Chlorophenol, photodegradation

L-type activated carbons on photocatalytic activity of TiOj in 4-chlorophenol photodegradation. 191 (2007) 122-131. 

The advantages of microreactors, for example, well-defined control of the gas-liquid distributions, also hold for photocatalytic conversions. Furthermore, the distance between the light source and the catalyst is small, with the catalyst immobilized on the walls of the microchannels. It was demonstrated for the photodegradation of 4-chlorophenol in a microreactor that the reaction was truly kinetically controlled, and performed with high efficiency [32]. The latter was explained by the illuminated area, which exceeds conventional reactor types by a factor of 4-400, depending on the reactor type. Even further reduction of the distance between the light source and the catalytically active site might be possible by the use of electroluminescent materials [19]. The benefits of this concept have still to be proven. 

Bell (1956) reported that the composition of photodegradation products formed were dependent upon the initial 2,4-D concentration and pH of the solutions. 2,4-D undergoes reductive dechlorination when various polar solvents (methanol, butanol, isobutyl alcohol, ferf-butyl alcohol, octanol, ethylene glycol) are irradiated at wavelengths between 254 to 420 nm. Photoproducts formed included 2,4-dichlorophenol, 2,4-dichloroanisole, 4-chlorophenol, 2- and 4-chlorophenoxy-acetic acid (Que Hee and Sutherland, 1981). 

Lipcz3mska-Kochany, E. and Bolton. J.R. Flash photolysis/HPLC applications. 2. Direct photolysis vs hydrogen peroxide mediation photodegradation of 4-chlorophenol as studied by a flash photolysis/HPLC technique. Environ. Sci Technol, 26(2) 259-262, 1992. 

Photodegradation rates of ortho derivates present good correlation with the thermodynamic stability of sigma-complexes formed between the aromatic ring and the surface OH-radicals. Rates decrease in the order -OCH3 (guiacol) > -Cl (2-chlorophenol) -H (phenol) > -OH (catechol). ... 

The deficiency of tlie LH equation was demonstrated also for pentachlorophenol [25] and by Cunningham [26-28]. The quantum yields for photodegradation of salicylate and other strongly sorbing substituted benzenes were less than those measured for nonsorbing chlorophenols. For salicylic acid, which chemisorbs, the photo-oxidation rates in air-saturated solutions are independent of the salicylic acid concentration, although the surface excess increases with the concentration [29]. 

Lettmann C, Hildenbrand K, Kisch H, Macyk W, Maier WF. Visible fight photodegradation of 4-chlorophenol with a coke-containing titanium dioxide photocatalysts. Appl Catal B Environ 2001 32 215-27. 

Barbeni M, Pramauro E, Pelizzetti E, Borgarello E, Gratzel M, Serpone N. Photodegradation of 4-chlorophenol catalyzed by titanium dioxide particles. Nouv J Chim 1984 8 547-50. 

Some of the topics addressed here have been reviewed by other authors. Studies of photoinduced processes (direct or indirect) of chlorophenols carried out before 1998 have been covered comprehensively [8] this work will only cursorily be treated here. A detailed overview of the photochemical behavior of phenylurea herbicides was in press at the time of writing this article [9]. The related subject of the photodegradation of pharmaceuticals in the aquatic environment has been reviewed very recently [10]. 

Further studies on the photodegradation of 4-chlorophenol were conducted, respectively, by combinations of flash irradiation and product analysis [25], and by fluorescence [26]. In the former study, a degradation quantum yield of = 0.24 was reported. 

A number of other, but minor primary photoproducts was also found, among them the products expected from a radical (photo-Claisen) rearrangement and from photohydrolysis of the ortho chlorine 2- and 4-chlorophenol were detected too, but their formation remained unexplained. The photodegradation quantum yield of dichlorprop did not depend on pH and was 50 times smaller than that of the anionic form of the related monohalo-genated compound mecoprop (see above) [77]. This is another example of the marked influence of the pattern of ring halogen substitution on the course and on the efficiency of photodegradation. 

In contrast, Lipczynska-Kochany and Bolton (19) reported that in photodegradation of 4-chlorophenol mediated by hydrogen peroxide the primary product was 4-chlorocatechol and hydroquinone was only a minor product. 

Figure 5.34 Spectral dependencies of the quantum yield (/> of photodegradation of phenol (1) and 4-chlorophenol (2) over Ti02 (P-25). Reprinted with permission from Emeline et al. (2000c). Copyright (2000) American Chemical Society.

This is demonstrated in Fig. 5.50. The scheme depicted in Fig. 5.51 (Emeline and Serpone, 2002c) illustrates the possible steps during the photodegradation of 4-chlorophenol (4-ClPhOH). Benzoquinone (BQ) is formed from interaction of 4-chlorophenol with electrons localised on the surface, whereas ClCat (chlorocatechol) is formed by interaction of the initial substrate 4-ClPhOH with the OH radicals formed by hole trapping by surface OH groups hydroquinone (HQ) is formed both... 

Figure 5.51 Scheme showing the proposed oxidative and reductive routes in the photodegradation of 4-chlorophenol to yield ClCat, HQ and BQ intermediates (see text). Reprinted with permission from Emeline and Serpone (2002). Copyright (2002) American Chemical Society. 

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