Selective Ethylene Oligomerization Research Articles

Ethylene Dimerization, Isomerization and Trimerization: Mechanistic Insights into Competing Pathways on Metal–Organic Framework Supported Metal Hydrides

AbstractCatalytic ethylene oligomerization is important in the petrochemical industry, as it produces valuable products like linear alpha‐olefins (LAOs) for the synthesis of polymers and specialty chemicals. Catalysts, typically transition metal complexes or metal‐containing materials, play a crucial role in selective oligomerization by providing active sites for ethylene activation and controlling product distribution. Recently, defective metal‐organic frameworks (MOFs) with well‐defined structures and unsaturated active sites have emerged as potential catalysts for ethylene oligomerization. In this study, we employ density functional theory calculations to investigate the energetic selectivity of competing pathways (including dimerization, isomerization and trimerization) during ethylene oligomerization on defective HKUST‐1 supported metal hydrides (H−M–DHKUST‐1, M: Co, Ni, Ru, Rh and Pd). On all the five hydrides, ethylene dimerization is found to be more preferential than trimerization and isomerization. In the microenvironment of defective paddlewheel of HKUST‐1, isomerization is highly unfavorable compared with dimerization because of high energy required for chain walk, and 1‐butene is expected to be the major product on all the five hydrides. Analysis of NBO charges reveals a notable reduction in cationic nature at the metal site on H−Ru–DHKUST‐1, which facilitates β‐hydride elimination for dimerization. The microscopic insights from this computational study might facilitate the rational design of new MOFs and other MOF‐supported catalysts for selective ethylene oligomerization.

Chromium Catalysts for Selective Ethylene Oligomerization Featuring Binuclear PNP Ligands.

A series of novel binuclear PNP ligands based on the cyclohexyldiamine scaffold were synthesized for this study. The experimental results showed that positioning the two PNP sites at the para-positions of the cyclohexyl framework led to a significant enhancement in the catalytic activity for selective tri/tetramerization of ethylene. The PNP/Cr(acac)3/MAO(methylaluminoxane) catalytic system exhibited relatively high catalytic activity (up to 3887.7 kg·g-1·h-1) in selective ethylene oligomerization with a total selectivity of 84.5% for 1-hexene and 1-octene at 40 °C and 50 bar. The relationship between the ligand structure and ethylene oligomerization performance was further explored using density functional theory calculations.

Ethylene trimerization using half‐sandwich titanium‐based catalysts supported on mesoporous silica modified with ionic liquids

Linear alpha olefins (LAOs) are produced industrially via ethylene oligomerization using catalytic methods. The cost‐effective separation process has sparked significant interest in the selective oligomerization of ethylene to produce alpha‐olefins, including 1‐hexene (1‐C6), in multi‐product commercial processes. In addition, the utilization of immobilized catalysts is crucial because of their reduced environmental impact, ease of catalyst separation, recyclability, and transportability. Furthermore, the use of immobilized catalysts simplifies the purification process, making it easier to isolate pure products. In the present study, mesoporous silica (MS) was first modified with ionic liquids (ILs) consisting BF4− and Br− counter‐anions to prepare IL‐BF4@MS and IL‐Br@MS, respectively. Then 12 catalysts were synthesized through immobilization of the half‐sandwich catalysts with different bridges on the surface of MS, IL‐Br@MS, and IL‐BF4@MS and characterized by BET, TGA, and SEM–EDX‐Mapping analyses. UV–Visible spectroscopy showed a tetrahedral structure for the synthesized complexes. The activity and selectivity of the catalysts for the production of 1‐hexene were studied under specific conditions, including an ethylene pressure of 8 bar, a temperature of 40 °C, and an Al/Ti ratio of 1:2000. The C1‐IL‐BF4@MS immobilized catalyst with cyclohexane middle bridge immobilized on MS modified with IL‐BF4 revealed the highest activity (1199 kg 1‐C6 molTi−1·h−1) at a catalyst concentration of 1.5 μmol. The lowest activity (138 kg 1‐C molTi−1·h−1) was obtained for both C3@MS and C4@MS catalysts.

Selective linear ethylene oligomerization over nickel-containing zeotypes with tetravalent framework heteroatoms

Nickel-containing Lewis acidic zeolites as highly selective ethylene oligomerization catalysts.

Cr/PCCP-catalysed selective ethylene oligomerization: analysis of various conformations and the hemilabile methoxy group

This work reports a DFT study on the effects of the hemilabile methoxy group in Cr/PCCP and Cr/PCCPOMe catalyzed ethylene tri- and tetramerization. The ensemble of conformers and the key geometrical features of the lowest conformer were discussed.

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.

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.

Tandem Synthesis of Ultra-High Molecular Weight Drag Reducing Poly-α-Olefins for Low-Temperature Pipeline Transportation.

Ultra-high molecular weight poly-α-olefins are widely used as drag reducing agents (DRAs) for pipeline transportation of oil and refined petroleum products. The synthesis of polyolefin DRAs is based on low-temperature Ziegler–Natta (ZN) polymerization of higher α-olefins. 1-Hexene based DRAs, the most effective at room temperature, typically lose DR activity at low temperatures. The use of 1-hexene copolymers with C8–C12 linear α-olefins appears to offer a solution to the problem of low-temperature drag reducing. The present work aims to develop two-stage synthesis of polyolefin DRAs that is based on selective oligomerization of ethylene in the presence of efficient chromium/aminodiphosphine catalysts (Cr-PNP), followed by polymerization of the olefin mixtures, formed at oligomerization stage, using efficient titanium–magnesium ZN catalyst. We have shown that oligomerization of ethylene in α-olefin reaction media proceeds faster than in saturated hydrocarbons, providing the formation of 1-hexene, 1-octene, and branched C10 and C12 olefins; the composition and the ratio of the reaction products depended on the nature of PNP ligand. Oligomerizates were used in ZN polymerization ‘as is’, without additional treatment. Due to branched character of C10+ hydrocarbons, formed during oligomerization of ethylene, resulting polyolefins demonstrate higher low-temperature DR efficiency at low polymer concentrations (~1 ppm) in comparison with benchmark polymers prepared from the mixtures of linear α-olefins and from pure 1-hexene. We assume that faster solubility and more efficient solvation of the polyolefins, prepared using ‘tandem’ ethylene-based process, represent an advantage of these type polymers over conventional poly(1-hexene) and linear α-olefin-based polymers when used as ‘winter’ DRAs.

Selective ethylene oligomerization based on Fe-containing metal-organic coordination polymer catalysts

One-dimensional, two-dimensional and three-dimensional Fe-containing metal-organic coordination polymer (1D/2D/3D-Fe-MOCP) catalysts have been solvothermally synthesized and evacuated at different temperatures to give a series of 1D-Fe, 2D-Fe and 3D-Fe catalysts used for ethylene oligomerization performed in slurry batch mode. Their catalytic performances were investigated with the aid of AlEt2Cl cocatalyst, and moderate catalytic activities and high selectivities to low carbon olefins C4, C6, C8, and C10 were observed. Moreover, the oligomerization product distributions differed depending on various factors such as the structural dimension, ligand type, framework flexibility, distances between metal atoms, et al. 1D-Fe catalyst evacuated at 150 °C for 8 hours showed high C8 olefin selectivity up to 94.90% with the catalytic activity of 6.28 × 105 g/mol(Fe)•h•atm. However, 2D-Fe and 3D-Fe catalysts showed relatively low catalytic activity and low selectivity for C8 olefin.

Chromium catalysts stabilized by alkylphosphanyl PNP ligands for selective ethylene tri-/tetramerization

The alkyl substituent(s) in the scaffold of the alkylphosphanyl PNP ligands of the form Ph2PN(cyclopentyl)PPhR, Ph2PN(cyclopentyl)PR2, and RPhPN(cyclopentyl)PPhR have been observed to have a marked impact on the catalytic performance of Cr(III) catalysts in ethylene oligomerization reactions. Precatalyst 6, bearing diisopropylphosphanyl substituent, afforded the highest catalytic activity of 4490 kg(product)·g(Cr)−1·h−1 and 51.0% C8 selectivity with good product purity under suitable conditions. Precatalyst 7, having diethylphosphanyl substituent in the PNP scaffold, exhibited 72.0% high C8 selectivity with 1717 kg(product)·g(Cr)−1·h−1 improved catalytic activity. Precatalysts based on monoalkylphosphanyl PNP (2–4) and symmetrical alkylphosphanyl PNP (9–10) exhibited poor catalytic performance toward selective ethylene oligomerization. Single crystal analysis suggested that the alkyl substituents in the scaffold of the alkylphosphanyl PNP ligands may effectively induce the complex structure and consequently the catalytic performance. Density functional theory (DFT) calculations revealed that in comparison to the trimerization pathway, the rate-determining states in the tetramerization pathways of precatalysts 6–7 may face low energy barriers and therefore may offer high C8 selectivity.

Chromium catalysts based on PNP(NR2)2 ligands for selective ethylene oligomerization

High active and selective 1–3 chromium catalysts based on the corresponding Ph2PN(cyclopentyl)P (NR2)2‐type L1–L3 ligands (L1: R = methyl, L2: R = ethyl, L3: R = isopropyl) have been explored for selective ethylene tri‐/tetramerization. We found that the ligand substituents and the experimental conditions considerably influenced the catalytic performance of 1–3 based catalysts. Combining a higher ethylene pressure can elevate the selectivity of 1‐octene (67.98%) of precatalyst 1 with high catalytic activity (9.3 × 106 g/[molCr h]). Furthermore, precatalysts 2–3 efficiently offer selective ethylene trimerization and afforded 97.76% 1‐hexene selectivity with 5.8 × 106 g/(molCr h) catalytic activity (in case precatalyst 3). A relatively low catalyst mass, high Al/Cr molar ratio, and high ethylene pressure display excellent catalytic activity and selectivity for ethylene oligomerization.

Ga-Ni supported catalytically active liquid metal solutions (SCALMS) for selective ethylene oligomerization.

Non-precious metal supported catalytically active liquid metal solutions exhibit attractive performance in ethylene oligomerization. It is found for the Ga–Ni system on silica that the performance depends strongly on the applied Ga/Ni ratio. Ga-rich systems forming liquid alloys exhibit a far higher Ni-based catalytic activity than solid intermetallic compounds or Ni nanoparticles.

Ni-Based Complexes in Selective Ethylene Oligomerization Processes

Linear alpha-olefins are widely used in the petrochemical industry and the world demand for these compounds increases annually. At present, the main method for producing linear alpha-olefins is the homogeneous catalytic ethylene oligomerization. This review presents modern nickel catalysts for this process, mainly systems for obtaining of one of the most demanded oligomer—1-butene—which is used for the production of linear low density polyethylene (LLDPE) and high density polyethylene (HDPE). The dependence of the catalytic performance on the composition and the structure of the used activated complexes, the electronic and coordination states of the nickel center was considered.

Molecular structure, magnetic properties and catalytic activity in selective ethylene dimerization of nickel (II) complexes with bis(3,5-dimethylpyrazol-1-yl)methane

Nickel(II) complexes with neutral bidentate nitrogen-containing ligands are of great interest as promising precatalysts for the selective oligomerization of ethylene. Complexes of compositions 0.75[LNiCl2]2•0.25[LNiClBr]22 and LNiBr23 where L = bis-(3,5-dimethylpyrazol-1-yl)methane have been synthesized and characterized by single crystal X-ray diffraction, IR and 1H NMR spectroscopies and elemental analysis. The 1H NMR spectra of paramagnetic complexes indicate that the structures of these complexes in solution are similar to ones in the solid state. Magnetic susceptibility of the compounds has been investigated and discussed in the context of their structures. Treatment of these complexes with diethylaluminum chloride (DEAC) or ethylaluminum sesquichloride (EASC) generates active catalysts that oligomerize ethylene to yield mostly butenes with activities up to 930 kg(oligomers)·mol[Ni]−1·h−1 (DEAC).

Zirconium Catalyzed Ethylene Oligomerization

A review of the scientific and patent literature on oligomerization of ethylene to linear olefins in the presence of homogenous or heterogeneous zirconium complexes and various aluminum organic compounds has been fulfilled for last twenty‐five years. Over these years, a wide range of studies had been conducted for selective oligomerization of ethylene into a narrow fraction of linear α‐olefins such as C4‐C8 and C6‐C10 fractions. During the discussion of these catalytic systems, exceptional attention is also paid to feature works such as the activity of the catalysts and the acquisition of clear liquid product without any trace of polymer, all of which play important roles in the selection of the best technology.

Research progress of iron-based catalysts for selective oligomerization of ethylene.

Linear α-olefins are widely used as raw materials in the chemical industry. Selective ethylene oligomerization is an important development direction of the linear α-olefin production process. Iron-based catalysts have become a research hotspot in selective ethylene oligomerization due to their advantages like high activity, high selectivity and convenience of adjusting their ligand structures. In this paper, the research progress of catalysts for selective oligomerization of ethylene was reviewed in terms of the cocatalysts, ligand structure, and immobilization of homogeneous catalysts.

PNPN‐H in Comparison to other PNP, PNPN and NPNPN Ligands for the Chromium Catalyzed Selective Ethylene Oligomerization

AbstractMany examples exist for the chromium catalyzed selective ethylene oligomerization in which the influence of ligands is essential for the formation of products. Regarding the tri‐ and tetramerization to 1‐hexene or 1‐octene mostly PNP ligands are responsible for the tetra‐ and some of such modified ligands for the trimerization. A very special case in these reactions are PNPN−H ligands, showing in most cases highly selective trimerization of ethylene to 1‐hexene. In this review all existing published information about these PNPN−H ligands is accumulated and compared to some other related PNP, PNPN and NPNPN ligands in the chromium catalyzed selective ethylene oligomerization with respect to the switch from tetra‐ to trimerization and back by different substituent pattern of PNP ligand. Mechanistic information and arguments are collected to explain the switch from tetra‐ to trimerization and back by substitution of functional groups in classical PNP to PNPN−H ligands as a result of mono‐ and dinuclear catalytic species.

Selective ethylene oligomerization bearing hyperbranched bispyridylamine chromium catalyst

Two new hyperbranched bispyridylamine ligands and multinuclear chromium complexes were synthesized with 1.0 G hyperbranched macromolecules, 2-chloropyridine, 2-chloro-4-methylpyridine and CrCl3(THF)3 as raw materials. The structures of hyperbranched ligands and chromium complexes were characterized by UV, FT-IR, 1H NMR, ESI-MS, and elemental analysis. These hyperbranched chromium complexes were evaluated as catalyst precursors by using MAO as activator in the oligomerization of ethylene. Effects of reaction temperature, reaction pressure, Al/Cr molar ratio, concentration of catalyst, solvent, and the structure of catalysts on the catalytic activity and product selectivity were investigated. The oligomerization results showed that with increase of reaction temperature, reaction pressure, and Al/Cr molar ratio, the catalytic activity increased and then decreased; the catalytic activity continuously decreased as the amount of catalyst increased. The products were mainly based on C6 and C8. Under optimized conditions, the catalytic system of hyperbranched NNN/Cr(III)/MAO led to activity of 1.26 × 105 g/(mol·Cr·h) and 63.34% selectivity for C6 and C8.

Selective Ethylene Oligomerization

Ethylene oligomerization to produce 1-alkenes is a cornerstone of organometallic research. The original α-olefin commercial catalyst system, triethylaluminum, has grown to become one of the highest volume organometallics. This tutorial covers the developmental arc of ethylene oligomerization research and explains the ongoing technological shift from full-range (C4–C30) to selective (C6, C8) catalytic systems. Catalyst design determines product selectivity, and the differing mechanisms underpinning catalyst performance, linear versus metallacyclic, are covered in detail. Despite the field’s maturity, there are still significant opportunities for exploration and discovery which are discussed at the conclusion.

Ligand Modification for Selectivity Control in Selective Ethylene Oligomerization

A series of aminophosphine ligands were synthesized by in situ combination of CrCl3(THF)3 and dry methylaluminoxane and examined for their catalytic behavior in chromium-catalyzed selective ethylene oligomerization. Variations of the ligand scaffold obtained by modifying the diarylphosphine moiety affected the steric bulkiness, which resulted in remarkable changes in the selectivity for 1-hexene vs. 1-octene. Based on our experimental results, we evaluated two different mechanisms proposed for selective ethylene oligomerization.