Olefin polymerization on Cr(III)/SiO2: Mechanistic insights from the differences in reactivity between ethene and propene

Delley, Murielle F.; Praveen, C. S.; Borosy, Andras; Núñez‐Zarur, Francisco; Comas‐Vives, Aleix; Copéret, Christophe

doi:10.1016/j.jcat.2017.08.016

Cited by 25 publications

(41 citation statements)

References 83 publications

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“…Among the evaluated tri-coordinated Ga-O pairs: II-O3, III-O2, V-O2 and V-O3 the Ga-O pairs involving site V are the most reactive ones, in line with previous finding regarding the C-H activation of ethylene and propene on analogous Cr sites. [37][38] For the Ga-O pair, this process on V-O2 is highly exoenergetic, with a reaction energy of -65.0 kcal.mol -1 and it is associated with an energy barrier of 14.5 kcal.mol - kcal.mol -1 , respectively. The correlation between energy barriers and reaction energies for the b-H transfer step for all the evaluated sites is similar than for the C-H activation step previous reaction step (R 2 =0.76).…”

Section: Step 1: C-h Bond Activation Of Propanementioning

confidence: 99%

Strain in Silica-Supported Ga(III) Sites: Neither Too Much nor Too Little for Propane Dehydrogenation Catalytic Activity

et al. 2021

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Well-defined Ga(III) sites on SiO2 are highly active, selective, and stable catalysts in the propane dehydrogenation reaction. In this contribution, we evaluate the catalytic activity towards propane dehydrogenation of tri-coordinated and tetra-coordinated Ga(III) sites on SiO2 by means of first principles calculations using realistic amorphous periodic SiO2 models. We evaluated the three reaction steps in propane dehydrogenation, namely the C-H activation of propane to form propyl, the b-hydride elimination transfer to form propene, and a Ga-hydride, and the H-H coupling to release H2, regenerating the initial Ga-O bond and closing the catalytic cycle. Our work shows how Brønsted-Evans-Polanyi relationships are followed for these three reaction steps on Ga(III) sites on SiO2 and highlights the role of the strain of the reactive Ga-O pairs on such sites of realistic amorphous SiO2 models. While highly strained sites are very reactive sites for the initial C-H activation, they are more difficult to regenerate. The corresponding less strained sites are not reactive enough, pointing to the need of a right balance in strain to be an effective site for propane dehydrogenation. Overall, our work provides an understanding of the intrinsic activity of acidic Ga single sites towards the propane dehydrogenation reaction and paves the road towards the design and prediction of better single-site catalysts on SiO2 for the propane dehydrogenation reaction.

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Section: Step 1: C-h Bond Activation Of Propanementioning

confidence: 99%

Strain in Silica-Supported Ga(III) Sites: Neither Too Much nor Too Little for Propane Dehydrogenation Catalytic Activity

et al. 2021

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“…The high temperature thermal treatment used would also lead to additional surface reactions including the formation of surface sites with different reactivities. [36][37] While no py-B(C 6 F 5 ) 3 is present in F6@SiO 2-700-thermal , surface oxygen atoms that can act as Lewis base might play a similar role in proton transfers. [38][39][40] It is noteworthy that F6@SiO 2-700 displays a significantly higher catalytic activity compared to its F6 homogeneous analogue ( Table 2).…”

Section: Scheme 2 Preparation Of Well-defined Silica-supported W(iv)mentioning

confidence: 99%

Well-Defined Silica-Supported Tungsten(IV)–Oxo Complex: Olefin Metathesis Activity, Initiation, and Role of Brønsted Acid Sites

et al. 2019

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Despite the importance of the heterogeneous tungsten-oxo based olefin metathesis catalyst (WO 3 /SiO 2) in industry, understanding of its initiation mechanism is still very limited. It has been proposed that reduced W(IV)-oxo surface species act as precatalysts. In order to understand the reactivity and initiation mechanism of surface W(IV)-oxo species, we synthesized a well-defined silica-supported W(IV)-oxo species (≡SiO)WO(OtBuF 6)(py) 3 (OtBuF 6 = OC(CH 3)(CF 3) 2 ; py = pyridine) (F6@SiO 2-700) via Surface Organometallic Chemistry (SOMC). F6@SiO 2-700 was shown to be highly active in olefin metathesis upon removal of pyridine ligands through the addition of tris(pentafluorophenyl)borane (B(C 6 F 5) 3) or thermal treatment under high vacuum. The metathesis activity towards olefins with and without allylic C−H groups, namely β-methylstyrene and styrene, respectively, was investigated. In the case of styrene, we demonstrate the role of surface OH groups in initiating metathesis activity. In particular, the presence of strong Brønsted acidic surface OH sites, revealed by 15 N-labeled pyridine, likely arises from the presence of adjacent W sites in the catalyst and assist styrene metathesis, an olefin having no allylic C-H bond. In contrast, initiation of olefins containing allylic C−H group (e.g. β-methylstyrene) is independent of surface OH group concentration and likely involves allylic C−H activation mechanism like the molecular W(IV)-oxo species. This study indicates that initiation mechanisms depend on the olefinic substrates and reveals the synergistic effect of Brønsted acidic surface sites and reduced W(IV) sites in the initiation of olefin metathesis.

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“…Secondly, only a portion, proposedly a maximum of 30 %, [1] of the Cr sites is active in ethylene polymerization. These two key‐points made necessary the use of various powerful analytical approaches, often avoiding industrial conditions and/or catalyst materials by use of, for example, model systems [13, 14, 23, 15–22] or well‐defined catalysts in which Cr 6+ is reduced by for example, CO: both examples limiting the heterogeneities of the surface sites, in the former case by rationally designing the Cr surface sites and in the latter by quantitative reduction to isolated Cr 2+ surface sites [24–32] …”

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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Olefin polymerization on Cr(III)/SiO2: Mechanistic insights from the differences in reactivity between ethene and propene

Cited by 25 publications

References 83 publications

Strain in Silica-Supported Ga(III) Sites: Neither Too Much nor Too Little for Propane Dehydrogenation Catalytic Activity

Strain in Silica-Supported Ga(III) Sites: Neither Too Much nor Too Little for Propane Dehydrogenation Catalytic Activity

Well-Defined Silica-Supported Tungsten(IV)–Oxo Complex: Olefin Metathesis Activity, Initiation, and Role of Brønsted Acid Sites

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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Olefin polymerization on Cr(III)/SiO2: Mechanistic insights from the differences in reactivity between ethene and propene

Cited by 25 publications

References 83 publications

Strain in Silica-Supported Ga(III) Sites: Neither Too Much nor Too Little for Propane Dehydrogenation Catalytic Activity

Strain in Silica-Supported Ga(III) Sites: Neither Too Much nor Too Little for Propane Dehydrogenation Catalytic Activity

Well-Defined Silica-Supported Tungsten(IV)–Oxo Complex: Olefin Metathesis Activity, Initiation, and Role of Brønsted Acid Sites

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