Direct Oxidation of Benzene to Phenol with Hydrogen Peroxide

Direct Oxidation of Benzene to Phenol with Hydrogen Peroxide 

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Balducci, L., Bianchi, D., Bortolo, R., D Aloisio, R., Ricci, M., Tassinari, R. and Ungarelli, R. (2003) Direct oxidation of benzene to phenol with hydrogen peroxide over a modified titanium silicalite. Angew. Chem. Int. Ed., 115, 5087-5090. 

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]. 

The direct oxidation of benzene into phenol constitutes one of the challenges in chemistry to substitute the cumene process at the industrial level. Such oxidation has also been achieved with several TpfCu complexes as catalysts, leading to moderate yields and high selectivity toward phenol, in a transformation using hydrogen peroxide as the oxidant and at moderate temperatures. The same catalytic system has been employed for the selective oxidation of anthracenes into anthraquinones (Scheme 24). 

The first example of the direct hydroxylation of arenes to phenols was reported in 1900 and described oxidation of benzene with a mixture of iron(II) sulfate and hydrogen peroxide (Fenton s reagent) [1], Low yields of phenol (21% yield based on H202) are typically obtained in such cases, in addition to significant amounts of biphenyl (24% yield based on H202) [1-3]. Both selectivity to phenol and overall... 

The further oxidation of phenol may also result in the formation of catechol, C,iH4(OH) (1 2). The transformation may be effected by fusion with sodinm hydroxide.85 The snbstance may also be obtained by oxidizing benzene with hydrogen peroxide in the presence of ferrous sulfate88 and by reducing o-benzoquinone with aqueous sulfurous acid in the cold.81 Quinol may be prepared from phenol by oxidation with potassium persulfate in alkaline solution.38 It can also be obtained directly from benzene by the electrolytic oxidation of an alcohol solution to which... 

The desire to convert benzene directly to phenol with 30% hydrogen peroxide was mentioned in Chap. 4. A polymer-supported salicylimine vanadyl complex (1 mol%) was used to catalyze this reaction. Phenol was obtained in 100% yield at 30% conversion.217 There was no leaching of the metal. The catalyst was recycled ten times after which it started to break up. Oxidation of ligands is often a problem with oxidation catalysts. Inorganic supports not subject to such oxidation need to be tried to extend the life of such catalytic agents. 

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