A Density Functional Embedded Cluster Study of Proposed Active Sites in Heterogeneous Ziegler−Natta Catalysts
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Bibliographic record
Abstract
A number of proposed models of the active sites in TiCl 4 /MgCl 2 heterogeneous Ziegler−Natta catalysts are examined using density functional methods. Using a number of different models for the surface, the sites formed when unreduced TiCl 4 is adsorbed onto MgCl 2 are predicted to be unstable. In contrast, TiCl 3 and TiCl 2 are found to bind strongly to MgCl 2 . Alternative models of the surface where one or all of the Mg atoms is replaced with Ti(II) give larger binding energies for TiCl 4 . Each step of the ethylene polymerization reaction is considered for each of the site models that are expected to be active. Two possible termination mechanisms, chain transfer to the monomer and β-hydrogen elimination, are also examined. The sites formed from TiCl 4 bind ethylene more weakly than those formed from TiCl 3 . No particular trends with respect to the type of site are found for the barrier to insertion, but the direction of approach of the ethylene molecule is important. The β-hydrogen elimination termination mechanism is predicted to be much less important than chain transfer to the monomer. From a comparison of ethylene insertion and termination barriers, the TiCl 3 -based sites on the TiCl 2 surface and the TiCl 3 -based Corradini site on MgCl 2 are shown to be very poor catalysts while the others, in particular the TiCl 3 -based edge site on MgCl 2, appear to be more promising models of the actual active sites.
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Codex and Gemma teacher scores by category
| Category | Codex | Gemma |
|---|---|---|
| Metaresearch | 0.000 | 0.000 |
| Meta-epidemiology (narrow) | 0.000 | 0.000 |
| Meta-epidemiology (broad) | 0.000 | 0.000 |
| Bibliometrics | 0.000 | 0.000 |
| Science and technology studies | 0.000 | 0.000 |
| Scholarly communication | 0.000 | 0.000 |
| Open science | 0.000 | 0.000 |
| Research integrity | 0.000 | 0.000 |
| Insufficient payload (model declined to judge) | 0.012 | 0.000 |
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Baseline scores from an immature model (maturity gate not passed, 7 training rounds). Scores rank; they never assert a category.
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