The coordinative state of aluminium alkyls in Ziegler–Natta catalysts

Blaakmeer, E.S.; Eck, Ernst R. H. van; Kentgens, A.P.M.

doi:10.1039/c8cp00603b

Cited by 12 publications

(18 citation statements)

References 54 publications

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“…Detailed sample preparations can be found in the Supporting Information. The industrial precatalyst CAT4 has been supplied by SABIC, and the ternary system TMC-M has been prepared as described elsewhere . Rotors were filled under inert nitrogen atmosphere inside a glovebox.…”

Section: Methodsmentioning

confidence: 99%

Preactive Site in Ziegler–Natta Catalysts

Blaakmeer¹,

Wensink²,

Eck³

et al. 2019

J. Phys. Chem. C

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Ziegler−Natta catalysts are one of the most important industrial catalysts for the production of isotactic polypropylene, yet they are still not properly understood at the molecular level. We explore the potential of 35,37 Cl and 47,49 Ti solid-state NMR to study the formation of catalytically relevant titanium sites. First, a systematic study of titanium chlorides (TiCl 2 , TiCl 3 , and TiCl 4 ) is undertaken to gain insight into the spectral characteristics for different titanium coordinations. For these materials, chlorine spectra can be relatively straightforwardly obtained, despite their strongly broadened quadrupolar line shapes. The sensitivity of titanium NMR to its local environment is exemplified by the TiCl 4 spectrum, for which a small quadrupolar interaction is found despite the nearly symmetric Ti coordination. Upon wet impregnation of MgCl 2 with TiCl 4 , TiCl 4 is immobilized at the surface, retaining its tetrahedral coordination. For a ball-milled binary MgCl 2 /TiCl 2 adduct, a ternary system where donors are added, and a ternary system where Al-alkyl cocatalysts are added, we obtain broad 47,49 Ti spectra after extensive signal averaging, showing that the local environment of Ti is substantially perturbed. The span of the signal is similar for all three samples, suggesting that most of the donor and the cocatalyst do not directly bind to the titanium. Nevertheless, the signal loss from reduction to Ti 3+ is obvious, indicating that a fraction of the titanium sites is activated.

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Section: Methodsmentioning

confidence: 99%

Preactive Site in Ziegler–Natta Catalysts

Blaakmeer¹,

Wensink²,

Eck³

et al. 2019

J. Phys. Chem. C

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“…Recent DFT studies highlighted that AlEt 2 Cl chemisorption onto MgCl 2 surface vacancies is exergonic, particularly when it occurs in bridging mode with adjacent Ti species. 15,33,35,60 As is well-known, molecular (nondissociative) chemisorption is typically a nonactivated process.…”

Section: ■ Materials and Methodsmentioning

confidence: 99%

Monitoring the Kinetics of Internal Donor Clean-up from Ziegler–Natta Catalytic Surfaces: An Integrated Experimental and Computational Study

et al. 2020

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Surface modification of MgCl 2 -supported Ziegler−Natta catalysts (ZNCs) by means of organic Lewis bases, either used as precatalyst components ("internal donors", ID) or in combination with the AlEt 3 cocatalyst ("external donors", ED), is key for achieving a high stereoselectivity in propene polymerization. In fourth-generation systems, which are the working horses of this important catalyst class, the ID is an (ortho-)dialkyl phthalate; under polymerization conditions, this reacts with AlEt 3 and must be replaced by an ED (typically an alkoxysilane) in order to obtain the desired performance. In a previous study, we investigated the molecular kinetics of the reaction between dibutyl phthalate and AlEt 3 in solution by means of integrated experimental and computational protocols. A similar approach has now been applied to monitor the progress of the reaction for the complete catalyst system. Compared with solution (ΔH # ≈ 15 kcal mol −1 ; ΔS # ≈ −28 cal mol −1 K −1 ), the activation parameters in heterogeneous phase (ΔH # ≈ 10 kcal mol −1 ; ΔS # ≈ −49 cal mol −1 K −1 ) indicate that phthalate reduction is less activated, but suffers from an augmented entropic penalty. This suggests that the ID reacts with AlEt 3 while still on the MgCl 2 surface, rather than in the liquid phase after desorption, and that the surface is not innocent. Whether or not an alkoxysilane ED was present in the system turned out to be immaterial on reaction kinetics. High-level DFT calculations on a wellestablished MgCl 2 /dibutyphthalate model cluster reproduced the experimental data with a remarkably good agreement (ΔH # ≈ 10 kcal mol −1 ; ΔS # ≈ −46 cal mol −1 K −1 ). Experiments and calculations agree on the first ethyl transfer to the reacting ester group representing the rate-determining step, in solution as well in heterogeneous phase. In both cases, a four-membered transition state (TS) appears to be involved; in heterogeneous phase, though, the ester carbonyl is extra-activated by the interaction with a Lewisacidic surface Mg center, rather than a second AlEt 3 molecule. In our opinion, the interest of the proposed approach goes beyond the present study; indeed, further applications can be proposed for the design and engineering of novel ZNCs with tailored behaviors.

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“…范志强等 [9] 也发现 AlEt 2 Cl 既可配位在活性中心 Ti 原子上, 降低活性中心数目和催化剂的活性, 又可吸附在活性中心 Ti 原子周围的 Mg 原子上, 形成高全同定向性的活性中心, 还可与某些无活性的二价钛配位, 形成新的活性中心. 因此烷基铝及其与 TiCl 4 反应形成的副产物 AlR 2 Cl 及 AlRCl 2 等对 Z-N 催化剂的活性和立构选择性有至关重要的影响 [10][11][12][13][14][15] .…”

Section: 引言unclassified