Photooxidation of toluene

Panov, A.G., Larsen, R.G., Totah, N.I., Larsen, S.C. and Grassian, V.H. (2000). Photooxidation of toluene and p-xylene in cation-exchanged zeolites X, Y, ZSM-5, and beta the role of zeolite physicochemical properties in product yield and selectivity. J. Phys. Chem. B 104, 5706-5714... 

Irradiation of toluene in the presence of chlorine yielded benzyl hydroperoxide, benzaldehyde, peroxybenzoic acid, carbon monoxide, carbon dioxide, and other unidentified products (Hanst and Gay, 1983). The photooxidation of toluene in the presence of nitrogen oxides (NO and NO2) yielded small amounts of formaldehyde and traces of acetaldehyde or other low molecular weight carbonyls (Altshuller et al, 1970). Other photooxidation products not previously mentioned include phenol, phthalaldehydes, and benzoyl alcohol (Altshuller, 1983). A carbon dioxide yield of 8.4% was achieved when toluene adsorbed on silica gel was irradiated with light X >290 nm) for 17 h (Freitag et ah, 1985). 

A variety of smaller multifunctional oxygenated compounds are also found as products of the gas-phase OH-aromatic reactions. Table 6.17 shows the yields of the smallest dicarbonyl compounds from these reactions, which, while small, are not insignificant. In addition to these products, a variety of other multifunctional compounds are typically found, the numbers, types, and concentrations of these products depending on the analytical methodologies used, the reaction conditions, and the skill and imagination of the experimentalist Table 6.18, for example, shows some products observed in the photooxidation of toluene in air where the loss is due to attack by OH (Dumdei et al., 1988). In this particular study, 44% of the reacted toluene could be accounted for by the products shown in Table 6.18. 

TABLE 6.18 Some Products Observed in the Photooxidation of Toluene"... 

Fig. 8.5 Effect of 02 initial concentration (0, 7.5, 15 and 20%) on formation of the products in the heterogeneous photooxidation of toluene for TiOj-toluene (80 ppm)-NOz (80 ppm) system at a reaction time of 10.3 min.

Bandow H, WashidaN, Akimoto H. 1985. Ring-cleavage reactions of aromatic hydrocarbons studied by FT-IR spectroscopy I. Photooxidation of toluene and benzene in the NOx-Air System. Bull Chem Soc Jpn 58 2531-2540. 

The coexistence of various pollutants does not have to be deleterious, but, in certain cases, can be quite beneficial. The first evidence for this claim came probably from the work of Lichtin et al. (1994) who found that the edition of 0.03% by volume to an air-stream containing 0.1% iso-octane caused an enhancement in the photocatalytic oxidation of the latter. Likewise, a significant rate enhancement was recorded in the photocatalytic degradation of chloroform and dichloromethane in the presence of TCE. Similar effects were recorded also with other chlorinated olefins, such as perchloroethylene (PCE) and trichloropropene (TCP), which enhanced the photooxidation of toluene in a manner similar to that of TCE (Sauer et al., 1995). 

Cicerone and Zellner have reviewed the atmospheric chemistry of HCN and discussed its photochemical properties. Penzhorn and Canosa obtained rate constants for the thermal reactions of NO2 with SOj and SO3 using second-derivative u.v. spectroscopy. The relevance of these reactions to aerosol formation in urban atmospheres was briefly discussed. A reassessment of the importance of atmospheric Na constituents has been provided by Kirchhoff and an updated chemical mechanism for the atmospheric photooxidation of toluene has been described by Leone and Seinfeld. ... 

Photocatalytic activation of allyl and benzyl ethers results either in carbon-carbon coupling or oxygenation [148-151]. The photocatalyst used for these conversions can be generated in situ, by photolysis of a zinc dithiolene salt, by preformed catalysts, or by particles supported within surfactant vesicles. Radical intermediates formed by hydrogen abstraction by photogenerated hydroxyl or hy-droperoxyl radicals may also be important in the photoelectrochemically induced oxidation of hydrocarbons. In the Ti02-sensitized photooxidation of toluene to cresols, for example, a photo-Fenton (radical) type mechanism has been suggested [150]. ... 

Kleindienst, T. E., Conver, T. S., Mclver, C. D., and Edney, E. O. (2004) Determination of secondary organic aerosol products from the photooxidation of toluene and their implications in ambient PM2.5, J. Atmos. Chem. 47, 79-100. 

Theoretical work on the oxidation reactions of aromatic hydrocarbons is scarce. Bartolotti and Edney [10] used a simple density functional approach with the local exchange correlation functional developed by Vosko-Wilk-Nusair [11] to identify potential intermediates produced in the OH addition initiated atmospheric photooxidation of toluene. Although their energy results were acknowledgely preliminary in nature, their calculations were able to confirm certain aspects of the proposed reaction mechanism [2,3] and to predict the importance of carbonyl compounds containing... 

We have investigated the photooxidation of toluene in several different zeolite hosts (BaX, BaY, CaY, BaZSM-5, and NaZSM-5) using in-situ Fourier Transform Infrared (FT-IR) spectroscopy and ex-situ Gas Chromatography (GC) to analyze product formation and product yields. This combined approach allows for a more detailed analysis of the product distribution. The product selectivity in these reactions appears to be governed by the presence of a small number of acid sites rather than by the framework composition or topology of the zeolite host. 

For the BaZSM-5 sample, the photoproducts were extracted in acetonitrile and analyzed by GC. The results are given in Table I. It can be clearly seen that the product selectivity to benzaldehyde observed for photooxidation of toluene in BaY is lost when the photoreaction is conducted in BaZSM-5. A number of different products have been identified. The three dominant products or product groups are benzaldehyde (31%), combined phenol and cresols (31%), condensation products (15%) and other oxygenated products (22%). Given that the FT-IR spectra are very similar for CaY, BaZSM-5, and NaZSM-5, a similar product distribution would be expected in each case. 

Despite the importance of solid-state nuclear magnetic resonance (NMR) spectroscopy for the characterization of solid catalysts, in situ studies related to photocatalysts are rare. Mills and O Rourke [59] monitored the selective photooxidation of toluene by in situ NMR using an NMR tube as the photoreactor. A Ti02 precursor paste was prepared by hydrolysis of titanium propoxide and following treatment at 228 °C. The obtained anatase-type titania was mixed with poly(vinyl alcohol). The obtained paste was coated on the walls of the NMR tube, rotated over night and calcined. In parallel, batch experiments were carried out. The reaction mixture containing the catalyst was directly placed into the tube, which was irradiated outside the spectrometer and then inserted into the NMR spectrometer. 

G6mez Alvarez et al. coupled PTR-MS with the European PHOtoREactor (EUPHORE) atmospheric simulation chamber (Valencia, Spain) to provide experimental confirmation of the dicarbonylic mechanism in the photooxidation of toluene and benzene [189]. The particular benefit of PTR-MS in this context is its relatively fast response time, which provides data that can be tested against a Master Chemical Mechanism (MCMv3.1) model. Differences in mass spectral fragmentation patterns also allowed PTR-MS to distinguish between cis- and fraws-butenedial, which are two of the products resulting from the photooxidation process. 

Sydnes, L. K., Burkow, 1. C., and Hanson, S. H., Photooxidation of toluene and xylenes concurrent formation of products from photooxygenation and photodimerization, Acta Chem. Scand., 39B, 829, 1985. 

In a smog chamber study of the photooxidation of toluene and the cresols, Atkinson et al. (1980) found that an assumed -value of 3 x 10 s for benzaldehyde photodecomposition was required to account for the observed benzaldehyde loss rates in their experiments (N02) 6.6 x 10 s They concluded that the product data were best fitted by assuming that benzaldehyde photolysis results in the formation of nonradical products, such as process (I). However, if process (I) was assumed to be the major photodecomposition mode, the amount of benzene observed was less than 1 % of the benzene yields predicted. 

Johnson, D., M.E. Jenkin, K. Wirtz, and M. Martin-Reviejo (2004), Simulating the formation of secondary organic aerosol from the photooxidation of toluene, Env. Chem., 1, 150-165. 

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