Ethylene Oligomerization Catalysts 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.

Nickel-containing catalysts for ethylene conversion to motor fuel components and light alkenes

Polyfunctional Ni-containing catalysts supported on the B2O3-Al2O3 and MoO3-Al2O3 were prepared by a sequential impregnation. They were evaluated in ethylene conversion to C5+ alkenes or propylene. The catalysts were characterized by X-ray diffraction, Fourier transformed infrared spectroscopy, Fourier transformed infrared spectroscopy of adsorbed CO, UV-VIS diffuse reflectance spectroscopy, temperature programmed reduction and temperature-programmed desorption of NH3. NiO/B2O3-Al2O3 samples containing Ni2+ cations chemically bonded to the acid support are the most effective catalysts for ethylene oligomerization. The NiO/MoO3-Al2O3 catalyst activity in the ethylene conversion to propylene is related with the presence of ethylene dimerization active sites, i.e. Ni2+ cations bonded to the support acidic sites, and active sites of metathesis in the form of monomolybdate species.

Sulfonic acid-modified MOFs as heterogeneous bifunctional catalysts for ethylene oligomerization at room temperature without cocatalysts

Ethylene oligomerization plays an important role in industrial production.

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

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

A comparison of the catalytic activities of oxovanadium(IV) and cobalt(II) complex compounds in the oligomerization of ethylene

The polymerization and oligomerization of olefins, especially ethylene, is a process of great industrial and scientific importance in polymer chemistry. Catalysts are an essential part of this reaction, and a new report of a catalyst improving this process is worthy of special attention. During this study, nine complex compounds based on oxovanadium(IV) and cobalt(II) ions in combination with organic ligands, e.g., dipicolinic acid, 2,2′-bipyridine, iminodiacetic acid, diglycolic acid, thiodiacetic acid, 1,10-phenanthroline were synthesized. The compound [VO(tda) · 0.5 H2O] is a new complex not previously reported in the literature, and its synthesis and structure are described in this article. The high catalytic activity values in other oligomerization processes in other olefins led us to use the synthesized compounds as precatalysts in the oligomerization reaction. Subsequently, their catalytic activity was also calculated in this process. The obtained oligomerization products were then analyzed by IR, MALDI-TOF-MS, TG, DSC, and SEM techniques to characterize the obtained products and verify the correctness of the oligomerization reaction. Based on the obtained data, the catalytic activity values were compared and the most active catalyst was selected. The best catalyst was a cobalt(II) compound containing an iminodiacetic acid anion and 2,2′-bipyridine in its structure. However, all complexes can be classified as highly active and active catalysts for ethylene oligomerization. The presented studies show novel catalysts that have in their structure other metal ions than those most commonly used in the literature so far. Additionally, coordination compounds of this type have not been previously used in ethylene oligomerization. Moreover, these studies are a source of knowledge on the correlation between catalyst structure and catalytic activity value, thanks to which they provide essential information necessary for the design of next-generation catalysts used for ethylene oligomerization and polymerization.

Localized structural and compositional effects on the kinetic performance of Ni-modified polyoxometalate catalysts for ethylene oligomerization

Ethylene oligomerization is an important step for upgrading light olefins to liquid fuels or value-added chemicals such as butene monomers. Ethylene oligomerization catalyzed by mono-Ni substituted polyoxometalate (POM) catalysts is promising due to the observed selectivity toward linear butene products and stability of these well-defined materials. In this work, two approaches to tailor the catalytic properties of isolated Ni sites in POMs were experimentally tested, both of which perturbed the surrounding molecular environment of the active sites. First, structural modifications were performed to alter the size of the polyoxometalates, evaluating between Wells-Dawson and Keggin structures while maintaining the same elemental components. When comparing these two structures, it was observed that the Ni active sites were kinetically independent from the effect of POM size, even when the POM consisted of the same elements. This was evidenced by the identical product distributions and kinetic parameters observed with both structures. However, a localized kinetic effect was observed when Keggin POM structures containing different internal heteroatoms (e.g, P, Si, Al) were included in close proximity to the Ni active site. Specifically, a notable periodic trend was observed between the electronegativity of internal heteroatoms and the measured apparent activation energy for the catalyzed ethylene coupling reaction to butene.

Engineering hollow covalent organic framework particle through self‐templated crystallization

AbstractNano/microparticles with hollow structures have many applications in various fields. However, engineering hollow covalent organic framework (COF) particles in a simple and efficient manner remains a significant challenge. In this study, we propose a self‐template crystallization method to prepare imine‐based COF microparticles with tailored shell thickness and high crystallinity. This method involves the post‐transformation of amorphous microparticles in a highly reversible reaction system containing monofunctional competitors. The amorphous precursors with high‐Gibbs free energy decompose and reconstruct to form highly crystalline COFs in situ on the particle surface. This process allows for the preparation of various Schiff‐base COF particles with good crystallinity and tailored hollow structures, with diameters ranging from 120 to 680 nm and shell thicknesses of 27–96 nm. These COF particles have well‐defined micropore sizes centered between 1.0 and 4.0 nm and a high‐specific surface area of up to 543 m2 g−1. The resulted COF particles could be used to support ethylene oligomerization catalysts for improving their activity and selectivity, that is, the mass fraction of α‐olefins above C10 increased from almost zero to 52.7% after COF supporting. Our method thus provides an efficient way to precisely tailor the microstructure and morphology of COF materials for advanced applications.

Alkylaluminum/Urea Hybrid Cocatalysts and Their Use in Iron‐Catalyzed Oligomerization of Ethylene

AbstractThe bis(imino)pyridyl iron‐based ethylene oligomerization catalysts have the advantages of mild reaction conditions, high activity and high selectivity to linear α‐olefins. However, barriers, such as high polymer content in the product mixture and large methylaluminoxane (MAO) dosage, are still limitations for the development of these systems at the industrial scale. In this study, new cocatalysts are prepared by the reaction of alkylaluminums with urea and its derivatives, and successfully used to activate iron‐based catalysts. It is found that the type and molar ratio of alkylaluminum and urea have significant influence on the performance of the cocatalysts. The activity of the preferred triisobutylaluminum/urea activated systems reaches more than 3.0 × 108 g (mol‐Fe)−1 h−1, and the polymer content can be controlled to be less than 2 wt.%. Notably, the catalytic system exhibits good high temperature stability, the activity can still reach 1.0 × 108 g (mol‐Fe)−1 h−1 even at 120 °C. Further 1H NMR studies show that alkylaluminum can react with the NH moieties of urea, forming complex alkylaluminum clusters, even aluminoxane‐like structures. This study offers a promising family of cocatalysts, making it possible to simultaneously improve their performance and reduce their cost.

Computer-assisted design of asymmetric PNP ligands for ethylene tri-/tetramerization: A combined DFT and artificial neural network approach

The combination of computational and experimental sciences accelerates the design and development of molecular catalysts. A general strategy for developing ethylene oligomerization catalysts is still lacking. Consequently, herein, we proposed a widely applicable strategy for designing ethylene oligomerization catalysts. We combined density functional theory (DFT) and an artificial neural network (ANN) to establish a relation between catalyst structure and performance. The structure optimization and electronic calculation of a series of asymmetric PNP/Cr active species were conducted using DFT, and the steric and electronic descriptors were extracted to establish datasets. The catalyst prediction model was constructed using ANN and the leave-one-out cross-validation (LOOCV) method was used to verify the generalization ability of the models. The optimized ANN-based models used to predict 1-hexene and 1-octene selectivity exhibited high R2 values, which indicates satisfactory prediction accuracy of the models. We designed new PNP ligands and successfully predicted the ethylene oligomerization performance of PNP/Cr precatalysts using ANN-based models, which were verified through experiments. In addition, we found that the steric properties more significantly affect the performance of precatalysts than the electronic properties.

Ni-H-Beta Catalysts for Ethylene Oligomerization: Impact of Parent Cation on Ni Loading, Speciation, and Siting

Ni-H-Beta catalysts for ethylene oligomerization (EO) were prepared by ion exchange of NH4-Beta and H-Beta zeolites with aqueous Ni(NO3)2 and characterized by H2-temperature-programmed reduction (TPR), NH3-temperature-programmed desorption (TPD), and diffuse-reflectance infrared Fourier-transform spectroscopy (DRIFTS). Quadruple exchange of NH4-Beta at 70 °C resulted in 2.5 wt.% Ni loading corresponding to a Ni2+/framework aluminum (FAl) molar ratio of 0.52. [NiOH]+ and H+ are the primary charge-compensating cations in the uncalcined catalyst, as evidenced by TPR and DRIFTS. Subsequent calcination at 550 °C in air yielded a Ni-H-Beta catalyst containing primarily bare Ni2+ ions bonded to framework oxygens. Quadruple exchange of H-Beta at 70 °C gave 2.0 wt.% Ni loading (Ni2+/FAl = 0.41). After calcination at 550 °C, the resulting Ni-H-Beta catalyst comprises a mixture of bare Ni2+ ions: [NiOH]+ and NiO species. The relative abundance of [NiOH]+ increases with the number of exchanges. In situ pretreatment at 500 °C in flowing He converted the [NiOH]+ species to bare Ni2+ ions via dehydration. The bare Ni2+ ions interact strongly with the Beta framework as evidenced by a perturbed antisymmetric T-O-T vibration at 945 cm−1. DRIFT spectra of CO adsorbed at 20 °C indicate that the Ni2+ ions occupy two distinct exchange positions. The results of EO testing at 225 °C and 11 bar (ethylene) suggested that the specific Ni2+ species initially presented (e.g., bare Ni2+, [NiOH]+) did not significantly affect the catalytic performance.

Ethylene oligomerization over NiOx/SiO2-Al2O3 catalysts prepared by a coprecipitation method

Thirty-three types of NiOx/SiO2-Al2O3 with different pore structures (BET specific surface area: 32–565 m2 g−1), acid properties (total acid sites: 0.32–1.50 mmol g−1), and NiO crystallite sizes (3.8–20.6 nm) were prepared and evaluated as catalysts for ethylene oligomerization. SiO2-Al2O3 support was prepared by a coprecipitation method, and Ni was loaded on it by an impregnation method. Catalysts were characterized by nitrogen adsorption, powder X-ray diffraction, scanning electron microscopy, temperature-programmed reduction under hydrogen, and temperature-programmed desorption of ammonia. Catalytic reactions were performed in a fixed-bed, down-flow reactor operated at 300 °C and 0.1 MPa. The preparation conditions of the SiO2-Al2O3 support (i.e., type of precipitant, Al2O3 content, and aging temperature) greatly influenced the structure and activity of the resulting NiOx/SiO2-Al2O3 catalysts. The highest activity was obtained when SiO2-Al2O3 was prepared using aqueous ammonia as the precipitant, the Al2O3 content was 15%, and the aging temperature was 70 °C. The activity of this NiOx/SiO2-Al2O3 catalyst was comparable with that of Ni-AlSBA-15, which was the highest activity heterogenous catalysts reported to date. The activity of NiOx/SiO2-Al2O3 increased linearly with increasing BET specific surface area and number of acid sites of the catalyst, but it decreased with increasing Ni surface density. We speculate that highly dispersed Ni species and (weak) acid sites together improve the activity of the developed catalysts.

Preparation of NiOx/SiO2–Al2O3 catalysts by a homogenous precipitation method and their catalytic activity for ethylene oligomerization

NiOx/SiO2–Al2O3 catalysts with different pore structures (BET specific surface area: 303–709 m2 g−1, average pore size: 2.2–5.1 nm), acid properties (total amount of acid sites: 0.99–1.66 mmol g−1), and NiO particle sizes (4.6–14.7 nm) were prepared, and the structural factors influencing their catalytic activity for ethylene oligomerization were studied. SiO2–Al2O3 support was prepared by means of a homogeneous precipitation method, and supports with different physicochemical properties were obtained by changing the preparation conditions. Ni was loaded by means of an impregnation method. Catalytic reactions were performed in a fixed-bed down-flow reactor operated at 300 °C and 0.1 MPa. NiOx/SiO2–Al2O3 catalysts prepared with the homogenous precipitation method had a higher surface area, more uniform pore size, and a larger amount of acid sites, and they showed a higher activity not only compared with NiOx/SiO2–Al2O3 catalysts prepared by means of our previous coprecipitation method but also compared with Ni-AlSBA-15, the best heterogenous catalyst for ethylene oligomerization reported to data. The catalytic activities of the NiOx/SiO2–Al2O3 catalysts strongly depended on the BET specific surface area and the amounts of acid sites. The optimal amounts of weak, medium-strength, strong, and total acid sites were 0.5, 0.3, 0.4, and 1.2 mmol g−1, respectively. Examination of the structure–activity relationships of the catalysts suggested that their catalytic activity was promoted by close contact between the Ni and acid sites.

Novel Ni-SIRAL Catalyst for Heterogeneous Ethylene Oligomerization

Novel Ni-SIRAL Catalyst for Heterogeneous Ethylene Oligomerization

Effect of catalyst support on cobalt catalysts for ethylene oligomerization into linear olefins

Here, we show that the oligomerization activity of a carbon-supported cobalt oxide catalyst is nearly twice as high when it is supported on a less oxidized carbon support.

Site Densities, Rates, and Mechanism of Stable Ni/UiO-66 Ethylene Oligomerization Catalysts.

Nickel-functionalized UiO-66 metal organic frameworks (MOFs) oligomerize ethylene in the absence of cocatalysts or initiators after undergoing ethylene-pressure-dependent transients and maintain stable oligomerization rates for >15 days on stream. Higher ethylene pressures shorten induction periods and engender more active sites for ethylene oligomerization; these sites exhibit invariant selectivity-conversion characteristics to justify that only one type of catalytic center is relevant for oligomerization. The number of active sites is estimated using in situ NO titration to disambiguate the effect of increased reaction rates upon exposure to increasing ethylene pressures. After accounting for augmented site densities with increasing ethylene pressures, ethylene oligomerization is first order in ethylene pressure from 100 to 1800 kPa with an activation energy of 81 kJ mol-1 at temperatures from 443-503 K on Ni/UiO-66. A representative Ni/UiO-66 cluster model that mimics high ethylene pressure process conditions is validated with ab initio thermodynamic analysis, and the Cossee-Arlman mechanism is posited based on comparisons between experimental and computed activation enthalpies from density functional theory calculations on these cluster models of Ni/UiO-66. The insights gained from experiment and theory help rationalize evolution in structure and stability for ethylene oligomerization Ni/UiO-66 MOF catalysts.

Polynuclear (α‐diimine) nickel(II) complex as catalyst for ethylene oligomerization

AbstractLate transition metal catalysts with single or multiple active centers have great attention in the ethylene oligomerization application owing to their unique properties. A new polynuclear (α‐diimine) nickel(II) complex (PS‐Ni complex) bearing the conjugated poly(Schiff‐base) ligand (PS‐ligand) was synthesized. The structures and the composition of the PS‐ligand and the PS‐Ni complex were characterized by Fourier transform infrared (FTIR) analysis, X‐ray diffraction (XRD), X‐ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), thermogravimetric (TG) analysis, nuclear magnetic resonance (NMR) and gel permeation chromatography (GPC). The PS‐Ni complex was used as precatalyst in ethylene oligomerization combined with methylaluminoxane (MAO) and has good catalytic activity for ethylene oligomerization under mild conditions. Compared with the corresponding mononuclear (α‐diimine) nickel(II) complex, the PS‐Ni complex showed higher activity of 17.62 × 105 g/(mol Ni•h), and the main products were C4 and C6 olefins in cyclohexane when the dosage of precatalyst was 2 μmol, the Al/Ni molar ratio was 500, the ethylene pressure was 0.5 MPa, the reaction time was 60 min, and the reaction temperature was 25°C. The main oligomers were α‐olefins, and the selectivity was up to 98.68%, due to the more electron‐deficient catalytic centers of the conjugated structure of poly(Schiff‐base) ligand.

Nickle-Schiff base covalently grafted to UiO-66-NH2 as heterogeneous catalyst for ethylene oligomerization

Metal organic frameworks (MOFs) UiO-66-NH2 had been modified by reaction of pyridine-2-carboxaldehyde with the amino groups to form a pyridineimine that act as ligand of metal Ni. The UiO-66-NH2 grafted pyridineimine nickel catalyst of post synthetic modification was assessed by fourier transform infrared spectroscopy (FT-IR), X-ray diffraction (XRD), scanning electron microscope (SEM), inductively coupled plasma mass spectrometry (ICP-MS) and nitrogen adsorption–desorption, and the catalytic performance of the UiO-66-NH2 grafted pyridineimine nickel catalyst in ethylene oligomerization was investigated. The results showed that the catalyst structure, reaction temperature, Al/Ni molar ratio and reaction pressure had a significant effect on the catalytic activity and products selectivity. The catalytic activity of 3.76 × 105 g·(mol Ni·h)−1 and 75.94% selectivity of butene were obtained when the reaction temperature was 25 ℃, Al/Ni molar ratio was 1000 and reaction pressure was 1.2 MPa.

Structure-Performance Evolution of Cobalt-Ammonia Activated Carbon Catalyst for Ethylene Oligomerization

Structure-Performance Evolution of Cobalt-Ammonia Activated Carbon Catalyst for Ethylene Oligomerization

One-step non-hydrolytic sol-gel synthesis of mesoporous SiO2-Al2O3-NiO catalysts for ethylene oligomerization

Mesoporous SiO2-Al2O3-NiO mixed oxide catalysts with different Si/Al ratio and a Ni content of ≈5.5 wt% have been prepared by a simple, one-step non-hydrolytic sol-gel route and evaluated in the oligomerization of ethylene. The catalysts were characterized by TEM-EDX, N2-physisorption, NH3-TPD, powder XRD, 29Si and 27Al MAS-NMR, XPS, and H2-TPR. The Si/Al ratio was found to influence both the texture and the acidity of the materials. According to XRD, part of the Ni formed NiO crystallites, but according to XPS, no bulky NiO was detected at the surface of the catalysts. XPS spectra and TPR experiments are consistent with the formation of dispersed Ni2+ species strongly interacting with the surface. The samples with high Al content displayed excellent performances with very high specific activity up to 270 mmol ethylene converted per g of catalyst per hour at 150 °C, and a percentage of butenes in the products of ≈70%. These catalysts compare well with model catalysts prepared by time-consuming multistep procedures from expensive ordered mesoporous silica supports.