Ethylene Dimerization Research Articles

Introducing Nonstructural Ligands to Zirconia-like Metal–Organic Framework Nodes To Tune the Activity of Node-Supported Nickel Catalysts for Ethylene Hydrogenation

Previous work has shown that introduction of hexafluoroacetylacetone (Facac) units as nonstructural ligands for the zirconia-like nodes of the eight-connected metal–organic framework (MOF), NU-1000, greatly alters the selectivity of node-supported oxy-nickel clusters for ethylene dimerization vs oligomerization. Here we explore a related concept: tuning of support/catalyst interactions, and therefore, catalyst activity, via parallel installation of organic modifiers on the support itself. As modifiers we focused on para-substituted benzoates (R-BA–; R = −NH2, −OCH3, −CH3, −H, −F, and −NO2) where the substituents were chosen to present similar steric demand, but varying electron-donating or electron-withdrawing properties. R-benzoate-engendered shifts in the node-based aqua O–H stretching frequency for NU-1000, as measured by DRIFTS (diffuse-reflectance infrared Fourier-transform spectroscopy), together with systematic shifts in Ni 2p peak energies, as measured by X-ray photoelectron spectroscopy, show tha...

Activated Niobium and Tantalum Imido Complexes: From Tuneable Polymerization to Selective Ethylene Dimerization Systems

AbstractThe niobium and tantalum imido complexes [CpMCl2(NDipp)], [MCl3(NR)(dme)] (R=tBu, Ph, 2,6‐iPr2C6H3 (Dipp), and Mes), and [TaCl3(NDipp)(tmeda)] were tested in combination with EtAlCl2 for the dimerization of ethylene. The niobium systems afforded dimers or polymers, depending on the nature of the imido ligand, with overall productivities in the range 720 to 13,720 (mol C2H4)(mol Nb)−1. The nature of the polyethylene produced (LDPE or HDPE) depended on the imido ligand and the niobium concentration at which catalysis was run. In contrast, the tantalum/dme systems all mediated ethylene dimerization with productivities of up to 4,503 (mol C2H4)(mol Ta)−1, with overall selectivities to butenes of between 73–81 wt %; selectivity within the dimer fraction to 1‐butene was in the range 72 to 100 %. The productivity of [TaCl3(NDipp)(tmeda)] was six times higher than that of its dme‐bearing counterpart, but at the cost of selectivity to 1‐butene. For the tantalum imido‐mediated ethylene dimerization the composition of the product slate formed is indicative of a metallacyclic mechanism being operative.

Ultrathin Nickel-Based Metal–Organic Framework Nanosheets as Reusable Heterogeneous Catalyst for Ethylene Dimerization

Ultrathin nickel-based metal–organic framework nanosheets (Ni-UMOFNs) were formed from NiCl2·6H2O and benzene-1,4-dicarboxylate through a sonication exfoliation method. In Ni-UMOFNs, the Ni atoms are octahedrally coordinated by six O atoms to form pseudo-octahedra which are further edge/corner connected with each other to form 2D layers separated by benzene-1,4-dicarboxylate linkers. On the Ni-UMOFN surfaces, Ni centers are partially five coordinated owing to the termination of benzene-1,4-dicarboxylate ligands, and these coordinated unsaturated metal sites can be used as catalytic active sites. In this work, Ni-UMOFNs were utilized as ethylene dimerization catalysts in the presence of an alkylaluminum cocatalyst for the first time. The Ni-UMOFN catalytic system showed moderate catalytic activity but high selectivity to 1-butene up to 92.8% under optimal conditions. The optimal Ni-UMOFN catalyst can be reused at least four times without considerable loss in the catalytic activity and selectivity to 1-butene. Extraordinarily, the outstanding level of reusable property is due to continuous exposure of fresh unsaturated metal active sites when the original metal centers on the catalyst surface are used and exfoliated, just like a snake’s molting.

Evidence for the Coordination–Insertion Mechanism of Ethene Dimerization at Nickel Cations Exchanged onto Beta Molecular Sieves

The mechanistic origin of alkene dimerization on Ni sites supported on aluminosilicates has been ascribed to both coordination–insertion (i.e., Cossee-Arlman) and metallacycle-based cycles; the latter is often invoked in the absence of externally supplied cocatalysts or activators that generate Ni-hydride or Ni-alkyl species to initiate coordination–insertion cycles. Determining the prevalent reaction mechanism at Ni sites is often complicated by the formation and consumption of alkene dimer products via oligomerization and other parallel reactions (e.g., cracking, isomerization) that occur at Bronsted acid sites on supports. Here, ethene dimerization (453 K) was studied on Beta zeolites synthesized to contain predominantly exchanged Ni2+ sites according to site balances determined by cation exchange, and to Ni structure determined by CO infrared, UV–visible, and Ni K-edge X-ray absorption spectroscopies. The catalytic behavior of Ni2+ sites was isolated by suppressing contributions from residual H+ sites...

Cobalt(II)-based ethylene dimerization catalysts with silicon-bridged diphosphine ligands

Cobalt(II)-based ethylene dimerization catalysts with silicon-bridged diphosphine ligands

Bis(Imido) Tungsten Complexes: Efficient Precatalysts for the Homogeneous Dimerization of Ethylene

The ethylene dimerization performance of a range of bis(imido)tungsten precatalysts, after activation with aluminum cocatalysts, has been assessed. The best-performing system achieved an activity of 409 410 (mol C2H4)(mol W)−1h–1 [79.6 wt % 1-butene] with a 1-butene selectivity of 82.4 wt % [290 770 (mol C2H4)(mol W)−1h–1 activity]. In addition to selected previously reported variants, the symmetrical bis(imido)tungsten(VI) complexes [WCl2(NPh)2(tmeda)] (3) and [WCl2(NR)2(dme)] [R = 2,4,6-trifluorophenyl, 7; 2,4,6-triphenylphenyl, 8; 2,4,6-tris(trifluoromethyl)phenyl, 9], were prepared. A series of unsymmetrical bis(imido)tungsten(VI) complexes [WCl2(NDipp)(NR)(dme)] (R = iPr, 11; tBu, 12; 2,4,6-trimethylphenyl, 13; 2,4,6-trichlorophenyl, 14; 2,4,6-trifluorophenyl, 15; 2,6-dinitrophenyl, 16; 2,4,6-triphenylphenyl, 17) were also synthesized using an improved version of a previous preparative route that eases purification. The molecular structures of complexes 5–13 and 15–17 were determined by X-ray crystal...

Exploration of Homogeneous Ethylene Dimerization Mediated by Tungsten Mono(imido) Complexes

In combination with EtAlCl2, complexes {[WCl3(μ-Cl)(N-2,6-iPr2C6H3)]2}, [WCl4(NR)(thf)] (R = Et, iPr, cHx, Ph, 2,6-iPr2C6H3, 4-F-C6H4, and 2,4,6-F3-C6H2), and [WCl2(NR′)(PMe3)3] (R′ = Ph, 2,6-iPr2C...

Ethene Dimerization and Hydrogenation over a Zeolite-Supported Rh(I)-Carbonyl Complex: Mechanistic Insights from DFT Modeling

Using DFT calculations within a quantum mechanical/molecular mechanical scheme, we present a model study on a zeolite-supported Rh(I) complex, [Rh(CO)(C2H4)]+, to rationalize the experimentally observed ethene hydrogenation and dimerization. Our computational results show that the coordination of an ethene to the Rh center of a [Rh(CO)(C2H4)]+ complex is thermodynamically favorable over H2 coordination. The diethyl complex [Rh(CO)(C2H5)2]+ resulting from hydrogenation acts as a branching point of two catalytic cycles of ethene conversion, to hydrogenation or dimerization. The Rh-acyl complex [Rh(COCH2CH3)(C2H5)(C2H4)]+ is the in situ-generated active species initiating the dimerization, as it entails a tremendous lowering of the C–C coupling barrier, by more than 100 kJ mol–1. Overall, free energy barriers of ethene hydrogenation (89–92 kJ mol–1) are calculated 4–7 kJ mol–1 lower than the barrier for dimerization, 96 kJ mol–1, in qualitative agreement with the experimentally observed selectivity. Finally, a side reaction of the Rh-acyl complex yields a qualitative explanation of the experimentally observed steady increase in butene selectivity.

Ethylene oligomerization promoted by nickel-based catalysts with silicon-bridged diphosphine amine ligands

A series of nickel complexes [Ni(L1)Br2] (C1), [Ni(L2)Br2] (C2) and [Ni(L3)Br2] (C3) (L1 = N-isopropyl-N-(((diphenylphosphanyl)methyl)dimethylsilyl)-1,1-diphenylphosphanamine, L2 = N-cyclopentyl-N-(((diphenylphosphanyl)methyl)dimethylsilyl)-1,1-diphenylphosphanamine, L3 = N-(2,6-diisopropylphenyl)-N-(((diphenylphosphanyl)methyl)dimethylsilyl)-1,1-diphenylphosphanamine) were synthesized and characterized by elemental analysis, mass spectrometry, spectroscopy and single-crystal X-ray diffraction. L1, L2 and L3 each act as bidentate ligands. Upon activation with ethylaluminum dichloride, these complexes produce efficient catalytic systems for selective dimerization of ethylene to 1-butene, with catalytic activities of 3.45 × 107 g/(molNi·h) and 95.6% butene selectivity containing 87.6% 1-butene fraction.

Structural diversity of metallacycle intermediates for ethylene dimerization on heterogeneous NiMCM-41 catalyst: a quantum chemical perspective

Nanocluster models were investigated to explore the diversity of metallacycle intermediates for ethylene dimerization over NiMCM-41 at B3LYP/6-311+G* and M06/Def2-TZVP. The thermodynamic favorability of the formation of matallacycle with respect to the ring size of silica varied in the sequence of 6T < 3T < 2T < 5T < 4T in terms of Gibbs free energy (ranging from − 10.01 to 16.66 kcal/mol at B3LYP/6-311+G*). The reaction cycle faced lower barriers on 3T and 2T clusters, however. The formation of the intermediate and π complexation of 1-butene led to positive total charges on the hydrocarbon segment of the complex, being maximized on four-membered sites and minimized on two-membered ones. Further insights are also provided with QTAIM, frontier orbital, and FTIR analyses.

Beyond the Active Site: Tuning the Activity and Selectivity of a Metal-Organic Framework-Supported Ni Catalyst for Ethylene Dimerization.

To modify its steric and electronic properties as a support for heterogeneous catalysts, electron-withdrawing and electron-donating ligands, hexafluoroacetylacetonate (Facac-) and acetylacetonate (Acac-), were introduced to the metal-organic framework (MOF), NU-1000, via a process akin to atomic layer deposition (ALD). In the absence of Facac- or Acac-, NU-1000-supported, AIM-installed Ni(II) sites yield a mixture of C4, C6, C8, and polymeric products in ethylene oligomerization. (AIM = ALD-like deposition in MOFs). In contrast, both Ni-Facac-AIM-NU-1000 and Ni-Acac-AIM-NU-1000 exhibit quantitative catalytic selectivity for C4 species. Experimental findings are supported by density functional theory calculations, which show increases in the activation barrier for the C-C coupling step, due mainly to rearrangement of the siting of Facac- or Acac- to partially ligate added nickel. The results illustrate the important role of structure-tuning support modifiers in controlling the activity of MOF-sited heterogeneous catalysts and in engendering catalytic selectivity. The results also illustrate the ease with which crystallographically well-defined modifications of the catalyst support can be introduced when the node-coordinating molecular modifier is delivered via the vapor phase.

Exposed Equatorial Positions of Metal Centers via Sequential Ligand Elimination and Installation in MOFs.

Metal-organic frameworks (MOFs) provide highly designable platforms to construct complex coordination architectures for targeted applications. Herein, we demonstrate that trans-coordinated metal centers with exposed equatorial positions can be placed in a MOF matrix. A Zr-based MOF, namely, PCN-160, was initially synthesized as a scaffold structure. Postsynthetic linker labilization was subsequently implemented to partially remove the original dicarboxylate linkers and incorporate pyridinecarboxylates. A pair of neighboring pyridyl groups was arranged at proper proximity within the framework to form trans-binding sites that accommodate different metal cations including Mn2+, Fe2+, Co2+, Ni2+, Cu2+, and Pd2+. Furthermore, the trans-coordinated Ni2+ sites in porous frameworks can be readily accessed by substrates along the equatorial plane, facilitating the catalysis as manifested by the superior activity in ethylene dimerization over that observed for a cis-chelated catalyst.

Broadly Applicable Stereoselective Syntheses of Serrulatane, Amphilectane Diterpenes, and Their Diastereoisomeric Congeners Using Asymmetric Hydrovinylation for Absolute Stereochemical Control.

A stereogenic center, placed at an exocyclic location next to a chiral carbon in a ring to which it is attached, is a ubiquitous structural motif seen in many bioactive natural products, including di- and triterpenes and steroids. Installation of these centers has been a long-standing problem in organic chemistry. Few classes of compounds illustrate this problem better than serrulatanes and amphilectanes, which carry multiple methyl-bearing exocyclic chiral centers. Nickel-catalyzed asymmetric hydrovinylation (AHV) of vinylarenes and 1,3-dienes such as 1-vinylcycloalkenes provides an exceptionally facile way of introducing these chiral centers. This Article documents our efforts to demonstrate the generality of AHV to access not only the natural products but also their various diastereoisomeric derivatives. Key to success here is the availability of highly tunable phosphoramidite Ni(II) complexes useful for overcoming the inherent selectivity of the chiral intermediates. The yields for hydrovinylation (HV) reactions are excellent, and selectivities are in the range of 92-99% for the desired isomers. Discovery of novel, configurationally fluxional, yet sterically less demanding 2,2'-biphenol-derived phosphoramidite Ni complexes (fully characterized by X-ray) turned out to be critical for success in several HV reactions. We also report a less spectacular yet equally important role of solvents in a metal-ammonia reduction for the installation of a key benzylic chiral center. Starting with simple oxygenated styrene derivatives, we iteratively install the various exocyclic chiral centers present in typical serrulatane [e.g., a (+)- p-benzoquinone natural product, elisabethadione, nor-elisabethadione, helioporin D, a known advanced intermediate for the synthesis of colombiasin and elisapterosin] and amphilectane [e.g., A-F, G-J, and K,L pseudopterosins] derivatives. A concise table showing various synthetic approaches to these molecules is included in the Supporting Information. Our attempts to synthesize a hitherto elusive target, elisabethin A, led to a stereoselective, biomimetic route to pseudopterosin A-F aglycones.

MP2-IQA: upscaling the analysis of topologically partitioned electron correlation

When electronic correlation energy is partitioned topologically, a detailed picture of its distribution emerges, both within atoms and between any two atoms. This methodology allows one to study dispersion beyond its more narrow definition in long-range Rayleigh-Schrödinger perturbation theory. The interacting quantum atoms (IQA) method was applied to MP2/6-31G(d,p) (uncontracted) wave functions of a wide variety of systems: glycine…water (hydration), the ethene dimer (π-π interactions), benzene (aromaticity), cyclobutadiene (antiaromaticity), and NH3BH3 (dative bond). Through the study of molecular complexes it turns out that dispersion energy is either important to a system’s stabilization (for the C2H4 dimer) or not important (for Gly…H2O). We have also discovered that the delocalization in benzene lowers the strength of Coulomb repulsion in the bonds, which has been quantified for the first time through IQA. Finally, we showed that the nature of the dative bond is much different from that of a regular covalent bond as it is not destabilized by electronic correlation. Finally, the conclusions obtained for these archetypical systems have implications for the future of the quantum topological force field FFLUX in the simulation of larger systems.Graphical abstractAtomic and bond dynamic correlation energies are now available thanks to IQA. Larger molecules can now be accessed to include resonance and solvation of FFLUX force field

Recent progress of transition metal based catalysts for the selective dimerization of ethylene

In this mini-review, the state-of-the-art of transition metal catalyzed selective dimerization of ethylene to butenes is presented. Catalytic activities, operating conditions, as well as the C4 and 1-butene/cis- and trans-2-butene selectivities, are discussed. The scope of the review is 2000-present.

Infrared spectra of C2H4 dimer and trimer

Spectra of ethylene dimers and trimers are studied in the ν11 and (for the dimer) ν9 fundamental band regions of C2H4 (≈2990 and 3100 cm−1) using a tunable optical parametric oscillator source to probe a pulsed supersonic slit jet expansion. The deuterated trimer has been observed previously, but this represents the first rotationally resolved spectrum of (C2H4)3. The results support the previously determined cross-shaped (D2d) dimer and barrel-shaped (C3h or C3) trimer structures. However, the dimer spectrum in the ν9 fundamental region of C2H4 is apparently very perturbed and a previous rotational analysis is not well verified.

Dimerization of Ethylene to Butenes Over Ordered Mesoporous Silica SBA-15 Supported Ni-Al Catalysts with Different Morphologies.

A series of ordered mesoporous silica (OMS) SBA-15 supports with different morphologies were prepared by different synthetic methods to investigate the effect of the characteristics of the morphology of OMS on the ethylene dimerization outcomes. After additions of Ni and Al species into the SBA-15 support, a dimerization reaction of ethylene was performed using a fixed-bed reactor. Rod-type Ni-Al-SBA-15 with a small micron size showed better catalytic performance compared to those of the other catalysts. From these catalytic results, the particle size and morphology of a SBA-15 support critically influenced the catalytic activities and lifetimes of the dimerization catalysts. The optimum reaction pathway in the Ni-Al-SBA-15 catalyst enhanced the overall catalytic performance due to the suppression of the further oligomerization of ethylene and butenes. Moreover, ethylene dimerization was investigated over the rod-type Ni-Al-SBA-15 catalyst to discover an optimum reaction condition. The maximum yield of butenes was 24.7% at 300 °C at 11.5 bar with a WHSV of 1.5 h-1.

Computational screening of MOF-supported transition metal catalysts for activity and selectivity in ethylene dimerization

Deposition of small metal-oxide clusters on the Zr-based nodes of a metal-organic framework has been demonstrated to provide access to a variety of single-site catalysts. Well-defined catalytic active sites are amenable to detailed computational studies of potential catalytic pathways, and they invite screening a wide range of metals to assess their expected activity. Here we report the application of density functional theory to a variety of transition metals (in particular TiIV, VII, VIV, CrII, CrIII, MnII, MnIV, FeII, FeIII, NiII, CoII, CoIII, CuII, CuIII, PdII, MoII, and WII) supported on NU-1000 inorganometallic nodes to evaluate their activity for ethylene dimerization. We found that the rate-determining step varies between different catalysts, which illustrates the importance of considering more than a single step when comparing catalytic cycles across a variety of metals. Our calculations are consistent with the known good activity of supported NiII for ethylene dimerization, and they predict that CrII and PdII are also potentially useful catalysts for this process. We also screen modifications to the organic linker of NiII-NU-1000 by considering the addition of Me, iPr, tBu, CF3 and NH2 groups to study the influence of sterically demanding and, the case of CF3 and NH2, respectively, electron-donating and -withdrawing, substituents on the activity for ethylene dimerization; we predict no improvements in activity or selectivity (for 1-butene) with such substitutions.

Catalytic descriptors and electronic properties of single-site catalysts for ethene dimerization to 1-butene

Six first-row transition metal cations (Mn2+, Fe2+, Co2+, Ni2+, Cu2+, Zn2+) were evaluated as catalysts for ethene dimerization to 1-butene. This is an important reaction in the chemistry of CC bond formation and in the conversion of natural gas to higher hydrocarbons. Two related classes of transition metal cation catalysts were investigated: 1) single transition metal cations supported on zirconium oxide nodes of the metal–organic framework NU-1000 and 2) small metal hydroxide clusters with two metal atoms (M2) that could be grown by atomic layer deposition on a support exhibiting isolated hydroxyl groups. Using scaling relations, the free energies of co-adsorbed hydrogen and ethene (i.e., (H/C2H4)*) and adsorbed ethyl (i.e., C2H5*) were identified as descriptors for ethene dimerization catalysis. Using degree of rate control analysis, it was determined that the rate controlling steps are either ethene insertion (CC bond forming) or β-hydride elimination (CH bond breaking), depending on the metal. Using degree of catalyst control analysis, it was determined that activity on all the catalysts studied could be improved by tuning the free energy of C2H5*.

Ethylene Dimerization and Oligomerization to 1-Butene and Higher Olefins with Chromium-Promoted Cobalt on Carbon Catalyst

Industrial dimerization of ethylene to 1-butene is achieved with catalysts bearing ligand structures and activated by a co-catalyst. Here, we report that a Cr-promoted cobalt oxide on carbon catalyst was able to produce 1-butene with 53.5–82.4% total selectivity at ethylene conversions between 8.9 and 31.5% without the use of a co-catalyst. The Cr-promoted catalyst showed enhanced activity and stability compared to the nonpromoted catalyst. The characterization results revealed that the incorporation of Cr into the cobalt oxide on carbon catalyst was able to decrease the cobalt oxide particle size. Charge transfer from Cr to Co was also observed in the Cr-promoted catalyst compared to monometallic Co and Cr on carbon. The kinetic model built to rationalize the experimental observations predicted a 50% increase in active site dispersion in the Cr-promoted catalyst, which explained the 1.6× apparent reaction rate increase compared to the nonpromoted catalyst.