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Hydrogenating in a Ball Mill

Here’s one to add to the “weird mechanosynthesis” pile. According to this paper, you can do hydrogenation reactions in a stainless-steel ball mill, without any sort of noble-metal catalyst. The hydrogen is produced when you add some n-alkane or diethyl ether to the mix (these actually get converted to gaseous methane and hydrogen under the milling conditions). Some pretty severe reductions are realized, for example, taking biphenyl all the way down to 1-cyclohexylcyclohexane. Olefins go to alkanes under these conditions, of course, and ketones and alcohols tend to just get erased: it’s a bulldozer.

And although it’s surprising, this reaction isn’t quite as much voodoo as it might appear. Well, not more than any other hydrogenation, anyway. The ball mill is, of course, a sealed vessel, and the authors show that the lower-boiling additives seem to be more effective, presumably because they increase the internal pressure. And it’s not like there’s no catalyst present, because you have the stainless steel. If you use zirconium oxide balls instead of the steel alloy ones, for example, you get no hydrogenation at all. Adding bits of different metals back in showed that the chromium is what’s producing the hydrogen from the alkanes or ether, while the nickel is acting as the hydrogenation catalyst itself. The stainless-steel alloy itself (304 steel) was the most effective combination, though.

The balls do get eroded a bit under the conditions, so what you’ve got is finely divided metals, under some heat from the mechanical energy of the milling, and under hydrogen pressure from the decomposition of the co-reactants. No wonder things get reduced! But it’s not a reaction that you would have predicted up front. And even if you’d stipulated that hydrogenation would start under these conditions, I don’t think you would have guessed how powerful it would be. The last time I hydrogenated down a whole aryl system (it’s been a while), I was running at something like 1600 psi (110 bar) at 120C with a rhodium catalyst, and the stuff came out looking – and smelling – like old lawnmower oil. This reaction probably doesn’t get above 60 degrees, by contrast. Admittedly, there’s no word in the paper about the appearance or aroma of the crude products, but I’m willing to bet that that part remains the same. . .

34 comments on “Hydrogenating in a Ball Mill”

    1. Me says:

      Yes that was my thought – along with ‘what’s a ball mill?’

      1. MS says:

        A rotating sealed barrel with some kind of grinding media inside, the ‘balls’ (though they aren’t necessarily spherical) that tumbles with whatever you are attempting to pulverize. Alternately you can polish things bey tumbling them with abrasive (rocks and sand for instance).

      2. Mark Thorson says:

        The only time I’ve ever seen a ball mill, my first comment to my collegue was “This looks like it could have been made in Colonial Willamsburg”. It was a ceramic pot with a ceramic lid and a metal clamp holding them together. They rotated on top of a set of rollers driven by a motor.

  1. myma says:

    this one sounds really useful.

  2. Darren says:

    My understanding of what happens in a ball mill is that you have very high local temperatures when two balls impact on each other during the milling process. So even though the global temperature may not be very high, the high local temperatures can drive reactions or other transformations.

    1. Scott says:

      Yeah, about 20 strikes with a hammer will get a ~1x3mm strip of steel red-hot. It’s how the Japanese light their charcoal forges.

  3. Bevis says:

    Heh… Balls…. Heh

    1. Joe Flenn says:

      Settle down Beavis

  4. Uncle Al says:

    When two hard curved surfaces meet at a point, the conditions are briefly cosmic for the very small area involved. Consider one psi tapered (not scaled) down to 0.1 mm² total area. That is 6452 psi, and dynamically with high shear.

    Pigment copper phthalocyanine does not dissolve (except by superacid protonation). Grinding it is ineffective given pi-stacking, Around a micron you are shuffling a deck of cards. A 30 wt-% solution (monomolecular dispersion) of CuPhth in Plexiglas obtains with energetic milling (attritor, zirconia). It is an awesome epsilon masterbatch dye for casting blue Plexiglas.

  5. milkshake says:

    If you want perhydrogenation without the high pressure autoclave, PtO2 and TFA as a solvent are pretty good, I was taking isoquinoline to E/Z-perhydroisoquinoline at room temp and 3 atm (Parr shaker)

    Also, if the squeak of Parr shaker bothers you, you probably do not want a ball mill within an earshot of your lab.

    1. Wile E Coyote, Genius says:

      You should hear one of the “ball mills” to polish large stones (6 inch diameter and larger). Horrendous racket.

      1. Marie says:

        How do you efficiently clean rust off a chain mail shirt? Put it in a 5-gallon bucket with a shovel of small gravel and a shovel of sand. Tape the lid securely shut and roll the bucket around for a while.
        It sounds as if the world (or at least the driveway) is coming apart beneath your feet, but works like a charm.

        1. Falanx says:

          Or leave it to soak in 5% citric in initially boiling water. Citric is strong acid for an organic, but still not enough to cause hydrogen embrittlement in ferritic/martensitic steels.

    2. Nick K says:

      If your Parr shaker is squeaking, you might like to oil the bearings. I found this out the hard way.

  6. Nate says:

    I worked on a project trying to make products using high-energy ball milling as the synthesis route. It works in some cases at a small scale, but when you get into the scale up details of it things get more complicated since the ratio of reactants to steel balls needs to be like 100:1 weight. If you want to make 1 kg of material you’re talking about spinning 100 kg of steel, which causes all kinds of other issues. God help you if you have to turn out 1 ton of material per day. The electricity costs to operate the mills scale somewhat linearly which messes with your economies of scale. For high-value products at low volumes it’s probably not an issue.

    1. Mark Thorson says:

      I wonder if a Wheelabrator shotpeening machine would have an application here.

    2. Med(iocre) Chemist says:

      I was halfway through a post about how you’d need to somehow incorporate flow chemistry, and pondering what that would look like until I realized I had just invented the flour mill.

      1. Mark Thorson says:

        If you looked at the flow sheet for any break system flour mill built in the last century, you’d be astounded by the complexity. More complex than an oil refinery. You’ve got all these roller mills, plansifters, air separators, etc. There are so many product flows, and it all has to be continuously adjusted for the input materials (different wheats mill differently) and the desired outputs based on market conditions.

        1. Barry says:

          and you want to mill the flour under an inert atmosphere. Flour mills–like grain elevators–are known to go boom.

  7. a. nonymaus says:

    This is reminiscent of the electrochemical Nozaki-Hiyama-Kishi reaction where the Ni and Cr are sourced from a sacrificial stainless steel anode:

  8. UudonRock says:

    This reminded me of a paper published earlier this year from Angewandte Chemie on oxidative mechanochemistry for the production of solvent free palladium and gold salts. A very useful concept for metalorganic catalysis. Mechanochemistry is really moving into some interesting areas.

  9. neo says:

    After your comment, Derek, I’d be interested to see what happens when you dissolve your compound in isooctane, then run it through a two-cycle engine.

  10. Some idiot says:

    The first time I saw ball-mill chemistry was in the late-1980s. I remember thinking at first that it sounded pretty spaced-out, but it is actually pretty logical. First time I have seen hydrogenation though (and pretty full-on hard-core hydrogenation at that…!).

  11. Med(iocre) Chemist says:

    Doesn’t this have implications for all the other mechanochemistry out there? If you’re trying to do some other chemistry on a molecule with reducible functional groups, you better make sure there’s no heptane or ether stuck in your starting material that would lead to reduction!

    1. Mark Thorson says:

      When I first ran across the term mechanochemistry, it was in relation to the colloid mill. I don’t think you’d see hydrocarbons breaking down in one of those. Could definitely affect things like proteins and other large molecules.

  12. ap says:

    Note that methane can’t be a major product of the process which generates hydrogen—it has a larger hydrogen/carbon atom ratio than the starting material. The major carbon containing product would have to be something like graphite or bucky balls or something else with a low hydrogen/carbon ratio.

    1. KazooChemist says:

      Did you read the article? With n-pentane there was almost three times as much methane produced as there was hydrogen. With diethyl ether the ratio was reversed.

      1. tlp says:

        So where all the extra carbon go? Maybe some steel turned into cast iron in the process? They report loss of 1 g of balls out of 22.9 g.

  13. aairfccha says:

    Would this also work with a ball mill containing a hydrogen atmosphere?

  14. Pliadisfoto says:

    Koks geras straipsnis!

  15. I was wondering what are the practical real life implication of this discovery? and where does the extra carbon goes during the hydrogenation process?

    1. Falanx says:

      At these pressures, it dissolves into the steel balls and vessel. Austenitic 3xx series steels have substantial solubility for carbon, even if we avoid making them with it to avoid weld decay in general engineering stock, and even high carbon corrosion resisting tool steels will happily soak up more.

  16. DrOcto says:

    I don’t understand how this works without a blue LED.

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