Oxidation of Phenol to Catechol and Hydroquinone

The hydroxylation of phenol to catechol and hydroquinone with H2O2, introduced in the 1970s, represented a major advance over earlier methods of production, which utilized the alkaline fusion of o-chlorophenol (catechol) and the stoichiometric oxidation of aniline with manganese dioxide (hydroquinone). The inorganic and organic wastes in both processes were of the order of several kg per kg of product. However, the hydroxylation of phenol too is not free of drawbacks one is the co-production of two chemicals, destined for two 

Three processes based on H2O2 are commercial they use different catalysts and show different performances (Table 2). The conversion of phenol is only partial to minimize further oxidations in which the expensive oxidant is also consumed (Equation 37). A compromise, different for each process, is normally made between the need to minimize the energy spent on the separation and recycle of phenol and that of maximizing the select vities. 

The hydroxylation of phenol on TS-1 is normally operated in a slurry reactor, at temperatures close to 100°C, with total consumption of the oxidant. The selectivities on phenol and H2O2 are generally in the ranges 90-95% and 80-90%, respectively. The hydroquinone to catechol ratio is well in excess of the statistical value of 1 2, owing to lower steric requirements for / -hydro-xylation and the faster diffusion of the p-substituted product (Table 2). Yields and kinetics are strictly related to the content of lattice Ti. It should be emphasized that any extra-framework Ti species, present as impurities on TS-1, are the major source of unproductive side reactions, such as H2O2 decomposition and unselective radical chain oxidations. 

Catalyst o-Jp- ratio Conversion (% phenol) Yields (% on H2O2) Yields (% on phenol) 

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New materials are also finding application in the area of catalysis reiated to the Chemicals industry. For example, microporous [10] materials which have titanium incorporated into the framework structure (e.g. so-calied TS-1) show selective oxidation behaviour with aqueous hydrogen peroxide as oxidizing agent (Figure 5). Two processes based on these new catalytic materials have now been developed and commercialized by ENl. These include the selective oxidation of phenol to catechol and hydroquinone and the ammoxidation of cyclohexanone to e-caproiactam. 

In phenol hydroxylation, remarkable selectivities to single products have been achieved using vanadium heteropolyacid catalysts.485 The use of the ZSM-5 titanium silicalite (TS-1)483 permits the oxidation of phenol to catechol and hydroquinone to be carried out on an industrial scale with a higher selectivity at a greater conversion of substrate that was not previously possible with strong acid catalysts. 

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