Catalytic Transfer of Ammonia New Ammonia Reaction Could Provide Sustainable Nitrogen Source

A major goal of chemistry is to produce amines from ammonia and unsaturated hydrocarbons in a simple way. Moreover, during catalytic addition, in which ammonia is activated and then transferred, no waste is produced, making it particularly sustainable. Researchers at the Karlsruhe Institute of Technology (KIT) have now come a step closer to this goal.

The molecule ammonia (NH₃), a compound of nitrogen and hydrogen, is one of the most produced chemicals worldwide and forms the starting material for the production of many other nitrogen-containing compounds. If it were possible to produce amines simply by adding ammonia to unsaturated hydrocarbons, chemistry would have made a decisive breakthrough. This is because amines, organic derivatives of ammonia, are in demand in many different areas: they serve, for example, as building blocks for agrochemicals and pharmaceuticals as well as for washing-active substances, dyes, lubricants and coatings. Amines are also used as catalysts in the production of polyurethanes. Another important application is gas scrubbing in refineries and power plants.

By breaking the strong bond between nitrogen and hydrogen, known as activation, the ammonia molecule can, at least in theory, be transferred to other molecules such as unsaturated hydrocarbons. For example, the transfer of ammonia to the alkene ethylene, an important substance in the chemical industry, would produce ethylamine. Chemists refer to this addition as hydroamination. However, ammonia and ethylene do not just react with each other — a catalyst must mediate the reaction. The usual and transition metal-based catalysts, however, have the disadvantage that they themselves react with ammonia and thus become inactive. “The hydroamination of non-activated alkenes with ammonia is therefore considered a great challenge, the Holy Grail of catalysis, so to speak,” explains Professor Frank Breher, research group leader of the Molecular Chemistry Department at the KIT Institute of Inorganic Chemistry (AOC).

Activation and Catalytic Transfer of Ammonia

Professor Frank Breher and Dr. Felix Krämer from KIT's AOC, supported by researchers from the University of Paderborn and the Complutense University of Madrid, have now come a great deal closer to achieving this challenging goal. ”We have developed a system for activating ammonia that is not based on transition metals, but on main group elements. During the 'atom-economic' process of activation and the subsequent transfer of ammonia, no waste is produced — this is of course particularly interesting from the point of view of sustainability,” says Breher. The researchers now report on their work in the journal Nature Chemistry.

The team created a so-called frustrated Lewis pair (FLP), consisting of an acid as an electron pair acceptor and a base as an electron pair donor. Both usually react with each other and produce an adduct. If adduct formation is prevented or at least restricted, a frustrated situation results, so to speak, and the molecule reacts readily with small molecules such as ammonia. “However, it is crucial to dampen the reactivity in such a way that the reaction with small molecules is as reversible as possible, because only then is it possible to also use such an FLP in catalysis. We succeeded in doing this for the first time with ammonia as a substrate,” reports Breher. The researchers showed that the title compound reacts readily with non-aqueous ammonia in a thermoneutral manner and reversibly cleaves the nitrogen-hydrogen bond of ammonia at room temperature. Furthermore, in their publication, they present for the first time NH₃ transfer reactions mediated by a main group element-based catalyst. “Although we have so far only converted activated substrates and not yet unsaturated hydrocarbons, we have come significantly closer to the 'dream reaction',” says Breher. “We expect that our first proof of principle will spur further work on the use of N-H-activated ammonia as a readily available and sustainable nitrogen source.”

Original Publication: Felix Krämer, Jan Paradies, Israel Fernández & Frank Breher: A crystalline aluminium–carbon-based ambiphile capable of activation and catalytic transfer of ammonia in non-aqueous media. Nature Chemistry, 2023. DOI: 10.1038/s41557-023-01340-9

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