The Fundamentals Behind the Use of Flow Reactors in Electrochemistry

Abstract

ConspectusIn the past decade, research into continuous-flow chemistry has gained a lot of traction among researchers in both academia and industry. Especially, microreactors have received a plethora of attention due to the increased mass and heat transfer characteristics, the possibility to increase process safety, and the potential to implement automation protocols and process analytical technology. Taking advantage of these aspects, chemists and chemical engineers have capitalized on expanding the chemical space available to synthetic organic chemists using this technology.Electrochemistry has recently witnessed a renaissance in research interests as it provides chemists unique and tunable synthetic opportunities to carry out redox chemistry using electrons as traceless reagents, thus effectively avoiding the use of hazardous and toxic reductants and oxidants. The popularity of electrochemistry stems also from the potential to harvest sustainable electricity, derived from solar and wind energy. Hence, the electrification of the chemical industry offers an opportunity to locally produce commodity chemicals, effectively reducing inefficiencies with regard to transportation and storage of hazardous chemicals.The combination of flow technology and electrochemistry provides practitioners with great control over the reaction conditions, effectively improving the reproducibility of electrochemistry. However, carrying out electrochemical reactions in flow is more complicated than just pumping the chemicals through a narrow-gap electrolytic cell. Understanding the engineering principles behind the observations can help researchers to exploit the full potential of the technology. Thus, the prime objective of this Account is to provide readers with an overview of the underlying engineering aspects which are associated with continuous-flow electrochemistry. This includes a discussion of relevant mass and heat transport phenomena encountered in electrochemical flow reactors. Next, we discuss the possibility to integrate several reaction steps in a single streamlined process and the potential to carry out challenging multiphase electrochemical transformations in flow. Due to the high control over mass and heat transfer, electrochemical reactions can be carried out with great precision and reproducibility which provide opportunities to enhance and tune the reaction selectivity. Finally, we detail on the scale-up potential of flow electrochemistry and the importance of small interelectrode gaps on pilot and industrial-scale electrochemical processes. Each principle has been illustrated with a relevant organic synthetic example. In general, we have aimed to describe the underlying engineering principles in simple words and with a minimum of equations to attract and engage readers from both a synthetic organic chemistry and a chemical engineering background. Hence, we anticipate that this Account will serve as a useful guide through the fascinating world of flow electrochemistry.

Accelerating sulfonyl fluoride synthesis through electrochemical oxidative coupling of thiols and potassium fluoride in flow

“…Typically, the electrode size is increased and larger amounts of supporting electrolyte are required to cope with the increase in Ohmic drop. Therefore, nearly all industrial electrochemical processes are carried out as flow processes in narrow-gap cells (inter-electrode gap = 0.5-10 mm) which are numberedup depending on the targeted throughput [16]. As can be seen from Fig.…”

“…Biphasic reaction conditions (acetonitrile/1 M HCl) are required and the reaction was carried out in batch. However, preliminary studies showed that the use of flow chemistry was of great benefit [16][17][18][19][20]. The reaction time could be reduced to the minute range in flow and no mass transfer limitations were observed.…”

Translating batch electrochemistry to single-pass continuous flow conditions: an organic chemist’s guide

“…In addition to reduced waste generation, the atom economy of electrochemical methods is most often superior to conventional procedures. The resurgence of organic electrochemistry has been accompanied by an increased attention to flow electrochemistry [6][7][8][9][10]. Implementation of flow cells is widely recognized as the appropriate method for the scale-up of electrochemistry.…”

“…Flow cells typically consist of two flat electrodes placed in parallel with a narrow distance (< 1 mm) between them [6][7][8][9][10]. The solution of reactants solution is then flown through the gap, where the electrolysis occurs.…”

“…This configuration, often called parallel plate cell, minimizes the resistance of the electrolyte, which is proportional to the interelectrode distance, enabling the application of higher currents and lower concentrations of supporting electrolyte. Mass transfer, of key importance in the kinetics of electrochemical reactions, is typically enhanced in flow cells [9,10]. Mass transfer can be further improved if the flow cell is equipped with "turbulence promoters."…”