Currently, mainstream approaches to chemosynthesis and chemical production in industry use thermal energy. Hence, the proactive use of energy forms other than conventional thermal energy is an important step to creating and establishing new synthetic and production methods, which will contribute to the reduction of greenhouse gas emissions.

Electrochemical reactions, which are driven by electricity, can generally be carried out under mild conditions (room temperature and ambient pressure). In addition, they do not require any hazardous reagents and produce less waste than other conventional chemical syntheses. Therefore, electrosynthesis is known to be a mild and clean method for organic synthesis and there has recently been renewed interest in its development. However, electrosynthesis also has some disadvantages. Ordinary chemical reactions are homogeneous, while the reaction field of electrolysis is a heterogeneous interface, therefore electrosynthesis has a productive drawback. In addition, a large amount of supporting electrolyte is usually required. Therefore, the availability of solvents is limited by the necessity for dissolution of a supporting electrolyte. The presence of the supporting electrolyte might also cause separation problems in the reaction workup. Moreover, in an ordinary electrolysis system, solution resistance is generated between the working and counter electrodes, resulting in a decrease in energy efficiency. To overcome these problems, we focused on a proton exchange membrane (PEM) reactor.

The PEM reactor was originally developed for fuel cell technologies. As shown in Figure 1(a), a membrane electrode assembly (MEA) is integrated into the central part of the reactor. The MEA consists of an ion exchange membrane (proton-conducting polymer) sandwiched between a pair of catalyst layers on the anode and cathode sides, having the dual roles of electrode and supporting electrolyte (Figure 1(b)). For this reason, the substrate solution does not require a supporting electrolyte and the cell resistance can be minimized. In addition, it is also possible to conduct electrochemical reactions of gaseous substances and use non-polar solvents. The catalyst layers are composed of noble metals, carbon black, and an ionomer. They have a highly porous structure in order to provide an effective triple-phase boundary for efficient electrochemical reaction. Moreover, the reaction is conducted in a flow operation. Thus, the PEM reactor system possesses many characteristics designed to overcome the disadvantages of conventional electrosynthetic processes.

Under these backgrounds, we have applied a PEM reactor system to electrosynthetic processes. In particular, product and stereoselectivity controls in fine chemical syntheses using a PEM reactor system have been investigated for this project.

Figure 1. Schematic image of (a) the PEM reactor system and (b) the MEA.