Preparation and Surface Science Characterization of Model Ziegler−Natta Catalysts. Role of Undercoordinated Surface Magnesium Atoms in the Chemisorption of TiCl4 on MgCl2 Thin Films

Abstract

Two new synthetic routes for the preparation of model Ziegler−Natta catalysts under UHV conditions are described. The exposure of metallic magnesium to TiCl4 produced titanium chloride films with Ti in the 4+, 3+, 2+, and 0 oxidation states. Stable titanium chloride films could also be obtained by TiCl4 and Mg codeposition on MgCl2 and Au. A TiCl4/TiCl2 film was obtained in this case. The reaction of these systems with AlEt3 produced model catalysts for the polymerization of both ethylene and propylene. XPS is a proper technique for the characterization of the oxidation state of Ti in a variety of titanium chloride surface species. The stability of MgCl2 surfaces with a high concentration of undercoordinated Mg atoms was studied in UHV by Mg gas-phase deposition on a MgCl2 multilayer film. The Mg adatoms were readily coordinated by the Cl ions diffusing from the halide bulk to the surface. Mg-containing MgCl2 faces are thermodynamically unstable, and the fast diffusion of the Cl ions in the MgCl2 matrix allows the recovery of the chlorine termination to lower the system surface energy. The high mobility of the chlorine ions is of central importance for the molecular level understanding of the dynamic equilibrium among the MgCl2 surface, TiCl4, and the electron donors used in the synthesis of high performance microporous Ziegler−Natta catalysts. The deposition of MgCl2 in the presence of TiCl4 was studied for the stabilization of high Miller index faces during the MgCl2 film growth. The interaction between the two halides is too weak to influence the MgCl2 deposition, and TiCl4 could not be chemisorbed at 300 K.