Photochemical oxidation of hydrocarbons

A few illustrative examples are the following. Photohydrogenation of acetylene and ethylene occurs on irradiation of Ti02 exposed to the gases, but only if TiOH surface groups are present as a source of hydrogen [319]. The pho-toinduced conversion of CO2 to CH4 in the presence of Ru and Os colloids has been reported [320]. Platinized Ti02 powder shows, in the presence of water, photochemical oxidation of hydrocarbons [321,322]. Some of the postulated reactions are ... 

Some pollutants fall in both categories. Nitrogen dioxide, which is emitted directly from auto exhaust, is also formed in the atmosphere photochemically from NO. Aldehydes, which are released in auto exhausts, are also formed in the photochemical oxidation of hydrocarbons. Carbon monoxide, which arises primarily from autos and stationary sources, is likewise a product of atmospheric hydrocarbon oxidation. 

Johnston, H. S., and K. Dev Jain. Sulfur dioxide sensitized photochemical oxidation of hydrocarbons. Science 131 1523-1524, 1960. 

In the lower atmosphere CH3O is an intermediate in the photochemical oxidation of hydrocarbons and may be important in the conversion of NO into NOg. Thus, in an extension of the work of Wiebe et al., CH3ONO was photolysed in the presence of NO, NO plus NO2, and NO plus Og. Nitro-methane has also been photolysed in the gas phase at 313 nm, the major products being CH3ONO = 0.22), HCHO = 0.20), CH3NO = 0.06), and NO = 0.10). When nitromethane decomposes thermally the same basic reaction occurs ... 

These osmium(VI) complexes, along with the ruthenium(VI) complex trans-[Ru02(L)] (L = m t>-l,2,6,10,ll-pentamethyl-2,6,10-triaza[ll]-(2,6)-pyridino-phane) can be used for the photochemical oxidation of hydrocarbons. The reaction has been used for the oxidation of 2-propanol to acetone, of both benzyl alcohol and toluene to benzaldehyde, of cyclohexane to both cyclohexanol and cyclohexanone, and of the epoxidation of cyclohexene, fran -stilbene, and norbomene. 

A. Carbonyl compounds (aldehydes and ketones) are usually the last organic species formed during the photochemical oxidation of hydrocarbons. 

Frei, H. (1997). Highly selective photochemical and dark oxidation of hydrocarbons by 02 in zeolites. In Studies in Surface Science and Catalysis. 3rd World Congress on Oxidation Catalysis, Grasselli, R.K., Oyama, S.T., Gaffney, A.M. and Lyons, J.E. (eds), Vol. 110, pp. 1041-1050. Elsevier Science, New York... 

Organic aerosols formed by gas-phase photochemical reactions of hydrocarbons, ozone, and nitrogen oxides have been identified recently in both urban and rural atmospheres. Aliphatic organic nitrates, such dicarboxylic acids as adipic and glutaric acids, carboxylic acids derived from aromatic hydrocarbons (benzoic and phenylacetic acids) and from terpenes emitted by vegetation, such as pinonic acid from a pinene, have been identified. The most important contribution in this held has been that of Schuetzle et al., who used computer-controlled... 

In many cases, a combination of physical, chemical, physico-chemical, and biotechnological treatments may be more efficient than one type of treatment (Table 3). Efficient pretreatment schemes, used prior to biotechnological treatment, include homogenization of sohd wastes in water, chemical oxidation of hydrocarbons by H2O2, ozone, or Fenton s reagent, photochemical oxidation, and preliminary washing of wastes by surfactants. 

Acetaldehyde is a natural product of combustion and photo-oxidation of hydrocarbons commonly found in the atmosphere. It is an important industrial chemical and may be released into the air or in wastewater during its production and use. It has been detected at low levels in drinking-water, surface water, rainwater, effluents, engine exhaust and ambient and indoor air samples. It is also photochemically produced in surface water. Acetaldehyde is an intermediate product in the metabolism of ethanol and sugars and therefore occurs in trace quantities in human blood. It is present in small amounts in all alcoholic beverages, such as beer, wine and spirits and in plant juices and essential oils, roasted coffee and tobacco smoke (lira et al., 1985 Hagemeyer, 1991 United States National Library of Medicine, 1998). 

In later experiments, Izumi et al. (47, 48) examined aerosol formation during photooxidation of a variety of hydrocarbons in an evacuable smog chamber. No seed particles were used in these experiments, but good estimates of the yield of aerosol from photochemical oxidation of the hydrocarbon precursors were obtained by using EAA data. In some cases, the volumetric yield was found to decrease with decreasing precursor concentration (Figure 4), so the finite vapor pressure of the reaction products limited nucleation, particle growth, or both. 

A study of die addition reactions of radicals to fiillerenes (C60/C70) by EPR has appeared and die dynamic effects in the EPR spectra of fidlerenyl radicals due to hindered rotation and the multi-addition of radicals to fidlerenes are described.9 Other review articles which have appeared this year include recent advances in the radical substitution reactions of alkyl, aryl, and vinyl halides10 and the substitution and photochemical reactions of heterocyclic A -oxidcs.11 The mechanisms for the oxidation of hydrocarbons, lipids, and low-density lipoproteins have been reviewed.12... 

Selective oxidation of small abundant hydrocarbons is the most important type of reaction in organic chemicals production. For example, essentially all building blocks for the manufacture of plastics and synthetic fibers are produced by oxidation of hydrocarbons [129], Among these, oxidations by molecular oxygen play a particularly important role [130,131], A key problem is the product specificity in the reactions of hydrocarbon with 02 here, photoassisted processes hold special promise. Photoinitiated reactions of 02 furnish access to products that in many cases cannot be obtained by a dark reaction of 02. Moreover, photochemical reactions can be conducted at or around ambient temperature, thus minimizing the chance for loss of product specificity due to secondary thermal chemistry of the initial products. 

For a long time, transport from the stratosphere to the troposphere was thought to be the dominant source of ozone in the troposphere. Early in the 1970s, it was first suggested that tropospheric ozone originated mainly from production within the troposphere by photochemical oxidation of CO and hydrocarbons catalysed by HO and NO c- These sources are balanced by in-situ photochemical destruction of ozone and by dry deposition at the earth s surface. Many studies, both experimental- and model-based have set about determining the... 

While SC>2 may be directly removed by rain [Beilke and Georgii (19)], plant absorption [Katz and Ledingham (134)], and direct deposition [Chamberlain (38)], the major sink is thought to be oxidation to sulfate. Though photoxidation in pure water appears to very slow [Robinson and Robbins (214)], the presence of metallic catalysts or dissolved ammonia produces very fast rates [Junge and Ryan (132)]. The photochemical oxidation of SC>2 in the presence of NC>2 and hydrocarbons has also been shown to produce considerable sulfuric acid aerosols [Schuck and Doyle (221)]. 

Figure 1. Scheme for photochemical production of co-reductant to drive the oxidation of hydrocarbons by mimicking the cytochrome-P450 cycle. The SnP sensitized photoredox cycle is on the left the P 50 catalytic cycle is shown on the right. 

Carbon monoxide (CO) strongly influences the concentration of the radical OH in the tropical atmosphere. CO oxidation can lead to either production or destruction of ozone, depending on the NOx mixing ratio. Tropical soils are either a sink or a weak source of CO, where photochemical oxidation of methane and other hydrocarbons and biomass burning emissions are the predominant CO sources. 

Paalme L., Irha N., Urbas E., Tsyban A., and Kirso U. (1990) Model studies of photochemical oxidation of carcinogenic polyaromatic hydrocarbons. Mar. Chem. 30, 105-111. 

Gasoline hydrocarbons volatilized to the atmosphere quickly undergo photochemical oxidation. The hydrocarbons are oxidized by reaction with molecular oxygen (which attacks the ring structure of aromatics), ozone (which reacts rapidly with alkenes but slowly with aromatics), and hydroxyl and nitrate radicals (which initiate side-chain oxidation reactions) (Stephens 1973). Alkanes, isoalkanes, and cycloalkanes have half-lives on the order of 1-10 days, whereas alkenes, cycloalkenes, and substituted benzenes have half- lives of less than 1 day (EPA 1979a). Photochemical oxidation products include aldehydes, hydroxy compounds, nitro compounds, and peroxyacyl nitrates (Cupitt 1980 EPA 1979a Stephens 1973). 

If we were to increase N02 concentrations (in a way that did not use 03), then the equilibrium could be maintained by increasing 03 concentrations. This happens in the photochemical smog through the mediation of hydroxyl (OH) radicals in the oxidation of hydrocarbons. Here we will use methane (CH4) as a simple example of the process ... 

Emissions of phosgene most commonly arise as a result of its release during manufacture and use, its formation from the decomposition of chlorinated hydrocarbons, and its formation from the photochemical oxidation of air-borne chlorinated organic materials, particularly the C, and C chloroalkanes, and chloroethenes. The location and estimation of air emissions from sources of phosgene have been described by the US Environmental Protection Agency [2088b], Catastrophic emissions and accidental spills and leaks are discussed in Section 3.6. 

The photochemical oxidation of chlorinated hydrocarbons into phosgene has been the subject of particular concern for the welding workshop [679,2217]. Solvent residues from degreasing procedures may remain on the part to be welded, particularly in cracks and cavities where the solvent may have been drawn by capillary action [627]. Upon exposure to the heat of the welding arc, the residues vaporize and although some thermal oxidation will then... 

The photochemical oxidation of methane is the most important source of formaldehyde. Atmospheric formaldehyde is also produced by the photochemical oxidation of non-methane hydrocarbons. The kinetics of the reaction HCHO + OH has been studied both experimentally and theoretically.151"162 Kinetic isotopic effects for some deuterated formaldehyde isotopomers have been reported.153"155 Results of experimental and theoretical studies151"162 indicate a complex reaction mechanism consisting of three competitive reaction channels... 

---