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Mechanosynthesis for beginners

Mechanosynthesis for beginners
23 December 2020 - Oddballs

Boswell2020 mechanosynthesis.PNGMechanosynthesis is an oddball. The Wikipedia page describes it in terms of nanotechnology. As originally proposed by K. Eric Drexler in 1981 any material can be assembled from the ground up, atom by atom, if only you can achieve the required precision. This was an outlandish idea at the time but but with the invention of the atomic force microscopy in 1993 less so. To biochemists on the other hand mechanosynthesis is nothing new. All enzymes are nanofactories, plucking a protein from an assembly line, hammer out a chemical transformation and releasing the product. And regular chemists? To them mechanosynthesis is all about the mortar and pestle, an ancient device used by alchemists, cooks and apothecaries. Primarily used for mixing solids but as chemists discovered a long time ago, also for bringing about chemical reactions. The driving force in this type of reaction is mechanical activation. A ball mill will also do the trick, the absence of a solvent is a plus.

It appears 2020 has been a good year for mechanosynthesis. Earlier this year a mortar and pestle was used in a surprise synthesis of dicarbon and nanotubes (blog link). And this month we can welcome reports on the mechanical synthesis of poly(difluoroacetylene) and ammonia. Both targets are relevant but also deceptively out of reach.

The synthesis of poly(difluoroacetylene) should be a straightforward polymerization of difluoroacetylene but as Boswell et al. explain in their article in Nature Chemistry, the monomer is pyrophoric and easily combustable and polymerisation does not yield a well-defined product, a shame because poly(difluoroacetylene) is predicted to be far more stable than it's naked counterpart polyacetylene which should be a premier conductive polymer but is plagued by poor oxidative stability.

As the acetylene polymerization was not a viable option, the solution the Stanford lab of Noah Burns arrived upon was not just outside of the box, it was well away from the box. In fact viewed from the Stanford lab the work took place the box was well over the horizon. Because how did ladderanes get involved? A ladderane is a molecule based on fused cyclobutane rings. The synthetic record holder is 13. Ordinarily an esoteric lab curiosity if it where not that the motif does also occur in nature, which got total synthesis people involved. The Burns lab is specialized in total synthesis, had already tackled a few naturally occurring ladderanes and in 2017 had already established that ladderanes can be triggered to open up by sonication in a tandem retro 2+2 ring opening to a polyacetylene. For the creation of a fluoroladderane a 40 year old obscure photochemical reaction was unearthed by which a ladderane was successfully joined with perfluorobenzene to a hexafluoroladderdiene. This monomer was then subjected to Ring-opening metathesis polymerisation / Grubbs catalyst to something of a side-chain polyladderane and then sonication yielded the fluoro polymer. Granted, it is more of a co-polymer with fluoroacetylene stretches and in case you are wondering if sonication is actually a form of mechanosynthesis, the article explains that with increasing moleculer weight of the polymer the reaction rate increases which is typical for mechanosynthesis. The gold-colored material was found to have a similar conductivity as regular polyacetylene combined with improved air stability.

The other bit of mechanosynthesis news this month concerns a new take on the Haber–Bosch process, the marriage of nitrogen and hydrogen to ammonia, which importance cannot be overstated (mankind depends on the stuff for fertilizer) and a frequent topic in this blog (2012,2016, 2018, 2019). An international research team led by Jong-Beom Baek report a new system (DOI) with the HB furnace hell of 450 degrees and 100 bar of pressure replaced by a friendly ball mill, 45 degrees, just 1 bar of pressure and an iron powder.

The iron was the "cheapest bulk iron powder". The economics: yields between 0.2 and 1 mole per hour per US dollar compared to 1 mole per hour per dollar for the commercial HB process. The concentration of nitrogen at the iron surface (16%) was much higher than in the commercial process One thing the ball mill does to the iron is creating defects from collisions which act as catalytic sites for the nitrogen dissociation step. The iron particles also become smaller (comminution). The researchers envision ball-mill production units at the individual farm level but are quick to point out energy costs will then be higher than in regular HB due to the advantage of scale.

Without any evidence at all I am predicting 2021 will be a hot (or cool?) mechanosynthesis year. Are you currently writing a grant proposal? Not sure it will be accepted? Make sure you mention mechanosynthesis somewhere. The research money will be flooding in!