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

Abstract

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.

Continuous Flow Esterification of a H-Phosphinic Acid, and Transesterification of H-Phosphinates and H-Phosphonates under Microwave Conditions

“…It has been a great challenge in the pharmaceutical industry to transform batch realizations of organic chemical reactions into continuous flow methods [1][2][3]. Kappe is one of the most prominent chemists who have elaborated flow chemical accomplishments that are welcome by the pharmaceutical industry in order to introduce up to date techniques, in the first approach, in the R&D segment [1].…”

“…Esterifications and transesterifications were chosen as suitable model reactions, as, in these cases, MW irradiation and IL additives proved to be useful in our earlier studies. (1) Before the flow chemical attempts, let us survey the precedents on the batch MW synthesis of alkyl phenyl-H-phosphinates (2). In the first round, phosphinic acid 1 was reacted with ethyl and other linear or branched C3-C5 and C8 alcohols applied in a 15-fold quantity at 160-200 °C to afford the esters 2a, 2b, 2d-i in yields of 73-90% (Table 1/Entries 1, 3, 7-9, 11, 13, 15, 17 and 19).…”

A Temperature-Control System for Continuous-Flow Microwave Heating Using a Magnetron as Microwave Source

“…Therefore, in order to improve the output of microwave-assisted chemical reaction and integrated microwave technology into the industrial chemical synthesis production, microwave continuous flow processing approach has been proposed. A large number of successful cases of microwave-assisted continuous flow chemical reactions have been reported, which show many advantages of continuous flow reactors, such as rapid temperature control, safety, and easy to scale up [6]- [9].…”

Practical and Scalable Organic Reactions with Flow Microwave Apparatus

“…However, a uniform heating profile should be realized in the thick tubular reaction vessel, a technology for which was developed by our research collaborators of SAIDA FDS. They constructed a bench‐top flow microwave reactor that uses a much larger reaction vessel compared to standard microreactors ( vide infra ), the details of which are described in a separate Personal Account by Barham and Yoshimura . To materialize the scale‐up of microwave‐assisted synthesis, we have investigated some organic reactions using the SAIDA flow microwave apparatus .…”