Sulfur dioxide aqueous-phase oxidation

Sulfur dioxide can be oxidized catalytically in an aqueous environment. The sample case introduces a catalytic oxidation process of SO2 in the liquid phase over an active carbon... 

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). 

Snia Viscosa. Catalytic air oxidation of toluene gives benzoic acid (qv) in ca 90% yield. The benzoic acid is hydrogenated over a palladium catalyst to cyclohexanecarboxyhc acid [98-89-5]. This is converted directiy to cmde caprolactam by nitrosation with nitrosylsulfuric acid, which is produced by conventional absorption of NO in oleum. Normally, the reaction mass is neutralized with ammonia to form 4 kg ammonium sulfate per kilogram of caprolactam (16). In a no-sulfate version of the process, the reaction mass is diluted with water and is extracted with an alkylphenol solvent. The aqueous phase is decomposed by thermal means for recovery of sulfur dioxide, which is recycled (17). The basic process chemistry is as follows ... 

Stockwell, W. R., The Effect of Gas-Phase Chemistry on Aqueous-Phase Sulfur Dioxide Oxidation Rates, J. Atmos. Chem., 19, 317-329 (1994). 

The above material is expected to have excellent diffusion resistance to gases such as sulfur dioxide, nitrogen oxides and saturated water vapour and outstanding chemical resistance to mineral acids, bases, aqueous phases and polar solvents and weatherproof. 

Working with a mutated bacterial strain, Isbister et al. (62) demonstrated a novel mechanism of aerobic oxidation of dibenzothiophene which involved the specific excision of the sulfur atom from the molecule (Figure 11). Studies with -labeled dibenzothiophene showed the release of the radioactivity into the aqueous phase and ion chromatography showed the appearence of sulfate. There was no radioactive carbon dioxide released when this microorganism was incubated with 14C-labeled dibenzothiophene. GC-MS analysis showed that the oxidation product was 2,2 -dihydroxybiphenyl. Kargi and Robinson (52) also report the release of sulfate from dibenzothiophene. This OSC served as the sole carbon and sulfur source in their cultures of the aerobic thermophile Sulfolobus acidocaldarius. 

Sulfur dioxide is oxidized in the atmosphere mainly within clouds, fogs and other aqueous-phase domains. The primary pathway [111-115] involves oxidation with H2O2 however, other reaction pathways are viable depending on pH. Some of the direct and indirect photochemical reactions of interest related to the fate of SO2 in the atmosphere include ... 

The oxidation reaction is interrupted by the addition of 10 ml of ethanol. After 10 min, the manganese dioxide is filtered off through a layer of water-washed fumed silica ( Kieselguhr , Fluka AG Switzerland) and washed with a small amount of sodium bicarbonate (1% solution). The aqueous phase is extracted with 2 x 50 ml of diethyl ether, the ether in turn is extracted with 15 ml of sodium bicarbonate (1% solution) and, after adjustment of the pH to 6.5 with 9M sulfuric acid, the combined aqueous extracts are evaporated to a small volume (30 ml). 

Sulfur dioxide (SO ) and nitrogen oxides (NO ) are oxidized to sulfate and nitrate aerosols either homogeneously rn the gas phase or heterogeneously in atmospheric microdroplets and hydrometeors Gas-phase production of nitric acid appears to be the dominant source of aerosol nitrate because the aqueous phase reactions of NO (aq) are slow at the nitrogen oxide partial pressures typically encountered in the atmosphere (5,i5). Conversely, field studies indicate that the relative importance of homogeneous and heterogeneous SO2 oxidation processes depends on a variety of climatological factors such as relative humidity and the intensity of solar radiation (4, -1 ). 

It has been shown recently that a hydroxyl radical may partition from the gaseous to the aqueous phase, or may be formed directly in the aqueous phase, and there effect the oxidation of sulfur dioxide. Sulfur dioxide can also be oxidized inside raindrops by hydrogen peroxide, H2O2, which is a minor atmospheric species formed principally by the disproportionation of HO2 radicals... 

Figure 6. Summary of gas phase and aqueous-gas phase oxidation of sulfur dioxide in atmosphere Bunce, 1994).

Atmospheric reactions modify the physical and chemical properties of emitted materials, changing removal rates and exerting a major influence on acid deposition rates. Sulfur dioxide can be converted to sulfate by reactions in gas, aerosol, and aqueous phases. As we noted in Chapter 17, the aqueous-phase pathway is estimated to be responsible for more than half of the ambient atmospheric sulfate concentrations, with the remainder produced by the gas-phase oxidation of S02 by OH (Walcek et al. 1990 Karamachandani and Venkatram 1992 Dennis et al. 1993 McHenry and Dennis 1994). These results are in agreement with box model calculations suggesting that gas-phase daytime S02 oxidation rates are l-5% per hour, while a representative in-cloud oxidation rate is 10% per minute for 1 ppb of H202. 

The indirect effect of aerosols on climate is exemplified by the processes that link S02 emissions to cloud albedo. Sulfur dioxide is oxidized in gas and aqueous phases to aerosol sulfate. Although increased S02 emissions can be expected to lead to increased mass of sulfate aerosol, the relation between an increased mass of aerosol and the corresponding change of the number concentration of aerosol is not well established. Yet, it is the aerosol number concentration that is most closely related to the cloud drop number concentration. Aerosol mass is created by gas-to-particle conversion, which can occur by growth of... 

The above reaction mechanisms for reduced sitlfmcom-potmds make evident that sulfur dioxide is the principal oxidation product in all cases. The further oxidation of sulfur dioxide to sulfuric acid occurs in the gas phase as well as in the aqueous phase of clouds. Aqueous reactions are discussed in Section XI. The gas-phase oxidation... 

Continental aerosol particles contain a significant fraction of minerals. The insoluble fraction consists mainly of the major crustal elements silicon, aluminum and trivalent iron, which occur as alumino-silicates, quartz, and iron oxides. Elements that are eluted from minerals by water are sodium, potassium, calcium (inpart), and magnesium. The water-soluble inorganic salt ftaction is dominated by am-monimn sulfate. Again, sulfate arises from the oxidation of sulfur dioxide, both by gas-phase and by aqueous phase reactions. Whereas the mineral components are mainly found in the coarse particle size range, ammonium sulfate resides mainly in the accumulation mode. Nitrate occurs partly in association with ammoniirm in the accumulation mode, and partly together with sodiirm and other cations in the coarse particle mode. Thus, nitrate often shows a bimodal size distribution. 

In aqueous phase several species of S(IV), hydrated sulfur dioxide, bisulfite and sulfite are formed according to the Association properties of S(IV)-oxides in aqueous solution. The distribution among the protonated and non protonated species is determined by the pH-value. 

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