(83x) A Continuous Flow Microwave-Assisted Microreactor System

1. Introduction Recently, chemical reaction processes with microwaves have attracted attention, and a lot of microwave effects in organic synthesis have been reported1), e.g., reducing chemical reaction times from hours to minutes, reducing side reactions, and increasing yields. However, most chemical reactors using microwaves that have already been developed are batch-types, and there are few flow-types. Accordingly, the objective of this study is to develop a flow-type microwave-assisted chemical reactor, and moreover, we try to apply the reactor to microreaction technology. We designed the reactor using electromagnetic simulation and examined its energy absorption efficiency by conducting a water heating test. We also examined the yields and the reaction times of the Suzuki-Miyaura coupling reaction2) under microwave heating and conventional (oil-bath) heating. 2. A continuous flow microwave-assisted microreactor system Figure 1 shows the developed microwave-assisted microreactor system. It consists of five parts: a microwave generator, waveguide (which transfers microwaves), pump unit, control unit, and monitor. A quartz helical tube, in which a reagent solution flows and is heated by microwaves, vertically penetrates the waveguide. A stab tuner projects down from the upper surface of the waveguide, and it is possible to adjust the impedance in the waveguide by changing the tuner's length and position to optimize the microwave energy absorption efficiency of the reagents. The waveguide was designed on the basis of analyses of electric field intensity and energy absorption efficiency by electromagnetic simulation. Figure 2 shows the electric field intensity distribution in the waveguide. When water was run through the quartz helical tube, the energy absorption efficiency of the water was 95% when the tuner's length and position were adjusted, whereas when there was no tuner, it was 63%. 3. Water heating test Figure 3 shows the results of water heating test. The rising temperature (the difference between the temperatures of the inlet and the outlet of the reactor) was measured by changing the water flow rate through the quartz helical tube and the power of the microwave irradiation. The solid line shows the results obtained from the electromagnetic simulation (energy absorption by water is 95%), and the marks show the experimental measurements. The simulation corresponds with the measurements within ±10% accuracy. 4. Evaluation experiment We examined Suzuki-Miyaura coupling reactions under microwave irradiation using the developed microwave-assisted microreactor. For comparison, we also examined the reactions under conventional (oil-bath) heating. The outlet temperature was kept at 95°C in both methods. The reaction time under microwave heating reduced to about an eighth of that under oil-bath heating (Fig. 4). 5. Conclusion We developed a prototype of a continuous flow microwave-assisted microreactor system. The waveguide was designed on the basis of electromagnetic simulations analyzing electric field intensity and energy absorption efficiency. Evaluation experiments showed that the simulation corresponded with the measurements within ±10% accuracy. We found that in the developed reactor, the reagent efficiently absorbed microwave energy and the energy absorption efficiency of the water was 95%. Moreover, by examining Suzuki-Miyaura coupling reactions, we found that the reaction time reduced to about an eighth of that when using conventional heating. This research was financially supported by the "Development of Microspace and Nanospace Reaction Environment Technology for Functional Materials" project run by the New Energy and Industrial Technology Development Organization (NEDO), Japan. References 1. C. O. Kappe (2004), "Controlled Microwave Heating in Modern Organic Synthesis," Angew. Chem. Int. Ed, 43, pp. 6250-6284. 2. N. Miyaura, A. Suzuki (1995), "Palladium-Catalyzed Cross-Coupling Reactions of Organoboron Compounds," Chem. Rev., 95, pp. 2457-2483.