C2 Oxygenates Research Articles

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.

Maximizing Active Fe Species in ZSM-5 Zeolite Using Organic-Template-Free Synthesis for Efficient Selective Methane Oxidation.

The selective oxidation of CH4 in the aqueous phase to produce valuable chemicals has attracted considerable attention due to its mild reaction conditions and simple process. As the most widely studied catalyst for this reaction, Fe-ZSM-5 demonstrates high intrinsic activity and selectivity; however, Fe-ZSM-5 prepared using conventional methods has a limited number of active Fe sites, resulting in low CH4 conversion per unit mass of the catalyst. This study reports a facile organic-template-free synthesis strategy that enables the incorporation of more Fe into the zeolite framework with a higher dispersion degree compared to conventional synthesis methods. Because framework Fe incorporated in this way is more readily transformed into isolated extra-framework Fe species under thermal treatment, the overall effect is that Fe-ZSM-5 prepared using this method (Fe-HZ5-TF) has 3 times as many catalytically active sites as conventional Fe-ZSM-5. When used for the selective oxidation of CH4 with 0.5 M H2O2 at 75 °C, Fe-HZ5-TF produced a high C1 oxygenate yield of 109.4 mmol gcat-1 h-1 (a HCOOH selectivity of 91.1%), surpassing other catalysts reported to date. Spectroscopic characterization and density functional theory calculations revealed that the active sites in Fe-HZ5-TF are mononuclear Fe species in the form of [(H2O)3Fe(IV)═O]2+ bound to Al pairs in the zeolite framework. This differs from conventional Fe-ZSM-5, where binuclear Fe acts as the active site. Analysis of the catalyst and product evolution during the reaction suggests a radical-driven pathway to explain CH4 activation at the mononuclear Fe site and subsequent conversion to C1 oxygenates.

Utilizing CO2 as a Reactant for C3 Oxygenate Production via Tandem Reactions.

One possible solution to closing the loop on carbon emissions is using CO2 as the carbon source to generate high-value, multicarbon products. In this Perspective, we describe four tandem reaction strategies for converting CO2 into C3 oxygenated hydrocarbon products (i.e., propanal and 1-propanol), using either ethane or water as the hydrogen source: (1) thermocatalytic CO2-assisted dehydrogenation and reforming of ethane to ethylene, CO, and H2, followed by heterogeneous hydroformylation, (2) one-pot conversion of CO2 and ethane using plasma-activated reactions in combination with thermocatalysis, (3) electrochemical CO2 reduction to ethylene, CO, and H2, followed by thermocatalytic hydroformylation, and (4) electrochemical CO2 reduction to CO, followed by electrochemical CO reduction to C3 oxygenates. We discuss the proof-of-concept results and key challenges for each tandem scheme, and we conduct a comparative analysis of the energy costs and prospects for net CO2 reduction. The use of tandem reaction systems can provide an alternative approach to traditional catalytic processes, and these concepts can be further extended to other chemical reactions and products, thereby opening new opportunities for innovative CO2 utilization technologies.

ZSM-5-confined Cr1–O4 active sites boost methane direct oxidation to C1 oxygenates under mild conditions

Cr1–O4 as an active site in the Cr1/ZSM-5 SAC can catalyze the DOM to form value-added C1 oxygenated products with a productivity of 21 100 μmol gcat−1 h−1 and a selectivity of 99.8% at 50 °C within 30 min, which outperforms most state-of-the-art catalysts.

Atomically Dispersed Nickel Anchored on a Nitrogen‐Doped Carbon/TiO2Composite for Efficient and Selective Photocatalytic CH4Oxidation to Oxygenates

AbstractDirect photocatalytic oxidation of methane to liquid oxygenated products is a sustainable strategy for methane valorization at room temperature. However, in this reaction, noble metals are generally needed to function as cocatalysts for obtaining adequate activity and selectivity. Here, we report atomically dispersed nickel anchored on a nitrogen‐doped carbon/TiO2composite (Ni−NC/TiO2) as a highly active and selective catalyst for photooxidation of CH4to C1 oxygenates with O2as the only oxidant. Ni−NC/TiO2exhibits a yield of C1 oxygenates of 198 μmol for 4 h with a selectivity of 93 %, exceeding that of most reported high‐performance photocatalysts. Experimental and theoretical investigations suggest that the single‐atom Ni−NC sites not only enhance the transfer of photogenerated electrons from TiO2to isolated Ni atoms but also dominantly facilitate the activation of O2to form the key intermediate ⋅OOH radicals, which synergistically lead to a substantial enhancement in both activity and selectivity.

Atomically Dispersed Nickel Anchored on a Nitrogen-Doped Carbon/TiO2 Composite for Efficient and Selective Photocatalytic CH4 Oxidation to Oxygenates.

Direct photocatalytic oxidation of methane to liquid oxygenated products is a sustainable strategy for methane valorization at room temperature. However, in this reaction, noble metals are generally needed to function as cocatalysts for obtaining adequate activity and selectivity. Here, we report atomically dispersed nickel anchored on a nitrogen-doped carbon/TiO2 composite (Ni-NC/TiO2 ) as a highly active and selective catalyst for photooxidation of CH4 to C1 oxygenates with O2 as the only oxidant. Ni-NC/TiO2 exhibits a yield of C1 oxygenates of 198 μmol for 4 h with a selectivity of 93 %, exceeding that of most reported high-performance photocatalysts. Experimental and theoretical investigations suggest that the single-atom Ni-NC sites not only enhance the transfer of photogenerated electrons from TiO2 to isolated Ni atoms but also dominantly facilitate the activation of O2 to form the key intermediate ⋅OOH radicals, which synergistically lead to a substantial enhancement in both activity and selectivity.

Selective Oxidation of Methane into Formic Acid over ZIF‐8‐Encapsulated Mononuclear Fe Species under Mild Conditions

AbstractThe direct catalytic oxidation of methane into C1 oxygenates is a promising approach for methane valorization. However, the difficulties in CH4 activation and inhibition of over‐oxidation hinder the development of efficient catalysts for this transformation. Herein, highly dispersed Fe species confined by ZIF‐8 are developed for the direct transformation of methane into value‐added C1 oxygenates with H2O2 as oxidant at 323 K. The as‐obtained Fe‐ZIF‐8 can afford an optimal turnover rate (TOR) of ∼45 molC1 molFe−1 h−1. Note that the selectivity to HCOOH is ∼97 %, outstanding among the reported Fe‐based heterogeneous catalysts. Combined analysis of X‐ray photoelectron spectroscopy, X‐ray absorption spectroscopy, ultraviolet‐visible diffuse reflectance spectroscopy and 57Fe Mössbauer spectroscopy suggests that the catalytically active sites in Fe‐ZIF‐8 are mononuclear six‐coordinated Fe3+ species. The confinement of mononuclear Fe species within a porous metal‐organic framework material can provide an exquisite, enzyme‐like performance for highly selective oxidation of methane towards formic acid under mild conditions.

Possible Fine-Tuning of Methane Activation toward C2 Oxygenates by 3d-Transition Metal-Ions Doped Nano-Ceria-Zirconia.

In this work, we demonstrate a simple sol-gel technique to prepare metal-ion(s)-doped ceria-zirconia solid solution for efficient catalytic methane activation. The cation-depicting formula units are Ce0.80Zr0.20 (CZ), Ce0.79Zr0.20M0.01 (CZM), and Ce0.79Zr0.20M0.005M10.005 (CZMM1) (M and M1 = V, Mn, Fe, Co, and Cu), employed for undoped, mono-metal-ion-doped, and bi-metal-ion-doped solid solutions, respectively. Methane activation with Mn, Fe, Cu mono-metal-ion-doped CZ favors the C1 product, while CZCo assists C-C coupling with the formation of acetaldehyde. On the other hand, the Co- and Fe-doped bi-metal-ion combination catalyst (CZCoFe) shows significant ethanol but predominant formic acid formation. This is further promoted by the Co + V bi-metal-ion combination (CZCoV) catalyst, and it shows ethanol as the major product along with methyl hydrogen peroxide, methanol, and formic acid as minor products. An impressive ethanol yield of 93 μmol/g h with 76% selectivity obtained with the CZCoV catalyst is at par with that obtained with noble-metal-based catalysts under comparable reaction conditions. When Co and V content was increased two and four times from 0.005 to 0.01 and 0.02, ethanol yield increased at the expense of formic acid. The 213 μmol/g h ethanol yield (86% selectivity) observed with Ce0.76Zr0.20Co0.02V0.02 is probably the highest observed. The partial oxidation of CH4 in Co-based bi-metal combinations (Co + V or Co + Fe) suggests the synergistic effect of doped metal ions owing to the heterogeneous near-neighbor environment. The present results are attributed to the surface heterogeneity between the host and the dopants, which selectively promotes methane activation as well as C-C coupling. This indicates a large scope to tune the activity of partial oxidation of methane and product selectivity with different metal-ion(s) combinations.

Low temperature catalytic conversion of CH4, CO2, and C2H4 to value-added C3 oxygenates and olefins via C1-C2 coupling on Pd-Au/CeO2

The catalytic conversion of CH4 and CO2 in the presence of more reactive co-reactants C2H4 and O2 on Pd-Au/CeO2 is achieved at 200 °C and elevated pressures. Propene and acetone were produced from the catalyzed reaction of CH4 +C2H4 +O2; addition of CO2 to the reactant stream produced methyl acetate (MAc) but it significantly reduced the C-selectivities of propene and acetone. DRIFTS experiments confirmed the formation of methoxy species from CH4 +CO2 at 200 °C, which is an intermediate in the formation of MAc. Control experiments with blanks, and with CeO2 did not show any propene, acetone, or MAc products. The complete oxidation of C2H4 was avoided; the catalyst is stable, and reactant conversions and product yields were sustained for the observed 1200 min time-on-stream, indicating that there is little or no carbon deposition and sintering. This direct coupling of CH4 and C2H4 intermediates to higher carbon-number products at 200 °C is significant.

(Invited, Digital Presentation) Methane Conversion to High Value Chemicals By Photocatalysis

As underlined in the COP26, methane as a greenhouse gas is nearly 80 time more potent than CO2 while its reserve is much more than the sum of coal, oil and natural gas. Thus methane conversion not only involves the environmental issue but also is related to high-value chemical production /clean energy supply, which has been attracting substantial interest over the last decades. However CH4 activation is energy intensive and kinetically very challenging so that methane activation is regarded as the “holy grail” in the catalytically chemical process. [1] Photocatalysis provides a cost efficient potential to activation of such small molecule under very mild conditions, while to achieve the potential is a huge challenge.[2]Stimulated by our research outcomes on the charge dynamics in inorganic semiconductor photocatalysis, which reveal that the low reaction efficiency is due to both fast charge recombination and large bandgap of an inorganic semiconductor [3,4], together with the recent findings on atomic catalysis [5], we developed novel material strategies for photocatalytic methane conversion to methanol. Highly dispersed atomic level iron species immobilised on a TiO2 photocatalyst show an excellent activity for methane conversion, resulting into ~97% selectivity towards alcohols operated under ambient conditions by a one-step chemical process [6]. Such photocatalyst is also very stable, promising an attractive industrial process of methane upgrade. The dominating function of the iron species has also been investigated in detail. Furthermore, we designed a flow system for relatively efficient methane to C2, achieving the benchmark results in this area.[7] In addition, C1 oxygenates can be produced with nearly 100% by photocatalytic methane conversion due to the synergy between Au and Cu cocatalysts loaded on ZnO.[8] References ： Li, X. Wang, C., Tang, J. Nature Reviews Materials , 2022, Doi: 1038/s41578-022-00422-3.Wang Y., Suzuki H., Xie, J., Tomita, O., Martin, D. J., Higashi, M., Kong, D., Abe R. Tang, J., Chem. Rev., 2018, 118: 5201-5241.Tang, J. Durrant J. R., Klug, R ., J. Am. Chem. Soc ., 2008, 130(42) : 13885-13891.Miao, T., Wang, C., Xiong, L., Li, X., Xie, X., Tang, J., ACS Catalysis , 2021, 11, 8226-8238.Wang, A. Li J., Zhang. T., Nat . Rev. Chem ., 2018, 2; 65-81.Xie, J., Jin, R. ,Li, A., Bi, Y., Sankar, G. , Ma , Tang, J. Nature Catalysis , 2018, 1: 889-896.Li, X., Xie J.,Rao, H., Wang, , Tang, J., Angewandte Chemie International Edition , 2020, 132: 19870-19875.Luo, L., Gong, Z., Xu, Y., Ma, J., Liu, H., Xing, J., Tang, J., Journal of the American Chemical Society , 2022, 144, 2, 740–750.

Toward abiotic sugar synthesis from CO2 electrolysis

Toward abiotic sugar synthesis from CO2 electrolysis

Boosting room-temperature conversion of methane via confining Cu atoms in ultrathin Ru nanosheets

Designing active centers for CH4 activation under mild conditions is pivotal for the low-energy conversion of natural gas, which is still a great challenge owing to the chemical inertness of CH4 and the difficulties in tuning coordination environments of active centers. Herein, we report that highly efficient room-temperature CH4 conversion to liquid C1 oxygenates is achieved over ultrathin Ru nanosheets with confined Cu atoms. Compared with previously reported catalysts, it delivers a superior performance of up to 1,533 mmol g−1Cu(surf.) h−1 productivity and 97.3 h−1 TOF with over 99% selectivity for C1 oxygenates, and the optimized CH4 conversion reaches 0.05% at 25°C. Multiple spectroscopic analysis and first-principles calculations reveal that bi-coordinated bridge-site oxygen species generated on the Ru edge-confined Cu sites can dissociate the C–H bond of CH4 with a moderately low energy barrier and thus enable the CH4 conversion at room temperature via a free radical mechanism.

Molecular beam mass spectrometry study on plasma-assisted low-temperature oxidation of ethylene

The chemical kinetics of plasma-assisted low-temperature oxidation of ethylene (C2H4) is explored by molecular beam mass spectrometry and kinetic modeling. In the experiments, a dielectric barrier discharge (DBD) flow reactor coupled with photoionization and electron ionization molecular beam mass spectrometry (PI-MBMS/EI-MBMS) is employed to provide detailed speciation information, including ions, radicals, and molecules in C2H4/O2/Ar systems (∼340 K, 30 Torr). A series of hydrocarbons and oxygenated intermediates including ethenol, methyl hydroperoxide, ethyl hydroperoxide, diacetylene, vinylacetylene, and hydroxyacetaldehyde is detected for the first time in this system. A kinetic mechanism that contains both combustion and plasma reaction networks is developed. The path fluxes of fuel consumption and oxygenates formation are obtained based on the rate of production analysis. Based on our experimental observations and modeling analysis, new insights into the kinetics of C2H4/O2/Ar plasmas are provided. The fuel is mainly consumed through OH addition, dissociation after e/Ar* collision, reactions with O/O(1D), and dissociative ionization. H abstraction reactions that are favored in alkane plasmas contribute little to the fuel consumption of ethylene. Formation pathways of C1 oxygenates are mainly initiated by CH3 produced by O/O(1D) reactions with fuel, and the subsequent pathways of OH/H addition to the fuel are responsible for the formation of most C2 oxygenates. Oxygen addition-elimination reactions of C2H4OH isomers are the sources of ethenol formation. The concentration profiles for some of the species under different oxygen mole fractions are measured. The two distinct patterns of species mole fraction profiles with increasing oxygen concentrations indicate the different dominating formation pathways, namely oxidation pathways and non-oxidation pathways. And the competition between oxidation effects and plasma effects is displayed by the non-monotonic mole fractions profiles of oxidation products.

Competing mechanisms of CO hydrogenation to ethanol over TM/Mo6S8 catalysts

In this work, the mechanism of the ethanol synthesis from CO hydrogenation on TM/Mo6S8 (TM = Fe, Ru) catalysts have been systematically investigated by using density functional theory calculations. The competition mechanism for the conversion of CO into CH4, CH3OH, and C2H6 formation is also considered. Our results indicate that an optimal route for the ethanol synthesis on TM/Mo6S8 (TM = Fe, Ru) catalysts started with an initial process of the dissociation and hydrogenation of CO to produce CH3 species. Subsequently, the HCO inserts into spontaneous CH3 species leads to the formation of CH3CHO, followed by successive hydrogenation to form ethanol via CH3CH2O intermediate. Comparing the two catalysts, Fe/Mo6S8 shows better catalytic performance than Ru/Mo6S8 for ethanol synthesis from CO hydrogenation, and Ru/Mo6S8 is more conducive to the production of CH3OH. The results of microkinetic modeling analysis of rate-determining step and selectivity and catalytic activity are in line with our energy barrier results. Simultaneously, the d-band center value of Fe/Mo6S8 is closer to EF than that of Ru/Mo6S8, which indicates that the catalytic activity of Fe/Mo6S8 is better. Mo6S8 in the TM/Mo6S8 catalysts mainly facilitates the formation of C2 oxygenates due to HCO is easy to insert into CH3 species. The present results will can provide clues for the catalyst design of oxygenate synthesis in some extent.

Synergy of Pd atoms and oxygen vacancies on In2O3 for methane conversion under visible light

Methane (CH4) oxidation to high value chemicals under mild conditions through photocatalysis is a sustainable and appealing pathway, nevertheless confronting the critical issues regarding both conversion and selectivity. Herein, under visible irradiation (420 nm), the synergy of palladium (Pd) atom cocatalyst and oxygen vacancies (OVs) on In2O3 nanorods enables superior photocatalytic CH4 activation by O2. The optimized catalyst reaches ca. 100 μmol h−1 of C1 oxygenates, with a selectivity of primary products (CH3OH and CH3OOH) up to 82.5%. Mechanism investigation elucidates that such superior photocatalysis is induced by the dedicated function of Pd single atoms and oxygen vacancies on boosting hole and electron transfer, respectively. O2 is proven to be the only oxygen source for CH3OH production, while H2O acts as the promoter for efficient CH4 activation through ·OH production and facilitates product desorption as indicated by DFT modeling. This work thus provides new understandings on simultaneous regulation of both activity and selectivity by the synergy of single atom cocatalysts and oxygen vacancies.

Highly Selective Photocatalytic Aerobic Oxidation of Methane to Oxygenates with Water over W‐doped TiO2

Highly selective conversion of methane to oxygenates with O2 as a green oxidant remains a great challenge. It is still difficult to suppress the generation of COx (x=1, 2) as undesired by-products due to unavoidable overoxidation reaction. Hence, tungsten-doped (W-doped) TiO2 photocatalysts were designed with a tunable band structure for photocatalytic oxidation of methane to C1 oxygenates using O2 at low temperature (30 °C). The W-doping effectively modified the electronic and band structure of pristine TiO2 to enhance photocatalytic performance. Liquid oxygenates productivity could reach as high as 12.2 mmol g-1 with high selectivity of 99.4 %. Moreover, COx selectivity was effectively decreased from 21.2 % over TiO2 to 0.6 % for W-doped catalyst. Detailed characterizations further disclosed that W-doping not only enhanced light absorption, but also promoted the separation of photo-generated carriers to improve methane conversion. This work provides new insights into the design of highly efficient photocatalysts for methane oxidation.

Intrinsic activity and selectivity enhancement of single-atom Rh in syngas-to-C2 oxygenates by engineering the local coordination atom

Noble-metal Rh catalyst is promising for syngas conversion to C2 oxygenates, and the single-atom Rh catalysts hold promise to minimize Rh contents. In this work, a series of single-atom Rh catalysts anchored by the local coordination atom B, N and P-doped graphene (Rh-C4, Rh-N4, Rh-N3C1, Rh-N3P1 and Rh-N3B1) were theoretically predicted and verified by the related reported experiment to tune the selectivity and intrinsic activity of syngas conversion to C2 oxygenates. Here, DFT calculations and microkinetic modeling were implemented. The Rh-N3P1 catalyst was screened out to have excellent intrinsic activity, selectivity and thermal stability at 525 K, attributing to the synergistic active sites between the local coordination atoms (N/P) and Rh to provide abundant CHx monomer, followed by CO/CHO insertion to C2 oxygenates. The small positive charge and moderate d-band center of single-atom Rh in SACs are the essential attributes to improve intrinsic activity, namely, the precise local coordination of heteroatom N/P significantly regulates the electronic structure of single-atom Rh and further tunes catalytic performance. This study provides new insights and theoretical basis for engineering the local coordination atoms of carbon-based material supported metal SACs to tune catalytic performance and expanding their applications in the field of other systems.

Reduced graphene oxide supported cobalt catalysts for ethylene hydroformylation: Modified cobalt-support interaction by rhodium

The metal-support interaction (MSI) alters the geometric morphologies, electronic properties, or distribution of metal nanoparticles, which is indeed to influence the heterogenous catalytic system. Two heterogenous ethylene hydroformylation catalysts, cobalt supported on reduced graphene oxide (RGO) with or without Rhodium (5%Rh-20%Co/RGO and 20%Co/RGO), were successfully synthesized. The effect of the MSI between Co and RGO and a MSI tuning strategy by Rh on the ethylene hydroformylation (EH) were investigated. CoO nanorods was present in fresh Rh-Co/RGO, while only CoO nano-particles were formed in fresh Co/RGO. Rh enhanced the reducibility of the cobalt species. After reduction and the ethylene hydroformylation reaction, Co2C was formed in spent Rh-Co/RGO, while a mixture of Co and CoO were present in the spent Co/RGO. Co2C grew to a large flat plane structure in spent Rh-Co/RGO, while Co and CoO formed smaller nano-particles in spent Co/RGO. The changes of the geometric morphologies and the phases of the active species made by the Rh-Co-RGO interaction significantly enhance the activity of the CO and the selectivity for the production of the C3 oxygenates. A Model on how the nanoparticles distributed on the RGO were hypothesized.

Identifying Monomeric Fe Species for Efficient Direct Methane Oxidation to C1 Oxygenates with H2O2 over Fe/MOR Catalysts

Exploring advanced catalysts and reaction systems operated at mild reaction conditions is crucial for conducting the direct methane oxidation reaction toward oxygenate products. Many efforts have been put into research on pentasil−type (MFI) zeolites based on mononuclear and/or binuclear iron sites, using H2O2 as the oxidant. In this work, we present a modified liquid ion−exchange method to better control Fe loading in a mordenite−type (MOR) zeolite with a Si/Al molar ratio of 9. The optimized Fe/MOR catalyst showed excellent performance in the direct methane oxidation reaction with turnover frequencies (TOFs) of 555 h−1 to C1 oxygenates, significantly better than the reported activity. Multiple comparative experiments were conducted to reveal the mechanism behind the performance. Strikingly, the active sites in the Fe/MOR catalyst were found to be mononuclear iron sites, confirmed by transmission electron microscopy (TEM), ultraviolet−visible diffuse reflectance spectroscopy (UV−vis DRS), and X-ray absorption spectroscopy (XAS). Increasing the iron loading led to the aggregation of the iron sites, which tend to trigger undesirable side reactions (i.e., H2O2 decomposition and over−oxidation), resulting in a significant decrease in TOFs to C1 oxygenates.

Rational design of ZSM-5 zeolite containing a high concentration of single Fe sites capable of catalyzing the partial oxidation of methane with high turnover frequency

We successfully synthesized a Fe/ZSM-5 catalyst enabling conversion of methane to C1 oxygenates in record yields, and demonstrated that the fraction of the single Fe cation, as well as the Al distribution, are the powerful activity descriptors.