Towards the Direct Oxidation of Benzene to Phenol

Marco Ricci, Daniele Bianchi, and Rossella Bortolo 

Phenol is one of the most important intermediates of the chemical industry. Its current global capacity is put at about 8 x 10 t yr and is forecast to grow near 4.5% per year through to 2009. 

Historically, several processes have been developed to an industrial scale to produce phenol, including (i) sulfonation of benzene and alkali fusion of the benzene sulfonate (ii) chlorination of benzene and hydrolysis of chlorobenzene (iii) the cumene process (Section 13.2) (iv) toluene oxidation to benzoic add and subsequent oxidative decarboxylation of the latter to phenol and (v) dehydrogenation of cyclohexanol-cyclohexanone mixtures. Today, however, only the cumene process and the toluene oxidation are still run on an industrial scale, all the other processes having been given up due to economic reasons or environmental problems. 

Of the two commercially operated technologies, the toluene oxidation route affords not only phenol but also the specialty chemicals benzaldehyde and benzoic acid (Equation 13.1)  

Sustainable Indu rial Processes. Edited by F. Cavani, G. Centi, S. Perathoner, and F. Trifiro Copyright 2009 WILEY-VCH Verlag GmbH Co. KGaA, Weinheim ISBN 978-3-527-31552-9 

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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). 

Starting from Benzene. In the direct oxidation of benzene [71-43-2] to phenol, formation of hydroquinone and catechol is observed (64). Ways to favor the formation of dihydroxybenzenes have been explored, hence CuCl in aqueous sulfuric acid medium catalyzes the hydroxylation of benzene to phenol (24%) and hydroquinone (8%) (65). The same effect can also be observed with Cu(II)—Cu(0) as a catalytic system (66). Efforts are now directed toward the use of Pd° on a support and Cu in aqueous acid and in the presence of a reducing agent such as CO, H2, or ethylene (67). Aromatic... 

To date, it seems that the most sustainable approach is the one that combines the use of cheap raw materials, for example, cyclohexane, benzene or phenol, with oxygen as the terminal oxidant. Within this context, a process that does not use acetic acid in the aerial oxidation of the KA Oil into AA or, even better, in the direct oxidation of cyclohexane to AA would represent a significant step forward towards a new and sustainable synthesis. On the other hand, recent examples demonstrate that even the traditional process making use of nitric acid for the oxidation of KA Oil may be turned into an intrinsically green one that is economically sustainable due to the use of the co-produced N2O in down-stream applications. 

The one-step hydroxylation ofbenzene represents an attractive alternative pathway for the direct synthesis of phenol and many studies are performed using different processes among which the photocatalytic reaction [45,46]. One of the main problem is the low selectivity of the process due to the higher reactivity of phenol towards the oxidation than benzene with the formation of oxidation by-products. In order to avoid these secondary products and to obtain the separation of the phenol from the oxidant reaction environment the use of a membrane system coupled with the photocatalytic process seems a useful solution. 

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