Phenols catalytic hydroxylation

Table 11. Catalytic hydroxylation of n-octane, 1-hexene and phenol with aqueous H2O2...

Stack and co-workers recently reported a related jx-rf / -peroxodi-copper(II) complex 28 with a bulky bidentate amine ligand capable of hydroxylating phenolates at - 80 °C. At - 120 °C, a bis(yu,-oxo)dicopper(III) phenolate complex 29 with a fully cleaved 0-0 bond was spectroscopically detected (Scheme 13) [190]. These observations imply an alternative mechanism for the catalytic hydroxylation of phenols, as carried out by the tyrosinase metalloenzyme, in which 0-0 bond scission precedes C - 0 bond formation. Hence, the hydroxylation of 2,4-di-tert-butylphenolate would proceed via an electrophilic aromatic substitution reaction. 

The mechanism of the catalytic hydroxylation of aromatic hydrocarbons by hydrogen peroxide has been reviewed (ref.8). The hydroxylation of benzene or toluene by peroxydiphosphate (ref.9) and peroxydisulphate, (more comnronly termed persulphate) (ref. 10) in aqueous (0.05-1.OM) acid in the presence of Cu(ll) has been found to be similar. Phenol (15.6%), diphenyl (5.2%), and 2-and 4-nitrophenols (11.7% and 5.6% respectively) resulted from a mixture of benzene and nitrobenzene in water at 80 C with peroxydisulphate. 

Catalytic hydroxylation of phenol by hydrogen peroxide. Kinetic study and comparison between solid acids and titanosilicates. 

Table 4.Catalytic hydroxylation of phenol over iM(lt)Al binary hydrotalcites...

Allian, M.. Germain. A., Cseri, T. and Figueras, F. (1993). Catalytic hydroxylation of phenol by hydrogen peroxide. Kinetic study and comparison between solid acids and litanosilicatcs. Stud. Surf. Sci. Catal., 78. 455-462. 

Rives, V., Dubey, A. and Kannan, S. (2001). Synthesis, characterization and catalytic hydroxylation of phenol over CuCoAl ternary hydrotalcites. Phys. Chem. Chem Phys., 3,4826-36. 

Ray, S., Mapolie, S.F., Darkwa, J. 2007. Catalytic hydroxylation of phenol using immobilized late transition metal salicylaldimine complexes. Journal of Molecular Catalysis A Chemical 267(1-2) 143-148. 

To satisfy environmental protection requirements and to meet the inereasing demand for phenol, a considerable effort has been devoted to the development of effective one-step processes for catalytic hydroxylation of benzene to phenol [11, 58, 59]. 

Liu CB, Zhao Z, Yang XG, Ye XK, Wu Y. (1996). Superconductor mixed oxides La2-xSrxCu04-l-/-lambda for catalytic hydroxylation of phenol in the liquid-solid phase. Chem Commun. 1019-20. 

During the 1980s few innovations were disclosed in the Hterature. The hydroxylation of phenol by hydrogen peroxide has been extensively studied in order to improve the catalytic system as well as to master the ratio of hydroquinone to catechol. Other routes, targeting a selective access to one of the dihydroxyben2enes, have appeared. World production capacities according to countries and process types are presented in Table 1. 

Starting from Benzene. In the direct oxidation of benzene [71-43-2] to phenol, formation of hydroquinone and catechol is observed (64). Ways to favor the formation of dihydroxybenzenes have been explored, hence CuCl in aqueous sulfuric acid medium catalyzes the hydroxylation of benzene to phenol (24%) and hydroquinone (8%) (65). The same effect can also be observed with Cu(II)—Cu(0) as a catalytic system (66). Efforts are now directed toward the use of Pd° on a support and Cu in aqueous acid and in the presence of a reducing agent such as CO, H2, or ethylene (67). Aromatic... 

The hydroxyl group of the resulting phenol is situated immediately adjacent to where the carboxyl group was previously located. This same Hquid-phase copper oxidation process chemistry has been suggested for the production of cresols by the oxidation of toluic acids. y -Cresol would be formed by the oxidation of either ortho or para toluic acids a mixture of 0- and -cresols would be produced from y -toluic acid (6). A process involving the vapor-phase catalytic oxidation of benzoic acid to phenol has been proposed, but no plants have ever been built utilizing this technology (27). 

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. 

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. 

The oxidation of phenol, ortho/meta cresols and tyrosine with Oj over copper acetate-based catalysts at 298 K is shown in Table 3 [7]. In all the cases, the main product was the ortho hydroxylated diphenol product (and the corresponding orthoquinones). Again, the catalytic efficiency (turnover numbers) of the copper atoms are higher in the encapsulated state compared to that in the "neat" copper acetate. From a linear correlation observed [7] between the concentration of the copper acetate dimers in the molecular sieves (from ESR spectroscopic data) and the conversion of various phenols (Fig. 5), we had postulated [8] that dimeric copper atoms are the active sites in the activation of dioxygen in zeolite catalysts containing encapsulated copper acetate complexes. The high substratespecificity (for mono-... 

This reaction was first demonstrated over V, Mo and W oxides [6]. At 823 K vanadium oxide provided phenol selectivity up to 71%, which was much higher than it had been ever achieved with O2. This result stimulated further efforts in searching for more efficient catalytic systems. As a result, in 1988 three groups of researchers [7-9] have independently discovered ZSM-5 zeolites to be the most efficient catalysts. They allowed the reaction to proceed at much lower temperature (573-623 K) with nearly a 100% selectivity. Later, more complex aromatic compounds were also hydroxylated in this way [2]. 

Crystallite dimensions play a role in determining the rates of reactions and their control is of fimdamental importance not only for the catalytic activity, but also for the selectivity, since, with low rates of the desired reaction, the relative importance of secondary reactions may be greater. The effects of crystallite dimensions have been demonstrated for 1-butene epoxidation and for phenol hydroxylation, and they are significant for many reactions carried out with hquid phase reactants [17]. 

The TS-1 catalysed hydroxylation of phenol to a 1 1 mixture of catechol and hydroquinone (Fig. 2.16) was commercialized by Enichem (Romano et ai, 1990). This process offers definite advantages, such as higher selectivities at higher phenol conversions, compared to other catalytic systems. It also illustrates another interesting development the use of solid, recyclable catalysts for liquid phase (oxidation) processes to minimize waste production even further. 

One of the exciting results to come out of heterogeneous catalysis research since the early 1980s is the discovery and development of catalysts that employ hydrogen peroxide to selectively oxidize organic compounds at low temperatures in the liquid phase. These catalysts are based on titanium, and the important discovery was a way to isolate titanium in framework locations of the inner cavities of zeolites (molecular sieves). Thus, mild oxidations may be run in water or water-soluble solvents. Practicing organic chemists now have a way to catalytically oxidize benzene to phenols alkanes to alcohols and ketones primary alcohols to aldehydes, acids, esters, and acetals secondary alcohols to ketones primary amines to oximes secondary amines to hydroxyl-amines and tertiary amines to amine oxides. 

We emphasize that the above results have been observed only in the oxidation of sulfides and phenols, reactions known to follow radical mechanisms. A thorough investigation of the catalytic potential of the materials in other oxidation reactions (epoxidation, hydroxylations, etc.) is warranted. 

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