Toluene Side-Chain Oxidation

Until 2005, DSM produced about 130 kt/a of phenol, used as a raw material to produce caprolactam from cyclohexanone. Phenol was produced by a copper-catalysed oxidation of benzoic acid. The raw material, benzoic acid, was produced in the same plant by the cobalt-catalysed oxidation of toluene, which also produced significant amounts of benzaldehyde (Fig. 16.19). 

At the end of 2004, phenol production at DSM stopped. The operating process of the existing toluene oxidation plant changed dramatically, producing much less benzoic acid but maintaining the same level of benzaldehyde production. 

From Fig. 16.20, it is also clear that at low toluene conversion, a significant increase of benzyl alcohol will be produced in the plant. The excess of benzyl alcohol is recovered downstream by distillation, and recycled back to the oxidation where it is oxidised to benzaldehyde. At first sight, this might be seen as a nice spin-off to produce additional benzaldehyde, but the reality was less promising the production of tar (i.e. benzyl benzoate) increased dramatically, as explained below. 

The products benzoic acid, benzaldehyde and benzyl alcohol are separated through distillation, by exposing these products to high temperatures, which normally results in the formation of benzyl benzoate. The common reaction pathway 

However, surprisingly, we have discovered that benzyl benzoate was predominantly formed by a new, unknown route the formation and decomposition of the acetal derived from benzaldehyde and benzyl alcohol. A proposed mechanism is shown in Fig. 16.21. 

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Side chain oxidation of alkylbenzenes is important in certain metabolic processes One way m which the body rids itself of foreign substances is by oxidation m the liver to compounds that are more polar and hence more easily excreted m the urine Toluene for example is oxidized to benzoic acid by this process and is eliminated rather readily... 

Nitration vs side-chain oxidation of toluene in dilute MA was investigated by Namba et al (Ref 69). They found that addition of sulfuric acid accelerated both reactions but nitration more than oxidation. Addition of water to the MA favors oxidation as does an increase in reaction temp... 

FIGURE 3.6 Degradation of toluene by side-chain oxidation. 

FIGURE 8.3 Aerobic degradation of toluene by (a) side-chain oxidation, (b) dioxygenation of the ring, and (c) monooxygenation. (From Neilson, A.H. and Allard, A.-S. The Handbook of Environmental Chemistry, Springer, 1998. With permission.)... 

Side-chain oxidations of alkyl aromatic compounds to aromatic carboxylic acids by electrogenerated and regenerated chromic acid have been studied extensively in the case of saccharin formation from o-toluene sulfonamide This... 

In addition to the synthesis of saccharin, also a number of other side-chain oxidations have been studied leading to aromatic carboxylic acids by indirect electrochemical oxidation using chromic acid as oxidizing agent. They include the oxidation of p-nitrotoluene 2,4-dinitrotoluene toluene, p-xylene, and p-tolualdehyde... 

Other aromatic molecules also suffer oxygenation on irradiated semiconductor powders. For example, both ring and side chain oxidation products can be observed when toluene is allowed to contact excited Ti02> eq. 91 (289) ... 

Cationic clays have also been used as supports for Cu. Cu-doped alumina-pillared montmorillonites have been employed in the oxidation of toluene and of xylenes with H2C>2. The pillaring and the Cu exchange are performed under acidic conditions at pH 2 and 3.5, respectively. It is unclear whether the Cu2+ remains fully associated with the clay in the presence of H2O2, which is itself acidic. Moreover, the reactions are unselective mixtures of ring-hydroxylated and side chain-oxidized products are obtained (180). 

Figure 3.65 Metal catalysed side-chain oxidation of toluenes.

Figure 3.69 Possible mechanisms operating during the side-chain oxidation of substituted toluenes employing the CAB/hydrogen peroxide system.

Table 3.6 Side-chain oxidation of substituted toluenes using PBS-1 in the CAB systema...

Figure 3.76 Side-chain oxidation of deactivated toluenes using the hydrogen peroxide/hv/ HBr system.

In many cases, peroxygen technology can be used to avoid the use of the transition metal altogether. Transition metal oxidants are traditionally used for alcohol or aldehyde oxidation and side chain oxidation of aromatic compounds such as toluenes. 

The degradation of toluene In this example, different pathways are used by the plasmid and chromosomally borne genes — side-chain oxidation to benzoate and ring 2,3-dioxygenation, respectively (Chapter 6, Section 6.2.1). 

Biodegradation of toluene (a) by side-chain oxidation, (b) with the methyl group intact, and (c) by hydroxylation. 

Fig. 6 Oxidation of toluene over V-MFI samples influence of duration of run on conversion and product distribution. A, V-MFI(A) B, V-MFI(B) ( ), side chain oxidation products and (A), cresols.

Hydroxylation of benzene to phenol using hydrogen peroxide in the presence of heteropoly compounds was observed in this study. Other aromatic hydrocarbons that were used as substrates were toluene, ethylbenzene, o,p-xylene and isopropyl benzene under homogeneous conditions. Both side chain oxidation and ring hydroxylation were observed in presence of hydrogen peroxide. For example, toluene gave benzyl alcohol, benzaldehyde, o,p-cresols in presence of hydrogen peroxide, whereas benzaldehyde and benzyl alcohol were observed in presence of t-BuOOH. 

Nevertheless, side chain oxidation also takes place with toluene, sumesting that the mechanism of oxidation is not influenced by the catalyst structure. With tert-butyl hydroperoxide, results are totally different. 

Let us take another and rather unusual example X type zeolite may be modified by Cs yielding a good catalyst for toluene side chain alkylation by methanol giving styrene and ethyl benzene. Cs" " element has been introduced in cationic exchange position and as tiny CS2O particles. A mechanistic study (66) has shown that such reaction follows a bifunctional mechanism and necessitates Cs both in cationic sites and in cesium oxides. A MAS-NMR study of 2 Si and 27ai has shown that Cs incorporation did not modify the material. 

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