Turning a Cr-based heterogeneous ethylene polymerisation catalyst into a selective ethylene trimerisation catalyst

Nenu, Cristina N.; Bodart, P.; Weckhuysen, Bert M.

doi:10.1016/j.molcata.2006.12.037

Cited by 15 publications

(9 citation statements)

References 33 publications

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“…This approach is ideal for the application of specific surface‐science methods such as X‐ray photoelectron spectroscopy and atomic force microscopy. Recently, a method to completely remove the heterogeneity of the Cr II /SiO 2 system was reported 9–11. By using the 1,3,5‐tribenzylhexahydro‐1,3,5‐triazine (TAC) ligand as a surface‐modifying agent, a single‐site Cr species was made that results in the formation of polyethylene with a very low polydispersity index.…”

Section: Methodsmentioning

confidence: 99%

Dichloromethane as a Selective Modifying Agent To Create a Family of Highly Reactive Chromium Polymerization Sites

et al. 2007

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II/SiO 2 Phillips catalyst, [1] although used in industrial plants since the 1960s, is still one of the most-debated systems with regard to both the molecular structure of the active sites and the related initiation mechanism, for which a unified picture is still missing. [2][3][4][5] The main reasons why these two strictly connected questions are not properly addressed are the high intrinsic heterogeneity of the Cr II sites formed at the surface of amorphous silica and the high degree of Cr dilution (typically less than 1 wt % Cr). Furthermore, catalytically inactive Cr 2 O 3 clusters are formed at higher Cr loadings. [6] The presence of at least three families of Cr II species that differ in their ability to coordinate ligand molecules and thus in their catalytic activity has been fully demonstrated, and preparative routes have been developed to minimize the formation of Cr 2 O 3 clusters. [3][4][5][6][7] Attempts to reduce the complexity of the catalyst surface have been continuously made over the last decades. In this respect, the annealing method of McDaniel, [2] further developed by Groppo et al., [4b] is a way to fine-tune the relative populations of Cr II sites. By grafting Cr species on a flat SiO 2 / Si(100) surface, van Kimmenade et al. [8] succeeded in obtaining better-defined Cr species. This approach is ideal for the application of specific surface-science methods such as X-ray photoelectron spectroscopy and atomic force microscopy. Recently, a method to completely remove the heterogeneity of the Cr II /SiO 2 system was reported.[9-11] By using the 1,3,5-tribenzylhexahydro-1,3,5-triazine (TAC) ligand as a surfacemodifying agent, a single-site Cr species was made that results in the formation of polyethylene with a very low polydispersity index.Herein we reveal that the much simpler CH 2 Cl 2 molecule acts as a surface-modifying agent for the Cr II /SiO 2 system. In this respect, it is important to recall the use of halocarbon promoters in the field of Ziegler-Natta-type polymerization catalysis, [12] although the true mechanism of promotion has not yet been fully reported. Indeed, results on, for example, vanadium-based catalysts suggest that the main role of the halogenated compounds may be due to their oxidizing character, as V II species formed during the polymerization process are re-oxidized to V III . [13,14] Herein we show that CH 2 Cl 2 has a dual function, that is, selectively enhancing the catalytic activity of a small fraction of Cr sites and poisoning the remainder. This novel approach represents a much simpler method to reduce the above-mentioned Cr heterogeneity and at the same time improves the catalytic performance of this industrially important catalyst system. Figure 1 shows the physicochemical processes taking place when CH 2 Cl 2 is brought into contact with Cr II /SiO 2 under different conditions, as probed by IR and diffusereflectance (DR) UV/Vis/NIR spectroscopy. When dosed at room temperature, CH 2 Cl 2 interacts with both the silica support and the Cr II sites. Interaction...

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Section: Methodsmentioning

confidence: 99%

Dichloromethane as a Selective Modifying Agent To Create a Family of Highly Reactive Chromium Polymerization Sites

et al. 2007

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“…[199] Cationic Ti dimethyl complexes were proposed to be the active species. [205,206] Whereas the homogeneous CrCl 3 (TAC)c omplex produces 1-hexene immediately,t he heterogeneous catalyst shows induction behavior, thus indi-Scheme 9. a) Ethylene trimerization catalyst Ti-1 supported on MAO-modified silica. [201] Alternatively,aw ell-defined surface complex Ti-2/SiO 2 (Scheme 9b), prepared by grafting the titanium precursor (h 5 -C 5 H 4 CMe 2 Ph)Ti(OiPr) 3 (Ti-2)o nto silica [202] and further activated with MAO, provides aselective ethylene trimeriza-tion catalyst that displays acatalytic performance comparable to the corresponding homogeneous catalyst, but leaches out under the reaction conditions.Incontrast, Ti-3/SiO 2 ,prepared by an SHC approach (Scheme 9b), is inactive in the trimerization of ethylene.C r[N(SiMe 3 ) 2 ] 3 supported on silica (Scheme 9b)a ctivated with isobutylaluminoxane yields ah ighly active (3170 g 1-hexene mmol Cr À1 h À1 )a nd selective ethylene trimerization catalyst, whereas the molecular complex Cr[N(SiMe 3 ) 2 ] 3 is almost inactive under the same conditions (4 g 1-hexene mmol Cr À1 h À1 ).…”

Section: Supported Group 4-6 Oligomerization Catalystsmentioning

confidence: 99%

Bridging the Gap between Industrial and Well‐Defined Supported Catalysts

et al. 2018

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Many industrial catalysts contain isolated metal sites on the surface of oxide supports. Although such catalysts have been used in a broad range of processes for more than 40 years, there is often a very limited understanding about the structure of the catalytically active sites. This Review discusses how surface organometallic chemistry (SOMC) engineers surface sites with well-defined structures and provides insight into the nature of the active sites of industrial catalysts; the Review focuses in particular on olefin production and conversion processes.

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“…Ein weiterer Ansatz für die Entwicklung von Oligomerisationskatalysatoren besteht im Modulieren der Reaktivität von Polymerisationskatalysatoren. Behandlung des reduzierten Phillips‐Polymerisationskatalysators, der hauptsächlich Cr II ‐Zentren enthält (siehe Abschnitt 6.1.1.2), mit 1,3,5‐Tribenzylhexahydro‐1,3,5‐triazin (TAC) liefert ein Material, das bei Aktivierung mit Al i Bu 3 bei tiefen Drücken dreimal aktiver als sein homogenes Analogon CrCl 3 (TAC) und selektiv für 1‐Hexen (bis 91 %) ist . Während der homogene CrCl 3 (TAC)‐Komplex sofort 1‐Hexen liefert, zeigt der heterogene Katalysator ein Induktionsverhalten, was einen Hinweis auf eine Umwandlung des aktiven Zentrums gibt.…”

Section: Alken‐oligomerisationunclassified

Eine Brücke zwischen industriellen und wohldefinierten Trägerkatalysatoren

et al. 2018

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Viele industrielle Katalysatoren enthalten isolierte Metallzentren an der Oberfläche von Oxidträgern. Derartige Katalysatoren werden seit über 40 Jahren bei verschiedensten Verfahren verwendet – die Struktur ihrer katalytischen Zentren bleibt aber bis heute nur ungenügend verstanden. Dieser Aufsatz zeigt, wie mit metallorganischer Oberflächenchemie (SOMC) Oberflächenzentren mit gut definierten Strukturen erzeugt werden können, und beschreibt die Beschaffenheit aktiver Zentren in industriellen Katalysatoren, insbesondere für die Herstellung und Umwandlung von Olefinen.

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Turning a Cr-based heterogeneous ethylene polymerisation catalyst into a selective ethylene trimerisation catalyst

Cited by 15 publications

References 33 publications

Dichloromethane as a Selective Modifying Agent To Create a Family of Highly Reactive Chromium Polymerization Sites

Dichloromethane as a Selective Modifying Agent To Create a Family of Highly Reactive Chromium Polymerization Sites

Bridging the Gap between Industrial and Well‐Defined Supported Catalysts

Eine Brücke zwischen industriellen und wohldefinierten Trägerkatalysatoren

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