CO Insertion Research Articles

Molybdenum tailored Co0/Co2+ active pairs on a perovskite-type oxide for direct ethanol synthesis from syngas

Selective synthesis of ethanol from syngas under the Co-based catalysts is still challenging due to the hard of regulating the active site Co0 and Co2+ ratio. In this work, a series of CaTi0.9−xCoxMo0.1O3 (x = 0, 0.1–0.4) and CaTi0.7Co0.3O3 catalysts were prepared by using citric acid complexation method to promote the synthesis of ethanol. It was found that Mo species in the perovskite lattice can regulate the Co0 and Co2+ ratio through the domain-limiting effect of perovskite and the degree of Co reduction could be adjusted by changing the Co/Mo molar ratio. Among these investigated catalysts, the total selectivity of alcohols over the catalyst with the optimal Co/Mo ratio CaTi0.6Co0.3Mo0.1O3 reached 39.1%, with ethanol accounting for 74.7%, which was ascribed to the moderate and tightly bound ratio of dissociative to non-dissociative adsorption sites on the surface and the balance of CHx-CHy coupling and CO insertion.

DFT Investigations of the Reaction Mechanism of Dimethyl Carbonate Synthesis from Methanol and CO on Various Cu Species in Y Zeolites

In this study, a density functional theory method is employed to investigate the reaction mechanisms of dimethyl carbonate (DMC) formation, through oxidative carbonylation of methanol, on four types of Y zeolites doped with Cu+, Cu2+, Cu2O and CuO, respectively. A common chemical route is found for these zeolites and identified as, first, the adsorbed CH3OH is oxidized to CH3O species; subsequently, CO inserts into CH3O to CH3OCO, which reacts with CH3O to form DMC rapidly; and finally, the adsorbed DMC is released into the gas phase. The rate-limiting step on Cu2+Y zeolite is identified as oxidation of CH3OH to CH3O with activation barrier of 66.73 kJ·mol−1. While for Cu+Y, Cu2O-Y and CuO-Y zeolites, the rate-limiting step is insertion of CO into CH3O, and the corresponding activation barriers are 63.73, 60.01 and 104.64 kJ·mol−1, respectively. For Cu+Y, Cu2+Y and Cu2O-Y zeolites, adsorbed CH3OH is oxidized to CH3O with the presence of oxygen, whereas oxidation of CH3OH on CuO-Y is caused by the lattice oxygen of CuO. The order of catalytic activities of these four types of zeolites with different Cu states follows Cu+Y ≈ Cu2O-Y > Cu2+Y > CuO-Y zeolite. Therefore, CuY catalysts with Cu+ and Cu2O as dominated Cu species are beneficial to the formation of DMC.

Insertion of CO2 in metal ion-doped two-dimensional covalent organic frameworks

Carbon capture is one of the essential low-carbon technologies required to achieve societal climate goals at the lowest cost. Covalent organic frameworks (COFs) are promising adsorbents for CO2 capture because of their well-defined porosity, large surface area, and high stability. Current COF-based CO2 capture is mainly based on a physisorption mechanism, exhibiting smooth and reversible sorption isotherms. In the present study, we report unusual CO2 sorption isotherms featuring one or more tunable hysteresis steps with metal ion (Fe3+, Cr3+, or In3+)-doped Schiff-base two-dimensional (2D) COFs (Py-1P, Py-TT, and Py-Py) as adsorbents. Synchrotron X-ray diffraction, spectroscopic and computational studies indicate that the sharp adsorption steps in the isotherm originate from the insertion of CO2 between the metal ion and the N atom of the imine bond on the inner pore surface of the COFs as the CO2 pressure reaches threshold values. As a result, the CO2 adsorption capacity of the ion-doped Py-1P COF is increased by 89.5% compared with that of the undoped Py-1P COF. This CO2 sorption mechanism provides an efficient and straightforward approach to enhancing the CO2 capture capacity of COF-based adsorbents, yielding insights into developing chemistry for CO2 capture and conversion.

Computational Study of Mechanisms and Tether Length Effects of Rh-Catalyzed [3+2] and [3+2+1] Reactions of Ene/Yne-Vinylcyclopropanes.

DFT calculations have been applied to study the mechanisms of [3+2] and [3+2+1] reactions of ene/yne-vinylcyclopropanes (shorted as ene/yne-VCPs). The [3+2] reactions of ene/yne-VCPs start from C-C cleavage of cyclopropane (CP cleavage) to form six-membered rhodacycle, followed by alkene/alkyne insertion and reductive elimination. The [3+2+1] reactions have two competing pathways, one is the [3+2+1] pathway (CP cleavage, ene/yne insertion, CO insertion and reductive elimination) and the other is the [3+1+2] pathway (CP cleavage, CO insertion, ene/yne insertion and reductive elimination). The length of tether in substrates affects the ene/yne insertion steps in these cycloadditions, making some reactions fail or changing the reaction pathways. The reasons for these tether length effects are discussed.

Exploring the Parameters Controlling Product Selectivity in Electrochemical CO2 Reduction in Competition with Hydrogen Evolution Employing Manganese Bipyridine Complexes.

Selective reduction of CO2 is an efficient solution for producing nonfossil-based chemical feedstocks and simultaneously alleviating the increasing atmospheric concentration of this greenhouse gas. With this aim, molecular electrocatalysts are being extensively studied, although selectivity remains an issue. In this work, a combined experimental-computational study explores how the molecular structure of Mn-based complexes determines the dominant product in the reduction of CO2 to HCOOH, CO, and H2. In contrast to previous Mn(bpy-R)(CO)3Br catalysts containing alkyl amines in the vicinity of the Br ligand, here, we report that bpy-based macrocycles locking these amines at the side opposite to the Br ligand change the product selectivity from HCOOH to H2. Ab initio molecular dynamics simulations of the active species showed that free rotation of the Mn(CO)3 moiety allows for the approach of the protonated amine to the reactive center yielding a Mn-hydride intermediate, which is the key in the formation of H2 and HCOOH. Additional studies with DFT methods showed that the macrocyclic moiety hinders the insertion of CO2 to the metal hydride favoring the formation of H2 over HCOOH. Further, our results suggest that the minor CO product observed experimentally is formed when CO2 adds to Mn on the side opposite to the amine ligand before protonation. These results show how product selectivity can be modulated by ligand design in Mn-based catalysts, providing atomistic details that can be leveraged in the development of a fully selective system.

Revealing and Regulating the Complex Reaction Mechanism of CO2 Hydrogenation to Higher Alcohols on Multifunctional Tandem Catalysts

Revealing and regulating the intricate reaction mechanism of direct CO2 hydrogenation to higher alcohols (C2+OH), especially for the crucial C–C coupling step, is still a great challenge. Herein, the specific reaction network on Co2C and CuZnAl multifunctional tandem catalysts is elucidated by designing subtly surface adsorption–desorption reactions, in situ chemical transient kinetics, and theory calculations. The key C–C coupling step for the formation of C2+OH over the sole Co2C catalyst was the insertion of CO into R-CHx, while the reaction mechanism can be modulated to the coupling of R-CH2 and CHO with a lower energy barrier on the tandem catalyst (Co2C||CuZnAl). R-CH2 was derived from the hydrogenation dissociation of olefins and coupled with the CHO from formate hydrogenation at the Cu/ZnAl2O4 interface. Such multifunctional tandem catalysts exhibited a high space–time yield of C2+OH of 2.2 mmol g–1 h–1. This work provides an effective strategy for studying complex mechanisms, contributing to the precise design of highly efficient catalysts and the optimization of reaction pathways.

Syngas-to-C2 oxygenates over the inverse Mo6C4/Cu catalyst: Identifying the role of synergistic effect

Syngas transformation into C2 oxygenates has attracted a long-term interest, however, it still faces the challenge of low selectivity and yield toward C2 oxygenates. In this study, the inverse Mo6C4/Cu catalyst is reasonably modeled and predicted to promote C2 oxygenates formation in syngas transformation using DFT calculations with the PBE functional. The results confirmed that the inverse Mo6C4/Cu catalyst not only greatly elevates catalytic performance toward CHx(x = 2, 3) monomer formation, but also facilitate C2 oxygenate production compared to the previously reported Cu4/β-Mo2C, β-Mo2C and Cu catalysts, which is attributed to the synergistic effect of Mo6C4 cluster with Cu catalyst, specifically, Mo6C4 cluster easily activates CO to produce CH2 monomer, Cu offers the undissociated CO migrated toward Mo6C4 cluster, then, CO insertion into CH2 to CH2CO over Mo6C4 cluster easily occurs. Our results offer a theoretical basis for the synergistic effect between Mo6C4 cluster with Cu catalyst in tuning catalytic performance of syngas transformation and gives a new way to rationally design the inverse catalyst with excellent performance for C2 oxygenate production from syngas.

Dihydroxybenzene‐derived ILs as Halide‐Free, Single‐component Organocatalysts for CO2 Insertion Reactions

AbstractA series of dihydroxybenzene‐derived ILs was synthesised via a halide‐free, eco‐friendly methodology and fully characterized. Their activity as single component catalyst towards synthesis of cyclic organic carbonates (COCs) via CO2 insertion into terminal epoxides was evaluated, observing that methyltrioctylammonium hydroquinolate, [N1888][HYD], was the most active catalyst in the proposed optimized conditions ([N1888][HYD] 10 % mol, T=120 °C, t=6 h, p0(CO2)=2.0 MPa, 12 examples, conversion >99 %, yield up to 98 %). Interestingly, [N1888][HYD] was also an active catalyst for CO2 insertion reactions with cyclohexene oxide (CHO), observing formation of both the COC and polycarbonate product. It is proposed that for p0(CO2)≥1.0 MPa, the catalytically active species is the hemicarbonate derivative of the hydroquinolate anion, active towards epoxide ring opening via an unusual hemicarbonate‐alkoxide pathway.

Small, Electron-Donating Substituents Give CO2 Activation by Permethylpentalene Zirconium Amido Complexes the Upper Hand: A DFT Study of Distortion and Interaction

An insight into factors controlling CO2 activation is necessary to develop molecular systems that utilize CO2 as a chemical feedstock. Two permethylpentalene zirconium cyclopentadienyl (mono)amido complexes, Pn*ZrCp(NR2), were previously assessed for CO2 activation, and a strong dependence on the amido substituent was observed. The R = Me analogue reacted rapidly and quantitatively at room temperature to form the carbamato complex, while the R = Ph species was inert. Here, we investigate the origin of this reactivity difference using DFT and the distortion-interaction model to characterize steric and electronic contributions to the activation barrier. We find that the barrier for CO2 insertion with R = Me (19.1 kcal/mol) is lower than with R = Ph (36.6 kcal/mol), explaining the inertness of the Ph-substituted analogue. The distortion energy trend follows the steric bulk of the amido substituents, and the bulkier Ph-substituted complex has a consistently higher distortion energy along its potential energy surface than that of the Me-substituted complex. The interaction energy trend follows the electronics, and a more electron-donating Me-substituted complex shows a consistently lower interaction energy. The balance of these effects at the corresponding TS gives a reduced activation barrier. Small, electron-donating substituents therefore facilitate CO2 activation in these complexes.

Insights into the Fischer–Tropsch mechanism on χ-Fe5C2(510) based on the hydrogen coverage effect

Investigating the mechanism of the Fischer–Tropsch synthesis is beneficial for modifying catalysts, which would result in an improved catalyst performance. However, theoretical calculations have mainly focused on several key elementary reactions and have thus provided fragmented information. Here, we select χ-Fe5C2(510) as representative facet and consider the elementary reactions on as comprehensively as possible via the density-functional theory (DFT)-based kinetic Monte Carlo (kMC) method to generate the fine structure of the Fischer–Tropsch mechanism. The results reveal that the CO activation mechanism is divided into cyclical pathways and termination pathways. The chain growth process and chain dissociation process coexist in the Fischer-Tropsch synthesis while the former follows the CO insertion mechanism, and the latter follows the carbide mechanism. The product methane and ethylene are produced major from the intermediates which are generated in the chain dissociation process rather than the chain grown process as the article reported before. The deep interpretation of the Fischer–Tropsch mechanism from the fine structure could be the guidance to experimentalists in mechanism verification or catalyst design.

Base-Mediated Direct Carboxylation of Heteroarenes with CO2

AbstractA base medium system, generated from a synergistic reaction between Cs2CO3 and t-BuOK, has been used for the direct carboxylation of imidazoles with CO2 in good yield. Oxygen- and thia-heterocyclic arenes also undergo the carboxylation with CO2 with the same reactivity. Density functional theory calculations show that construction of t-BuOCs in situ is an endothermic process of 38.31 kJ/mol. t-BuOCs can promote the deprotonation of substrates by forming an active C–Cs bond via a rather low HOMO (t-BuOCs)–LUMO (imidazole) gap of 0.28 eV, which favors the electrophilic insertion of CO2 by releasing 94.13 kJ/mol of heat.

Formation of Value-Added Cyclic Carbonates by Coupling of Epoxides and CO2by Ruthenium Pincer Hydrazone Complexes under Atmospheric Pressure

Transforming atmospheric CO2 into industrially valuable cyclic carbonates is of great importance and one of the attractive approaches for reducing CO2 concentration in the biosphere. However, the selectivity and conversion of ruthenium catalysts for cyclic carbonate formation via CO2 insertion to epoxides are low at high pressure. Here, a series of novel ruthenium complexes supported by ONN pincer-type hydrazone ligands were successfully generated, and their potential for CO2 valorization to cyclic carbonates was investigated. NMR, Fourier transform infrared (FT-IR), mass spectrometry, elemental analysis, and single-crystal diffraction analysis further confirmed the structures of these ruthenium compounds. These compounds were utilized for the CO2/coupling reaction after thorough characterization. Fortunately, these complexes outperform the reported ruthenium catalysts with a turn-over frequency of 833 h–1 and >99% selectivity at atmospheric pressure without any assisted cocatalyst. The designed catalyst showed consistent catalytic performance over five repeating cycles.

Selectivity of Oxygen Evolution Reaction on Carbon Cloth-Supported δ-MnO2 Nanosheets in Electrolysis of Real Seawater.

Electrolysis of seawater using solar and wind energy is a promising technology for hydrogen production which is not affected by the shortage of freshwater resources. However, the competition of chlorine evolution reactions and oxygen evolution reactions on the anode is a major obstacle in the upscaling of seawater electrolyzers for hydrogen production and energy storage, which require chlorine-inhibited oxygen evolution electrodes to become commercially viable. In this study, such an electrode was prepared by growing δ-MnO2 nanosheet arrays on the carbon cloth surface. The selectivity of the newly prepared anode towards the oxygen evolution reaction (OER) was 66.3% after 30 min of electrolyzer operation. The insertion of Fe, Co and Ni ions into MnO2 nanosheets resulted in an increased number of trivalent Mn atoms, which had a negative effect on the OER selectivity. Good tolerance of MnO2/CC electrodes to chlorine evolution in seawater electrolysis indicates its suitability for upscaling this important energy conversion and storage technology.

Rhodium‐Catalyzed Transannulation of 1,2,3‐Thiazoles with Alkynes for the Direct Synthesis of Thiochromenones

Comprehensive SummaryA rhodium‐catalyzed denitrogenative carbonylation of 1,2,3‐thiadiazole with alkynes to construct (benzo)thiochromenones is developed. Both terminal and internal alkynes were compatible in this [3+2+1] cycloaddition, affording a broad range of poly‐substituted thiochromenones. Unsymmetrical alkyne substrates exhibited good regioselectivity. It is noteworthy that the present strategy could be used to construct both monocyclic and benzo‐thiochromenones, and it is a novel example of denitrogenative transannulation with CO insertion.

Crown ether as organocatalyst for reductive upgrading of CO2 to N-containing benzoheterocyclics and N-formamides

Constructions of heterocyclics by incorporation of CO2 into N-, O- and S-nucleophiles represent advance of synthetic methodology by using CO2 as a C1 feedstock. Herein, we reported crown ether (e.g. 18-crown-6)-catalyzed reductive functionalization of CO2 to various N-containing benzoheterocyclics and N-formamides in the presence of PhSiH3 under metal-free and mild conditions. Lewis acid–base interaction between PhSiH3 and 18-crown-6 enhanced hydride-donating ability of the PhSiH3, thus facilitating the CO2 insertion into the Si–H bond. A positive correlation was observed between electron-donating ability and catalytic activity of the investigated crown ethers on CO2 reduction. Crown ethers thus represent a novel organocatalytic activation model for CO2 reduction, demonstrating an important alternative for transition-metal-based catalysts.

Comparative study of CO2 insertion into pincer supported palladium alkyl and aryl complexes.

The insertion of CO2 into metal alkyl bonds is a crucial elementary step in transition metal-catalyzed processes for CO2 utilization. Here, we synthesize pincer-supported palladium complexes of the type (tBuPBP)Pd(alkyl) (tBuPBP = B(NCH2PtBu2)2C6H4-; alkyl = CH2CH3, CH2CH2CH3, CH2C6H5, and CH2-4-OMe-C6H4) and (tBuPBP)Pd(C6H5) and compare the rates of CO2 insertion into the palladium alkyl bonds to form metal carboxylate complexes. Although, the rate constant for CO2 insertion into (tBuPBP)Pd(CH2CH3) is more than double the rate constant we previously measured for insertion into the palladium methyl complex (tBuPBP)Pd(CH3), insertion into (tBuPBP)Pd(CH2CH2CH3) occurs approximately one order of magnitude slower than (tBuPBP)Pd(CH3). CO2 insertion into the benzyl complexes (tBuPBP)Pd(CH2C6H5) and (tBuPBP)Pd(CH2-4-OMe-C6H4) is significantly slower than any of the n-alkyl complexes, and CO2 does not insert into the palladium phenyl bond of (tBuPBP)Pd(C6H5). While (tBuPBP)Pd(CH2CH3) and (tBuPBP)Pd(CH2CH2CH3) are resistant to β-hydride elimination, we were unable to synthesize complexes with n-butyl, iso-propyl, and tert-butyl ligands due to β-hydride elimination and an unusual reductive coupling, which involves the formation of new C-B bonds. This reductive process also occurred for (tBuPBP)Pd(CH2C6H5) at elevated temperature and a related process involving the formation of a new H-B bond prevented the isolation of (tBuPBP)PdH. DFT calculations provide insight into the relative rates of CO2 insertion and indicate that steric factors are critical. Overall, this work is one of the first comparative studies of the rates of CO2 insertion into different metal alkyl bonds and provides fundamental information that may be important for the development of new catalysts for CO2 utilization.

Insertion of CO2 and CS2 into Bi-N bonds enables catalyzed CH-activation and light-induced bismuthinidene transfer.

The uptake and release of small molecules continue to be challenging tasks of utmost importance in synthetic chemistry. The combination of such small molecule activation with subsequent transformations to generate unusual reactivity patterns opens up new prospects for this field of research. Here, we report the reaction of CO2 and CS2 with cationic bismuth(iii) amides. CO2-uptake gives isolable, but metastable compounds, which upon release of CO2 undergo CH activation. These transformations could be transferred to the catalytic regime, which formally corresponds to a CO2-catalyzed CH activation. The CS2-insertion products are thermally stable, but undergo a highly selective reductive elimination under photochemical conditions to give benzothiazolethiones. The low-valent inorganic product of this reaction, Bi(i)OTf, could be trapped, showcasing the first example of light-induced bismuthinidene transfer.

Synthesis of mono-, di- and tripalladated 1,3,5-benzenetristyryl complexes. CO insertion to give a dipalladated indenone.

The tribrominated arenes 1,3,5-C6(E-CHCHAr)3Br3 (Ar = Ph, (I), p-To (I')), add oxidatively to [Pd(dba)2] ([Pd2(dba)3]·dba) in the presence of two equivalents of a phosphine (PPh3 or PMe2Ph) to form the monopalladated complexes trans-[Pd{C6(E-CHCHAr)3Br2}Br(L)2] (Ar = Ph, L = PPh3 (1a), Ar = p-To, L = PPh3 (1a'), Ar = Ph, L = PMe2Ph (1b)), while the reaction in a 1 : 2 : 4 arene : Pd : PMe2Ph molar ratio affords the dipalladated complex [{trans-PdBr(PMe2Ph)2}2{μ2-C6(E-CHCHPh)3Br}] (2b). Both I and I' add oxidatively to 3 equivalents of [Pd(dba)2] in the presence of the chelating N-donor ligand tmeda (N,N,N',N'-tetramethylethylenediamine) to form the tripalladated complexes [{PdBr(tmeda)}3{μ3-C6(E-CHCHAr)3}] (Ar = Ph, (3c), p-To (3c')). Complex 3c reacts with PMe3 to form [{trans-PdBr(PMe3)2}3{μ3-C6(E-CHCHPh)3}] (3d). Compound 3c also reacts with CO to give the novel dipalladated indenone [2-Ph-4,6-{PdBr(tmeda)}2-5,7-(E-CHCHPh)2-inden-1-one] (4). The crystal structures of 1a' and 1b were determined by X-ray diffraction studies.

Theoretical insights into the catalytic mechanism of propylene hydroformylation over Co-N-C materials.

M-N-C was recently reported to be a high activity catalyst for hydroformylation compared with a metal nanocluster. However, the nature of M-N-C sites and the dominant path of propylene hydroformylation on M-N-C sites with different structures are poorly understood. In this work, five different Co-N-C models (Co-N3-C, Co-N4-C, 0N-bridged Co2-N6-C, 1N-bridged Co2-N7-C and 2N-bridged Co2-N6-C) were constructed to simulate the Co active sites with different coordination that may exist on the surface of MOF-derived Co-based carbon materials. DFT combined with kinetic Monte Carlo (kMC) methods were used to study the catalytic performance for hydroformylation of different Co-N-C models. The results of the DFT calculations show that the coordination number and mode of N atoms could regulate the electronic density of the Co sites. The electronic density of the Co sites further affects the catalytic activity. The higher the electronic density is, the lower the energy barrier for partial hydrogenation of propylene and CO insertion reactions. Besides, the catalytic activity is also affected by the strong interaction in closer neighboring Co atoms in some Co2-Nx-C models. The strong interaction affects the adsorption state and energy of species, which also reduces the overall reaction energy barrier. The kMC simulation results further showed that the dominant path of propylene hydroformylation was the n-butylaldehyde path for the 0N-bridged model, and the isobutylaldehyde path for Co-N3-C and 2N-bridged models.

A supported pyridylimine-cobalt catalyst for N-formylation of amines using CO2.

N-Formylation of amines with CO2 as a cheap and non-toxic C1-feedstock and hydrosilane reducing agent is a practical and environment friendly method to synthesize formamides. This study describes an efficient and chemoselective mono-N-formylation of amines using CO2 and phenylsilane under mild conditions using a porous metal-organic framework (MOF)-supported single-site cobalt catalyst (pyrim-UiO-Co). The pyrim-UiO-Co MOF has a UiO-topology, and its organic linkers bear a pyridylimine ligated Co catalytic moiety. A wide range of aliphatic and aromatic amines are transformed into desired N-formamides in moderate to excellent yields under 1-5 bar CO2. Pyrim-UiO-Co is tolerant to various functional groups and could be recycled and reused at least 10 times. Mechanistic investigation using kinetic, spectroscopic and density functional theory studies suggests that the formylation of benzylamine proceeds sequentially via oxidative addition of PhSiH3 and CO2 insertion, followed by a turn-over limiting reaction with an amine. Our work highlights the importance of MOF-based Earth-abundant metal catalysts for the practical and eco-friendly synthesis of fine chemicals using cheap feedstocks.