Photocatalytic oxidation of benzene

Figure 7 Apparent deactivation during the continuous photocatalytic oxidation of benzene, toluene, and zw-xylene. (From Ref. 50.)...

This kinetic model was used to simulate the photocatalytic oxidation of benzene and zw-xylene at 50 rng/rn , and toluene over a range of gas-phase concentrations (10-100 mgW) [50,57]. The model system was found to be reasonably capable of reproducing the trends observed in experimental data (b ig. 10). 

Figure 12 Photocatalytic oxidation of benzene cofed with TCE. (From Ref. 17.)...

Although TCE cofeeds are unable to increase the photocatalytic oxidation of benzene, the effects on the oxidation of branched aromatics are significant. However, practical considerations will hinder the use of TCE cofeeds. First, TCE and branched aromatic contaminants are not often present in the same airstream. Adding TCE or similar chlorinated olefins to an environment or airstream containing branched aromatic contaminants is not practical, because TCE is, itself, classi-... 

By means of preliminary batch tests some important parameters that influence the photocatalytic oxidation of benzene to phenol were investigated. In particular, the obtained results showed an increase of phenol production depending on the pH of the aqueous Ti02 suspensions, the catalyst concentration and the radiation intensity. 

Silva, C.G., and L. Faria, Photocatalytic oxidation of benzene derivatives in aqueous... 

Korologos CA, Philippopoulos CJ, Poulopoulos SG (2011) The effect of water presence on the photocatalytic oxidation of benzene, toluene, ethylbenzene and m-xylene in the gas-phase. Atmos Environ 45 7089-7095... 

Lin T, Pi Z, Gong MC, Zhong JB, Wang JL, Chen YQ (2007) Gas-phase photocatalytic oxidation of benzene over titanium dioxide loaded on Bii2 TiOaO. Chin Chem Lett... 

Scheme 8.1 Photocatalytic oxidation of benzene to phenol and subsequent decomposition of phenol on Ti02 (Reproduced with permission [26], Copyright 2008 Elsevier)...

In all four cases, tlie initial reaction rates at the start of illumination in the continuous-feed photoreactor were higher than the pseudo-steady-state reaction rates the reaction rates declined over time until pseudo-steady-state operation was achieved. Tliis apparent deactivation phenomenon, often observed with aromatic contaminants, is discussed in Sec. III.E. In a transient reaction system, the time required to reach pseudo-steady-state operation also appears to increase in the same order as the reaction rates. For example, for the continuous photocatalytic oxidation of aromatic contaminants at 50 mg/m in a powder-layer photoreactor, the time required for pseudo-steady-state operation to be achieved was reported to be approximately 90 min for benzene, 120 min for toluene, and as long as 6 hr for wz-xylene [50,51]. Under such conditions, the difference in reaction rates between the aromatic contaminants is magnified by the fact that the more reactive aromatics retain their higher transient reaction rates for longer periods (Fig. 7). 

Blount and Falconer [54] further examined the photocatalytic oxidation of toluene using TPH. During TPH analysis of used catalyst samples, the strongly bound intermediates observed by Larson and Falconer [43] were reported to be hydrogenated and desorbed predominantly as toluene, along with smaller quantities of benzene. This indicated that the intermediate species responsible for apparent catalyst deactivation during toluene photooxidation retained an aromatic ring structure. 

Figure 13 Photocatalytic oxidation of (a) toluene (from Ref. 68) and (b) benzene (from Ref. 56) on HCl-pretreated TiO, catalysts.

The photocatalytic oxidation of alkylbenzenes and alkenylbenzenes has been widely reported. The data concerning alkylbenzenes have shown that the reactivity of toluenes is low when compared to other monosubstituted benzenes (Somarani et al., 1995). The effect of adding a zeolite, which is an acid solid catalyst, by Ti02/UV on various 4-substituted toluenes was studied by Somarani et al. (1995). The compounds of interest were prepared at 0.03 M in solutions containing TiOz. The effect of a zeolite was also studied by adding HY-type zeolites with various Si/Al ratios. The solutions were irradiated with a 125-W mercury lamp emitting light at 330 nm. Samples were taken at 48 hours and analyzed by GC/mass spectroscopy (MS) to determine percent conversions of the toluenes to the desired products. 

Palmisano et al. [41] in a study on the selectivity of hydroxyl radical in the partial oxidation of different benzene derivatives have investigated how the substituent group affect the distribution of the hydroxylated compounds. The reported results show that the primary photocatalytic oxidation of compounds containing an electron donor group (phenol, phenylamine, etc.) leads to a selective substitution in ortho and para positions of aromatic molecules while in the presence of an electron-withdrawing group (nitrobenzene, benzoic acid, cyanobenzene, etc.) the attack of the OH radicals is nonselective, and a mixture of all the three possible isomers is obtained. 

Here the intervention of the hydrocarbon radical cation seems possible. Hydrocarbon photocatalyzed oxidations seem to depend significantly on the relative positions of the valence-band edge of the active photocatalyst and the oxidation potential of the substrate. For example, in contrast to the photocatalytic oxidation of toluene described above, lower activity was observed in neat benzene, despite the fact that its oxidation potential lies at or slightly below the valence-band edge. This observation implies the importance of radical cation formation (by photoinduced electron transfer across the irradiated interface) as a preliminary step to hydrocarbon radical formation. If the benzene is dispersed into a benzene-saturated aqueous solution into which the semiconductor is suspended, complete mineralization is attained [158]. Thus, to observe selective photoelectrochemistry, it is necessary to avoid primary formation of the highly reactive, nonselective hydroxyl radical (formed by water oxidation) by the use of an unreactive, but polar, organic solvent. 

Photocatalytic systems based on the plasmon-induced charge separation can be used for oxidation of alcohols, aldehydes, and phenol [8, 13] mineralization of carboxylic acids [14] oxidation of benzene to phenol [15] release of hydrogen from alcohols and ammonia [16] and oxidation and reduction of water (but not water splitting) [17]. The photocatalytic system can also be applied to hydrophilic/hydrophobic patterning based on photocatalytic removal of a hydrophobic thiol adsorbed on metal nanoparticles [18]. 

Wu ZB, Gu ZL, Zhao WR, Wang HQ (2007) Photocatalytic oxidation of gaseous benzene over nanosized Ti02 prepared by solvothermal method. Chin Sd Bull 52(22) 3061-3067. doi 10.1007/sl 1434-007-0456-x... 

Zhang G, Yi J, Shim J, Lee J, Choi W (2011) Photocatalytic hydroxylation of benzene to phenol over titanium oxide entrapped into hydrophobically modified siliceous foam. Appl Catal B 102(1) 132-139... 

The photocatalyzed oxidation of gas-phase contaminants in air has been demonstrated for a wide variety of organic compounds, including common aromatics like benzene, toluene, and xylenes. For gas-phase aromatic concentrations in the sub-lOO-ppm range, typical of common air contaminants in enclosed spaces (office buildings, factories, aircraft, and automobiles), photocatalytic treatment leads typically to complete oxidation to CO2 and H2O. This generality of total destruction of aromatic contaminants at ambient temperatures is attractive as a potential air purification and remediation technology. 

The one-step hydroxylation ofbenzene represents an attractive alternative pathway for the direct synthesis of phenol and many studies are performed using different processes among which the photocatalytic reaction [45,46]. One of the main problem is the low selectivity of the process due to the higher reactivity of phenol towards the oxidation than benzene with the formation of oxidation by-products. In order to avoid these secondary products and to obtain the separation of the phenol from the oxidant reaction environment the use of a membrane system coupled with the photocatalytic process seems a useful solution. 

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