Highly selective electrocatalytic alkynol semi-hydrogenation for continuous production of alkenols

Electrochemical semi-hydrogenation of adiponitrile over copper nanowires as a key step for the green synthesis of nylon-6

The industrial production of nylon 6 usually includes synthesizing caprolactam through the cyclohexanone-hydroxylamine route. This approach requires complex protocols, elevated temperatures, noble metal catalysts and the use of hazardous strong acids or hydroxylamine. Additionally, a significant quantity of ammonium sulphate is generated during the synthesis procedure. This study aims to develop an electrochemical reduction system for the conversion of ADN generated from the electrolytic dimerization of acrylonitrile (AN) to 6-aminocapronitrile (ACN), a precursor of nylon 6. This system utilizes a cost-effective Cu nanomaterial under eco-friendly conditions, avoiding lengthy and harsh processes, eliminating NH2OH use, and reducing low-value ammonium sulfate generation. This electrosynthesis method maintains approximately 85% ACN selectivity at 40-100 mA cm-2 when passing the charge required for 37% theoretical conversion. When extending the reaction time to achieve an 80% conversion, ACN selectivity still reached 81.6%, exceeding the theoretical value of non-selective hydrogenation by 20%. The pseudo-first-order reaction kinetic modeling proves that the reaction rate constant for ADN hydrogenation is significantly greater than that for ACN hydrogenation, highlighting the selectivity advantage of the system for ACN. This study establishes the foundation for developing a continuous electrolysis process to produce the nylon 6 precursor from AN feedstock.

Alloying and confinement effects on hierarchically nanoporous CuAu for efficient electrocatalytic semi-hydrogenation of terminal alkynes

Electrocatalytic alkynes semi-hydrogenation to produce alkenes with high yield and Faradaic efficiency remains technically challenging because of kinetically favorable hydrogen evolution reaction and over-hydrogenation. Here, we propose a hierarchically nanoporous Cu50Au50 alloy to improve electrocatalytic performance toward semi-hydrogenation of alkynes. Using Operando X-ray absorption spectroscopy and density functional theory calculations, we find that Au modulate the electronic structure of Cu, which could intrinsically inhibit the combination of H* to form H2 and weaken alkene adsorption, thus promoting alkyne semi-hydrogenation and hampering alkene over-hydrogenation. Finite element method simulations and experimental results unveil that hierarchically nanoporous catalysts induce a local microenvironment with abundant K+ cations by enhancing the electric field within the nanopore, accelerating water electrolysis to form more H*, thereby promoting the conversion of alkynes. As a result, the nanoporous Cu50Au50 electrocatalyst achieves highly efficient electrocatalytic semi-hydrogenation of alkynes with 94% conversion, 100% selectivity, and a 92% Faradaic efficiency over wide potential window. This work provides a general guidance of the rational design for high-performance electrocatalytic transfer semi-hydrogenation catalysts.

Electron Divergence of Cuδ- and Pdδ+ in Cu3Pd Alloy-Based Heterojunctions Boosts Concerted C≡C Bond Binding and the Volmer Step for Alkynol Semihydrogenation

Electrocatalytic semihydrogenation of alkynols presents a sustainable alternative to conventional thermal methodologies for the high-value production of alkenols. The design of efficient catalysts with superior catalytic and energy efficiency for semihydrogenation poses a significant challenge. Here, we present the application of an electron-divergent Cu3Pd alloy-based heterojunction in promoting the electrocatalytic semihydrogenation of alkynols to alkenols using water as the proton source. The tunable electron divergence of Cuδ- and Pdδ+, modulated by rectifying contact with nitrogen-rich carbons, enables the concerted binding of active H species from the Volmer step of water dissociation and the C≡C bond of alkynols on Pdδ+ sites. Simultaneously, the pronounced electron divergence of Cu3Pd facilitates the universal adsorption of OH species from the Volmer step and alkynols on the Cuδ- sites. The electron-divergent dual-center substantially boosts water dissociation and inhibition of completing hydrogen evolution to give a turnover frequency of 2412 h-1, outperforming the reported electrocatalysts' value of 7.3. Moreover, the continuous production of alkenols at industrial-related current density (-200 mA cm-2) over the efficient and durable Cu3Pd-based electrolyzer could achieve a cathodic energy efficiency of 45 mol kW·h-1, 1.7 times the bench-marked reactors, promising great potential for sustainable industrial synthesis.

Tandem Electrocatalytic Alkyne Semihydrogenation over Bicomponent Catalysts through Hydrogen Spillover

Electrocatalytic alkyne semihydrogenation under mild conditions is a more attractive approach for alkene production than industrial routes but suffers from either low production efficiency or high energy consumption. Here, we describe a tandem catalytic concept that overcomes these challenges. Component (i), which can trap hydrogen effectively, is partnered with component (ii), which can readily release hydrogen for hydrogenation, to enable efficient generation of active hydrogen on component (i) at low overpotentials and timely (i)-to-(ii) hydrogen spillover and facile desorptive hydrogenation on component (ii). We examine this concept over bicomponent palladium-copper catalysts for the production of representative 2-methyl-3-butene-2-ol (MBE) from 2-methyl-3-butyne-2-ol (MBY) and achieve a record high MBE production rate of 1.44 mmol h-1 cm-2 and a Faraday efficiency of ~88.8 % at a low energy consumption of 1.26 kWh kgMBE -1. With these catalysts, we further achieve 60 h continuous production of MBE with record high profit space.

Recent Advances in Electrocatalytic Hydrogenation Reactions on Copper-Based Catalysts

Hydrogenation reactions play a critical role in the synthesis of value-added products within the chemical industry. Electrocatalytic hydrogenation (ECH) using water as the hydrogen source has emerged as an alternative to conventional thermocatalytic processes for sustainable and decentralized chemical synthesis under mild conditions. Among the various ECH catalysts, copper-based (Cu-based) nanomaterials are promising candidates due to their earth-abundance, unique electronic structure, versatility, and high activity/selectivity. Herein, recent advances in the application of Cu-based catalysts in ECH reactions for the upgrading of valuable chemicals are systematically analyzed. The unique properties of Cu-based catalysts in ECH are initially introduced, followed by design strategies to enhance their activity and selectivity. Then, typical ECH reactions on Cu-based catalysts are presented in detail, including carbon dioxide reduction for multicarbon generation, alkyne-to-alkene conversion, selective aldehyde conversion, ammonia production from nitrogen-containing substances, and amine production from organic nitrogen compounds. In these catalysts, the role of catalyst composition and nanostructures toward different products is focused. The co-hydrogenation of two substrates (e.g., CO2 and NOx n, SO3 2-, etc.) via C─N, C─S, and C─C cross-coupling reactions are also highlighted. Finally, the critical issues and future perspectives of Cu-catalyzed ECH are proposed to accelerate the rational development of next-generation catalysts.

Membrane-Free Selective Semi-Hydrogenation of Alkynes Over an In Situ Formed Copper Nanoparticle Electrode

Selective semi-hydrogenation of alkynes is a significant reaction for preparing functionalized alkenes. Electrochemical semi-hydrogenation presents a sustainable alternative to the traditional thermal process. In this research, affordable copper acetylacetonate is employed as a catalyst precursor for the electrocatalytic hydrogenation of alkynes, using MeOH as the hydrogen source in an undivided cell. Good to excellent yields for both aromatic and aliphatic internal/terminal alkynes are obtained under constant current conditions. Notably, up to 99% Z selectivity is achieved for various internal alkynes. Mechanistic investigations revealed the formation of copper nanoparticles (NPs) at the cathode during electrolysis, acting as the catalyst for the selective semireduction of alkynes. The copper NPs deposited cathode demonstrated reusable for further hydrogenation.

Hydrogen Spillover Accelerates Electrocatalytic Semi-hydrogenation of Acetylene in Membrane Electrode Assembly Reactor

Electrocatalytic acetylene semi-hydrogenation (EASH) offers a promising and environmentally friendly pathway for the production of C2H4, a widely used petrochemical feedstock. While the economic feasibility of this route has been demonstrated in three-electrode systems, its viability in practical device remains unverified. In this study, we designed a highly efficient electrocatalyst based on a PdCu alloy system utilizing the hydrogen spillover mechanism. The catalyst achieved an operational current density of 600 mA cm-2 in a zero-gap membrane electrode assembly (MEA) reactor, with the C2H4 selectivity exceeding 85%. This data confirms the economic feasibility of EASH in real-world applications. Furthermore, through in situ Raman spectroscopy and theoretical calculations, we elucidated the catalytic mechanism involving interfacial hydrogen spillover. Our findings underscore the economic viability and potential of EASH as a greener and scalable approach for C2H4 production, thus advancing the field of electrocatalysis in sustainable chemical synthesis.

Electroplated electrodes for continuous and mass-efficient electrochemical hydrogenation

Electrocatalytic hydrogenations (ECH) enable the reduction of organic substrates upon usage of electric current and present a sustainable alternative to conventional processes if green electricity is used. Opposed to most current protocols for electrode preparation, this work presents a one-step binder- and additive-free production of silver- and copper-electroplated electrodes. Controlled adjustment of the preparation parameters allows for the tuning of catalyst morphology and its electrochemical properties. Upon optimization of the deposition protocol and carbon support, high faradaic efficiencies of 93 % for the ECH of the Vitamin A- and E-synthon 2-methyl-3-butyn-2-ol (MBY) are achieved that can be maintained at current densities of 240 mA cm-2 and minimal catalyst loadings of 0.2 mg cm-2 , corresponding to an unmatched production rate of 1.47 kgMBE  gcat -1  h-1 . For a continuous hydrogenation process, the protocol can be directly transferred into a single-pass operation mode giving a production rate of 1.38 kgMBE  gcat -1  h-1 . Subsequently, the substrate spectrum was extended to a total of 17 different C-C-, C-O- and N-O-unsaturated compounds revealing the general applicability of the reported process. Our results lay an important groundwork for the development of electrochemical reactors and electrodes able to directly compete with the palladium-based thermocatalytic state of the art.

Developing electrochemical hydrogenation towards industrial application

Electrochemical hydrogenation reactions gained significant attention as a sustainable and efficient alternative to conventional thermocatalytic hydrogenations. This tutorial review provides a comprehensive overview of the basic principles, the practical application, and recent advances of electrochemical hydrogenation reactions, with a particular emphasis on the translation of these reactions from lab-scale to industrial applications. Giving an overview on the vast amount of conceivable organic substrates and tested catalysts, we highlight the challenges associated with upscaling electrochemical hydrogenations, such as mass transfer limitations and reactor design. Strategies and techniques for addressing these challenges are discussed, including the development of novel catalysts and the implementation of scalable and innovative cell concepts. We furthermore present an outlook on current challenges, future prospects, and research directions for achieving widespread industrial implementation of electrochemical hydrogenation reactions. This work aims to provide beginners as well as experienced electrochemists with a starting point into the potential future transformation of electrochemical hydrogenations from a laboratory curiosity to a viable technology for sustainable chemical synthesis on an industrial scale.

Electrochemical Reversible Reforming between Ethylamine and Acetonitrile on Heterostructured Pd-Ni(OH)2 Nanosheets

Rational design of electrocatalysts is essential to achieve desirable performance of electrochemical synthesis process. Heterostructured catalysts have thus attracted widespread attention due to their multifunctional intrinsic properties, and diverse catalytic applications with corresponding outstanding activities. Here, we report an in situ restoration strategy for the synthesis of ultrathin Pd-Ni(OH)2 nanosheets. Such Pd-Ni(OH)2 nanosheets exhibit excellent activity and selectivity towards reversible electrochemical reforming of ethylamine and acetonitrile. In the acetonitrile reduction process, Pd acts as reaction center, while Ni(OH)2 provide proton hydrogen through promoting the dissociation of water. Also ethylamine oxidation process can be achieved on the surface of the heterostructured nanosheets with abundant Ni(II) defects. More importantly, an electrolytic cell driven by solar cells was successfully constructed to realize ethylamine-acetonitrile reversible reforming. This work demonstrates the importance of heterostructure engineering in the rational synthesis of multifunctional catalysts towards electrochemical synthesis of fine chemicals.