A periodic hybrid DFT approach (including dispersion) to MgCl ₂-supported Ziegler-Natta catalysts - 1: TiCl ₄ adsorption on MgCl ₂ crystal surfaces

The adsorption of TiCl ₄ on the surfaces of MgCl ₂ crystals has been investigated by means of state-of-the-art periodic hybrid DFT methods, as the first step of a comprehensive study aiming to elucidate the structure of the active species in industrial MgCl ₂-supported Ziegler-Natta catalysts for ethene and propene polymerization. A first distinctive feature of the approach was the thorough evaluation of dispersion forces, crucial because the binding of TiCl ₄ on MgCl ₂ surfaces turned out to be essentially dispersion-driven. Also important was a careful investigation of the effects of different choices on basis set and density functional (DF) on the quantitative aspects of the results; this allowed us to trace the unusually large disagreement in the previous literature and identify unambiguous trends. In particular, three full sets of calculations were run adopting the B3LYP(-D), PBE0(-D) and M06 approximations; to the best of our knowledge, the last represents the first case of M06 functional implementation in a periodic code (CRYSTAL) of widespread use. The results consistently indicated that the adsorption of TiCl ₄ on well-formed MgCl ₂ crystals under conditions relevant for catalysis can only occur on MgCl ₂(1 1 0) or equivalent lateral faces, whereas the interaction with MgCl ₂(1 0 4) - for decades claimed as the most important surface in stereoselective catalysts - is too weak for the formation of stable adducts. The implications of these findings for catalysis are discussed.