Products of the initial reduction of the Phillips catalyst by olefins

Joseph, Jincy; Potter, Kelsey C.; Wulfers, Matthew J.; Schwerdtfeger, Eric D.; McDaniel, Max P.; Jentoft, Friederike C.

doi:10.1016/j.jcat.2019.07.057

Cited by 12 publications

(7 citation statements)

References 49 publications

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“…This at least excludes the presence of exclusively naked Cr 2 + ions and indicates that reduction by-products remain in the proximity of the Cr active sites, hereby affecting the d-d transition and CT bands. [16,[57][58][59][60][61][62][63] In order to ensure a proper fit, an additional band was required at 24000 cm À 1 , which is expected to be a CT band. This 24000 cm À 1 CT band, which is absent in the CO reduced material, can be explained by reduction by-products remaining in the coordination sphere of the Cr active site: affecting the location the d-d transition bands and the CT bands.…”

Section: Ethylene Polymerization After Pre-treatment With 150 Molecumentioning

confidence: 99%

Tuning the Redox Chemistry of a Cr/SiO₂ Phillips Catalyst for Controlling Activity, Induction Period and Polymer Properties

et al. 2020

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The Cr/SiO2 Phillips catalyst has taken a central role in ethylene polymerization ever since its discovery in 1953. This catalyst is unique compared to other ethylene polymerization catalysts, since it is active without the addition of a metal‐alkyl co‐catalyst. However, metal‐alkyls can be added for scavenging poisons, enhancing the catalyst activity, reducing the induction period and altering polymer characteristics. Despite extensive research into the working state of the catalyst, still no consensus has been reached. Here, we show that by varying the type of metal‐alkyl co‐catalyst and its amount, the Cr redox chemistry can be tailored, resulting in distinct catalyst activities, induction periods, and polymer characteristics. We have used in‐situ UV‐Vis‐NIR diffuse reflectance spectroscopy (DRS) for studying the Cr oxidation state during the reduction by tri‐ethyl borane (TEB) or tri‐ethyl aluminum (TEAl) and during subsequent ethylene polymerization. The results show that TEB primarily acts as a reductant and reduces Cr6+ with subsequent ethylene polymerization resulting in rapid polyethylene formation. TEAl generated two types of Cr2+ sites, inaccessible Cr3+ sites and active Cr4+ sites. Subsequent addition of ethylene also revealed an increased reducibility of residual Cr6+ sites and resulted in rapid polyethylene formation. Our results demonstrate the possibility of controlling the reduction chemistry by adding the proper amount and type of metal‐alkyl for obtaining desired catalyst activities and tailored polyethylene characteristics.

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Section: Ethylene Polymerization After Pre-treatment With 150 Molecumentioning

confidence: 99%

Tuning the Redox Chemistry of a Cr/SiO₂ Phillips Catalyst for Controlling Activity, Induction Period and Polymer Properties

et al. 2020

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“…At about 400 • C, the chromium is oxidized into the hexavalent form, which spreads out and becomes anchored onto the support as chromate or dichromate surface esters [3,[5][6][7][8][9][10][11][12][13][14]. These species are then reduced by ethylene to a lower valence, expanding the potential coordination sphere, alkylating the Cr, and forming the coordinatively unsaturated active sites [3][4][5][15][16][17][18][19][20]. Consequently, and somewhat unusually for inorganic industrial catalysts, the active sites are individually attached to the support.…”

Section: Introductionmentioning

confidence: 99%

Comparison of Support Effects on Phillips and Metallocene Catalysts

Yang

McDaniel

2021

Catalysts

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Both metallocene and Phillips chromium catalysts are used in the commercial manufacture of polyethylene. Unlike most other commercial metallocene systems, the Chevron Phillips Chemical (CPC) platform does not use methylaluminoxane or fluoroorganic boranes. Instead, the support itself serves to activate (ionize) the metallocenes, which then polymerize ethylene at high activity. Most of these solid acid supports can also be used to anchor Cr to make a Phillips catalyst. This provides an interesting opportunity to compare the polymerization responses by these two disparate systems, Phillips Cr and CPC metallocene, when supported on the same solid acid carriers. In this study, both chromium oxide and several metallocenes were deposited onto a variety of solid oxides, under a variety of conditions, and the resulting support effects were observed and compared. Although using seemingly different chemistries, the two catalyst systems exhibited a surprising number of similarities, which can be attributed to the acidity and porosity of these diverse supports.

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“…Scott and co-workers studied the active site of chromium-based catalysts and found the effects of resonant enhancement on the normal modes of vibration are associated with the dioxoCr(VI) sites rather than mono-oxoCr(VI) sites . Recently, a group of oxidation products formed under various reduction conditions have been detected, and some oxidation products did not desorb readily from the surface of the support. , Although an understanding of the active site of the industrial polyethylene catalyst remains controversial, various new reduction routes have been put forward by spectroscopic experiments, with CO 2 , ester, or methylformate being the oxidative products. − Jin et al reported that a modified vanadium-oxide catalyst can catalyze ethylene polymerization to produce ultrahigh-molecular-weight polyethylene with the assistance of the cocatalyst . Interestingly, the VO x catalyst, prepared by the incipient wetness impregnation of ammonium metavanadate with γ-Al 2 O 3 , can generate a vanadium(III) active site, which is capable of catalyzing propane dehydrogenation with proper propylene selectivity and considerable activity .…”

Section: Introductionmentioning

confidence: 99%

“…13 Recently, a group of oxidation products formed under various reduction conditions have been detected, and some oxidation products did not desorb readily from the surface of the support. 14,15 Although an understanding of the active site of the industrial polyethylene catalyst remains controversial, various new reduction routes have been put forward by spectroscopic experiments, with CO 2 , ester, or methylformate being the oxidative products. 16−18 Jin et al reported that a modified vanadium-oxide catalyst can catalyze ethylene polymerization to produce ultrahigh-molecular-weight polyethylene with the assistance of the cocatalyst.…”

Section: Introductionmentioning

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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Products of the initial reduction of the Phillips catalyst by olefins

Cited by 12 publications

References 49 publications

Tuning the Redox Chemistry of a Cr/SiO₂ Phillips Catalyst for Controlling Activity, Induction Period and Polymer Properties

Tuning the Redox Chemistry of a Cr/SiO₂ Phillips Catalyst for Controlling Activity, Induction Period and Polymer Properties

Comparison of Support Effects on Phillips and Metallocene Catalysts

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

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Products of the initial reduction of the Phillips catalyst by olefins

Cited by 12 publications

References 49 publications

Tuning the Redox Chemistry of a Cr/SiO2 Phillips Catalyst for Controlling Activity, Induction Period and Polymer Properties

Tuning the Redox Chemistry of a Cr/SiO2 Phillips Catalyst for Controlling Activity, Induction Period and Polymer Properties

Comparison of Support Effects on Phillips and Metallocene Catalysts

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

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Product

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Tuning the Redox Chemistry of a Cr/SiO₂ Phillips Catalyst for Controlling Activity, Induction Period and Polymer Properties

Tuning the Redox Chemistry of a Cr/SiO₂ Phillips Catalyst for Controlling Activity, Induction Period and Polymer Properties