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Michael Seth and Tom Ziegler University of Calgary

Density Functional Theory Study of the Polymerization of Ethylene on the Classical TiCl 4 /MgCl 2 Ziegler-Natta Catalyst. Michael Seth and Tom Ziegler University of Calgary. The Nature of the Catalytic System. Not very well understood for the classical heterogeneous catalysts.

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Michael Seth and Tom Ziegler University of Calgary

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  1. Density Functional Theory Study of the Polymerization of Ethylene on the Classical TiCl4/MgCl2 Ziegler-Natta Catalyst Michael Seth and Tom Ziegler University of Calgary

  2. The Nature of the Catalytic System Not very well understood for the classical heterogeneous catalysts. Polymerization Mechanism

  3. A number of different active sites. • Ti is present in several oxidation states. • XAS/EXAFS – Suggest Ti IV • XPS – Suggest TiIV. Or not. • EPR results –Ti III • Added Lewis bases change rate/stereospecificity. • Termination proceeds mainly by H-transfer to the monomer.

  4. Theoretical Approach • Studying low-concentration impurities on a surface is still difficult. • MgCl2 is an insulator. An advantage. Three commonly used approaches: Ignore the surface completely Cluster methods Slab methods

  5. DFT-QM/MM Cluster • Treat part of surface using DFT • Rest of surface described using MM for steric purposes. “Mechanical Embedding”

  6. Proposed Sites • Based upon the crystal structure of MgCl2 • Ti(IV), Ti(III), Ti(II) • Single Ti active sites

  7. MgCl2

  8. Sites Corradini Edge Slope

  9. Binding Energies DST52 kJ/mol

  10. Further Calculations Experiment:  30-40 kJ/molb a Monaco et al Macromolecules 33, 8953, 2000. bSomorjai et al Appl. Surf. Sci. 89, 187, 1995 and J. Phys. Chem. B. 102, 8788, 1998

  11. New Model Magni and Somorjai J. Phys. Chem. 35, 14786, 1996 Heat of desorption  155 kJ/mol

  12. TiCl2 MgCl2 crystal parameters: 3.596Å, 17.589Å TiCl2 crystal parameters: 3.561Å, 5.875Å

  13. Binding Energies

  14. The MechanismResting States/MgCl2 Slope IV / TiCl2 Corradini III/ MgCl2 Alkylation energies slightly exothermic Slope III/ MgCl2

  15. Ethylene p-Complex Slope IV Back/TiCl2 Edge III front/ MgCl2 Complexation energies: ~ 45 kJ/mol (III) ~ -5 kJ/mol (IV)

  16. Transition States/MgCl2 Slope front III/MgCl2 Slope back IV/TiCl2 Typically, R(Ca-C1) III < R(Ca-C1) IV Internal energy barrier lower for IV but overall barrier lower for III (~0)

  17. Products Total reaction ~100 kJ/mol exothermic

  18. Termination/MgCl2 Edge III/MgCl2 Slope back IV/TiCl2 Corradini III/MgCl2 Reaction not exactly symmetric but close. Energy barriers similar to insertion reaction. H-transfer to Ti does not appear to go.

  19. Insertion vs Termination Height of termination energy barrier minus height of insertion energy barrier (kJ/mol).

  20. Conclusions • TiCl4 does not bind well to MgCl2 but TiCl3 does • TiCl4 binding to TiCl2/mixed TixMg(1-x)Cl2 is an alternative model • Low barriers to termination suggest that many of the sites are not active.

  21. Acknowledgements • Eastman Chemical Company • Ziegler Group

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