Hydroxylation benzene

The conversion of benzene to phenol on an industrial scale is realized via the Cumene Process, which is based on the facile autoxidation of cumene (1) to phenol and acetone [1-3]. 

There is considerable interest in the direct conversion of benzene to phenol to decrease the number of steps and to avoid the acetone by-product which could become a serious drawback in the future. Fenton type reagents utilizing are the most suitable for effecting this 

The search for catalysts capable of effecting direct aromatic hydroxylation has been motivated by interest in the modeling of monooxygenase enzymes of both heme and non-heme type [5,6]. The active species in working models may be the hydroxyl radical [7] or oxenoid (oxometal) species [8-10]. 

The nickel(II) complex of a macrocyclic pentaamine (2) catalyzes the oxygenation of benzene to phenol at room temperature in borate buffer [11]. Tracer studies show that the phenolic 0-atom is derived entirely from dioxygen. The formation of a Ni -0 type superoxo 

Fenton type systems, producing OH radicals have been extensively used for hydroxylating aromatic rings. 

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Benzene hydroxylation to Phenol with Iron impregnated Activated carbon Catalysts... 

Key words Activated carbon. Benzene hydroxylation, Phenol, Transition metal 1. INTRODUCTION... 

Phenol is the starting material for numerous intermediates and finished products. About 90% of the worldwide production of phenol is by Hock process (cumene oxidation process) and the rest by toluene oxidation process. Both the commercial processes for phenol production are multi step processes and thereby inherently unclean [1]. Therefore, there is need for a cleaner production method for phenol, which is economically and environmentally viable. There is great interest amongst researchers to develop a new method for the synthesis of phenol in a one step process [2]. Activated carbon materials, which have large surface areas, have been used as adsorbents, catalysts and catalyst supports [3,4], Activated carbons also have favorable hydrophobicity/ hydrophilicity, which make them suitable for the benzene hydroxylation. Transition metals have been widely used as catalytically active materials for the oxidation/hydroxylation of various aromatic compounds. 

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. 

Catalytic activity in benzene hydroxylation ofTS-1 samples (Si/Ti = 33 particle size = 0.2-0.3 p,m) prepared by different methods... 

It was reported independently by three research groups that MFI-type zeolites selectively catalyze the reaction of N20 with benzene to give phenol C6H6 + N20 —> C6H5OH + N2 [93-96]. Fe/ZSM-5 shows remarkable performance in benzene hydroxylation to phenol with N20 as oxidant, which is the first example of a successful gas phase direct phenol synthesis from benzene [97]. No other catalysts show similar high performances to the Fe/ZSM-5 catalyst. At present, iron is the sole element capable of catalyzing the benzene-to-phenol reaction [98]. Direct oxidation of benzene to phenol by N20 has been commercialized in the so-called AlphOx process in Solutia Inc., US A, where N20 is obtained as a by-product in adipic acid production with nitric acid [97, 99, 100] a selectivity >95% to phenol is achieved at >40% conversion at around 4000 C. But the process is cost-effective only if N20 can be obtained cheaply as a by-product in adipic acid production. 

In this connection, of greatest interest are catalytic systems based of Fe3+ complexes (the Hamilton system) with phenol, pyrocatechol, hydroquinone, etc. These compounds provide for higher yields at benzene hydroxylation in the Hamilton system compared with... 

For benzene hydroxylation an analytical system [37] was successfully used at the interface. This system contains Fe3+ hydrophobic complexes, which promote the process intensification. It is shown [38, 39] that compared with hydrophobic complexes, Fe3+ complexes with the phase transfer—tertiary ammonium salts and crown ethers—display more effective action. At 20-50 °C, owing to the use of trimethylacetylammonium bromide as the phase transferring agent, benzene is successfully hydroxylated in the two-phase water-benzene system in the presence of Fe3+ ions [40], Hence, it is Shilov s opinion [41] that in the case of cytochrome P-450 a radical reaction is probable. It produces radicals, which then transform in the cell, as follows ... 

The hydrophobicity of TS-1 could also explain why the oxidation of hydrocarbons in aqueous H2C>2 is faster without added organic solvent (triphase catalysis) than in organic solution (biphase catalysis) e.g. benzene hydroxylation under triphase conditions was up to 20 times faster than in acetonitrile or acetone (biphase conditions).1741 Indeed, benzene competes more favourably with water than with organic solvents for adsorption within the micropores of hydrophobic TS-1, as furthermore confirmed through adsorption experiments.1471... 

In all cases, some of the V was released into solution this amount can be decreased by cosupporting Cu on the catalyst (50). Kumar et al. (51) used a polymer-bound Schiff base to chelate V02+ the resulting polymer was active for benzene hydroxylation with H2O2 ... 

Benzene hydroxylation to give phenol has been performed with Mo-substituted mesoporous silicas and H2O2 in the absence of solvent (267). However, as explained earlier, reports of anchoring of Mo in an inorganic support must be treated with great caution, particularly if there is no clear concept for immobilizing both Mo and peroxo Mo. The same holds true for the Mo silicalite MoS-1, which has been used for sulfide oxidation with H2O2 (268). 

Other chemicals that induce specific isoenzymes of cytochrome P-450 can increase the rate of benzene metabolism and may alter metabolism pathways favoring one over another. Ikeda and Ohtsuji (1971) presented evidence that benzene hydroxylation was stimulated when rats were pretreated with phenobarbital and then exposed to 1,000 ppm of benzene vapor for 8 hours per day for 2 weeks. Additionally, phenobarbital pretreatment increased the rate of metabolism by 40% in rats and 70% in mice (Pawar and Mungikar 1975). In contrast, rats exposed to phenobarbital showed no effects on the metabolism of micromolar amounts (35-112.8 pmol) of benzene in vitro (Nakajima et al. 1985). 

Inoue et al. 1988b). This was due to competitive inhibition of the oxidation mechanisms involved in the metabolism of benzene. Phenobarbital pretreatment of the rats alleviated the suppressive effect of toluene on benzene hydroxylation by the induction of oxidative activities in the liver. This effect has been observed in other studies in rats (Purcell et al. 1990). 

Catalytic activity in benzene hydroxylation (Table LIV), on the other hand, followed the total concentration of the various superoxo species, which increased in the order TS-1 (with anatase) < TS-1 (without anatase) < TS-1 (fluoride). The total concentration of the superoxo species was obtained from the integrated intensity of all the EPR signals representing superoxo species. This intensity in various solvents increases in the order acetone < methanol water. 

Benzene hydroxylation on Ti,Al-MOR was studied by three different groups [57-59]. Despite the spaciousness of its pores, the activity of this catalyst was lower than that ofTS-1, as expected for a more hydrophilic catalyst. Accordingly, increase in the A1 content caused a decrease in the conversion, probably because of reduced adsorption of benzene. 

Table 2.15 summarizes the main reactions of industrial interest in the liquid and vapor phases, the type of catalysts used, conversions and selectivities in the industrial processes. While O2 is the only oxidizing agent in the second sub-class (apart from the recent case of benzene hydroxylation to phenol using N2O as the oxidant), O2 and oxygen transfer agents (alkyl hydroperoxide or H2O2) are used in the liquid phase. 

All zeolite samples were synthesized by the hydrothermal method described in detail in [6]. The experiments were performed in a completely automated laboratory setup including an integrally operated plug flow tubular reactor. Reaction components were analyzed by on-line gas chromatography with FID and TCD [5-7]. Table 1 summarizes the reaction conditions for the benzene hydroxylation on the H-Ga-ZSM5 catalyst. Nitrogen was used as balance. 

Based on these reaction engineering results several of kinetic models were developed. The first step was to develop a power law model describing the experimental data. Especially, the activation energy of benzene hydroxylation to phenol was of particular interest. The following... 

B. Louis, L. Kiwi-Minsker, P. Reuse, A. Renken, ZSM-5 coatings on stainless steel grids in one-step benzene hydroxylation to phenol by N20 Reaction kinetics study, Ind. Eng. Chem. Res. 40 (2001) 1454. 

Polyoxometalate Catalysts on Benzene Hydroxylation with Hydrogen-Peroxide and 2 Reaction Types with and Without an Induction Period J. Mol. Catal. A. 152, 55 (2000). 

Iron-zeolite catalysts present an important type of materials with broad application for selective oxidations (i.e. benzene hydroxylation) and environmentally important processes, like SCR reduction of NOx or N2O decomposition. In the case of SCR reaction they could provide a convenient substitution of the vanadia-based system using environmentally problematic ammonia, by more convenient paraffin as a reducing agent. Unfortunately, the efficiency in utilization of paraffin is inferior in comparison to ammonia, namely due to paraffin nonselective oxidation by oxygen catalyzed by unspecified iron-oxide type species typically present in the iron-zeolite catalysts. The mostly used preparation processes include impregnation from water solutions, ion exchange procedures, both in water solution or solid state, as well as gas phase CVD. 

A feature encountered during benzene hydroxylation is the appearance of both radical and cationic mechanisms. This feature is associated with the stability of the cationic species and will appear in alkane hydroxylation and alkene epoxidation whenever the corresponding radical center has a sufFiciently low ionization energy to transfer an electron to the heme. In such an event, the reactivity will be dominated by the LS state. The result of Newcomb and Toy , which indicates the presence of carbocations may well belong to this category. Such results are in progress. 

De Visser, S.P. and S. Shaik (2003). A proton-shuttle mechanism mediated by the porphyrin in benzene hydroxylation by eytochrome P450 enzymes. J.Am. Chem. Soc. 125, 7413-7424. 

When turning to transition-metal-containing compounds the situation changes somewhat. Yoshizawa et al.40 studied the benzene hydroxylation by iron-oxo species. As an initial study to this, they considered the dissociation energy of FeO+. Whereas the experimental value lies about 81 kcal mol 1, various Cl-like methods yielded values between 25 and 75 kcal mol 1, and a hybrid method gave 75 kcal mol 1. Thus, the error is larger, but the density-functional calculations gave some of the most accurate results. Therefore,... 

As the final example we show in Figure 7 results by Yoshizawa et al.40 on the benzene hydroxylation by FeO+. They found that the energy barriers depend significantly on the spin state of the system, as shown in Figure 7. 

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