Sciencemadness Discussion Board

Idea for homeade makeshift microwave reactor...thoughts??

Leben - 26-10-2014 at 15:14

I've always wanted a microwave reactor that didn't cost an arm and a leg.... some reactions can be done in microwaves, taking out periodically for stirring. There are these magnetic-microwave hotplates that I found that use a microwave rotary tray mechanism to power the magnetic stirring, but I am not sure how strong these would be.

What if you just drill two small holes in the microwave door, and then feed plastic tubing through it, through which you pump your reaction contents. Since it is the microwave energy itself directly activating the bonds, and not the heat (right??), then you may not even need a vessel inside. Maybe you could use an externally stirred vessel, and just have plastic tubing coiled inside the microwave, and pump the reaction contents through it. The tubing should never get hot enough to melt, but the reactants should absorb microwave energy to cause a reaction to occur. Cooling could of course be easily implemented in this design, by simply running the coin through a bucket of ice water as it exits the microwave.

It almost seems too good to be true.... although most microwave reactions I've read about seem mind blowingly efficient. The only real concern I would have in this design is that the trapping of the microwaves in the microwaver may be disturbed by the holes, but if the holes are small, I suspect this may be minimal, and probably easily fixed with a number of small modifications. The reaction time may also be longer than usual, since you are only exposing a small amount of the reaction mix to the microwaves at a time, but you still get the benefit of microwaves, which can activate some reactions that no easily attainable amount of heat can.

Any thoughts or criticisms on this? I have been searching for a small-medium scale microwave reactor that doesn't cost tens of thousands of dollars.... Is there such a thing as a small microwave chamber that can be affixed to a stirring hotplate, or anything of the such, of reasonable cost?

unfrozen - 26-10-2014 at 16:33

Here is a link to a paper describing a microwave oven like that:
"Continuous Transethylation of Vegetable Oils by Microwave Irradiation"
http://e-nett.sut.ac.th/download/RE/RE11.pdf

ziqquratu - 26-10-2014 at 16:37

Many serious researchers do microwave reactions in beakers or flasks in a regular microwave. This method has its limitations (poor stirring, limited control over the power provided to the reaction mixture, boiling over, and so forth), but it does work.

I've also seen designs for providing a hole in the top for a condenser, and I'm sure you could replace the turntable motor with one suited for magnetic stirring if you were keen.

As for the mechanism behind the reaction... more and more evidence is mounting that in most cases there is no specific "microwave effect" as it is (used to be?) called. In the vast majority of cases, it seems that the supposed increase in rate is simply due to an increase in temperature above that achieved in a classical reflux setup (and that the use of inappropriate techniques for measuring the temperature - such as turning off the microwave, taking out the beaker and sticking in a thermometer, or using inaccurate infra-red sensors - explains why it looks like a non-thermal rate increase). There are some cases where an effect may be due to using microwaves - for example, in a mixed phase solid-liquid reaction, if the solid significantly absorbs the microwave energy, then the surface - where the reaction takes place - may be MUCH hotter than the bulk temperature, resulting in an increase in the rate (although this is still a temperature-related rate increase, it can only occur due to the microwaves and so may be considered a "microwave effect"). In general, though, it seems that microwaves are mostly just another way of heating stuff up.

I have given this some thought over the years, and done some limited experimenting (largely with one of those $10k units, though!). If you use something like Teflon tubing, you should be able to create this type of flow reactor - it resists quite respectable temperatures, and has excellent chemical resistance to boot! Glass tubing, of course, would also work, but could be more dangerous in case of failure - and sooner or later, your flow system WILL get blocked and blow out the tubing!

j_sum1 - 26-10-2014 at 16:39

If possible go for a modern induction microwave. You have a much better control over the energy levels. (Magnetron tubes are merely on/off) The distribution of microwaves within the oven is more uniform also. You won't be spending your time trying to find the sweet spot where the microwaves are concentrated.

If you need to drill holes that shouldn't be too much of a problem provided they are small. Larger than 1cm and I would expect some microwave leakage. You might mitigate against this if you need larger diameter tubing if you are able to place some kind of metal sieve inside your tubing or use metal tubing with a bend or shield with some kind of foil.

Leben - 26-10-2014 at 20:40

Quote: Originally posted by ziqquratu  

As for the mechanism behind the reaction... more and more evidence is mounting that in most cases there is no specific "microwave effect" as it is (used to be?) called. In the vast majority of cases, it seems that the supposed increase in rate is simply due to an increase in temperature above that achieved in a classical reflux setup.


There are tons of studies that strongly suggest there is indeed a "microwave effect"
There are several effects of microwave catalysis...... in some regards not unlike the effect of ultrasound in which pressure cavities can be formed in the liquid, creating pressure conditions of thousands of PSI's, allowing reactions to happen that would not normally happen at that temperature or ambient pressure. With microwave catalysis, there is a similar superheating effect that occurs amongst molecules, having tons of energy being absorbed by bonds equivalent to hundreds of degrees celcius, without the backlash of that much heat actually being produced...

In spite of this theoretical idea, there are tons of reactions documented that are accelerated to almost unreal speeds at low temperatures that could not be achieved at higher temperatures or pressure without causing compound decomposition. You might could think of microwave catalysis as one of the more efficient waves to injecting energy directly into bonds.

http://www.organic-chemistry.org/topics/microwave-synthesis....
Quote:
The water molecule is the target for microwave ovens in the home; like any other molecule with a dipole, it absorbs microwave radiation. Microwave radiation is converted into heat with high efficiency, so that "superheating" (external link) becomes possible at ambient pressure. Enormous accelerations in reaction time can be achieved, if superheating is performed in closed vessels under high pressure; a reaction that takes several hours under conventional conditions can be completed over the course of minutes.


Thermal vs. Nonthermal Effects

Excitation with microwave radiation results in the molecules aligning their dipoles within the external field. Strong agitation, provided by the reorientation of molecules, in phase with the electrical field excitation, causes an intense internal heating. The question of whether a nonthermal process is operating can be answered simply by comparing the reaction rates between the cases where the reaction is carried out under irradiation versus under conventional heating. In fact, no nonthermal effect has been found in the majority of reactions, and the acceleration is attributed to superheating alone. It is clear, though, that nonthermal effects do play a role in some reactions.




ziqquratu - 26-10-2014 at 22:55

There are indeed many studies which suggest the existence of a non-thermal "microwave effect" leading to rate acceleration.

A growing body of literature, however, indicates that the origin of this increase in reaction rate is purely thermal, and the difference can be explained by the use of poor methods for measuring the temperature of the reaction. Even the quote you provided above states:
Quote:
The question of whether a nonthermal process is operating can be answered simply by comparing the reaction rates between the cases where the reaction is carried out under irradiation versus under conventional heating. In fact, no nonthermal effect has been found in the majority of reactions, and the acceleration is attributed to superheating alone. It is clear, though, that nonthermal effects do play a role in some reactions.


It also mentions superheating or the use of higher pressure to reach higher temperatures - pure thermal effects (well, pressure, too, but the effect of pressure on rate is usually minimal!).

That's not to say that a different method of heating can't have different effects - certainly, "internal" heating of a solution via microwaves could indeed result in differences as compared to "external" heating with an oil bath - more even heat distribution and less chance of charring on the walls of the flask due to local hot spots jumps to mind. This, in turn, could effect the yields and the presence of impurities in your product mixture. Also, you can generally heat a mixture faster in the microwave than via an oil bath, which may have beneficial effects on your reaction. And there's also the possibility of selective heating of polyphasic reactions, such as the solid-liquid example in my previous post.

But, to my reading of the modern literature, it would appear that whilst the "microwave effect" theory is still broadly held by a lot of chemists, the consensus amongst those who have studied microwave chemistry in detail is that there's no such effect.

In any case, this has little to do with reactor design; if you're keen to try it, go ahead, and I'm sure myself and others can provide some insight and critique. I'm just suggesting you shouldn't expect magic from it...

[Edited on 27-10-2014 by ziqquratu]

Leben - 27-10-2014 at 08:29

ziqquratu

You may be correct that it is technically a thermal effect, in that energy is being directly absorbed by bonds from the microwaves, instead of being transferred there by thermal convection. In this sense, this is, broadly, a "microwave effect", that typically cannot be identically replicated by application of heat through typical means (heating mantle, hot plate, flame). Microwave chemistry arguably does enable reactions that would not be possible under identical circumstances of typical heat application, which is why it is so attractive in many regards.

Dr.Bob - 27-10-2014 at 09:28

I have run 1000's of reactions, and 100's of microwave reactions and I have not seen any real evidence for reactions that worked in a microwave not working by thermal heating given a similar comparison.

The most useful version of the microwave reactor that I have used are the ones that heat a small, sealed tube, using pressure sensors and IR temperature sensing to control the amount of heating. Since the reaction tube is sealed in most of my cases, almost all reactions are done at a temperature higher than the BP of the solvent, routinely at 40 C or more above it. That alone would provide a 2 ^ 4 = 16 fold increase in reaction rate, allowing a reaction to go from 1 day to 1-2 hours, plus the microwave certainly gets to the target temperature much faster, thus increasing the actual time at the desired temperature.

I am not saying that the microwave heating systems are not useful, they are very useful, but what they really are is small automated pressure reactor systems. Using an old pressure reactor (often called a steel bomb reactor) waqs often a painful experience, as the reactor was large, heavy, hard to stir (try stirring a magnetic stirbar inside a steel vessel), took hours to heat and an hour to cool, and often was heated via an oil bath with is messy itself.

The newer microwave systems replace all of that with a 5-100 ml glass vessel, which is clear to see through, can be stirred with a simple magnetic stirbar, are cheap enough to be considered disposable (thus not needing to be washed), you can buy them in cases, allowing parallel reaction setup, and they have automated temperature and pressure control, and heat and cool rapidly.

The main problem is that they cost a lot, and break easily if mistreated. Using a home microwave can work in a few cases, but is hard to really get repeatable results from, and often over or under heats the reaction due to the challenge of monitoring temperature and/or pressure.

But in answer to the thermal verses microwave question, I have taken the exactly same glass tubes reactions and stuck them in a heating block and the results are the same, except I have to put the heating block behind a safety shield since the glass tube is not inside a safe enclosure that way, and not as well controlled via temp and pressure. It is also harder to stir well, so that works best on homogeneous reactions.

Heterogeneous reactions often due well in microwaves as some solids absorb microwaves very well, thus they do superheat, which can be shown as it is easy to overheat solids and melt or char them even when the average temperature of the reaction (as monitored by IR temperature sensor) is lower than the mp of the solid.

But to best imitate a microwave, just get some glass high pressure tubes (Ace glass makes nice ones), run the reactions at 40C over the normal BP (behind a safety shield), and stir well magnetically, and you will find nearly identical results as to microwave heating.

[Edited on 27-10-2014 by Dr.Bob]

Lambda-Eyde - 27-10-2014 at 09:46

I remember having a paper lying around on modifying commercial microwave ovens for chemistry purposes, but it must have been lost in my HD crash a while ago. It should be on the forum somewhere, for those interested.

Leben - 30-10-2014 at 22:30

Dr.Bob,

Those are interesting comments you make. In scavenging my Universities research database, I have found a lot of documented microwave reactions that had the microwave equipped with a reflux condenser, and even in some cases, the bulk of the reaction mixture cooled (resulting in no slow down of reaction rate). Many of these reactions containing only homogenous compounds (no ions, metals, etc), experiencing drastic accelerations (8 hours -> 5 seconds, for example)

In fact, what I have had trouble finding is any mention of the use of a "sealed" (pressure safe) vessel in many of these reactions. Which is curious because in many forum posts here I've read about microwaves, it seems to be assumed.

I'm just having trouble wrapping my head around why so many people, especially those who seem to have worked with these microwave reactors, seem to be fixated only on the pure thermal/pressure effects of the microwaves. While there seems to be tons of evidence (outside of "bad" propaganda green science) suggesting the Microwaves themselves effect the molecule differently when absorbed as a source of energy, in comparison to molecules bumping into each other to pass heat/energy.

There have been tons of extremely well documented reactions tested side by side in microwave reactors, and subjected to similar pressure/temperature conditions in a sealed vessel heated by conventional means. Rarely, if ever, do the rates compare with each other (with the MW being still exponentially faster). This of course suggests that a microwave reactor is more than just a fast way of heating a sample. The effect certainly isn't magical by any means, as many have pointed out, but this does not mean it doesn't exist.

[Edited on 31-10-2014 by Leben]