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.

No abstract

Operando-sensitive spectroscopic techniques were employed for investigating the changes in the molecular structure of the Cr sites in the Cr/SiO Phillips catalyst during ethylene polymerization. Practically, the most arduous barrier to be overcome was the separation of the chromates reduction carried out by ethylene from the subsequent polymerization. By carefully tuning the experimental parameters we succeeded in observing these two events separately. We found that the sites involved in ethylene polymerization are mainly divalent Cr ions in a 6-fold coordination, in interaction with the oxygenated byproduct (mostly methylformate, generated from the disproportionation of two formaldehyde molecules). Unreduced Cr species are also present during ethylene polymerization as well as reduced Cr species (either Cr or Cr) acting as spectators. Our results challenge the old vision of "naked" chromium species (i.e., low coordinated) as the active sites and attribute a fundamental role to external (and flexible) oxygenated ligands that resemble the ancillary ligands in homogeneous polymerization catalysis.