Lithium-Ammonia Nitrogen Fixation

Summary

6  Li⁰ +  N_{2 (g)}    ----------------->   2  Li₃N

2  Li₃N  +  6 H₂O   ---------------->  6 LiOH  +  2 NH_{3 (g)}

6  LiOH  +  electricity  ---------------->  6  Li⁰   +  1.5  O_2 (g)  + 3 H₂O

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Net :   N_{2 (g)} +  3 H₂O  + electricity   -------------->  2  NH_{3 (g)} +  1.5  O_{2 (g)} ;

This is the reverse of combustion :  4  NH_{3 (g)}  + 3  O_{2 (g)}    ------------->   2  N_{2 (g)}  + 6 H₂O .

Discussion

Step 1 :  Lithium metal reacts with nitrogen gas at an elevated temperature (200 – 450 ºC) to give a quantitative yield of lithium nitride :

6  Li⁰    +    N_{2 (g)}    ----------------->   2  Li₃N   (100% yield)

m.p. 180.5 ºC         “reddish-brown hexagonal crystals”, m.p. 813 ºC

– E. Masdupuy and F. Gallais , Inorg. Synth. 4 , pp. 1-5 (1953) ; F. Schönherr , A. Köhler , and G. Pfrommer , Inorg. Synth. 30 , Nonmolecular Solids , pp. 38-45 (1995) .

Step 2 :  The lithium nitride is dissolved in water to give ammonia and lithium hydroxide . The ammonia is distilled from solution , dried , and liquefied for storage . The water is evaporated from the solution to give dry lithium hydroxide . Schönherr et al. mention (p. 41) , “The compound [lithium nitride] forms NH₃ in humid air” ; that is , it is rapidly and quantitatively hydrolysed :

Li₃N   +   3  H₂O   ----------------->   3  LiOH  +  NH_{3 (g)}

Step 3 :  The dry lithium hydroxide is melted (m.p. 450 ºC) and electrolyzed to give molten lithium metal and hydrogen gas at the cathode , and oxygen gas at the anode . Analogous electrolyses of molten sodium and potassium hydroxides were used in 1807 by the British chemist Sir Humphry Davy (1778-1829) to isolate the alkali metal elements sodium and potassium , respectively , for the first time . The redox reactions involved are as follows :

Cathode :   4 Li¹⁺   +  4 e-    ----------------->    4 Li⁰     E⁰_red = – 3.040 V

Anode :    4  OH-   –   4 e-    ----------------->    O_{2 (g)}   +   2  H₂O _(g)

                                                                                   E⁰_ox = – 0.401 V

The water molecules diffuse to the cathode , where they react with the lithium to give hydrogen gas and lithium hydroxide (which is recycled internally) :

2  Li⁰   +  2  H₂O _(g)   --------------->   2  LiOH  +  H_{2 (g)}

Net :  2  Li⁺   +  2  OH-   --------------->   2  Li⁰  +  H_{2 (g)} +  O_{2 (g)}  ; E⁰_T = – 3.441 V .

The molten lithium metal (m.p. 181 ºC) is tapped off from the electrolysis cell and recycled back to the nitrogen reactor . The oxygen and hydrogen by-products can be either vented , or collected , dried , and liquefied for storage and sale .

The lithium electrolysis cell could be designed and operated in a manner similar to the Castner cell , in which sodium hydroxide is melted and electrolysed to produce sodium metal . The Castner cell is described in the following references :

J.R. Partington , A Text-Book of Inorganic Chemistry , sixth ed. , Macmillan , London (UK) , 1957 ; pp. 683-685 ; and ,

C.H. Lemke , “Sodium and Sodium Alloys”, pp. 181-204 in the Kirk-Othmer Encyclopedia of Chemical Technology , vol. 21 , third ed. , M. Grayson and D. Eckroth (eds.) , John Wiley , New York (1983) ; see especially pp. 187-189 for a description of the Castner cell .

Note that lithium metal usually isn't produced in a Castner cell , but rather only in a Downs cell , which uses a molten chloride salt electrolyte consisting of 55% LiCl and 45% KCl :

R. Bach and J.R. Wasson , “Lithium and Lithium Compounds”, pp. 448-476 in the Kirk-Othmer Encyclopedia of Chemical Technology , vol. 14 (1981) ; esp. p. 456 .

I think it prudent in this approach to ammonia to avoid any chemistry involving halide salts or halogens .

The high theoretical cell potential , – 3.441 V (the thermodynamic ideal , ignoring the inevitable overvoltages) , indicates a considerable amount of electricity will be required for the electrolysis of molten lithium hydroxide . Additionally , heat energy will be required for all three steps (the nitriding reaction , drying the ammonia and LiOH , and melting the LiOH in the electrolysis cell) , and in general plant operations . The overall lithium-ammonia nitrogen fixation process will thus be very electrical and thermal energy-intensive .

The ammonia product would probably be quite costly , and could not compete economically with the ammonia produced from the hydrogenation of atmospheric nitrogen , usually by the Haber-Bosch or a related process . Unfortunately , the hydrogen used for ammonia production is almost entirely obtained from the steam reforming of either natural gas (mostly in North America) , or coal (elsewhere in the world) . The by-product of such steam reforming of carbon or hydrocarbons is carbon dioxide , which is usually vented to the atmosphere . The above lithium-mediated ammonia synthesis is entirely carbon-free , and produces no atmospheric pollutants . However , it would need large quantities of green energy (electrical and thermal) to become a practical reality . The production of such green energy from hydroelectric , solar , wind , geothermal , tidal , and ocean thermal (OTEC) sources , replacing the present-day fossil fuel economy , lies well in the future .

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Recently I became aware of research carried out in this area by Japanese electrochemists (several of their papers are cited below) , who discovered that nitrogen gas can be cathodically reduced to nitride anions , in a high yield , in molten salt mixtures similar to those used in the production of lithium metal in the Downs cell . They extended this finding to design a practical synthesis of ammonia by forcing steam into the cathode chamber , to hydrolyse the nitride anions and distil ammonia gas out of the cell . A possible modification of their process is sketched below , in which the three steps discussed above for the lithium-mediated synthesis of ammonia are combined in a single electrolysis cell :

Lithium and its compounds , including lithium hydroxide , are quite expensive . It might be possible to use a mixture of LiOH “diluted down” with the much cheaper NaOH (and maybe also KOH) as the molten salt electrolyte , which remains unchanged in the process . The added NaOH and KOH might also form a eutectic mixture with the LiOH , which would melt at a much lower temperature than pure LiOH (450 ºC) . Note that under normal conditions , only lithium metal is reactive enough to reduce nitrogen molecules to nitride anions . The other alkali metals don't usually react at all with nitrogen gas , even when heated . Thus , the presence of lithium in the molten salt electrolyte may be critical to the success of the overall process , with the formation of the transient , but crucial , lithium metal at the cathode .

References

D.R. Safrany , “Nitrogen Fixation”, Scientific American 231 (4) , pp. 64-80 (October , 1974) ; discussion of lithium nitride on p. 74 ; economic analysis of the Li₃N process on p. 80 .

T. Goto and Y. Ito , “Electrochemical Reduction of Nitrogen Gas in a Molten Chloride System”, Electrochim. Acta 43 (21-22) , pp. 3379-3384 (1998) .

T. Murakami , T. Nishikiori , T. Nohira , and Y. Ito , “Electrolytic Synthesis of Ammonia in Molten Salts Under Atmospheric Pressure”, J. Amer. Chem. Soc. 125 (2) , pp. 334-335 (2003) .

T. Murakami et al. , “Electrolytic Ammonia Synthesis from Water and Nitrogen Gas in Molten Salt Under Atmospheric Pressure”, Electrochim. Acta 50 (27) , pp. 5423-5426 (2005) .

R.B. Steele , “A Proposal for an Ammonia Economy”, Chemtech 29 (8) , pp. 28-34 (August , 1999) . In this article (p. 34) I proposed another ammonia synthesis scheme involving the reduction of nitrogen gas in aqueous solution by V(OH)₂ and Mg(OH)₂ [catalyst] at ~ 70 ºC .

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