Triangular carbon molecules are packed with energy, but that is precisely why they are difficult to make. Researchers put bacteria to work, which they succeeded anyway. This could lead to a renewable fuel for aviation.
Bacteria can make a difficult-to-synthesize triangular molecule that contains enough energy to serve as jet fuel. That report microbiologist Pablo Cruz-Morales from the Technical University of Denmark near Copenhagen and colleagues in the magazine Joules†
Cyclopropanes, triangular carbon molecules, are among the most energy-rich cyclic molecules in existence. The four bonds of a carbon atom are normally at an angle of 109.5 degrees. But in a triangular molecule it’s 60 degrees, so the bonds are under tension.
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Elastic
Compare it to elastic stretched as far as possible. When released, it springs back with force to its original, unstressed situation. The energy stored in the molecule is released when the bonds are broken, for example during combustion. But that triangle is also the reason that production is so difficult. Because how do you push the atoms of a molecule into such a tense position?
As is often the case, nature is more skilled than the chemist, so the researchers took a look at the bacteria of the species Streptomyces rose overticillatuswhich live in the ground† They make the substance from sugar jawsamycin, a molecule containing a number of cyclopropane rings. It is named after the film classic Jaws, because of the triangles that are reminiscent of shark teeth. The bacteria use a protein that adds the triangular parts in the molecules.
Bacteria at work
The researchers genetically modified bacteria of a different species to make this protein, and many variants of it. Those bacteria then produced an array of new molecules full of triangular elements. The researchers chose the substance that seemed most suitable for aviation, and again made clever genetic adjustments that made the bacteria another 22 times more productive. Calculations show that the final fuel contains enough energy to replace kerosene.
Aviation currently flies almost exclusively on kerosene. That fossil fuel is a mixture of substances that are also full of cyclic structures, including triangular ones. There is always a lot of energy in that. But the combustion of kerosene releases many greenhouse gases, while aviation must be climate neutral by 2050. Alternative renewable fuels are a possible solution for this.
Kerosene
‘It is interesting and clever that the researchers have been able to make compounds with a higher energy density than kerosene’, says Adrie Straathof† He is researching the production of renewable fuels and chemicals with micro-organisms at TU Delft.
But there is an important caveat to be made, he says. ‘The production method still needs enormous improvement. Moreover, aviation does not need a higher energy density, but a much larger amount of renewable kerosene substitutes than currently available. The biggest challenge for aviation is to quickly take responsibility for the very large-scale production of a kerosene substitute.’