Redox-mediated electrosynthesis of ethylene oxide from CO2 and water

Electrocatalytic water-to-oxygenates conversion: redox-mediated versus direct oxygen transfer

Electrocatalytic oxygenation of hydrocarbons with high selectivity has attracted much attention for its advantages in the sustainable and controllable production of oxygenated compounds with reduced greenhouse gas emissions. Especially when utilizing water as an oxygen source, by constructing a water-to-oxygenates conversion system at the anode, the environment and/or energy costs of producing oxygenated compounds and hydrogen energy can be significantly reduced. There is a broad consensus that the generation and transformation of oxygen species are among the decisive factors determining the overall efficiency of oxygenation reactions. Thus, it is necessary to elucidate the oxygen transfer process to suggest more efficient strategies for electrocatalytic oxygenation. Herein, we introduce oxygen transfer routes through redox-mediated pathways or direct oxygen transfer methods. Especially for the scarcely investigated direct oxygen transfer at the anode, we aim to detail the strategies of catalyst design targeting the efficient oxygen transfer process including activation of organic substrate, generation/adsorption of oxygen species, and transformation of oxygen species for oxygenated compounds. Based on these examples, the significance of balancing the generation and transformation of oxygen species, tuning the states of organic substrates and intermediates, and accelerating electron transfer for organic activation for direct oxygen transfer has been elucidated. Moreover, greener organic synthesis routes through heteroatom transfer and molecular fragment transfer are anticipated beyond oxygen transfer.

Electrifying oxidation of ethylene and propylene

Ethylene and propylene, as essential precursors in the chemical industry, have been playing a pivotal role in the production of various value-added chemicals that find wide applications in diverse sectors, such as polymer synthesis, lithium-ion battery electrolytes, antifreeze agents and pharmaceuticals. Nevertheless, traditional methods for olefin functionalization including chlorohydrination and epoxidation involve energy-intensive steps and environment-detrimental by-products. In contrast, electrocatalysis is emerging as a promising and sustainable approach for olefin oxidation via utilizing renewable electricity. Recent advancements in energy storage and conversion technologies have intensified the research efforts toward designing efficient electrocatalysts for the selective oxidation of ethylene and propylene, highlighting the shift towards more sustainable production methods. Herein, we summarize recent progress in the electrocatalytic oxidation of ethylene and propylene, focusing on achievement in catalyst design, reaction system selection and mechanism exploration. We figure out the advantages of different oxidation methods for improved performance and discuss the various types of catalysts like noble metals, non-noble metals, metal oxides and carbon-based materials, in facilitating the electrochemical oxidation of ethylene and propylene. Finally, we also provide an overview of current challenges and problems requiring further works.

Solid Electrolytes for Low-Temperature Carbon Dioxide Valorization: A Review

One of the most promising approaches to address the global challenge of climate change is electrochemical carbon capture and utilization. Solid electrolytes can play a crucial role in establishing a chemical-free pathway for the electrochemical capture of CO2. Furthermore, they can be applied in electrocatalytic CO2 reduction reactions (CO2RR) to increase carbon utilization, produce high-purity liquid chemicals, and advance hybrid electro-biosystems. This review article begins by covering the fundamentals and processes of electrochemical CO2 capture, emphasizing the advantages of utilizing solid electrolytes. Additionally, it highlights recent advancements in the use of the solid polymer electrolyte or solid electrolyte layer for the CO2RR with multiple functions. The review also explores avenues for future research to fully harness the potential of solid electrolytes, including the integration of CO2 capture and the CO2RR and performance assessment under realistic conditions. Finally, this review discusses future opportunities and challenges, aiming to contribute to the establishment of a green and sustainable society through electrochemical CO2 valorization.

Redox Molecular Junction Metal-Covalent Organic Frameworks for Light-assisted CO2 Energy Storage

Visible-light sensitive and bi-functionally favored CO2 reduction (CRR)/evolution (CER) photocathode catalysts that can get rid of the utilization of ultraviolet light and improve sluggish kinetics is demanded to conquer the current technique-barrier of traditional Li-CO2 battery. Here, a kind of redox molecular junction sp2c metal-covalent organic framework (i.e. Cu3-BTDE-COF) has been prepared through the connection between Cu3 and BTDE and can serve as efficient photocathode catalyst in light-assisted Li-CO2 battery. Cu3-BTDE-COF with redox-ability, visible-light-adsorption region, electron-hole separation ability and endows the photocathode with excellent round-trip efficiency (95.2 %) and an ultralow voltage hysteresis (0.18 V), outperforming the Schiff base COFs (i.e. Cu3-BTDA-COF and Cu3-DT-COF) and majority of the reported photocathode catalysts. Combined theoretical calculations with characterizations, Cu3-BTDE-COF with the integration of Cu3 centers, thiazole and cyano groups possess strong CO2 adsorption/activation and Li+ interaction/diffusion ability to boost the CRR/CER kinetics and related battery property.

Boosting Electrocatalytic Ethylene Epoxidation by Single Atom Modulation

The electrochemical synthesis of ethylene oxide (EO) using ethylene and water under ambient conditions presents a low-carbon alternative to existing industrial production process. Yet, the electrocatalytic ethylene epoxidation route is currently hindered by largely insufficient activity, EO selectivity, and long-term stability. Here we report a single atom Ru-doped hollandite structure KIr4O8 (KIrRuO) nanowire catalyst for efficient EO production via a chloride-mediated ethylene epoxidation process. The KIrRuO catalyst exhibits an EO partial current density up to 0.7 A cm-2 and an EO yield as high as 92.0 %. The impressive electrocatalytic performance towards ethylene epoxidation is ascribed to the modulation of electronic structures of adjacent Ir sites by single Ru atoms, which stabilizes the *CH2CH2OH intermediate and facilitates the formation of active Cl2 species during the generation of 2-chloroethanol, the precursor of EO. This work provides a single atom modulation strategy for improving the reactivity of adjacent metal sites in heterogeneous electrocatalysts.

Spatial decoupling of bromide-mediated process boosts propylene oxide electrosynthesis

The electrochemical synthesis of propylene oxide is far from practical application due to the limited performance (including activity, stability, and selectivity). In this work, we spatially decouple the bromide-mediated process to avoid direct contact between the anode and propylene, where bromine is generated at the anode and then transferred into an independent reactor to react with propylene. This strategy effectively prevents the side reactions and eliminates the interference to stability caused by massive alkene input and vigorously stirred electrolytes. As expected, the selectivity for propylene oxide reaches above 99.9% with a remarkable Faradaic efficiency of 91% and stability of 750-h (>30 days). When the electrode area is scaled up to 25 cm2, 262 g of pure propylene oxide is obtained after 50-h continuous electrolysis at 6.25 A. These findings demonstrate that the electrochemical bromohydrin route represents a viable alternative for the manufacture of epoxides.

One-Step Electrochemical Ethylene-to-Ethylene Glycol Conversion over a Multitasking Molecular Catalyst

Ethylene glycol is an essential commodity chemical with high demand, which is conventionally produced via thermocatalytic oxidation of ethylene with huge fossil fuel consumption and CO2 emission. The one-step electrochemical approach offers a sustainable route but suffers from reliance on noble metal catalysts, low activity, and mediocre selectivity. Herein, we report a one-step electrochemical oxidation of ethylene to ethylene glycol over an earth-abundant metal-based molecular catalyst, a cobalt phthalocyanine supported on a carbon nanotube (CoPc/CNT). The catalyst delivers ethylene glycol with 100% selectivity and 1.78 min-1 turnover frequency at room temperature and ambient pressure, more competitive than those obtained over palladium catalysts. Experimental data demonstrate that the catalyst orchestrates multiple tasks in sequence, involving electrochemical water activation to generate high-valence Co-oxo species, ethylene epoxidation to afford an ethylene oxide intermediate via oxygen transfer, and eventually ring-opening of ethylene oxide to ethylene glycol facilitated by in situ formed Lewis acid site. This work offers a great opportunity for commodity chemicals synthesis based on a one-step, earth-abundant metal-catalyzed, and renewable electricity-driven route.

Membrane-Free Electrosynthesis of Epichlorohydrins Mediated by Bromine Radicals over Nanotips

The industrial manufacture of epichlorohydrin (ECH) often suffers from excessive corrosive chlorine and multistep processes. Here, we report a one-pot membrane-free Br radical-mediated ECH electrosynthesis. Bromine radicals electro-oxidized from Br- ions initiate the reaction and then eliminate HBr from bromohydrin to give ECH and release Br- ions for reuse. A high energy barrier for *OH oxidation and isolated Br adsorption sites enables NiCo2O4 to suppress the competitive oxygen and bromine evolution reactions. The high-curvature nanotips with an increased electric field concentrate Br- and OH- ions to accelerate ECH electrosynthesis. This strategy delivers ECH with a Faradaic efficiency of 47% and a reaction rate of 1.4 mol h-1 gcat-1 at a high current density of 100 mA cm-2, exceeding the profitable target from the techno-economic analysis. Economically profitable electrosynthesis, methodological universality, and the extended synthesis of epoxide-drug blocks highlight their promising potential.

Recent Advances in Hybrid Seawater Electrolysis for Hydrogen Production

Seawater electrolysis (SWE) is a promising and potentially cost-effective approach to hydrogen production, considering that seawater is vastly abundant and SWE is able to combine with offshore renewables producing green hydrogen. However, SWE has long been suffering from technical challenges including the high energy demand and interference of chlorine chemistry, leading electrolyzers to a low efficiency and short lifespan. In this context, hybrid SWE, operated by replacing the energy-demanding oxygen evolution reaction and interfering chlorine evolution reaction (CER) with a thermodynamically more favorable anodic oxidation reaction (AOR) or by designing innovative electrolyzer cells, has recently emerged as a better alternative, which not only allows SWE to occur in a safe and energy-saving manner without the notorious CER, but also enables co-production of value-added chemicals or elimination of environmental pollutants. This review provides a first account of recent advances in hybrid SWE for hydrogen production. The substitutional AOR of various small molecules or redox mediators, in couple with hydrogen evolution from seawater, is comprehensively summarized. Moreover, how the electrolyzer cell design helps in hybrid SWE is briefly discussed. Last, the current challenges and future outlook about the development of the hybrid SWE technology are outlined.

Electrochemical Epoxidation of Propylene to Propylene Oxide via Halogen-Mediated Systems

As one of the most important derivatives of propylene, the production of propylene oxide (PO) is severely restricted. The traditional chlorohydrin process is being eliminated due to environmental concerns, while processes such as Halcon and hydrogen peroxide epoxidation are limited by cost and efficiency, making it difficult to meet market demand. Therefore, achieving PO production through clean and efficient technologies has received extensive attention, and halogen-mediated electrochemical epoxidation of alkene is considered to be a desirable technology for the production of alkylene oxide. In this work, we used electrochemical methods to synthesize PO in halogen-mediated systems based on a RuO2-loaded Ti (RuO2/Ti) anode and screened out two potential mediated systems of chlorine (Cl) and bromine (Br) for the electrosynthesis of PO. At a current density of 100 mA·cm-2, both Cl- and Br-mediated systems delivered PO Faradaic efficiencies of more than 80%. In particular, the Br-mediated system obtained PO Faradaic efficiencies of more than 90% at lower potentials (≤1.5 V vs RHE) with better electrode structure durability. Furthermore, detailed product distribution investigations and DFT calculations suggested hypohalous acid molecules as key reaction intermediates in both Cl- and Br-mediated systems. This work presents a green and efficient PO production route with halogen-mediated electrochemical epoxidation of propylene driven by renewable electricity, exhibiting promising potential to replace the traditional chlorohydrin process.

Next-generation feedstocks methanol and ethylene glycol and their potential in industrial biotechnology

Microbial fermentation processes are expected to play an important role in reducing dependence on fossil-based raw materials for the production of everyday chemicals. In order to meet the growing demand for biotechnological products in the future, alternative carbon sources that do not compete with human nutrition must be exploited. The chemical conversion of the industrially emitted greenhouse gas CO2 into microbially utilizable platform chemicals such as methanol represents a sustainable strategy for the utilization of an abundant carbon source and has attracted enormous scientific interest in recent years. A relatively new approach is the microbial synthesis of products from the C2-compound ethylene glycol, which can also be synthesized from CO2 and non-edible biomass and, in addition, can be recovered from plastic waste. Here we summarize the main chemical routes for the synthesis of methanol and ethylene glycol from sustainable resources and give an overview of recent metabolic engineering work for establishing natural and synthetic microbial assimilation pathways. The different metabolic routes for C1 and C2 alcohol-dependent bioconversions were compared in terms of their theoretical maximum yields and their oxygen requirements for a wide range of value-added products. Assessment of the process engineering challenges for methanol and ethylene glycol-based fermentations underscores the theoretical advantages of new synthetic metabolic routes and advocates greater consideration of ethylene glycol, a C2 substrate that has received comparatively little attention to date.

Electrocatalytic CO2 Reduction to Ethylene: From Advanced Catalyst Design to Industrial Applications

The value-added chemicals, monoxide, methane, ethylene, ethanol, ethane, and so on, can be efficiently generated through the electrochemical CO2 reduction reaction (eCO2 RR) when equipped with suitable catalysts. Among them, ethylene is particularly important as a chemical feedstock for petrochemical manufacture. However, despite its high Faradaic efficiency achievable at relatively low current densities, the substantial enhancement of ethylene selectivity and stability at industrial current densities poses a formidable challenge. To facilitate the industrial implementation of eCO2 RR for ethylene production, it is imperative to identify key strategies and potential solutions through comprehending the recent advancements, remaining challenges, and future directions. Herein, the latest and innovative catalyst design strategies of eCO2 RR to ethylene are summarized and discussed, starting with the properties of catalysts such as morphology, crystalline, oxidation state, defect, composition, and surface engineering. The review subsequently outlines the related important state-of-the-art technologies that are essential in driving forward eCO2 RR to ethylene into practical applications, such as CO2 capture, product separation, and downstream reactions. Finally, a greenhouse model that integrates CO2 capture, conversion, storage, and utilization is proposed to present an ideal perspective direction of eCO2 RR to ethylene.

Direct time-resolved observation of surface-bound carbon dioxide radical anions on metallic nanocatalysts

Time-resolved identification of surface-bound intermediates on metallic nanocatalysts is imperative to develop an accurate understanding of the elementary steps of CO2 reduction. Direct observation on initial electron transfer to CO2 to form surface-bound CO2•- radicals is lacking due to the technical challenges. Here, we use picosecond pulse radiolysis to generate CO2•- via aqueous electron attachment and observe the stabilization processes toward well-defined nanoscale metallic sites. The time-resolved method combined with molecular simulations identifies surface-bound intermediates with characteristic transient absorption bands and distinct kinetics from nanosecond to the second timescale for three typical metallic nanocatalysts: Cu, Au, and Ni. The interfacial interactions are further investigated by varying the important factors, such as catalyst size and the presence of cation in the electrolyte. This work highlights fundamental ultrafast spectroscopy to clarify the critical initial step in the CO2 catalytic reduction mechanism.

Bromine-Enhanced Generation and Epoxidation of Ethylene in Tandem CO2 Electrolysis Towards Ethylene Oxide

The indirect electro-epoxidation of ethylene (C2 H4 ), produced from CO2 electroreduction (CO2 R), holds immense promise for CO2 upcycling to valuable ethylene oxide (EO). However, this process currently has a mediocre Faradaic efficiency (FE) due to sluggish formation and rapid dissociation of active species, as well as reductive deactivation of Cu-based electrocatalysts during the conversion of C2 H4 to EO and CO2 to C2 H4 , respectively. Herein, we report a bromine-induced dual-enhancement strategy designed to concurrently promote both C2 H4 -to-EO and CO2 -to-C2 H4 conversions, thereby improving EO generation, using single-atom Pt on N-doped CNTs (Pt1 /NCNT) and Br- -bearing porous Cu2 O as anode and cathode electrocatalysts, respectively. Physicochemical characterizations including synchrotron X-ray absorption, operando infrared spectroscopy, and quasi in situ Raman spectroscopy/electron paramagnetic resonance with theoretical calculations reveal that the favorable Br2 /HBrO generation over Pt1 /NCNT with optimal intermediate binding facilitates C2 H4 -to-EO conversion with a high FE of 92.2 %, and concomitantly, the Br- with strong nucleophilicity protects active Cu+ species in Cu2 O effectively for improved CO2 -to-C2 H4 conversion with a FE of 66.9 % at 800 mA cm-2 , superior to those in the traditional chloride-mediated case. Consequently, a single-pass FE as high as 41.1 % for CO2 -to-EO conversion can be achieved in a tandem system.

Electrified hydrocarbon-to-oxygenates coupled to hydrogen evolution for efficient greenhouse gas mitigation

Chemicals manufacture is among the top greenhouse gas contributors. More than half of the associated emissions are attributable to the sum of ammonia plus oxygenates such as methanol, ethylene glycol and terephthalic acid. Here we explore the impact of electrolyzer systems that couple electrically-powered anodic hydrocarbon-to-oxygenate conversion with cathodic H₂ evolution reaction from water. We find that, once anodic hydrocarbon-to-oxygenate conversion is developed with high selectivities, greenhouse gas emissions associated with fossil-based NH₃ and oxygenates manufacture can be reduced by up to 88%. We report that low-carbon electricity is not mandatory to enable a net reduction in greenhouse gas emissions: global chemical industry emissions can be reduced by up to 39% even with electricity having the carbon footprint per MWh available in the United States or China today. We conclude with considerations and recommendations for researchers who wish to embark on this research direction.

Killing Two Birds with One Stone: Upgrading Organic Compounds via Electrooxidation in Electricity-Input Mode and Electricity-Output Mode

The electrochemically oxidative upgrading reaction (OUR) of organic compounds has gained enormous interest over the past few years, owing to the advantages of fast reaction kinetics, high conversion efficiency and selectivity, etc., and it exhibits great potential in becoming a key element in coupling with electricity, synthesis, energy storage and transformation. On the one hand, the kinetically more favored OUR for value-added chemical generation can potentially substitute an oxygen evolution reaction (OER) and integrate with an efficient hydrogen evolution reaction (HER) or CO₂ electroreduction reaction (CO₂RR) in an electricity-input mode. On the other hand, an OUR-based cell or battery (e.g., fuel cell or Zinc-air battery) enables the cogeneration of value-added chemicals and electricity in the electricity-output mode. For both situations, multiple benefits are to be obtained. Although the OUR of organic compounds is an old and rich discipline currently enjoying a revival, unfortunately, this fascinating strategy and its integration with the HER or CO₂RR, and/or with electricity generation, are still in the laboratory stage. In this minireview, we summarize and highlight the latest progress and milestones of the OUR for the high-value-added chemical production and cogeneration of hydrogen, CO₂ conversion in an electrolyzer and/or electricity in a primary cell. We also emphasize catalyst design, mechanism identification and system configuration. Moreover, perspectives on OUR coupling with the HER or CO₂RR in an electrolyzer in the electricity-input mode, and/or the cogeneration of electricity in a primary cell in the electricity-output mode, are offered for the future development of this fascinating technology.

Coupling Value-Added Anodic Reactions with Electrocatalytic CO2 Reduction

Electrocatalytic CO₂ reduction features a promising approach to realize carbon neutrality. However, its competitiveness is limited by the sluggish oxygen evolution reaction (OER) at anode, which consumes a large portion of energy. Coupling value-added anodic reactions with CO₂ electroreduction has been emerging as a promising strategy in recent years to enhance the full-cell energy efficiency and produce valuable chemicals at both cathode and anode of the electrolyzer. This review briefly summarizes recent progresses on the electrocatalytic CO₂ reduction, and the economic feasibility of different CO₂ electrolysis systems is discussed. Then a comprehensive summary of recent advances in the coupled electrolysis of CO₂ and potential value-added anodic reactions is provided, with special focus on the specific cell designs. Finally, current challenges and future opportunities for the coupled electrolysis systems are proposed, which are targeted to facilitate progress in this field and push the CO₂ electrolyzers to a more practical level.

Electrocatalytic CO2 Upgrading to Triethanolamine by Bromine-Assisted C2 H4 Oxidation

The electrocatalytic carbon dioxide (CO₂ ) reduction is a promising approach for converting this greenhouse gas into value-added chemicals, while the capability of producing products with longer carbon chains (C_n >3) is limited. Herein, we demonstrate the Br-assisted electrocatalytic oxidation of ethylene (C₂ H₄ ), a major CO₂ electroreduction product, into 2-bromoethanol by electro-generated bromine on metal phthalocyanine catalysts. Due to the preferential formation of Br₂ over *O or Cl₂ to activate the C=C bond, a high partial current density of producing 2-bromoethanol (46.6 mA⋅cm^-2 ) was obtained with 87.2 % Faradaic efficiency. Further coupling with the electrocatalytic nitrite reduction to ammonia at the cathode allowed the production of triethanolamine with six carbon atoms. Moreover, by coupling a CO₂ electrolysis cell for in situ C₂ H₄ generation and a C₂ H₄ oxidation/nitrite reduction cell, the capability of upgrading of CO₂ and nitrite into triethanolamine was demonstrated.