(Digital Presentation) Development of Linear Paired Electrolysis for the Oxidation of Benzyl Alcohol
Authors: Horiguchi Genki , Kamiya Hihidero , Okada Yohei 
Published: July 2022

Electrochemical processes are always coupled reactions, with anodic oxidation and cathodic reduction. Paired electrolysis system can produce valuable products at both anode and cathode, and maximizes the efficient use of applied energy. Especially, the conversion of a single raw material to a single valuable product by paired electrolysis referred to as linear paired electrolysis. Since the electron transfers at both electrodes can produce a single valuable product in the linear paired electrolysis system, a single compound is generated with a current efficiency of 200%, theoretically. The linear paired electrolysis is appearing electrochemical process, however, it remains a challenging topic in organic electrochemical synthesis because it is complicated system. Herein, we develop the synthesis process of benzaldehyde by oxidation of benzyl alcohol using linear paired electrolysis. Electrochemical synthesis of carbonyl compounds by oxidation of alcohol is a challenging topic due to the selectivity and efficiency. We use aqueous media as the electrolyte and oxygen as the terminal oxidant to increase the selectivity and efficiency in the electrochemical transformation.

Electrochemical oxidation of benzyl alcohol was performed by following method. A 4.0 mmol of tetraethylammonium perchlorate (Et4NClO4) was dissolved in 20 mL of acetonitrile (CH3CN) and 20 mL of H2O (Milli-Q water). The resultant mixture was used as an electrolyte in this study. The electrolyte was purged with O2 or Ar for 30 min, and then a 0.5 mmol of benzyl alcohol was dissolved in the electrolyte. A 20 mL of cyclohexane was added to the electrolyte to prevent overoxidation of the product. Electrolysis was conducted at room temperature with three electrodes method: a 2 cm2 of carbon felt (CF) anode, a 2 cm2 of various cathode materials, and a Ag/AgCl reference electrode.

In the present study, transformation of 4-tert-butyl-benzyl alcohol (peak potential of oxidation: +1.76 V vs. Ag/AgCl) to 4-tert-butyl-benzaldehyde was used as a model reaction for optimization of reaction condition. First, we carried out constant potential electrolysis at +1.8 V vs. Ag/AgCl. Electrolysis using O2-saturated electrolyte resulted in a higher yield of the benzaldehyde (71%) than that using an Ar-saturated electrolyte (45%) when the coulomb amount was 2 F. This result suggested that O2 promoted the desired conversion. Cyclic voltammograms of the electrolyte suggested that reduction of O2 could proceed at cathode. Cathodic reduction of O2 could produce reactive oxygen species (ROS) which have strong oxidizing power. As a result, cathode in our electrochemical cell could have high oxidation potential. Therefore, it was indicated that desired oxidation progressed using ROS in our electrochemical system. In next step, we conducted electrochemical reactions under constant potential electrolysis at −0.7 V vs. Ag/AgCl which was suitable for the reduction of O2. In this condition, benzaldehyde was obtained in 86% yield when the coulomb amount was 2 F, and no byproduct was observed. From this result, we proposed the indirect oxidation pass using ROS generated at cathode. Interestingly, when the cathode (working electrode) was set to −0.7 V vs. Ag/AgCl, the measured potential of the anode (counter electrode) was +1.5 to +1.8 V vs. Ag/AgCl. Therefore, when the reduction of O2 proceeded at cathode, potential of anode reached sufficient value to oxidize the benzyl alcohol. It meant both direct oxidation and indirect oxidation progressed in a single electrochemical cell. It was linear paired electrolysis system of oxidation of benzyl alcohol.

We considered the reaction on cathode was important to progress desired oxidation, and the effect of different cathode materials was also investigated. In this investigation, carbon felt, glassy carbon, Pt, and Ni could be used as cathodes in the linear paired electrolysis system. In contrast, a sharp decrease in current was observed immediately after the start of electrolysis using Al as the cathode. Observation and analysis of the used Al electrodes suggested that surface of Al cathodes was passivated. It was indicated that electrochemically generated ROS with high oxidizing power might promote the passivation of the cathode materials. The observed passivation also supported the generation of ROS at cathode in our electrochemical cells.

Finally, we demonstrated the versatility of developed linear paired electrolysis with various benzyl alcohols. Especially, 4-Methoxybenzaldehyde was not only obtained in high yields (94%) but also with maximum current efficiencies of 146%. The exceeding 100% current efficiency also supported the linear paired electrolysis system.

In summary, we successfully developed a process for oxidation of benzyl alcohols via linear paired electrolysis with high selectivity and efficiency. Since desired reaction progressed both electrodes and aqueous media and O2 were used, this system offered green and sustainable chemical synthesis process.

Figure 1