https://doi.org/10.15255/KUI.2004.019
Published: Kem. Ind. 54 (3) (2005) 149–153
Paper reference number: KUI-19/2004
Paper type: Review
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Synthesis of Ammonia (Nitrogen Fixation) – “Never Ending Story”

S. Ašperger+ and B. Cetina-Čižmek

Abstract

It is well known that the molecular nitrogen (N2) is very inert. The dissociation of N2 into atoms involves the breakage of a triple bond, which requires a high energy. This requirement is partly compensated by newly formed bonds of the stable NH3 product. The reaction is exothermic and at room temperature the equilibrium is shifted well in the direction of the NH3 formation (4 vol. of the reactants yield 2 vol. of product), but under this conditions the reaction is exceedingly slow. Even at 700 °C the yield of ammonia is very small, but at about 200 bar and 400 to 550 °C, in the presence of a catalyst, the process can be successfully carried out. It is the well-known Haber- Bosch procedure. Applying this procedure, ammonia is nowadays globally produced in quantities of over 100 million tons annually. Nevertheless, over the years chemists have tried to develop an alternative procedure to the Haber-Bosch process that will enable nitrogen fixation under mild conditions, as in the biological nitrogen fixation under the action of the enzyme nitrogenase. The present knowledge of mechanism of nitrogenase action is described but we are still very far from the final understanding of its action. It is generally considered that the successful nitrogen fixation under mild conditions should be found in reactions involving binuclear complex-intermediates, as are complexes of the type metal- NN-metal. Coordination of N2 to transition metal complexes can activate and even rupture the strong N–N bond under mild conditions. Thus, coordinated N2 in a zirconium intermediate, prepared by P. J. Chirik at al. at Cornell University, Ithaca, New York, yielded small quantities of ammonia. The authors extended their work on zirconium complexes containing cyclopentadienyl ligands and have shown that adjustment of the ligands allows direct observation of N–H bond formation from N2and H2. Subsequent warming of the complex cleaves the N–N bond at 45 °C, and continued hydrogenation at 85 °C resulted in complete fixation to ammonia. This appeared very promising considering, that at room temperature the procedure is not yet catalysed, but there is hope that in the near future an adequate catalyst will be found. In spite of the promising success it appears that it is still a long way from practical solving of the nitrogen fixation at room temperature because the catalytic procedure, when (if) found, has to be cheap to compete with the mighty industry producing ammonia by the Haber-Bosch procedure.

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Keywords

nitrogen fixation, nitrogenase, metal complexes, biological nitrogen fixation, environment