Ammonia: Confronting a Primal Trend Blaise J. Arena Allied Signal Engineered Materials Research Center, 50 East Algonquln Road, Box 5016, Des Plaines, IL 60017
Ammonia, NHs, is a simple assembly as molecules go. But, this plainness belies the crucial role i t has played in the ability of man'to feed his brethren as well as make war on him and indeed, ammonia has allowed the very existence of life on earth. The richness of ammonia chemistry and its impact on natural historv and human events will become ev;htnt uirh a survey of its invdvcment since the beginning. The collection of matter which was to hecome our la net was the result of the condensation of heavy elements from a cosmic dust cloud centering about our new sun. The continuing accretion of cosmic debris and collisions with meteorites and comets provided primitive earth with a storehouse of organic compounds including HCN and CHsCN. Ammonia probably became an early descendant of these nitriles upon hydrolysis and accumulated in the early oceans and probably was a key ingredient in the "primordial soup". There was no shortage in our early atmosphere of energy sources such as thermal, gamma rays, shock waves, UV radiation, and electric discharges. Any of these may have fired the comhination of ammonia with CHq and Hz0 to form aldehydes, hydroxy acids, and amino acids. These can be viewed as molecular building blocks having future hiological importance, culminating several billion years later in the first living cells. As Harold Urey a t the University of Chicago put it, "If God had not created life under these conditions he would have missed a sure thing." In 1953 Urey's student, Stanley Miller conducted a now classic experiment in which he subjected mixtures of ammonia, methane, water, and hydrogen to an electric discharge in order to simulate lightning. The product of this was found to contain a varietv of nat"ral1y &curring a-amino acids. This offered the iirst experimental evidence that biomolecules could have been formed under primitive earth conditions. Eventually, all of the Earth's atmospheric ammonia was converted, possibly by solar energy photolysis, to give Np and H Z followed by oxidation of HZ to water. The atmosphere was left rich in Nz but depleted of the ammonia needed to build biomolecules. One primitive single-cell organism, the cvanobacteria. acauired an imnortant comnetiiive advantage by developing both the lapahility t o use Hz0 as a COz reductant in the formation of
sugars and also a process for nitrogen fixation, i.e., theability to convert atmospheric Np to ammonia. Thus, they were freed of dependence of already "fixed" nitrogen for the biosynthesis of amino acids. The chemistry of nitrogen fixation in living systems is complex even by hiocbemical standards. Many details of this process still elude investigators.but the general sequence of chemical events is known. At the heart of the system is an enzyme complex called nitrogenase composed of two protein components, I and 11. Although the focus of considerable research effort, the active sites of these components are not well characterized. Component I contains two Mo atoms, 30 Fe atoms, and as many S atoms probahly arrayed in four Fe-S centers with two Fe-Mo-S cofactors. The simpler component I1 is centered around an Fe-S site containing four Fe and four S atoms. In an attempt to gain some understanding of the chemical mechanism involved in nitrogenase catalysis, workers have developed model compounds or "mimics" of nitrogenase enzymes. Researchers a t the Nitrogen Fixation Unit of the University of Sussex, England, have been pioneers in this field. For example, they have found that
Figure 1. Biological ammonia synthesis
Figure 2. The nitmgen cycle.
1040
Journal of Chemical Education
4(PR3)
+ Mow compounds + NH3 + Nz
J
thus demonstrating that a t room temperature and pressure nitroaen. - . suitablv coordinated to Mo. can be reduced to ammonia. The catalvtic Dower of the nitroaenase enzvme svstem is appreciated-when one recalls thevinertness-of the N=N bond, 225 kcallmol, which must be broken to accomplish the overill reaction
-
Nz + 3H2
2NH3
The energy for this formidable task is derived from the
breakdoumn of the phm,synrhetir produrt glucose 10 supply adenosine triphosphate (A'I'I'), the energy currcnry 01 the d~~~~,~~~ l !see Fie. I ,I. The burden on the rell is rosrlv with 12-24 ATP molecules being required for the fixation of each molecule of NP.In fact, the consumption of carbon by the cell may increase by 50% during Ng fixation. The importance of fixed nitrogen to alivingsystem whether single cell or higher plant, is due, in part, to its role in amino acid biosvnthesis. T o accom~lishthis, a-ketodutarate undergoes reductive amination iatalyzed by the enzyme glutamate dehydrogenase to yield L-glutamate. Nicotanamide adenine dinucleotide phosphate (NADPH) serves as a hydride transfer agent:
-
c-o-
I1
One way in which other amino acids can he synthesized is by transamination. For example, L-glutamate can supply NHs for the conversion:
I
I
c-oII
0 pyruvate
L-glutam"
o-ketaglutarafe
- 1 transaminas.
I HC-NH3
c-oII 0
L-alanine
Since the ability to fix atmospheric nitrogen has been retained in only a comparatively few primitive bacterial genera, higher plants must rely on avariety of sources of already fixed nitrogen. Referred to as the nitrogen cycle, a dynamic interrelationship of processes provides fixed nitrogen to living plants and returnsNg to theatmosphere. Many ammonia producing hacteria are natural inhabitants of the soil and make a laree contribution to the needed sunulv. .." Some of these species have evolved into a symbiotic relationship with certain ~ l a n t sthe . most i m ~ o r t a nof t which are the legumes such as soyhe&, alfalfa, pea, clover, and peanut. ~ h e s ~ p l a n t roots are infected by the nitrogen-fixing symhionts and a tumor-like growth or nodule is formed. The nodule encloses a colony of hacteria that supply the plant with needed ammonia and are repaid with carbohydrate nutrients from the host plant's photosynthetic machinery. Additional fixed nitrogen is provided by atmospheric fixation during lightning discharges, the presence of animal waste and decaying oreanisms. These sunnlies . . of fixed nitroeen are continuouslv depleted by natural leachingof the soii'hy rainwater and 1); thr soil nonulation ofdenitritvine bacteria that art on fixed nitrogen ti regenerate Ng ha& tothe atmosphere. The cycle is completed (see Fig. 2). Early agrarian societies realized the benefits of applying nitrogen fertilizer to their farm crops. Fixed nitrogen in the form of manure, guano, and natural deposits of saltpeter (NaN03)was effective in improving crop yields. The ancient Romans were sophisticated enough in 200 B.C. t o recommend adding manure and growing legumes in rotation with other crops. But it was not until about the year 1800 that chemists began rational studies of the factors hearing on plant nutrition and soil fertility. Key among these was the formulation of the "Law of the Minimum" by Justus von
Liehie in 1840. which. out simnlv. said that ulant ernwth is limit& by whkever n2rient i;i; the shortes't sup2y. I t was soon learned that except during periods of drought, fixed nitrogen is commonly the limiting nutrient. Several decades before. Thomas Malthus in his "Essav on the Princinle of Population" (17981, had proposed that populations would increase at a rate far greater than that of the rate of food production thus outstripping their ability to feed themselves. This was prophetic; the population of Europe increased dramatically, with Britain itself trehling its population during the 1800's. Food supplies were adequate, hut only with imports from the United States and the Ukraine. European farmers were making heavy use of manure fertilizer and imnorted Chilean saltneter. . . hut.. nevertheless. fixed nitrogen remained the limiting nutrient. The situation o r o m ~ t e dWilliam Crookes in his 1898 address to the British ksso'iation for the Advancement of Science to warn of the threatening nitrogen fertilizer shortage and of the accompanying stra& being put on crop soil. He challenged that "the fixation of atmospheric nitrogen would he the solution and that it would he bne of the great discoveries awaiting the ingenuity of chemists". A number of chemists scattered throughout Europe confronted this problem in the early 1900's but it was to he Fritz Haher (1868-1934) who would eventually realize Crnokes' vision of a synthetic ammonia process. Haher, a young professor a t the Technische Hochschule in Karlsruhe. Germany, agreed to the 1904request of some Austrian huskessmen to investigate the question of ammonia synthesis from the elements. Haher embarked on a program comprising the fertile union of empirical and theoretical research, trial and error shaped by the new discipline of thermodynamics. Over the next several vears he determined the equilibrium Ng + 3H2 2NH3 at-various temperatures and-pressures, concluding that high pressure and temperature were needed to shift the equilibrium toward the product while affording a satisfactory rate. This work was accompanied hy a search for an effective solid catalyst, one that might lower the activation energy enough to allow a lower-temperature reaction. Bv a thousand trial-and-error exueriments he hit unon fine1;divided iron mixed with traces of uranium andosmium oxides as a satisfactory catalyst, which in 1908 a t 200 atm, 500 "C, produced high yields of ammonia. In 10 years he would he awarded the Nobel Prize for this work. Haher's synthetic ammonia process accomplishes the same result as that of its natural counterpart in biological nitrogen fixation
-
by using a simple iron catalyst, hut under extreme conditions in place of the complicated nitrogenase enzyme. The nature of ammonia synthesis over the Fe catalyst is not completely resolved even after 70 years, hut certain explanations for the phenomenon have been proposed and supported. A fundamental aspect of the problem involves the mechanism by which hydrogen adddto nitrogen. Two extreme possibilities, the associative and dissociative mechanisms, dominate current thinking and are outlined below. Associative N,
+ Catalyst * N,*
N2*+ H2 + NZH1* N,H,* + Hz= NgH,* N,H,* H, * 2NHs
+
(adsorbs on catalyst and is activated)
(desorbs from catalyst)
Dissociatiue N,
+ catalyst * ZN*
H,
+ catalyst + 2H*
(N2and Hzare adsorhed on catalyst, are activated and dissociated)
Volume 63 Number 12 December 1966
1041
N* NH* NH,'
+ H*=NH* + H* = NH,'
+ H = NH,
(desorbs from catalyst)
Many variations and combinations are possible here hut the central question remains: Does the N2 bond break before or during addition of hydrogen once i t is activated by adsorption on the Fe surface? Some have suggested that each mechanism is operative in a different temperature range. For examole. . . the associative mechanism mav be favored a t low temperatures (perhaps a t the nitrogenase enzyme?) while it seems reasonable that hieh t e m ~ e r a t u r ~ e o u l favor d the dissociative mechanism. h he iron k s t a l s that catalyze the reaction by whatever mechanism are composed of a variety of crystal faces or planes, each having its own unique spatial arrangement of Fe surface atoms. The proper geometry of Fe atoms is crucial to the catalytic activity, and there is evidence that surface Fe atoms coordinated to seven others possess the highest ammonia synthesis activity, i.e., the reaction is highly sensitive to the local microstructure of the catalyst surface. Haber's catalytic reaction soon attracted the attention of industrv. in narticular the German eiant Badische Anilin Soda F ~ h r i k ' ~ l Ma~company ~), u,i';h the a,herewithal to confront the heavy financial and ~eshnoloricaldemands of a new process. Several years of refinements to the catalyst, development, and plant design led to the first ammonia synthesis in 1913 a t Op6au producing 9000 tonslyear. The interest of German business in this new process was exceeded by that of the German military whoforesaw an important military advantage in this process. Another catalytic process had recently been developed for the oxidation of ammonia to nitrate by
+ 50,+ 4 N 0 + 6H20 2NO + O2e 2N02 H,O + 3N0, = ZHNO, + NO HNO, + NH,OH + NH4N03+ H,O 4NH,
Ammonium nitrute und nitric acid, key ingredients in the manufacture of explosiws such as TN'C, gunpowder, and cellulose nitrate, could now be p r o d u c e d h the heart of Germany. She was relieved of her dependence on imported nitrates from Chile. Shortly after the outbreak of World War I in 1914, the Allies cut off Chilean nitrate shipments to Germany helieving this would surely smother their enemy's war-making Dower. German industrv countered hv accelerating - the construction of Haber process plants so that synthetic ammonia production crew from 9000 tonslvear in 1913 to 95,000 tons/ year in 191i a t war's end. This is an unimaginable growth rate for any new process and was a major factor in allowing Germany to survive in the war as long as she did. This was not lost on the Allies, who were eager to acquire this new technology after the war. The United States had commissioned the construction of a calcium cyanamid plant during the war a t Muscle Shoals, Alabama, for the production of 110,000 tons of ammonium nitrate per year. However, Haber's ammonia svnthesis from the elements had such a tremendous economic ad\,antagr that the \luscle Shoals plant nwer rot into actunl ~nnluction.The United States was produckg ammonia with the Haber process by 1921 and other industrialized nations quickly followed suit. By the beginning of World War 11, world ammonia capacity stood a t 3.7 million tonslyear. The second war accelerated plant start-ups again and the U S . itself increased world capacity to 4.5 million tonslyear by the end of the war. The post-war world saw innovations in the engineering and desipn of even larrer, more efficient ammonia svnthesis plants. A recent example of this continuing effortwas the 1042
Journal of Chemical Education
introduction in 1983 of a "reduced energy ammonia process" by the M. W. Kellogg Co. Kellogg, a leading American designer of ammonia plants, examined every stage in the process train for possible application of energy saving strategies. Included in the design was a proprietary horizontal synthesis reactor (see Fig. 3) and a catalyst with a 10-year lifetime! The process has been proven out as state-of-the-art technology with the start-up of the 400,000-tonslyear Sherritt Gordon Mines, Ltd., plant at Fort Saskatchewan, Canada (see Fig. 4). The operating efficiency of this plant was so improved that it uses 25% less energy (feed fuel) than conventional designs making it the most energy-efficient in the world. Today, ammonia produced by the Haber process accounts for 40% of all nitrogen supplied t o agricultural land worldwide. The remainder is still largely the result of natural biological fixation processes. Almost every developed nation has the technolow -" for ammonia manufacture bv the Haber process, with world production capacity now standing a t about 120 million tonslyear worth $150lton. An $18 billion1 year industry! Crookes' vision of abundant synthetic nitroeen fertilizer. thoueh fostered hv war. was achieved. However, the cost df e n e k both as source of H Zfeed and as a supplier of fuel to the process plant remains the major cost factor in ammonia manufacture. In spite of sophisticated design improvements, making ammonia is truly an energy intensive proposition. Those areas without access to supplies of coal, natural gas, or petroleum have severely lagged hehind the rest of the world in ammonia production. Demand for nitrogen fertilizer will continue to increase as populations increase worldwide. One analysis predicts a world ammonia deficit of 12 million tons by 1995 with the trend continuing into the new century. The Haher process has served us well in the 20th century and will continue to fill a need. However, there lies on the horizon a new approach to the problem of nitrogen as the limiting nutrient. This ap-
+
a
Figure 3. Horizonfa1ammonia synthesis reactor
Figure 4. The Sharrin Oordon Ammonia Plant, Fort Saskatchewan. Canada. (Courtesy of M. W. Kellogg Co.)
proach, if fruitful, would provide fixed nitrogen a t no cost in depletion of non-renewable fossil fuels. It would also make the high crop vields, possible onlv with an abundance of . . t'lxed nirn~gen,a\,ailablr w e n in underdrveloped cuuntrier. Suppose we cuuld ~xploitthe biochemistry of the living re11 in &der to confer nitpogen-fixing capacity on all crop plants? Could we manipulate Np fixing bacteria into forming relationships with crops other thailegnmes? Not long ago these questions would have had little meaning, much less answers, but now they may hold the promise of erasing the threat of nitrogen famine forever. Modern geneticists have identified and manned . . the cluster of 17 genes nmhich carry the cude for nitrogen-fixation activity in living crlls. Desicnnred ss the nil. .. arnes. this chromosome section of a nitrogvn firing microorganism regulate3 the synthesis ot all enz\,mes involved in fixation includinp. the central nitrogenase system. In fact, the nif genes c&trol the entire fixation process by turning it on or off in response to demand for ammonia by the organism. In 1972, Dixon and Postgate of the University of Sussex, England, performed an experiment that may prove to he of landmark importance. They were successful in transferring the genetic material containing the nif genes from the nitrogen-fixing bacterium Klebsiella pneumoniae t o another microorganism Escherichin coli. E. coli, not naturally a nitrogen fixer, became endowed with the biochemical machinery to fix Np. The consequences of this result were striking then and remain so today. They evoke the possibility of someday transferring the nif gene segment to crops such as wheat or corn allowing them to produce high yields without added nitrogen fertilzer. Today, the development of powerful genetic engineering techniques has made the exploitation of biochemical nitrogen fixation a serious pursuit for industry and university researchers. The work can be divided into three approaches of increasing levels of difficulty. Almost certain t o b e accomplished in the relative short term are attempts to manipulate genetically those microorganisms which live symbiotically with legumes. By recombinant-DNA techniques, nif gene seements. altered for maximum nitroeen-fixine oroductivitv could he transferred to highly compkitive n&de-forming Rhizobium bacteria. This techniaue could simificantlv increase the yield of legume crops. once crop l&d was i&ulated with these super nitrogen fixers, no further effort would be needed. ~ G h e r would he achieved perpetually. A second and more exciting possibility is that of the genetic redesign of nitrogen-fixing microorg&sms and nonlegume crops, ie., corn, wheat, etc., to allow them to form a svmbiotic relationshin. This is a considerablv more comnlex task. Several operations must he regulated by the microorganism and the cereal plant. Once the plant root is infected, nodule formation rather than rejection must be induced, and a transport mechanism for NH? from bacteria to host plant must be constructed as well askutrient flow to the nodule colonv. But here again, once inoculated. the soil can continue to serve up nitrog&fi'xing bacteria for association with corn and wheat and so on. The ultimate challenge in this undertaking is to fulfill the dream of designing a cereal plant, such as corn, that can fix Nz without the aid of an associated microorganism. This looms as an enormously complicated task and although manv vears awav is thought to be theoreticallv oossible. ~ e v & formidadle i obstacres lie in the path. T o Gegin with, techniaues will have to be develooed to transfer the nif genes to the cells of a higher plant, a more demanding g o y t h a n transfer to other bacterial snecies. However. rapidlv developing tissue-culture techniiues, where entire plants can be regenerated from single cells, will make this possible. A second prob1em;nvolve~ the nature of the nitrogenase enzyme system itself. Oxygen, obviously plentiful in most living systems, is a rapid and irreversible poison for nitrogenase, rendering it catalytically inactive. Nitrogen-fixing
organisms have developed a varietv of methods for isolatine ni&ogenase from 0 2 . FOI example; certain species have t h i abilitv to maintain extreme metabolic rates so that virtuallv all 0; entering the cell is reduced to Hp0. ~itrogen-fixing algae have specialized cells called heterocysts which maintain nitrogenase in an 02-free environment. Those bacteria residing in nodule colonies of legume roots are bathed in the plant kingdom's only version of hemoglobin. Called leghemoglohin, this red protein is structurally and functionally similar to blood hemoglobin and is synthesized by the host plant as part of its symbiotic contribution. Leghemoglobin serves as an Op "sink" by rapidly binding O2 in the nodule colony thereby providing an oxygen-free environment. Finally, some species simply cannot fix nitrogen except when they happen to reside in 02-freeenvironment. Unfortunately, a corn plant, for instance, possesses none of these Oz safeguards. This problem still awaits a truly rational approach to its solution. \Ye kniw that the nitrogcnasc enzvme is a mnlyhdenumrich material and even early u.~rkerswere ahlr to show that most nitrogen fixers shut down in Mo-depleted environments. A cereal crop plant that has the Np-fixing system in place must be able to sunoort .. it with a transoort anoaratus . for remwing Mu t'rom the soil and supplying if fur nitrogenase synthesis rct~uldMI, somedav bewme the limiting nutrient?!. Now, finally, we address the problems of enerev . . that is a heavy requircmwt regardless or the method o r the organism used 10 manuiactl~rrNH,. I(. lserman ifitrinaly . . of RASP) has surveyed the eoergetirs of nitrogen nutrition t o rrnp plnnti and bhows that hidopical nitrogen fixation in nonsymhioric urganism.; costs 101 mJ/kg of nitrugen. This wollld he a suhstontiul drain on any standard crop plant, enough so that n ulant endowed with N,-fixine abilitv. mieht generate copious quantities of ammonia hut use up large portions of its stored carbohvdrates to fuel the orocess. thus leadine to luw cnrp yield
