Mechanochemistry – getting chemical reactions to occur by pressing, pulling, and grinding solid substances – continues to produce weird and interesting results. Here are a couple of recent ones from the same issue of Angewandte Chemie, both from a group at McGill.
This paper is about making soluble compounds of the noble metals (such as palladium, platinum, and gold). That’s generally a rather painful procedure – they got the nickname “noble” because they’re quite chemically resistant. People have been willing to use ridiculous amounts of cyanide just to get gold into solution for mining and extraction purposes, for example, because the metal laughs at normal solubilizing conditions (short of say, freshly prepared aqua regia, which I don’t count as normal).
The group shows that if you take one of these metals (powder, wire, what have you) and mechanically mill it with either potassium chloride or ammonium chloride along with the weak oxidant Oxone (potassium peroxymonosulfate), all as powders, you get a powder at the end that completely dissolves in water. That’s a startling thought to a chemist, grinding up palladium powder or gold wire and having it form just a clear colored solution at the end. That solution contains ammonium tetrachloroaurate (or the corresponding other salts), which is already a known soluble compound, of course – it’s just that in the past, if you wanted your gold in that form, you had to sort of go all the way around the barn to get it. If you add other ligands to the grinding mixture, you can form triphenylphosphine complexes and many others directly.
So what’s going on here? You’d have to think surface area and fresh metal exposure, right off. The mechanical stress is constantly roughening the surface of the metal samples and removing any chemically passivated layers (which makes me think that ball-milling a thermite mixture might be a spectacularly bad idea). When you really zoom in, metal atoms that are on the surface are certainly different from the ones down in the bulk phase, and there are different kinds of surface atoms, too (imagine one sitting up on the top of a microscopic peak in the material – it’s surely more reactive, since it’s interacting with far fewer partners).
As an aside, I remember when I was about ten years old, and an elementary school teacher asked me if I thought that when a person touched a piece of metal, if some of the atoms from either our fingers or the metal ended up getting moved from one to the other. I have held that thought for over 45 years, coming back to it as I learned more and more chemistry, and here it is again).
The second paper looks at the digestion of cellulose into glucose, which (as you’d imagine) is a process that’s been investigated a great deal over the years. Cellulose is normally pretty sturdy, which is probably why plants make so much use of it, but there are several species of fungus that produce various cellulase enzymes that will break it down. Stirring cellulose powder in water with the (commercially available) powders derived from these species will indeed convert the polymer to glucose, albeit slowly. Actually, as this work shows, just mixing the dry powders together with a small amount of water (1 microliter/mg) will do the same thing. But taking the same ingredients and ball-milling them gives much better conversions, and further experimentation showed that a combination of five minutes of milling, followed by 55 minutes of “aging” was exceptionally effective, giving up to 50% conversion to soluble sugars after 12 hours.
The technique was still effective on going from microcrystalline cellulose powder (which is rather well-processed beforehand) to plain old cedar-wood sawdust (which is not!) I am no expert on cellulose degradation, but the group does seem to have substantially improved the reported yield/time figures, and with a rather simple technique. In this case as well, surface area effects must be a big part of the picture. It’s hard to picture what the local microenvironment around the enzyme molecules is like under these conditions – are they still clumped together into microparticles and able to assist each other somehow in a way that’s less likely in solution? (Remember, there’s more than one kind of cellulose-degrading enzyme present in these fungal powders). At any rate, it’s clear (once again) that there’s a whole world of surface chemistry and nanoscale solid phase chemistry that we don’t really understand and can be taking more advantage of.