by Vaclav Smil
Ammonium nitrate—a compound that was first made by J. R. Glauber in 1659 by reacting ammonium carbonate with nitric acid—is difficult to detonate, but it is a powerful oxidizing agent, and its heating under confinement results in highly destructive explosions. Mixtures of ammonium nitrate and carbonaceous materials (charcoal, sawdust) were patented as explosives by C. J. Ohlsson and J. H. Norbin in 1867, but mixtures of the compound with fuel oil (generally referred to as ANFO) became commonly used only during the 1950s (Marshall 1917; Clark 1981). Since that time, explosives based on ammonium nitrate that are inexpensive and safe to use have captured most of the blasting market by replacing dynamite; some 2.5 Mt of these mixtures accounted for 99% of U.S. industrial explosives sales during the late 1990s (Kramer 1997).
Unfortunately, ANFO has been used not only as a common industrial explosive but also in powerful car and truck bombs by terrorists. The two worst terrorist attacks that took place in the United States before September 11, 2001—the bombing of the World Trade Center in New York City on February 26, 1993 (when, fortunately, only six people were killed), and the destruction of the Alfred Murrah Federal Office Building in Oklahoma City on April 19, 1996 (169 adults and children killed)—involved truck bombs using ANFO mixtures. The bombing also resulted in a renewed interest in additives able to desensitize ammonium nitrate, and new tests showed that this option does not work very well with larger volumes of explosives that are used in terrorist acts: the Oklahoma bomb contained about i.8 t of nitrate (Hands 1996).
Although Nobel was frequently reproached for inventing such a destructive substance, dynamite did not take place of gunpowder for military uses. While it made it easier to commit terrorist attacks—in 1882 Czar Alexander II was among the first victims of a dynamite bomb—slower acting, and preferably smokeless propellants were needed for guns. Such explosives were produced during the 1880s: poudre B (nitrocellulose is first gelatinized in ether and alcohol and then extruded and hardened to produce a propellant that could be handled safely) by Paul Vieille (1854–1934) in 1884, Nobel’s own Ballistite (using nitroglycerine instead of ether and alcohol) in 1887, and cordite, patented in England by Frederick Abel and James Dewar in 1889. The combination of these new powerful propellants and better guns made from new alloys resulted in considerably longer firing ranges.
Destructiveness of these weapons was further increased by introducing new explosive fillings for shells. Ammonium picrate was prepared in 1886; trinitrotoluene (TNT), which was synthesized by Joseph Wilbrand in 1863 and which must be detonated by a high-velocity initiator, was first manufactured in Germany in 1891 and by 1914 became a standard military compound; and the most powerful of all prenuclear explosives, cyclotrimethylenetrinitramine, known either as cyclonite or RDX (for royal demolition explosive), was first made by Hans Henning in 1899 for medicinal uses by treating a formaldehyde derivative with nitric acid, and its widespread use came only during WWII.
Structural formulas of these three powerful explosives clearly show that they are variations on the common theme, namely, a benzene ring with three nitro groups (figure 4.15). Cyclonite production took off only after cheap supplies of formaldehyde became available, and its combination of high velocity of detonation (8,380 m/s) and large volume of liberated gas make it an unsurpassed choice for bursting charges. Consequently, this substance was produced and used on a massive scale during WWII, as was TNT, which was deployed in more than 300 million antitank mines.
Ammonia from Its Elements
Inclusion of this item may seem puzzling, as the synthesis of the simplest triatomic nitrogen compound does not figure on commonly circulating lists of the greatest invention in modern history. Whether they were assembled by the canvassing of experts or of the general public, these compilations (a few of them were noted in chapter 1) almost invariably contain automobiles, telephones, airplanes, nuclear energy, space flight, television, and computers as the most notable technical inventions of the modern era. And yet, when measured by the most fundamental of all yardsticks, that of basic human physical survival, none of these innovations has had such an impact on our civilization as did the synthesis of ammonia from its elements (Smil 2001). The two keys needed to unlock this puzzle are one of the peculiar attributes of human metabolism and immutable realities of nitrogen’s presence and cycling in the biosphere.
In common with other heterotrophs, humans cannot synthesize amino acids, the building blocks of proteins, and have to ingest them (eight essential amino acids for adults, and nine for children) in plant or animal foods in order to grow all metabolizing tissues during childhood and adolescence and to maintain and replace them afterward. Depending on the quality of their diets, most adults need between 40 and 80 g of food proteins (which contain roughly 6–13 g of nitrogen) each day. Crops, or animal foods derived from crop feeding, provide nearly 90% of the world’s food protein, with the rest of the nutrient coming from grasslands, fresh waters, and the ocean. In order to produce this protein, plants must have an adequate supply of nitrogen—and this macro-nutrient is almost always the most important factor that limits the yields in intensive cropping. Unfortunately, the natural supply of nitrogen is restricted. Nitrogen makes up nearly 80% of Earth’s atmosphere, but the molecules of dinitrogen gas (N2) are nonreactive, and only two natural processes can split them and incorporate the atomic nitrogen into reactive compounds: lightning and biofixation.
Lightning, producing nitrogen oxides, is a much smaller source of reactive nitrogen than is biofixation, whereby free-living cyanobacteria and bacteria and, more important, bacteria symbiotic with legumes (above all, those belonging to genus Rhizobium), use their unique enzyme, nitrogenase, to cleave the atmospheric dinitrogen’s strong bond and to incorporate the element first into ammonia and eventually into amino acids. In addition, reactive nitrogen is easily lost from soils through leaching, volatilization, and denitrification. As a result, shortages of nitrogen limit the crop yields more than shortages of any other nutrient. Traditional agricultures could improve the nutrient’s supply only by cultivating leguminous crops or by recycling crop residues and animal and human wastes. But this organic matter has only very low concentrations of nitrogen and even its (utterly impracticable) complete recycling would be able to support only a finite number of people even if they were to subsist on a largely vegetarian diet.
In contrast, it is much easier to cover any shortages of phosphorus and potassium. The latter nutrient is not commonly deficient in soils, and it is easily supplied just by mining and crushing potash. And the treatment of phosphate rocks by diluted sulfuric acid, the process pioneered by John Bennett Lawes (1814–1900) in England during the 1860s, yields a fairly concentrated source of soluble phosphorus in the form of ordinary superphosphate. But at the close of the 19th century, there was still no concentrated, cheap, and readily available source of fertilizer nitrogen needed to meet higher food requirements of expanding populations. And not for the lack of effort.
Two new sources were commercially introduced during the 1840s—guano (bird droppings that accumulated on some arid islands, mainly in the Pacific) and Chilean nitrate (NaNO3) extracted from near-surface deposits in the country’s northern desert regions—but neither had a high nitrogen content (15% in nitrate, up to 14% in the best and less than 5% in typical guanos), and both sources offered only a limited supply of the nutrient. So did the recovery of ammonium sulfate from coking ovens, which was introduced for the first time in Western Europe starting in the 1860s: the compound contained only 21% N. As the 19th century was coming to its close, there was a growing concern about the security of future global food supply, and no other scientist summarized these worries better than William Crookes (1832–1919), a chemist (as well as physicist) whom we will again encounter in chapter 5 in relation to his comments on wireless transmission.
Crookes based his presidential address to the British Association for the Advancement of Science that he delivered in September 1898 (Crookes 1899) on “stubborn facts”—above all
, the limited amount of cultivable land, growing populations, and relatively small number of food-surplus countries. He concluded that the continuation of yields that prevailed during the late 1890s would lead to global wheat deficiency as early as in 1930. Higher yields, and hence higher nitrogen inputs, were thus imperative, and Crookes saw only one possible solution: to tap the atmosphere’s practically unlimited supply of non-reactive nitrogen in order to produce fixed nitrogen that could be assimilated by plants.
He made the existential nature of this challenge quite clear (Crookes 1899: 45–46):
The fixation of nitrogen is vital to the progress of civilised humanity. Other discoveries minister to our increased intellectual comfort, luxury, or convenience; they serve to make life easier, to hasten the acquisition of wealth, or to save time, health, or worry. The fixation of nitrogen is a question of the no far-distant future.
And he had no doubt as to how this will happen: “It is the chemist who must come to rescue . . . It is through the laboratory that starvation may ultimately be turned into plenty” (Crookes 1899:3).
FIGURE 4.16. Fritz Haber’s portrait (left) taken during the early 1920s. Photograph courtesy of Bibliothek und Archiv zur Geschichte der Max-Planck-Gessselschaft, Berlin. Carl Bosch’s photograph (right) from the early 1930s. Photograph courtesy of BASF Unternehmensarchiv, Ludwigshafen.
But in 1898 there was no technical breakthrough in sight. The German cyanamide process—in which coke was reacted with lime and the resulting calcium carbide was combined with pure nitrogen to produce calcium cyanamide (CaCN2)—was commercialized in that year, but its energy requirements were too high to become a major producer of nitrogen fertilizer. That the sparking of nitrogen and oxygen can produce nitrogen oxides (readily convertible to nitric acid) was known for more than a century, but very high temperatures needed for this process (up to 3,000°C) could be produced economically only with very cheap hydroelectricity.
The first commercial installation of this kind (the Birkeland-Eyde process) was set up in Norway in 1903, and it was enlarged before 1910, but the total output remained small. The breakthrough came 11 years after Crookes’s memorable speech, when Fritz Haber (1868–1934; figure 4.16) demonstrated his process of ammonia synthesis from its elements. And just four years after that, his small bench-top apparatus was transformed into a large-scale commercial process by a dedicated engineering team led by another outstanding German chemist, Carl Bosch (1874–1940; figure 4.16).
Genesis of the Haber-Bosch Process
Fritz Haber was born to a well-off merchant family (dyes, paints, chemicals) in Breslau, and as a young man took advantage of the advancing integration of well-educated Jews into privileged German society. He studied in Berlin, where Carl Liebermann (1842–1914), best known for the synthesis of alizarin (a red dye), was one of his supervisors, and in Heidelberg under Robert Wilhelm Bunsen. Afterward, he went through a number of jobs before Hans Bunte offered him an assistantship at the Technische Hochschule in Karslruhe, where Haber stayed for the next 17 years, working initially in electrochemistry and then in thermodynamics of gases.
His first experiments with ammonia took place in 1904 at the request from the Österreichische Chemische Werke in Vienna, and as noted in his 1919 Nobel lecture, the challenge was only seemingly simple (Haber 1920:326):
We are concerned with a chemical phenomenon of the simplest possible kind. Gaseous nitrogen combines with gaseous hydrogen in simple quantitative proportions to produce ammonia. The three substances involved have been well known to the chemist for over a hundred years. During the second half of the last century each of them has been studied hundreds of times in its behaviour under various conditions during a period in which a flood of new chemical knowledge became available.
But none of the many illustrious chemists who attempted to perform that simple reaction—N2 + 3H2 ↔ 2NH3—during the 19th century had succeeded in the task, and by the beginning of the 20th century it appeared that such a synthesis may not be possible, a perception that deterred many new attempts. Haber’s first experiments to synthesize ammonia at 1,000°C and at atmospheric pressure were soon abandoned due to very low yields obtained with iron catalyst, which he chose after testing more than 1,000 materials. He returned to the task only three years later, in a large part in order to answer an unjustified criticism that Walther Nernst, one of the leading chemists of the early 20th century, leveled against his previous experiments. But Nernst was right in pointing out that increased pressure should lower the reaction temperature, and almost immediately after Haber, with his English assistant Robert Le Rossignol, began new experiments under a pressure of 3 MPa, he obtained much higher, although still clearly noncommercial, yields (Smil 2001).
His new calculations showed that a pressure of 20 MPa, at that time the limit he could obtain in his laboratory, and a temperature of 600°C would yield about 8% ammonia—but he had no catalyst that would work at such a relatively low temperature. By 1908, his search for ammonia synthesis received assistance from BASF, at that time the world’s largest chemical company, which was interested in advancing research on nitrogen fixation by the Birkeland-Eyde process. As already noted, this technique used a high-voltage electric arc to produce nitrogen oxide that could be later converted to nitric acid. In 1907, Haber and his pupil A. König published a paper on that topic, and their work attracted the attention of BASF. On March 6, 1908, the company concluded two agreements with Haber: the first one provided fairly generous financial assistance to do more work on the electric arc process; the second one, a smaller sum of money to continue his previous work on high-pressure ammonia synthesis.
Nothing pathbreaking ever came out of arc experiments, while the high-pressure catalytic synthesis, thanks to Le Rossignol’s skills in constructing the necessary apparatus (including a small double-acting steel pump) and Haber’s perseverance in searching for better catalysts and for the best combination of pressure and temperature, began soon yielding promising results. Haber’s first German patent for ammonia synthesis (Patent 235,421 valid since October 13, 1908) is generally known as the circulation patent, and its basic principle is still at the core of every ammonia plant today (BASF 1911). Commercialization of the process could not be done without catalysts that could support rapid conversion at temperatures of 500–600°C.
Initially Haber believed that osmium would be the best choice, and the finely divided metal supporting ammonia yields of about 6% and higher was clearly of economic interest; hence, he advised BASF to buy up all the osmium that was at that time in the possession of a company that used it to produce incandescent gas mantles. Haber filed his osmium catalyst patent on March 31, 1909, and continued to search for other catalysts; uranium nitride looked particularly promising. But above all, he concentrated on perfecting his laboratory apparatus built to demonstrate the potential of high-pressure synthesis with gas recirculation, and by the end of June the setup was finally ready.
Convincing demonstration of ammonia synthesis is one of those rare great technical breakthroughs that can be dated with great accuracy. On July 3, 1909, Haber sent a detailed letter to the BASF directors (now in the company’s Ludwigshafen archives) that described the events of the previous day when Alwin Mittasch and Julius Kranz witnessed the demonstration of Haber’s synthesis in his Karslruhe laboratory (Haber 1909):
Yesterday we began operating the large ammonia apparatus with gas circulation in the presence of Dr. Mittasch, and we were able to keep its uninterrupted production for about five hours. During this whole time it had functioned correctly and it produced liquid ammonia continuously … The steady yield was 2 cm3/minute and it was possible to raise it to 2.5 cm3. This yield remains considerably behind the capacity for which the apparatus has been constructed because we have used the catalyst space very insufficiently.
FIGURE 4.17. Laboratory apparatus that was used by Haber and Le Rossignol in their successful experiments to synthesize ammonia from its elements. Reproduced from Smil (2001).
; The apparatus used in the experiment is shown in figure 4.17. Inside an iron tube kept under the pressure of 20.3 MPa was a nickel heating coil used to raise the gas temperature to a desired level; the synthesized ammonia was separated from the flowing gas at a constant high pressure, and the heat from the exothermic reaction could be removed from exhaust gases and used to preheat the freshly charged gas replacing the synthesis gas removed by the conversion. Rossignol designed a small double-acting steel pump to circulate the gases, and their recycling over the catalyst made it possible to sustain relatively high production rates even though the thermodynamic equilibrium was not particularly favorable. Haber’s third key German patent application (Patent 238,450 submitted on September 14, 1909, and issued on September 28, 1911) detailed the synthesis under very high pressure (Haber 1911).
Once there was no doubt about the feasibility of high-pressure catalytic synthesis of ammonia from its elements, BASF moved swiftly to commit its resources to the commercialization of the process. This challenge called for scaling up Haber’s bench-top apparatus—a pressurized tube that was just 75 cm tall and 13 cm in diameter—into a large-throughput assembly operating under pressures that were at that time unprecedented in industrial synthesis and filling it up with a relatively inexpensive yet highly effective catalyst. And it also called for, first and foremost, producing economically large volume of the two feedstock gases. Rapid success could not have happened without Carl Bosch’s decisive managerial leadership and his technical ingenuity.