Benzene to phenol with hydrogen peroxide

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. 

Direct Oxidation of Benzene to Phenol with Hydrogen Peroxide... 

Balducci, L., Bianchi, D., Bortolo, R., D Aloisio, R., Ricci, M., Tassinari, R. and Ungarelli, R. (2003) Direct oxidation of benzene to phenol with hydrogen peroxide over a modified titanium silicalite. Angew. Chem. Int. Ed., 115, 5087-5090. 

Bianchi, D., Balducci, L., Bortolo, R., et al (2007). Oxidation of Benzene to Phenol with Hydrogen Peroxide Catalyzed by a Modified Titanium Silicalite (TS-IB), Adv. Synth. Catal, 349, pp. 979-986. 

Benzoquinone [106-51-4], C6H402 (quinone) has been reported as a by-product of benzene oxidation at 410—430°C. Benzene can be oxidized to phenols with hydrogen peroxide and reducing agents such as Fe(II) and Ti(II). Frequendy ferrous sulfate and hydrogen peroxide are used (Fenton s reagent), but yields are generally low (12) and the procedure is of limited utility. Benzene has also been oxidized in the vapor phase to phenol in low yield at 450—800°C in air without a catalyst (13). 

To ascertain whether syringyl derivatives IV, V, and VI could be converted to phenoxy radicals under biological conditions, we treated water solutions of the phenols with hydrogen peroxide and peroxidase. All three phenols formed transient green species in contrast to the vanillin compounds (which yielded brown precipitates). The aqueous solutions turned red within a few minutes after initial reaction. If these solutions were quickly frozen in liquid nitrogen a few seconds after the reactants had been mixed, a blue solid was obtained. When the solid was examined by EPR spectrometry, radicals were observed whose half-lives were estimated at 30-60 seconds. The radicals appeared to be identical to those generated in benzene. 

In this work, catalysts containing iron supported on activated carbon were prepared and investigated for their catalytic performance in the direct production of phenol fiom benzene with hydrogen peroxide and the effect of Sn addition to iron loaded on activated carbon catalyst were also studied. 

Catalysts were prepared with 0.5, 1.0, 2.0 and 5.0 wt% of iron loaded on activated carbon. Benzene hydroxylation with hydrogen peroxide as oxidant was carried out. The conversion of benzene, selectivity and yield of phenol for these catalysts are shown in Fig. 4. As the weight of loaded metal increased the benzene conversion increased by about 33% but the selectivity to phenol decreased. The yield of phenol that was obtained with S.OFe/AC was about 16%. 

The preparation of iron impregnated activated carbon as catalysts and the catalytic performance of these catalysts were studied in benzene hydroxylation with hydrogen peroxide as oxidant. 5.0Fe/AC catalyst containing 5.0 wt% iron on activated carbon yielded about 16% phenol. The addition of Sn on 5.0Fe/AC catalyst led to the enhancement of selectivity towards phenol. 

Analogous to the acid catalyzed hydroxylation of phenol on H-ZSM-5 with hydrogen peroxide [92], benzene and chlorobenzene can be oxidized with dinitrogen oxide on HZSM-5 [125-126]. In the hydroxylation of benzene, the selectivity for phenol is high at conversions below 10%, with selectivities in N2O of about 30%. A small amount of ortho-diphenol or catechol was formed. The hydroxylation of chlorobenzene was also ortho-selective (58%). The reaction was proposed to proceed via scheme 7. 

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 reaction of hydrogen peroxide with copper(I) salts produces a Fenton-like hydroxylating system involving reactive hydroxyl radical intermediates (equation 265).486,491 Hydroxylation of benzene to phenol can be achieved by air in the presence of copper(I) salts in an acidic aqueous solution.592 593 This reaction is not catalytic (phenol yields are ca. 8% based on copper(I) salts) and stops when all copper(I) has been oxidized to copper(II). A catalytic transformation of benzene to phenol can occur when copper(II) is electrolytically reduced to copper(I) (equation 266).594,595... 

Let us consider the reaction of benzene oxidation with hydrogen peroxide in the Fenton system as the classical situation [30], In the absence of iron ions benzene does not in practice interact with H202. The addition of bivalent iron salt to the system C6H6-H202-H20 induces benzene oxidation to phenol and diphenyl according to the following mechanism ... 

In 1983, Mimoun and co-workers reported that benzene can be oxidized to phenol stoichiometrically with hydrogen peroxide in 56% yield, using peroxo-vana-dium complex 1 (Eq. 2) [20]. Oxidation of toluene gave a mixture of ortho-, meta-and para-cresols with only traces of benzaldehyde. The catalytic version of the reaction was described by Shul pin[21] and Conte [22]. In both cases, conversion of benzene was low (0.3-2%) and catalyst turned over 200 and 25 times, respectively. The reaction is thought to proceed through a radical chain mechanism with an electrophilic oxygen-centered and vanadium-bound radical species [23]. 

Fenton reagent generated in situ — Indirect electrochemical oxidation of aromatic compounds (e.g., benzene to phenol) proceeds with the Fenton reagent generated in situ electrochemically at the cathode by the reduction of ferric to ferrous salt and by the reduction of oxygen to hydrogen peroxide... 

The direct oxidation of benzene to phenol is usually affected by a poor selectivity due to the lack of kinetic control. Indeed, phenol is more reactive towards oxidation than benzene itself and consecutive reactions occur, with substantial formation of overoxidized products like catechol, hydroquinone, benzoquinones and tars. This is the usual output of the oxidation of aromatic hydrocarbons by the classical Fenton system, a mixture of hydrogen peroxide and an iron(II) salt, usually ferrous sulfate, most often used in stoichiometric amounts [8]. 

The further oxidation of phenol may also result in the formation of catechol, C,iH4(OH) (1 2). The transformation may be effected by fusion with sodinm hydroxide.85 The snbstance may also be obtained by oxidizing benzene with hydrogen peroxide in the presence of ferrous sulfate88 and by reducing o-benzoquinone with aqueous sulfurous acid in the cold.81 Quinol may be prepared from phenol by oxidation with potassium persulfate in alkaline solution.38 It can also be obtained directly from benzene by the electrolytic oxidation of an alcohol solution to which... 

The need for a process for the direct conversion of benzene to phenol was mentioned in the foregoing. A polymer-supported vanadyl chelate has been used (at 1 mol%) to obtain phenol (100% selectivity at 30% conversion) by treatment with 30% hydrogen peroxide.209 There was no leaching of the metal ion. The catalyst could be recycled ten times before it started breaking up. Further work with an inorganic support might allow the development of a commercial process. 

The desire to convert benzene directly to phenol with 30% hydrogen peroxide was mentioned in Chap. 4. A polymer-supported salicylimine vanadyl complex (1 mol%) was used to catalyze this reaction. Phenol was obtained in 100% yield at 30% conversion.217 There was no leaching of the metal. The catalyst was recycled ten times after which it started to break up. Oxidation of ligands is often a problem with oxidation catalysts. Inorganic supports not subject to such oxidation need to be tried to extend the life of such catalytic agents. 

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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