(657d) 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