Effect of surface hydroxylation on the catalytic activity of a Cr(II)/SiO2 model system of Phillips catalyst

Budnyk, Andriy P.; Damin, Alessandro; Groppo, Elena; Zecchina, Adriano; Bordiga, Silvia

doi:10.1016/j.jcat.2015.01.016

Cited by 19 publications

(26 citation statements)

References 32 publications

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“…The molecular structure of the active center in heterogeneous catalysts depends on its grafting sites. , The molecular models for the mononuclear and dinuclear chromium sites were first constructed by Espelid and Børve in a series of theoretical studies on the Phillips-type catalysts. , In the past two decades, cluster models have been built to mimic the real surface of the heterogeneous catalyst in many theoretical calculations, which contributes to the understanding of the Phillips CrO x /SiO 2 catalyst. ,− In this computational work, three typical vanadium sites including a monomeric site (Model A) and two dimeric sites (Model B and Model C) were designed to investigate the reduction mechanism and alkylation, assisted by the hydroxyl group as the capping group (Figure ). These cubelike models represent different VO, V–O–V, or V–O–Si grafting sites on the surface of the catalyst, which determine the polymerization activity and the properties of polyolefins.…”

Section: Resultsmentioning

confidence: 99%

Mechanistic Studies of Reduction and Initiation over the Vanadium-Oxide Polyethylene Catalyst

Liu

Cheng

et al. 2021

J. Phys. Chem. C

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Supported vanadium oxides are one of the most promising candidates for ultrahigh molecular weight polyethylene, and cocatalysts play a key role in this system. In this work, a fundamental insight into the reduction and initiation of the heterogeneous vanadium-oxide catalyst has been gained. Both cocatalyst and ethylene are utilized as reductants to investigate the distinction between the Phillips catalyst and the vanadium-oxide catalyst. This study also describes the possible active metal centers existing on the surface of the silica support and their feasible interactions with reductants. The energy barrier difference demonstrates that there is a distinction between cocatalyst reduction and ethylene reduction. The V−O−V site is the most active site among the three different sites with a relatively low Gibbs free energy according to density functional theory calculations, which is in agreement with the wide molecular weight distribution, and alkylation with a cocatalyst seems to be more feasible than ethylene with the breaking of the V−O bond to promote ethylene insertion. The results highlight the significance of the cocatalyst for the vanadium-oxide catalyst compared with ethylene.

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Section: Resultsmentioning

confidence: 99%

Mechanistic Studies of Reduction and Initiation over the Vanadium-Oxide Polyethylene Catalyst

Liu

Cheng

et al. 2021

J. Phys. Chem. C

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“…Only a few works employed larger SiO 2 clusters having an amorphous structure, ,, or periodic models of amorphous silica. ,− Larger and/or periodic models are usually more flexible and account for the presence of siloxane ligands nearby the chromium sites. Both properties are very important in affecting the structure of the chromium sites and, consequently, their reactivity and the type of polymer they produce.…”

Section: Open Debates Currently Inflaming the Scientific Communitymentioning

confidence: 99%

The Active Sites in the Phillips Catalysts: Origins of a Lively Debate and a Vision for the Future

et al. 2018

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In this work, we summarize and critically compare some of the experimental results recently published on the Phillips catalyst, in the attempt to make the point on a few particularly debated questions that have recently animated the specialized literature; in particular, we discuss the structure of the active chromium sites and how ethylene polymerization initiates on them. The data collected in this article unequivocally demonstrate that the structural and electronic properties of the chromium sites strongly depend on the strain of the silica surface, which in turns is affected by both the activation treatment and the chromium loading. This explains, at least partially, the differences of results obtained in different research groups. Another fundamental message is the need of applying the largest possible set of characterization methods, including theoretical calculation on large and flexible models. Our final purpose (and hope) is to promote a positive and constructing discussion on this catalyst, as a premise to create a solid scientific base useful to both the young researchers approaching this field and the industrial researchers who daily work with it.

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“…Another extraordinary aspect, and long‐standing point of discussion, is the large variety of PEs that can be produced with the Phillips catalyst and the multitude of strategies, such as calcination temperature, support type, co‐catalyst addition and catalyst pre‐treatment, one can employ in tailoring the final PE product properties [43, 47–55] . The diversity of produced PEs, in terms of PE‐type as well as the broad molecular weight distributions (MWD), is ascribed to the large variety of Cr surface sites.…”

Section: Introductionmentioning

confidence: 99%

Influence of Metal‐Alkyls on Early‐Stage Ethylene Polymerization over a Cr/SiO₂ Phillips Catalyst: A Bulk Characterization and X‐ray Chemical Imaging Study

Jongkind

Meirer

Bossers

et al. 2020

Chemistry A European J

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The Cr/SiO2 Phillips catalyst has taken a central role in ethylene polymerization since its invention in 1953. The uniqueness of this catalyst is related to its ability to produce broad molecular weight distribution (MWD) PE materials as well as that no co‐catalysts are required to attain activity. Nonetheless, co‐catalysts in the form of metal‐alkyls can be added for scavenging poisons, enhancing catalyst activity, reducing the induction period, and tailoring polymer characteristics. The activation mechanism and related polymerization mechanism remain elusive, despite extensive industrial and academic research. Here, we show that by varying the type and amount of metal‐alkyl co‐catalyst, we can tailor polymer properties around a single Cr/SiO2 Phillips catalyst formulation. Furthermore, we show that these different polymer properties exist in the early stages of polymerization. We have used conventional polymer characterization techniques, such as size exclusion chromatography (SEC) and 13C NMR, for studying the metal‐alkyl co‐catalyst effect on short‐chain branching (SCB), long‐chain branching (LCB) and molecular weight distribution (MWD) at the bulk scale. In addition, scanning transmission X‐ray microscopy (STXM) was used as a synchrotron technique to study the PE formation in the early stages: allowing us to investigate the produced type of early‐stage PE within one particle cross‐section with high energy resolution and nanometer scale spatial resolution.

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Effect of surface hydroxylation on the catalytic activity of a Cr(II)/SiO2 model system of Phillips catalyst

Cited by 19 publications

References 32 publications

Mechanistic Studies of Reduction and Initiation over the Vanadium-Oxide Polyethylene Catalyst

Mechanistic Studies of Reduction and Initiation over the Vanadium-Oxide Polyethylene Catalyst

The Active Sites in the Phillips Catalysts: Origins of a Lively Debate and a Vision for the Future

Influence of Metal‐Alkyls on Early‐Stage Ethylene Polymerization over a Cr/SiO₂ Phillips Catalyst: A Bulk Characterization and X‐ray Chemical Imaging Study

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Effect of surface hydroxylation on the catalytic activity of a Cr(II)/SiO2 model system of Phillips catalyst

Cited by 19 publications

References 32 publications

Mechanistic Studies of Reduction and Initiation over the Vanadium-Oxide Polyethylene Catalyst

Mechanistic Studies of Reduction and Initiation over the Vanadium-Oxide Polyethylene Catalyst

The Active Sites in the Phillips Catalysts: Origins of a Lively Debate and a Vision for the Future

Influence of Metal‐Alkyls on Early‐Stage Ethylene Polymerization over a Cr/SiO2 Phillips Catalyst: A Bulk Characterization and X‐ray Chemical Imaging Study

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Influence of Metal‐Alkyls on Early‐Stage Ethylene Polymerization over a Cr/SiO₂ Phillips Catalyst: A Bulk Characterization and X‐ray Chemical Imaging Study