penta-coordinated ones and provides a clear experimental evidence that TiCl4 binds to the (110) and (015) surfaces. The (015) surface was never considered in the past and is characterized by an unusual flexibility in the presence of adsorbates, which detach the Mg cations from the Cl underneath, leaving a coordination vacancy available for the binding of asymmetric titanium sites. Since the recent literature identified a tetra-coordinated Mg as a site of election for the deposition of the Ti species relevant in olefin polymerization, the presence of two eligible Mg sites for Ziegler-Natta catalysis is highly interesting.