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

Abstract

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.

Formation of Highly Active Ziegler–Natta Catalysts Clarified by a Multifaceted Characterization Approach

“…On the contrary, IR spectroscopy provides new insights into the fate of the donor after TEAl activation, which is one of the main open questions in ZN catalysis. As a matter of fact, depending on the temperature, a more or less large fraction of DBP is known to be cleaned up by TEAl (a minimal fraction at room temperature, almost the totality at about 100 °C), 91 but the state of the molecules remaining on the catalyst surfaces is still unclear. Figure 7b,c displays the magnification of the ν(CO) region for C and F before and after TEAl activation at room temperature, revealing an analogous trend for the samples corresponding to the initial DBP addition and to the sample at the end of the synthesis (the quantification of the integrated areas for all the individual ν(CO) components are reported in Table S3).…”

Understanding the Role of Sulfonyl Amine Donors in Propylene Polymerization Using MgCl2-Supported Ziegler–Natta Catalyst

“…More importantly, the suitable internal donor and external donor pair can enhance the stereoregularity of the polymer product. The internal donor is typically added during the catalyst preparation, e.g., ethyl benzoate (EB) (third generation), dialkyl phthalate (fourth generation), 1,3-diether (fifth generation), and dialkyl succinate (sixth generation). − The external donor is added during the olefin polymerization process in combination with the cocatalyst AlR 3 , e.g., silane, and alkoxysilane. − The appropriate internal donor and external donor pair can control the regularity of the polypropylene and greatly increase the yield of the isotactic polypropylene. , In addition, the alkylation product AlR n Cl 3– n generated from AlR 3 could also be stably coordinated in the vicinity of the Ti active site, promoting the stereo- and regioselectivity of propylene insertion. , …”

The atomic defects on the (104) and (110) surfaces of the MgCl2-supported Ziegler–Natta catalyst: a periodic DFT study

“…So far, most studies have focused on the ideal (110) and (104) MgCl 2 surfaces. 19–34 However, it should be noted that surface defects, such as edges, corners, vacancies, steps, etc. , could be formed during the synthesis of the Ziegler–Natta catalyst under extremely drastic conditions.…”

“…So far, most studies have focused on the ideal (110) and (104) MgCl 2 surfaces. [19][20][21][22][23][24][25][26][27][28][29][30][31][32][33][34] However, it should be noted that surface defects, such as edges, corners, vacancies, steps, etc., could be formed during the synthesis of the Ziegler-Natta catalyst under extremely drastic conditions. 16,18,[35][36][37][38][39][40][41] Experiments have confirmed the existence of edge crystallites on the MgCl 2 support by atomic force microscopy (AFM), scanning electron microscopy (SEM), and HR-TEM.…”