Structural Characterization of the EtOH–TiCl₄–MgCl₂ Ziegler–Natta Precatalyst

Abstract

The Ziegler-Natta polymerization is one major example of application of catalysis in industry. Since the first discovery of the Ziegler and Natta, several modifications of the catalyst have been developed, in order to improve its performances. Nowadays, a typical Ziegler-Natta catalyst for polyethylene synthesis consists of a precatalyst, composed by TiCl 4 supported on MgCl 2 in presence of a Lewis base, activated by organoaluminium.The atomic-scale characterization of the Ziegler-Natta catalyst is crucial for further improvement of the catalyst. Here, the precatalyst TiCl 4 -MgCl 2 with EtOH as internal Lewis base is characterized combining solid-state NMR spectroscopy and periodic density functional theory calculations. From total energy and NMR spectra, 8 surface species were proposed showing EtO -ligands on the Ti and EtOH/EtO -on the surface Mg species. These species lead to a complete interpretation of the NMR 2D spectra.Hence a detailed molecular scale description of the pre-catalyst was obtained.

Genesis of MgCl2‐based Ziegler‐Natta Catalysts as Probed with Operando Spectroscopy

“…Also in this case the first spectrum is dominated by the spectroscopic manifestations of the pure alcohol, whereas the last spectrum presents only a few bands due to residual ethoxy byproducts, whose formation has already been assessed in literature. [66,93] The whole set of experimental data discussed in this section converges in showing that the stepwise dealcoholation of the MgCl 2 -6ROH adducts occurs through the formation of a large number of intermediate species (some of them crystalline) and following a series of side reactions often neglected in the specialized literature. The slightly different evolution of the two alcoholate precursors may explain the different properties of the produced MgCl 2 , in terms of specific surface area, particles size and morphology.…”

“…Such a procedure is quite unusual in the industrial practice, but it is fundamental to unravel the whole sequence of chemical reactions taking place during the formation of the MgCl 2 support material, affecting its final properties. [66] All these studies represent a fundamental benchmark for investigating the formation of MgCl 2 from alcoholate precursors, though the industrial situation is complicated by the co-presence of several intermediate states. Among the most relevant studies, we cite the pioneering work of Di Noto et al reporting a detailed XRD and FT-IR characterization of several MgCl 2 -nEtOH adducts, [62] followed by the Raman spectroscopic investigation of Tewell et al, [63] the particle size distribution analysis of Ye et al, [64] and the NMR structural characterization of Sozzani et al, who first proposed also an analytical methodology to recognize the pure MgCl 2 -nEtOH compounds within a mixture.…”

“…For this reason, D'Anna et al adopted exactly the MgCl 2 -2EtOH adduct as the model system to study the successive titanation step and the structure of the Ziegler-Natta pre-catalysts. [66] All these studies represent a fundamental benchmark for investigating the formation of MgCl 2 from alcoholate precursors, though the industrial situation is complicated by the co-presence of several intermediate states. In this sense, the works of Bart and Roovers and of Jalali Dil et al [67,68] are much closer to the industrial scenario.…”

The Influence of Alcohols in Driving the Morphology of Magnesium Chloride Nanocrystals

“…We will call these sites “weakly acidic Mg 2+ sites”. No absorption bands owing to CO adsorbed on defects (expected at ≈2200 cm −1 ) are observed, likely as a consequence of the persistence of a minor amount of alcohol molecules (or some byproducts) on the most energetic surface sites. The absolute intensity of the (CO) absorption bands at the maximum θ CO is smaller for MgCl 2‐EtOH (Figure b) than for MgCl 2‐MeOH (Figure c), which is in fair agreement with the specific surface area determined by BET analysis (Figure a).…”

Eine Brücke zwischen industriellen und wohldefinierten Trägerkatalysatoren

“…B. THF oder Ethanol. Bei Festkörper‐NMR‐Untersuchungen von TiCl 4 /MgCl 2 (THF) 1.5 wurde gefunden, dass der THF‐Ligand eine Lewis‐Säure‐vermittelte Ringöffnung zu Ti S Cl 3 (O(CH 2 ) 4 Cl)MgCl x erfährt (Schema a). Ähnliche Ti S Cl 3− y (OEt) y (MgCl x )‐Spezies werden auch für TiCl 4 /MgCl 2 (EtOH) 1.5 beobachtet (Schema b), wobei die Ethoxygruppen durch Deprotonierung von Ethanol gebildet werden.…”