Process for the hydroxylation of aromatic hydrocarbons

The invention relates to a process for the hydroxylation of aromatic hydrocarbons by direct oxidation with hydrogen peroxide. The process is carried out in the presence of a catalyst comprising: PA1 iron, administered as an inorganic salt; PA1 a carboxylic acid of an aromatic compound containing nitrogen, in particular a pyrazin-2-carboxylic acid or derivative; PA1 acidifying agent, especially trifluoracetic acid, and a solvent system comprising an organic phase consisting of a substrate and acetonitrile and an aqueous phase containing the catalyst and hydrogen peroxide.

The invention relates to a process for the hydroxylation of aromatic 
hydrocarbons by direct oxidation of hydrogen peroxide. 
More specifically, it relates to a process for the preparation of phenols 
in which the oxidation of the substrate with hydrogen peroxide is carried 
out in the presence of a particular catalytic system containing iron, an 
iron ligand and an acidifying agent and a biphasic solvent system. 
The invention also relates to the catalyst used for the oxidation. 
The production of phenol based on the hydroxylation of benzene has been 
studied since the 70's. 
The considerable number of publications on the matter is indicative of the 
efforts made in research in this field. In particular, research has been 
based on the use of transition metals and relative complexes as catalysts 
in the oxidation of hydrocarbons. (Ed. D. H. R. Barton et al., Plenum, New 
York, 1993; G. B. Shul'pin et al., J. Chem. Soc. Perkin Trans, 1995, 
1459). 
Although illustrating the catalytic capacities of these compounds, the 
articles show how the low conversion of the substrate, the presence of 
undesired by-products and the low selectivity of the oxidating agent make 
industrial embodiment of direct oxidation reaction unsatisfactory. 
The critical points indicated mainly concern the oxidating agent, the 
catalyst (metal-ligand complex) and the solvent system. 
Hydrogen hydroperoxide is considered as being one of the most promising 
among oxidating reagents owing to its low cost and its capacity of 
producing only water as by-product. 
It is, in fact, widely used in oxidation reactions of alkanes, 
alkyl-aromatic compounds and arenes carried out in the presence of 
catalysts consisting of complexes of transition metals. (R. A. Sheldon et 
al., "Metal-Catalyzed Oxidations of Organic Compounds" Academic Press, New 
York, 1981; G. B. Shul'pin et al., J. Cat. 1993, 142, 147). 
The solvent system is the basis of the control not only of the yields but 
also the ratios between the products obtained. 
The catalyst regulates the reaction rate. 
Its activity is influenced by the metal and posssible ligand of which it is 
composed. 
Among the reaction systems which have been developed for the oxidation of 
hydrocarbons, the Fenton and Gif systems are the most widely studied. The 
Fenton system, consisting of Fe.sup.II /H.sub.2 O.sub.2 in water at pH 2, 
is based on the production of the hydroxyl radical which seems to be the 
active oxidative species (J. Stubble et al., Chem. Rev., 1987, 87, 1107). 
The reaction is exploited to oxidate aromatic hydrocarbons; in the case of 
benzene the hydroxyl radical directly attacks the aromatic ring with the 
consequent formation of phenol. 
This reaction is accompanied by other collateral reactions with a 
consequent decrease in the selectivity with respect to the phenol. 
The formation of undesired products, such as biphenyl or polyhydroxylated 
compounds which tend to polymerize (via formation of quinones), form a 
definite limit of this system for the production of phenol, for the 
purposes of industrial development. 
The Gif system like the Fenton system comprises the use of a catalyst based 
on iron and H.sub.2 O.sub.2 as oxidating agent. The characteristic element 
of this system is the mixture of pyridine and acetic acid used as solvent. 
It is generally used for the conversion of saturated hydrocarbons in 
ketones (H. R. Barton et al., J. Am. Chem. Soc. 1992, 114, 2147; C. Sheu 
et al., J. Am. Chem. Soc. 1990, 112, 1936). 
The role of pyridine as blocking agent of the hydroxylic radicals which are 
formed in the reaction medium, is considered fundamental for overcoming 
the limits relating to the Fenton system. In general, with the different 
substrates used, such as cyclohexane and adamantane, the use of iron in 
the form of simple salts is less effective than in the form of complex 
salts. In these complexes the iron is in the presence of ligands of the 
picolinic acid type which is by far the most widely used. 
In spite of the various studies carried out, the effective nature of the 
catalyst in solution and the actual role of the ligand is still not clear 
(D. H. R. Barton et al., Tetrahedron Lett. 1996, 37,1133). 
The importance of the solvent system is evident from the investigations of 
Sheu et al. in which substitution with acetonitrile of the pyridine/acetic 
acid system determines a reduction in the efficiency and selectivity (C. 
Sheu et al., J. Am. Chem. Soc. 1990, 112, 1936). In this context 
subsequent data in literature show that acetonitrile can be an effective 
solvent provided the presence of pyridine is ensured in adequate 
quantities (D. H. R. Barton et al., Tetrahedron Lett. 1996, 37, 8329). 
Experiments carried out by Menage et al. show that benzene is not oxidated 
in this system (S. Menage et al., J. Mol. Cat. 1996, 113, 61). 
A process has now been found for the preparation of phenols by the direct 
oxidation of an aromatic substrate which enables much higher selectivity 
values of hydrogen peroxide and conversions of the substrate and 
productivity to be obtained with respect to the processes described of the 
known art. 
In particular the present invention relates to a process for the 
preparation of phenols having the formula: 
##STR1## 
wherein R is a group selected from hydrogen, a C.sub.1 -C.sub.8 linear or 
branched alkyl, a C.sub.1 -C.sub.8 alkoxy, a halogen, a carbonate, nitro, 
by the direct oxidation with hydrogen peroxide of an aromatic compound 
having the formula: 
##STR2## 
wherein R has the meaning previously defined, characterized in that the 
oxidation reaction is carried out in the presence of a catalytic system 
and a solvent system consisting of an organic phase made up of the 
aromatic compound and acetonitrile and an aqueous phase containing the 
catalytic system and hydrogen peroxide. 
The invention also relates to the catalyst used for the oxidation. 
The double phase of which the solvent consists, extracts the phenol from 
the aqueous phase, where the reaction takes place, reducing the 
possibility of subsequent oxidations. 
The reaction system used determines a high conversion of the aromatic 
hydrocarbon, of up to 15%, and a high selectivity with respect to the 
hydrogen peroxide, of up to 90%. 
These values are much higher than those quoted in literature. 
In the reaction system of the present invention, the phenols are produced 
by the direct oxidation in liquid phase of the aromatic hydrocarbon with 
hydrogen peroxide in the presence of a catalyst based on iron. 
The catalyst can be administered both as Fe.sup.+2 and as Fe.sup.+3 in the 
form of chloride, sulfate, nitrate or perchlorate, preferably as sulfate 
and more preferably as FeSO.sub.4 *7H.sub.2 O. 
Iron ligands which can be used are carboxylic acids of heteroaromatic 
compounds containing nitrogen, such as picolinic acid, dipicolinic acid, 
isoquinolin-1-carboxylic acid, pyrazin-2-carboxylic acid, 
5-methylpyrazin-2-carboxylic acid N oxide, preferably pyrazin-2-carboxylic 
acid and its N oxide. 
The acidifying agent can be either an inorganic acid, such as sulfuric acid 
or an organic acid, such as p-toluene-sulfonic acid, methane-sulfonic 
acid, pyrazin-2-carboxylic acid and trifluoracetic acid, preferably 
trifluoracetic acid. 
The liquid phase consists of a double phase in which the organic phase is 
made up of the aromatic hydrocarbon and an organic solvent, the aqueous 
phase contains the catalyst and hydrogen peroxide. 
Acetonitrile has proved to be the most effective organic solvent; for the 
purposes of the yield and selectivity it is considered a determinant 
component for the particular reaction system. 
The volume ratio in the organic phase between the aromatic substrate and 
organic solvent can be between 1 and 6, preferably close to 5. 
The organic phase and the aqueous phase can be distributed according to 
different volumetric ratios; it is preferable to have a ratio of the 
phases close to 1 in the reaction mixture. 
The reagents, aromatic hydrocarbon and hydrogen peroxide can be present in 
the reaction mixture according to a molar ratio of between 10 and 3; it is 
preferable to have a ratio approximate to 10. 
For the formation of the catalyst the molar ratio between the iron and 
ligand can be between 2 and 5, preferably 4; the molar ratio between the 
acid and iron can vary from between 6 and 2, and is preferably approximate 
to 2. 
For the activation of the hydrogen peroxide the molar ratio between H.sub.2 
O.sub.2 and the iron can be between 20 and 100, preferably 50. 
The reaction temperature can be between 40 and 80 degrees centigrade, 
preferably 70. 
The reaction time, under the best conditions, can be between 3 and 15 
minutes. 
The recovery of the phenol and the possibility of recycling the catalyst 
are particularly facilitated in this reaction system. 
The normal physico-chemical techniques can be used for recovering the 
phenol from the organic phase in which it is present in about 90%. Due to 
the removal of the product, it is possible to recycle the catalyst 
situated in the aqueous phase.