Hydroquinone, photocatalytic degradation 4-chlorophenol

Li X, Cubbage JW, TetzlaffTA, Jenks WS. Photocatalytic degradation of 4-chlorophenol 1. The hydroquinone pathway. J Org Chem 1999 64 8509-8524. 

Mills et al. performed extensive investigations into the photocatalytic degradation of 4-chlorophenol. These included studies on the effects of different titania samples [102], effects of annealing temperature on the photocatalytic efficiency of titania [ 103] and a mechanistic study of the decomposition process. The rate of chlorophenol destruction was found to drop when using titania photo catalysts that had been heated above 600 °C. This was believed to be due to a build up of the rutile phase and a reduction of surface area following heat treatment above these temperatures. A number of intermediates were reported including 4-chlorocatechol, hydroquinone, benzoquinone and 4-chlororesorcinol [104],... 

Photocatalytic degradation of 4-chlorophenol in TiOz aqueous suspensions produces 4-chlorocatechol, an ortho hydroxylated product, as the main intermediate. This result disagrees with data reported by other researchers, who proposed the formation of a para-hydroxylated product, hydroquinone, as the major intermediate. Results also indicated that further oxidation of 4-chlorocatechol yields hydroxy hydroquinone, which can readily be oxidized and mineralized to carbon dioxide. Complete dechlorination and mineralization of 4-chlorophenol can be achieved. In contrast, direct photolysis of 4-chlorophenol produces hydroquinone and p-benzoquinorie as the main reaction products. The photocatalytic oxidation reaction, initially mediated by TiO2, is generated by an electrophilic reaction of the hydroxyl radical attacking the benzene ring. 

Figure 5. Mass spectra of derivatized intermediates from the photocatalytic degradation of 4-chlorophenol. Experimental conditions 4-chlorophenol = 10 3 M, TiO2 = 1 g/L, pH = 4.0, I = 5 X 10 2 M NaN03, oxygen atmosphere, temperature = 25 °C. Peak A is 4-Chlorophenol, peak B is hydroquinone, peak C is 4-chlorocatechol, and peak D is 4-chlororesorcinol.

Figure 5.50 Spectral dependencies of the initial selectivity toward formation of (1) hydroquinone, (2) benzoquinone, and (3) chlorocathecol during 4-chlorophenol photocatalytic degradation over Ti02 (Degussa P25). Reprinted with permisison from Emehne and Serpone (2002). Copyright (2002) American Chemical Society.

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. 

The mechanism of photoinduced oxidation of aromatic compounds mediated by Ti02 in aqueous media is demonstrated by the reaction of 4-chlorophenol (601). Its degradation is principally based on oxidation by photocatalytically produced hydroxyl radicals, most likely adsorbed on the surface of a semiconductor catalyst.1554,1555 The initial reaction affords a 4-chlorodihydroxycyclodienyl radical 602, which releases the chlorine atom to form hydroquinone in a radical substitution reaction or loses the hydrogen atom via... 

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