4-chlorophenol, reaction mechanisms

Reaction mechanism for the degradation of 4-chlorophenol by UV/Ti02. (From Martin et al., Environ. Sci. Teclmol., 30, 2535, 1996. With permission.)... 

To date, numerous model compounds simulating the pollutants in common waste streams have been studied under laboratory-scale conditions by many researchers to determine their reactivities and to understand the reaction mechanisms under supercritical water oxidation conditions. Among them, hydrogen, carbon monoxide, methanol, methylene chloride, phenol, and chlorophenol have been extensively studied, including global rate expressions with reaction orders and activation energies [58-70] (SF Rice, personal communication, 1998). 

Canizares, P., Garcia-Gomez, J., Saez, C. and Rodrigo, M. A. (2003b) Electrochemical oxidation of several chlorophenols on diamond electrodes Part I. Reaction mechanism. J. Appl. Electrochem. 33, 917-927. 

Reaction Mechanisms. Our analysis of intermediates and reactions reported by other researchers leads to proposed reaction pathways describing the photocatalytic oxidation of 4-chlorophenol in TiOz aqueous suspensions. The photocatalytic oxidation reaction is brought about by OH radicals, which are formed mainly from water decomposition on the Ti02 surface upon UV light irradiation (9-13). The OH radicals can either directly react with the adsorbed organic species on the TiOa surface or diffuse to the solution and then react with the dissolved organic species in the solution phase. Both reactions lead to formation of hydroxylated products such as 4-chlorocatechol, hydroquinone, 4-chlororesorcinol, and hydroxyhydroquinone as the initial products (Figure 6). Eventually, the reaction will mineralize these interme-... 

Comparison with Direct Photolysis Process. The Ti02-mediated photocatalytic oxidation reaction involves a complex free-radical reaction mechanism in which OH radicals are responsible for the oxidation of 4-chlorophenol. The initial reaction step produces 4-chlorocatechol as the main product. In contrast, the direct photolysis of 4-chlorophenol produces a different set of reaction products. Figure 8 shows that the direct photolysis of... 

The Ti02-mediated photocatalytic oxidation process can readily degrade 4-chlorophenol in aqueous solutions, with a complete mineralization to carbon dioxide and chloride ions, whereas the direct photolysis of 4-chlorophenol generates only a small amount of carbon dioxide. The distribution of intermediates during the course of the reaction shows that the reaction mechanism of the photocatalytic oxidation process is clearly different from that of the direct photolysis reaction. 

The Ti02-mediated photocatalytic oxidation reaction can be described by the radical mechanism involving OH as the major reaction species. The reaction mechanism follows the ortho pathway, so that the main intermediate found is 4-chlorocatechol, whereas the formation of 4-chlororesorcinol and hydroquinone is only a minor pathway. Further degradation of 4-chloroca-techol leads to production of hydroquinone, which can be further oxidized and mineralized to carbon dioxide. In contrast, the direct photolysis of 4-chlorophenol follows the para pathway, which leads to the formation of hydroquinone and p-benzoquinone as the major products. 

A complete pathway for the mineralization of 4-chlorophenol can be described by the hydroxylation reaction through dechlorination, ring cleavage, and mineralization. Reaction kinetics of the 4-chlorophenol and its intermediates can be reasonably well approximated by using a complex parallel and consecutive first-order reaction mechanism. 

The effect of pH on 4-chlorophenol oxidation is shown in Figure 4. The results show that oxidation is favored under both acidic and basic conditions. This fact implies that different reaction mechanisms may be operative and that photocatalytic oxidation is affected by both H+ and OH- ions. At high pH, the number of hydroxyl ions on the TiO surface increases because of the abundance of OH- ions, thereby increasing the population of -OH radicals. Hickling and Hill (25) suggested that, at high pH, the adsorbed OH- group can be readily converted to OH radical upon irradiation. 

The mechanism of the aqueous photochemistry of 4-chlorophenol has been reviewed earlier [5,8]. Its basic features are the same as those of the carbene pathway described above for 2-bromophenol. The main differences are the fact that this is the only photolytic reaction of 4-chlorophenol and that its quantum yield is considerably higher than that of the 2-sub-stituted analogues

triplet carbene, 4-oxocyclohexa-2,5-dienylidene (A.max = 384 and 370 nm) from aqueous 4-chlorophenol (see Fig. 1) [20]. Photoproduct analysis yielded p-benzo-quinone (in the presence of O2), phenol (in the presence of an alcohol), hydroquinone and isomeric chlorodihydroxybiphenyls, which could all be accounted for by carbene reactions [20]. 

In the discussion of the mechanism of product formation from the 2-haloanilines, a strong similarity to the behavior of the 2-halophenols was noted. The two major monomeric products correspond to the processes of photohydrolysis and ring contraction which are also characteristic for this group of compounds (see above). The photohydrolysis reaction was proposed to be a concerted process of halide elimination and H20 attack. Two possibilities were noted for the ring contraction process, the first one similar to the mechanism observed for 2-chlorophenol, possibly via a singlet car-bene, the other one involving the intermediate formation of phenylnitrene. No direct proof could be obtained for either mechanism, not least because it was impossible to observe any transients by nanosecond spectroscopy. The... 

In a more recent study. Nelson and Yang [494] pre.sented a surface complex-ation model to describe the effect of pH on adsorption equilibria of chlorophe-nols, i.e., the electrostatic effect they also discussed the potential importance of 7t-7t interactions and donor-acceptor complex formation but could not distinguish between the two and concluded, somewhat vaguely, that [t]hese proposed mechanisms provide plausible explanations for the surface complexation reactions between chlorophenols (neutral or anionic forms) and the surface of activated carbon (acidic or basic sites). ... 

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