Copper-based Catalysts For Electrocatalytic Transfer Semi-hydrogenation Of Alkynes In Water

Olefins are components of many natural products as well as biological macromolecules,and are key intermediates used in the production of many fine and functional chemicals.Selective hydrogenation of alkynes is one of the most common methods for the production of olefins.Most conventional hydrogenation methods involve or directly use gaseous H₂,which,due to its flammability and explosiveness,poses additional difficulties in terms of storage and transport,especially in large-scale commercial applications where,for example,an H₂ leak would cause significant economic losses and environmental hazards.Electrocatalytic conversion offers several advantages over conventional chemical processes,such as workability in aqueous feedstocks,elimination of external reducing or oxidizing reagents,and relatively mild reaction conditions.In particular,electrocatalytic semi-hydrogenation（ECSH）can be performed at mild temperatures and pressures relative to conventional Thermocatalytic conversions,and the reaction conditions in ECSH can be easily controlled by the applied electrical potential or current.In addition,hydrogen can be obtained in situ by hydrolysis,thus eliminating the need for an external supply of high-purity hydrogen.In this paper,Cu-CA-CP and Cu-ZnO electrocatalysts based on transition metal copper were prepared for the selective electrocatalytic semi-hydrogenation of alkynes to olefins in KOH solution with 4-ethynyl aniline as the substrate and water as the hydrogen source.Cu-CA-CP electrocatalysts were synthesized by a simple and easy-to-operate constant-current electrodeposition method for the highly selective reduction of alkynes to olefins.Due to the exceptional catalytic activity,Cu-CA-CP exhibited excellent performance in the electrocatalytic semi-hydrogenation of 4-ethynylaniline to 4-vinylaniline in 1 M KOH solution,and was able to obtain up to 99%olefin selectivity and 97%alkyne conversion.The ECSH expansion experiments were carried out on Cu-CA-CP electrocatalyst under the same mild conditions with a series of typical terminal alkynyl compounds with electrically absorbing and powered substituents on the aryl ring as substrates,and the results showed that the Cu-CA-CP catalyst cathode has promising applications in ECSH and the catalyst system can effectively selectively reduce alkynes.Meanwhile,five electrolysis cycles at a potential of-0.56 V vs.RHE maintained good selectivity and conversion,indicating that the Cu-CA-CP electrocatalyst has excellent catalytic activity and durability.However,competing hydrogen precipitation reactions on the Cu-CA-CP cathode resulted in low FE for alkyne semihydrogenation,which inevitably led to energy waste.Subsequently,a loaded Cu-ZnO electrocatalyst was designed and synthesized for the alkyne transfer semi-hydrogenation reaction in alkaline solution,and was able to achieve 70%FE while maintaining 99%selectivity for 3 h at-0.55 V vs.RHE applied potential in 0.013 M alkyne,15 m L of 1.0 M KOH electrolyte,which largely improved the electrical energy utilization efficiency.The crystal structure of the catalyst was determined by X-ray powder diffractometry,the morphology of the catalyst was observed by scanning electron microscopy and transmission electron microscopy,and the valence state of Cu in Cu-ZnO was determined by X-ray photoelectron spectroscopy.The hydrogenation performance of Cu-ZnO,Cu-ZnO-B,Cu-ZnO-N and ZnO-B catalysts was compared,and the introduction of Cu improved the electrohydrogenation performance,which may be related to the unique synergistic interaction between Cu and ZnO.A series of Cu-ZnO catalysts with different Cu loadings were synthesized under different ZnO:Cu NO₃（mol/mol）conditions and electrolyzed under the same conditions to compare the electrocatalytic hydrogenation performance and to determine the optimal ZnO to Cu NO₃ ratio.Meanwhile,five electrolysis cycles were continued to demonstrate the good stability of Cu-ZnO.The ECSH method was used to refine alkynes to high value olefins with high selectivity,avoiding the risk of flammability and explosion in conventional thermal hydrogenation and providing a green synthetic route.This study provides an efficient catalytic hydrogenation strategy for the reduction of alkynes.