Using boron radicals to convert nitrogen to ammonia in solution
Ammonia is obtained industrially using the Haber–Bosch process, which
requires a lot of energy and hydrogen gas. A much milder approach has
now been reported by a team of researchers in the journal Angewandte Chemie.
According to this research, reactive boron compounds can efficiently
target atmospheric nitrogen and convert it to ammonium chloride after
the addition of an acid. This conversion takes place in solution, at
room temperature, and without the need for metals or hydrogen gas.
© Wiley-VCH, re-use with credit to 'Angewandte Chemie' and a link to the original article (open access).
Nitrogen makes up 77% of the air we breathe, and so, in theory, it is
virtually infinitely available for ammonia synthesis. However, in
practice, it only reacts extremely slowly with other elements. In the
Haber–Bosch process, which was developed over 100 years ago, metal
catalysts accelerate this sluggish reaction. They activate the nitrogen
which is then reacted with hydrogen under high pressure and temperature,
giving ammonia.
Ammonia is used industrially for producing nitrate fertilizers. It
can also be used as a hydrogen store when hydrogen is used as a source
of energy. To date, microbiological methods for nitrogen fixation have
been the predominant milder alternative proposed for the Haber–Bosch
process. However, exploiting bacteria for biotechnological ammonia
production is still quite inefficient.
A team of researchers headed by Nicolas Mézailles of the Université
Paul Sabatier, CNRS, in Toulouse, France, have now discovered that
reactive boron compounds can very efficiently target and activate
molecular nitrogen. The team explained their initial thinking: “We
reasoned that the use of high-energy radicals might provide a
kinetically and thermodynamically favorable pathway to nitrogen
functionalization.”
The team’s theoretical calculations then highlighted boron-centered
radicals as suitable candidates. The researchers produced these
boron-centered radicals by adding a strong reducing agent to organic
boron halides. The resulting substances converted molecular nitrogen at
room temperature to borylamines, which in turn reacted with aqueous acid
to give ammonium chloride.
Mézailles and the team have now described a novel approach to
nitrogen fixation in solution using radical compounds. The researchers
observed that the boron-centered radicals they produced efficiently
broke down the stable triple bond in molecular nitrogen, making it
possible to functionalize molecular nitrogen under mild conditions. This
radical-based approach opens up further possibilities for ammonia
production without having to rely on fossil-based raw materials.
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About the Author
Dr. Nicolas
Mézailles is a CNRS Research Director (UMR 5069, “Laboratoire
Hétérochimie Fondamentale et Appliquée”) at the Université Paul
Sabatier, Toulouse, and a Professor at the École Polytechnique,
Palaiseau, France. His research team studies the synthesis and
reactivity of high-energy species, ranging from metal complexes to main
group high-energy compounds. He has co-founded a startup company Swan-H
aiming to industrialize this discovery.