Phenol oxidation rates

A first set of experimenfs was aimed at evaluating the effect of Fe on the phenol oxidation rate at a concentration of 20 ppm C and at a pH of 3.2. The initial concentration of Fe cations was varied using different amoimts of a solution containing Fe ions. 

Oxidation of the same set of substituted phenols listed in Table 17.3 was also undertaken employing the organic oxidant, cumylperoxyl radical (Cum ). Unlike the metal based oxidation. Fig. 17.11 shows that the phenol oxidation rate constants do not correlate well with the substrate army better than with substrate BDE. This behavior implicates a more synchronous HAT mechanism for the organic radical oxidant. 

In the experiment involving oxidative enzyme HRP (EC 1.11.1.7, RZ 1.9, 240 purpuro gallin (units/mg)) [89] for the enzymatic treatment and ultrasonic waves of 423 kHz and 5.5 W, the phenol degradation rate was found to increase. The ultrasound assisted biodegradation method has been found to be more efficient method than the sonolysis and enzyme treatment when operated individually. 

The nitrosophenol (10), which may be isolated, is oxidised very rapidly by nitric acid to yield the p-nitrophenol (11) and nitrous acid more nitrous acid is produced thereby and the process is progressively speeded up. No nitrous acid need be present initially in the nitric acid for a little of the latter attacks phenol oxidatively to yield HN02. The rate-determining step is again believed to be the formation of the intermediate (9). Some direct nitration of such reactive aromatic compounds by N02 also takes place simultaneously, the relative amount by the two routes depending on the conditions. 

At alkaline conditions (pH=l 1) no phenol, hydroquinone were detected. This is possibly due to the fact that the rate of phenol oxidation increases under alkaline conditions with optimum pH between 9.5 and 13 [17] and so once it is formed, it is readily oxidised. The absence of phenol and increased concentration of p-hydroxybenzoic acid could be also explained by reduced decarboxylation rates under conditions of high pH, which would result in the oxidation of p-hydroxybenzaldehyde to form p-hydroxybenzoic acid. 

Assuming that ks = k9 = 3x 108Lmol 1s 1 (the key value), the diversity of the rate constants of the reactions of phenols and phenoxyl radicals (7, —7, 10, 11, and 12) can be reduced to only two parameters, k7 and T. This allows one to get the universal formulae for the oxidation rate v, into which these parameters enter as functions of k2, k7, T, and ambient conditions (Table 14.7). When considering this table, it should be taken into account that mechanism VII is possible only for 2,4,6-tris-alkylphenols, while mechanism IX holds only for o- and p-alkoxyphenols. 

Phenolic compounds naturally occurring in plants have induced many physiological responses that duplicate those reported for ozone and/or peroxyacetylnitrate (PAN). Chlorogenic acid is a competitive inhibitor of lAA-oxidase (35) and plant growth is adversely affected by increased concentrations of auxins (36). Concentrations of chlorogenic acid are increased in tobacco tissue exposed to ozone ( ) Phenols inhibit ATP synthesis (37), oxidative phosphorylation ( ) and SH enzyme activity (27) they increase respiration (38), reduce CO2 fixation (22), modify both membrane permeability (40) and oxidation rate of reduced NADH... 

On the other hand, an attempt to accelerate the step of coordination of the substrate to the Cu catalyst was successful because it used the hydrophobic domain of the polymer ligand156 That was the oxidation catalyzed by polymer-Cu complexes in a dilute aqueous solution of phenol, which occurred slowly. The substrate was concentrated in the domain of the polymer catalyst and was effectively catalyzed by Cu in the domain. A relationship was found to exist between the equilibrium constant (Ka) for the adsorption of phenol on the polymer ligand and the catalytic activity (V) of the polymer-ligand-Cu complex for various polymer ligands K a = 0.21 1/mol and V = 1(T6 mol/1 min for QPVP, K a = 26 and V = 1(T4 for PVP, K a = 52 and V = 10-4 for the copolymer of styrene and 4-vinylpyridine (PSP) (styrene content 20%), and K a = 109 and V = 10-3 for PSP (styrene content 40%). The V value was proportional to the Ka value, and both Ka and V increased with the hydrophobicity of the polymer ligand. The oxidation rate catalyzed by the polymer-Cu complex in aqueous solutions depended on the adsorption capacity of the polymer domain. 

The reactivity of phenols in the Cu-complex-catalyzed oxidation was studied by measuring the oxidation rate, the rate constant ke, and the redox potentials of the Cu complex and of the phenol (Fig. 29)158). The logarithm of the oxidation rate is proportional to log ke, which supports the assumption that the electron-transfer step is rate-determining. A linear relationship is observed between log ke and Hammett s o value of the phenol, which is proportional to the oxidation potential of the phenol. 

Fig. 29. Relationship between rate constant of electron-transfer step (ke), oxidation rate, and Hammett s o value of phenol for the pyridine-Cu-catalyzed oxidation of phenols1 ...

That is, hox(AtXH)= - / (ArX ). For this type of reaction, E (ArX ) is positive. Hence, the more positive this value, the more difficult it is to oxidize the compound. For many phenols and anilines, polarographic half-wave potentials, Zs1/2(ArX"), determined at pH values where the compound is present in its neutral form, are available. These values should reasonably parallel the oxidation potentials of the compounds, and therefore can also be used to relate oxidation rate constants ... 

It has been found that oxygen-containing substitutents (particularly phenols) and alkyl substitutents on an aromatic nucleus increase dramatically the susceptibility to oxidation of the original aromatic compound (18, 24, 25). Oxidation of the aromatic nuclei in coal undoubtedly takes place at periphery sites hence, an increase in size of aromatic nuclei would be expected to lower the oxidation rate owing to a decrease in the number of oxidation sites per unit area of coal surface. 

The coexistence of NH3 is indispensable for selective benzene oxidation. Neither benzene oxidation nor combustion proceeded in the absence of NH3 (Table 2.5). Fe/ZSM-5 has been reported to be active and selective for phenol synthesis from benzene using N20 as an oxidant [97], but selective benzene oxidation did not proceed with N20 instead of 02. The addition of H20 to the system gave no positive effects on the catalytic performance, either. In addition, other amine compounds such as pyridine and isopropyl amine did not produce phenol. The phenol formation rate and selectivity increased with increasing NH3 pressure because the coexisting NH3 produces active Re clusters, as described below, and reached maximum conversion and selectivity at a partial pressure of NH3 of around 35—42kPa. 

Tratnyek and Hoigne (1994) investigated 25 substituted phenoxide anions for QSARs that can be used to predict rate constants for the reaction of additional phenolic compounds oxidized by chlorine dioxide (OCIO). Correlating oxidation rates of phenols in aqueous solution is complicated by the dissociation of the phenolic hydroxyl group. The undissociated phenol and the phenoxide anion react as independent species and exhibit very different properties. The correlation analysis should be performed on the two sets of rate constants separately. 

Based on their chemical structure, the organic chemicals were divided into a number of categories alkanes, alkenes, amines, aromatic hydrocarbons, benzenes, carboxylic acids, halides, phenols, and sulfonic acid. Linear regression analysis has been applied using the method of least-squares fit. Each correlation required at least three datapoints, and the parameters chosen were important to ensure comparable experimental conditions. Most vital parameters in normalizing oxidation rate constants for QSAR analysis are the overall liquid volume used in the treatment system, the source of UV light, reactor type, specific data on substrate concentration, temperature, and pH of the solution during the experiment. 

Correlation of oxidation rate constants of halogenated phenols with log P. TABLE 7.7... 

Reviewed previous SCWO research with model pollutants and demonstrated that phenolic compounds are the model pollutants studied most extensively under SCWO conditions Studied supercritical water oxidation of aqueous waste Explored reaction pathways in SCWO of phenol Studied catalytic oxidation in supercritical water Explored metal oxides as catalysts in SCWO Studied decomposition of municipal sludge by SCWO Investigated the SCWO kinetics, products, and pathways for CH3- and CHO-substituted phenols Determined oxidation rates of common organic compounds in SCWO... 

Phenol is commonly present in industrial streams and is classified as a priority pollutant. At temperatures of 380 to 440°C and pressures of 190 to 270 atm, oxidation rates were calculated from kinetic Equation 10.16 by Minok et al. (1997). Their results showed that, under the designed system conditions, the rate of phenol destruction was dependent only on temperature, concentration of water, oxygen, and phenol but not on pressure. Water acts in the system as a reactant and was considered to be a reactive radical producer. The destruction rate of phenol can be expressed as follows ... 

By using the fraction of phenol ionized (a) with pH and pKa, together with rate constants for phenol and phenolate oxidations, it becomes possible to estimate the observed rate constant (kobs) for oxidation of phenols as a function of pH ... 

Haag and Mill (1987) reported rate constants and correlation equations for oxidation of a series of phenylazonaphthol dyes, where oxidation of the naphthol and naphtholate ion are accounted for mainly by reaction with singlet oxygen, with some contribution from R02. Naphtholate anion is about ten times more reactive than naphthol toward oxidants. The correlation equations for these compounds are similar to those used for phenol oxidations (Tratnyek and Hoigne, 1991) ... 

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