Bimetallic Cobalt Research Articles

Enantioselective Polymerization of Epoxides Using Biaryl-Linked Bimetallic Cobalt Catalysts: A Mechanistic Study

The enantioselective polymerization of propylene oxide (PO) using biaryl-linked bimetallic salen Co catalysts was investigated experimentally and theoretically. Five key aspects of this catalytic system were examined: (1) the structural features of the catalyst, (2) the regio- and stereoselectivity of the chain-growth step, (3) the probable oxidation and electronic state of Co during the polymerization, (4) the role of the cocatalyst, and (5) the mechanism of monomer enchainment. Several important insights were revealed. First, density functional theory (DFT) calculations provided detailed structural information regarding the regio- and stereoselective chain-growth step. Specifically, the absolute stereochemistry of the binaphthol linker determines the enantiomer preference in the polymerization, and the interaction between the salen ligand and the growing polymer chain is a fundamental aspect of enantioselectivity. Second, a new bimetallic catalyst with a conformationally flexible biphenol linker was synthesized and found to enantioselectively polymerize PO, though with lower enantioselectivity than the binaphthol linked catalysts. Third, DFT calculations revealed that the active form of the catalyst has two active exo anionic ligands (chloride or carboxylate) and an endo polymer alkoxide which can ring-open an adjacent cobalt-coordinated epoxide. Fourth, calculations showed that initiation is favored by an endo chloride ligand, while propagation is favored by the presence of two exo carboxylate ligands.

Magnetic properties of small cobalt–copper clusters

Accurate first-principle calculations on bimetallic cobalt–copper clusters of up to six atoms (Pérez et al 2012 J. Nanopart. Res. 14 933) revealed a close similarity of the ground-state magnetic properties to the ultimate jellium model, provided that a 2D to 3D geometric transition was invoked. We discuss this relationship in terms of partial occupancies of the valence electrons in both cases, with the jellium results described by nonperturbative spherical wavefunctions. Based upon this, we propose a scheme to predict magnetic properties of cobalt–copper clusters of up to twenty atoms using arguments of dimensionality and charge localization, and confirm some of these results with other independent density-functional calculations and experimental available data. The comparison with experiments is carried out for neutral and singly ionized cobalt clusters. Furthermore, a many-body tight-binding pseudopotential is used with Monte Carlo techniques to verify the stability of these new first-principle solutions.

Reversible Carbon–Carbon Bond Formation Induced by Oxidation and Reduction at a Redox-Active Cobalt Complex

The electronic structure of the diamagnetic pyridine imine enamide cobalt dinitrogen complex, ((iPr)PIEA)CoN2 ((iPr)PIEA = 2-(2,6-(i)Pr2-C6H3N═CMe)-6-(2,6-(i)Pr2-C6H3NC═CH2)C5H3N), was determined and is best described as a low-spin cobalt(II) complex antiferromagnetically coupled to an imine radical anion. Addition of potential radical sources such as NO, PhSSPh, or Ph3Cl resulted in C-C coupling at the enamide positions to form bimetallic cobalt compounds. Treatment with the smaller halocarbon, PhCH2Cl, again induced C-C coupling to form a bimetallic bis(imino)pyridine cobalt chloride product but also yielded a monomeric cobalt chloride product where the benzyl group added to the enamide carbon. Similar cooperative metal-ligand addition was observed upon treatment of ((iPr)PIEA)CoN2 with CH2═CHCH2Br, which resulted in allylation of the enamide carbon. Reduction of Coupled-((iPr)PDI)CoCl (Coupled-((iPr)PDI)CoCl = [2-(2,6-(i)Pr2-C6H3N═CMe)-C5H3N-6-(2,6-(i)Pr2-C6H3N═CCH2-)CoCl]2) with NaBEt3H led to quantitative formation of ((iPr)PIEA)CoN2, demonstrating the reversibility of the C-C bond forming reactions. The electronic structures of each of the bimetallic cobalt products were also elucidated by a combination of experimental and computational methods.

Benzene activation and H/D isotope effects in reactions of mixed cobalt platinum clusters: The influence of charge and of composition

A dedicated ion trap served to investigate the reaction with benzene and benzene-d6 under single collision conditions of size selected bimetallic cobalt platinum clusters ConPtm± in the size range n+m≤8. Dehydrogenation by cationic cobalt clusters Con+ is sparse, it is effective in small bimetallic clusters ConPtm+, n+m≤3. Single platinum atoms promote benzene dehydrogenation while further cobalt atoms quench it. Mixed triatomic clusters Co2Pt1− and Co1Pt2− are special in causing effective reactions in a cooperative way. Kinetic isotope effects KIE(n) in total reaction rates are inverse and – in part – large, dehydrogenation isotope effects DIE(n) are normal. A multistep model of adsorption and stepwise dehydrogenation from the precursor adsorbate proves suitable to rationalize the found KIEs and DIEs in principle. The pronounced influence of cluster charge and of cluster composition demands for further investigation.

Pd–Co Coating onto Cordierite Monoliths as Structured Catalysts for Methane Catalytic Combustion

Alumina-supported mono- and bimetallic cobalt (Co) and palladium (Pd) monolithic catalysts were prepared and tested for methane catalytic combustion. The activity test showed that the catalyst containing 0.1 wt % Pd and 0.25 wt % Co was the most promising, with a high activity that did not decline with time on stream. With the aim to explain the relationship between activity–stability and structure, the catalysts were characterized by scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), and temperature-programmed oxidation (TPO) techniques. The high stability displayed by the bimetallic catalysts is attributed to the influence of Co upon hindering the decomposition of PdO into metallic palladium, as a consequence of the increase in oxygen mobility in the spinel phase of cobalt oxides. The intrinsic kinetics of the methane oxidation reaction over various Pd–Co mono- and bimetallic catalysts has been measured in a fixed-bed reactor in the absence of internal and external diffusions, a...

Correlation between Fischer-Tropsch catalytic activity and composition of catalysts

This paper presents the synthesis and characterization of monometallic and bimetallic cobalt and iron nanoparticles supported on alumina. The catalysts were prepared by a wet impregnation method. Samples were characterized using temperature-programmed reduction (TPR), temperature-programmed oxidation (TPO), CO-chemisorption, transmission electron microscopy (TEM), field emission scanning electron microscopy (FESEM-EDX) and N2-adsorption analysis. Fischer-Tropsch synthesis (FTS) was carried out in a fixed-bed microreactor at 543 K and 1 atm, with H2/CO = 2 v/v and space velocity, SV = 12L/g.h. The physicochemical properties and the FTS activity of the bimetallic catalysts were analyzed and compared with those of monometallic cobalt and iron catalysts at similar operating conditions.H2-TPR analysis of cobalt catalyst indicated three temperature regions at 506°C (low), 650°C (medium) and 731°C (high). The incorporation of iron up to 30% into cobalt catalysts increased the reduction, CO chemisorption and number of cobalt active sites of the catalyst while an opposite trend was observed for the iron-riched bimetallic catalysts. The CO conversion was 6.3% and 4.6%, over the monometallic cobalt and iron catalysts, respectively. Bimetallic catalysts enhanced the CO conversion. Amongst the catalysts studied, bimetallic catalyst with the composition of 70Co30Fe showed the highest CO conversion (8.1%) while exhibiting the same product selectivity as that of monometallic Co catalyst. Monometallic iron catalyst showed the lowest selectivity for C5+ hydrocarbons (1.6%).

Exploration of Cocatalyst Effects on a Bimetallic Cobalt Catalyst System: Enhanced Activity and Enantioselectivity in Epoxide Polymerization

Organic ionic compounds were synthesized and investigated as cocatalysts with a bimetallic cobalt complex for enantioselective epoxide polymerization. The identities of both the cation and the anion were systematically varied, and the subsequent reactivity was studied. The nature of the ionic cocatalyst dramatically impacted the rate and enantioselectivity of the catalyst system. The ionic cocatalyst [P(N═P(N(CH2)4)3)4+][tBuCO2–] in combination with a bimetallic cobalt complex produced a catalyst system that exhibited the greatest activity and selectivity for a variety of monosubstituted epoxides.

Use of pyrazolyl ligands for the formation of a bimetallic cobalt–ruthenium complex

A straightforward route to Ru(pyz) 2(Hpyz) 4 (Hpyz = pyrazole) has been developed. This has been found to react readily with a modified Co(pyrazolylborate)Cl complex in the presence of triethylamine as an HCl acceptor to yield a bimetallic cobalt–ruthenium complex, in which the two metals are each six-coordinate, and bridged by three pyrazolyl anions. Both species have been characterized structurally, as has a modified Co(pyrazolylborate) 2 complex.

Enantioselective Epoxide Polymerization Using a Bimetallic Cobalt Catalyst

A highly active enantiopure bimetallic cobalt complex was explored for the enantioselective polymerization of a variety of monosubstituted epoxides. The polymerizations were optimized for high rates and stereoselectivity, with s-factors (k(fast)/k(slow)) for most epoxides exceeding 50 and some exceeding 300, well above the threshold for preparative utility of enantiopure epoxides and isotactic polyethers. Values for mm triads of the resulting polymers are typically greater than 95%, with some even surpassing 98%. In addition, the use of a racemic catalyst allowed the preparation of isotactic polyethers in quantitative yields. The thermal properties of these isotactic polyethers are presented, with many polymers exhibiting high T(m) values. This is the first report of the rapid synthesis of a broad range of highly isotactic polyethers via the enantioselective polymerization of racemic epoxides.

Reactions of 2-chlorobutane with polynuclear bimetallic aluminum and cobalt chloride complexes: low-temperature synthesis and structure of adducts

Co-condensation of aluminum and cobalt chlorides and 2-chlorobutane leads to the formation of triple complexes. Density functional method PBE was used for the quantum chemical calculations of structures and harmonic vibration frequencies. The structures of associates were established by the comparison of experimental and theoretical IR spectra. Introduction of cobalt chloride into the system 2-chlorobutane-aluminum chloride leads to the formation of a triple molecular associate, in which coordination of the organic substrate with the bimetallic complex is accomplished through the transition metal atom. The triple complex is stable in the temperature range 80–240 K and in contrast to the binary complex of butyl chloride with aluminum halide is not capable of self-ionization to form sec-butyl cation.

Bimetallic Cobalt Based Catalysts

Highly dispersed, oxide- or zeolite-supported bimetallic catalysts are widely used in the catalytic industry, such as in catalytic reforming, nitrogen industry and gas-to-liquid technology. The paper highlights the nano-sized Co-based bimetallic system in terms of correlation between structure and reactivity/selectivity promoted by the second metal. Opposite to the bulk alloys nano-sized bimetallic catalysts are extremely sensitive to the structure, morphology, valence state of the supporting oxide material in which the nano-particles are embedded. In this case, one of the less reducible components, which strongly interact with the supports, may stabilize the second, more noble metals, and thus the latter can be stabilized in highly dispersed state. Conversely, addition of noble metal to the hardly reducible component may facilitate reduction, which causes the retardation of the deactivation process of some hydrocarbon reaction. The future trend is the application of bimetallic nano-particles although careful consideration and experimentation should be taken to elucidate the structure of such type of catalyst. The various effects of Co-based bimetallic particles, such as particle size, metal/support interface, morphology and electronic effects, on the activity/selectivity in given catalytic reactions will be discussed.

Isospecific polymerization of racemic epoxides: a catalyst system for the synthesis of highly isotactic polyethers

A highly active bimetallic cobalt catalyst system is reported for the polymerization of racemic terminal epoxides to yield isotactic polyethers.

Cooperative Activation in the Hydrolytic Kinetic Resolution of Epoxides by a Bis-Cobalt(III)salen-Calix[4]arene Hybrid

A chiral, bimetallic cobalt(III)salen-calix[4]arene hybrid structure was prepared and tested in the hydrolytic kinetic resolution (HKR) of racemic epoxides. Kinetic studies have revealed that the two catalytic units on the upper rim of the calix-arene scaffold are able to activate the reactants in a cooperative and primarily intramolecular mode. High enantioselective behaviour was observed and besides, a higher stability was found for the bimetallic catalyst as compared to a monometallic reference complex.

Enantioselective Polymerization of Epoxides: A Highly Active and Selective Catalyst for the Preparation of Stereoregular Polyethers and Enantiopure Epoxides

A chiral, bimetallic cobalt catalyst was discovered that is highly active and enantioselective for epoxide polymerization. The enantiomerically pure catalyst system exhibits a stereoselectivity factor (s = k(fast)/k(slow)) of 370 for propylene oxide, allowing enantiomerically pure epoxide to be recovered in nearly the maximum theoretical yield. In addition, the racemic catalyst forms highly isotactic poly(propylene oxide) in quantitative yield. The catalyst is active and selective for other epoxides, such as 1-butene oxide, 1-hexene oxide, and styrene oxide.

Oriented magnetic nanowires with high coercivity

CoNi magnetic nanowires with a mean diameter of 7 nm and a mean length in the range 100–300 nm have been prepared by reduction of a mixture of cobalt and nickel salts in a sodium hydroxide solution in a liquid polyol. The shape control is related to the particle growth rate which is strongly dependent on the basicity and on the Co–Ni composition. We show that high growth rate favours cobalt urchin-like particles while lower growth rate favours dumbbell-like bimetallic cobalt–nickel particles. Wire formation corresponds to an optimization of the growth conditions and is obtained in a narrow range of basicity and Co–Ni composition. Structural studies showed well crystallized wires with their long axis corresponding to the crystallographic c-axis of the hexagonal close-packed (hcp) phase. Multipods are also observed in which several wires have grown from a single nucleus presenting a cubic structure. Addition of surfactants (trioctyl phosphine and oleic acid) allows the modification of the wire surface and favours their dispersion in toluene. Square hysteresis curves are obtained on wires aligned under magnetic field and frozen in toluene or in PMMA with remanence to saturation ratio close to 1 and coercivity of 6.5 kOe. This value is much higher than was previously obtained on anisotropic particles prepared by the polyol process and results from an improvement of the shape control and the good crystallinity of the wires.

Fischer–Tropsch synthesis on mono- and bimetallic Co and Fe catalysts in fixed-bed and slurry reactors

Abstract Mono- and bimetallic cobalt and iron oxide nanoparticles deposited on the surface of a silica substrate were tested in a fixed-bed reactor and stirred-tank slurry reactor under different process conditions, for the Fischer–Tropsch reaction, this comparison is presented for the first time, being presented in this work a comparison of above systems in both reactor types. For the fixed-bed reaction, the monometallic iron-based catalyst presented the highest activity at low reaction temperatures. By contrast, the monometallic cobalt catalyst is the less active for all reaction temperatures. This catalyst records high values for the α parameter, whereas the iron catalyst reveals major selectivity to alcohols. Moreover, the introduction of iron produces a decrease in hydrocarbon chain growth and an increase in alcohol selectivity. This behaviour might be due to the formation of a Co–Fe alloy, observed by several characterization techniques. In the slurry studies, CO conversion is greater for the cobalt-based catalyst and lower for bimetallic systems, whilst the iron catalyst deactivates in a few hours, which is explained by the formation of carbonaceous deposits. The bimetallic catalysts record a decrease in CO conversion and an enhancement in the selectivity of methane and light hydrocarbons with the increase in iron amount. The absence of alcohols in this type of reactor is explained by the greater solubility of hydrogen, as well as by the greater contact time of reactants, which decreases the possibility of non-dissociative CO insertion.

A kinetic study of multi-walled carbon nanotube synthesis by catalytic chemical vapor deposition using a Fe–Co/Al 2O 3 catalyst

A kinetic study was performed to describe the initial specific rate of multi-walled carbon nanotube synthesis by catalytic chemical vapor deposition (CCVD) on a bimetallic cobalt–iron catalyst at high temperature using ethylene decomposition to solid carbon and gaseous hydrogen. The study uses a mass spectrometer that allows reaction rate to be inferred from the exhaust gas composition measurements. The aim is to obtain a better understanding of the elementary steps involved in the production of carbon nanotubes so as to derive phenomenological kinetic models in agreement with experimental data. The best models assume the elimination of the first hydrogen atom from adsorbed ethylene as rate determining step and involve a hydrogen adsorption weak enough to be neglected. It was proved that hydrogen partial pressure has no influence on initial reaction rate of carbon nanotube synthesis with the catalyst used for this study. Activation energy and ethylene adsorption enthalpy were found to be equal to around 130 and −130 kJ mol −1, respectively.

Solvent dependent assembly and disassembly of a hydrogen bonded helical structure in a Co–Mo bimetallic complex

The effect of solvent on the assembly and disassembly of a hydrogen bonded helical structure in a bimetallic cobalt complex of a molybdenum-containing metalloligand and their spontaneous resolution has been demonstrated.

Ethylene/1,3‐Cyclohexadiene Copolymerization by Means of Methylaluminoxane Activated Half‐Sandwich Complexes

AbstractEthylene copolymerization with 1,3‐cyclohexadiene (CHD) was investigated by using methylaluminoxane (MAO) activated single‐site catalysts including bridged and non‐bridged titanium half‐sandwich and bimetallic cobalt(I) complexes. MAO activated CpTiCl2[NP(tBu)3] (Cp = cyclopentadienyl, tBu = tert‐butyl) gave high molecular weight CHD copolymers without encountering catalyst activity losses. According to the NMR microstructure investigation the resulting copolymers are highly regioregular.

Bimetallic Cobalt Catalysts for the Synthesis of Hydrocarbons from CO and H2

AbstractFor Abstract see ChemInform Abstract in Full Text.