Microwave Flow: A Perspective on Reactor and Microwave Configurations and the Emergence of Tunable Single‐Mode Heating Toward Large‐Scale Applications

• Published in: Chemical record Vol. 19; no. 1; pp. 188 - 203
• Main Authors: Barham, Joshua P., Koyama, Emiko, Norikane, Yasuo, Ohneda, Noriyuki, Yoshimura, Takeo
• Format: Journal Article
• Language: English
• Published: United States Wiley Subscription Services, Inc 01.01.2019
• Subjects: Chemical reactions; Chemical reactors; Chemistry; Configurations; Continuous flow; Dependence; Energy efficiency; flow chemistry; Heating; microwave chemistry; Microwave heating; Microwaves; Organic chemistry; Penetration depth; Process control; Process controls; process intensification; Reactors; scale up; Wave propagation; continuous flow; process intensification; microwave chemistry; scale up; flow chemistry
• ISSN: 1527-8999; 1528-0691
• DOI: 10.1002/tcr.201800104

Microwave heating in chemical reactions was first reported in 1986. There have since been many reports employing microwave heating in organic chemistry, where microwave heating has afforded higher yields of products in shorter time periods. However, such reactions are challenging to scale in batch due to the limited penetration depth of microwaves as well as the wave propagation dependence on cavity size. Continuous flow has addressed both these issues, enabling scalability of microwave processes. As such, a host of reports employing microwave flow chemistry have emerged, employing various microwave heating and reactor configurations in the context of either custom‐built or commercial apparatus. The focus of this review is to present the benefits of microwave heating in the context of continuous flow and to characterize the different types of microwave flow apparatus by their design (oscillator, cavity type and reactor vessel). We advocate the adoption of tunable, solid‐state oscillator single‐mode microwave flow reactors which are more versatile heaters, impart better process control and energy efficiency toward laboratory and larger‐scale synthetic chemistry applications. We present the benefits of microwave (MW) heating in the context of continuous flow and characterize different types of MW flow apparatus by their design. Various MW flow reactors and reaction examples (including our own) are presented on g/h to 100’s g/h scale. We advocate for tunable, solid‐state oscillator single‐mode MW flow reactors which are more versatile heaters, impart better process control and energy efficiency toward laboratory and larger‐scale synthetic chemistry applications.