Ethylene Tetramerization Research Articles

Synthesis of Branched α-Olefins via Trimerization and Tetramerization of Ethylene.

α-Olefins are very important bulk and fine chemicals and their synthesis from ethylene, an abundantly available and inexpensive feedstock, is highly attractive. Unfortunately, the direct or on-purpose synthesis of olefins from ethylene is limited to three examples, 1-butene, 1-hexene, and 1-octene, all having a linear structure. Herein, the direct synthesis of 3-methylenepentane and 4-ethylhex-1-ene, branched trimerization, and tetramerization products of ethylene, respectively, is reported. Different molecular titanium catalysts, all highly active, with a selectivity toward the formation of the branched ethylene trimer or tetramer, the employment of different activators, and different reaction conditions are the key to selective product formation. The long-time stability of selected catalysts employed permits upscaling as demonstrated for the synthesis of 4-ethylhex-1-ene (52g isolated, TON(ethylene) 10.7 · 106).

High-temperature stable chromium complexes bearing fluorinated PNP ligands for selective ethylene tetramerization toward 1-octene

A series of new fluorinated PNP ligands of bis(ortho-fluorophenyl)-type (L1-L2) and ortho-fluorophenyl-type (L3-L4) have been synthesized and assessed for the selectivity of ethylene oligomerization. Compared to the non-fluorinated analogue ligand L6, the fluorinated ligand L1 exhibited approximately three times high catalytic activity. The bis(ortho-fluorophenyl)-type ligand L2 with a diethylphosphine substituent offered the highest catalytic performance of 3548 kg∙g−1∙h−1 and the 1-octene selectivity is 67.6 %. Both the L1 and L2 ligands exhibited excellent the activity of catalyst at a high temperature of 100 ℃, with ligand L2 providing the catalytic performance of 1587 kg∙g−1∙h−1 and good selectivity of 47.5 % for 1-C8=. The ortho-fluorophenyl substituent enhances the catalytic activity, while the para-fluorophenyl substituent reduces the catalytic activity. In addition, density functional theory (DFT) calculations and UV–vis spectroscopy have been used to rationalize the role of the fluorine effect in promoting catalytic performance. Our results indicated that the strong inductive effect of fluorine atoms and the non-covalent interaction between fluorine atoms and active centers jointly enhance the stability of the ortho-fluorophenyl-based catalyst and promote its overall activity.

Preparation of chromium complexes for ethylene tetramerization catalyst

AbstractOne of the remaining challenges in the commercialization of a highly efficient ethylene tetramerization catalyst, [1‐CrCl2][B(C6F5)4] (1=iPrN[P(C6H4Si(Octyl)3)2]2), is the generation of small amounts of polyethylene (PE). To address this issue, we explored different activator options and found that (Octyl)3Al was the most effective choice. Building on the observation that [iPrN(PPh2)2]CrCl3(THF) was not completely alkylated by the action of excess Me3Al to form ([iPrN(PPh2)2]Cr(μ2‐Cl)(ClMe2AlClAlMe3))2, we synthesized [ortho‐(MeO)C6H4CH2‐η1C:κO]3Cr, developing a catalytic system comprising this precursor, PNP ligand 1, and [MeN(H)(C18H37)2][B(C6F5)4]. However, this catalytic system exhibited a long induction time (~30 min) and generated a significant amount of PE. Returning to the catalytic system of [1‐CrCl2][B(C6F5)4], we successfully achieved the conversion of [(EtOH)4CrCl2][B(C6F5)4] to [(CH3CN)4CrCl2][B(C6F5)4] at a large scale by designing a specialized glass apparatus. Finally, we discovered that the generation of PE could be reduced by adjusting the reaction conditions during the synthesis of [1‐CrCl2][B(C6F5)4].

Highly efficient PCCP/Cr(III) ethylene tetramerization catalyst designed using artificial neural network approach

Chromium complexes are feasible for inducing ethylene selective oligomerization to generate valuable linear α-olefins, such as 1-hexene and 1-octene. The current theoretical calculation techniques can effectively accelerate the design of molecular catalysts. In this work, we combined density functional theory (DFT) and artificial neural network (ANN) methods to model vinyl-bridged PCCP/Cr active species and used an ANN-based model to predict and screen the designed novel PCCP ligands. DFT was used to optimize the structure of active species and calculate electronic properties. The datasets were established using steric descriptors, electronic descriptors, and experimental data. The ANN-based models were trained based on the established datasets. We found that the optimized ANN-based models ANN-HT and ANN-OT, respectively for the prediction of 1-hexene and 1-octene selectivity, provide satisfactory predictive performance, with R2 values of 0.962 for ANN-HT and 0.976 for ANN-OT. We investigated the contribution of steric and electronic descriptors on catalytic performance, and the results showed that steric descriptors have a greater contribution to controlling product selectivity compared to electronic descriptors. The similarity between the ethylene oligomerization test results of the experimentally prepared L28/Cr catalyst and the ANN-based model prediction results proved the accuracy of the models. The experimentally prepared L28/Cr exhibited catalytic activity of 2181 kg∙g−1∙h−1 and 65.8 % 1-C8 selectivity under suitable reaction conditions.

Unveiling meta-Alkyloxy/-Silyloxy-Substituted N-Aryl PNP Ligands for Efficient Cr-Catalyzed Ethylene Tetramerization.

Novel N-aryl-functionalized PNP ligands (1-4) bearing m-alkyloxy/-silyloxy substituents were prepared and evaluated for chromium-catalyzed ethylene oligomerization using MMAO-3A as an activator. The selected Cr/PNP system under optimized condition exhibited high 1-octene-selective (up to 70 wt %) ethylene tetramerization at a remarkable rate (over 3000 kg gCr-1 h-1). More importantly, the undesirable polyethylene selectivity was restricted to a minimum level of ∼1-2 wt % for pre-catalysts derived with ligands 1 and 2. Employing chlorobenzene as a reaction medium yielded best productivity in conjunction to the total α-olefin (1-C6 + 1-C8) selectivity (∼88 wt %). N-aryl PNP ligands (3 and 4) incorporating m-silyloxy substituents in the phenyl ring exhibited relatively poorer tetramerization performance while yielding higher PE fraction as compared to their m-alkyloxy derivatives. A detailed molecular structure of the best-performing pre-catalyst 1-Cr was established by single-crystal X-ray diffraction analysis. The stability of 1/Cr-based catalyst system was investigated for a reaction time of up to 2 h under optimized condition.

A Density Functional Study on Ethylene Trimerization and Tetramerization Using Real Sasol Cr-PNP Catalysts.

To gain molecular-level insight into the intricate features of the catalytic behavior of chromium-diphosphine complexes regarding ethylene tri- and tetramerizations, we performed density functional theory (DFT) calculations. The selective formation of 1-hexene and 1-octene by the tri- and tetramerizations of ethylene are generally accepted to follow the metallacycle mechanism. To explore the mechanism of ethylene tri- and tetramerizations, we used a real Sasol chromium complex with a nitrogen-bridged diphosphine ligand with ortho- and para-methoxyaryl substituents. We explore the trimerization mechanism for ethylene first and, later on for comparison, we extend the potential energy surfaces (PES) for the tetramerization of ethylene with both catalysts. The calculated results reveal that the formation of 1-hexene and 1-octene with the ortho-methoxyaryl and para-methoxyaryl Cr-PNP catalysts have nearly similar potential energy surfaces (PES). From the calculated results important insights are gained into the tri- and tetramerizations. The tetramerization of ethylene with the para-methoxyaryl Cr-PNP catalyst lowers the barrier height by ~2.6 kcal/mol compared to that of ethylene with the ortho-methoxyaryl Cr-PNP catalyst. The selectivity toward trimerization or tetramerization comes from whether the energy barrier for ethylene insertion to metallacycloheptane is higher than β-hydride transfer to make 1-hexene. The metallacycle mechanism with Cr (I)-Cr (III) intermediates is found to be the most favored, with the oxidative coupling of the two coordinated ethylenes to form chromacyclopentane being the rate-determining step.

Selective ethylene tetramerization: an overview

1-octene is used to produce linear low-density polyethylene (LLDPE), polyolefin elastomers (POE)etc. This review provides an overview of the history, recent advancements and future opportunities in selective ethylene tetramerization.

Insights on the Mechanism for Ethylene Tetramerization

Insights on the Mechanism for Ethylene Tetramerization

Highly Efficient Ethylene Tetramerization Using Cr Catalysts Constructed with Trifluoromethyl-Substituted N-Aryl PNP Ligands.

Tetramerization of ethylene by chromium catalysts stabilized with functionalized N-aryl phosphineamine ligands C6H4(m-CF3)N(PPh2)2 (1), C6H4(p-CF3)N(PPh2)2 (2), C6H4(o-CF3)N=PPh2-PPh2 (3), and C6H3(3,5-bis(CF3))N(PPh2)2 (4) was evaluated. The parameter optimization includes temperature, co-catalyst, and solvent. Upon activation with MMAO-3A, the new catalyst system especially with m-functional PNP ligand (1) exhibited high 1-octene selectivity and productivity while giving minimum undesirable polyethylene and C10+ olefin by-products. Using PhCl as a solvent at 75 °C led to a remarkable α-olefin (1-C6 + 1-C8) selectivity (>90 wt %) at a reaction rate of 2000 kg·gCr–1·h–1. Under identical conditions, analogous PNP ligands bearing −CH3, −Et, and −Cl functional moieties at the meta position of the N-phenyl ring displayed significantly lower reactivity. The catalyst with p-functional ligand (2) exhibited lower activity and comparable selectivities, while the Cr/PPN (with ligand 3) system gave no noticeable reactivity. The molecular structure of the precatalyst (1-Cr), exhibiting a monomeric structural feature, was elucidated with the aid of single-crystal X-ray diffraction study.

Catalytic tri- and tetramerization of ethylene: a mechanistic overview

ABSTRACT On purpose, selective oligomerization of ethylene to its linear trimer and tetramer such as 1-hexene and 1-octene has assumed significant commercial interest. Catalytic systems based on chromium and titanium have shown remarkable selectivity and productivity for the tri and tetramerization of ethylene to 1-hexene or 1-octene. Chromium-based catalysts are the most selective and active and show the highest structural diversity. This article discusses the most recent mechanistic approaches regarding active catalytic species that determine the selectivity to either hexene-1 or octene-1 and reaction parameters that control selectivity of byproducts. Isotopic labeling protocols, in-situ spectroscopic investigations and DFT studies on selected chromium-based catalyst systems are reviewed.

A rational approach towards selective ethylene oligomerization via PNP-ligand design with an N-triptycene functionality.

Novel PNP ligands bearing an N-triptycene backbone were developed and evaluated for selective ethylene oligomerization. Upon activation with MMAO-3A, the pre-catalyst mixture containing Cr(acac)3/ligand efficiently promotes ethylene tetramerization with remarkably high productivities (up to 1733 kg gCr-1 h-1) and C8 olefin selectivities (up to 74.1 wt%). More importantly, ligands with a PNP moiety connecting at the 1- or 1,4-position of the triptycene molecule could achieve exceptionally high alpha (1-C6 + 1-C8) selectivities, exceeding 90 wt%, as a result of high 1-C6 purity (>90 wt%) in the C6 fraction. Based on comparative catalytic studies employing various PNP ligands with or without an N-triptycene backbone, we illustrate the fact that a rational design of PNP ligands with an optimum degree of steric profile around the N-center could provide C6 cyclics controlled highly α-selective ethylene oligomerization.

Preparation of Extremely Active Ethylene Tetramerization Catalyst [iPrN(PAr2)2−CrCl2]+[B(C6F5)4]– (Ar = −C6H4-p-SiR3)

Numerous efforts have been made to achieve “on-purpose” 1-octene production since Sasol discovered a Cr-based selective ethylene tetramerization catalyst in the early 2000s. By preparing a series of bis(phosphine) ligands iPrN(PAr2)2 where the Ar contains a bulky –SiR3 substituent (Ar = −C6H4-p-Si(nBu)3 (1), −C6H4-p-Si(1-hexyl)3 (2), −C6H4-p-Si(1-octyl)3 (3), −C6H4-p-Si(2-ethylhexyl)3 (4), −C6H4-p-Si(3,7-dimethyloctyl)3 (5)), we obtained an extremely active catalyst that meets the criteria for commercial utilization. The Cr complexes [iPrN(PAr2)2−CrCl2]+[B(C6F5)4]–, obtained by reacting ligands 1–5 with [(CH3CN)4CrCl2]+[B(C6F5)4]–, showed high activity exceeding 6000 kg/g-Cr/h, when combined with the inexpensive iBu3Al, thus avoiding the use of expensive modified methylaluminoxane (MMAO). The bulky –SiR3 substituents played a key role in the success of catalysis by blocking the formation of inactive species (Cr complexes coordinated by two iPrN(PAr2)2 ligands, that is, [(iPrN(PAr2)2)2−CrCl2]+[B(C6F5)4]–). Among the complexes prepared, [3−CrCl2]+[B(C6F5)4]– exhibited the highest activity (11,100 kg/g-Cr/h, 100 kg/g-catalyst) with high 1-octene selectivity (75 wt%) and, moreover, mitigated the generation of undesired > C10 fractions (10.7 wt%). A 10-g-scale synthesis of 3 was developed, as well as a facile and low-cost synthetic method for [(CH3CN)4CrCl2]+[B(C6F5)4]–.

Synthesis of a Ni Complex Chelated by a [2.2]Paracyclophane-Functionalized Diimine Ligand and Its Catalytic Activity for Olefin Oligomerization.

A diimine ligand having two [2.2]paracyclophanyl substituents at the N atoms (L1) was prepared from the reaction of amino[2.2]paracyclophane with acenaphtenequinone. The ligand reacts with NiBr2(dme) (dme: 1,2-dimethoxyethane) to form the dibromonickel complex with (R,R) and (S,S) configuration, NiBr2(L1). The structure of the complex was confirmed by X-ray crystallography. NiBr2(L1) catalyzes oligomerization of ethylene in the presence of methylaluminoxane (MAO) co-catalyst at 10–50 °C to form a mixture of 1- and 2-butenes after 3 h. The reactions for 6 h and 8 h at 25 °C causes further increase of 2-butene formed via isomerization of 1-butene and formation of hexenes. Reaction of 1-hexene catalyzed by NiBr2(L1)–MAO produces 2-hexene via isomerization and C12 and C18 hydrocarbons via oligomerization. Consumption of 1-hexene of the reaction obeys first-order kinetics. The kinetic parameters were obtained to be ΔG‡ = 93.6 kJ mol−1, ΔH‡ = 63.0 kJ mol−1, and ΔS‡ = −112 J mol−1deg−1. NiBr2(L1) catalyzes co-dimerization of ethylene and 1-hexene to form C8 hydrocarbons with higher rate and selectivity than the tetramerization of ethylene.

H2 promoting effect in Cr/PNP‐catalyzed ethylene tetramerization: A density functional theory study

AbstractIt is well known that hydrogen promotes catalyst activity in Cr/PNP‐catalyzed ethylene tetramerization, but the mechanism of this effect is unclear. A density functional theory (DFT) study was conducted to explore this effect, and conformation changes were carefully taken into consideration to build a clear reaction pathway. Three components in the ethylene tetramerization catalytic cycle were examined in detail: the production of 1‐hexene from metallacycloheptane, the production of 1‐octene from metallacyclononane, and the formation of an active centre on the Cr/PNP catalyst. This result indicates that the formation of an active centre on the catalyst becomes more favorable upon the imposition of hydrogen, where hydrogen functions as a second ligand. This easing effect could be the key factor leading to the outperformed Cr/PNP catalyst activity in ethylene tetramerization.

Computer-assisted catalyst development via automated modelling of conformationally complex molecules: application to diphosphinoamine ligands

Simulation of conformationally complicated molecules requires multiple levels of theory to obtain accurate thermodynamics, requiring significant researcher time to implement. We automate this workflow using all open-source code (XTBDFT) and apply it toward a practical challenge: diphosphinoamine (PNP) ligands used for ethylene tetramerization catalysis may isomerize (with deleterious effects) to iminobisphosphines (PPNs), and a computational method to evaluate PNP ligand candidates would save significant experimental effort. We use XTBDFT to calculate the thermodynamic stability of a wide range of conformationally complex PNP ligands against isomeriation to PPN (ΔGPPN), and establish a strong correlation between ΔGPPN and catalyst performance. Finally, we apply our method to screen novel PNP candidates, saving significant time by ruling out candidates with non-trivial synthetic routes and poor expected catalytic performance.

Replacement of the Common Chromium Source CrCl3(thf)3 with Well-Defined [CrCl2(μ-Cl)(thf)2]2.

CrCl3(thf)3 is a common starting material in the synthesis of organometallic and coordination compounds of Cr. Deposited as an irregular solid with no possibility of recrystallization, it is not a purity guaranteed chemical, causing problems in some cases. In this work, we disclose a well-defined form of the THF adduct of CrCl3 ([CrCl2(μ-Cl)(thf)2]2), a crystalline solid, that enables structure determination by X-ray crystallography. The EA data and XRD pattern of the bulk agreed with the revealed structure. Moreover, its preparation procedure is facile: evacuation of CrCl3·6H2O at 100 °C, treatment with 6 equivalents of Me3SiCl in a minimal amount of THF, and crystallization from CH2Cl2. The ethylene tetramerization catalyst [iPrN{P(C6H4-p-Si(nBu)3)2}2CrCl2]+[B(C6F5)4]− prepared using well-defined [CrCl2(μ-Cl)(thf)2]2 as a starting material exhibited a reliably high activity (6600 kg/g-Cr/h; 1-octene selectivity at 40 °C, 75%), while that of the one prepared using the impure CrCl3(thf)3 was inconsistent and relatively low (~3000 kg/g-Cr/h). By using well-defined [CrCl2(μ-Cl)(thf)2]2 as a Cr source, single crystals of [(CH3CN)4CrCl2]+[B(C6F5)4]− and [{Et(Cl)Al(N(iPr)2)2}Cr(μ-Cl)]2 were obtained, allowing structure determination by X-ray crystallography, which had been unsuccessful when the previously known CrCl3(thf)3 was used as the Cr source.

Characterization of Cr-Hydrocarbyl Species via Pulse EPR in the Study of Ethylene Tetramerization Catalysis

The characterization of complexes involved in chromium catalysis is challenging due to the paramagnetism of Cr in its common oxidation states. Here, we demonstrate the utility of pulse electron paramagnetic resonance (pulse EPR) techniques in assigning structural features of Cr organometallic complexes relevant to ethylene tetramerization. An S = 3/2, Cr(III) bisaryl-methyl ethylene tetramerization precatalyst (1) has been selected for characterization by CW and pulse EPR spectroscopies. Using an isotopically labeled Cr-CD3 complex (1-d3), the methyl ligand was confirmed to remain bound to Cr in solution by detection of 2H couplings in X-band hyperfine sublevel correlation (HYSCORE) spectroscopy. Protonolysis of 1-d3 led to an S = 3/2, Cr(III) product (2-d3) that maintained spectroscopic features in HYSCORE for the CD3 group, indicative of retention of the Cr-alkyl bond. Following protonolysis of 1-h3 and subsequent reaction with ethylene, an S = 1/2 Cr(I) species with an ethylene-derived ligand was generated, supporting a mechanism involving this Cr oxidation state. Additionally, the pulse EPR characterization of a Cr(I) allyl-diene complex was performed for comparison. This is the first direct observation of hydrocarbyl ligands on Cr using pulse EPR methods. The methods described here are broadly applicable to Cr, first-row transition metals and other open-shell organometallic catalytic systems.

Ethylene Tetramerisation: A Structure-Selectivity Correlation.

The effect of ethylene tetramerisation ligand structures on 1-octene selectivity is well studied. However, by-product formation is less understood. In this work, a range of PNP ligand structures are correlated with the full product selectivity and with catalyst activity. As steric bulk on the N-substituent increases, the product selectivity shifts from >10 % to < 3% of both C6 cyclics and C16+ by-products. 1-Octene peaks at ca. 70%. Thereafter, only 1-hexene increases. Similar selectivity changes were observed for ortho-Ph-substituted PNP ligands. The C10-14 selectivity was less affected by the ligand structure. The ligand effect on the changes in selectivity is explained mechanistically. Lastly, an increase in ligand steric bulk was found to improve catalyst activity and reduce polymer formation by an order of magnitude. It is proposed that steric bulk promotes formation of cationic catalytic species which are responsible for selective ethylene oligomerisation.

Phospholane-Based Ligands for Chromium-Catalyzed Ethylene Tri- and Tetramerization

Chromium complexes with bis(phospholane) ligands were synthesized and evaluated for ethylene tetramerization in a high-throughput reactor. Three ligand parameters—the phospholane substituent, the l...

Evaluation of Bis(phosphine) Ligands for Ethylene Oligomerization: Discovery of Alkyl Phosphines as Effective Ligands for Ethylene Tri- and Tetramerization

Fifty-three bis(phosphines) were evaluated as ligands for chromium-catalyzed ethylene tetramerization in a high-throughput reactor. Selected ligands previously reported in the literature gave high ...