World's first ammonia synthesis method that does not require organic solvents developed by the University of Tokyo and Kyoto University

A research group led by Professor Yoshiaki Nishibayashi of the University of Tokyo's Graduate School of Engineering and Professor Hisao Yoshida of Kyoto University's Graduate School of Human and Environmental Studies have successfully synthesized ammonia from nitrogen gas by a mechanochemical reaction using a ball mill under normal temperature and pressure without using organic solvents. The results are published in Nature Synthesis.

Ammonia is essential for fertilizers and pharmaceuticals and, in recent years, has attracted attention as a hydrogen carrier. Ammonia is currently produced by the Haber-Bosch process at high temperatures and pressures. The consumption of large amounts of energy through this process is considered a problem.

The Nishibayashi Laboratory is developing a catalytic synthesis method for ammonia production from nitrogen gas, water, or alcohol under mild reaction conditions at room temperature and atmospheric pressure. This method uses samarium iodide as a reductant in the presence of a molybdenum complex with a pincer-type ligand. However, since this is a homogeneous reaction in which reactants are dissolved in an organic solvent, one of the concerns for practical application is that producing a large amount of ammonia requires the use of large amounts of expensive, highly toxic organic solvents.

In their study, the research group examined whether the reaction of generating ammonia from nitrogen gas that employs a molybdenum complex bearing a pincer-type ligand could proceed under ball-milling conditions. They found that even under mechanochemical conditions without any organic solvents, nitrogen gas at 1 atm (atmospheric pressure) reacted with samarium iodide (solid) as a reductant and water or alcohol (solid or liquid) as a hydrogen source in the presence of a molybdenum complex catalyst to produce ammonia in a high yield (up to 860 equivalents per molybdenum).

This is the world's first example of a nitrogen fixation reaction using molecular catalysts under mechanochemical conditions without using any solvents. Interestingly, ammonia was also produced efficiently when cellulose, the main component of plants and paper with natural abundance, was used as the hydrogen source. Because cellulose is almost insoluble in organic solvents, no ammonia was produced in conventional reaction systems that used organic solvents. In addition, ammonia produced through the proposed method can be recovered without distillation, which is an energy-consuming process.

The mechanism of ammonia production under mechanochemical conditions was investigated in detail. It was found that the key to reaction progression was the nitrogen-nitrogen bond cleavage reaction that occurred at the gas-solid interface and the nitrogen-hydrogen bond formation reaction that occurred in the solid reagents. It is notable that the synthesis reaction of ammonia from nitrogen gas using a molecular catalyst has been modified from the conventional homogeneous reactions based on solvents to a heterogeneous reaction occurring between a gas and a solid. This development opens up new possibilities for more practical reaction systems, including electrochemical ammonia synthesis at the gas-solid interface.

Journal Information
Publication: Nature Synthesis
Title: Mechanochemical nitrogen fixation catalysed by molybdenum complexes
DOI: 10.1038/s44160-024-00661-y