Efficient Carboxylic Acid Synthesis through Electrolytic Reduction of CO2 using BDD Electrodes
Authors: Okamura Ryoya , Inaba Toya , Ishihara Keizo , Takagi Kai , Hunge Yuvaraj Mohan , Guan Sujun , Uetsuka Hiroshi , Terashima Chiaki 
Published: December 2024

Recent chemical advancements have brought a variety of products but also increased emissions of harmful substances, prompting the search for cleaner synthesis methods like organic electrolytic synthesis methods, using electrical energy for reactions. Meanwhile, with the urgent need to reduce CO2 emissions and achieve carbon neutrality, recycling and reducing technologies for CO2 have gained significant attention 1). This study focuses on electrolytic carboxylation, utilizing CO2 as a raw material, which presents a promising avenue for organic electrolytic synthesis, allowing the synthesis of high-value compounds from cost-effective materials using electrical energy as a reducing agent. Notably, it offers the advantage of producing less waste, positioning it as a robust option for industrial adoption centered on green chemistry. However, since electrocarboxylation reactions occur on electrode surfaces, conventional approaches utilize organic solvents and precious metals like platinum (Pt), prized for their corrosion resistance and durability. To address this challenge, we have directed our focus towards synthesizing boron-doped diamond (BDD) electrodes, incorporating boron into the diamond structure. BDD electrodes boast a wide potential window, high durability, and corrosion resistance, making them capable of inhibiting undesirable reactions and facilitating sustained synthesis. Herein, it aims to address challenges in electrolytic carboxylation reactions, enhance energy efficiency, analyze reactions using diamond electrodes, ultimately establish a new synthesis method contributing to carbon neutrality.

A comparative analysis of BDD and platinum electrodes was conducted to confirm the superior performance of BDD. BDD electrodes were prepared using the microwave plasma chemical vapor deposition method, employing acetone and methanol as carbon sources and boron trioxide as the boron source. Plasma irradiation of a mixture of raw materials and hydrogen gas was used to deposit BDD on a silicon substrate. Prior to the experiment, the BDD surface underwent ultraviolet ozone treatment with oxygen to enhance stability and establish consistent initial conditions. Under N2 or CO2 atmospheric conditions, cyclic voltammetry (CV) was performed with a scanning rate of 10 mV/s in Bu4NBF4/acetonitrile (0.1 M) and acetophenone (0.01 M) to investigate the electrochemical behavior of BDD and Pt electrodes. CV studies were utilized to investigate the reduction potential. Finally, acetophenone was synthesized via constant current electrolysis under a CO2 atmosphere.

Figure 1 displays the CV curves of a) BDD and b) Pt electrodes in acetonitrile solvent and acetophenone electrolyte, respectively. In the electrocarboxylation reaction, the electrochemical reduction of acetonitrile and CO2 occurs as a side reaction. Our findings indicate that the reduction of acetonitrile and CO2 does not occur at the reduction potential of acetophenone when using a BDD electrode. Conversely, when a Pt electrode is employed, acetonitrile is slightly reduced by the reduction potential of acetophenone, and CO2 reduction occurs as a side reaction. Figure 2(a) depicts the yield of atrolactic acid obtained using both BDD and Pt electrodes. In case of Pt electrode 13 % yield of atrolactic acid while in case of BDD electrode 25 % yield of atrolactic acid observed. These results demonstrate that the electrocarboxylation reaction efficiency of BDD electrodes is higher than that of platinum electrodes. Figure 2 (b) shows a schematic diagram of the electrocarboxylation reaction. Initially, acetophenone undergoes one electron reduction. The resulting substance reacts directly with CO2 and undergoes further reduction by one electron 2). This study showcases the potential of BDD electrodes for organic synthesis, particularly in the electrolytic carboxylation of acetophenone.

References

1) S. Wang, T. Feng, Y. Wang, Y. Qiu, Chem. Asian. J. 17, e202200543 (2022).

2) M. A. Stalcup, et al., ACS Sustainable Chem. Eng. 9, 10431-10436 (2021).

Figure 1