A “hidden variable” that can determine the success or failure of a chemical reaction has been found in the immediate vicinity of electrodes.
A research team led by Professor Jaeyune Ryu of the Department of Chemical and Biological Engineering at Seoul National University College of Engineering has developed a method for improving the efficiency and selectivity of organic compound synthesis using oxygen and electricity by controlling “local pH,” or the acidity in the immediate vicinity of an electrode. Without separately adding peroxide oxidants, the researchers synthesized lactones and epoxides, which serve as feedstocks for plastics and fine chemicals, and increased target-product selectivity from approximately 16% to 97% through reactor design that controls electrode spacing and electrolyte flow.
The compounds examined in this study are closely connected to materials used in everyday life. Lactones are used as feedstocks for biodegradable plastics and polyurethanes, while epoxides are key feedstocks for adhesives, coatings, and epoxy resins used in electronic materials. Both are also widely used as intermediates in the production of pharmaceuticals and fine chemicals. Improving the way these widely used compounds are synthesized could therefore contribute to the development of safer and more sustainable chemical processes.
The findings were published in the Journal of the American Chemical Society (JACS), a leading international chemistry journal published by the American Chemical Society (ACS).
Organic electrosynthesis is a technology that uses electricity to produce organic compounds. It has attracted attention as an alternative approach to sustainable chemical processing because it can reduce the use of oxidizing and reducing agents required in conventional chemical synthesis. Previous studies, however, have focused primarily on optimizing bulk-solution conditions such as voltage, electrode materials, and electrolyte composition. By comparison, the microscopic environment immediately surrounding the electrode—where molecules actually exchange electrons and undergo reactions—has received relatively little attention.
The researchers focused on the possibility that this microscopic environment could determine the outcome of a synthesis reaction. When protons are generated or consumed at an electrode faster than they can be transported through the solution, the local pH can differ substantially from the pH of the bulk solution. The team found that this difference affects not only the generation of the reactive oxygen species required for the reaction, but also the reactivity of organic molecules and the stability of the products. The researchers then proposed a way to control this environment through reactor design.
The team also examined how broadly this phenomenon may occur in organic electrosynthesis. After analyzing more than 600 related papers published since 2010, the researchers found that in approximately 89% of the studies examined, reactions at at least one electrode involved the transfer not only of electrons but also of protons, or hydrogen ions. This suggests that local changes in acidity may not be limited to a handful of specialized reactions but could be a variable that should be considered across organic electrosynthesis more broadly.
The differences were also clear in electrochemical measurements. Even when the bulk solution was nominally neutral, the environment near the cathode was estimated to become alkaline at approximately pH 11, while the region near the anode became acidic at approximately pH 2.5. Under such extreme local conditions, the supply of reactive oxygen species required for the reaction can decrease, the chemical form of the starting material can change, and degradation of the resulting product can be accelerated. This explains why adjusting the pH of the bulk solution alone is not sufficient and why the microscopic environment surrounding the electrode, where the reaction actually takes place, must also be considered.
To demonstrate this principle, the researchers used the Baeyer–Villiger oxidation as a representative reaction. Baeyer–Villiger oxidation is an organic oxidation reaction that inserts an oxygen atom into a ketone to convert it into an ester or lactone. Lactones, in particular, are cyclic ester compounds used as feedstocks for materials including biodegradable plastics and polyurethanes.
Conventional Baeyer–Villiger oxidation often relies on highly reactive oxidants such as peroxyacids, creating challenges in reagent handling and byproduct treatment. Instead of adding such oxidants externally, the researchers established an electrochemical reaction pathway in which oxygen gas is activated at the electrode and an oxygen atom is transferred to an organic molecule. Isotope-tracing experiments confirmed that the oxygen atom incorporated into the product originated from oxygen gas rather than water.
The researchers also identified a pathway in which reactive oxygen species generated by oxygen reduction at the cathode initiate the oxygen-transfer reaction, while the hydrogen peroxide produced in the process is converted back into reactive oxygen species at the anode. In other words, the two electrodes do not perform entirely separate roles; rather, they work together to maintain the supply of the reactive species required for the reaction. The team confirmed that this process is highly sensitive to local pH, providing a mechanistic basis for controlling acidity near the electrodes.
The researchers combined computer simulations with electrochemical measurements to analyze changes in local pH. Based on these findings, they designed a reactor that narrows the distance between the electrodes and allows the electrolyte to flow between them. The principle is to reduce extreme pH deviations by allowing the acidic environment near the anode and the alkaline environment near the cathode to counterbalance each other.
The effect of this control strategy was quantitatively evaluated by varying electrode spacing and electrolyte flow conditions. Compared with conditions in which local pH deviations were not mitigated, the proportion of the target product formed selectively increased from approximately 16% to approximately 97%. The amount produced per unit time also increased steadily as the electrolyte flow rate increased, with an approximately 14-fold difference between the lowest and highest flow rates tested. When the electrode spacing was reduced from 20 mm to 4 mm, the production rate increased by approximately 2.2-fold. Notably, when the solution was not allowed to flow, almost none of the target product was obtained even though the voltage and solution composition remained the same. These results demonstrate that the success or failure of a reaction can depend strongly not only on the electrode material and applied voltage, but also on the local reaction environment.
Under the conditions established through this approach, the researchers synthesized lactones from 18 ketone compounds, obtaining the representative product ε-caprolactone at an 82% yield. ε-Caprolactone is a feedstock for the biodegradable polymer polycaprolactone, which is used in medical materials including surgical sutures and drug-delivery systems. The same local pH control principle was also applied to epoxide synthesis, yielding products from six alkene substrates. This demonstrates the potential to apply the principle to the synthesis of a wide range of chemical feedstocks, including those used in adhesives, coatings, and resins for electronic materials.
The significance of this study lies in establishing both a reaction pathway and a reactor-design principle for synthesizing organic compounds using oxygen and electricity while reducing reliance on externally added peroxide oxidants. In particular, the reaction proceeds at room temperature and atmospheric pressure, eliminating the need for separate heating or pressurization equipment. Because electricity serves as the energy source, the approach may also have potential for future chemical processes coupled with electricity generated from renewable energy.
From an industrial perspective, potential applications could be explored in areas such as pharmaceutical and fine-chemical intermediates. Because these products have high added value and require careful safety management, they align with the advantages of electrochemical processes, which can reduce the burden associated with hazardous oxidants and can be operated using relatively compact equipment. However, further process optimization and validation at production scale will be required before commercial application. More broadly, an important aspect of the study is that it expands organic electrosynthesis beyond optimizing voltage or solution composition toward “designing the environment in which molecules actually react.” The researchers expect that considering local pH in other electrosynthetic reactions involving proton transfer could reveal new design opportunities for improving reaction efficiency and selectivity.
Professor Jaeyune Ryu, who supervised the research, said, “This study transforms ‘local pH’ near the electrode from a hidden variable into a controllable design principle.” He added, “To improve the efficiency of organic electrosynthesis, we need to design not only the reaction substrates and catalysts, but also the environment in which the molecules actually react.”
Seonghyeon Min and Junghoon Lee, M.S. students in the Department of Chemical and Biological Engineering at Seoul National University, participated in the study as co-first authors, while Hanju Kim, an integrated M.S./Ph.D. student, participated as a co-author. Professor Jaeyune Ryu supervised the research. Min and Lee plan to build on their research into organic electrosynthesis that takes local reaction environments into account and expand their work toward electrochemical reactions and processes applicable to the synthesis of a wide range of organic compounds.
The research was supported by Seoul National University, the National Research Foundation of Korea, and the Institute for Basic Science (IBS).
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Journal of the American Chemical Society
Experimental study
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The authors declare no competing financial interest.