Toluene, oxidation mechanism

Figure 1. Schematic representations of toluene oxidation mechanisms. Upper panel MCMv3, lower panel MCMv3.1.

The following outer-sphere oxidation mechanism is favoured over the radical-cation formation which is a feature of similar oxidations by Mn(III) acetate (p. 375). With toluene... 

A related study reports the use of the complexes [Ru(02)X(N0)(PPh3)2] (X = CN, NCS, Cl,or OH) as homogeneous catalysts for the oxidation of triphenylphosphine. Only the CN" and NCS complexes were found to-be effective, although [Ru(02)Cl(N0XPPh3)2] oxidized the corresponding AsPhj in boiling toluene. Kinetic studies suggested " the oxidation mechanism shown in Scheme 5. 

El Bakali et al. noted an overall similarity between the chemistry of benzene and the chemistry of toluene, based on their oxidation mechanisms. Ethylbenzene has also been the subject of mechanistic studies,most recently by Ergut et al. ... 

So, our data and the known organic chemistry suggest, according to the literature (8,12), that ammoxidation and oxidation mechanisms are interplayed. We can consequently propose the following mechanism for the surface reactions relative to ammoxidation of toluene ... 

Such a reaction mechanism could initiate toluene oxidation and increase the observed removal rates and overall conversion of the toluene. 

Oxidation of Benzene Because of the complexity of the aromatics chemistry, large uncertainties remain in the oxidation mechanisms for even the simplest aromatic species, benzene and toluene [12,113]. The overall oxidation behavior (i.e., the fuel consumption rate) can be calculated with some confidence, but predictions of the concentration of intermediate species may be off by orders of magnitude. 

Conversion increases from toluene to p-xylene to p-methoxytoluene due to the electron-donating effects of the substituents. The increased number of electron-donating substituents creates a positive charge on the reaction center and activates the ring for increased oxidation rates. This effect is in agreement with the radical cation oxidation mechanism proposed by Som-rani et al. (1995). Electron density on the reaction center was shown to play an important role in determining oxidation rates of toluenes. 

Similar results from product studies were observed with toluene as the substrate (Ross et al., 1970). Voltammetric curves from substrates with relatively high oxidation potentials were in all cases markedly displaced towards higher potentials upon addition of the toluene, a phenomenon ascribed to blocking of anode sites by adsorption of toluene. Hexamethylbenzene, with Ex j2 considerably lower than the discharge potential of nitrate ion, gives a mixture of the a-nitrate and acetate upon oxidation at 1 1 V versus see, demonstrating at least in this case that these can be products of a direct oxidation mechanism. In all cases small amounts of bibenzyl derivatives were formed, which of course necessitates the intervention of benzyl—but not nitrate—radicals at some stage of the mechanism. 

Inoue et al. 1988b). This was due to competitive inhibition of the oxidation mechanisms involved in the metabolism of benzene. Phenobarbital pretreatment of the rats alleviated the suppressive effect of toluene on benzene hydroxylation by the induction of oxidative activities in the liver. This effect has been observed in other studies in rats (Purcell et al. 1990). 

Studies of benzene and toluene oxidation in the turbulent flow reactor at Princeton University have provided valuable information on the mechanisms of oxidation at temperatures in excess of 1000 K [213]. The first extensive study of benzene and toluene at temperatures of about 750 K was made by Burgoyne [35]. Apart from CO and CO2, the major products of benzene oxidation that were detected were phenols and acids. An autocatalytic reaction was observed by Burgoyne [35] presumably driven by H2O2 formation and decomposition. Amongst the main products of toluene slow oxidation were benzyl alcohol, benzaldehyde and benzoic acid. Phenolic compounds were also reported. This reaction also showed an autocatalytic development. An equilibrium constant for the equilibrium between benzyl and benzylperoxy radicals has been measured by Fenter et al. [214], but this cannot be followed by an isomerization in the way that is possible in alkanes. 

In particular, the study was centered on the analysis of the behavior in toluene selective oxidation of a series of V-containing micro- and mesoporous materials [mesoporous (MCM-41), MFI (2SM-5) and P type]. The formation of phenol as a by-product has also been observed in some cases. This product does not form in toluene oxidation over vanadium oxide supported on AI2O3 or TiOa [2-6] and is an interesting first example of the possibility of direct synthesis of phenol from toluene using gaseous O2. A further objective of this work therefore was to identify the key aspects in this reaction as well as the possible reaction mechanism. 

The detailed oxidation mechanisms of aromatic hydrocarbons in the MCM have been revised and updated based on the latest available experimental data and is available via the MCM website ( http //mcm.leeds.ac.uk/MCMl as MCMvS.l. A series of chamber experiments were carried out to investigate the details of key areas of aromatic oxidation mechanisms, and these were primarily focused on toluene oxidation (Bloss et al, 2005b). 

Fig. 6-11. Oxidation mechanism for toluene. The pathway leading to ring opening is not yet fully understood.

Kinetic studies of the oxidation of aromatic hydrocarbtSns with potassium permanganate show that reaction is first order with respect to each reactant. With toluene, oxidation takes place almost exclusively at the methyl group to give successively benzyl alcohol, benzaldehyde, and benzoic acid with ethyl benzene, attack is predominantly at the alpha carbon atom to yield acetophenone and with n- and isopropylbenzene, the products are propiophenone and acetophenone, respectively, plus benzoic acid. These results are of interest primarily for mechanism studies. 

We have performed laboratory measurements of rate constants of a few first steps of the tropospheric oxidation mechanism of some monocyclic aromatic hydrocarbons the following reactions have been studied OH radical with benzene or toluene (+M) benzyl radicals (from OH abstraction pathway) with O2, NO and NO2 on the other band, the branching ratios (abstraction/addition + abstraction) have been measured by a direct spectroscopic technique. All these measurements have been achieved thanks to the Discharge Flow (DF) technique with detection of OH by Resonance Fluorescence (RF) and other radicals by Laser Induced Fluorescence (LIF). 

FIGURE 5 Oxidation mechanism for toluene. The pathway leading to ring opening is not fully understood, only one of several possibilities is indicated. [From Warneck, P. (1999). Chemistry of the Natural Atmosphere, Academic Press, San Diego.]... 

Benzaldehyde has been identified as a major oxidation product of pure toluene or alkane toluene mixtures in studies at temperature below 800 K in a static reactor between 723 and 788 in a jet stirred reactor between 580 and 620 and in a rapid compression machine at 750 The reaction sequence R1-R3-R4 is often suggested to explain the formation of benzaldehyde in the low-temperature oxidation mechanism of toluene. 

D. A. Bittker, Oxidation Mechanisms of Toluene and Benzene, NASA Technical Paper 3546, National Aeronautics and Space Administration, Washington, DC, 1995. 

Birdsall, A.W., Dreoni, J., Elrod, M.J. Investigation of the role of bicyclic peroxy radicals in the oxidation mechanism of toluene. J. Phys. Chem. A 114, 10655-10663 (2010)... 

Suh, L, Zhang, R., Molina, L.T., Molina, M.J. Oxidation mechanism of aromatic peroxy and bicyclic radicals from OH-toluene reactions. J. Am. Chem. Soc. 125, 12655-12665 (2003)... 

Acids are minor products of the oxidation of aromatic compounds. For example, benzoic acid is formed in small yield from the oxidation of benzaldehyde, itself a minor product of toluene oxidation, as shown in the following reaction mechanism (mechanism VI-F-1) note that only reactions leading from toluene to benzaldehyde to benzoic acid are shown in mechanism VI-F-1. 

Synthetic phenol capacity in the United States was reported to be ca 1.6 x 10 t/yr in 1989 (206), almost completely based on the cumene process (see Cumene Phenol). Some synthetic phenol [108-95-2] is made from toluene by a process developed by The Dow Chemical Company (2,299—301). Toluene [108-88-3] is oxidized to benzoic acid in a conventional LPO process. Liquid-phase oxidative decarboxylation with a copper-containing catalyst gives phenol in high yield (2,299—304). The phenoHc hydroxyl group is located ortho to the position previously occupied by the carboxyl group of benzoic acid (2,299,301,305). This provides a means to produce meta-substituted phenols otherwise difficult to make (2,306). VPOs for the oxidative decarboxylation of benzoic acid have also been reported (2,307—309). Although the mechanism appears to be similar to the LPO scheme (309), the VPO reaction is reported not to work for toluic acids (310). 

Meluch et al.10 reported that high-pressure steam hydrolyzes flexible polyurethane foams rapidly at temperatures of 232-316°C. The diamines are distilled and extracted from the steam and the polyols are isolated from the hydrolysis residue. Good results were obtained by using reclaimed polyol in flexible-foam recipes at file 5% level. Mahoney et al.53 reported the reaction of polyurethane foams with superheated water at 200°C for 15 min to form toluene diamines and polypropylene oxide. Gerlock et al.54 studied the mechanism and kinetics of the reaction... 

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