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.

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Presentation transcript:

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.

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.

Theoretical Approach Studying low-concentration impurities on a surface is still difficult. MgCl 2 is an insulator. An advantage. Three commonly used approaches: Ignore the surface completely Cluster methods Slab methods

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

Proposed Sites Based upon the crystal structure of MgCl 2 Ti(IV), Ti(III), Ti(II) Single Ti active sites

MgCl 2

Corradini Edge Slope Sites

Binding Energies SiteB.E. [kJ/mol] Ti IVCorr.18 Edge20 Slope27 Ti IIICorr.80 Edge86 Slope84 Ti IICorr176 Edge93 Slope104  ST  52 kJ/mol

Further Calculations SiteQM Mech QM (Fr) cluster a QM cluster QM Slab Corr Edge2023- Slope a Monaco et al Macromolecules 33, 8953, b Somorjai et al Appl. Surf. Sci. 89, 187, 1995 and J. Phys. Chem. B. 102, 8788, 1998 Experiment:  kJ/mol b

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

TiCl 2 MgCl 2 crystal parameters: 3.596Å, Å TiCl 2 crystal parameters: 3.561Å, 5.875Å

Binding Energies SiteB.E. [kJ/mol] Ti IVCorr.116 Edge103 Slope111 Ti IIICorr.178 Edge178 Slope143

The Mechanism Resting States/MgCl 2 Corradini III/ MgCl 2 Slope IV / TiCl 2 Slope III/ MgCl 2 Alkylation energies slightly exothermic

Ethylene  Complex Slope IV Back/TiCl 2 Edge III front/ MgCl 2 Complexation energies: ~ 45 kJ/mol (III) ~ -5 kJ/mol (IV)

Transition States/MgCl 2 Slope front III/MgCl 2 Slope back IV/TiCl 2 Typically, R(C  -C 1 ) III < R(C  -C 1 ) IV Internal energy barrier lower for IV but overall barrier lower for III (~0)

Products Total reaction ~100 kJ/mol exothermic

Termination/MgCl 2 Corradini III/MgCl 2 Edge III/MgCl 2 Reaction not exactly symmetric but close. Energy barriers similar to insertion reaction. H-transfer to Ti does not appear to go. Slope back IV/TiCl 2

Insertion vs Termination SiteMgCl 2 TiCl 2 Slope Fr. IV310 Slope Ba. IV63 Edge Fr. IV189 Edge Ba. IV1213 Slope Fr. III0-38 Slope Ba. III7-25 Edge Fr. III280 Edge Ba. III21-4 Corr. III-3-6 Height of termination energy barrier minus height of insertion energy barrier (kJ/mol).

Conclusions TiCl 4 does not bind well to MgCl 2 but TiCl 3 does TiCl 4 binding to TiCl 2 /mixed Ti x Mg (1-x) Cl 2 is an alternative model Low barriers to termination suggest that many of the sites are not active.

Acknowledgements Eastman Chemical Company Ziegler Group