Ethylene Trimerization Research Articles

Catalytic Oligomerization of Ethylene to Higher Linear α-Olefins Promoted by Cationic Group 4 Cyclopentadienyl-Arene Active Catalysts: A DFT Investigation Exploring the Influence of Electronic Factors on the Catalytic Properties by Modification of the Hemilabile Arene Functionality

Hessen and co-workers described Ti-based precatalysts of the title class bearing a hemilabile ancillary arene functionality for ethylene trimerization. The analogues with the heavier group 4 metals Zr and Hf as the active center have been recently explored computationally. This study suggested the Zr-based system as a promising catalyst for production of an oligomer mixture containing 1-octene besides the prevalent 1-hexene. Herein, we have presented a detailed computational analysis of the influence of modifications of the hemilabile arene functionality on the catalytic abilities of cationic group 4 [(η5-Cp-(CMe2R‘-bridge)-C6HyRx)MII(C2H4)2]+ active species (M = Zr, Hf) for linear ethylene oligomerization, employing a gradient-corrected DFT method. This comprised the arene substitution with prototypical electron-releasing (R = Me) and electron-withdrawing (R = F, CF3) groups in various combinations and also the enlargement of the Cp-arene connecting bridge by an additional methylene group (R‘ = CH2). The...

Catalytic oligomerization of ethylene to higher linear alpha-olefins promoted by the cationic group 4 [(eta 5-Cp-(CMe2-bridge)-Ph)MII(ethylene)2]+ (M = Ti, Zr, Hf) active catalysts: a density functional investigation of the influence of the metal on the catalytic activity and selectivity.

A detailed theoretical analysis is presented of the catalytic abilities of heavier group 4 (M = Zr, Hf) metals for linear ethylene oligomerization with the cationic [(eta(5)-C(5)H(4)-(CMe(2)-bridge)-C(6)H(5))M(IV)(CH(3))(2)](+) complex as precatalyst, employing a gradient-corrected DFT method. The parent Ti system has been reported as a highly selective catalyst for ethylene trimerization. The mechanism involving metallacycle intermediates, originally proposed by Briggs and Jolly, has been supported by the present study to be operative for the investigated class of group 4 catalysts. Metallacycle growth through bimolecular ethylene uptake and subsequent insertion is likely to occur at uniform rates for larger cycles that are furthermore comparable for Ti, Zr, and Hf catalysts. Ethylene insertion into the two smallest five- and seven-membered cycles is found to become accelerated for Zr and Hf catalysts, which is due to geometrical factors. In contrast, electronic effects act to raise the barrier for metallacycle decomposition, affording alpha-olefins upon descending group 4. This process is furthermore predicted to be kinetically more difficult for larger metallacycles. The oligomer distribution of the Zr-mediated reaction is likely to comprise predominantly 1-hexene together with 1-octene, while 1-butene and alpha-olefins of chain lengths C(10)-C(18) should occur only in negligible portions. A similar composition of alpha-olefins having C(6)-C(18) chain lengths is indicated for the Hf catalysts, but with long-chain oligomers and polymers as the prevalent fraction. Between the group 4 catalysts of the investigated type, the Zr system appears as the most promising candidate having catalytic potential for production of 1-octene, although not selectively. The influence of temperature to modulate the oligomer product composition has been evaluated.

A DFT Study toward the Mechanism of Chromium-Catalyzed Ethylene Trimerization

A theoretical study of a mechanism for ethylene trimerization, catalyzed by Cr-pyrrolyl complexes, is proposed and investigated with density functional theory methods. The selective formation of 1-hexene is generally accepted to follow a metallacycle mechanism. A detailed spin state analysis for active species in the mechanism shows that the triplet spin state represents the ground spin state for all stationary points. Complete Gibbs free energy (298.15 K) surfaces are mapped for both η5- and σ-bonding modes of pyrrole, as well as a stripped-down Cl anion model and a full ClAlMe3 anion model. From the calculated results it is shown that the proposed metallacycle mechanism is energetically favorable, with metallacycle growth identified as the rate-determining step. In addition, it is demonstrated that different bonding modes of pyrrole are preferred at different stages in the proposed mechanism, effectively suggesting that ring slippage of the pyrrole occurs on the minimum energy path on the potential energy surface. From the calculated results important insight is gained into the hemilabile nature of the pyrrole ring in the mechanism, which in turn sheds light on the general requirements for an effective ligand in Cr-catalyzed ethylene trimerization.

Catalytic trimerization of ethylene by half-sandwich titanium complexes bearing a pendant ethereal group

Three half-sandwich titanium complexes with a pendant ethereal group, CH3OCH2CH2CpTiCl3, CH3OCH(CH3)CH2CpTiCl3, and tetrahydrofurfurylcyclopentadienyltitanium trichloride activated by MAO have been found to trimerize ethylene with high selectivity and moderate activity. Compared with the catalytic behavior of CpTiCl3/MAO and n-Pr-CpTiCl3/MAO, we can identify that the pendant ethereal group plays an important role in this selective trimerization process. Their different catalytic behavior toward temperature is discussed.

Mechanistic Studies of the Ethylene Trimerization Reaction with Chromium−Diphosphine Catalysts: Experimental Evidence for a Mechanism Involving Metallacyclic Intermediates

A system for catalytic trimerization of ethylene utilizing CrCl3(THF)3 and a diphosphine ligand PNPOMe [= (o-MeO-C6H4)2PN(Me)P(o-MeO-C6H4)2] has been investigated. The coordination chemistry of chromium with PNPOMe has been explored, and (PNPOMe)CrCl3 and (PNPOMe)CrPh3 (3) have been synthesized by ether displacement from chromium(III) precursors. Salt metathesis of (PNPOMe)CrCl3 with o,o'-biphenyldiyl Grignard affords (PNPOMe)Cr(o,o'-biphenyldiyl)Br (4). Activation of 3 with H(Et2O)2B[C6H3(CF3)2]4 or 4 with NaB[C6H3(CF3)2]4 generates a catalytic system and trimerizes a 1:1 mixture of C2D4 and C2H4 to give isotopomers of 1-hexene without H/D scrambling (C6D12, C6D8H4, C6D4H8, and C6H12 in a 1:3:3:1 ratio). The lack of crossover supports a mechanism involving metallacyclic intermediates. The mechanism of the ethylene trimerization reaction has also been studied by the reaction of trans-, cis-, and gem-ethylene-d2 with 4 upon activation with NaB[C6H3(CF3)2]4.

Ethylene trimerisation catalyst based on substituted cyclopentadienes

A four-component homogeneous catalyst system comprising a substituted cyclopentadiene ligand, a chromium compound, an alkyl aluminium and a halogen source is effective for the trimerisation of ethylene to 1-hexene in good selectivity and activity. In a typical example a catalyst system containing chromium(III) tris (2-ethylhexanoate), 1,2,3,4,5-pentaphenylcyclopenta-1,3-diene, hexachloroethane and triethylaluminium produced 1-hexene in up to 76 mass%, at an activity of 40 700 g of product/(g Cr h). Approximately 15 mass per cent of internal hexenes are also produced as well as smaller quantities of C-10’s and polyethylene. The effect of changes in temperature, pressure and reagent ratios has been investigated as well as the use of other substituted cyclopentadienes.

The Effect of Aromatic Ethers on the Trimerisation of Ethylene using a Chromium Catalyst and Aryloxy Ligands

AbstractA catalyst system consisting of a chromium source, a 2,6‐disubstituted phenol and an aromatic ether solvent, on activation with triethylaluminium, is active and selective for the trimerisation of ethylene to form 1‐hexene. The aromatic ether appears to play a role as both solvent and reagent and is pivotal in ensuring both the good activity of the system as well as selectivity to the desired product, 1‐hexene.

Hemilabile Ligand Induced Selectivity: a DFT Study on Ethylene Trimerization Catalyzed by Titanium Complexes

In this computational study, we propose a detailed mechanism, which has been explored by density functional theory simulations, for the trimerization reaction of ethylene to give selectively 1-hexene using a [(η5-C5H4CMe2C6H5)TiCl3/MAO] catalyst. For ring-opening reactions we distinguish between agostic assisted β-hydrogen transfer and hydride formation. With the B3LYP functional it was found that the rate-determining step is the ring-opening reaction of the seven-membered metallacycle, exhibiting a barrier ΔG⧧(298.15 K) of 18.4 kcal/mol. It appears that the selectivity of the reaction results from two effects: the stabilizing effect of the hemilabile phenyl ligand and the ring size of the metallacycle. Upon interchange of the phenyl group by the labile methyl group, the calculations predict the formation of polyethylene, which is in agreement with the experimental data.

Investigation of monomer ratios in random and block copolymers of ethylene and propylene using pyrolysis–gas chromatography/mass spectrometry

Polyolefins such as polyethylene and polypropylene degrade during pyrolysis to form a wide distribution of oligomers, producing pyrograms which are complex, but characteristic. Polyethylene produces mainly straight-chain hydrocarbons, including alkanes, alkenes and dienes, while polypropylene produces methyl-branched hydrocarbon oligomers. The pyrogram of a block copolymer of polyethylene and polypropylene will contain oligomers of both homopolymers, and simple relationships may be made between peaks for oligomers of each polymer to the relative amount of each component in the copolymer. When a random copolymer of ethylene and propylene is pyrolyzed, however, mixed oligomers are produced which are not seen in the homopolymer pyrolysate. Depending on the relative amounts of the two monomers, homo-oligomers may also be present, especially for short sequences such as trimer and tetramer. Although the pyrograms of random copolymers appear different when different monomer ratios are examined, the relationship of specific peaks to the monomer concentration is not so straightforward as in block copolymers. This paper presents analytical results for random and block copolymers of ethylene and propylene, relating the peaks for various oligomers to the monomer ratio in the polymer. In the case of random copolymers, the mathematical probability of finding three ethylene units adjacent to one another on the polymer molecule is compared with the analytical determination of the ethylene trimer 1-hexene in the pyrograms.

Novel Cr‐PNP Complexes as Catalysts for the Trimerization of Ethylene.

AbstractFor Abstract see ChemInform Abstract in Full Text.

Mechanism of Ethene Trimerization at an ansa-(Arene)(cyclopentadienyl) Titanium Fragment

A theoretical study of ethene trimerization at a cationic (C6H5CH2C5H4)Ti fragment generally supports the metallacycle mechanism proposed earlier for this reaction. However, the crucial formation of the 1-hexene complex from a titanacycloheptane intermediate occurs by direct Cβ → Cα‘ hydrogen transfer rather than by the more traditional β-elimination/reductive elimination sequence. The pendant arene moiety “breathes” during the reaction, being more strongly bound at the TiII stage than at the TiIV stage of the reaction. Its main role is to make the olefin complex formation more endothermic, thus increasing the barriers for formation of titanacyclopentane and titanacycloheptane intermediates. For the “naked” (C5H5)Ti system, which lacks this effect, further ring growth wins over hexene formation. But even for the bridged (C6H5CH2C5H4)Ti system, we find that the various reactions are very delicately balanced.

First Cr(III)-SNS complexes and their use as highly efficient catalysts for the trimerization of ethylene to 1-hexene.

Cr(III) complexes of tridentate SNS ligands have been prepared and evaluated as catalysts for ethylene trimerization, with several giving very high activity and excellent selectivity toward 1-hexene when activated with methylaluminoxane. The new complexes illustrate the potential of sulfur-based ligands on early transition metals for catalysis.

Why Trimerization? Computational Elucidation of the Origin of Selective Trimerization of Ethene Catalyzed by [TaCl3(CH3)2] and An Agostic‐Assisted Hydride Transfer Mechanism

Why Trimerization? Computational Elucidation of the Origin of Selective Trimerization of Ethene Catalyzed by [TaCl₃(CH₃)₂] and An Agostic‐Assisted Hydride Transfer Mechanism

Why Trimerization? Computational Elucidation of the Origin of Selective Trimerization of Ethene Catalyzed by [TaCl3(CH3)2] and An Agostic‐Assisted Hydride Transfer Mechanism

Why Trimerization? Computational Elucidation of the Origin of Selective Trimerization of Ethene Catalyzed by [TaCl₃(CH₃)₂] and An Agostic‐Assisted Hydride Transfer Mechanism

Novel Cr-PNP complexes as catalysts for the trimerisation of ethylene.

Cr(III) complexes of tridentate PNP ligands have been prepared and evaluated as catalysts for ethylene trimerisation, with several giving high activity and excellent selectivity towards 1-hexene when activated with methylaluminoxane.

Ethylene trimerization with a half-sandwich titanium complex bearing a pendant thienyl group.

A half-sandwich titanium complex with a pendant thienyl group, activated by methylalumoxane (MAO), can trimerize ethylene to 1-hexene with considerable activity and high selectivity; the coodination of the sulfur atom to the titanium center is proposed to be responsible for the selective trimerization of ethylene.

Catalytic Trimerization of Ethene with Highly Active Cyclopentadienyl−Arene Titanium Catalysts

The mono(cyclopentadienyl−arene)titanium complexes [η5-C5H3R−(bridge)−Ar]TiCl3, activated by methylalumoxane (MAO) cocatalyst, form a family of highly active catalysts for the trimerization of ethene, giving 1-hexene as the main product. Concomitant cotrimerization of ethene and 1-hexene, to give mainly 5-methylnon-1-ene, is also observed. The selectivity for trimerization depends on the presence of a pendant arene group on the cyclopentadienyl ligand and the nature of the bridge between these two ligand moieties. In the absence of a pendant arene, polyethene is the main product. The highest activity and selectivity for trimerization was observed for catalysts with a disubstituted C1 bridge between the cyclopentadienyl and arene ligand moieties. A SiMe3 substituent on the cyclopentadienyl ligand improves catalyst activity and selectivity, whereas methyl substitution of the arene decreases activity. Nevertheless, combining cyclopentadienyl SiMe3 substitution with arene Me substitution gives rise to a catal...

ChemInform Abstract: High Activity Ethylene Trimerization Catalysts Based on Diphosphine Ligands.

AbstractChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.

Silica-supported Cr[N(SiMe 3) 2] 3/isobutylalumoxane catalyst for selective ethylene trimerization

A new silica-supported chromium complex catalyst for ethylene trimerization having high activity and high selectivity is presented. The catalyst was prepared by supporting tris[bis(trimethylsilyl)amido]Cr(III) Cr[N(SiMe 3) 2] 3 and isobutylalumoxane on silica calcined at 600 °C. The catalyst containing 1.0 wt.% Cr with an isobutylalumoxane/Cr molar ratio of 24 gave a 74.2% 1-hexene selectivity at an activity of 3170 g/(mmol Cr h) in heptane solvent at 120 °C and 0.7 MPa ethylene partial pressure. The main by-product was decenes while small amounts of 1-butene, 1-octene and polyethylene were also obtained. To improve the 1-hexene selectivity, 1,2-dimethoxyethane was added as an electron donor, which gave an 86.4% 1-hexene selectivity at a 1,2-dimethoxyethane/Cr molar ratio of 0.25 under the same reaction conditions, but the activity decreased to 1580 g/(mmol Cr h). With the addition of 1,2-dimethoxyethane, the 1-hexene selectivity increased at the expense of decenes. The presence of the silica as a support and the silica calcination temperature were the key factors for the ethylene trimerization. The trimerization activity greatly decreased when the silica support was not used or calcined at low temperature. The trimerization activity obeyed first-order kinetics on the ethylene partial pressure. The ethylene trimerization mechanism, which includes the metallacycle intermediates supported on silica by one SiOCr bond, is proposed.

High activity ethylene trimerisation catalysts based on diphosphine ligands.

Chromium complexes of ligands of the type Ar2PN(Me)PAr2 (Ar = ortho-methoxy-substituted aryl group), on activation with MAO, are extremely active and selective catalysts for the trimerisation of ethylene.