278814 Design of a Microreactor for Microwave Organic Synthesis
Microwave-assisted organic synthesis has become increasingly popular due to the numerous advantages brought about by the unique mechanisms of microwave heating. Microwave heating dramatically increases the heating rate compared with conventional heating since molecules in the reaction mixtures can absorb the microwave energy directly within a microwave-transparent vessel. Novel reaction pathways and product distributions that differ from conventional heating can also be achieved through selective heating. The ability to collect data rapidly and potential to expand chemical space make microwave-assisted organic synthesis attractive to the fields of kinetic studies, high-throughput synthesis, and reaction optimization. However, most microwave synthesis is done in batch mode, which requires time, labor, and materials. There is also challenges in scaling up microwave reactions due to the limited penetration depth of microwave and the reduction of energy efficiency in turning electricity into microwave irradiation when going up to large volumes. Furthermore, there are ongoing debates on the existences of non-thermal microwave effects. Microreactors are a promising approach to overcome the above issues by conducting microwave reactions in continuous flow formats and allowing for better understanding of the basic phenomena of microwave heating through accurate kinetic studies.
In this presentation, we will discuss the issues of the microreactor setup designed for microwave organic synthesis and demonstrate how simulation of microwave heating is used to improve the microreactor design. The original setup includes a micoreactor made of borosilicate glass, PEEK compressive packaging, and a Teflon holder for a fiber optic temperature detector (Fig. 1). Validation reactions show unexpected low conversions that stem from an uneven temperature distribution across the microreactor and temperature limitations. In order to tackle the heating issues, COMSOL simulations is used to understand the mechanism of microwave heating.
The heating source in microwave heating is the power dissipation caused by the interaction between the electric field and the materials. The heating rate therefore depends on the magnitude of the electric field and the dielectric loss of the material. Using the COMSOL software, we solve the Maxwell equations that govern the electromagnetic field and then use the calculated heating rate to simulate the heat transfer scheme of the materials.
The results agree with the experimental observation of the uneven temperature distribution (Fig. 2), and show that this is because the electric field strength is not uniform across the entire microwave cavity. The simulations also show that the electric field distribution changes with the size, position, and material of the objects inside the microwave cavity. The heating limitation of the original microreactor is due to low electrical field strength caused by the thinness of the reactor. By changing the thickness of the reactors in the simulation, we can find a design that will induce higher electric field strengths and lead to higher heating rates. The simulations allow us to improve the design of the microreactor to overcome heating limits and obtain uniform temperature distribution in the reactor. We present chemical synthesis examples with new reactor design based on our simulations.
Keywords: Microwave Reactions, Microwave Heating, Microwave Simulation