Microwaves Confirmed To Enhance Reaction Rates

Microwave ovens have become popular tools to facilitate chemical reactions, but it has been unclear if observed enhanced reaction rates result from rapid heating or from interactions of microwaves with the bonds of the reactants. Roshan Jachuck and Dinesh K. Selvaraj of Clarkson University, Potsdam, N.Y., and Rajender S. Varma of EPA's National Risk Management Research Laboratory, Cincinnati, have now shown that microwave radiation indeed has a significant impact on reaction rates (Green Chem. 2006, 8, 29).

Microwave reactions typically are carried out in a vessel placed inside a commercial microwave oven—a batch reaction. As part of Jachuck's work on industrial process intensification, however, his group in the department of chemical and biomolecular engineering designed a continuous capillary microreactor that can be maintained at a constant temperature while being irradiated with microwaves. The researchers used the reactor to study the iron-catalyzed conversion of benzyl alcohol to benzaldehyde.

The reactor produced benzaldehyde in about 75% yield under optimized conditions, which equated to a reaction time of about 15 seconds and a flow rate of about 1 mL/minute. The key observation was that increasing the microwave intensity at constant temperature increased the yield of benzaldehyde, Jachuck notes.

The researchers are looking for possible business partners to commercialize their microwave microreactors, including versions that would allow tuning the microwave frequency for specific reactions, Jachuck says. He envisions that the reactors could be used for lab- or pilot-scale projects or even for production of low-volume pharmaceutical active ingredients.