Antinucci, Giuseppe
(2018)
Heterogeneous Ziegler-Natta catalysts: experimental and computational study by means of resonance-based techniques.
[Tesi di dottorato]
| Tipologia del documento: |
Tesi di dottorato
|
| Lingua: |
English |
| Titolo: |
Heterogeneous Ziegler-Natta catalysts: experimental and computational study by means of resonance-based techniques |
| Autori: |
| Autore | Email |
|---|
| Antinucci, Giuseppe | giuseppe.antinucci@unina.it |
|
| Data: |
8 Gennaio 2018 |
| Numero di pagine: |
124 |
| Istituzione: |
Università degli Studi di Napoli Federico II |
| Dipartimento: |
dep19 |
| Dottorato: |
phd078 |
| Ciclo di dottorato: |
30 |
| Coordinatore del Corso di dottorato: |
| nome | email |
|---|
| Paduano, Luigi | lpaduano@unina.it |
|
| Tutor: |
| nome | email |
|---|
| Busico, Vincenzo | [non definito] |
|
| Data: |
8 Gennaio 2018 |
| Numero di pagine: |
124 |
| Parole chiave: |
Ziegler-Natta, DFT, solid-state NMR |
| Settori scientifico-disciplinari del MIUR: |
Area 03 - Scienze chimiche > CHIM/03 - Chimica generale e inorganica |
[error in script]
[error in script]
| Depositato il: |
24 Gen 2018 10:18 |
| Ultima modifica: |
14 Mar 2019 11:29 |
| URI: |
http://www.fedoa.unina.it/id/eprint/12279 |
Abstract
Ziegler-Natta Catalysts (ZNC) for the industrial production of isotactic polypropylene were introduced over 60 years ago. Compared with the original TiCl3-based systems, present-day versions are much more complex: they are comprised of a solid precatalyst, in which TiCl4 and a suitable Lewis Base (LB; e.g. an ester or ether) are co-adsorbed on a nano-structured MgCl2 support, and a soluble cocatalyst made of a trialkyl-Al compound and another LB (typically an alkoxysilane). Despite a tremendous commercial success, the question of the exact nature and structure of the active species in such formulations remains open. This hampered rational catalyst design; as a matter of fact, progress has been – and still is – mostly based on trial-and-error.
In this Ph.D. project, which is part of the research programme of the Dutch Polymer Institute (DPI), real-world ZNC and model systems thereof were investigated with an integrated experimental and computational approach. Advanced solid-state Quadrupolar Nuclear Magnetic Resonance (QNMR) and Electron Spin Resonance (ESR) characterizations were backed by state-of-the-art periodic and cluster dispersion-corrected Density Functional Theory (DFT-D) models. The latter were aimed to calculate relevant spectroscopic parameters of use for the interpretation of the spectroscopic data. The project was a collaboration between the host Institution, namely the Federico II University of Naples (Italy; research group of Prof. Vincenzo Busico), Radboud Universiteit Nijmegen (The Netherlands; research group of Prof. Arno Kentgens), the University of Turin (Italy; research group of Prof. Elio Giamello), and ETH Zurich (Switzerland; research group of Prof. Christophe Copéret).
Owing to the complexity of ZNC, a bottom-up approach was applied. The first step was an investigation of the neat MgCl2 support, and simple MgCl2/TiCl4 or MgCl2/LB binary adducts, so as to achieve a deeper insight into the pairwise interactions between components. The investigation was then extended to ternary MgCl2/TiCl4/LB systems (including industrial precatalysts), and finally to their activation process.
Chapter 1 of the present thesis provides a brief historical overview of ZNC, and introduces the challenges of their mechanistic study.
Chapter 2 discusses the question how to prepare dry samples of ‘activated’ (i.e. high-surface-area) MgCl2 and MgCl2/LB adducts for meaningful spectroscopic studies. MgCl2 is an exceedingly hygroscopic solid, and water adsorption can lead to flawed results. Two MgCl2 drying protocols were applied, and evaluated comparatively: one entailed treatment of MgCl2 with SiCl4 in aliphatic hydrocarbon slurry; another consisted in prolonged exposure to a flow of dry N2 at 250°C. The thus obtained dry samples, either neat or after chemisorption of a LB, were characterized by FTIR and/or solid-state NMR. The conclusion was that it is virtually impossible to prevent water from re-adsorbing on (activated) dry MgCl2 under non-UHV conditions (UHV = Ultra High Vacuum), even when the samples are manipulated inside a high-performance glovebox. On the other hand, activated MgCl2/LB adducts featured only minor water contents (despite a much lower average particle size); we trace this finding to an effective ‘shielding’ of the surface by the chemisorbed LB molecules.
Chapter 3 reports on the characterization of MgCl2/LB model adducts by means of advanced solid-state (Q)NMR techniques. The study included DFT-D calculations of quadrupole coupling constants and chemical shift tensors for systems at variable degree of surface coverage, as an aid to the interpretation of the experimental spectra. All results consistently indicated that certain LB (e.g. 1,3-diethers) chemisorb in preference on MgCl2 crystal terminations exposing tetra-coordinated Mg, whereas others (e.g. phthalates) do not feature any strong preference for a particular crystal edge.
Chapter 4 addresses the long-standing question of the chemical activation of the precatalyst by means of the Al-alkyl cocatalyst. DFT-D evaluations of epitaxial TixCl3x (x = 1 or 2) adsorbates on MgCl2(104) and MgCl2(110) terminations, as originally proposed by Corradini and coworkers, were carried out using high-end periodic and cluster methods. The relative stability of various Ti(III) model species via reduction and alkylation of Ti(IV) precursors by Et3Al was calculated, along with the Ti(III) ESR parameters for comparison with experimental data for model and real-world catalysts. Overall, the results pointed to mononuclear Ti(III) adsorbates on MgCl2(110) terminations as the most plausible ZNC active species.
Finally, a brief summary and an outlook of the study are given in Chapter 5.
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