Nature Of Cocatalyst Research Articles

Cocatalyst effects in Hf-catalysed olefin polymerization: taking well-defined Al-alkyl borate salts into account.

Hafnium catalysts for olefin polymerization are often very sensitive to the nature of cocatalysts, especially if they contain "free" aluminium trialkyls. Herein, cocatalyst effects in Hf-catalysed propene polymerization are examined for four Hf catalysts belonging to the family of CS-symmetric (Hf-CS-Met) and C2-symmetric (Hf-C2-Met) metallocenes, as well as of octahedral (Hf-OOOO) and pentacoordinated (Hf-PyAm) "post-metallocenes". The performance of the recently developed {[iBu2(PhNMe2)Al]2(μ-H)}+[B(C6F5)4]- (AlHAl) cocatalyst is compared with that of established systems like methylalumoxane, phenol-modified methylalumoxane and trityl borate/tri-iso-butylaluminium. The worst catalytic performance is observed with MAO. Conversely, the best cocatalyst varies depending on the Hf catalyst used and the performance indicator of interest, highlighting the complexity and importance of selecting the right precatalyst/cocatalyst combination. AlHAl proved to be a suitable system for all catalysts tested and, in some cases, it provides the best performance in terms of productivity (e.g. with hafnocenes). Furthermore, it generally leads to high molecular weight polymers, also with catalysts enabling easy chain transfer to Al like Hf-PyAm. This suggests that AlHAl has a low tendency to form heterodinuclear adducts with the cationic active species, therefore preventing the formation of dormant sites and/or termination events by chain transfer to Al.

Chromium Catalysts Based on Unsymmetrical PNP Ligands for Selective Ethylene Tri-/Tetramerization: Effect of Electron-Withdrawing/Donating Substituents on Catalytic Performance

The in situ formation and activation of Cr(III) catalysts based on unsymmetrical PNP ligands yield efficient catalytic systems for selective ethylene tri-/tetramerization. The electronic nature (electron-withdrawing or electron-donating) and position (para or meta) of the substituents over the phenyl rings of the PNP, the nature of cocatalyst (DMAO/AlEt3 and MMAO-3A), and reaction conditions have been observed to have a marked impact on catalytic performance, particularly catalytic activity. Ligand L2, bearing 4-(trifluoromethyl)phenyl substituents, yielded 33.6 kg(product).g(Cr)−1·h−1 catalytic activity with 57.7% C8 selectivity under optimal conditions. Ligand L4, having para-tolyl substituents, yielded 43.3 kg(product).g(Cr)−1·h−1 with 59.0% C8 selectivity under optimum conditions. Changing the positions of both the electron-withdrawing and electron-donating substituents from para to meta over the phenyls of the PNP may lead to both catalytic systems exhibiting poor performance.

Decorating vertically aligned MoS2 nanoflakes with silver nanoparticles for inducing a bifunctional electrocatalyst towards oxygen evolution and oxygen reduction reaction

Abstract Catalysts capable of improving the performance of oxygen evolution reaction (OER) and oxygen reduction reactions (ORR) are essential for the advancement of renewable energy technologies. Herein, Ag-decorated vertically aligned MoS2 nanoflakes are developed via magnetron co-sputtering and investigated as electrocatalyst towards OER and ORR. Due to the presence of silver, the catalyst shows more than 1.5 times an increase in the roughness-normalized rate of OER, featuring a very low Tafel slope (58.6 mv dec−1), thus suggesting that the catalyst surface favors the thermodynamics of hydroxyl radical (OH•) adsorption with the deprotonation steps being the rate-determining steps. The improved performance is attributed to the strong interactions between OOH intermediates and the Ag surface which reduces the activation energy. Rotating ring disk electrode (RRDE) analysis shows that the net disk currents on the Ag-MoS2 sample are two times higher at 0.65 V compared to MoS2, demonstrating the co-catalysis effect of silver doping. Based on the rate constant values, Ag-MoS2 proceeds through a mixed 4 electron and a 2 + 2 serial route reduction mechanism, in which the ionized hydrogen peroxide is formed as a mobile intermediate. The presence of silver decreases the electron transfer number and increases the peroxide yield due to the interplay of a 2 + 2 electron reduction pathway. A 2.5–6 times faster conversion rate of peroxide to OH- observed due to the presence of silver, indicating its effective cocatalyst nature. This strategy can help in designing a highly active bifunctional catalyst that has great potential as a viable alternative to precious-metal-based catalysts.

New Catalytic Systems Based on Pd(0) Complexes for Addition Polymerization of Norbornene

Addition polynorbornenes are the promising class of polymers for macromolecular design of various engineering polymers. This work has shown for the first time the fundamental possibility to obtain addition polynorbornenes in the presence of catalytic systems based on Pd(0) complexes activated by organic cocatalysts (aryl halides). Addition polymerization of norbornene has been studied in the presence of different cocatalysts, which allowed one to assess the effect of cocatalyst nature on the activity of the catalytic system.

Nickel-containing nanophases as the carriers of catalytic active sites in the ethylene oligomerization in the presence of systems based on Ni(acac)2 and organoaluminum compounds

The nature and size of particles, formed under conditions of the ethylene oligomerization in the systems based on nickel bis(acetylacetonate) with Et2AlCl or Et2AlCl·Cl2AlEt (Al: Ni = 50: 1), have been determined using ex situ HRTEM, electron diffraction and EDAX in combination with EPR spectroscopy and kinetic studies. It was revealed that the average sizes of Ni-containing particles depend on the nature of cocatalyst and are ∼3.5 and ∼6 nm for the systems derived from Et2AlCl and Et2AlCl·Cl2AlEt, respectively. It was shown that these nanoparticles act as the carriers of catalytically active sites during the C2H4 conversion.

Cobalt Phosphate Cocatalyst Loaded-CdS Nanorod Photoanode with Well-Defined Junctions for Highly Efficient Photoelectrochemical Water Splitting

Cocatalysts play important roles in photocatalytic and photoelectrochemical water splitting reactions. However, the formation of well-defined junctions between low dimensional semiconductors and cocatalysts is still challenging. In this study, CdS nanorod photoanodes loaded with cobalt phosphate (CoPi) cocatalyst were synthesized by a facile two-step route, in which CdS nanorods were prepared using a hydrothermal method followed by photo-assisted electrodeposition of CoPi. It was found that the formation of intimate junctions between CoPi and CdS nanorods in the form of Co–S bonding effectively facilitated the charge separation and lowered the activation energy of the water oxidation reaction. This resulted in highly efficient and stable photoelectrochemical water splitting on the CdS photoanode. The optimal CdS/CoPi photoanode showed a maximum photocurrent of 4.7 mA/cm2 at 0 V versus reversible hydrogen electrode under an AM 1.5 G solar simulator, which was 5.5-fold higher than that of bare CdS photoanode. This work expands the potential application of the cocatalyst CoPi in CdS photoanode systems and improves our understanding of the nature of cocatalysts with well-defined interface junctions in semiconductors. Well-defined interfacial junction with Co–S bonding over cobalt phosphate cocatalyzed CdS nanorod photoanode facilitates the charge separation and lowers the activation energy, thus achieving a considerable photocurrent of 4.7 mA/cm2 at 0 V vs. RHE.

Selective Ethylene Dimerization into 2-Butenes Using Homogeneous and Supported Nickel(II) 2-Iminopyridine Catalysts

In this work, we report a series of novel nickel(II) dibromide and dichloride complexes with 2-iminopyridine and 2-iminoquinoline ligands bearing electron-withdrawing substituents (F, Cl, CF3), that have demonstrated high ethylene dimerization activity [up to 19.2 × 106 g of oligomers·(mol Ni)−1 h−1] in the presence of MAO or Et2AlCl, affording predominantly a mixture of 1-butene and cis- and trans-2-butene (C4 selectivity varies from 89 to 100%; 2-butenes selectivity in the C4 fraction approaches 96%). The effects of ligand substituents and the cocatalyst nature on the activity and selectivity of the nickel(II) complexes in ethylene dimerization have been established. Several nickel complexes were supported on silica-alumina and the resulting heterogeneous catalysts were probed towards ethylene dimerization. These catalysts also afford 2-butenes, with the activity and C4 selectivity being comparable to that of homogeneous catalysts.

Addition Homo‐ and Copolymerizations of Dicyclopentadiene and 5‐n‐Hexylnorbornene in the Presence of Pd‐N‐Heterocyclic Carbene Complexes

AbstractHerein for the first time addition homopolymerization of dicyclopentadiene (DCPD) and its copolymerization with 5‐n‐hexylnorbornene in the presence of different Pd‐N‐heterocyclic carbene complexes activated by a borate (Na+[B(3,5‐(CF3)2C6H5)4]− (NaBARF)) or methylaluminoxane (MAO) are investigated. It is found that systems based on Pd‐N‐heterocyclic carbene complex in combination with NaBARF or MAO are effective catalysts and a Pd‐N‐heterocyclic carbene complex/NaBARF system allows for the addition polymerization of these monomers in air. The influence of the N‐heterocyclic carbene structure, the nature of cocatalyst, and solvent on the catalyst activity of a system based on Pd‐N‐heterocyclic carbene complex are determined. The properties of the obtained addition polydicyclopentadiene (poly(DCPD)) such as thermal stability, solid‐state structure, and porosity are studied. The synthesized poly(DCPD) possesses Brunauer, Emmett, and Teller surface area as high as 155 m2 g−1 and so addition polymerization of DCPD can be considered as a single‐step and convenient way to synthesize of porous polymers from a norbornene derivative.

Enhanced photocatalytic hydrogen production of AgMO3 (M = Ta, Nb, V) perovskite materials using CdS and NiO as co-catalysts

The synthesis of AgMO3 (M = V, Nb, Ta) perovskite materials by solid state reaction and the additional deposition of CdS and NiO particles as co-catalysts using hydrothermal and impregnation methods, respectively, were performed in order to investigate their photocatalytic activities for hydrogen production. The structural and morphological properties were characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM). UV–vis spectroscopy (UV–vis) and physical adsorption analysis by BET method were used for optical and textural properties. The photocatalytic activities for hydrogen production were carried out under UV light in a Na2SO3 aqueous solution. The effect of loading amount and the nature of co-catalyst used is also discussed. The results showed that the electronic band structure of the AgMO3 (M = Ta, Nb, V) perovskite materials and the co-catalysts play an important role in the photocatalytic activity due to their band positions that allow an improvement in the photogenerated charge carrier transfer and in the hydrogen production amount. The best photocatalytic hydrogen production (4645 μmol g−1) was obtained with 5 wt% loading of CdS co-catalyst onto AgTaO3 perovskite compared to AgTaO3 material loaded with 2 wt% of NiO co-catalyst (1847 μmol g−1).

Effect of imidazolium ionic liquids anions on copolymerization of CO2 with cyclohexene oxide by CrIII(Salen)Cl

The copolymerization of carbon dioxide and cyclohexene oxide was investigated without using solvent in the presence of ionic liquids based on 1-butyl-3-methylimidazolium with anions Cl− (BMI.Cl) and BF4− (BMI.BF4) were used as cocatalyst. The catalyst used was chloro (1R,2R)-(−)-[1,2-cyclohexanediamine-N,N‘-bis(3,5-di-tert-butylsalicilidene)] chromium (III) [(Cr(Salen)Cl)]. Parameters like molar ratio catalyst/cocatalyst, pressure of CO2 and the nature of cocatalyst were evaluated. The imidazolium ionic liquids were effective as cocatalyst. The BMI.Cl provided better results than when used BMI.BF4. Poly(cyclohexene carbonate) PCHC with high carbonate units (98%) and lower polydispersity (1.4) was obtained under pressure of 30bar of carbon dioxide and using 10 equivalents of BMI.Cl.

A systematic study of Nickel (II) α-diimine complex performance on ethylene polymerization: influence of cocatalyst nature

Ethylene polymerization was carried out with 1,4-bis(2,6-diisopropylphenyl) acenaphthene diimine-dibromo nickel (II) complex (1) combined with methyl aluminoxane (MAO) and ethylaluminium sesquichloride (EASC). The effect of three factors on three levels [temperature (10, 30, 50 °C), ethylene pressure (3, 5, 7 bar) and cocatalyst to catalyst ratio (CC) (1000, 2000, 3000)] on the polymerization process was investigated individually for both cocatalysts using regression models of responses [catalyst activity, weight average molecular weight (M w) and crystallinity of polymer] and visualized via the response surface method (RSM). For both cocatalysts, activity responses show a second-order variation with temperature, while their pressure dependence is different. The maximum activity of catalyst 1 in the presence of EASC {3907.78 kg [(mol Ni)−1 bar−1 h−1} is higher than maximum activity in the presence of MAO {1091.89 kg [(mol Ni)−1 bar−1 h−1}. Polymerization conditions for reaching the maximum M w are the same for both cocatalysts (10 °C, 7 bar, 3000); it is about 1,330,000 g mol−1 for MAO and 1,512,297 g mol−1 for EASC. Crystallinity responses reveal that EASC concentration has more significant influence on branching content of final polymer especially at lower polymerization temperature. For example at 10 °C and 3 bar, the increase of EASC concentration from 1000 to 3000 leads to the increase of crystallinity from 32 to 45 %. The obtained models provided a promising tool for designing the polymer properties by replacement of MAO with other cocatalysts.

Titanium Bis(amidinates) Bearing Electron Donating Pendant Arms as Catalysts for Stereospecific Polymerization of Propylene

A series of titanium bis(amidinate) complexes containing pendant arms as one of the amidinate N substituents have been prepared and studied in the polymerization of propylene after their activation with MAO and other cocatalysts. The type of pendant arm greatly influences the reactivity and stereospecificity of the resulting polymers. The effect of the cocatalyst nature, its amount, and the time of the reaction have a dramatic effect on the reactivity of a titanium bis(amidinate) bis(dimethylamido) precatalyst containing a furyl group at the pendant arm.

Plasmon Induced Nano Au Particle Decorated over S,N-Modified TiO2 for Exceptional Photocatalytic Hydrogen Evolution under Visible Light

Nano Au deposited mesoporous S,N-TiO2 (SNT) nanocomposites have been fabricated through deposition precipitation technique by employing urea as the hydrolyzing agent. To investigate the structural, optical, and electronic properties, the photocatalysts are characterized through X-ray diffraction (XRD), UV-vis diffuse reflectance spectra, transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), and photoelectrochemical measurements. Here in addition to the co-catalyst nature of nano Au particles, surface plasmon resonance (SPR) effect in visible region enhances the light harvestation ability as well as transfer electrons to the conduction band of SNT. Furthermore, easy channelization of photogenerated charge carriers through sulfate facilitated redox couple makes the system more potential towards H2 evolution. TEM study exhibits well interconnective morphology in the matrix which helps easy channelization of electrons in the SNT nanocomposites. The photocatalytic activities have been evaluated for hydrogen generation under the irradiation of visible light and an enhanced activity has been observed for the Au promoted SNT due to the presence of nano Au particles, that is, 3.5 nm. The hydrogen generation activity of 3Au-SNT is nearly 9 times higher than that of neat SNT, and the energy conversion efficiency was found to be 17.6 %.

Complejos pincer y su comportamiento como iniciadores en polimerización de etileno

En esta revisión se describe la síntesis, caracterización de complejos pincer con metales de transición Pd(II), Ni(II) y Cr(III), su comportamiento catalítico como iniciadores en reacciones de polimerización de etileno en presencia de un cocatalizador. Además, se discute cómo influye en las actividades y propiedades del polímero: la naturaleza y cantidad de cocatalizador, la estructura y propiedades electrónicas de los ligandos y la variación en la temperatura. Los polímeros que se obtienen son lineales, de alta densidad y caracterizados mediante FT-IR, DSC y GPC.

Ethylene polymerization using tris(pyrazolyl)borate titanium(IV) catalyst supported in situ on MAO-modified silica

Abstract The immobilization “in situ” of soluble catalyst TpMs*TiCl3 (1) on MAO-modified silica (4.0 wt.% Al/SiO2) yields active supported catalyst for ethylene polymerization. This supported catalyst showed activities between 132 and 461 kg of PE/mol[Ti] h atm which varies according to the nature of cocatalyst (MAO > TMA > TiBA > IPRA). Upon activation with MAO, no significant impact on the activity has been observed at 30 and 60 °C; however at higher temperature (90 °C) the activity decreases substantially as result of a partial catalyst deactivation. Studies related to the influence of the [Al]/[Ti] molar ratio using TMA as cocatalyst have shown that this supported catalyst presented higher activities using Al concentrations as low as 25:1. The viscosity-average molecular weights ( M ¯ v ) of the PE's produced with 1/SMAO-4 are strongly influenced by the Al concentration varying from 0.41 to 13.6 × 105 g/mol. Resulting polyethylenes can replicate the support morphology. The scanning electron microscopy (SEM) studies revealed that the spherical morphology of the supported catalyst is replicated in the polyethylene particles.

Elastomeric Poly(propylene) from “Dual‐side” Metallocenes: Reversible Chain Transfer and its Influence on Polymer Microstructure

AbstractSummary: The influence of the cocatalyst nature on the distribution of the stereoerrors along the polymer chain has been studied using either MAO or [(C6H5)3C+] [(C6F5)4B−] to activate a C1‐symmetric (Flu‐Ind) complex in propene polymerization experiments. The in situ activation with borate indicated the chain back‐skip as the decisive mechanism responsible for stereoerror formation. When MAO is used for activation, additionally the reversible chain transfer to aluminum occurs, which can be called into account as a second mechanism for stereoerror formation. By the combination of 13C NMR, DSC, WAXS and SFM, it was shown that the differences in polymerization mechanisms result in variations of stereoerror formation. Due to this, the isotactic block length niso as well as their distribution along the chain changes. Using MAO activation, polypropenes with crystallizable blocks consisting of 23–32 monomers in isotactic sequences were generated, which co‐crystallized in α‐ and γ‐phase lamellae. When the reversible chain transfer was occluded (in situ borate activation) the bimodal distribution of crystalline lamellae strongly referred to a homogeneous random distribution of stereoerrors. In this case, two crystalline populations were present. The prevailing one, which crystallized in the orthorhombic γ‐modification, contained 23 consecutive isotactic blocks. Additionally, small amounts of α‐phase lamellae were present consisting of longer isotactic blocks (niso > 35). The different crystalline modifications resulted in different polymer morphologies. These changes caused in turn variations in the mechanical properties, such as elasticity and mechanical strength. This clearly shows that, by using different cocatalysts for activating C1‐symmetric complexes, the properties of poly(propylenes) with statistically distributed stereoerrors can be tailored. magnified image

Titanium Complexes of Tartarate for Ethylene Polymerization

Titanium complexes of tartarate were synthesized by complexation of titanium tetrachloride with tartarate. The isolated products were characterized by compositional, infrared and thermogravimetric analysis. Synthesized complexes in combination with methylaluminoxane as cocatalyst polymerize ethylene to produce high‐density polyethylene. The efficiency of catalysts is found to be dependent on polymerization parameters and nature of cocatalyst. The polyethylene was characterized for melting temperature and molecular weight characteristics by DSC and GPC, respectively.

Ionic and coordination diene polymerization and organic derivatives of main group metals

The effects of the nature of an organic derivative of a main group metal (cocatalyst), its composition, the cocatalyst : transition metal compound ratio, and the way of introducing the cocatalyst on the formation and operation of the active sites in the Ziegler—Natta catalytic systems in the polymerization of conjugated dienes are discussed. A correlation between the cocatalyst nature and the number and kinetic heterogeneity of the active sites is shown.

Kinetic investigation of ethylene polymerization catalyzed by nickel-diimine catalysts

The effect of ethylene pressure on the activity of ethylene polymerization with the complex (1,4-bis(2,6-diisopropylphenyl))-acenaphtenediimine-dichloronickel(II) ( 1) and trimethylaluminum was evaluated. At low ethylene concentrations, the polymerization rate is first order with respect to monomer concentration. At higher ethylene concentrations, the polymerization rate has a negative order with respect to monomer concentration. We propose a mechanism where the active sites are in a dynamic equilibrium with latent states, the later having two monomer molecules coordinated to the metal center. In situ spectroscopic observations corroborate the proposed mechanism and show that the cocatalyst nature might affect the ion pair formation.

The effect of cocatalysts on the oligomerization and cyclization of ethylene catalyzed by zirconocene complexes

Under certain conditions (150°C, P(C 2H 4)=1.4 MPa), the oligomerization of ethylene catalyzed by Cp 2ZrCl 2/EAO (ethylaluminoxane), Et 3Al or (i-Bu) 3Al afforded methylenecyclopentane (MCP) along with chain oligomers. The nature of cocatalysts and the Al/Zr ratios as well as the hydrolysis extent of Et 3Al have tangible effects on the selectivity of the cyclic oligomer. With Et 3Al as cocatalyst under optimal conditions, the oligomerization of ethylene gave MCP and C 4–C 10 chain olefins in a ratio of 45/55.