Sciencemadness Discussion Board

Microwave generator for small scale chemistry?

Leben - 27-9-2014 at 12:01

I've seen lots of interesting reactions that utilize microwave catalysis to achieve some incredible results, but putting some reactions into a microwave doesn't seem possible or practical.

Is there any sort of commercially manufactured microwave-gun sort of thing that can be used for small scale chemistry?

aga - 27-9-2014 at 13:59

ISTR that microwave emitters are available for Amateur Radio usage (10Ghz ?)
Perhaps they would be of some use.

The Power you require and the Focusing will be the main factors relating to what equipment you need.

Chemosynthesis - 28-9-2014 at 12:33

I did novel microwave synthesis reactions for awhile. We had a fancy MARS reactor, but for proof of concept, we often used a home microwave. As long as you do not heat food in it anymore, the one in your domicile is probably just fine for plenty of home chemistry, believe it nor not. Ventilating can be an issue if it's not in a fume hood. That initially caused us some problems.

FireLion3 - 28-9-2014 at 16:28

Quote: Originally posted by Chemosynthesis  
I did novel microwave synthesis reactions for awhile. We had a fancy MARS reactor, but for proof of concept, we often used a home microwave. As long as you do not heat food in it anymore, the one in your domicile is probably just fine for plenty of home chemistry, believe it nor not. Ventilating can be an issue if it's not in a fume hood. That initially caused us some problems.


How do you manage to fit a condenser, hotplate, or stirrer, into your microwave?

Chemosynthesis - 28-9-2014 at 16:44

Quote: Originally posted by FireLion3  

How do you manage to fit a condenser, hotplate, or stirrer, into your microwave?

We didn't. Microwaving even just a stir bar will end badly. The easiest way to test a reaction for proof of concept is to put the largest Erlenmeyer you can in the microwave with your reactants, possibly loosely cover the top with a smaller beaker, and then microwave for 10-30s increments to avoid overflow. We were able to shorten 24 hour reactions to about two minutes, and change solvents in several instances to more environmentally friendly ones. Triturate and chromatography resolve the mixture or products and reactants if necessary and then NMR. Obviously the latter portion is outside hobby chemistry.

Additionally, we had microwaves with holes cut in the top for condensers, or door latches wedged to allow operation with the shielded door open (point in safe direction as you will run the risk of instant and permanent blindness if not careful), and other kind of modifications, but I can't even remember using them more than perhaps once. All of these seem potentially feasible and I would probably be most interested in setting up an open microwave proximate a reflux app. or something similar with plenty of ventilation.

FireLion3 - 28-9-2014 at 16:57

Quote: Originally posted by Chemosynthesis  
Quote: Originally posted by FireLion3  

How do you manage to fit a condenser, hotplate, or stirrer, into your microwave?

We didn't. Microwaving even just a stir bar will end badly. The easiest way to test a reaction for proof of concept is to put the largest Erlenmeyer you can in the microwave with your reactants, possibly loosely cover the top with a smaller beaker, and then microwave for 10-30s increments to avoid overflow. We were able to shorten 24 hour reactions to about two minutes, and change solvents in several instances to more environmentally friendly ones. Triturate and chromatography resolve the mixture or products and reactants if necessary and then NMR. Obviously the latter portion is outside hobby chemistry.

Additionally, we had microwaves with holes cut in the top for condensers, or door latches wedged to allow operation with the shielded door open (point in safe direction as you will run the risk of instant and permanent blindness if not careful), and other kind of modifications, but I can't even remember using them more than perhaps once. All of these seem potentially feasible and I would probably be most interested in setting up an open microwave proximate a reflux app. or something similar with plenty of ventilation.


That has always interested me. What is it exactly that allows microwave radiation to speed up the reaction so much? How is the energy absorbed differently from microwaves when compared to thermal convection?

Chemosynthesis - 28-9-2014 at 18:14

Quote: Originally posted by FireLion3  

That has always interested me. What is it exactly that allows microwave radiation to speed up the reaction so much? How is the energy absorbed differently from microwaves when compared to thermal convection?

It depends on your reactants, and in particular the solvent situation of your reaction. We mostly focused on single solvent phase reactions, but we did a couple solid state solvent less ones. Basically, there was a theory that wasn't accepted at the lab I worked, whereby special excitatory states were induced in the reactants that allowed abnormal reaction. The general repeatability of non-microwave reactions in a microwave don't really fit this paradigm.

What I believe is more commonly accepted now is the rapid and more homogenous heating of a phase, or gradient between phases, allows faster kinetics without the wasteful conduction of your vessel and then convection of solvent. The rapidity of heating and ability to add pressure in specialized reactors also allows solvent superheating effects to speed up molecular collisions, relative more pedestrian lab heating methods.
Even at atmospheric pressure, how your microwaves are focused can allow tiny pockets of superheating to occur in your solvent, which rapidly equilibrate, which may also affect your reaction rate. It can also lead to quite a bit of tar flecks in otherwise pristine solvent, but we weren't interested in scale or optimization of the technique, so I am not sure how easy power modulation or refocusing would be to predict, as we just gradually heated for fewer and fewer time increments to see if we could take a 48hr. rxn to 30s.

Dr.Bob - 29-9-2014 at 06:04

I used lab microwaves for many years, still use them occasionally. The only real effect that I saw was that if you used the specific pressure tubes provided, you could seal them and heat them to higher temperatures than normal as they were sealed tubes. The microwave had pressure and temp sensors which allowed real time monitoring and control, which was the real benefit of them, allowing small sealed tube reactions to be heated and cooled rapidly, which is not very easy in a steel pressure reactor.

That allowed, for example, a reaction in isopropanol to be done at 150C instead of the BP of about 82C, which speed the reaction up by about 2 ^ 6 or 2^ 7 times, which followed the normal guideline of doubling the reaction rate (or halving the time needed to react) for every 10C of temp increase. They were also nice for Pd chemistry like Susuki's, as you could set up the reaction in the small tube, purge it with N2 or Argon easily, and then the tube was sealed under that inert environment, so much simpler than trying to keep an STP reaction inert overnight. They followed the same rules in that heating the reaction about 40 C over the normal BP sped them up by about 15 fold, allow "overnight" reactions to be run and tested within an hour.

You might be able to find a used CEM or Biotage cheap now, as there are many surplus ones from industry, if the Chinese have not bought them all up.

Trying to build a good and safe one from scratch would be tough, fixing a broken one might be easier. You can do some reactions in a home microwave, but don't expect much magic, they will still heat quickly, but controlling the temperature if much tougher, easy to overheat things.