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When we perform a organic reaction without any addtional catalyst except that the reaction is carried out under microwave condition we can accelerate this reaction in comparison with one under heating only. So, microwaves can be considered a form of catalysis according to the concept of reaction kinetics?

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  • $\begingroup$ A number of people, scientists, have claimed that. What convinced you? $\endgroup$ – Karl Nov 17 '20 at 22:56
  • $\begingroup$ There are several factors affecting microwave synthesis. Can you elaborate the method convinced you as @Karl's comment? $\endgroup$ – Mathew Mahindaratne Nov 17 '20 at 23:06
  • $\begingroup$ Apparently, much literature has been contributed in this area. See my answer and cited references therein. $\endgroup$ – AJKOER Nov 18 '20 at 2:47
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    $\begingroup$ If the question is "can microwave speed reactions?" then the answer is yes, it also has a large literature. If the question is "can we call this catalysis?" then I would say, I do not think so, we generally call something else catalysis, but one can argue that it is a similar scenario to photocatalysis or electrocatalysis. $\endgroup$ – Greg Nov 18 '20 at 3:40
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    $\begingroup$ @Poutnik Beware, microwave heating does not work via resonant absorption. $\endgroup$ – Karl Nov 18 '20 at 8:34
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Here is an example of an entire online GoogleBook dedicated to the topic Microwave Assisted Organic Synthesis, apparently based on some 2,000 papers.

On page 3 of the cited reference, notes to quote:

Interaction between microwave radiation and solutions of polar molecule polar molecules may be adequately described using classical models, which may be derived from Maxwell's equations.

And further:

Although this analysis has concentrated on the oscillating components of the electromagnetic field of the microwave radiation, it is also possible to get interaction between the oscillating magnetic field and the magnetic dipoles in the sample. The high-heating rates for Fe304 may be attributed to such interactions.

With respect to benefits, per page 23 to quote:

Reaction rates are generally very high and yields in many cases can be greatly improved, as competing side reactions can be minimized...

I would also add that the use of microwave-based dielectric heating is, at times, more than just fast and efficient and heating, as it can, in the presence of activated carbon, for example, introduce surface-based radicals into the reaction mix. See, for example, Generation of hydroxyl radical in aqueous solution by microwave energy using activated carbon as catalyst and its potential in removal of persistent organic substances.

As such, I described the general use of this new heating technology as microwave-assisted synthesis, and not per se, as catalytic, albeit depending on the reaction mix, it can be (per last cited reference).

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    $\begingroup$ Can I (genuinely) ask why you mentioned EM fields and Maxwell equations in respect to answering the question? $\endgroup$ – dval98 Nov 18 '20 at 3:05
  • $\begingroup$ Yes, its relates to the underlying mathematics used in employing microwaves to achieve max efficiency in likely a commercial synthesis application. Read the first 26 available pages of the ebook. A question you should ask yourself is why the book mentions it and you apparently have an issue. $\endgroup$ – AJKOER Nov 18 '20 at 3:12
  • $\begingroup$ I think your misinterpreting my intentions. Don’t confuse my inquiry about your answer with a challenge to you intellectual abilities. I was just curious. $\endgroup$ – dval98 Nov 18 '20 at 3:24
  • $\begingroup$ Additionally, the full book is not accessible on google for me. $\endgroup$ – dval98 Nov 18 '20 at 3:26
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    $\begingroup$ In other words, any effect except the purely thermal occurs under very special circumstances and/or is highly disputed. $\endgroup$ – Karl Nov 18 '20 at 8:18
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If you look at the Arrhenius equation for reaction rates,

$k=Ae^{\frac{-E_a}{RT}}$

we see that the rate is dependent on collision frequency (A) and the portion of molecules with sufficient energy to overcome the activation energy barrier ($E_a$) [1].

By heating the solution with microwaves, we are increasing T, therefore, increasing the portion of molecules with enough energy to overcome the $E_a$.

Catalysts lower the $E_a$ or provide a different mechanism with a lower $E_a$. This is not how microwaves increase reaction rate as irradiation does not change the activation energy.

Edit

Some studies suggest that non-thermal effects are present (i.e. interactions between the EMR field and dipole moment of molecules) however it is still under debate whether they actually exist. A recent article publish in 2018 suggests that a majority of research disputes the existence of non thermal effects or suggest that non thermal effects arise from enhanced diffusion.

Either way, microwaves aren’t a catalyst.

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  • $\begingroup$ I feel it is not the effect of temperature only. $\endgroup$ – M. Farooq Nov 18 '20 at 3:09
  • $\begingroup$ @M.Farooq what did you have on mind? $\endgroup$ – dval98 Nov 18 '20 at 3:13
  • $\begingroup$ See microwave effects en.wikipedia.org/wiki/…. $\endgroup$ – M. Farooq Nov 18 '20 at 3:16
  • $\begingroup$ @M.Farooq The reference is much appreciated . It said somewhere that non-thermal effects aren’t present in liquids. I just thought that was interesting. $\endgroup$ – dval98 Nov 18 '20 at 3:39

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