Microwave irradiation assisted chemical reactions

[gilden]

Having read several articles about microwave assisted chemical reactions, it is not completely clear for me whether the reactions are not just enhanced by the temperature increase provoked by the microwave irradiation, or is there really a pure microwave effect? In other words, would this enhancement still take place if the reaction mixture is cooled during irradiation? Who can help me?

 

[kenvlach]

Microwave radiation provides kinetic energy to the atoms taking part in the reaction, enabling them to more readily attain the transition state. I believe that current thinking is that this kinetic energy is equivalent to that produced by a temperature increase. For example, in the microwave-assisted sintering of alumina, the microwave radiation was the equivalent of 250[sup]o[/sup]C of temperature:
J. Amer. Ceramic Soc., Vol. 86, No. 8, August 2003
"Microwave Sintering of Alumina at 2.45 GHz"
from the abstract: "With no hold at sintering temperature, microwave-sintered samples reached 95% density at 1350°C versus 1600°C for conventionally heated samples...The grain size/density trajectory was independent of the heating source. It is concluded that the enhanced densification with microwave heating is not a consequence of fast-firing and therefore is not a result in the change in the relative rates of surface and grain boundary diffusion in the presence of microwave energy."

http://www.ceramicjournal.org/issues/current/abs/6633.html

So, to answer your question, yes, microwave radiation can stimulate a reaction at lower temperature by producing atomic vibrations equal to a higher temperature.

 

[hduncan]

I worked on my thesis in college in this area. What I have seen and read is that the enhancement comes from the high temperatures seen by the reactants. Microwaves work well with polar solvents, but not so well with non-polar organics. The subject is not easy to explain, because you can design a system which has high power, but the geometry of what your heating can be a huge factor. For example, if you heat a catalyst particle that is 3/8"--you might see a center temperature of 300 F with a given incident power density. Change the diameter to 1" and this can go to 1500 F. This is also similar with cylinders. I had to build my entire experiment from scratch.....I learned a lot about microwaves and power transmission. The dielectric loss factors of the materials that your heating also play an important part. Any more questions, you can email me at hduncan@apex-engineering.com