Catalytic Reduction of Nitrogen Gas by Cheap Iron Catalyst under Mild Conditions

Catalytic formation of ammonia and hydrazine from nitrogen gas

nitrogen gas
Catalytic formation of ammonia and hydrazine from dinitrogen gas by using iron-dinitrogen complexes © 2016 Yoshiaki Nishibayashi.

A University of Tokyo and Kyushu University joint research group designed and synthesized a novel iron-dinitrogen complex that catalyzed the direct conversion of nitrogen gas to ammonia at normal atmospheric pressure. Further, the group also discovered a novel reaction to create hydrazine, a fuel, directly from nitrogen gas.

Nitrogen is one of the fundamental elements essential for all living organisms, being included in DNA, amino acids, and proteins. However, although nitrogen is very abundant in the atmosphere as nitrogen gas (gaseous molecular dinitrogen, N2), it is highly unreactive and cannot be used as a direct source of nitrogen. Currently, industrial conversion of nitrogen gas into ammonia as a usable nitrogen source is achieved by the Haber–Bosch process, which requires high temperatures (300-500 C) and high pressures (200-300 atmospheres) and consumes vast quantities of energy. Consequently, researchers have been searching for alternative methods of synthesizing ammonia under milder conditions.

On the other hand, bacterial nitrogenase enzymes perform biological nitrogen fixation (the conversion of nitrogen into ammonia) under ambient pressure and temperature. Because the structure of the active site of nitrogenase that catalyzes the conversion has been identified, research and development of alternative nitrogen fixation methods has focused on metal complexes and nitrogen complexes modeled on nitrogenase. In a previous study, the research group has developed a reaction to synthesize ammonia from nitrogen gas under mild reaction conditions catalyzed by a dinitrogen-bridged dimolybdenum-dinitrogen complex. While molybdenum used in the catalyst is cheap, it is a rare metal so researchers were searching for catalysts employing other cheap and more abundant metals.

Now, the research group of Professor Yoshiaki Nishibayashi at the University of Tokyo Graduate School of Engineering, working with the research group of Professor Kazunari Yoshizawa at Kyushu University Institute for Materials Chemistry and Engineering, has developed an inexpensive iron catalyst modeled on the active site of nitrogenase. The catalyst is an iron-dinitrogen complex with a negatively charged PNP-pincer ligand (a tridentate ligand that combines pyrrole and two phosphine groups) and works as an effective catalyst for catalytic nitrogen fixation, producing ammonia under very mild reaction conditions. In addition, altering the solvent enabled production of hydrazine.

The present reaction system is the first reported reaction converting nitrogen gas catalytically and directly into hydrazine and serves as a guide to the development of a new class of nitrogen fixation catalysts. This method offers great potential and is an important step towards the development of an energy-saving next-generation nitrogen fixation method to replace the Haber-Bosch process. In addition, the findings described in this paper provide an important insight into the as yet unknown reaction mechanism by which nitrogenase converts nitrogen gas into ammonia under normal temperature and pressure conditions.

“In this study, our research group including Dr. Kuriyama, who was then a graduate student, has achieved the catalytic formation of ammonia and hydrazine, which are important chemical feedstocks for nitrogen-containing materials and fuels, directly from dinitrogen gas under mild reaction conditions using these newly developed iron catalysts,” says Nishibayashi. He continues, “We have emulated nitrogenase enzymes, which are responsible for the biological nitrogen fixation, to prepare the present iron-catalysts. I believe that the effort of Dr. Kuriyama and his co-workers lead to the present achievement.”