Cossee-Arlman Mechanism Research Articles

Mechanochemical regulation of the nickel species coordination environment on Ni/Beta catalysts to enhance propylene dimerization

Propylene oligomerization over nickel-based catalysts has been developed as an attractive route to produce high-value fine chemicals and clean fuels. In this work, the coordination environment of nickel species on high-loading Ni/Beta catalysts is successfully regulated by mechanochemistry, exhibiting a remarkable promotion in activity for propylene dimerization. The optimized ball-milled catalyst with rich four-coordinated nickel species exhibits a higher initial rate of 139.6 molC3=/(molNi·h) than the conventional impregnated catalyst (57.5 molC3=/(molNi·h)). Mechanochemistry induces a strong metal-support interaction by transforming more Ni species into the zeolite channel to generate four-coordinated Ni cations, boosting the adsorption and activation of propylene. Additionally, in-situ Fourier Transform Infrared experiments reveal that propylene dimerization complies with the Cossee-Arlman mechanism via the initial π-adsorption of propylene instead of σ-adsorption. The steric hindrance of the Ni-alkyl intermediate is critical for product selectivity, driving the dimer distribution away from thermodynamic equilibrium and resulting in preferential 1,2-insertion and 2,2-insertion products.

Ultra-Highly Active Ni-Doped MOF-5 Heterogeneous Catalysts for Ethylene Dimerization.

Here, an ultra-highly active Ni-MOF-5 catalyst with high Ni loading for ethylene dimerization is reported. The Ni-MOF-5 catalysts are synthesized by a facile one-pot co-precipitation method at room temperature, where Ni2+ replaces Zn2+ in MOF-5. Unlike Zn2+ with tetrahedral coordination in MOF-5, Ni2+ is coordinated with extra solvent molecules except for four-oxygen from the framework. After removing coordinated solvent molecules, Ni-MOF-5 achieves an ethylene turnover frequency of 352000h-1 , corresponding to 9040g of product per gram of catalyst per hour, at 35°C and 50bar, far exceeding the activities of all reported heterogeneous catalysts. The high Ni loading and full exposure structure account for the excellent catalytic performance. Isotope labeling experiments reveal that the catalytic process follows the Cossee-Arlman mechanism, rationalizing the high activity and selectivity of the catalyst. These results demonstrate that Ni-MOF-5 catalysts are very promising for industrial catalytic ethylene dimerization.

Structure and Site Evolution of Framework Ni Species in MIL-127 MOFs for Propylene Oligomerization Catalysis

A mixed-valence oxotrimer metal-organic framework (MOF), Ni-MIL-127, with a fully coordinated nickel atom and two iron atoms in the inorganic node, generates a missing linker defect upon thermal treatment in helium (>473 K) to engender an open coordination site on nickel which catalyzes propylene oligomerization devoid of any cocatalysts or initiators. This catalyst is stable for ∼20 h on stream at 500 kPa and 473 K, unprecedented for this chemistry. The number of missing linkers on synthesized and activated Ni-MIL-127 MOFs is quantified using temperature-programmed oxidation, 1H nuclear magnetic resonance spectroscopy, and X-ray absorption spectroscopy to be ∼0.7 missing linkers per nickel; thus, a majority of Ni species in the MOF framework catalyze propylene oligomerization. In situ NO titrations under reaction conditions enumerate ∼62% of the nickel atoms as catalytically relevant to validate the defect density upon thermal treatment. Propylene oligomerization rates on Ni-MIL-127 measured at steady state have activation energies of 55-67 kJ mol-1 from 448 to 493 K and are first-order in propylene pressures from 5 to 550 kPa. Density functional theory calculations on cluster models of Ni-MIL-127 are employed to validate the plausibility of the missing linker defect and the Cossee-Arlman mechanism for propylene oligomerization through comparisons between apparent activation energies from steady-state kinetics and computation. This study illustrates how MOF precatalysts engender defective Ni species which exhibit reactivity and stability characteristics that are distinct and can be engineered to improve catalytic activity for olefin oligomerization.

Influence of 1-Butene Adsorption on the Dimerization Activity of Single Metal Cations on UiO-66 Nodes.

Grafting metal cations to missing linker defect sites in zirconium-based metal-organic frameworks, such as UiO-66, produces a uniquely well-defined and homotopic catalytically active site. We present here the synthesis and characterization of a group of UiO-66-supported metal catalysts, M-UiO-66 (M = Ni, Co, Cu, and Cr), for the catalytic dimerization of alkenes. The hydrogen-deuterium exchange via deuterium oxide adsorption followed by infrared spectroscopy showed that the last molecular water ligand desorbs from the sites after evacuation at 300 °C leading to M(OH)-UiO-66 structures. Adsorption of 1-butene is studied using calorimetry and density functional theory techniques to characterize the interactions of the alkene with metal cation sites that are found active for alkene oligomerization. For the most active Ni-UiO-66, the removal of molecular water from the active site significantly increases the 1-butene adsorption enthalpy and almost doubles the catalytic activity for 1-butene dimerization in comparison to the presence of water ligands. Other M-UiO-66 (M = Co, Cu, and Cr) exhibit 1-3 orders of magnitude lower catalytic activities compared to Ni-UiO-66. The catalytic activities correlate linearly with the Gibbs free energy of 1-butene adsorption. Density functional theory calculations probing the Cossee-Arlman mechanism for all metals support the differences in activity, providing a molecular level understanding of the metal site as the active center for 1-butene dimerization.

Ni-Loaded 2D Zeolitic Imidazolate Framework as a Heterogeneous Catalyst with Highly Activity for Ethylene Dimerization

Metal–organic frameworks (MOFs) hold great potential in heterogeneous catalysis due to their unique properties and tunable structures. Zeolitic imidazolate frameworks (ZIFs) are series of classical MOFs that have been widely studied, where Zn2+ is coordinated with four imidazoles to form a tetrahedral structure. ZIF-L, one type of ZIF, is known for its unique two-dimensional (2D) leaf-like morphology and large external specific surface area. In this work, we reported a facile and green synthesis of the Ni-loaded ZIF-L (Ni-ZIF-L) catalysts, where water as solvent and product collection by filtration make the preparation easy for large-scale industrial production. Considering the incompatibility of the d8 electron configuration of Ni with the three-dimensional (3D) framework of ZIF-L, Ni can only be selectively anchored to the surface of ZIF-L. The Ni-ZIF-L has shown high crystallinity and 2D leaf-like microflakes similar to those of pure ZIF-L. After systematic analysis, we speculate that the Ni sites in Ni-ZIF-L samples have a square planar configuration, which should be coordinated by two imidazole groups from the framework, one NO3– group, and one free imidazole group. Ni-ZIF-L possesses fully exposed Ni active sites and a large specific surface area, accounting for the good performance for ethylene dimerization. In addition, the large particle size of the Ni-ZIF-L catalyst is beneficial for separation. With the assistance of cocatalyst, Ni-ZIF-L achieves an average ethylene turnover frequency of 330 320 moles of ethylene per mole of Ni per hour (1-butene selectivity >90%)) under 40 °C and 30 bar, comparable to the activity of the benchmark heterogeneous catalyst. The isotope experiments are used to illustrate that the ethylene dimerization catalyzed by Ni-ZIF-L follows the Cossee-Arlman mechanism, which also rationalizes the high catalytic activity and the small amount of isomerization and trimerization product formation.

Deep reaction network exploration at a heterogeneous catalytic interface

Characterizing the reaction energies and barriers of reaction networks is central to catalyst development. However, heterogeneous catalytic surfaces pose several unique challenges to automatic reaction network characterization, including large sizes and open-ended reactant sets, that make ad hoc network construction the current state-of-the-art. Here, we show how automated network exploration algorithms can be adapted to the constraints of heterogeneous systems using ethylene oligomerization on silica-supported single-site Ga3+ as a model system. Using only graph-based rules for exploring the network and elementary constraints based on activation energy and size for identifying network terminations, a comprehensive reaction network is generated and validated against standard methods. The algorithm (re)discovers the Ga-alkyl-centered Cossee-Arlman mechanism that is hypothesized to drive major product formation while also predicting several new pathways for producing alkanes and coke precursors. These results demonstrate that automated reaction exploration algorithms are rapidly maturing towards general purpose capability for exploratory catalytic applications.

The Role of Nickel and Brønsted Sites on Ethylene Oligomerization with Ni-H-Beta Catalysts

The present work studies the adsorption of ethylene on Ni-H-Beta particles to unravel the roles of nickel and Brønsted sites in the catalytic oligomerization of ethylene. Three models (i.e., two based on the Cossee–Arlman mechanism and one based on the metallacycle mechanism) are examined in terms of the nature of the active sites and the adsorption mechanism involved in the ethylene coordination step. The results are consistent with the participation of two active sites in the formation of [Ni(II)-H]+ Cossee–Arlman centers and also suggest that ethylene dissociates upon adsorption on [Ni(II)-H]+ sites. Further characterization of Ni-H-Beta catalysts prepared at different nickel loadings and silica-to-alumina ratios reveals that highly dispersed Ni2+ exists on the catalyst surface and interacts with the catalyst’s lattice oxygen and free NiO crystals. At the same time, the kinetic results indicate that Brønsted sites may form isolated nickel-hydride ([Ni(II)-H]+) centers on the catalyst surface. In addition, the presence of residual, noncoordinated Ni2+ and Brønsted sites (not involved in the formation of [Ni(II)-H]+ sites) shows a reduced probability of the formation of nickel-hydride sites, hindering the conversion rate of ethylene. A mechanism for forming [Ni(II)-H]+ centers is proposed, involving ethylene adsorption over Ni2+ and a Brønsted site. This research has important implications for improving ethylene oligomerization processes over nickel-based heterogeneous catalysts.

Visualization of on-surface ethylene polymerization through ethylene insertion.

Polyethylene production through catalytic ethylene polymerization is one of the most common processes in the chemical industry. The popular Cossee-Arlman mechanism hypothesizes that the ethylene be directly inserted into the metal-carbon bond during chain growth, which has been awaiting microscopic and spatiotemporal experimental confirmation. Here, we report an in situ visualization of ethylene polymerization by scanning tunneling microscopy on a carburized iron single-crystal surface. We observed that ethylene polymerization proceeds on a specific triangular iron site at the boundary between two carbide domains. Without an activator, an intermediate, attributed to surface-anchored ethylidene (CHCH3), serves as the chain initiator (self-initiation), which subsequently grows by ethylene insertion. Our finding provides direct experimental evidence of the ethylene polymerization pathway at the molecular level.

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.

Highly Active Heterogeneous Catalyst for Ethylene Dimerization Prepared by Selectively Doping Ni on the Surface of a Zeolitic Imidazolate Framework.

The production of 1-butene by ethylene dimerization is an important chemical industrial process currently implemented using homogeneous catalysts. Here, we describe a highly active heterogeneous catalyst (Ni-ZIF-8) for ethylene dimerization, which consists of isolating Ni-active sites selectively located on the crystal surface of a zeolitic imidazolate framework. Ni-ZIF-8 can be easily prepared by a simple one-pot synthesis method in which site-specific anchoring of Ni is achieved spontaneously because of the incompatibility between the d8 electronic configuration of Ni2+ and the three-dimensional framework of ZIF-8. The full exposure and square-planar coordination of the Ni sites accounts for the high catalytic activity of Ni-ZIF-8. It exhibits an average ethylene turnover frequency greater than 1 000 000 h-1 (1-butene selectivity >85%) at 35 °C and 50 bar, far exceeding the activities of previously reported heterogeneous catalysts and many homogeneous catalysts under similar conditions. Moreover, compared to molecular Ni complexes used as homogeneous catalysts for ethylene dimerization, Ni-ZIF-8 has significantly higher stability and shows constant activity during 4 h of continuous reaction. Isotopic labeling experiments indicate that ethylene dimerization over Ni-ZIF-8 follows the Cossee-Arlman mechanism, and detailed characterizations combined with density functional theory calculations rationalize this observed high activity.

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.

Co-catalyst free ethene dimerization over Zr-based metal-organic framework (UiO-67) functionalized with Ni and bipyridine

Ni functionalized metal organic frameworks (MOF) are promising heterogeneous ethene dimerization catalysts. Activities comparable to or higher than Ni-aluminosilicates have been reported in literature. However, unlike the Ni-aluminosilicates, those Ni-MOFs require a large excess of co-catalyst to initiate the dimerization process and some catalysts generate polymers which lead to catalyst deactivation. Herein, we report a series of Ni(II) and 2,2′-bipyridine-5,5′-dicarboxylate (bpy) functionalized UiO-67 MOF that catalyze the ethene dimerization reaction co-catalyst free. The catalysts were active for ethene dimerization (up to 850 mg butene gcat−1 h−1) after activation at 300 °C in 10 % O2 for 360 min and subsequent exposure to flowing ethene (P(ethene) =26 bar, 250 °C) for 240 min. The catalysts yielded up to 6 % conversion with 99 % selectivity to linear 1- and 2-butenes, which formed in non-equilibrated ratios. Overall, the test data indicate that all three linear butenes are formed on a single active site, in accordance with the Cossee-Arlman mechanism. Ex situ XAS and CO FT-IR spectroscopy studies point towards Ni monomers or, plausibly, low-nuclearity Ni-multimers, docked at bpy linkers with Ni-Ni distances > 4 Å, as the main active site for the ethene dimerization reaction.

Ethene oligomerization on nickel microporous and mesoporous-supported catalysts: Investigation of the active sites

Nickel-containing mesoporous (using Al2O3 or SiO2-Al2O3 support) and microporous (using nano- or micro-crystalline Beta zeolite support) catalysts were prepared by ion exchange and characterized by XRD, SEM, N2-adsorption, MP-AES and FTIR. The samples were subjected to testing as ethene oligomerization catalysts at T=120°C, Ptotal=29bar, Pethene=11.6–25.1bar. All catalysts were active for ethene oligomerization, and linear butenes were the main gaseous products. However, catalyst deactivation due to retained long-chain alkenes was observed. Ethene partial pressure variation experiments showed that the reaction order in ethene for butene formation was 1.5–1.7 for the mesoporous, and 2.0 for the microporous catalysts. Contact time variation experiments carried out with the microporous catalysts at Pethene=18bar showed that product selectivity was independent of ethene conversion, and suggested that 1-butene and 2-butenes are primary products. This result is consistent with the Cossee-Arlman mechanism. The nature of the active sites was investigated with FTIR spectroscopy with CO as probe molecule. The results point to Ni2+-counterions as the preeminent active sites, while we suggest that Ni-sites grafted on silanol groups and NiO particles are spectators.

Mechanism of Single-Site Molecule-Like Catalytic Ethylene Dimerization in Ni-MFU-4l.

A recently developed metal-organic framework (MOF) catalyst for the dimerization of ethylene has a combination of selectivity and activity that surpasses that of commercial homogeneous catalysts, which have dominated this important industrial process for nearly 50 years. The uniform catalytic sites available in MOFs provide a unique opportunity to directly study reaction mechanisms in heterogeneous catalysts, a problem typically intractable due to the multiplicity of coordination environments found in many solid catalysts. In this work, we use a combination of isotopic labeling studies, mechanistic probes, and DFT calculations to demonstrate that Ni-MFU-4l operates via the Cossee-Arlman mechanism, which has also been implicated in homogeneous late transition metal catalysts. These studies demonstrate that metal nodes in MOFs mimic homogeneous catalysts not just functionally, but also mechanistically. They provide a blueprint for the development of advanced heterogeneous catalysts with similar degrees of tunability to their homogeneous counterparts.

Mechanistic Studies on the Switching from Ethylene Polymerization to Nonselective Oligomerization over the Triphenylsiloxy Chromium(II)/Methylaluminoxane Catalyst

Our previous experimental report showed a switching behavior from ethylene polymerization to nonselective oligomerization by a novel triphenylsiloxy complex of chromium(II) [(Ph3SiO)Cr·(THF)]2(μ-OSiPh3)2 (1) together with methylaluminoxane (MAO) as a cocatalyst. In this work, combined experimental and computational studies were carried out to shed some light on the nature of the active species and their fascinating switching mechanism. The experimental results and DFT calculations suggested that (i) the chain propagation and chain transfer processes proceed via a Cossee–Arlman mechanism and β-hydrogen transfer to the chromium center, respectively; (ii) the trivalent cationic model [(Ph3SiO)CrIIIMe]+ and [(η6-toluene)CrIIIMe2]+, which could be generated by a disproportionation reaction, are the most plausible active species for ethylene polymerization, and the divalent cationic model [(η6-toluene)CrIIMe]+ might be responsible for ethylene nonselective oligomerization. A switching mechanism from ethylene polymerization to nonselective oligomerization in the 1/MAO catalyst system was proposed on the basis of DFT calculations. These results may have useful implications for studying active species and the mechanism of transition-metal-catalyzed olefin polymerization and oligomerization.

α-Agostic Interactions and Olefin Insertion in Metallocene Polymerization Catalysts

In the 40 years since the discovery of transition metal catalysts for olefin polymerization by Ziegler and Natta a substantial amount of research has been directed toward understanding the basic mechanistic steps of this important industrial process. Attention has focused in particular on the process of monomer enchainment, which is generally agreed to occur through olefin coordination followed by insertion into a metal-carbon bond. Four general mechanistic proposals for the nature of this insertion have emerged (Scheme 1). The first is alkyl migration to the coordinated olefin and is known as the Cossee-Arlman mechanism. The second model, proposed by Rooney and Green, involves an oxidative 1,2-hydrogen shift from the R-carbon of the polymer chain, generating a metal-alkylidene hydride. This species then reacts with an olefin to generate a metallacyclobutane, and reductive elimination completes the propagation sequence. Somewhat intermediate to the first two proposals is a mechanism proposed by Green, Rooney, and Brookhart where a hydrogen atom on the R-carbon of the growing polymer chain interacts with the metal center throughout the catalytic cycle. This three-center, two-electron covalent bond, termed an “agostic interaction”, occurs when the hydrogen atom is simultaneously bonded to both a carbon and a metal atom. The fourth mechanistic possibility is olefin insertion where an R-hydrogen interacts with the metal center only during the transition state of the C-C bond forming step. This mechanism is a hybrid of the Cossee-Arlman and modified Green-Rooney mechanisms.

Detection of an alkyl ethylene complex during ethylene polymerization by a cobalt(III) catalyst. Energetics of the .beta.-migratory insertion reaction

The mechanism of carbon-carbon bond formation in transition-metal-catalyzed olefin polymerization reactions is though to be a {beta}-migratory insertion reaction occurring via an intermediate alkyl olefin complex, 1 {yields} 2 {yields} 1{prime} (the Cossee-Arlman mechanism). Although the stepwise polymerization of ethylene (and other alkenes) in homogeneous systems has been studied spectroscopically, intermediate alkyl ethylene complexes have not been detected. Thus, activation energies for the {beta}-migratory insertion reactions of alkyl olefin complexes in polymerizing systems are unknown. They report here spectroscopic detection of an intermediate cobalt alkyl ethylene complex during ethylene polymerization by a Co(III) catalyst together with a kinetic study which allows estimation of {Delta}G{double dagger} for the {beta}-migratory insertion reaction.