Tuning the Ti3+ and Al3+ Synergy in an Al2O3/TiClx Catalyst To Modulate the Grade of the Produced Polyethylene

Piovano, Alessandro; Morra, Elena; Chiesa, Mario; Groppo, Elena

doi:10.1021/acscatal.7b01284

Cited by 18 publications

(6 citation statements)

References 64 publications

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“…86 At the same time, a broad absorption grows up centered at about 24000 cm −1 , whose position corresponds to a Ti 3+ d−d transition, but whose intensity is much higher. Similar bands have been already observed in the past for TEAlactivated ZN catalysts and assigned to intersite d−d transition involving two Ti 3+ sites bridged by a Cl − ligand, thus assuming a partial CT character; 89 for this reason, they have been considered as a proof for the formation of TiCl 3 clusters. In A, the formation of TiCl 3 clusters is favored by the high mobility of physisorbed TiCl x (OEt) 4−x species.…”

Section: Resultssupporting

confidence: 80%

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

et al. 2021

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Although the formation of nanosized and defective δ-MgCl2 is essential for the performance of Ziegler–Natta catalysts, the process has not sufficiently been elucidated due to certain limitations in characterization. Here, the formation of nanostructures and active surfaces of Ziegler–Natta catalysts was investigated in detail based on a multifaceted set of characterization techniques represented by X-ray total scattering and various spectroscopies in correlation with chemical composition analysis and polymerization tests. Solid samples were extracted in the course of catalyst preparation from Mg(OEt)2 and subjected to the analysis. Several interesting results were found. The addition of TiCl4 almost spontaneously converts Mg(OEt)2 into MgCl2 seeds mainly exposing the {001} basal surface, whose dimensions are below 2 nm; a large Ti amount remains on the material as physisorbed 4-fold-coordinated TiCl x (OEt)4–x species. The heating treatment removes the physisorbed TiCl x (OEt)4–x and/or convert them into chemisorbed 6-fold-coordinated TiCl x (OEt)4–x , while the subsequent addition of an internal donor (here dibutyl phthalate, DBP) promotes a substantial reconstruction and growth of MgCl2 seeds to almost the same size as the final catalyst (ca. 6 nm), with the exposure of the more catalytically relevant lateral surfaces. DBP is in one part adsorbed on MgCl2 surfaces and in the other part complexed with Ti sites. This complex is only partially removed in the following steps of the synthesis. The second TiCl4 addition replaces the chemisorbed TiCl x (OEt)4–x with 6-fold-coordinated TiCl4 species, but it also causes side reactions with DBP, as testified by the formation of phthaloyl chloride. After activation by triethylaluminum (TEAl), the activity per Ti for ethylene was almost constant throughout the whole preparation process after the initial TiCl4 addition, whereas the activity for propylene was negligible before the addition of the donor and increased dramatically in the subsequent steps of the preparation. This was further investigated based on spectroscopies for TEAl-activated samples in order to individuate the active Ti species responsible for the catalysis and to monitor the fate of DBP upon TEAl reaction. The multifaceted characterization approach allowed us to integrate information on the formation of δ-MgCl2, their surfaces, and adsorbed species, providing us with deep insights into the meaning of each step within an industrial catalyst preparation method that has been empirically refined over a long history.

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Section: Resultssupporting

confidence: 80%

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

et al. 2021

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“…In that spectrum TEA dosage is testified by the appearance of some absorption bands in the n(CH x ) (2950-2800 cm À1 ) and d(CH x ) (1500-1350 cm À1 ) regions, which are related to the vibrational modes of the alkyl chains deriving from TEA. [119,120] Unfortunately, a clear distinction among all the different species could not be addressed here. [119,120] Unfortunately, a clear distinction among all the different species could not be addressed here.…”

Section: Genesis Of Active Sites On Mgclmentioning

confidence: 99%

“…These bands are very similar to the fingerprints of pure TEA (inset in Figure 3b), [118] and account for the coexistence of alkylated TiCl x R y species, AlR x Cl y byproducts, and unreacted AlR 3 , as already observed for other Ziegler-Natta catalysts. [119,120] Unfortunately, a clear distinction among all the different species could not be addressed here. Finally, a few changes are observed in the range between 1100 and 1000 cm À1 , likely associated to the reaction of TEA with the above mentioned Ti alkoxide species.…”

Section: Genesis Of Active Sites On Mgclmentioning

confidence: 99%

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

et al. 2018

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Ziegler-Natta catalysts for olefin polymerization are intrinsically complex multi-component systems. The genesis of the active sites involves several simultaneous and sequential steps, making the individual steps and interconnections difficult to be unraveled in an unambiguous manner. In this work, we combine X-ray diffraction and spectroscopy to probe each step of the birth and life of a MgCl -based Ziegler-Natta catalyst, namely the formation of high surface area MgCl by dealcoholation of an alcoholate precursor, the TiCl grafting, and the subsequent activation by triethylaluminum as co-catalyst. The so-prepared catalyst was tested towards ethylene polymerization, leading to the production of mainly crystalline high-density polyethylene. The use of operando characterization techniques allowed probing the transient details that are difficult to be dissected in the aftermath, but can radically affect the overall catalytic process.

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“…Nevertheless, DR UV–vis spectroscopy has been only rarely used for investigating ZN catalysts, , and often the spectra were not sufficiently (or properly) interpreted. In the last decade, some of the authors systematically exploited DR UV–vis spectroscopy to investigate a series of ZN (pre)catalysts and related systems, − never reaching, however, such a high level of details as in the present work, which benefits the complementarity with NEXAFS spectroscopy and density-functional theory (DFT) calculation. As far as NEXAFS (in the soft X-ray range) is concerned, the short attenuation length of soft X-rays has long prevented the application of NEXAFS in reaction conditions, and in situ observations of surface reactions were limited only in the presence of gases with pressures lower than 10 –6 Torr.…”

Section: Introductionmentioning

confidence: 99%

Electronic Properties of Ti Sites in Ziegler–Natta Catalysts

et al. 2021

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Although Ziegler–Natta (ZN) catalysts play a major role in the polyolefin market, a true understanding of their properties at the molecular level is still missing. In particular, there is a lack of knowledge on the electronic properties of Ti sites. Theoretical calculations predict that the electron density of the Ti sites in the precatalysts correlates with the activation energy for olefin insertion in the Ti-alkyl bond generated at these sites after activation by Al-alkyls. It is also well known that the effective charge on the Ti sites in the activated catalysts affects the olefin π-complexation. In this contribution, we exploit two electronic spectroscopies, UV–vis and Ti L2,3-edge near-edge X-ray absorption fine structure (NEXAFS), complemented with theoretical simulation to investigate three ZN precatalysts of increasing complexity (up to an industrial system) and the corresponding catalysts activated by triethylaluminum (TEAl). We provide compelling evidence for the presence of monomeric 6-fold-coordinated Ti4+ species in all of the precatalysts, which however differ in the effective charge on the Ti sites. We also unambiguously demonstrate that these sites are reduced by TEAl to two types of monomeric 5-coordinated Ti3+, either alkylated or not, and that the former are involved in ethylene polymerization. In addition, small TiCl3 clusters are formed in the industrial catalyst, likely due to the occurrence of severe reducing conditions within the catalyst pores. These data prove the potential of these two techniques, coupled with simulation, in providing an accurate description of the electronic properties of heterogeneous ZN catalysts.

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Tuning the Ti³⁺ and Al³⁺ Synergy in an Al₂O₃/TiCl_x Catalyst To Modulate the Grade of the Produced Polyethylene

Cited by 18 publications

References 64 publications

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

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

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

Electronic Properties of Ti Sites in Ziegler–Natta Catalysts

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Tuning the Ti3+ and Al3+ Synergy in an Al2O3/TiClx Catalyst To Modulate the Grade of the Produced Polyethylene

Cited by 18 publications

References 64 publications

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

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

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

Electronic Properties of Ti Sites in Ziegler–Natta Catalysts

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Tuning the Ti³⁺ and Al³⁺ Synergy in an Al₂O₃/TiCl_x Catalyst To Modulate the Grade of the Produced Polyethylene

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