Several processes are described in the art for the hydroxylation of phenol to hydroquinone and catechol using hydrogen peroxide as the oxidant and transition metals as catalysts. In the prior art, catalytic conversion of phenol to catechol and hydroquinone has been achieved by the following procedures.
European patent 0266825 (1988), 0265018; U.S. Pat. No. 4,396,783 (1983), 5493061 (1996), UK patent No. 2116974; Japanese patent No. JP 10291948, JP 2001158756, Chinese patent CN 1268502, CN 1129607 all disclose various methods for the use of titanium silicate molecular sieves as catalysts for the hydroxylation of phenol with aq. H2O2.
Chinese Patent CN 1125642 describes hydroxylation of phenol using Y-zeolite containing transition metals such as Cu, Mn, Fe, Cr, Co with cyclic ligands. Use of metal oxides (including transition, alkali and alkaline earth metals) as catalysts for the oxidation of phenol with H2O2 in synthesizing benzenediol is reported in Chinese patent No. CN 1134313.
In a continuous process for dihydric phenols using peroxides, Japanese patent JP 55069529 reports 82% conversion of phenol to catechol and hydroquinone with (CH3)2HPO4 as catalyst whereas German patent DE 2638559 reports 9% conversion using peracid in the presence of acetylacetone.
European patent [EP C07CO39-08, C07CO37-60, C07CO37-82] claims 55% catechol, 34% hydroquinone by hydroxylation of preheated phenol with propionic acid using ion-exchange resin. A Chinese patent [CN 1167012] reports that the hydroxylation of phenol with aq. H2O2 has been achieved in low conversion by using nano metal oxide particles and a microporous ion exchanged resin. The reaction of phenol with organic solutions of peroxycarboxylic acids in the presence of chelating agents such as malonic, glutamic, citric or tartaric acid to yield catechol and hydroquinone has been reported in a Brazilian patent
German patent DE 2658545 describes phenol hydroxylation using H3PO4 and HClO4 as catalyst in the presence of benzaldehye to give hydroquinone and catechol. German patent DE 26330302 describes phenol hydroxylation with H2O2 in CF3SO3H containing a small amount of H3PO4 giving 51% hydroquinone and 23.7% catechol. Slovakian patents [SK 278582 and SK 278569] describe the preparation of benzoquinones by the oxidation of phenols with oxygen in the presence of 2,2′ bipyridine-Cu complex in acetonitrile.
Synthesis of 1,4-benzoquinone in 95% yield has been achieved using MnO2 in the presence of aniline. [Zh. Org. Khim. 1990, 26, 2460]. Catalytic oxidation of phenol to p-benzoquinone is reported by using cobalt Schiff base complexes [Fenzi Cuihua 1990, 4, 306 (Chinese)]. The oxidation of phenol in electrochemical reactor with modulated AC voltage produced benzoquinone (43% yield) [J. Appl. Electrochem. 1989, 19, 459].
The abovementioned processes in the prior art are known to be useful for the conversion of phenol to quinones, catechol and hydroquinone. However they suffer from the following drawbacks:    1. Oxidation of phenol with Y-zeolites containing transition metals such as Cu, Mn, Fe, Cr, etc. makes use of expensive cyclic ligands for effective conversion of phenol to hydroquinone; separation of these cyclic ligands is also tedious.    2. H3PO4, and HClO4, which are corrosive and environmentally hazardous, are used for the oxidation of phenol.    3. Use of sulfonic acid and peroxy carboxylic acids often results in mixture of catechol and hydroquinone with poor selectivity, which makes the separation of products more difficult.    4. Use of benzaldehye adds to the cost of the process and also generates benzoic acid as the side product. This leads to an additional raw material consumption and separation issue.    5. Use of Cu with 2,2-bipyridine or phenanthroline ligands in CH3CN is a homogeneous process in which catalyst separation from the product is tedious and the catalyst cannot be reused.    6. MnO2 is used as the oxidant in presence of aniline; here again the process is homogeneous wherein the separation of product becomes difficult.    7. Co-Schiff base complexes are also used under the homogeneous conditions. The ligands have to be prepared using multi step reaction sequences.    8. In addition to the desired products (quinones and catechol) significant amounts of heavy oxidation products known as tar are also formed in all the processes.
In view of all the above disadvantages of the prior act, it is desirable to provide a process that is safe, inexpensive, heterogeneous and simple to perform.