Oxidation by Nitrous Oxide in the Gas Phase

Catalytic Oxidation by Nitrous Oxide in the Gas Phase 221 Table 7.4 Methane oxidation by N2O over 1.7% Mo03/Si02. ... 

Although the pathway of Eq. (1) is now based on much evidence (Section 111) and is unambiguous in the case of at least one bacterium [Pseudomonas stutzeri strain Zobell (f. sp. P. perfectomarina)], there have been alternative hypothesis. One hypothesis, advanced by the Hollocher group (Garber and Hollocher, 1981 St. John and Hollocher, 1977), considered NO as a likely intermediate, but one that remained at least partly enzyme-bound and was not entirely free to diffuse. This view was based on the outcome of certain kinetic and isotope experiments which can be summarized as follows. When denitrifying bacteria were challenged simultaneously with [ N]nitrite and ordinary NO, the cells reduced both compounds concomitantly to N2 (or to N2O in the presence of acetylene which is a specific inhibitor (Balderston et al., 1976 Yoshinari and Knowles, 1976) of nitrous oxide reductase). In the process, little NO was generally detected in the gas phase pool of NO and there was relatively little isotopically mixed N2O formed. That is, most of the N and N reduced to NjO appeared as N2O... 

The first extensive attempt to use nitrous oxide as a selective oxidant in the liquid phase was made in the early 1950s by ICI researchers Bridson-Jones et al. [167]. The gas-phase... 

The flame is a chemical reaction which takes place in the gas phase. The ideal flame for atomic absorption would generate the correct amount of thermal energy to dissociate the atoms from their chemical bonds. The most commonly used flames are aii -acetylene and nitrous oxide—acetylene. The choice of oxidant depends upon the flame temperature and composition required for the production of free atoms. These temperatures vary the molecular or chemical form of the element. Air and acetylene produce flame temperatures of about 2300°C and permit the analysis by atomic absorption of some thirty or so elements. The nitrous oxide—acetylene flame is some 650°C hotter and extends the atomic absorption technique to around 66 elements. It also permits the successful analysis of most elements by flame atomic emission, in many cases at fractional parts per million levels, providing adequate spectral resolution is available. 

Not being aware of the earlier work, the present author first noticed the phenomenon in 1981. Geiger and Huber10 had photolyzed dimethylnitrosamine in the gas phase at 1 Torr and under 100 Torr N2 buffer. This compound fragments from the first excited singlet state into dimethylaminyl radicals and nitrous oxide NO with unity quantum yield, but neither photoproducts nor a decrease of the initial compound pressure were observed. Even after 20 h photolysis the back-reaction was complete to more than 99.9% (Scheme 6). This seemed quite puzzling because sterically unhindered aminyl radicals are transient and readily self-terminate by coupling and disproportionation. 

The reaction is run in the gas phase at 350 °C and, at 27% of benzene conversion, selectivity for phenol is claimed to be 98% [5]. The main by-products are dihydrox-ybenzenes (about 1%) and carbon oxides (0.2-0.3%). Selectivity is of paramount importance for this process, since 15 molecules of nitrous oxide are consumed for the total oxidation of just one molecule of benzene (Equation 13.5) ... 

A special case is the hydroxylation of benzene with nitrous oxide as oxidant, for which commercialization has been announced [55]. The reaction occurs on Fe-silicalite-1, in the gas phase, at temperatures close to 400 °C, producing molecular nitrogen as by-product. Other zeolites and supported metals and metal oxides are less satisfactory catalysts. Toluene, chlorobenzene, and fluorobenzene are similarly hydroxylated, yielding all three possible isomers. Phenol produces catechol and hydroquinone. 

The radiation cells were made from 9-mm. o.d. borosilicate glass tubing. The concentration of nitrous oxide was calculated by assuming that the amount in the gas phase was negligible since the solutions filled approximately 90% of the cells. The samples were thermostatted at 35°C. and irradiated for one hour in a G0Co y-ray facility at a dose rate of approximately 1.3 X 1019 e.v. liter1 sec."1. Ferrous sulfate dosimetry was used. Since we wished to compare different alkanes, samples were prepared in pairs one sample of the pair always contained cyclohexane and the other some other alkane. Each sample of each pair contained the same amount of nitrous oxide. 

Photoionization of the hydrocarbon followed by dissociative electron attachment (Reaction 1) should be considered since the ionization potential of a molecule is less in the liquid phase than it is in the gas phase. For hydrocarbons the ionization potential is 1 to 1.5 e.v. less in the liquid phase (24). The photon energy at 1470 A. is about 1.4 e.v. below the gas-phase ionization potentials of cyclohexane and 2,2,4-trimethylpentane (14). Some ionization may therefore occur, but the efficiency of this process is expected to be low. Photoionization is eliminated as a source of N2 for the following reasons. (1) If photoionization occurred and the electron reacted with nitrous oxide, then O" would be formed. It has been shown in the radiolysis of cyclohexane-nitrous oxide solutions that subsequent reactions of O result in the formation of cyclohexene and dicyclohexyl (I, 16, 17) and very little cyclohexanol (16, Table III). In the photolysis nitrous oxide reduces the yield of cyclohexene and does not affect the yield of dicyclohexyl. This indicates that O is not formed in the photolysis, and consequently N2 does not result from electron capture. (2) A further argument against photoionization is that cyclohexane and 2,2,4-trimethylpentane have comparable gas-phase ionization potentials but exhibit quite different behavior with respect to N2 formation. 

Table 8-9 lists processes leading to the oxidation of N02 to nitrous or nitric acid in aqueous solution. The significance of these reactions in clouds may be assessed by comparing their rates with that for the oxidation of N02 by ozone, which is the major dark reaction in the gas phase. Table 8-9 includes the latter process for comparison. 

The reaction of sulphur dioxide with oxygen to form sulphur trioxide is industrially the most significant of all its reactions because of its importance in sulphuric acid production. In the gas phase, it will only take place at elevated temperatures and, for a satisfactory yield of sulphur trioxide it requires the presence of a catalyst. In aqueous solution, sulphur dioxide is oxidized to sulphuric acid at low temperatures by air in the presence of activated coke or nitrous gases or by oxidizing agents like hydrogen peroxide. 

If nitrogenous organic compounds, no matter what their type, are strongly heated along with excess MnOg, nitrous oxides result. The latter can be detected in the gas phase by the Griess reaction (see page 364). 

When in a gas phase reaction a liquid condenses, the liquid phase may absorb reactants, and reactions may proceed in the liquid phase with much higher rates than in the gas phase. In many cases the liquid phase will form a mist that is difficult to separate. The temperature in the mist particles may rise, promoting certain reactions within the particles. An example is the oxidation of nitrous oxide in moist air, where eventually nitric acid mist is formed. The contribution of the mist phase to the total conversion may be considerable. However, the estimation of reaction rates in this situation is usually very difficult, since often the relative volume of the mist and the size of the droplets cannot be predicted. This makes this type of process difficult to control. It may be better to avoid this situation, either by raising the temperature so that no mist will form, or by lowering the temperature so much that a well controlled amount of liquid will condense and scrub the mist. 

Mist formation may also occur in gas/liquid reactions reactants may evaporate, react in the gas phase and form a liquid mist that will not coalesce with the other liquid phase. The temperature of the mist may rise, and uncontrolled reactions may take place in the mist particles. Examples of these are found in the processes for the manufacture of nitric and sulfuric acids, and in the preparation of ammonium nitrite from ammonium carbonate solution and nitrous oxides. In the nitric acid production, the product in the mist may be recovered by an effective separation of the mist particles (with demisters, wet scrubbers or electrostatic filters). But in the ammonium nitrite process, the product formed in the mist phase may subsequently decompose into nitrogen and water, thus reducing the process yield. 

Adsorption of nitric and sulfuric acids on ice particles provides the sol of the nitrating mixture. An important catalyst of aromatic nitration, nitrous acid, is typical for polluted atmospheres. Combustion sources contribute to air pollution via soot and NO emissions. The observed formation of HNO2 results from the reduction of nitrogen oxides in the presence of water by C—O and C—H groups in soot (Ammann et al. 1998). As seen, gas-phase nitration is important ecologically. 

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