Development of microreactor setups for microwave organic synthesis

Abstract

The main contribution of this work is the understanding of microwave heating and the microreactor design challenges involved through both chemical experiments and computational models. The original goal of this research is to develop a microreactor system in order to carry the microwave organic synthesis in continuous flow format and to understand the basic phenomena of microwave heating through accurate kinetic studies. Several heating issues were observed with the first microreactor setup, including an uneven temperature distribution across the microwave irradiated area and a temperature limitation that depends on the position of the reactor. To find the root of these problems, the electromagnetic field and the heat transfer scheme of the microwave system were modeled with COMSOL. The simulations show that there are three main causes to the heating issues: (1) the electric field has an inherent resonance structure and thus has an uneven magnitude within the microwave cavity, (2) the electric field changes with not only the material, but also the sizes and positions of the objects in the microwave cavity, (3) the air gaps in the microwave waveguide creates a large natural convection heat loss. The simulations gave us a deeper understanding of the microwave heating phenomena and were used to find the optimum design of the microreactor. A second multiple-layered glass reactor was designed accordingly to overcome the heating limitation and minimized the temperature unevenness. However, the non-uniform heating rate cannot be eliminated since it is inherent in the resonance structure of microwaves. Both the experimental results and the simulations of the final reactor show that even though the reactor can reach the desired temperature, the temperature range across the reactor could be up to 20 *C. In addition, it was found that the flow rate of the reaction greatly affects the thermal equilibrium of the reaction volume. Accurate temperature control is therefore still a challenge for kinetic studies to be feasible with the current single-mode microwave system. The benefit of microwave heating is therefore still in the qualitative screening of chemical compounds, a feature which was demonstrated with a Fischer-Indole screening in the final setup.