A review of the existing and alternative methods for greener nitrogen fixation

• All main nitrogen fixation methods were overviewed.
• Energy consumption of the methods were compared.
• The Haber–Bosch and non-thermal plasma processes are industrially promising.
• Metallocomplex nitrogen fixation is more energy intensive than the Haber–Bosch.

The conversion of atmospheric nitrogen into valuable substances such as fertilisers and fine chemicals is essential for agriculture and many other processes that sustain life on the planet. Although the Haber–Bosch process is the most important method of nitrogen fixation, the process is associated with major environmental concerns because it is very energy intensive and requires non-renewable feedstock to generate hydrogen. Hence, alternative ways of nitrogen fixation are being studied, from plasma synthesis and biological processes to metallocomplex catalysis, while existing methods are being improved using novel catalysts. This review covers all of the major areas of nitrogen fixation, discusses the industrial feasibility of each process, the reaction mechanisms, and provides a comparative evaluation of the various nitrogen fixation processes in terms of energy efficiency. Considering energy efficiency, the Haber–Bosch process and non-thermal plasma nitrogen fixation are promising methods for green industrial nitrogen fixation. Although metallocomplex nitrogen fixation takes place at ambient pressures, energy estimations show that this method does not provide higher energy efficiency than biological nitrogen fixation or the Haber–Bosch process. Biological nitrogen fixation on the other hand, has energy efficiency comparable to that of the Haber–Bosch process.

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