Catalysis of metalloporphyrins for selective hydroxylation of phenol by H₂O₂

Abstract

Liquid phase catalytic selective hydroxylation of phenol to catechol and hydroquinone was carried out in the presence of metalloporphyrins using hydrogen peroxide as oxidant and water as solvent. Five kinds of metal tetra(p-chlorophenyl)porphrin (T(p-Cl)PPMCl, M = Fe, Co, Mn, Cu, Zn) were studied. It was found that T(p-Cl)PPFeCl had high catalytic activity and diphenol selectivity for the hydroxylation of phenol to catechol and hydroquinone. The influence of various reaction parameters, namely, reaction temperature, solvent, ratio of substrate and oxidant, substrate concentration, the amount of catalyst, reaction time and pH value were investigated systematically. When water was used as solvent (10 mL), the optimum conditions were following: pH = 7, the concentration of phenol was 0.3 g/mL, the molar ratio of phenol and H 2 O 2 was 1/2, the molar ratio of catalyst and phenol was 7/100000, the reaction temperature was 65 °C and the reaction time was 1.5 h. Under above optimum conditions, the phenol conversion was up to 55.1%, and the selectivity of diphenol was almost up to 99.9%, the molar turnover numbers of the catalyst was about 7500. A possible mechanism was also proposed.

Enzyme‐Like Selectivity on Metalloporphyrin‐Catalyzed Oxidation by a Linear Homopolymer

“…The product 2 a can undergo 2‐electron oxidation to afford 1,4‐benzoquinone ( 3 ), for this reason 2 a and 3 were considered jointly as 1,4 dioxygenated products. 1,2‐Benzoquinone was not detected, as expected from another report on phenol oxidation . Table presents phenol conversion and quantification of the regioselectivity in terms of molar ratio between 1,4 and 1,2 dioxygenated products, i. e. [C( 2 a )+C( 3 )]/C( 1 a ), with C( i ) being molar concentration of i .…”

“…The similarity indicate that this species is formed from a similar intermediate in both cases: acylperoxo‐manganese(III). This result unifies the mechanism propositions envolving H 2 O 2 /carboxylic acid and m CPBA into the scheme shown in Figure a. Finally, the substrate consumption (step f ) can proceed by reaction with Mn V ‐oxo species originated either from heterolytic clevage of acylperoxo‐manganese(III) or by disproportionation of Mn IV ‐oxo [44] (step e ).…”

“…The use of MnTPFPP without PAA (#3) increased the conversion to 49 % and already showed preferred formation of 2 a and 3 over 1 a . Phenol oxidation using meso ‐tetra( p ‐chloro‐phenyl)porphyrinatoiron(III) chloride with H 2 O 2 was reported to have regioselectivity dependent on the solvent used, probably due to solvent‐substrate interactions. Interestingly, when PAA was included (#4) the conversion raised to 64 % and the regioselectivity increased by 20 %.…”

Phenol Hydroxylation over Alkaline Treated TS-1 Catalysts

“…/Co 2? catalyst, and larger than 1.6 for metalloporphyrin complexes [14]. Two hypotheses have been proposed to explain CT/HQ product selectivity.…”

“…While TS-1 is limited to the application of those substrates and oxidants smaller than its pore opening of 5.4 Å , many new types of catalysts were proposed for the catalysis of large molecules, such as mixed oxides Cu-Bi-V-O superconductor [8], mesoporous titanosilicates [9], mesoporous Fe-HMS [10], vanadium-substituted phosphomolybdate [11], Fe-containing pillared clays [12], Fe/KIT-polystyrene composite [13], and metalloporphyrin complexes [14]. Otherwise, introducing mesoporosity into zeolite can improve the diffusivity of zeolite [15].…”