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

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.

• MgCl2 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 MgCl2

• Ti(IV), Ti(III), Ti(II)• Single Ti active sites

MgCl2

Corradini Edge

Slope

Sites

Binding EnergiesSite B.E.

[kJ/mol]

Ti IV Corr. 18

Edge 20

Slope 27

Ti III Corr. 80

Edge 86

Slope 84

Ti II Corr 176

Edge 93

Slope 104

ST52 kJ/mol

Further Calculations

Site QM Mech

QM (Fr)

clustera

QM cluster

QM Slab

Corr. 18 56 18 31

Edge 20 23 -1 -

Slope 27 21 6 4

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

Experiment: 30-40 kJ/molb

New Model

Magni and Somorjai J. Phys. Chem. 35, 14786, 1996

Heat of desorption 155 kJ/mol

TiCl2

MgCl2 crystal parameters: 3.596Å, 17.589ÅTiCl2 crystal parameters: 3.561Å, 5.875Å

Binding Energies

Site B.E. [kJ/mol]

Ti IV Corr. 116

Edge 103

Slope 111

Ti III Corr. 178

Edge 178

Slope 143

The MechanismResting States/MgCl2

Corradini III/MgCl2

Slope IV /TiCl2

Slope III/ MgCl2

Alkylation energies slightlyexothermic

Ethylene Complex

Slope IV Back/TiCl2

Edge III front/MgCl2

Complexation energies: ~ 45 kJ/mol (III) ~ -5 kJ/mol (IV)

Transition States/MgCl2

Slope front III/MgCl2 Slope back IV/TiCl2

Typically, R(C-C1) III < R(C-C1) IVInternal energy barrier lower for IV butoverall barrier lower for III (~0)

Products

Total reaction ~100 kJ/mol exothermic

Termination/MgCl2

Corradini III/MgCl2 Edge III/MgCl2

Reaction not exactly symmetric but close.Energy barriers similar to insertion reaction.H-transfer to Ti does not appear to go.

Slope back IV/TiCl2

Insertion vs TerminationSite MgCl2 TiCl2

Slope Fr. IV 3 10

Slope Ba. IV 6 3

Edge Fr. IV 18 9

Edge Ba. IV 12 13

Slope Fr. III 0 -38

Slope Ba. III 7 -25

Edge Fr. III 28 0

Edge Ba. III 21 -4

Corr. III -3 -6

Height of termination energy barrier minus height of insertion energy barrier (kJ/mol).

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.

Acknowledgements

• Eastman Chemical Company• Ziegler Group