Oxidation Fenton system

In the presence of metal catalysts, hydrogen peroxide oxidations proceed in improved yields. The most common catalyst is an iron(II) salt which produces the well-known Fenton system or reagent. Dimethyl sulphoxide is oxidized to the sulphone using this system although a range of unwanted side-products such as methanol and methane are produced Diphenyl sulphoxide does not react using this reagent due to its insolubility and in all cases some iron(III) is formed by other side-reactions. 

Based on his own experimental results and their analysis, Traube has made the correct (in terms of modem understanding) conclusion that H202 displays high reactivity in the presence of a catalyst in the system. This is the pathway of various substrate oxidations by hydrogen peroxide, proceeding in the Fenton system (iron ion + H202), and some biochemical processes. 

Critical review of oxidation reactions in the Fenton system. 

Let us consider the reaction of benzene oxidation with hydrogen peroxide in the Fenton system as the classical situation [30], In the absence of iron ions benzene does not in practice interact with H202. The addition of bivalent iron salt to the system C6H6-H202-H20 induces benzene oxidation to phenol and diphenyl according to the following mechanism ... 

As noted in the previous chapter, substrate interaction with OH and H02 radicals produces different products, which composition depends on H202 dilution. Hence, OH radicals participate in nonselective oxidation, whereas H02 promotes selective gas-phase oxidation of substrates with hydrogen peroxide [32], This situation is observed for both low H202 concentrations in the reaction mixture and liquid-phase oxidation in the Fenton system, where the OH radical is the key active site. In the Fenton system, connection channels between two reactions are set with the help of a general intermediate, the OH radical, which represents a nonselective active site due to the ability to attack another complex molecule by various... 

As follows from the above, there is no principal possibility of implementing high selectivity oxidation of the substrate in the Fenton system. 

Although there is still debate as to whether hydroxyl radicals or ferryl species are the key oxidants in Fenton systems, most literature reports on the mechanisms of degradation of organic compounds invoke the hydroxyl radical. Based on the reports discussed above, it seems likely that hydroxyl radical is a major oxidant during Fenton degradations. Although ferryl ions or other highly oxidized forms of iron may occur, either to a limited extent or more abundantly under specific conditions, this section will deal with documented reaction pathways and kinetics for hydroxyl radical or species assumed to be hydroxyl radical. The reader should keep in mind that ferryl pathways may need to be considered under certain conditions. 

Savinova ER, Kuzmin AO, Frusteri F, Parmaliana A, Parmon VN. Partial oxidation of ethane in a three-phase Electro-Fenton system. In Parmaliana A, ed. Natural Gas Conversion Y. Studies in Surface Science and Catalysis 1998 119 429 134. 

As a unique reaction with the Fenton system, the alkylation of heteroaromatics with alkyl iodide, hydrogen peroxide, and dimethyl sulfoxide in the presence of FeS04 can be carried out. This reaction comprises of the initial formation of reactive HO by the reaction of FeS04 and hydrogen peroxide, reaction of HO on the sulfur atom of dimethyl sulfoxide to form CH 3 and methanesulfinic acid (SH2 reaction), reaction of CH3 on the iodine atom of alkyl iodide via SH2 pathway to form more stable R and methyl iodide, and then addition of R to the a-position of y-picoline (1) to form an addition-intermediate radical which is rearomatized under oxidative conditions to 2-alkyl-4-methylpyridine (2) [15, 16]. 

The feasibility of applying solar radiation as a source of UV-visible radiation has made the photo-Fenton system an economical and competitive process. Within this context an alternative method has been developed based on solar photocatalytic oxidation and natural processes of wastewater treatment [5], as well as sunlight-driven degradations of many compounds, such as EDTA [30], phenols [7,13], pesticides [31-33], surfactants [34], diclofenac [24], formic acid [22], azo-dyes [19], non-biodegradable chlorinated solvents [35], nitroaniline [16], and other organic compounds [21, 36]. 

The facile oxidation of organic substrates in the presence of a Fenton system is believed to be due to the production of the hydroxyl radical, as mentioned earlier. Consequently, a number of techniques have been employed to generate the species independently of any iron centres. Two techniques worthy of a mention are photolysis and radiolysis. Photolytic activation can be used to cause homolysis of the peroxygen bond because peroxides have a relatively broad absorption band above 300 nm (Figure 2.13).29... 

On the face of it, the Fenton system seems ideally suited for the oxidation of organic species. As a consequence of the high reactivity of the hydroxyl radical (approx. 109 M-1 s 1), there is very little selectivity and this limits the application of Fenton chemistry in fine organic synthesis. It is, however, efficacious as an oxidation system for the removal of organic effluents from water courses. These systems will be further discussed in Chapter 5. Some organic syntheses have been carried out in the presence of Fenton s reagent and a few illustrative examples are outlined in Figure 2.14. 

The direct oxidation of benzene to phenol is usually affected by a poor selectivity due to the lack of kinetic control. Indeed, phenol is more reactive towards oxidation than benzene itself and consecutive reactions occur, with substantial formation of overoxidized products like catechol, hydroquinone, benzoquinones and tars. This is the usual output of the oxidation of aromatic hydrocarbons by the classical Fenton system, a mixture of hydrogen peroxide and an iron(II) salt, usually ferrous sulfate, most often used in stoichiometric amounts [8]. 

Using FeS04 (1.67 x 10 M) in conjunction with equimolar amounts of methyl-pyrazine-5-carboxylic acid N-oxide and trifluoroacetic acid, in a water-acetonitrile-benzene (5 5 1 v/v/v) biphasic system, with benzene-H202-FeS04 = 620 60 1, a benzene conversion of 8.6% is achieved (35 °C 4h). Hydrogen peroxide conversion is almost complete (95%) and selectivities to phenol are 97% (based on benzene) and 88% (based on H2O2) [13]. These values are definitely higher than those described in the literature for the classical Fenton system [14], whereas iron complexes with pyridine-2-carboxylic acid derivatives are reported to be completely ineffective in the oxidation of benzene under the well-knovm Gif reaction conditions [15]. 

Regarding the possible mechanism, notably, toluene, ethylbenzene and terf-butylbenzene are less reactive than benzene, which is not consistent with the expected order for an electrophilic aromatic substitutions, such as that found with the classic Fenton reagent. There are also other differences with respect to the Fenton chemistry. In particular, under biphase conditions the reaction is definitely more selective although comparisons are difficult due to the huge amount of data, sometimes inconsistent, on the Fenton system (for which most of the data have been obtained with the iron used in stoichiometric amounts) it seems that selectivities dose to those observed under biphase conditions are only attained at a conversion around of 1%. Furthermore, in the biphase system, only a negligible amount (<1%) of biphenyl was detected among secondary products, whereas in the classic Fenton oxidation this compound is formed by radical dimerization of hydroxycydohexadienyl radicals in typical yields ranging from 8 to 39%. 

Nonselective and efficient consecutive oxidation reactions ultimately lead to nontoxic mineralization products, such as CO2 and H20.1519,1520 For example, an improved version of the photo-Fenton system, utilizing ferrioxalate ion, very efficiently oxidizes organic compounds present in the aqueous solution.1528 This process affords a reactive C204 intermediate, which generates a superoxide radical anion (02 ) from dissolved oxygen or directly attacks relatively inert molecules such as CCI4 (Scheme 6.288).1529... 

Catalytic wet peroxide oxidation of organic compound in wastewater streams Fe- and Cu-containing zeolites (ZSM-5 and Y supports) Successful replacement of homogeneous Fenton systems [72]... 

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