Hydroxylation, aromatics phenol oxidation, hydrogen peroxide

Besides a variety of other methods, phenols can be prepared by metal-catalyzed oxidation of aromatic compounds with hydrogen peroxide. Often, however, the selectivity of this reaction is rather poor since phenol is more reactive toward oxidation than benzene itself, and substantial overoxidation occurs. In 1990/91 Kumar and coworkers reported on the hydroxylation of some aromatic compounds using titanium silicate TS-2 as catalyst and hydrogen peroxide as oxygen donor (equation 72) . Conversions ranged from 54% to 81% with substituted aromatic compounds being mainly transformed into the ortho-and para-products. With benzene as substrate, phenol as the monohydroxylated product... 

The incorporation of vanadium(V) into the framework positions of silicalite-2 has been reported by Hari Prasad Rao and Ramaswamy . With this heterogeneons oxidation catalyst the aromatic hydroxylation of benzene to phenol and to a mixtnre of hydroqninone and catechol conld be promoted. A heterogeneons ZrS-1 catalyst, which has been prepared by incorporation of zirconinm into a silicalite framework and which catalyzes the aromatic oxidation of benzene to phenol with hydrogen peroxide, is known as well in the literature. However, activity and selectivity were lower than observed with the analogous TS-1 catalyst. 

Hydroxylation of benzene to phenol using hydrogen peroxide in the presence of heteropoly compounds was observed in this study. Other aromatic hydrocarbons that were used as substrates were toluene, ethylbenzene, o,p-xylene and isopropyl benzene under homogeneous conditions. Both side chain oxidation and ring hydroxylation were observed in presence of hydrogen peroxide. For example, toluene gave benzyl alcohol, benzaldehyde, o,p-cresols in presence of hydrogen peroxide, whereas benzaldehyde and benzyl alcohol were observed in presence of t-BuOOH. 

Peracids have found wide application in the oxidation of aromatic species. In situ peracids have been used in the presence of strong acids to affect the hydroxylation of phenol in a similar manner to the method using hydrogen peroxide. Trifluoroacetic acid has also been shown to be highly effective for the... 

The hydroxylation of the aromatic nucleus by hydroxyl radicals, generated by decomposition of hydrogen peroxide in the presence of iron(II) ions, may be applied to the electrochemical synthesis of phenol fiom benzene, since the concentration of the iron(II) ions can be controlled by the cathodic reduction of iron(III) ions formed by oxidation of iron(II) ions wiA H2O2. 

A promising and cleaner route was opened by the discovery of titanium silica-lite-1 (TS-1) [1,2]. Its successful application in the hydroxylation of phenol started a surge of studies on related catalysts. Since then, and mostly in recent years, the preparation of several other zeolites, with different transition metals in their lattice and of different structure, has been claimed [3]. Few of them have been tested for the hydroxylation of benzene and substituted benzenes with hydrogen peroxide. Ongoing research on suppoi ted metals and metal oxides has continued simultaneously. As a result, knowledge in the field of aromatic hydroxylation has experienced major advances in recent years. For the sake of simplicity, the subject matter will be ordered according to four classes of catalyst medium-pore titanium zeolites, large-pore titanium zeolites, other transition metal-substituted molecular sieves, and supported metals and mixed oxides. 

The hydroxylation of benzene on TS-1 produces phenol as the primary product. Conversion is generally kept low, because introduction of a hydroxyl group activates the aromatic nucleus to further oxidation to hydroquinone, catechol, and eventually to tarry products (Eq. 2). Acetone, methanol, 2-butanone or just water are suitable reaction media [2,16,17]. In aqueous solution, benzoquinone was also found, in appreciable amounts, among the products. Hydroxylation of benzene with a mixture of hydrogen and oxygen, an in situ source of hydrogen peroxide, can be achieved on Pd-containing TS-1 [18]. This is, in principle, an easier route to phenol than that based on the preformed oxidant [19]. In practice, it proved less effective, because of faster catalyst decay (maximum TON... 

Direct oxidation or hydroxylation of aromatics and particularly alkylbenzenes, is a complex reaction consisting of a variety of parallel and consecutive reactions. Low temperature procedures involving hydrogen peroxide as the oxidant have been reported. They include hydroxylation over Ti-silicalites TS-1 d TS-2. Perego et al. [1] reported that toluene and phenol could be oxidized at 80 C into a mixture of cresols and dihydroxybenzenes respectively. More recently, Hari Prasad Rao et al. observed that phenol [2] and toluene [3] could also be oxidized over VS-2, the vanadium substituted silicalite-2. 

Another metallocene, namely, decamethylosmocene, (Mc5C5)20s (catalyst 1.2), turned out to be a good precatalyst in a very efficient oxidation of alkanes with hydrogen peroxide in acetonitrile at 20 — 60 °C [9]. The reaction proceeds with a substantial lag period that can be reduced by the addition of pyridine in a small concentration. Alkanes, RH, are oxidized primarily to the corresponding alkyl hydroperoxides, ROOH. TONs attain 51,000 in the case of cyclohexane (maximum turnover frequency was 6000 h ) and 3600 in the case of ethane. The oxidation of benzene and styrene afforded phenol and benzaldehyde, respectively. A kinetic study of cyclohexane oxidation catalyzed by 1.2 and selectivity parameters (measured in the oxidation of n-heptane, methylcyclohexane, isooctane, c -dimethylcyclohexane, and trans-dimethylcyclohexane) indicated that the oxidation of saturated, olefinic, and aromatic hydrocarbons proceeds with the participation of hydroxyl radicals. 

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