High C2 Selectivity Research Articles

Delocalized Orbitals over Metal Clusters and Organic Linkers Enable Boosted Charge Transfer in Metal–Organic Framework for Overall CO2 Photoreduction

AbstractThe conversion of CO2 to C2 through photocatalysis poses significant challenges, and one of the biggest hurdles stems from the sluggishness of the multi‐electron transfer process. Herein, taking metal–organic framework (MOF, PFC‐98) as a model photocatalyst, we report a new strategy to facilitate charge separation. This strategy involves matching the energy levels of the lowest unoccupied node and linker orbitals of the MOF, thereby creating the lowest unoccupied crystal orbital (LUCO) delocalized over both the node and linker. This feature enables the direct excitation of electrons from photosensitive linker to the catalytic centers, achieving a direct charge transfer (DCT) pathway. For comparison, an isoreticular MOF (PFC‐6) based on analogue components but with far apart frontier energy level was synthesized. The delocalized LUCO caused the presence of an internal charge‐separated (ICS) state, prolonging the excited state lifetime and further inhibiting the electron‐hole recombination. The presence of ICS state prolongs the excited state lifetime and further inhibits the electron‐hole recombination. Moreover, it also induced abundant electrons accumulating at the catalytic sites, enabling the multi‐electron transfer process. As a result, the material featuring delocalized LUCO exhibits superior overall CO2 photocatalytic performance with high C2 production yield and selectivity.

Delocalized Orbitals over Metal Clusters and Organic Linkers Enable Boosted Charge Transfer in Metal-Organic Framework for Overall CO2 Photoreduction.

The conversion of CO2 to C2 through photocatalysis poses significant challenges, and one of the biggest hurdles stems from the sluggishness of the multi-electron transfer process. Herein, taking metal-organic framework (MOF, PFC-98) as a model photocatalyst, we report a new strategy to facilitate charge separation. This strategy involves matching the energy levels of the lowest unoccupied node and linker orbitals of the MOF, thereby creating the lowest unoccupied crystal orbital (LUCO) delocalized over both the node and linker. This feature enables the direct excitation of electrons from photosensitive linker to the catalytic centers, achieving a direct charge transfer (DCT) pathway. For comparison, an isoreticular MOF (PFC-6) based on analogue components but with far apart frontier energy level was synthesized. The delocalized LUCO caused the presence of an internal charge-separated (ICS) state, prolonging the excited state lifetime and further inhibiting the electron-hole recombination. The presence of ICS state prolongs the excited state lifetime and further inhibits the electron-hole recombination. Moreover, it also induced abundant electrons accumulating at the catalytic sites, enabling the multi-electron transfer process. As a result, the material featuring delocalized LUCO exhibits superior overall CO2 photocatalytic performance with high C2 production yield and selectivity.

Tuning the CO2 Reduction Selectivity of an Immobilized Molecular Ag Complex beyond CO.

The electrochemical reduction of carbon dioxide (CO2) to produce fuels and chemicals has garnered significant attention. However, achieving control over the selectivity of the resulting products remains a challenging task, particularly within molecular systems. In this study, we employed a molecular silver complex immobilized on graphitized mesoporous carbon (GMC) as a catalyst for converting CO2 into CO, achieving an impressive selectivity of over 90% at -1.05 V vs RHE. Notably, the newly formed silver nanoparticles emerged as the active sites responsible for this high CO selectivity rather than the molecular system. Intriguingly, the introduction of copper ions into the restructured Ag-nanoparticle-decorated carbon altered the product selectivity. At -1.1 V vs RHE in 0.1 M KCl, we achieved a high C2 selectivity of 75%. Furthermore, not only the Ag-Cu bimetallic nanoparticle but also the small-sized Ag-Cu nanocluster decorated over GMC was proposed as active sites during catalytic reactions. Our straightforward approach offers valuable insights for fine-tuning the product selectivity of immobilized molecular systems, extending beyond C1 products.

Activating *CO by Strengthening Fe–CO π‐Backbonding to Enhance Two‐Carbon Products Formation toward CO2 Electroreduction on Fe–N4 Sites

AbstractMany non‐precious metal‐nitrogen (M–Nx)‐containing catalysts are highly efficient for electrochemical reduction of CO2 to CO and yet encounter challenges in further converting CO to more valuable two‐carbon products (C2), such as ethanol and acetic acid. The ambiguous structure‐activity relationship of the M–Nx moieties toward CO2 reduction reaction (CO2RR) results in difficulties in regulating the CO2RR product selectivity on the M–Nx‐containing catalysts. Herein, by using fluorinated iron phthalocyanines with axial‐coordinated ligands (L–FePc–F) as an M–N4‐based model electrocatalyst for CO2RR, a correlation between the electronic structure and C2 selectivity of Fe–N4 is revealed and a comprehensive descriptor based on the Fe–CO π‐backbonding is proposed for guiding the regulation of M–N4 toward higher C2 selectivity. Based on the regulation principle, the Br‐axial‐coordinated FePc–F (Br–FePc–F) remarkably increases the Faradic efficiency (FE) of C2 products from 0% (i.e., the FEC2 of FePc–F) to 34% due to the strengthened Fe–CO π‐backbonding stemming from the elevated 3dxz/dyz orbital energy and enhanced electron‐donating ability of the Fe centers of Fe–N4. This work provides a strategy and mechanism insights on the regulation of the C2 selectivity of CO2RR on the Fe–N4 moieties, which may be inspiring for precise construction and regulation of the M–Nx‐containing catalysts for specific CO2RR products.

Manipulating electrochemical CO2 reduction pathway by engineering energy level of Cu/Zn dual-metal single atom catalysts

Dual-metal single atom catalysts (DSACs) demonstrate outstanding catalytic performance in the CO2 reduction reaction. However, the complexity of catalyst structures and interface environments has led to an unclear understanding of the synergistic catalysis mechanism involved. Here, we developed a series of catalysts with atomically dispersed Cu–N4 and Zn–N4 sites on N-doped carbon supports (Cu1-xZnx-NC), which can efficiently reduce CO2 to C2 products by optimizing the Zn loading. Specifically, a remarkable faradaic efficiency of 65 % at −1.1 VRHE for the C2 products on the Cu0.75Zn0.25-NC catalyst has been obtained. Structural characterizations and density functional theory calculations demonstrate that Cu/Zn DSACs synergistically lower the energy barrier of the rate-determining step and facilitate proton transfer kinetics. Among them, Zn atoms activate CO2, while Cu atoms aid in the adsorption and dissociation of intermediates. The electron energy redistribution induced by adjacent Cu–N4 and Zn–N4 site optimizes the 3d orbitals of Cu centers, thereby accelerating and promoting C–C coupling. The energy level optimization of Cu/Zn DSACs results in Cu0.75Zn0.25-NC with a high C2 selectivity and activity, improved kinetics. In summary, these findings provide new insights into the development of highly efficient DSACs and its synergistic catalysis mechanism in CO2 reduction.

Microenvironment of the HMOR catalyst leads to high ethylene selectivity from ketene conversion: Insights from ab initio molecular dynamics simulations

The origin of high ethylene selectivity in ketene conversion with the HMOR catalyst is revealed using ab initio molecular dynamics simulations, and the microenvironment of catalyst pores of 8-member ring (8MR) and 12MR are found to be playing a crucial role. The higher activity of ketene conversion in 8MR than that in 12MR is because the 8MR side pocket pore microenvironment has a better stabilization effect on reaction intermediates. In addition, the free energy barrier of methylation of ethylene is higher than that of ketene in 8MR, which is due to the stronger interaction between hydrogen in ethylene and the framework of 8MR side pocket, resulting in relatively high C2 selectivity in 8MR. Similarly, the microenvironment of 12MR would facilitate the further methylation of CH3CH2CO+ to isopropyl, decreasing the selectivity of ethylene. These results strengthen the importance of understanding microenvironment of HMOR catalyst in improving ethylene selectivity from ketene conversion.

Effect of different valence metals doping on methane activation over La2O3(001) surface

La2O3 as a catalyst is used for oxidative coupling of methane (OCM) reactions due to its excellent stability and high C2 selectivity, but poor activity on methane dissociation limits its wide application. Different valence metals are doped on the La2O3(001) surface to improve the methane conversion activity, and the activation of methane on metal-doped La2O3(001) surfaces has been investigated via the density functional theory (DFT) calculations. The relationship between the valence states of doped metals and the methane conversion activities shows that doping low valence metals (Li, Na, K, Mg, Ca, Sr and Ba) and equivalent metals (Al, Ga, In) can significantly improve the conversion activity of methane. Among them, the activation energy of methane on the Li-La2O3(001) surface is the lowest, which is only 13.0 kJ/mol. However, doping of high valence metals (Zr, Nb, Re and W) cannot improve the CH4 dissociation activity. Furthermore, the relationships between surface oxygen vacancy formation energies, acid-base properties and the activation energies of CH4 have also been investigated. The results show that with the increase of metal valence state, the oxygen vacancy formation energy increases, while the dissociation activity of CH4 decreases. The introduction of alkali and alkaline earth metals increases the alkalinity of La2O3(001) surface, and the alkalinity of La2O3(001) doped with the alkali metal is stronger than that with the alkaline earth metal, exhibiting higher dissociation activity of CH4. Our research may provide a guide for improving methane conversion activity on La2O3 catalysts.

Hydroxypillar[5]arene‐Confined Silver Nanocatalyst for Selective Electrochemical Reduction of CO2 to Ethanol

AbstractCO is usually the dominant product on silver‐based catalysts in electrochemical CO2 reduction reaction (CO2RR) possibly due to weak *CO adsorption. In this report, a hydroxypillar[5]arene‐extended porous polymer‐confined silver catalyst (PAF‐PA5‐Ag‐0.8) for electrochemical CO2RR which can selectively produce ethanol with a maximum Faradaic efficiency of 55% at 11 mA cm−1 is described. The study reveals that the hydroxypillar[5]arene‐confined Ag clusters are the active sites for ethanol formation. Moreover, temperature‐programmed desorption measurements demonstrate an enhanced adsorption strength of CO* on PAF‐PA5‐Ag‐0.8 compared with that on commercial Ag nanoparticles, which is favored by the C‐C coupling to form ethanol. The density functional theory study indicates that the confined Ag clusters in PAF‐PA5‐Ag‐0.8 contribute to high C2 selectivity in CO2RR through facilitating *COOH formation, stabilizing *CO intermediates, and inhibiting hydrogen evolution. This work provides a new design strategy by modulating *CO adsorption strength on non‐copper electrocatalysts in converting CO2 into “green” C2 products.

Bifunctional NHC-Promoted C2-Alkylation of Pyridine via Ni–Al Bimetallic-Catalyzed Hydroarylation of Unactivated Alkenes

Achieving site-selective alkylation with pyridine under transition metal catalysis has remained a long-standing challenge, especially when using unactivated alkene as an alkylating reagent. We herein describe a site-selective C2-alkylation of pyridine under Ni–Al bimetallic catalysis and directing bifunctional N-heterocyclic carbene (NHC) as ligand through hydroarylation of unactivated alkenes. A broad range of C2-alkylated pyridines with various functionalities could be prepared in high efficiency (up to 99% yield) and high C2-selectivity, and the scope of heterocycles is not limited to pyridines but can be expanded to other azines like pyrazines and pyrimidines. Mechanistic studies suggest that a bifunctional NHC–Ni–Al–pyridine complex is responsible for high site selectivity control and that reductive elimination might be the rate-determining step.

Palladium-Catalyzed C2-Selective Oxidative Olefination of Benzo[b]thiophene 1,1-Dioxides with Styrenes and Acrylates.

Here, we disclose a novel Pd(II)-catalyzed oxidative Heck reaction of benzo[b]thiophene 1,1-dioxides with styrenes and acrylates. This transformation features broad functional group tolerance and high C2 selectivity. Furthermore, the photoluminescence properties of C-2 alkenylated products have been characterized, which illustrates the potential usefulness of our protocol in constructing π-conjugated fluorescent molecules.

Identifying an Interfacial Stabilizer for Regeneration-Free 300 h Electrochemical CO2 Reduction to C2 Products.

The electrochemical CO2 reduction reaction (CO2RR) to produce high value-added hydrocarbons and oxygenates presents a sustainable and compelling approach toward a carbon-neutral society. However, uncontrollable migration of active sites during the electrochemical CO2RR limits its catalytic ability to simultaneously achieve high C2 selectivity and ultradurability. Here, we demonstrate that the generated interfacial CuAlO2 species can efficiently stabilize the highly active sites over the Cu-CuAlO2-Al2O3 catalyst under harsh electrochemical conditions without active sites regeneration for a long-term test. We show that this unique Cu-CuAlO2-Al2O3 catalyst exhibits ultradurable electrochemical CO2RR performance with an 85% C2 Faradaic efficiency for a 300 h test. Such a simple interfacial engineering design approach unveiled in this work would be adaptable to develop various ultradurable catalysts for industrial-scale electrochemical CO2RR.

A Self-Forming CO2/O2 Co-Transport Dual-Phase Membrane for Oxidative Coupling of Methane

Direct methane conversion (DMC) has garnered considerable interest from both industries and academia in recent years due to its great economic potential created by significantly increased price differential between low-cost natural gas (NG) and the derived value-added chemicals. Oxidative coupling of methane (OCM), which transforms CH4 into ethylene (C2H4) with molecular O2 as the oxidant in a single step, is one of the most studied DMCs. However, a major technical challenge for the OCM process is the difficulty to achieve high CH4 conversion at high C2 (ethane or ethylene) selectivity. The rudimentary cause for this “tradeoff” behavior is that the products (C2H6 or C2H4) are chemically more reactive than the reactant (CH4). To overcome this thermodynamic hurdle, minimizing the oxidizing power of the oxidant or lowering the local oxygen partial pressure is key. In this presentation, we show a CO2/O2 co-transport membrane reactor for OCM conversion. The results explicitly show that the permeated O2 can react preferably with CH4 as in the conventional OCM to form C2H6; the latter then undergoes thermal cracking into C2H4 and H2. The roles of the permeated CO2 are assumed to be reacting with H2 via reverse water gas shift (RWGS) reaction to shift the C2H6 thermal cracking; another role of CO2 is assumed to be mitigating coke formation via Boudouard (RB) reaction.

Engineering Supramolecular Binding Sites in a Chemically Stable Metal-Organic Framework for Simultaneous High C2 H2 Storage and Separation.

Developing porous materials to overcome the trade-off between adsorption capacity and selectivity for C2 H2 /CO2 separation remains a challenge. Herein, we report a stable HKUST-1-like MOF (ZJU-50a), featuring large cages decorated with high density of supramolecular binding sites to achieve both high C2 H2 storage and selectivity. ZJU-50a exhibits one of the highest C2 H2 storage capacity (192 cm3 g-1 ) and concurrently high C2 H2 /CO2 selectivity (12) at 298 K and 1 bar. Single-crystal X-ray diffraction studies on gas-loaded ZJU-50a crystal unveil that the incorporated supramolecular binding sites can selectively take up C2 H2 molecule but not CO2 to result in both high C2 H2 storage and selectivity. Breakthrough experiments validated its separation performance for C2 H2 /CO2 mixtures, providing a high C2 H2 recovery capacity of 84.2 L kg-1 with 99.5 % purity. This study suggests a novel strategy of engineering supramolecular binding sites into MOFs to overcome the trade-off for this separation.

Oxidative coupling of methane (OCM) conversion into C2 products through a CO2/O2 co-transport membrane reactor

Oxidative coupling of methane (OCM), which transforms CH4 into C2 products (C2H6 and C2H4) with molecular O2 as the oxidant, is one of the most studied direct methane conversions (DMCs). However, a major technical hurdle to the OCM process is to achieve high CH4 conversion at high C2 (C2H6 and C2H4) selectivity. One rudimentary cause for this “tradeoff” behavior is the high chemical reactivity of the products (C2H6 or C2H4), which can be re-oxidized by O2. To overcome this thermodynamic challenge, minimizing the oxidizing power of the oxidant and lowering the local oxygen partial pressure are keys. In this work, we demonstrate a new membrane reactor that capture CO2/O2 from a flue gas and uses it for OCM conversion. The results show that the co-captured CO2/O2 mixture converts CH4 into C2H6 in the presence of a 2%Mn–5%Na2WO4/SiO2 catalyst, followed by thermal cracking of C2H6 into C2H4 and H2. The presence of CO2 decreases the local partial pressure of O2, thus reducing the propensity of C2-products re-oxidation and leading to a higher C2 selectivity. We show that a small button-type membrane reactor can achieve 12% C2 yield with ∼57% C2-selectivity using a diluted CH4-Ar mixture as the feedstock. We expect higher C2 yield with tubular plug-flow membrane reactors in the future. We also highlight the unique advantage of the membrane reactor in intensifying CO2 capture from both flue gas and OCM purification process into one single step.

Oxidative Coupling of Methane over Pt/Al2O3 at High Temperature: Multiscale Modeling of the Catalytic Monolith

At high temperatures, the oxidative coupling of methane (OCM) is an attractive approach for catalytic conversion of methane into value-added chemicals. Experiments with a Pt/Al2O3-coated catalytic honeycomb monolith were conducted with varying CH4/O2 ratios, N2 dilution at atmospheric pressure, and very short contact times. The reactor was modeled by a multiscale approach using a parabolic two-dimensional flow field description in the monolithic channels coupled with a heat balance of the monolithic structure, and multistep surface reaction mechanisms as well as elementary-step, gas phase reaction mechanisms. The contribution of heterogeneous and homogeneous reactions, both of which are important for the optimization of C2 products, is investigated using a combination of experimental and computational methods. The oxidation of methane, which takes place over the platinum catalyst, causes the adiabatic temperature increase required for the generation of CH3 radicals in the gas phase, which are essential for the formation of C2 species. Lower CH4/O2 ratios result in higher C2 selectivity. However, the presence of OH radicals at high temperatures facilitates subsequent conversion of C2H2 at a CH4/O2 ratio of 1.4. Thereby, C2 species selectivity of 7% was achieved at CH4/O2 ratio of 1.6, with 35% N2 dilution.

Protecting the state of Cu clusters and nanoconfinement engineering over hollow mesoporous carbon spheres for electrocatalytical C-C coupling

Copper-based catalysts are widely used to adjust the activity and selectivity of CO2 electroreduction reactions (CO2RR). In this article, we choose to use hollow mesoporous carbon spheres (HMCS) to confine and protect Cu clusters to achieve high C2 selectivity. The electrocatalytic results show that when the amount of Cu clusters confined by HMCS reaches 20% (Cu/HMCS5-20%), the selectivity of C2 products reach 88.7% at − 1.0 V vs. RHE. In situ Fourier transform infrared spectroscopy (FTIRS) shows that Cu clusters confined and protected by HMCS is beneficial to the conversion of *CO to *CHO, while the nanocavities formed by HMCS can effectively confine the in situ formed *CHO carbon intermediates, which facilitates the C-C bond coupling to form C2H4 and C2H5OH. We proposes a method to improve the C2 selectivity of CO2RR and reduce the amount of Cu in CO2RR by using the confinement effect of HMCS.

Intermediate enrichment effect of porous Cu catalyst for CO2 electroreduction to C2 fuels

Electrochemical carbon dioxide (CO2) reduction reaction (CO2RR) to obtain C2/multi-carbon fuels is an appealing technology to reduce carbon emission and store intermittent renewable electricity. The C–C coupling process, as the most critical step for generating C2/multi-carbon fuels, strongly depends on the concentration of *CO intermediates around the active sites. In this work, we prepared porous copper nanospheres (P-Cu) that greatly improved the C–C coupling process by enriching *CO intermediates in the pore structure. The specific pore features of P-Cu were characterized in detail by scanning electron microscopy (SEM), transmission electron microscopy (TEM), and nitrogen adsorption/desorption tests. Obviously, the P-Cu exhibits higher specific surface area and richer pore structure than the control sample compact copper nanosphere (C-Cu). The CO gas sensor and temperature programmed desorption (TPD) tests prove that the P-Cu has better CO adsorption and enrichment capacity than C-Cu due to the rich pore structure. As a result, P-Cu exhibited a high C2 selectivity with a Faradaic efficiency (FEC2) of 57.22% at −1.3 V (vs. RHE), which is about 2.5-fold than the C-Cu (FEC2 of 22.71%). This work provides an effective strategy to improve the activity and selectivity of C2 products in CO2RR by optimizing the adsorption/enrichment property of intermediates.

Construction of Indoline/Indolenine Ring Systems by a Palladium-Catalyzed Intramolecular Dearomative Heck Reaction and the Subsequent Aza-semipinacol Rearrangement.

The palladium-catalyzed intramolecular dearomative Heck reaction of 2,3-disubstituted indoles serves as an access to spiro-indoline products. Herein, we report an efficient construction of indoline/indolenine core stuctures via a dearomative Heck reaction of simple 2,3-disubstituted indoles with all-carbon tethers and the subsequent aza-semipinacol rearrangement. The Heck reaction features a high C2-selectivity, and the stereospecific aryl/alkyl migration selectivity has been investigated by DFT calculations. Using this method, we accomplished the formal total synthesis of akuammiline alkaloids vincorine.

Effect of facile nitrogen doping on catalytic performance of NaW/Mn/SiO2 for oxidative coupling of methane

The oxidative coupling of methane (OCM) has great potential as a cost-effective pathway to directly convert methane to C2 species such as ethane and ethylene. Although NaW/Mn/SiO2 is one of the most promising catalysts for OCM, an improvement in its catalytic performance is required to achieve high C2 yield. Herein, we show that a relatively simple pyridine treatment of NaW/Mn/SiO2 leads to nitrogen-doping of the active sites, resulting in significantly high CH4 conversion rate while maintaining high C2 selectivity. Various characterisation techniques such as X-ray diffraction analysis, X-ray photoelectron spectroscopy, temperature-programmed reduction analysis, transmission electron microscopy, and density functional theory calculations are employed to understand the effect of N doping on the performance of NaW/Mn/SiO2 for OCM.

Coking mechanism of Mo/ZSM-5 catalyst in methane dehydroaromatization

Methane dehydroaromatization is a potential technique in converting natural gas into value-added aromatics and clean hydrogen. However, severe coking hinders industrial application of conventionally prepared Mo/ZSM-5 catalyst for methane dehydroaromatization. Usually, Bronsted acid sites (BAS) were considered the key for coking. To gain deeper insight into the coke, here we present a simple post-impregnation of Na+ to Mo/HZSM-5 to regulate its surface acidity. Influences of BAS on catalytic performance and coking behavior were thus exclusively studied. Na+ modified catalysts exhibit lower naphthalene and higher C2 selectivity, while selectivity for benzene is not obviously changed. These results contradict conventional bi-functional pathway that methane is dehydrogenated on Mo while BAS act as the center for intermediates cyclization. Characterizations and calculations indicate restricted growth of hydrocarbon pools confined in zeolite channels of Na+ modified catalysts. Hence, it is proposed that aromatization is the intrinsic property of Mo, and the Mo-associated carbonaceous species act as precursors for both coke and aromatics production.