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ScienceWeek
CHEMISTRY: ON THE SYNTHESIS OF AMMONIA
The following points are made by Michael D. Fryzuk (Nature 2004 427:498):
1) The synthesis of ammonia from its constituent elements, nitrogen and hydrogen, ranks as one of the most important discoveries in industrial catalysis(1). It merited the award of two separate Nobel prizes -- first to Fritz Haber (1868-1934) in 1918 and then to Carl Bosch (1874-1940) in 1931, recognizing the discovery of the process and its implementation, respectively. For more than 70 years, millions of tons of ammonia have been produced annually through the Haber-Bosch process; fertilizer made from this ammonia is estimated to be responsible for sustaining roughly 40% of the world's population and is the source for 40-60% of the nitrogen in the human body.
3) Pool et al(2) have traced the activation of molecular nitrogen by an organometallic complex. The complex consists of a zirconium ion to which are attached two substituted cyclopentadienyl rings (C5Me4H). When molecular nitrogen, or dinitrogen, is introduced, a side-on N2 fragment is generated, bridging between two zirconium centers. This in itself is not surprising, but what happens next to the complex, in pentane solution and in the presence of hydrogen, is unprecedented: a new complex forms, in which hydrogen atoms are added to the dinitrogen bridge. Although predicted(4) a few years ago, this is the first observation of such a transformation.
4) Why has it taken so long? The answer is probably that molecular nitrogen is not that good a ligand. It is so chemically inert that even binding it to metal complexes in solution, just as Pool et al.(2) have done, was a decades-long challenge for inorganic chemists(5). When it can be coerced to coordinate to a metal, in many cases the interaction is so weak that other (better) ligands can easily displace the dinitrogen unit.
5) In truth, it is unlikely that any homogeneous catalytic process will ever compete on an industrial scale with the heterogeneous Haber-Bosch reaction and its modern variants. But now that ammonia has been produced from its elements in solution, one can only begin to imagine what other kinds of transformation might be possible for molecular nitrogen.
References (abridged):
1. Tamaru, K. in Catalytic Ammonia Synthesis (ed. Jennings, J. R.) 1-18 (Plenum, New York, 1991)
2. Pool, J. A., Lobkovsky, E. & Chirik, P. J. Nature 427, 527-530 (2004)
3. Schlögl, R. Angew. Chem. Int. Edn 42, 2004-2008 (2003)
4. Basch, H., Musaev, D. G. & Morokuma, K. Organometallics 19, 3393-3403 (2000)
5. Shaver, M. P. & Fryzuk, M. D. Adv. Synth. Catal. 345, 1061-1076 (2003)
Nature http://www.nature.com/nature
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CHEMISTRY: DINITROGEN TO AMMONIA
The following points are made by G. Jeffery Leigh (Science 2003 301:55):
1) The reactions by which dinitrogen, N2, is converted to ammonia by *nitrogen-fixing organisms are one of the continuing mysteries of chemistry and biology. Whereas the industrial *Haber-Bosch process uses temperatures as high as 400 deg C and pressures of several hundred atmospheres, microorganisms can reduce dinitrogen at ambient conditions.
2) Since the first dinitrogen complex was discovered in 1965 and the basic structure of the active site of conventional molybdenum nitrogenases was unraveled, efforts have been made to combine chemistry and biology to explain the mechanism of biological nitrogen fixation at the atomic level. One problem is that dinitrogen in a synthetic complex may not approach the reactivity of dinitrogen at an enzyme site that is receiving a constant flux of electrons and protons. This issue may be addressed by attaching the dinitrogen complex to an electrode, but even then it is often difficult to define precisely the molecular species involved in the electrochemical cycle. It should be possible to mimic the enzyme reaction more accurately if the range of metal oxidation states during the reduction of dinitrogen in a synthetic complex can be restricted. Yandulov and Schrock (Science 2003 301:76) did just that, and their results may finally allow us to draw realistic and empirically based chemistry parallels with dinitrogenase reductions.
3) Dinitrogen chemistry in synthetic complexes involves many different structural arrangements and transition metals. Very little of the reactivity observed to date is likely to occur at a nitrogenase active site. The best-defined sequence of protonation reactions in a synthetic complex involves molybdenum or tungsten. This series cannot model the reduction of dinitrogen by nitrogenase, because the metal is in oxidation state zero when it binds dinitrogen. Biological reducing agents are probably not strong enough to bring this about. In addition, a wide range of oxidation states is exhibited by the single metal center. It is unlikely that a single metal atom could cover these oxidation states in nitrogenase while it reduces dinitrogen without considerable rearrangement.
Science http://www.sciencemag.org
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ON AMMONIA AND THE POPULATION EXPLOSION
Ammonia [NH(sub3)], a nitrogen hydride, is a colorless gas with a rather interesting human history that ranges from its discovery by the remarkable chemist Joseph Priestley (1733-1804) to the first large-scale synthetic production and use of ammonia in synthetic fertilizers and explosives in the 20th century.
The human requirement for synthetic fertilizers and explosives is an instance of irony in the application of science, since the major use of synthetic fertilizers is in the production of crops to feed people, and the major use of explosives is in the production of weapons to kill people.
Nitrogen compounds are essential to fertilizers and explosives, but in the early 20th century the best large-scale source of such compounds was in the nitrate deposits of Chile [*Note #1], which at that time was quite remote from Europe. Another possible source of nitrogen compounds, only theoretical at the time, was Earth's atmosphere, since the atmosphere is mostly nitrogen gas and therefore constitutes an inexhaustible supply.
If atmospheric nitrogen could be converted to ammonia, the ammonia could be used in the synthesis of various nitrogen compounds, including fertilizers and explosives. Fritz Haber (1868-1934) and Carl Bosch (1874-1940) are credited with the discovery of the Haber-Bosch process for the synthesis of ammonia from its elements, a discovery that literally altered the course of 20th century history. The basis of the process is the combining of nitrogen and hydrogen at high pressure over a catalyst. Haber, who first demonstrated the synthesis in 1909, received the Nobel Prize for Chemistry in 1918; Bosch, who engineered the application of the method to the large-scale production of ammonia, received the Nobel Prize for Chemistry in 1931 [*Note #2]
The following points are made by Vaclav Smil (Nature 1999 400:415):
1) The author poses the question: What is the most important invention of the 20th century? The usual answers include airplanes, nuclear energy, space flight, television, and computers, but none of these are critical to human well-being. The synthesis of ammonia from its elements, however, is critical: the world's population could not have grown from 1.6 billion in 1900 to the 6 billion of today without the Haber-Bosch process.
2) The synthesis of ammonia belongs to that special group of discoveries -- including Edison's light bulb and the Wright brothers' flight -- for which we can pinpoint the date of the decisive breakthrough. The archives of Badische Anilin-Und Soda-Fabrik (BASF) contain a letter from Haber, at that time Professor of Physical Chemistry at Technische Hochschule in Karlsruhe, to the company directors, a letter in which Haber recounts how the previous day the first demonstration to company scientists of the synthesis of ammonia from nitrogen and hydrogen was made: "All parts of the apparatus were tight and functioned well, so it was easy to conclude that the experiment could be repeated."
3) Although a number of company officials lacked confidence in the application of Haber's method because of the high pressure (over 100 atmospheres) required, Carl Bosch, who managed the BASF nitrogen-fixation research, was apparently confident: "I believe it can go. I know exactly the capability of the steel industry. It should be risked." It was Bosch who was responsible for the development of the proper steel housing necessary for large-scale ammonia production.
4) The present world output of ammonia amounts to approximately 130 million metric tons per year, and 80 percent of this goes into fertilizers, of which urea is the most important. The ammonia is absolutely essential to sustain today's population: rich countries might fertilize much less by cutting excessive food production and by eating fewer animals, but even the most assiduous recycling of organic wastes and the widest planting of *nitrogen-fixating legumes could not supply enough nitrogen for land-scarce, poor and populous nations. For several decades now, virtually all the fixed nitrogen added to the fields of China, Egypt, and Indonesia has come from synthetic fertilizers.
5) The author concludes: "Without this [the Haber-Bosch process], almost two-fifths of the world's population would not be here --and our dependence will only increase as the global count moves from 6 to 9 or 10 billion people."
Nature http://www.nature.com/nature
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Notes:
Note #1: During World War I (1914-1918) access to the Chilean nitrate deposits by Germany was almost impossible, with imports of nitrates blocked by the British navy. The German military needed explosives, which required nitrates, which required a source of usable nitrogen. This was the main impetus for the development of the large-scale production of ammonia by Bosch and BASF. Many historians believe that if Germany had had to depend only on Chilean nitrates for explosives, World War I would have ended in 1916, with several million lives saved.
Note #2: The personal story of Fritz Haber is interesting. Haber became a prominent chemist following his discovery of the synthesis of ammonia from nitrogen and hydrogen. He was extremely patriotic, and during the war he devoted great efforts to the development of gas warfare, directing the first warfare use of chlorine gas in 1915, and of mustard gas in 1917. In the history of war, the beginning of gas warfare is dated as April 22, 1915, "the day at Ypres when Haber's gas blowing process surprised and overpowered the enemy lines for the first time." Because of his work in gas warfare, there were many protests when Haber was awarded the Nobel Prize after the war ended. Following the war, and the huge reparations demanded from Germany by the Allies, Haber worked to isolate gold from seawater in order to pay the reparations. The yield was too small and research failed. In 1933, when the Nazis came to power in Germany, Haber's patriotic services in ammonia synthesis for explosives, gas warfare, and the attempted isolation of gold from seawater were dismissed as irrelevant because Haber was a Jew, and Haber was forced to give up his post and flee Germany. He went first to England, then decided to go to Palestine, but he died in Switzerland on his way south. Carl Bosch had a different fate: Bosch, who was not a Jew, remained in Germany as a prominent scientist. In 1933, Bosch actually cautioned Hitler against the policy of dismissing non-Aryan scientists, pointing out to Hitler the severe damage which this policy threatened to inflict on the pursuit of chemistry and physics in Germany. Hitler's response: "Then we'll just get along without physics and chemistry for a hundred years!" In 1935, as the Nazi era continued, Bosch succeeded Max Planck as head of the Kaiser Wilhelm Society (now called the Max Planck Society).
nitrogen-fixating legumes: In leguminous plants such as beans and peas, the symbiotic bacteria Rhizobium form characteristic root nodules, the bacteria supplying the plant with usable nitrate obtained from atmospheric nitrogen, while the bacteria obtain carbohydrates from the plant. In general, the term "nitrogen-fixation" refers to any fixation of nitrogenous compounds from atmospheric nitrogen. In nature, this is achieved by the normal metabolism of specialized soil bacteria (e.g., Rhizobium), and also by the electric discharges of lightening in the atmosphere. The Haber-Bosch process is industrial nitrogen-fixation.
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