CO Insertion Step Research Articles

Non-Aqueous Liquid Phase Synthesis of Acetic Acid via Ionic Liquid Promoted Homogeneous Carbonylation of Methanol over Ir(III) Catalysts

Abstract Carbonylation of methanol to acetic acid is an aqueous homogeneous catalytic process widely used in industry. In the existing methanol carbonylation industry, a large amount of water (14–15 wt.%) is required in the Monsanto process to inhibit catalyst deactivation, which also consumes a lot of energy to separate the water from the acetic acid product. Here, non-aqueous liquid phase synthesis of acetic acid was carried out by ionic liquid promoted homogeneous carbonylation of methanol over Ir(III) catalysts. It was found that 1-butyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide ([Bmim]Tf2N) and N-butyl pyridinium bis(trifluoromethanesulfonyl)imide ([BPy]Tf2N) could promote the acetic acid selectivity (>98%) and methanol conversion (>99%) under a relatively mild reaction condition of 160 °C and 3.0 MPa. In the reaction, Tf2N− formed an Ir*[Tf2N] complex with Ir to promote the stability of Ir and enhance the activation of CO. From DFT calculation results, the CO insertion was a rate controlling step on the Ir-base catalyst in the reaction cycle, determining the conversion and selectivity of the reaction. The oxygen and nitrogen groups from Tf2N− interacted with Ir to form an Ir*[Tf2N] complex, which could effectively reduce the energy barrier of the CO insertion step, enhancing the selectivity of HAc. Moreover, the catalyst system could be easily recycled and reused with the methanol conversion of 89.66% after five cycles. The methanol carbonylation based on the ionic liquid promoted catalyst is a promising non-aqueous liquid-phase sustainable process.

Mechanistic understanding of methanol carbonylation: Interfacing homogeneous and heterogeneous catalysis via carbon supported Ir[sbnd]La

The creation of heterogeneous analogs to homogeneous catalysts is of great importance to many industrial processes. Acetic acid synthesis via the carbonylation of methanol is one such process and it relies on a difficult-to-separate homogeneous Ir-based catalyst. Using a combination of density functional theory (DFT) and attenuated total reflectance-Fourier transform infrared (ATR-FTIR) spectroscopy, we determine the structure and mechanism for methanol carbonylation over a promising single-site IrLa/C heterogeneous catalyst replacement. Here, the Ir center is the active site with the acetyl-Ir complex being a rate controlling intermediate. Furthermore, the La both atomically disperses the Ir and acts as a Lewis acid site. In fact, the La promoter in the IrLa/C catalyst was found to behave similarly to homogeneous promoters by abstracting an iodine from the Ir center and accelerating the CO insertion step. Overall, this work provides key insight into the atomistic nature of the IrLa/C single-site catalyst and allows for the further design and optimization of single-site heterogeneous catalysts.

Tuning the Fischer–Tropsch reaction over CoxMnyLa/AC catalysts toward alcohols: Effects of La promotion

Selective synthesis of mixed alcohols from syngas (CO + H2) via Fischer–Tropsch synthesis (FTS) is an alternative one-step one-pot method. Here we report a series of Co-based catalysts supported by activated carbon (AC) and promoted by Mn and La, namely, CoxMnyLa/AC, over which the selectivity to alcohols is usually ∼26.8% and up to 31.7% at elevated reaction pressure, accompanied by a large amount of olefin byproducts (∼30%) that can be further converted to alcohols via a hydroformylation process. Detailed characterizations are performed for the structural properties of catalysts, including physisorption, chemisorption, microscopy, crystal structure, and surface element compositions. The active site of CoxMnyLa/AC catalysts is the highly dispersed metallic Co on the surface of Co2C nanoparticles (Co@Co2C), similar to that of CoxMn/AC catalysts reported previously, while that of CoyLa/AC catalysts is a combination of metallic Co and Co2C phases (Co–Co2C). The result indicates that Mn promoter plays a determinative role in the formation of active sites over CoxMnyLa/AC catalysts. Nevertheless, the La promoter tends to aggregate on the surface of cobalt nanoparticles in the form of La2O3 to promote the CO insertion step and enhance the H2 chemisorption, so that it can both facilitate the formation of alcohols and make the FTS product class shift to light components. The result proves that the structure of active sites can be established by adding a strong promoter such as Mn, and a second promoter, such as La, can be used to tune the FTS reaction toward alcohols by further modifying its surface properties.

Ga-promoted CO insertion and C–C coupling on Co catalysts for the synthesis of ethanol and higher alcohols from syngas

This work focuses on Ga promotion to Co sites in supported CoGa catalysts on CO insertion and C–C coupling in syngas conversion. The CoGa catalysts were developed in our group for syngas conversion; they possess uniform dispersion of CoGa particles and pseudo-homogeneous distribution of Co and Ga elements. The Ga atoms contiguous to Co atoms not only contributed to isolating Co centers but also donated electrons to neighboring Co sites. The isolated Co sites were responsible for linearly nondissociative CO adsorption, boosting the CO insertion step to afford alcohol products. Electronic donation from Ga to Co sites promoted dissociative CO adsorption, enhancing carbon-chain growth. The match of CO insertion and C–C coupling gave rise to both enhanced activity in the syngas conversion and impressive selectivity to ethanol and higher alcohols.

Computational Study of Rh-Catalyzed Carboacylation of Olefins: Ligand-Promoted Rhodacycle Isomerization Enables Regioselective C-C Bond Functionalization of Benzocyclobutenones.

The mechanism, reactivity, regio- and enantioselectivity of the Rh-catalyzed carboacylation of benzocyclobutenones are investigated using density functional theory (DFT) calculations. The calculations indicate that the selective activation of the relatively unreactive C1-C2 bond in benzocyclobutenone is achieved via initial C1-C8 bond oxidative addition, followed by rhodacycle isomerization via decarbonylation and CO insertion. Analysis of different ligand steric parameters, ligand steric contour maps, and the computed activation barriers revealed the origin of the positive correlation between ligand bite angle and reactivity. The increase of reactivity with bulkier ligands is attributed to the release of ligand-substrate repulsions in the P-Rh-P plane during the rate-determining CO insertion step. The enantioselectivity in reactions with the (R)-SEGPHOS ligand is controlled by the steric repulsion between the C8 methylene group in the substrate and the equatorial phenyl group on the chiral ligand in the olefin migratory insertion step.

Chain growth mechanism on bimetallic surfaces for higher alcohol synthesis from syngas

Density function theory calculations are performed to investigate the chain growth mechanism on bimetallic surfaces during the syngas conversion. The weighted d-band center correlates well with the adsorption energy of two reactants on bimetallic surface. The boundary between Cu and Co domains facilitates the association reaction of chain growth. Particularly, the reduction of barrier for CO insertion step accelerates the formation of acyl intermediate and thus provides paths to higher alcohol synthesis. The present work demonstrates the synergistic effect in the bimetallic surface from the microscopic view.

A DRIFTS study of CO adsorption and hydrogenation on Cu-based core–shell nanoparticles

Core–shell nanoparticles are being considered for various applications due to their controllable atomic structure and improved properties compared to their bulk counterparts. In the present work, we have synthesized Cu@Mn3O4 and Cu@Co3O4 (core@shell) nanocatalysts using wet-chemical synthesis methods involving organic surfactants, and probed their surfaces using CO and H2 under reaction conditions using diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS). The surfactant ligands used in the synthesis of the nanoparticles must be removed to allow access to the active catalyst sites. These ligands can be removed by oxidation, allowing adsorption of CO and H2. This work reports the DRIFTS results of CO adsorption and hydrogenation on Cu@Mn3O4 and Cu@Co3O4 nanoparticles after removing the ligands. The CO hydrogenation results were in agreement with the DRIFTS results, which suggested that the Cu@Co3O4 nanoparticles adsorb CO both dissociatively and associatively, creating a balance between molecular CO required for CO insertion and dissociated surface carbon species required for chain growth. This resulted in higher selectivities towards C2+ alcohols on this catalyst. On the other hand, the Cu@Mn3O4 nanoparticles showed a higher CO uptake and a lower CO dissociation activity, which resulted in a lower CHx concentration on the surface, thus limiting the rate of the CO insertion step required to form higher alcohols.

Oxidative addition of methyl iodide to [Rh(PhCOCHCOPh)(CO)(P(OCH2)3CCH3)]: an experimental and computational study

Abstract The reaction rate of the oxidative addition and CO insertion steps of methyl iodide with [Rh(PhCOCHCOPh)(CO)(P(OCH2)3CCH3)] are presented. Large negative experimental values for the activation entropy and results from a density functional theory computational chemistry study indicated trans addition of the CH3I to [Rh(PhCOCHCOPh)(CO)(P(OCH2)3CCH3)]. A study of the molecular orbitals gives insight into the flow of electrons during the oxidative addition reaction. CO insertion leads to a square pyramidal [Rh(PhCOCHCOPh)(P(OCH2)3CCH3)(COCH3)(I)] acyl product with the COCH3 moiety in the apical position. The strong electron donation of the P(OCH2)3CCH3 ligand accelerates the oxidation addition step of methyl iodide to [Rh(PhCOCHCOPh)(CO)(P(OCH2)3CCH3)] by ca. 265 times faster (at 35°C) than that of the Monsanto catalyst, but inhibits the CO insertion step.

Oxidative addition of methyl iodide to [Rh(CH3COCHCOCH3)(CO)(P(OCH2)3CCH3)

The reaction rate of the oxidative addition and the following CO insertion step of methyl iodide with [Rh(acac)(CO)(P(OCH2)3CCH3)] is determined. The key finding is that while [Rh(acac)(CO)(P(OCH2)3CCH3)] oxidatively adds methyl iodide ca 300 times faster than the Monsanto catalyst, the CO insertion step is much slower. However, the rate-determining step of the oxidative addition reaction of the phosphorus-containing acetylacetonato-rhodium(I) complex, the carbonyl insertion step, is still in the same order or faster than the rate-determining oxidative addition step of iodomethane to [Rh(CO)2I2]−.

Generation of α-Phosphinocarbene Complexes and Their Evolution: New Light on Relevant Isomerization Pathways

The η1-α-phosphinocarbene complex Cp(CO)2Mn═CR(PHMes) (R = Ph, Me) is generated by nucleophilic attack of MesPH2 at the electrophilic center of the cationic Mn−carbyne complex [Cp(CO)2Mn≡CR]+, followed by deprotonation. One of its identified isomerization pathways to η1-phosphaalkene derivatives is shown to be both base-catalyzed and stereoselective, and to involve a transient phosphidocarbene species engaged in an acid-catalyzed migratory CO-insertion step.

Insertion and carbonylation reactions of styrene promoted by [HRh(CO)x‐ (PMe3)3−x] (x = 1, 2) compounds: A theoretical investigation

AbstractIn this work we have applied Quantum Mechanical calculations to investigate the first two elementary steps (olefin insertion and carbonylation) in the hydroformylation of styrene, using the model catalysts of the type [HRh(CO)x(PMe3)3−x] (x = 1, 2), which are supposed to be the catalytic species that will be present depending on the CO pressure. The migratory styrene insertion reaction and CO insertion reaction into the metal‐alkyl bond were investigated at the MP4(SDQ) level using the BP86 optimized geometries. Additionally the Spin Component Scale procedure on the MP2 and MP3 energies (SCS‐MP2 and SCS‐MP3) was also applied. It is shown that, at normal hydroformylation conditions and normal CO and H2 pressure, the active catalytic species is preferentially formed in trans arrangement, trans‐[HRh(CO)(PMe3)2], 2b. Because of the greater steric hindrance around the rhodium atom, the electronic effects of the phosphine do not contribute significantly to the stability of the catalyst. The MPn calculations overestimate the stability of the π‐complexes, in comparison with the BP86 value, as much as 25 kcal/mol, with large fluctuations along the perturbation series. The activation energies predicted by the BP86 method, in comparison with the MPn results, are underestimated about 5 kcal/mol. The reaction and the coordination energies are very sensitive to the theoretical level employed. Our results indicate that the competitive trapping of the styrene by different catalytic species, one leading to the branched and the other leading to the linear species, as was experimentally proposed to explain the selectivity, only seems to hold if we consider the subsequent CO insertion step. This assumption based only on the olefin insertion reaction, as the selectivity determining step, does not apply since the branched product will always preferentially be formed. All methods employed here predict the selectivity in good agreement with the experimental selectivity of 70–95% in favor of the branched product. © 2008 Wiley Periodicals, Inc. Int J Quantum Chem, 2008

Cooperative effect between iridium and platinum in the carbonylation of methanol to acetic acid

The iodocarbonyl monomer [PtI2(CO)2] 7 promotes the iridium catalyzed carbonylation of methanol to acetic acid at low water contents. Studies based on low pressure or high pressure NMR and the use of labeled reactants were conducted close to the real conditions of catalysis in order to get a deeper insight into this system. Carbonylation of CH3I at low water contents proceeds slowly and the migratory CO insertion step, leading from H[IrI3(CH3)(CO)2] 2-H to H[IrI3(COCH3)(CO)2] 6-H is rate limiting. The dimer [PtI2(CO)]2 7′ reacts immediately with [PPN][IrI3(CH3)(CO)2] 2-PPN (PPN is Ph3P=N+=PPh3) under nitrogen to afford a mixture of species, among which the key heterobinuclear [Ir–Pt] intermediate [PPN][IrI2(CH3)(CO)2(μ-I)PtI2(CO)] 8-PPN has been identified; [PPN][IrI2(CH3)(CO)2(μ-I)PtI2(CO)] 8-PPN can in its turn lead to the formation of [PPN][PtI3(CO)] 9-PPN, [IrI2(CH3)(CO)2(solv)] 10, [Ir2I2(CH3)2(μ-I)2(CO)4] 3′ and [PPN][Ir2I4(CH3)2(μ-I)(CO)4] 11-PPN; all of these species have been characterized. Under CO pressure, [PPN][IrI2(CH3)(CO)2(μ-I)PtI2(CO)] 8-PPN is a short-lived species that quickly leads to [IrI2(CH3)(CO)3] 4 and [PPN][PtI3(CO)] 9-PPN showing that the main role of the platinum promoter is to abstract an I− ligand from [PPN][IrI3(CH3)(CO)2] 2-PPN. Under catalytic conditions, I− is abstracted from H[IrI3(CH3)(CO)2] by [PtI2(CO)2] 7 and the rate determining step is accelerated; the relevant species H[IrI3(CH3)(CO)2] 2-H, H[IrI3(COCH3) (CO)2] 6-H and H[PtI3(CO)] 9-H have been observed under 30 bar of CO. A catalytic cycle is proposed, which depicts the cooperative effect between the iridium catalyst and the platinum promoter.

Kinetics of CO Insertion and Acetyl Group Transfer Steps, and a Model of the Acetyl-CoA Synthase Catalytic Mechanism

Acetyl-CoA synthase/carbon monoxide dehydrogenase is a Ni-Fe-S-containing enzyme that catalyzes the synthesis of acetyl-CoA from CO, CoA, and a methyl group. The methyl group is transferred onto the enzyme from a corrinoid-iron-sulfur protein (CoFeSP). The kinetics of two steps within the catalytic mechanism were studied using the stopped-flow method, including the insertion of CO into a putative Ni(2+)-CH(3) bond and the transfer of the resulting acetyl group to CoA. Neither step had been studied previously. Reactions were monitored indirectly, starting with the methylated intermediate form of the enzyme. Resulting traces were analyzed by constructing a simple kinetic model describing the catalytic mechanism under reducing conditions. Besides methyl group transfer, CO insertion, and acetyl group transfer, fitting to experimental traces required the inclusion of an inhibitory step in which CO reversibly bound to the form of the enzyme obtained immediately after product release. Global simulation of the reported datasets afforded a consistent set of kinetic parameters. The equilibrium constant for the overall synthesis of acetyl-CoA was estimated and compared to the product of the individual equilibrium constants. Simulations obtained with the model duplicated the essential behavior of the enzyme, in terms of the variation of activity with [CO], and the time-dependent decay of the NiFeC EPR signal upon reaction with CoFeSP. Under standard assay conditions, the model suggests that the vast majority of active enzyme molecules in a population should be in the methylated form, suggesting that the subsequent catalytic step, namely CO insertion, is rate limiting. This conclusion is further supported by a sensitivity analysis showing that the rate is most sensitively affected by a change in the rate coefficient associated with the CO insertion step.

(Diphosphane monosulfide)platinum(II) Complexes for Hydroformylation Reactions: Their Catalytic Activity and a High‐Pressure NMR Mechanistic Study

AbstractThe neutral complexes of formula [κ2P,S‐(dppmS)Pt(CH3)(Cl)] (1), [κ2P,S‐(dppeS)Pt(CH3)(Cl)] (2) and [κ2P,S‐(dpppS)Pt(CH3)(Cl)] (3) [dppmS = Ph2PCH2P(S)Ph2; dppeS = Ph2P(CH2)2P(S)Ph2; dpppS = Ph2P(CH2)3P(S)Ph2] are active catalyst precursors for the hydroformylation of 1‐octene in methyl isobutyl ketone. The order of reactivity found is3 > 2 > 1. Surprisingly, the cationic complexes [κ2P,S‐(dppeS)Pt(CH3)(CH3CN)]BF4 (4a) and [{κP,μ‐κS‐(dppeS)Pt(CH3)}2][BF4]2 (5) are less active than the analogous neutral complex 2. High‐pressure NMR studies revealed that, at 20 °C under 1 to 50 bar of syngas, and in the presence of SnCl2, both 2 and 4a react immediately to form the same acetyl complex (8). However, in the absence of SnCl2, the methyl carbonyl complex [κ2P,S‐(dppeS)Pt(CH3)(CO)]+ (9), which is formed from 4a and CO, does not undergo insertion to give the acetyl complex, even under 50 bar of syngas. Thus, the role of SnCl2 is not only to create a vacant site for CO coordination and to lower the energy barrier for the hydrogenolysis, but also to assist migration of the alkyl group in the CO insertion step. High‐pressure NMR studies of the working reaction solution, under steady‐state conditions, found no evidence for intermediates in which the phosphane sulfide group shows hemilabile behaviour during catalysis. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2006)

Analysis of the Synergistic Effect of Carbonylplatinum Complexes on the Iridium‐Catalysed Carbonylation of Methanol to Acetic Acid

AbstractHigh‐pressure NMR mechanistic investigations into the promoting role of dicarbonyldiiodoplatinum complex in the iridium‐catalysed carbonylation of methanol to acetic acid are consistent with iodide abstraction from the [IrI3(CH3)(CO)2]– resting state species by [PtI2(CO)2], unbolting the rate‐limiting migratory CO insertion step. A new [Ir–Pt] heterodimetallic complex, which presumably represents the key species involved in the abstraction of I–, has been characterised by NMR in tandem with FAB‐MS. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2006)

Detection of Intermediates in Cobalt-Catalyzed Hydroformylation Using para-Hydrogen-Induced Polarization

para-Hydrogen-induced polarization methods are shown to enable the in situ detection of linear and branched monophosphine-containing intermediates during hydroformylation when Co(eta3-C3H5)(CO)2(PCy3) is the catalyst precursor. The NMR signal characteristics of the alkyl arms of these species provide direct evidence for the rapid interconversion of linear and branched cobalt alkyls prior to the CO insertion step. The observation of additional para-hydrogen-enhanced signals for the corresponding linear and branched aldehydes enables the reactions selectivity to be rapidly monitored as a function of H2 and CO pressure or reaction temperature.

On the importance of electronic and steric effects in the migratory CO insertion step of rhodium–diphosphine catalyzed methanol carbonylation

To provide further insight into electronic and steric factors and to quantify their relative importance, we studied in detail the migratory CO insertion step for RhMe(CO)I2(L–L) systems (L–L = dppms (PPh2CH2P(S)Ph2) or dppe (PPh2CH2CH2PPh2)), Monsanto catalysts and some electronically unsymmetrical diphosphine model systems. The difference in the reaction rates of dppms and dppe has a clear electronic origin that reflects the different properties of sulfide phosphine (π-donor) and phosphine (π-acid) ligands. Molecular orbital calculations clearly show that dppms strongly increases back-bonding to CO and favors the overlap between CO and methyl. Steric effects modulate the barrier, which decreases more for dppe than it does for dppms. For dppms, the electronic contribution that phenyl phosphine substituents make to lower the barrier is greater than that made by purely steric effects. The sulfide phosphine ligand dppms accelerates the carbonyl insertion because of its π-donor capability. For the diphosphine ligands we studied, the energy barrier varied gradually as basicity varied, and the slowest kinetics is shown by the most electron-donating ligand. The basicity dependence is stronger when the phosphine ligand occupies a trans position to CO. On the other hand and in unsymmetrical diphosphine complexes, phosphine basicity affects stability and reactivity in opposite ways.

DFT Study of the Ethylene Hydroformylation Catalytic Cycle Employing a HRh(PH3)2(CO) Model Catalyst

The potential energy hypersurface for ethylene hydroformylation catalyzed by HRh(PH3)2(CO) was mapped out at the CCSD(T)//B3LYP and B3LYP//B3LYP levels of theory using effective core potentials. Combining the results obtained for each elementary step there are a number of possible pathways for the hydroformylation catalytic cycle, originating from the trans (2a) and cis (2b) isomers of the active catalyst. At both levels of theory employed, a preference was predicted for the pathways originating from the trans isomer of the active catalyst, 2a. The alternative pathways originating from the cis isomer, 2b, were discounted because of the large activation barriers predicted for the two migratory insertion reactions, arising from unfavorable interactions between the equatorial phosphine ligands and the migrating axial ligand. Considering only those reaction paths originating from the trans isomer of the active catalyst, 2a, a strong preference was identified for the oxidative addition of H2 to the unsaturated Rh-acyl complex (6a) on the same side as the ethyl moiety of the acyl ligand, as opposed to the addition on the opposite face (i.e., on the same side as the acyl oxygen). In the final aldehyde reductive elimination step an energetic preference was predicted for the migration of the hydride ligand trans to the CO ligand in the most stable H2 oxidative addition products; however, this migration would lead to the generation of the cis catalyst instead of the trans catalyst. Therefore, either the less electronically favored hydride ligand trans to the PH3 ligand migrates to the acyl carbon, thereby regenerating 2a, or there must be some interconversion between the pathways originating from 2a and 2b. This interconversion would most likely occur at either the η2-olefin adduct (3b) or the CO addition intermediate (5b), since previous research indicates that complexes of this type can undergo facile pseudorotation. For the energetically feasible catalytic cycle, the CO insertion step is predicted to be the rate-determining step with predicted activation barriers of 20.4 and 14.9 kcal/mol, at the CCSD(T)//B3LYP and B3LYP//B3LYP levels of theory, respectively. The experimental enthalpy of hydroformylation (−28 kcal/mol), corresponding to the energy difference between the end aldehyde product and the constituent reactant species, C2H4, CO, and H2, is overestimated by about 7 kcal/mol (−34.7 kcal/mol) at the B3LYP//B3LYP level. However, recomputing the energies of the species with the CCSD(T) methodology yields a value of −24.4 kcal/mol, which is more in accord with experiment.

Annulation With and Without CO-Insertion: Striking a Balance in Reactivity of Chromium Carbene Complexes with Chalcogen-Stabilized Iron Cluster Appendage

Two new sets of products [(CO)6Fe2EE′{μ-indanone}] (2a: E, E′=Se; 2b: E=S, E′=Se) and [(CO)6Fe2EE′{μ-(2-ethoxy, α-naphthol)}] (3a: E, E′=Se; 3b: E=S, E′=Se; 3c: E=S, E′=Te) have been isolated from the thermolysis of the cluster-bound Fischer carbene complex, [(CO)6Fe2EE′{μ-C(Ph)C–C(OEt)Cr(CO)5}], 1a–c (1a: E, E′=Se; 1b: E=S, E′=Se; 1c: E=S, E′=Te) in THF. The products are formed by annulation pathway with and without the intermediacy of CO insertion step, from the same substrate. They have been characterized by IR and 1H, 13C, 77Se and 125Te NMR spectroscopy and the structural types were unequivocally established by crystallographic analysis of compounds 2a and 3c.

Chiral CO-emulating ligands: From arene chromium chemistry to enantioselective catalysis

Abstract