density functional theory study of the polymerization of ethylene on the classical ticl 4 /mgcl 2...
Post on 15-Jan-2016
222 views
TRANSCRIPT
![Page 1: 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](https://reader035.vdocument.in/reader035/viewer/2022062309/56649d5f5503460f94a3f071/html5/thumbnails/1.jpg)
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
![Page 2: 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](https://reader035.vdocument.in/reader035/viewer/2022062309/56649d5f5503460f94a3f071/html5/thumbnails/2.jpg)
The Nature of the Catalytic System
Not very well understood for the classical heterogeneous catalysts.
![Page 3: 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](https://reader035.vdocument.in/reader035/viewer/2022062309/56649d5f5503460f94a3f071/html5/thumbnails/3.jpg)
• 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.
![Page 4: 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](https://reader035.vdocument.in/reader035/viewer/2022062309/56649d5f5503460f94a3f071/html5/thumbnails/4.jpg)
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
![Page 5: 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](https://reader035.vdocument.in/reader035/viewer/2022062309/56649d5f5503460f94a3f071/html5/thumbnails/5.jpg)
DFT-QM/MM Cluster
• Treat part of surface using DFT• Rest of surface described using MM
for steric purposes.
“Mechanical Embedding”
![Page 6: 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](https://reader035.vdocument.in/reader035/viewer/2022062309/56649d5f5503460f94a3f071/html5/thumbnails/6.jpg)
Proposed Sites
• Based upon the crystal structure of MgCl2
• Ti(IV), Ti(III), Ti(II)• Single Ti active sites
![Page 7: 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](https://reader035.vdocument.in/reader035/viewer/2022062309/56649d5f5503460f94a3f071/html5/thumbnails/7.jpg)
MgCl2
![Page 8: 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](https://reader035.vdocument.in/reader035/viewer/2022062309/56649d5f5503460f94a3f071/html5/thumbnails/8.jpg)
Corradini Edge
Slope
Sites
![Page 9: 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](https://reader035.vdocument.in/reader035/viewer/2022062309/56649d5f5503460f94a3f071/html5/thumbnails/9.jpg)
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
![Page 10: 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](https://reader035.vdocument.in/reader035/viewer/2022062309/56649d5f5503460f94a3f071/html5/thumbnails/10.jpg)
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
![Page 11: 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](https://reader035.vdocument.in/reader035/viewer/2022062309/56649d5f5503460f94a3f071/html5/thumbnails/11.jpg)
New Model
Magni and Somorjai J. Phys. Chem. 35, 14786, 1996
Heat of desorption 155 kJ/mol
![Page 12: 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](https://reader035.vdocument.in/reader035/viewer/2022062309/56649d5f5503460f94a3f071/html5/thumbnails/12.jpg)
TiCl2
MgCl2 crystal parameters: 3.596Å, 17.589ÅTiCl2 crystal parameters: 3.561Å, 5.875Å
![Page 13: 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](https://reader035.vdocument.in/reader035/viewer/2022062309/56649d5f5503460f94a3f071/html5/thumbnails/13.jpg)
Binding Energies
Site B.E. [kJ/mol]
Ti IV Corr. 116
Edge 103
Slope 111
Ti III Corr. 178
Edge 178
Slope 143
![Page 14: 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](https://reader035.vdocument.in/reader035/viewer/2022062309/56649d5f5503460f94a3f071/html5/thumbnails/14.jpg)
The MechanismResting States/MgCl2
Corradini III/MgCl2
Slope IV /TiCl2
Slope III/ MgCl2
Alkylation energies slightlyexothermic
![Page 15: 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](https://reader035.vdocument.in/reader035/viewer/2022062309/56649d5f5503460f94a3f071/html5/thumbnails/15.jpg)
Ethylene Complex
Slope IV Back/TiCl2
Edge III front/MgCl2
Complexation energies: ~ 45 kJ/mol (III) ~ -5 kJ/mol (IV)
![Page 16: 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](https://reader035.vdocument.in/reader035/viewer/2022062309/56649d5f5503460f94a3f071/html5/thumbnails/16.jpg)
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)
![Page 17: 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](https://reader035.vdocument.in/reader035/viewer/2022062309/56649d5f5503460f94a3f071/html5/thumbnails/17.jpg)
Products
Total reaction ~100 kJ/mol exothermic
![Page 18: 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](https://reader035.vdocument.in/reader035/viewer/2022062309/56649d5f5503460f94a3f071/html5/thumbnails/18.jpg)
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
![Page 19: 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](https://reader035.vdocument.in/reader035/viewer/2022062309/56649d5f5503460f94a3f071/html5/thumbnails/19.jpg)
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).
![Page 20: 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](https://reader035.vdocument.in/reader035/viewer/2022062309/56649d5f5503460f94a3f071/html5/thumbnails/20.jpg)
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.
![Page 21: 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](https://reader035.vdocument.in/reader035/viewer/2022062309/56649d5f5503460f94a3f071/html5/thumbnails/21.jpg)
Acknowledgements
• Eastman Chemical Company• Ziegler Group