Ethylene Tetramerization Research Articles

Ethylene tetramerisation: Subtle effects exhibited by N-substituted diphosphinoamine ligands

Since the first report of ethylene tetramerisation to 1-octene using Cr-bis(diphenylphosphino)amine (PNP) catalysts, numerous studies have investigate the influence of ligand structure on the outcome of the catalysis. Although these studies have highlighted some key “ligand structure–reaction selectivity” relationships, a systematic study of the effect of steric bulk and basicity of various N-substituted PNP ligands has not been reported to date. The present study characterises in more detail the subtle effects of varying the N-substituent on overall oligomerisation performance and selectivity over a broad range of Cr-PNP based catalyst systems. A clear distinction was made between the basicity and steric bulk effects of each N-substituent, with the latter being the dominant parameter influencing the reaction selectivity. The experimental evidence suggests that for alkyl PNP ligands, branching (i.e., further substitution) of the carbon atom directly attached to the N-atom of the PNP ligand was essential for obtaining highly selective tetramerisation catalysts. The best systems achieved total alpha selectivity as high as 88% with exceptional catalyst activities (up to 3,200,000 g/(gCr h)).

Cocatalyst Influence in Selective Oligomerization: Effect on Activity, Catalyst Stability, and 1-Hexene/1-Octene Selectivity in the Ethylene Trimerization and Tetramerization Reaction

The trimerization and tetramerization of ethylene to 1-hexene and 1-octene with a Cr/PNP/AlEt3 catalyst system, in combination with a variety of cocatalysts, has been investigated. The cocatalysts B(C6F5)3 (1), Al(OC6F5)3 (2), [(Et2O)2H][Al(OC6F5)4] (3), [Ph3C][Ta(OC6F5)6] (4), (Et2O)Al{OCH(C6F5)2}3 (5), (Et2O)Al{OC(CF3)3}3 (6), [Ph3C][Al{OC(CF3)3}4] (7), [Ph3C][AlF{OC(CF3)3}3] (8), [Ph3C][{(F3C)3CO}3Al−F−Al{OC(CF3)3}3] (9), and [Ph3C][CB11H6Br6] (10) have been evaluated. The relative selectivity to 1-hexene and 1-octene obtained shows a strong dependence on the nature of the cocatalyst, and a range of selectivities from <5% C8 (90% C6) to 72% C8 have been observed. The stability of several cocatalysts toward AlEt3 has been studied, and the poor performance of 1 and 2 is linked to degradation of the cocatalyst through ethyl group exchange with AlEt3. In contrast, the [Al{OC(CF3)3}4]- anion in 7 is much more stable and gives rise to a highly active and longer lived catalyst. The overall productivity and selectivity of the catalyst is dependent upon both cocatalyst stability and the nature of the anion present, and a reason for this effect has been suggested. Selectivity control by the cocatalyst has been ascribed to interaction of the anion with the active Cr center.

Ethylene Tetramerization with Cationic Chromium(I) Complexes

Various [Cr(CO)4(P∩P)]+ complexes have been synthesized and tested for the tetramerization of ethylene to give principally 1-octene. The effect of the anion has been investigated, and it has been found that for successful catalysis an extremely weakly coordinating anion is required. This study shows that when [Al(OC(CF3)3)4]- is employed as the anion, along with triethylaluminum and PNPiPr as the coordinated diphosphine, the system is active for the tetramerization of ethylene. This is the first example of CrI being used as a catalyst precursor for selective tetramerization and gives evidence toward the catalytic cycle acting through a CrI → CrIII cationic mechanism.

Effects of halide in homogeneous Cr(III)/PNP/MAO catalytic systems for ethylene tetramerization toward 1-octene

In homogeneous Cr(III)/methylaluminoxane/cyclopentyl-bisphosphineamine (PNP)/halide ethylene tetramerization catalyst systems, the effects of halide on the 1-octene formation selectivity and the catalytic activity were investigated. The comparative studies showed that both 1-octene formation selectivity and catalytic activity of the four-member catalytic systems containing dichloromethane were higher than those of containing trichloromethane and tetrachoromethane. 1,1,2-Trichloroethane showed much higher 1-octene formation selectivity improvement than 1,1,1-trichloroethane. The improvement of chloride on 1-octene formation selectivity and catalytic activity was much better than that of a corresponding bromide. So we can draw a conclusion that the steric hindrance, the molecular stability, the halides group configuration, and the types of the halides are important factors for ethylene tetramerization toward 1-octene. Some specific interaction modes of halogen groups with active Cr species in the catalytic cycle are proposed to explain the 1-octene selectivity improvement effects of halide in the ethylene tetramerization reaction.

Role of MAO in Chromium-Catalyzed Ethylene Tri- and Tetramerization: A DFT Study

The successful [Ph2PN(iPr)PPh2]Cr-catalyzed trimerization and tetramerization of ethylene to 1-hexene and 1-octene requires the presence of a cocatalyst, of which methylaluminoxane (MAO) is particularly relevant. Density functional theory (DFT) calculations are reported on the interaction of various MAO models with chromacycloheptane intermediates. Chromacycloheptanes are well established to be important intermediates during the selective chromium-catalyzed trimerization and tetramerization of ethylene, effectively resembling appropriate models for a study of MAO interaction with chromium complexes during active catalysis. A systematic study is presented evaluating different (AlOMe)n cage structure models for MAO, as both “classic” MAO cages and cages activated by interaction with trimethylaluminium (TMA), comparing methylation aptitudes of TMA versus MAO models and evaluating the interaction of MAO models with chromacycloheptane intermediates. From the results the importance of the use of realistic ligand and large MAO models is shown to be a prerequisite for obtaining accurate catalyst activation data. In particular, use of a “stripped-down” ligand [Me2PN(Me)PMe2] on chromacycloheptane in combination with a relatively small MAO cage [(AlOMe)6·(AlMe3)] results in the optimization of formally coordinated chromacycloheptane−MAO complexes, even with increased steric congestion on chromium upon coordination of an additional ethylene moiety. In contrast, use of the “full” ligand [Ph2PN(iPr)PPh2] and larger MAO cages [(AlOMe)9·(AlMe3)] shows that while the formation of formally coordinated chromacycloheptane−MAO complexes are successfully optimized in the absence of additional ethylene, only dissociated ion-pair complexes are present when an additional ethylene molecule is introduced. From these results important insight is gained on the role of MAO during catalysis, as well as the model requirements for both MAO and chromium complexes to conduct fundamental theoretical studies in selective chromium-catalyzed ethylene oligomerization.

Chromium(iii) catalysed ethylene tetramerization promoted by bis(phosphino)amines with an N-functionalized pendant

Several N-functionalized bis(phosphino)amine ligands with ether, thioether and pyridyl tethers [(R'')2PN(R')P(R'')2=PNP] () have been synthesized. They react with CrCl3(THF)3 in CH2Cl2 to give dinuclear chloro bridged Cr2(micro-Cl)2Cl4(PNP)2 () which converts to the corresponding mononuclear solvento complexes fac-CrCl3(PNP)(NCR) (). The structures of the ligand with R'=-(CH2)3SCH3 and R''=Ph, and the complexes with R=CH3 () and C2H5 (), R'=-(CH2)3SCH3 and R''=Ph) have been established by single-crystal X-ray crystallography. All ligands are active towards ethylene tetramerization in the presence of Cr(III) and excess MAO at 80 degrees C in toluene. The ligand with thioether pendant Et2PN(CH2CH2CH2SCH3)PEt2 () shows the highest selectivity (55% weight in liquid product distribution) towards 1-octene. Complexes and are active towards ethylene polymerization under thermal conditions.

N-substituted diphosphinoamines: Toward rational ligand design for the efficient tetramerization of ethylene

Bis(diphenylphosphino)amine (PNP) ligands with different alkyl and cycloalkyl substituents attached to the N atom of the ligand backbone were synthesised and tested together with chromium as ethylene tetramerization catalysts. On activation with a methylaluminoxane-based activator, the catalysts displayed good activity and selectivity toward 1-octene and 1-hexene, with the best ligand systems containing cyclopentyl or cyclohexyl moieties. In addition, it was established that substitution at the 2 position of the cyclohexyl skeleton and, more importantly, an increase in steric bulk at that point, led to a drastic reduction of side product formation (i.e., methyl- and methylenecyclopentane). Interestingly, additional methyl substitution in the 6 position of the cyclohexyl ring changed the selectivity of the catalyst from predominantly tetramerization to a 1:1 mixture of 1-hexene and 1-octene. Structurally similar ligands, such as cyclohexylmethyl and cyclohexylethyl PNP, were also tested and were also found to yield efficient tetramerization catalysts. It was concluded that structural fine tuning of the N-alkyl moiety of the PNP ligand is essential for obtaining efficient tetramerization catalysts, with the best systems achieving combined selectivities as high as 88% (1-octene and 1-hexene) with exceptionally high activities exceeding 2,000,000 g/(g-Cr h).

Performance of various aluminoxane activators in ethylene tetramerization based on PNP/Cr(III) catalyst system

Ethylene tetramerization with PNP/Cr(III)/aluminoxanes was investigated. By comparing catalytic activity, selectivity to 1-octene and selectivity to methylenecyclopentane in ethylene tetramers, the effects of various aluminoxanes on ethylene tetramerization were discussed. The results showed that MMAO(TMA-depleted), MAO and EAO were all effective cocatalyst for ethylene tetramerization toward 1-octene. PNP/Cr(III)/MMAO catalytic systems afford the highest catalytic activity and the highest selectivity to 1-octene. The selectivities to methylenecyclopentane was decreased by using i -BAO and EAO as cocatalyst, which indicated that different mechanism for ethylene tetramerization has been occurred in such cases.

Nitrogen-Linked Diphosphine Ligands with Ethers Attached to Nitrogen for Chromium-Catalyzed Ethylene Tri- and Tetramerizations

A series of bis(diphenylphosphino)amine ligands with a donor group attached to the nitrogen linker have been prepared. Metalation of these ligands with chromium trichloride provides precursors to highly active, relatively stable, and selective catalysts for trimerization and tetramerization of ethylene. It has been demonstrated in oligomerization reactions performed at 1 and 4 atm of ethylene that these new systems increase total productivity by enhancing catalyst stability, as compared with those lacking a donor group on the diphosphine ligand. Furthermore, the use of chlorobenzene solvent (rather than toluene) significantly improves productivity, stability, and selectitvity. The product distributions and minor byproducts provide information relevant to mechanistic issues surrounding these types of reactions.

Preparation of 1-octene by the selective tetramerization of ethylene

A diphosphinoamine ligands has been synthesized. In combination with Cr(III), methylalumoxane (MAO) and tetrachloroethane (TCE), they generate active catalytic systems for ethylene tetramerization toward 1-octene. The effects of reaction temperature, reaction pressure, molar ratio of Al/Cr, TCE/Cr, ligand/Cr, reaction time and sorts of cocatalyst on catalytic activity and selectivity to 1-octene were studied. The results show that the diphosphinoamine/Cr(III)/TCE/MAO four-membered catalytic systems for ethylene tetramerization have improved the selectivity to 1-octene, but catalytic activity is lower than that of three component system. MAO is requisite in forming active species for ethylene tetramerization toward 1-octene. TCE shows significant promotion effect to assist chromium center in improving selectivity toward 1-octene.

Reaction kinetics of an ethylene tetramerisation catalyst

This paper describes a study on the reaction kinetics of an ethylene tetramerisation catalyst comprising Cr(acac) 3, a bis(diphenylphosphino)isopropylamine ligand and MAO as catalyst activator. The reaction rate was found to be dependent on temperature as well as on ethylene concentration to the order of 1.57. The catalyst deactivation rate was found to be a function of temperature. A reaction rate model was developed providing good description of reaction performance in the temperature range of 35–45 °C and pressure range of 30–45 bar. Selectivity data is provided which illustrates the effects of pressure, temperature and time on stream. Increased temperature and decreased pressure result in increased 1-hexene and decreased 1-octene selectivity, while time on stream data indicates the occurrence of secondary incorporation reactions of 1-hexene and 1-octene.

Preparation of 1-octene by ethylene tetramerization with high selectivity

Two novel PNP ligands have been synthesized and characterized by 1H-NMR, elemental analysis, and mass spectra. In combination with Cr(III) and cocatalyst MAO, they generate active catalytic systems that tetramerize ethylene with both high catalytic activity and high selectivity to produce 1-octene. The results show that these catalyst systems are able to catalyze ethylene tetramerization, with high catalytic activity up to 0.89×106 g/mol Cr.h, and the selectivity of C8 in products is 72.52%, and the percentage of 1-olefins in the C8 cut is 97.87%.

Isolation of a Cationic Chromium(II) Species in a Catalytic System for Ethylene Tri- and Tetramerization

Treatment of the ethylene tetramerization catalyst precursor [PNP]CrCl3 with Me3Al afforded the new divalent catalyst precursor {[PNP]2Cr(m-Cl)AlMe3}[ClAlMe3]0.34[Me4Al]0.66·0.125(hexane)·0.25(toluene) (1). The formation of 1 implies reduction to the divalent state, cationization, and ligand scrambling. The fact that this species displays, upon activation with MAO, a catalytic activity similar to that of the trivalent precursor indicates that reduction of the metal to the divalent state is a step toward the formation of the actual catalytically active species. The failure of CrCl2(THF)2 to react with [PNP] indicates that cationization and acquisition of the second ligand and of a bridging ClAlMe3 residue are central to the stabilization of this new divalent catalyst precursor.

Mechanistic Investigations of the Ethylene Tetramerisation Reaction

The unprecedented selective tetramerisation of ethylene to 1-octene was recently reported. In the present study various mechanistic aspects of this novel transformation were investigated. The unusually high 1-octene selectivity in chromium-catalyzed ethylene tetramerisation reactions is caused by the unique extended metallacyclic mechanism in operation. Both 1-octene and higher 1-alkenes are formed by further ethylene insertion into a metallacycloheptane intermediate, whereas 1-hexene is formed by elimination from this species as in other reported trimerisation reactions. This is supported by deuterium labeling studies, analysis of the molar distribution of 1-alkene products, and identification of secondary co-oligomerization reaction products. In addition, the formation of two C6 cyclic products, methylenecyclopentane and methylcyclopentane, is discussed, and a bimetallic disproportionation mechanism to account for the available data is proposed.

Ethylene Tetramerization: A New Route to Produce 1-Octene in Exceptionally High Selectivities

Linear alpha-olefins, such as 1-hexene and 1-octene, are important comonomers in the production of linear low-density polyethylene (LLDPE). The conventional method of producing 1-hexene and 1-octene is by oligomerization of ethylene, which yields a wide spectrum of linear alpha-olefins (LAOs). While there exists several processes for producing 1-hexene via ethylene trimerization, a similar route for the selective production of 1-octene has so far been elusive. We now, for the first time, report an unprecedented ethylene tetramerization reaction that produces 1-octene in selectivities exceeding 70%, using an aluminoxane-activated chromium/((R2)2P)2NR1 catalyst system.