Hydroxylation of Benzene and Substituted Benzenes

Compared with phenol, benzene and substituted benzenes have received less attention. Their oxidation was occasionally mentioned, with few details, in studies mostly focusing on other aspects of TS-1 chemistry. Only recently have benzene and alkylbenzenes provoked sufficient interest to become the main subject of a few papers. 

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 

The oxidation of hydrocarbon compounds in aqueous hydrogen peroxide, without added organic solvent (triphase catalysis solid TS-1 + two immiscible liquids) has been found to be faster than in organic solution (biphase catalysis solid TS-1 -I- one liquid). Similarly, the initial rate of epoxidation of propylene was also the highest when conducted in totally aqueous hydrogen peroxide (M. G. Clerici, unpublished results). 

This specific aspect, i. e. the omission of organic solvent from the reaction mixture, has been the subject of recent detailed investigations on the hydroxylation of aromatic compounds [20-23]. According to Kumar et al., the hydroxylation of 

The hydrophobicity index HI), introduced by Weitkamp and coworkers, is most useful in this context [24]. According to the HI, benzene is expected to compete more favorably with water than with acetonitrile, acetone, or methanol for adsorption in TS-1 micropores, thus maximizing under triphase catalysis the probability of interaction with active sites. This interpretation is supported by competitive adsorption experiments which revealed that the amount of adsorbed benzene in the system TS-l-benzene-H20 was almost ten times greater than for TS-l-benzene-CH3CN [22]. 

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Aromatic hydrocarbons can be oxidized to the corresponding phenols by transition metal peroxo complexes and, in particular, vanadium(V) peroxo complexes, which act either as electrophilic oxygen transfer reagents or as radical oxidants -, depending on the nature of the ligands coordinated to the metal and on the experimental conditions. Vanadium picolinato peroxo complex (V0(02)PIC(H20)2) (39) (PIC = picoUnic acid anion) has been reported to be particularly effective in the hydroxylation of benzene and substituted benzenes (equation 50) . Accordingly, 39 smoothly oxidizes substituted benzenes 38 to the corresponding monophenols 40 in acetonitrile at room temperature. 

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. 

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