The effect of temperature on ethylene polymerization over flat Phillips model catalysts

Kimmenade, E.M.E. van; Loos, Joachim; Niemantsverdriet, J.W.; Thüne, Peter C.

doi:10.1016/j.jcat.2006.03.002

Cited by 29 publications

(10 citation statements)

References 21 publications

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“…These properties place the Phillips catalyst in a unique position in the polyethylene market when compared to its competitors . Direct spectroscopic methods applied on model versions of the catalyst, such as the CO-reduced form (Cr II /SiO 2 ), ,,− or systems obtained from well-defined Cr VI– n organometallic precursors, − or even planar models suitable for surface science investigation, − have played the major role in the characterization of the active sites at a molecular level, while the mechanism of ethylene polymerization has been mainly investigated by means of theoretical calculation. ,− …”

Section: Introductionmentioning

confidence: 99%

Photoinduced Ethylene Polymerization on the Cr^VI/SiO₂ Phillips Catalyst

et al. 2018

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Herein we report on two photoinduced reactions occurring at the surface of the CrVI/SiO2 Phillips catalyst. The first is the photoreduction of CrVI/SiO2 in CO at room temperature, that leads to the reduction of a fraction of the CrVI sites to CrIV(O) intermediates and then to CrII. With respect to the thermal reduction of CrVI/SiO2 in CO, the relative amount of highly uncoordinated CrII sites is larger, suggesting that the relaxation of the CrII sites at the silica surface strongly depends on the temperature at which they are formed. The second is the photoinduced ethylene polymerization on CrVI/SiO2 at room temperature that is reported here for the first time. We demonstrate that UV–vis light has the potential to trigger the reduction of chromates by ethylene and the successive ethylene polymerization. Diffuse reflectance (DR) UV–vis and FT-IR spectroscopies allowed us to identify the nature of the Cr sites involved in the polymerization (i.e., 6-fold coordinated CrII) and of the oxidized byproducts remaining in their coordination sphere, which are methylformate and ethylene oxide. While the former was previously observed during the thermally induced ethylene polymerization on CrVI/SiO2, the latter was never observed.

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Section: Introductionmentioning

confidence: 99%

Photoinduced Ethylene Polymerization on the Cr^VI/SiO₂ Phillips Catalyst

et al. 2018

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“…[23] In addition to model studies employing high surface area silicas, planar models suited for "surface science" studies have also been suggested, most notably by Niemantsverdriet and coworkers. [24][25][26][27] Thermally oxidized silica films grown on Si(100) wafers were used as supports for chromia deposited from chromate solution using spin coating impregnation.…”

Section: Introductionmentioning

confidence: 99%

Planar model system of the Phillips (Cr/SiO2) catalyst based on a well-defined thin silicate film

Pan

Shaikhutdinov

et al. 2018

Journal of Catalysis

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The Phillips catalyst (Cr/SiO 2 ) is successfully used in the large-scale production of polyethylene and has attracted a great interest in catalytic community over the last sixty years.However, the atomic structure(s) of the active site(s) and the reaction mechanism remain controversial, in particular due to the structural complexity and surface heterogeneity of the amorphous silica. In this work, we used a well-defined, atomically flat silicate bilayer film grown on Ru(0001) as a support offering the opportunity to investigate mechanistic aspects at the atomic scale. To fabricate a planar Cr/SiO 2 model system suitable for surface science studies, chromium was deposited using physical vapor deposition onto the hydroxylated silica film surface. Structural characterization and adsorption studies were performed by infrared reflection absorption spectroscopy (IRAS) and temperature programmed desorption (TPD).Hydroxyls groups seem to serve as anchoring cites to Cr ad-atoms. As monitored by IRAS, hydroxyls consumption correlated with the appearance of the new band at ~ 1007 cm -1 typical for Cr=O vibrations. In addition, CO titration experiments suggested also the presence of "naked" Cr, which transforms into mono-and di-oxo chromyl species and their aggregation upon oxidation treatments. TPD experiments of ethylene adsorption at low temperatures under UHV conditions showed the formation of butane as one of the main products. The resultant surfaces are thermally stable, at least, up to 400 K which allows to investigate ethylene polymerization further under more realistic conditions.

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“…First of all, the addition amount of cocatalyst (0.25 mmol) is excessive, and the redundant cocatalyst will increase the high molecular weight tail in GPC curves, as we have mentioned above. Additionally, low temperature usually increases molecular weight and enhances the high molecular weight part. − On the other hand, the decrease of temperature also resulted in an increase of Mw for polymers made by an iV catalyst, while the peak position of the MWD curves remains unchanged.…”

Section: Resultsmentioning

confidence: 99%

Peculiarities of Ethylene Polymerization Kinetics with an Imido-Vanadium/Silyl-Chromate Bimetallic Catalyst: Effect of Polymerization Conditions

Bao

Tian

Zhao

et al. 2017

Ind. Eng. Chem. Res.

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The effects of various polymerization conditions such as cocatalyst concentration, pressure, temperature, and reaction time on ethylene homopolymerization over a SiO2-supported imido-vanadium/silyl-chromate (Cr-iV) bimetallic catalyst were systematically investigated. The monometallic silyl-chromate (S-2) and imido-vanadium (iV) catalyst were also employed for comparison. It was found that the S-2 catalyst produced a polymer with relatively low molecular weight, while the iV catalyst produced ultrahigh-molecular-weight polyethylene (UHMWPE). The bimetallic catalyst was capable of producing reactor blends, with bimodal molecular weight distribution (MWD) and a considerable amount of UHMWPE. Increasing cocatalyst concentration and decreasing polymerization temperature both enhanced the high molecular weight part of the bimodal MWD while the position of the two peaks remained unchanged. The polymerization rates all showed first-order dependences with respect to ethylene pressure for the three catalysts. Ethylene pressure variations caused no changes in the MWD of polymers made by the three catalysts, indicating that transfer to the monomer is the main chain transfer mechanism.

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The effect of temperature on ethylene polymerization over flat Phillips model catalysts

Cited by 29 publications

References 21 publications

Photoinduced Ethylene Polymerization on the Cr^VI/SiO₂ Phillips Catalyst

Photoinduced Ethylene Polymerization on the Cr^VI/SiO₂ Phillips Catalyst

Planar model system of the Phillips (Cr/SiO2) catalyst based on a well-defined thin silicate film

Peculiarities of Ethylene Polymerization Kinetics with an Imido-Vanadium/Silyl-Chromate Bimetallic Catalyst: Effect of Polymerization Conditions

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The effect of temperature on ethylene polymerization over flat Phillips model catalysts

Cited by 29 publications

References 21 publications

Photoinduced Ethylene Polymerization on the CrVI/SiO2 Phillips Catalyst

Photoinduced Ethylene Polymerization on the CrVI/SiO2 Phillips Catalyst

Planar model system of the Phillips (Cr/SiO2) catalyst based on a well-defined thin silicate film

Peculiarities of Ethylene Polymerization Kinetics with an Imido-Vanadium/Silyl-Chromate Bimetallic Catalyst: Effect of Polymerization Conditions

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Photoinduced Ethylene Polymerization on the Cr^VI/SiO₂ Phillips Catalyst

Photoinduced Ethylene Polymerization on the Cr^VI/SiO₂ Phillips Catalyst