Oxidations of ammonia

Several ammonia-oxidizing bacteria are known, including Nitrosomonas europaea, Nitrosomonas eutropha, Nitrosococcus oceanus, Nitrosospira briensis, 

Nitrosovibrio tenuis, and Nitrosolobus multiformis (Watson et al., 1989 Koops et al., 1991). Among these, N. europaea has been the most studied. 

The ammonia-oxidizing bacteria oxidize ammonia to nitrous acid via hydroxyl-amine (NH2OH) (Lees, 1952 Hofman and Lees, 1953) ammonia is first oxidized to hydroxylamine by the catalysis of ammonia monooxygenase (AMO) (Dua et al., 1979 Hollocher et al., 1981). In this reaction, molecular oxygen is utilized. Then, hydroxylamine formed is oxidized to nitrous acid by the catalysis of hydroxylamine oxidoreductase (HAO). 

Ammonia monooxygenase is very unstable when the cells of N. europaea are destroyed the enzyme is mostly inactivated (Suzuki and Kwok, 1970 Suzuki et al., 1981). Therefore, little is known about the properties of the enzyme. The copper atom seems important for the enzyme to function, as the enzyme is inactivated by many cuprous chelating agents (Hooper and Terry, 1973 Wood, 1986), and the enzyme seems to be activated by cuprous ions (Ensign et al., 1993). Moreover, the enzyme purified from the heterotrophic nitrifier Paracoccus denitrificans has been found to be activated by the cuprous ion (Moir et al., 1996). However, the P. denitrificans enzyme may be a little different from the N. europaea enzyme, because it is not inhibited by acetylene (Moir et al., 1996), while the N. europaea enzyme is inhibited by the compound (Hooper and Terry, 1973). Electron paramagnetic resonance (EPR) studies show that iron is also important for the function of the enzyme (Zahn et al., 1996). The amino acid sequence of the N. europaea enzyme is deduced from DNA (Hyman and Wood, 1985 McTavish et al., 1993 Bergmann and Hooper, 1994), although firm evidence has not been obtained that the DNA really encodes the enzyme. 

Ammonia monooxygenase requires 2[H] (two hydrogen atoms or electrons) to oxygenate ammonia, as shown by equation (3.1). As the oxidation of ammonia to hydroxylamine by the cell-free extracts of N. europaea is activated by addition of cytochrome c-554 (Yamanaka and Shinra, 1974), the 2 [H] are thought to be supplied to the enzyme through the cytochrome (Suzuki and Kwok, 1981). Incidentally, the substrate for ammonia monooxygenase is ammonia, but not ammonium ion (Suzuki et al., 1974). 

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Industrially nitrogen monoxide is prepared by the catalytic oxidation of ammonia as an intermediate in the manufacture of nitric acid (p. 238). The molecule of nitrogen monoxide contains an odd number of electrons and can be represented as... 

On the large scale, nitric acid is now made in large quantities by the catalytic oxidation of ammonia, employing the reaction ... 

Hydrazine [302-01-2] (diamide), N2H4, a colorless liquid having an ammoniacal odor, is the simplest diamine and unique in its class because of the N—N bond. It was first prepared in 1887 by Curtius as the sulfate salt from diazoacetic ester. Thiele (1893) suggested that the oxidation of ammonia (qv) with hypochlorite should yield hydrazine and in 1906 Raschig demonstrated this process, variations of which constitute the chief commercial methods of manufacture in the 1990s. 

Raschig Process. The Raschig process (92) is based on the oxidation of ammonia with hypochlorite according to the following overall... 

KetaZine Processes. The oxidation of ammonia by chlorine or chloramine in the presence of ahphatic ketones yields hydrazones (36), ketazines (37), or diaziddines (38), depending on the pH, ketone ratios, and reaction conditions (101). 

Alternative approaches to nitric oxide formation include irradiation of air in a nuclear reactor (72) and the oxidation of ammonia to nitric oxide in a fuel cell generating energy (73). Both methods indicate some potential for commercial appHcation but require further study and development. 

The synthetic ammonia industry of the latter part of the twentieth century employs only the Haber-Bosch process (12—15), developed in Germany just before World War 1. Development of this process was aided by the concurrent development of a simple catalyzed process for the oxidation of ammonia to nitrate, needed at that time for the explosives industry. N2 and H2 are combined direcdy and equiUbrium is reached under appropriate operating conditions. The resultant gas stream contains ca 20% ammonia. 

Above pH 9, decomposition of ozone to the reactive intermediate, HO, determines the kinetics of ammonia oxidation. Catalysts, such as WO, Pt, Pd, Ir, and Rh, promote the oxidation of dilute aqueous solutions of ammonia at 25°C, only two of the three oxygen atoms of ozone can react, whereas at 75°C, all three atoms react (42). The oxidation of ammonia by ozone depends not only on the pH of the system but also on the presence of other oxidizable species (39,43,44). Because the ozonation rate of organic materials in wastewater is much faster than that of ammonia, oxidation of ammonia does not occur in the presence of ozone-reactive organics. 

A major step in the production of nitric acid [7697-37-2] (qv) is the catalytic oxidation of ammonia to nitric acid and water. Very short contact times on a platinum—rhodium catalyst at temperatures above 650°C are required. 

Industrial. Nitric acid is itself the starting material for ammonium nitrate, nitroglycerin [55-63-0] trinitrotoluene [118-96-7]., nitroceUulose [9004-70-0] and other nitrogen compounds used in the manufacture of explosives (see Explosives and propellants). Nitric acid is made by oxidation of ammonia to nitrogen dioxide [10102-44-0] which is subsequently absorbed by water. 

Nitrate and Nitrite. Nitrate is usually present in trace quantities in surface waters but occasionally occurs in high concentrations in some groundwaters. If present in excessive amounts, it can contribute to the illness infant methemoglobinemia. Nitrate is an essential nutrient for many photosynthetic autotrophs. Nitrite is an intermediate in the reduction of nitrate as well as in the oxidation of ammonia it is also used as a corrosion inhibitor in some industrial processes. 

Allied-Signal Process. Cyclohexanone [108-94-1] is produced in 98% yield at 95% conversion by liquid-phase catal57tic hydrogenation of phenol. Hydroxylamine sulfate is produced in aqueous solution by the conventional Raschig process, wherein NO from the catalytic air oxidation of ammonia is absorbed in ammonium carbonate solution as ammonium nitrite (eq. 1). The latter is reduced with sulfur dioxide to hydroxylamine disulfonate (eq. 2), which is hydrolyzed to acidic hydroxylamine sulfate solution (eq. 3). 

Hydroxylamine sulfate is produced by direct hydrogen reduction of nitric oxide over platinum catalyst in the presence of sulfuric acid. Only 0.9 kg ammonium sulfate is produced per kilogram of caprolactam, but at the expense of hydrogen consumption (11). A concentrated nitric oxide stream is obtained by catalytic oxidation of ammonia with oxygen. Steam is used as a diluent in order to avoid operating within the explosive limits for the system. The oxidation is followed by condensation of the steam. The net reaction is... 

The precious-metal platinum catalysts were primarily developed in the 1960s for operation at temperatures between about 200 and 300°C (1,38,44). However, because of sensitivity to poisons, these catalysts are unsuitable for many combustion apphcations. Variations in sulfur levels of as Httle as 0.4 ppm can shift the catalyst required temperature window completely out of a system s operating temperature range (44). Additionally, operation withHquid fuels is further compHcated by the potential for deposition of ammonium sulfate salts within the pores of the catalyst (44). These low temperature catalysts exhibit NO conversion that rises with increasing temperature, then rapidly drops off, as oxidation of ammonia to nitrogen oxides begins to dominate the reaction (see Fig. 7). 

Usually they are employed as porous pellets in a packed bed. Some exceptions are platinum for the oxidation of ammonia, which is in the form of several layers of fine-mesh wire gauze, and catalysts deposited on membranes. Pore surfaces can be several hundred mVg and pore diameters of the order of 100 A. The entire structure may be or catalytic material (silica or alumina, for instance, sometimes exert catalytic properties) or an active ingredient may be deposited on a porous refractory carrier as a thin film. In such cases the mass of expensive catalytic material, such as Pt or Pd, may be only a fraction of 1 percent. 

Tests on pig gut contents using molecular probes to detect the presence of (aerobic) ammonia oxidizers proved negative. Recently, the anaerobic oxidation of ammonia coupled to nitrate reduction has been demonstrated in... 

In 1838, Frederic Kuhlmann discovered die formation of nitrogen oxide (NO) during die catalytic oxidation of ammonia. Wilhelm Ostwald developed die production mediods in 1902 and established die base for today s major commercial processes. However, industrial production began only after Haber and Bosch developed the synthesis of ammonia around 1916. 

Modern processes for strong acid are based on direct oxidation of ammonia with air or oxygen, with the first two steps being similar to the weak acid process. Various processes exist allowing co-production of weak and strong acid. 

Nitric oxide is made commercially by oxidation of ammonia above 500°C in the presence of platinum, or by reduction of nitrous acid with ferrous sulphate or feiTous halides. The physical ... 

The overall stoiehiometrie reaetion for the oxidation of ammonia to nitrie oxide is ... 

This reaetion is very rapid and has been diffieult to study meehan-istieally. The direet oxidation of ammonia, NH3, to nitrie oxide, NO, over platinum eatalyst is one of the major steps in the manufaeture of nitrie aeid, HNO3. 

Nitric oxide is the simplest thermally stable odd-electron molecule known and, accordingly, its electronic structure and reaction chemistry have been very extensively studied. The compound is an intermediate in the production of nitric acid and is prepared industrially by the catalytic oxidation of ammonia (p. 466). On the laboratory scale it can be synthesized from aqueous solution by the mild reduction of acidified nitrites with iodide or ferrocyanide or by the disproportionation of nitrous acid in the presence of dilute sulfuric acid ... 

There is also considerable current environmental interest in hyponitrite oxidation because it is implicated in the oxidation of ammonia to nitrite, an important step in the nitrogen cycle (p. 410). Specifically, it seems likely that the oxidation proceeds from ammonia through hydroxylamine and hyponitrous acid to nitrite (or N2O). 

Nitric acid is one of the three major acids of the modem chemical industiy and has been known as a corrosive solvent for metals since alchemical times in the thirteenth centuiy. " " It is now invariably made by the catalytic oxidation of ammonia under conditions which promote the formation of NO rather than the thermodynamically more favoured products N2 or N2O (p. 423). The NO is then further oxidized to NO2 and the gases absorbed in water to yield a concentrated aqueous solution of the acid. The vast scale of production requires the optimization of all the reaction conditions and present-day operations are based on the intricate interaction of fundamental thermodynamics, modem catalyst technology, advanced reactor design, and chemical engineering aspects of process control (see Panel). Production in the USA alone now exceeds 7 million tonnes annually, of which the greater part is used to produce nitrates for fertilizers, explosives and other purposes (see Panel). 

Hydrazine is produced by the oxidation of ammonia using the Rashig process. Sodium hypochlorite is the oxidizing agent and yields chloramine NH2CI as an intermediate. Chloramine further reacts with ammonia producing hydrazine ... 

Consider a more complicated example—the oxidation of ammonia, NHS ... 

Nitric Add by the Oxidation of Ammonia. Here, the catalytic oxidation of ammonia under press using a Pt catalyst maintained at a temp of 900—1000° is the process used. The reaction press is the rate determining step, being directly proportional to the product nitric acid concn (Refs 6, 22, 26, 30, 34, 36, 37 41). 

Instead of using air for the oxidation of ammonia, some investigators, for instance Cze and Chi Wu (Ref 21), advise using oxygen, especially in cases where oxygen is available cheaply. According to Thorpe (Ref 22, p 571),... 

Eng 20, 470-477 (1919) (Description of ammonia oxidation process beginning with Kuhl-mann s method of 1839 and ending with the cyanamide process at Muscle Shoals) 7) C.L. Parsons, 1EC 11,541 (1919) (Oxidation of ammonia to nitric acid as well as the prepn of nitric acid from Chile saltpeter) 8) F.C. Zeis-berg, ChemMetEng 24, 443-45 (1921) (Manuf of nitric acid from Chilean saltpeter brief description) 9) G.B. Taylor, IEC 26,1217-19 (1922) (Some economic aspects of ammonia oxidation) 10) Ministry of Munitions, Manufacture of Nitric Acid from Nitre and Sulfuric Acid , London (1922) (Book No 7 of Technical Records of Explosives Supply, 1915—1919)... 

In the manufacture of nitric acid by the oxidation of ammonia, the first product is nitric oxide, which is then oxidized to nitrogen dioxide. From the standard reaction enthalpies... 

We can conclude that the oxidation of ammonia is spontaneous at 25°C under standard conditions. 

Hydrazine, NH2NH2, is an oily, colorless liquid. It is prepared by the gentle oxidation of ammonia with alkaline hypochlorite solution ... 

Step 1 Oxidation of ammonia nitrogen s oxidation number increases from —3 to +2 ... 

NO. Nitric oxide, or nitrogen monoxide, is a colorless gas at room temperature. As we have already seen, it is industrially produced by the oxidation of ammonia. However, with respect to the urban envirorunent, a more significant process is the high temperature reaction of N2 with O2 (as in a car engine) to produce NO. 

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