Single-site heterogeneous Cr-based catalyst for the selective trimerisation of ethylene

Nenu, Cristina N.; Weckhuysen, Bert M.

doi:10.1039/b417938m

Cited by 47 publications

(28 citation statements)

References 18 publications

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“…In a recent communication and related patent application, [23,24] we reported briefly on a novel TAC-modified Cr/ SiO 2 catalyst that is very active and selective in the trimerization of ethylene, with activities well above those of its homogeneous counterpart. Specific tests also showed that the catalyst is truly heterogeneous and, therefore, its remarkable catalytic activity is due to newly Cr-assembled surface species.…”

Section: Introductionmentioning

confidence: 99%

Controlled Assembly of a Heterogeneous Single‐Site Ethylene Trimerization Catalyst as Probed by X‐ray Absorption Spectroscopy

Nenu

Lingen

Groot

et al. 2006

Chemistry A European J

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X-ray absorption spectroscopy at the Cr K- and L(2,3)-edges was used to study the assembling process of a heterogeneous Cr-based single-site catalyst. The starting point was a Phillips-type system with monochromate species anchored on a silica surface, which was first reduced to a variety of different surface Cr(II) species. The reduced sample was modified with a 1,3,5-tribenzylhexahydro 1,3,5-triazine (TAC) ligand in the presence of CH(2)Cl(2) as solvent to yield a heterogeneous single-site Cr-based catalyst active in the trimerization of ethylene. The molecular structure of the resultant catalytic material consists of distorted octahedral Cr(III) species. The extended X-ray absorption fine-structure (EXAFS) spectroscopy fitting procedure in R space up to 2.5 A showed that the synthesis leads to coordination with a TAC ligand. The fit also shows that it was possible to complete the six-fold environment around Cr(III) with two oxygen atoms and one chloride ligand. This chloride ligand is formed in a redox process from the solvent and is responsible for the oxidation of surface Cr(II) to Cr(III). The obtained geometry and the local environment of the surface complex are discussed in light of its homogeneous counterpart and confirm the single-site characteristics of the prepared catalytic material.

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Section: Introductionmentioning

confidence: 99%

Controlled Assembly of a Heterogeneous Single‐Site Ethylene Trimerization Catalyst as Probed by X‐ray Absorption Spectroscopy

Nenu

Lingen

Groot

et al. 2006

Chemistry A European J

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“…In this respect, the above-mentioned annealing method (Groppo et al 2005d) is a way to fine tune the relative populations of Cr II sites, and thus the molecular weight distribution of the polyethylene product. More recently, based on the addition of specific surface modifying agents, finer methods have been adopted (Nenu & Weckhuysen 2005;Nenu et al 2006Nenu et al , 2007. In addition, the possibility of changing the catalytic activity of Cr/SiO 2 by altering the chemical composition of the support has been known since the 1970s.…”

Section: (C) Engineering Of the Ligand Sphere By Means Of Activatorsmentioning

confidence: 99%

“…The active catalytic centre originated by both precursors is assumed to be the same (figure 1d) because the polyethylene obtained in the two cases is the same. By analogy with the Cossee-Arlman mechanism (Arlman & Cossee 1964;Cossee 1964), proposed for classical Ziegler-Natta catalysts (Ziegler et al 1955;Natta 1956;Busico et al 1999), as well as for metallocene catalysts (Resconi et al 2000;Kaminsky & Laban 2001), it is arbitrarily assumed that the catalytic centre contains a coordination vacancy (coloured shadow in figure 1d), where the olefin molecule can insert, and a growing hydrocarbon chain. However, the initiation steps leading to the formation of the active centres constituted a matter of debate for many years, and were the subject of many investigations, also by means of surface science methods (Groppo et al 2005b).…”

Section: (B) Mechanism Of Initiation Of Ethylene Polymerizationmentioning

confidence: 99%

Surface chromium single sites: open problems and recent advances

Zecchina

Groppo

2012

Proc. R. Soc. A.

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The results of decades of studies on the Phillips chromium (Cr)/silica polymerization catalyst are briefly summarized. The application of several characterization methods has allowed a detailed knowledge of the structure and reactivity of Cr centres to be obtained. In particular, many aspects of this apparently simple single-site catalyst, including the heterogeneity, the modification of the ligand sphere upon interaction with many molecules and the initiation mechanism of the ethylene polymerization reaction, have been clarified. It is shown that based on the acquired knowledge, it is now possible to proceed further towards the intelligent modification of the ligand sphere with the scope to increase the reaction rate and selectivity. It is also illustrated that, besides polymerization/oligomerization reactions, it is possible to extend the study of Cr II reactivity towards new reactions.

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“…The obtained value of 324 amu indicates that the solid product is more a heavy oligomer of ethylene than a polymer. The solid product obtained at 1 bar presents a remarkable narrow distribution of the average molecular weight (M w ) that indicates equal-length polymeric chains [25]. This feature of the solid product reflects the single-site behaviour of the TAC-CrCl/SiO 2 catalyst.…”

Section: Trimerisation Versus Polymerisation Activitymentioning

confidence: 99%

“…By coordinating a TAC ligand onto a coordinatively unsaturated Cr II site, a trimerisation site was assembled on a silica surface [25]. Its structure consists of a coordinated TACring at Cr centre and one chloride ligand, besides two links with the silica carrier [25,26].…”

Section: Introductionmentioning

confidence: 99%

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

Nenu

Bodart

Weckhuysen

2007

Journal of Molecular Catalysis A: Chemical

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A Phillips Cr/SiO 2 polymerisation catalyst was converted into an ethylene trimerisation catalyst after assembling new active sites on the silica surface in the presence of TAC (1,3,5-triphenylhexahydro 1,3,5-triazacyclohexane) as ligand and CH 2 Cl 2 as solvent. The reaction conditions play a role in tuning the catalytic activity of this system. It is a true trimerisation catalyst at low ethylene pressures, while, at high pressures, the polymerisation activity becomes dominant. Moreover, at high ethylene pressures, polyethylene characterised by a high crystallinity degree is obtained. Instead, a highly branched heavy oligomer of ethylene characterised by a very narrow molecular weight distribution is obtained at 1 bar. 1-Hexene, formed as the main product in the ethylene trimerisation reaction at low pressures, is partially incorporated into this growing oligomer making redundant the use of a second comonomer feedstock and an expensive catalyst.

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Single-site heterogeneous Cr-based catalyst for the selective trimerisation of ethylene

Abstract: TAC-Cr3+/SiO2 complexes are highly active and selective ethylene trimerisation catalysts and possess single-site catalytic behaviour, an unusual property for heterogeneous catalysts.

Cited by 47 publications

References 18 publications

Controlled Assembly of a Heterogeneous Single‐Site Ethylene Trimerization Catalyst as Probed by X‐ray Absorption Spectroscopy

Controlled Assembly of a Heterogeneous Single‐Site Ethylene Trimerization Catalyst as Probed by X‐ray Absorption Spectroscopy

Surface chromium single sites: open problems and recent advances

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

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