Ethylene Oligomerization beyond Schulz–Flory Distributions

Ethylene Dimerization and Oligomerization Using Bis(phosphino)boryl Supported Ni Complexes

We report the dimerization and oligomerization of ethylene using bis(phosphino)boryl supported Ni(II) complexes as catalyst precursors. By using alkylaluminum(III) compounds or other Lewis acid additives, Ni(II) complexes of the type (^RPBP)NiBr (R = tBu or Ph) show activity for the production of butenes and higher olefins. Optimized turnover frequencies of 640 mol_ethylene·mol_Ni^-1·s^-1 for the formation of butenes with 41(1)% selectivity for 1-butene using (^PhPBP)NiBr, and 68 mol_ethylene·mol_Ni^-1·s^-1 for butenes production with 87.2(3)% selectivity for 1-butene using (^tBuPBP)NiBr, have been demonstrated. With methylaluminoxane as a co-catalyst and (^tBuPBP)NiBr as the precatalyst, ethylene oligomerization to form C₄ through C₂₀ products was achieved, while the use of (^PhPBP)NiBr as the pre-catalyst retained selectivity for C₄ products. Our studies suggest that the ethylene dimerization is not initiated by Ni hydride or alkyl intermediates. Rather, our studies point to a mechanism that involves a cooperative B/Ni activation of ethylene to form a key 6-membered borametallacycle intermediate. Thus, a cooperative activation of ethylene by the Ni-B unit of the (^RPBP)Ni catalysts is proposed as a key element of the Ni catalysis.

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 g_Cr^-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.

Olefin-Surface Interactions: A Key Activity Parameter in Silica-Supported Olefin Metathesis Catalysts

Molecularly defined and classical heterogeneous Mo-based metathesis catalysts are shown to display distinct and unexpected reactivity patterns for the metathesis of long-chain α-olefins at low temperatures (<100 °C). Catalysts based on supported Mo oxo species, whether prepared via wet impregnation or surface organometallic chemistry (SOMC), exhibit strong activity dependencies on the α-olefin chain length, with slower reaction rates for longer substrate chain lengths. In contrast, molecular and supported Mo alkylidenes are highly active and do not display such dramatic dependence on the chain length. State-of-the-art two-dimensional (2D) solid-state nuclear magnetic resonance (NMR) spectroscopy analyses of postmetathesis catalysts, complemented by Fourier transform infrared (FT-IR) spectroscopy and molecular dynamics calculations, evidence that the activity decrease observed for supported Mo oxo catalysts relates to the strong adsorption of internal olefin metathesis products because of interactions with surface Si-OH groups. Overall, this study shows that in addition to the nature and the number of active sites, the metathesis rates and the overall catalytic performance depend on product desorption, even in the liquid phase with nonpolar substrates. This study further highlights the role of the support and active site composition and dynamics on activity as well as the need for considering adsorption in catalyst design.

Influence of N- and P-substituents in N-aryl-phosphinoglycine ligands on the selectivity of Ni-catalysed ethylene oligomerization

Quantum-chemical calculations were performed to rationalize the experimental molecular weight distribution of α-olefin products, revealing the main mechanistic models of the process.

Single- and double-bridged PNP ligands in chromium-catalysed ethylene oligomerisation

Chromium catalysts with diazaphospholane ligands have shown good activities and selectivities for ethylene tri- and tetramerisation. Oligomerisations with a doubly N-bridged cyclodiphosphazane result in a Schulz–Flory distribution of α-olefins.

Olefin oligomerization by main group Ga3+ and Zn2+ single site catalysts on SiO2

In heterogeneous catalysis, olefin oligomerization is typically performed on immobilized transition metal ions, such as Ni²⁺ and Cr³⁺. Here we report that silica-supported, single site catalysts containing immobilized, main group Zn²⁺ and Ga³⁺ ion sites catalyze ethylene and propylene oligomerization to an equilibrium distribution of linear olefins with rates similar to that of Ni²⁺. The molecular weight distribution of products formed on Zn²⁺ is similar to Ni²⁺, while Ga³⁺ forms higher molecular weight olefins. In situ spectroscopic and computational studies suggest that oligomerization unexpectedly occurs by the Cossee-Arlman mechanism via metal hydride and metal alkyl intermediates formed during olefin insertion and β-hydride elimination elementary steps. Initiation of the catalytic cycle is proposed to occur by heterolytic C-H dissociation of ethylene, which occurs at about 250 °C where oligomerization is catalytically relevant. This work illuminates new chemistry for main group metal catalysts with potential for development of new oligomerization processes.

Controlled di-lithiation enabled synthesis of phosphine-sulfonamide ligands and implications in ethylene oligomerization

Catalyst design for ethylene oligomerization has attracted significant interest. Herein, we report the synthesis of phosphine-sulfonamide-derived palladium complexes and examine their performance in ethylene oligomerization. Arresting a dilithiation intermediate of N-(2-bromophenyl)-4-methylbenzenesulfonamide (1) at -84 °C selectively produced N-(2-(bis(2-methoxyphenyl)phosphanyl)phenyl)-4-methylbenzenesulfonamide (L1A). However, the same reaction at -41 °C delivered a different ligand; 2-(bis(2-methoxyphenyl)phosphanyl)-4-methyl-N-phenylbenzenesulfonamide (L2A). The generality of our strategy has been demonstrated by preparing N-(2-(diphenylphosphanyl)phenyl)-4-methylbenzenesulfonamide (L1B) and 2-(diphenylphosphanyl)-4-methyl-N-phenylbenzenesulfonamide (L2B). Subsequently, L1A and L1B were treated with a palladium precursor to yield 5-membered complexes C1 and C2, respectively. In contrast, L2A upon treatment with palladium produced a 6-membered metal complex C3. Thus, a small library of 7 palladium complexes (C1-C7) were synthesized by varying the donor moiety (pyridine, DMSO, and acetonitrile). The identity of palladium complexes was unambiguously ascertained using a combination of spectroscopic and analytical methods, including single-crystal X-ray diffraction. The performance of the complexes C1-C7 was investigated in ethylene oligomerization and almost all of them were found to be active. The resultant ethylene oligomers were characterized using 1H and 13C NMR, MALDI-ToF-MS, and GPC. Detailed screening of reaction parameters revealed 100 °C and 40 bars ethylene to be optimal conditions. Complex C5 outperformed other complexes and produced ethylene oligomers with a molecular weight of 1000-1900 g mol-1.

Chromium complexes supported by bidentate thioether-imine [N,S] ligands: synthesis and ethylene oligomerization studies

Chromium complexes bearing thioether-imine ligands were synthesized and their catalytic behavior in ethylene oligomerization has been investigated, evaluating the effect of the ligand and the experimental parameters on the activity, selectivity, and product distribution.

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.

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.

Selective Trimerization of α-Olefins with Immobilized Chromium Catalyst for Lubricant Base Oils

The demand for poly(α-olefin)s (PAOs), which are high-performance group IV lubricant base oils, is increasingly high. PAOs are generally produced via the cationic oligomerization of 1-decene, wherein skeleton rearrangement inevitably occurs in the products. Hence, a transition-metal-based catalytic process that avoids rearrangement would be a valuable alternative for cationic oligomerization. In particular, transition-metal-catalyzed selective trimerization of α-olefins has the potential for success. In this study, (N,N′,N″-tridodecyltriazacyclohexane)CrCl3 complex was reacted with MAO-silica (MAO, methylaluminoxane) for the preparation of a supported catalyst, which exhibited superior performance in selective α-olefin trimerization compared to that of the corresponding homogeneous catalyst, enabling the preparation of α-olefin trimers at ~200 g scale. Following hydrogenation, the prepared 1-decene trimer (C30H62) exhibited better lubricant properties than those of commercial-grade PAO-4 (kinematic viscosity at 40 °C, 15.1 vs. 17.4 cSt; kinematic viscosity at 100 °C, 3.9 vs. 3.9 cSt; viscosity index, 161 vs. 123). Moreover, it was shown that 1-octene/1-dodecene mixed co-trimers (i.e., a mixture of C24H50, C28H58, C32H66, and C36H74), generated by the selective supported Cr catalyst, exhibited outstanding lubricant properties analogous to those observed for the 1-decene trimer (C30H62).

Nickel Catalyzed Olefin Oligomerization and Dimerization

Brought to life more than half a century ago and successfully applied for high-value petrochemical intermediates production, nickel-catalyzed olefin oligomerization is still a very dynamic topic, with many fundamental questions to address and industrial challenges to overcome. The unique and versatile reactivity of nickel enables the oligomerization of ethylene, propylene, and butenes into a wide range of oligomers that are highly sought-after in numerous fields to be controlled. Interestingly, both homogeneous and heterogeneous nickel catalysts have been scrutinized and employed to do this. This rare specificity encouraged us to interlink them in this review so as to open up opportunities for further catalyst development and innovation. An in-depth understanding of the reaction mechanisms in play is essential to being able to fine-tune the selectivity and achieve efficiency in the rational design of novel catalytic systems. This review thus provides a complete overview of the subject, compiling the main fundamental/industrial milestones and remaining challenges facing homogeneous/heterogeneous approaches as well as emerging catalytic concepts, with a focus on the last 10 years.

The Mathematics of Ethylene Oligomerisation and Polymerisation

Linear α-olefins or LAOs are produced by the catalytic oligomerisation of ethylene on a multimillion ton scale annually. A range of LAOs is typically obtained with varying chain lengths which follow a distribution. Depending on the catalyst, various types of distributions have been identified, such as Schulz–Flory, Poisson, alternating and selective oligomerisations such as ethylene trimerisation to 1-hexene and tetramerisation to 1-octene. A comprehensive mathematical analysis for all oligomer distributions is presented, showing the relations between the various distributions and with ethylene polymerisation, as well as providing mechanistic insight into the underlying chemical processes.

Ethylene Oligomerization over Nickel Supported Silica-Alumina Catalysts with High Selectivity for C10+ Products

The nickel (II) loading silica-alumina under various treatments in terms of aging temperature, Si/Al ratio and activation temperature were investigated by XRD, N2 adsorption-desorption, TEM, UV-Vis, NH3-TPD and XRF and then applied to catalyze the ethylene oligomerization. High aging temperature, low Si/Al ratio and high activation temperature were beneficial to high selectivity for C10+ products because of a reasonable match between Ni active sites and acid sites, high Ni loading content and less octahedral coordination Ni2+ species, respectively. Ni loading content was more important than the number of acid sites for high yield of C10+ products, and less octahedral coordination Ni2+ species favored less by-products produced at high reaction temperature. In addition, other experimental conditions, such as reaction temperature, weight hourly space velocity (WHSV) and nickel precursor were discussed in the paper.

Mechanistic insights into alkene chain growth reactions catalyzed by nickel active sites on ordered microporous and mesoporous supports

This mini-review discusses the mechanistic details underlying the activation and deactivation behavior, and the kinetics and selectivity among alkene isomer products, observed on Ni-based ordered porous materials during light alkene oligomerization.

Electrochemical Synthesis of Zirconium Pre-Catalysts for Homogeneous Ethylene Oligomerization

The catalytic activity of electrochemically synthesized zirconium carboxylates was studied in the process of ethylene oligomerization. Zirconium carboxylates were electrochemically synthesized directly from metallic zirconium and corresponding carboxylic acids (acetic, octanoic and lauric). A comprehensive study (element analysis, nuclear magnetic resonance (NMR) and infrared (IR) spectroscopy, powder X-ray diffraction (PXRD)) of the synthesized zirconium carboxylates showed that these species contain bidentate carboxylate moieties. It was shown that obtained zirconium carboxylates, in combination with Et3Al2Cl3 (Al/Zr = 20), have a moderate activity of (7.6–9.9) × 103 molC2H4⋅molZr−1⋅h−1 in terms of ethylene oligomerization (at T = 80 °C, p = 20 bar), leading to even-numbered C4–C10 linear alpha-olefins.

Spectroscopic Investigation of the Activation of a Chromium-Pyrrolyl Ethene Trimerization Catalyst

1-Hexene is an important α-olefin for polyethylene production and is produced from ethene. Recent developments in the α-olefin industry have led to the successful commercialization of ethene trimerization catalysts. An important industrially applied ethene trimerization system uses a mixture of chromium 2-ethylhexanoate, AlEt₃, AlEt₂Cl, and 2,5-dimethylpyrrole (DMP). Here, we have studied the activation of this system using catalytic and spectroscopic experiments (XAS, EPR, and UV–vis) under conditions employed in industry. First, chromium 2-ethylhexanoate was prepared and characterized to be [Cr₃O(RCO₂)₆(H₂O)₃]Cl. Next, the activation of chromium 2-ethylhexanoate with AlEt₃, AlEt₂Cl, and DMP was studied, showing immediate reduction (<5 ms) of the trinuclear Cr(III) carboxylate and formation of a neutral polynuclear Cr(II) carboxylate complex. Over time, this Cr(II) carboxylate complex is partially converted into a chloro-bridged dinuclear Cr(II) pyrrolyl complex. In cyclohexane, small quantities of an unknown Cr(I) complex (∼1% after 1 h) are observed, while in toluene, the quantity of Cr(I) is much higher (∼23% after 1 h). This is due to the formation of cationic bis(tolyl)Cr(I) complexes, which likely leads to the observed inferior performance using toluene as the reaction solvent. Catalytic studies allow us to exclude some of the observed Cr(I) and Cr(II) complexes as the active species in this catalytic system. Using this combination of techniques, we have been able to structurally characterize complexes of this selective Cr-catalyzed trimerization system under conditions which are employed in industry.

Chromium complexes supported by the bidentate PN ligands: synthesis, characterization and application for ethylene polymerization

Chromium-based complexes are among the most important catalysts in the field of ethylene polymerization and oligomerization. Heterogeneous Cr Phillips catalysts account for more than one-third of the commercialized high density polyethylene (HDPE). In this contribution, chromium complexes, LCrCl3 (Cr1-Cr4: L = 2,6-R1-4-R2-C6H2-N[double bond, length as m-dash]CH-C6H4-2-PPh2; Cr1: R1 = H, R2 = H; Cr2: R1 = Me, R2 = H; Cr3: R1 = iPr, R2 = H; Cr4: R1 = Ph2CH, R2 = iPr), have been synthesized and characterized by elemental analysis, ESI and IR spectroscopy. The molecular structures of Cr3 and Cr4 are defined by X-ray diffraction, revealing a distorted octahedral geometry around the chromium center in both structures. In the presence of an aluminum cocatalyst, complexes Cr1-Cr4 show moderate to high activities toward ethylene polymerization. The nature of the catalysts and various reaction conditions, such as the nature and the amount of cocatalyst, reaction time and temperature, are investigated in detail. The results show that the title complexes have good thermal stability and the substituents on the ligands significantly affect the catalytic properties. Particularly, complex Cr4 can produce HDPE with a high molecular weight up to 68.3 × 104 g mol-1 due to the suppression of the chain transfer/termination by the introduction of bulky Ph2CH groups.

From alternating to selective distributions in chromium-catalysed ethylene oligomerisation with asymmetric BIMA ligands

The oligomerisation of ethylene with chromium-based catalysts containing asymmetric BIMA (bis(benzimidazole)methylamine) ligands produces linear alpha olefins (LAOs) that follow an alternating distribution.

Ethylene oligomerization in metal–organic frameworks bearing nickel(ii) 2,2′-bipyridine complexes

The metal–organic frameworks Zr₆O₄(OH)₄(bpydc)₆ (1; bpydc²⁻ = 2,2′-bipyridine-5,5′-dicarboxylate) and Zr₆O₄(OH)₄(bpydc)_0.84(bpdc)_5.16 (2; bpdc²⁻ = biphenyl-4,4′-dicarboxylate) were readily metalated with Ni(DME)Br₂ (DME = dimethoxyethane) to produce the corresponding metalated frameworks 1(NiBr₂)₆ and 2(NiBr₂)_0.84. Both nickel(ii)-containing frameworks catalyze the oligomerization of ethylene in the presence of Et₂AlCl. In these systems, the pore environment around the active nickel sites significantly influences their selectivity for formation of oligomers over polymer. Specifically, the single-crystal structure of 1(NiBr₂)_5.64 reveals that surrounding metal–linker complexes enforce a steric environment on each nickel site that causes polymer formation to become favorable. Minimizing this steric congestion by isolating the nickel(ii) bipyridine complexes in the mixed-linker framework 2(NiBr₂)_0.84 markedly improves both the catalytic activity and selectivity for oligomers. Furthermore, both frameworks give product mixtures that are enriched in shorter olefins (C_4–10), leading to deviations from the expected Schulz–Flory distribution of oligomers. Although these deviations indicate possible pore confinement effects on selectivity, control experiments using the nickel-treated biphenyl framework Zr₆O₄(OH)₄(bpdc)₆(NiBr₂)_0.14 (3(NiBr₂)_0.14) reveal that they likely arise at least in part from the presence of nickel species that are not ligated by bipyridine within 1(NiBr₂)_5.64 and 2(NiBr₂)_0.84.

Functional Short-Bite Ligands: Synthesis, Coordination Chemistry, and Applications of N-Functionalized Bis(diaryl/dialkylphosphino)amine-type Ligands

The aim of this review is to highlight how the diversity generated by N-substitution in the well-known short-bite ligand bis(diphenylphosphino)amine (DPPA) allows a fine-tuning of the ligand properties and offers a considerable scope for tailoring the properties and applications of their corresponding metal complexes. The various N-substituents include nitrogen-, oxygen-, phosphorus-, sulfur-, halogen-, and silicon-based functionalities and directly N-bound metals. Multiple DPPA-type ligands linked through an organic spacer and N-functionalized DRPA-type ligands, in which the PPh2 substituents are replaced by PR2 (R = alkyl, benzyl) groups, are also discussed. Owing to the considerable diversity of N-functionalized DPPA-type ligands available, the applications of their mono- and polynuclear metal complexes are very diverse and range from homogeneous catalysis with well-defined or in situ generated (pre)catalysts to heterogeneous catalysis and materials science. In particular, sustained interest for DPPA-type ligands has been motivated, at least in part, by their ability to promote selective ethylene tri- or tetramerization in combination with chromium. Ligands and metal complexes where the N-substituent is a pure hydrocarbon group (as opposed to N-functionalization) are outside the scope of this review. However, when possible, a comparison between the catalytic performances of N-functionalized systems with those of their N-substituted analogs will be provided.