Peroxide Decoloration of Azo Dyes Catalyzed by Polyethylene Glycol

and Department of Science of Living, Osaka Kun'ei Women's Junior College, .... Coloration Technology 2000 116 (10.1111/cte.2000.116.issue-2), 42-4...
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Langmuir 1998, 14, 3272-3278

Peroxide Decoloration of Azo Dyes Catalyzed by Polyethylene Glycol-Linked Manganese Halogenated Porphyrins Mamoru Nango,* Toyota Iwasaki, Yoshito Takeuchi, Yukihisa Kurono,† Junko Tokuda,‡ and Ritsuko Oura‡ Department of Applied Chemistry, Nagoya Institute of Technology, Gokiso-cho, Showa-ku, Nagoya 466, Japan, Pharmaceutics, Faculty of Pharmaceutical Science, Nagoya City University, 3-1 Tanabe-dori, Mizuho-ku, Nagoya 467, Japan, and Department of Science of Living, Osaka Kun’ei Women’s Junior College, Shojaku, Settu, Osaka 566, Japan Received June 17, 1997. In Final Form: March 30, 1998

Polyethylene glycol (PEG)-linked manganese halogenated porphyrins (Chart 1) catalyzed oxidation of azo dyes (Chart 2) by H2O2 under mild conditions such as pH 8.0 at 25 °C especially when imidazole was present, causing the decoloration of azo dyes. The decoloration of azo dyes by synthetic manganese porphyrins under mild conditions was first reported. The decoloration rate depended on the structures of the porphyrins, in which the largest rate was observed in the presence of PEG-MnDCPP. The decoloration may be contributed by radical species rather than electrophilic species, consistent with the side-chain oxidation of toluene. Kinetics on polyethylene glycol-linked manganese porphyrin-catalyzed decoloration of C.I. Acid Orange 7 by hydrogen peroxide revealed that the decoloration was contributed at the oxidation process by manganese porphyrins with hydrogen peroxide in the polymer domain rather than the complexformation process between manganese porphyrins and azo dyes.

Introduction The synthetic porphyrin model is useful in providing insight into possible reactions of porphyrin complexes, in which porphyrin pigments play the key role in oxidative electron-transfer systems such as peroxidases and cytochromes.1-3 Many oxidation systems have been reported to mimic cytochrome P-450-dependent monooxygenases and peroxidases with metalloporphyrins, where quantitative epoxidations of alkenes and alkane hydroxylation catalyzed by synthetic porphyrin complexes have * Address correspondence to this author at Nagoya Institute of Technology. Telephone and fax: 81-52-735-5226. E-mail: nango@ ach.nitech.ac.jp. † Nagoya City University. ‡ Osaka Kun’ei Women’s Junior College. (1) (a) Nango, M.; Kryu, H.; Loach, P. J. Chem. Soc., Chem. Commun. 1988, 697-698. (b) Nango, M.; Higuchi, M.; Gondo, H.; Hara, M. J. Chem. Soc., Chem. Commun. 1989, 1550-1553. (c) Nango, M.; Mizusawa, A.; Miyake, T.; Yoshinaga, J. J. Am. Chem. Soc. 1990, 112, 1640-1642. (d) Nango, M.; Iida, K.; Kawakita, T.; Matsuura, M.; Harada, Y.; Yamashita, K.; Tsuda, K.; Kimura, Y. J. Chem. Soc., Chem. Commun. 1992, 545-547. (e) Nango, M. MEMBRANE 1992, 17, 105-114. (f) Iida, K.; Nango, M.; Hikita, M.; Tajima, T.; Kurihara, T.; Yamashita, K.; Tsuda, K.; Dewa, T.; Komiyama, J.; Nakata, M.; Ohtsuka, Y. Chem. Lett. 1994, 1157-1160. (g) Iida, K.; Nango, M.; Okada, K.; Matsumoto, S.;Yamashita, K.; Tsuda, K.; Kurono, Y.; Kimura, Y. Chem. Lett. 1994, 1307-1310. (h) Iida, K.; Nango, M.; Hikita, M.; Hattori, A.; Yamashita, K.; Yamauchi, K.; Tsuda, K. Chem. Lett. 1994, 753-756. (i) Iida, K.; Nango, M.; Okada, K.; Hikita, M.; Matsuura, M.; Kurihara, T.; Tajima, T.; Hattori, A.; Ichikawa, S.; Yamashita, K.; Tsuda, K.; Kurono, Y. Bull. Chem. Soc. Jpn. 1995, 68, 1959-1968. (2) (a) Dewa, T.; Satoh, M.; Komiyama, J.; Nango, M.; Tsuda, K. Macromol. Chem. Phys. 1994, 195, 2917-2929. (b) Dewa, T.; Mitsuru, S.; Komiyama, J.; Nango, M.; Tsuda, K. Macromol. Chem. Phys. 1994, 195, 1031-1041. (3) (a) Nango, M.; Dannhauser, T.; Huang, D.; Spears, K.; Morrison, L.; Loach, P. A. Macromolecules 1984, 17, 1898-1902. (b) Dannhauser, T.; Nango, M.; Oku, N.; Anzai, K.; Loach, P. J. Am. Chem. Soc. 1986, 108, 5865-5871. (c) Nango, M.; Iida, K.; Yamaguchi, M.; Yamashita, K.; Tsuda, K.; Mizusawa, A.; Miyake, T.; Masuda, A.; Yoshinaga, J. Langmuir 1996, 12, 1981-1988. (d) Iida, K.; Nango, M.; Matsuura, M.; Yamaduchi, M.; Sato, K.; Tanaka, K.; Akimoto, K.; Yamashita, K.; Tsuda, K.; Kurono,Y. Langmuir 1996, 12, 450-458.

been developed.4-12 It is interesting to note that some peroxidases such as lignin-peroxidase, manganese peroxidase, and horseradish peroxidase catalyze the decoloration of azo dyes because the peroxidases are expected to be bleaching agents that are not only more active than those currently available but also environmentally safe.15 Recently, Hage et al. reported that non-heme manganese ion complex catalyzed low-temperature decoloration of tea-colored cottton by hydrogen peroxide as an bleaching agent.15 Namboodri et al. reported the decoloration of azo dyes by a hot peroxide catalyzed by a copper phthalocyanine derivative.16 However, no report was observed for the catalytic effect of synthetic porphyrin complexes upon the peroxidative decoloration of azo dyes under mild conditions such as neutral pH at 25 °C. We now report biomimetic peroxidative-oxidation of azo dyes (Chart 2) (4) Collmann, J. P.; Kodadek, T.; Raybuck, S. T.; Meunier, B. Proc. Natl. Acad. Sci. USA 1983, 80, 7039-7041. (5) Ortiz de Montellano, P. R. Cytochrome P-450, Structure, Mechanism and Biochemistry; Plenum Press: New York and London, 1986. (6) Nolte, R. J. M.; Razenberg, J. A. S. J.; Schuurman, R. J. Am. Chem. Soc. 1986, 108, 2751-2752. (7) Pattioni, P.; Renaud, J. P.; Bartoli, J. F.; Reina-Artiles, M.; Fort, M.; Mansuy, D. J. Am. Chem. Soc. 1988, 110, 8462-8470. (8) Carrier, M.-N.; Scheer, C.; Gounine, P.; Bartoli, J.-F.; Barttioni, P.; Mansuy, D. Tetrahedron Lett. 1990, 31, 6645-6648. (9) Groves, J. T.; Ungashe, S. B. J. Am. Chem. Soc. 1990, 112, 77967797. (10) Groves, J. T.; Fate, G. D.; Lahiri, S. J. Am. Chem. Soc. 1994, 116, 5477-5478. (11) Chang, C. K.; Ebina, F. J. Chem. Soc., Chem. Commun. 1981, 778. (12) Lindsey-Smith, J. R.; Sleath, P. R. J. Chem. Soc., Perkin Trans. 2 1982, 1009-1015. (13) Chivukula, M.; Spadarao, J. T.; Renganathan, V. Biochemistry 1995, 34, 7765-7772. (14) Tokuda, J.; Oura, R.; Nango, M. Submitted. (15) Hage, R.; Iburg, J.; Kerschner, J.; Koek, J.; Lempers, L.; Martens, R.; Racherla, U.; Russell, S.; Swarthoff, T.; van Vliet, M.; Warnaar, J.; van der Wolf, L.; Krijnen, B. Nature 1994, 369, 637-639. (16) Namboodri, C. G.; Walsh, W. K. Am. Dyest. Rep. 1995, 84, 8695.

S0743-7463(97)00644-6 CCC: $15.00 © 1998 American Chemical Society Published on Web 05/19/1998

Peroxide Decoloration of Azo Dyes Chart 1. Polyethylene Glycol-Linked Manganese Porphyrin Derivatives

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bleaching benefit in comparison to the use of H2O2 alone.15 Thus, to study the catalytic effect of polyethylene glycollinked manganese porphyrins on the decoloration rates of azo dyes by hydrogen peroxide is useful to provide insight into the modest catalytic bleaching benefit. Experimental Section

Chart 2. Azo Dyes

catalyzed by polyethylene glycol (PEG)-linked manganese halogenated porphyrins or mesoporphyrins (Chart 1) with H2O2 in aqueous solution under mild conditions. The kinetics of polyethylene glycol-linked manganese halogenated porphyrin catalyzed decoloration of C.I. Acid Orange 7 by hydrogen peroxide were analyzed. Furthermore, polyethylene glycol-linked manganese halogenated porphyrin-catalyzed peroxidation of simple aromatic hydrocarbons such as toluene was examined to elucidate the catalytic mechanism of the halogenated manganese porphyrins for decoloration. The decoloration of azo dyes by synthetic halogenated manganese porphyrins under mild conditions was the first reported. The key to the peroxidative decoloration is the usefulness of providing an insight into the catalytic effect of porphyrin structures on decoloration. Polyethylene glycol-linked manganese halogenated porphyrins were selected because of their water solubility and their well-defined properties, in which manganese halogenated porphyrins caused an enhanced oxidation of aromatic hydrocarbons by hydrogen peroxide and the halogenated porphyrins must be chemically stable because of the steric or electrodeficient effect of halogen portions of the porphyrin ring, as reported preliminarily.1g,i,3d,c Furthermore, we reasoned that the PEG moiety on the compound was chemically stable against the oxidants.3d,18 The detergents of the next century will be routinely required to contain bleaching agents that are not only more active than those currently available but also environmentally safe and cost-effective.15 Hydrogen peroxide, a traditional bleaching agent, loses its activity as the washing temperature decreases. Manganese or ferric complex is expected to have a modest catalytic (17) Fendler, J. H.; Fendler, E. J. Catalysis in Micellar and Macromolecular Systems; Academic Press: New York, 1975. (18) Kimura, Y.; Nango, M.; Kuroki, N.; Ihara, Y.; Klotz, I. M. J. Polym. Sci., Polym. Symp. 1984, 71, 167-182.

General Methods. All reactions and chromatographic separations were carried out in minimum room light. Benzene, chloroform, dichloromethane, pyridine, dimethyl sulfoxide (DMSO), tetrahydrofuran (THF), and methanol were distilled and stored over molecular sieves. Other solvents used were spectral grade or better quality. The silica gel used for dry column chromatography was Woelem Silica TSC, activity III/30 mm, obtained from ICN Pharmaceuticals. Merck silica gel GF Uniplates of 1000- and 250-µm thickness were used for preparative or analytical thin layer chromatography. 1H NMR spectra were taken with Jeol JNM-GX-270 and Jeol JNM-GX-400 instruments with tetramethylsilane as an internal standard for CDCl3 and d6-DMSO. The UV absorption spectra were recorded on Hitachi 124 and Hitachi-Perkin Elmer MPF-4 spectrophotometers, respectively. Mass spectra (MS) were obtained in the FAB mode, 3-nitrobenzyl alcohol or thioglycerol matrix, with a JMS-DX300 Jeol instrument. Polyethylene glycol having one primary amine group (PEG-NH2) with a molecular weight of 5000 or 550 was provided by Nippon Oil Co. LTD, Tokyo, Japan. The primary amine content was determined by the TNBS (trinitrobenzenesulfonic acid) method.16 Polyethylene glycol was purified by ultrafiltration in an Amicon Diaflo ultrafiltration apparatus. Hydrogen peroxide was provided by Mitubisigasu Chemicals, and it was a 31% solution. The pH was adjusted with tris(hydroxymethyl)aminomethane. The azo dyes C.I. Acid Orange 7, Acid Orange 52, and C.I. Basic Orange 33 were provided by Tokyo Kasei and purified by recrystallization from aqueous ethanol. The chemical structures of the azo dyes are shown in Chart 2. Synthetic Procedure. 5-(4-Carboxyphenyl)-10,15,20-tris(2,6-dichlorophenyl)porphyrin, H2DCPPCOOH, and its manganese or zinc complex, MnDCPPCOOH or ZnDCPPCOOH, were prepared as described in our previous paper.1i Zinc 5-(4-Carbomethoxyphenyl)-10,15,20-tris(2,6-dichlorophenyl)-2,3,7,8,12,13,17,18-octabromoporphyrin, ZnDCPPBr8COOH. ZnDCPPCOOH (800 mg, 0.807 mmol) and N-bromosuccinimide (2.87 g, 16.1 mmol) were dissolved in carbon tetrachloride (80.7 mL) in a 300-mL three-neck bottle. Trifluoroacetic acid (2.47 mL, 32.3 mmol) was added to the solution, and the resulting solution was brought to reflux for 5 h and allowed to stand at room temperature. Sodium sulfate was added, and the reaction mixture was neutralized with sodium bicarbonate and washed with distilled water. Zinc acetate dihydrate (886 mg, 4.04 mmol) dissolved in methanol (20 mL) was added, and the reaction mixture was stirred for 10 min. The resulting solution was washed with distilled water and dried over MgSO4, and the solvent was removed under reduced pressure. After evaporation of the solvent, the resulting solid was purified by silica gel column chromatography using chloroform. The yield of ZnDCPPBr8COOH was 28.1%. UV/vis (CHCl3): 466 nm, 600. MS (FAB): m/z 1560 (MH+). 5-(4-Carbomethoxyphenyl)-10,15,-20-tris(2,6-dichlorophenyl)-2,3,7,8,12,13,17,18-octabromoporphyrin, H2DCPPBr8COOH. ZnDCOOPPBr8COOH (500 mg, 0.308 mmol) was dissolved in chloroform (20 mL) in a 100-mL two-neck bottle. Trifluoroacetic acid (2.36 mL, 30.8 mmol) was added to the solution, and the resulting solution was stirred for 1 h at room temperature. Distilled water (10 mL) was added to the solution in an ice bath, and the reaction mixture was brought to and allowed to stand at room temperature for 3 h. The organic phase was washed with distilled water and neutralized with sodium bicarbonate. The organic phase was again washed with distilled water and dried over MgSO4. The solvent was removed under reduced pressure. After evaporation of the solvent, the resulting solid was purified by silica gel column chromatography using chloroform. The yield of DCPPBr8COOH was 48.4%. 1H NMR (CDCl3): δ -1.4 (2H, s, pyrrole-NH), 7.7 (2H, d, Ar 3-H and

3274 Langmuir, Vol. 14, No. 12, 1998 5-H), 7.9 (2H, d, Ar 2-H and 6-H), 7.7-7.9 (9H, Ar 3-H, 4-H, and 5-H). MS (FAB): m/z 1560 (MH+). UV/vis (CHCl3): 464.0 nm ( 194 mM-1 cm-1), 560.0 (17.3), 601.0 (10.3). Manganese Dichlorinated Porphyrin Complexes, MnDCPPBr8COOH. H2DCPPBr8COOH (0.140 mmol) was dissolved in chloroform (14 mL), and Mn(C5H7O2)2 (172 mg, 0.700 mmol) was added. The solution was stirred at room temperature for 30 min. Distilled water was added to the solution, and the precipitate was filtered with a membrane filter. The precipitate was dissolved in chloroform and dried over magnesium sulfate. The chloroform was removed under reduced pressure, and the sample was purified by silica gel chromatography (chloroform1% MeOH). The yield of MnDCPPBr8COOH was 68.3%. UV/ vis (CHCl3): λmax 468 nm, 569.5. MS (FAB): m/z 1497 (M+). Polyethylene Glycol-Linked Manganese Chlorinated Porphyrin Derivatives, PEG-MnDCPP, PEG(550)-MnDCPP, and PEG-MnDCPPBr8. For example, PEG-MnDCPP was prepared as described as follows. MnDCPPCOOH (0.051 mmol) was dissolved in benzene (5.1 mL), and thionyl chloride (0.37 mL, 5.10 mmol) was added. The mixture was brought to reflux for 1 h. The solvent was removed under reduced pressure. The residue was redissolved in benzene (2 mL) and once again taken to dryness under reduced pressure to remove traces of thionyl chloride. The acid chloride was dissolved in chloroform (5.1 mL), and PEG(5000)-NH2 (208 mg, 0.0408 mmol) in chloroform (4.08 mL) containing a few drops of triethylamine was added dropwise. The solution was stirred at room temperature for 1 h, and the solvent was removed under pressure. The sample was purified by Sephadex LH-20 gel chromatography (EtOH). PEG-MnDCPP. The yield was 61.8%. UV/vis (CH2Cl210% EtOH): λmax 465.5 nm ( 158 mM-1 cm-1), 560.0 (17.9). PEG(550)-MnDCPP. The yield was 63.0%. UV/vis (CH2Cl2-10% EtOH): λmax 465.5 nm ( 158 mM-1 cm-1), 560.0 (17.9). PEG-MnDCPPBr8. The yield was 77.9%. UV/vis (CH2Cl210% EtOH): λmax 473.0 nm ( 94.4 mM-1 cm-1), 592.5 (13.0). Peroxide Oxidation of Toluene. Hydroxylation of toluene catalyzed by manganese halogenized porphyrins with 31% H2O2 was carried out in CH2Cl2-H2O (1:1) during 2 h at 40 °C under following conditions: catalyst, 0.77 mmol dm-3; oxidant, H2O2, 8.2 mmol dm-3; substrate, 2.3 mol dm-3; imidazole, 54 mmol dm-3. In these reactions, imidazole was used as a cocatalyst, as it has been shown to have a beneficial effect in oxidations by H2O2.1g Their reaction products were identified by the methods of GC-Mass spectrometry and HPLC. Hydroxylation of toluene gave cresol, which comes from direct hydroxylation of the aromatic ring and benzyl alcohol, benzylaldehyde, and benzoic acid, which come from the side-chain oxidation.1g Peroxidative Decoloration of Azo Dyes. The peroxide oxidation of azo dyes by hydrogen peroxide in the presence of manganese porphyrins was evaluated by determining the decoloration curve of the dye in aqueous solution at pH 7 or 8.0 at 25 °C. The required amount of an aqueous solution of the azo dye adjusted to the desired pH was added to 25 mL of the bleaching solution at 25 °C, so as to obtain the initial concentrations of the azo dye of 1 × 10-4 mol dm-3 and of hydrogen peroxide of 3 × 10-2 mol dm-3. To obtain each decoloration curve, each of the solutions was placed as quickly as possible in a covered optical quartz cell, and the concentration of the coloring matter in the bleaching solution was determined by a spectrophotometric method using a Shimazu spectrophotometer UV-160A equipped with a thermostat at appropriate time intervals and the desired temperature. There is no interference at an absorption maximum of the coloring matter by the products formed during decomposition. Under these conditions, the calibration line follows Beer’s law. Absorptions of C.I. Acid Orange 7, C.I. Acid Orange 52, and C.I. Basic Orange 33 were measured at 484, 474, and 476 nm, respectively. Cyclic Voltammogram (CV) Measurement. Cyclic voltammetry was measured with a Yanako P-900. A standard calomel electrode (SCE) and Ag/AgCl served as reference electrodes, glassy carbon served as the working electrode, and platinum functioned as the counter electrode. Solutions containing porphyrins were prepared in DMSO containing 0.1 M tetra-nbutylammomium perchlorate (TBAP). The solutions were degassed by argon bubbling. The data were summarized in Table 1.3d

Nango et al. Table 1. Redox Potentials (V) vs SCE of PEG-Linked Halogenated Manganese Porphyrinsa redox potential (V) vs SCE Mn porphyrin

Mn(II)/Mn(III)

Mn(II)/Mn(III) with addition of imidazoleb

PEG-MnPFPPBr8 PEG-MnDCPPBr8 PEG-MnPFPP PEG-MnDCPP

+0.330 +0.130 -0.084 -0.160

+0.284 +0.113 -0.127 -0.235

a Conditions: redox potentials (V) vs SCE of porphyrin derivatives in DMSO containing 0.1 M TBAP. Redox potential was calculated from the midpoint between anodic and cathodic peak potentials. b Concentration of imidazole is 2.21 M.

Results and Discussion PEG-MnPFPP and PEG-MnPFPPPBr8 were prepared as described in the previous paper.3d Synthetic sequences leading to the compounds PEG-MnDCPP and PEGMnDCPPBr8 were prepared by the following synthetic sequence of steps. The manganese complexes of the porphyrins 5-(p-carboxylphenyl)-10,15,20-tris(2,6-dichlorophenyl)porphyrin and 5-(4-carbomethoxyphenyl)-10,15,20-tris(2,6-dichlorophenyl)-2,3,7,8,12,13,17,18-octabromoporphyrin, MnDCPPCOOH and DCPPBr8COOH, were prepared as described in the Experimental Section.1i MnDCPPCOOH and MnDCPPBr8COOH were treated with ethyl chloroformate in chloroform and reacted with PEG to give PEG-MnDCPP and PEG-MnDCPPBr8, followed by Sephadex LH-20 gel chromatographic separation. PEG-linked porphyrin complexes were very soluble in water at various pH values and in typical organic solvents such as ethanol and chloroform. Characterization and verification of the compounds synthesized were done by mean of UV/vis absorption spectroscopies. The visible spectra of PEG-MnDCPP and PEG-MnDCPPBr8 in chloroform showed the presence of a characteristic manganese porphyrin, as shown in the Experimental Section. The UV/vis absorption spectra indicated one porphyrin group per polymer. The data for the PEG-linked manganese porphyrins agree well with previous spectral data for manganese dichlorophenylporphyrin, MnDCPPCOOH, and manganese dichlorophenyloctabromoporphyrin, MnDCPPBr8COOH (see the Experimental Section). Stability of Poly(ethylene glycol)-Linked Manganese Porphyrins against Oxidant. The decomposition of the porphyrin moiety on PEG-linked manganese porphyrins in aqueous solutions containing H2O2 (2.8 × 10-1 M) and imidazole (1.4 × 10-3 M) at room temperature was measured spectroscopically to see the effects of halogen portions on the porphyrin ring on the stability against the oxidant. The data indicated that no or less decomposition of the porphyrin was observed for PEGMnDCPPBr8 even after 2 h, consistent with the data for PEG-MnPFPPBr8, as described previously.3d However, 80% and 85% decompositions of the porphyrin were observed for PEG-MnDCPP and PEG-MnPFPP, respectively, under the same conditions. This result implies that the bromo portions of the pyrrole group for the manganese porphyrins play an important role on the stability against the oxidant in the aqueous solution.3d Peroxidative Decoloration of Azo Dye. To examine the effect of the porphyrin structure on the chemoselectivity variations of the biomimetic peroxide oxidation, we investigated the peroxidative decoloration of azo dyes catalyzed by PEG-linked manganese porphyrins. Figure 1 illustrated that semilogarithmic plots of C0/Ct against decoloration time gave a straight line passing through the origin for C.I. Acid Orange 7 in the presence and

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Figure 1. Plot of C0/Ct against decoloration time at 25 °C and at pH 8.0 for C.I. Acid Orange 7 in the presence of manganese porphyrin derivatives. C0 and Ct are the initial concentration and the concentration at time t of coloring matter, respectively (initial concentration of C.I. Acid Orange 7 ) 1 × 10-4 mol dm-3, H2O2 ) 3 × 10-2 mol dm-3, manganese porphyrin derivatives ) 1 × 10-5 mol dm-3, imidazole ) 1 × 10-3 mol dm-3): 0, control; b, PEG-MnPFPP; O, PEG-MnDCPP. Scheme 1. Products Identified from Peroxidative Oxidation of C.I. Acid Orange 7 (see ref 13)

absence of PEG-linked manganese porphyrins at 25 °C and pH 8.0. C0 and Ct are the dye concentrations in the initial solution and at time t in Figure 1. Similar straight lines were observed for all cases, in which the increased line was observed due to the presence of manganese porphyrins. This result is the first reported that manganese porphyrins catalyze the decoloration of the azo dye by hydrogen peroxide in aqueous solution under mild conditions. The decoloration rates were first-order with respect to the dye concentration for the times. Equation 1 gives numerical values that can be used later to compare the influence of various conditions, such as pH and temperature:

ln(C0 /Ct) ) kobst

Figure 2. Effect of imidazole on the decoloration rate of C.I. Acid Orange 7 catalyzed by PEG-Mn porphyrins at 25 °C, pH 8.0, respectively (initial concentration of C.I. Acid Orange 7 ) 1 × 10-4 mol dm-3, H2O2 ) 3 × 10-2 mol dm-3, PEG-Mn porphyrin ) 1 × 10-5 mol dm-3): O, PEG-MnDCPP; b, PEGMnPFPP; 0, PEG-MnDCPPBr8; 9, PEG-MnMPMME. Table 2. Rate Constants for Decoloration of C.I. Acid Orange 7 Catalyzed by Manganese Porphyrin Derivatives in the Absence and Presence of Imidazole with H2O2 at pH 8.0 and 25 °Ca kobs (102 min-1) manganese porphyrin derivative

none

imidazole

none PEG-MnDCPPBr8b PEG-MnDCPPb PEG-MnPFPPc PEG-MnPFPPBr8c

0.05 0.05 0.13 0.26 0.05

0.05 0.05 14.0 0.34 0.05

a H O , 3 × 10-2 mol dm-3; manganese porphyrin derivatives, 2 2 1 × 10-5 mol dm-3. b Imidazole, 1 × 10-3 mol dm-3. c Imidazole, 1 × 10-2 mol dm-3.

Scheme 2. Tentative Catalytic Cycle for Peroxidative Hydroxylation by H2O2 Catalyzed by Mn Porphyrins and Ligand (L)

(1)

The rate constant kobs based on the first-order kinetics is given in units of min-1. The decoloration products of the azo dye by hydrogen peroxide are very complex. For example, Chivukula et al. reported that the peroxide oxidation of C.I. Acid Orange 7 caused the decoloration products such as quinone derivatives, as shown in Scheme 1.13 The mechanism for peroxide decoloration of dyes with hydrogen peroxide is not fully understood, but the mechanism is believed to be hydroxyl radicals reacting with the organic coloring agent and destroying the chromophore, as described in the section Peroxidation of toluene.13,16 The decoloration rate of C.I. Acid Orange 7 with hydrogen peroxide catalyzed by PEG-linked manganese porphyrin derivatives was examined at various concentrations of imidazole at pH 8.0. For example, Figure 2 showed the rate constant plotted against the concentration of imidazole, in which the maximum rate was observed, especially for PEG-MnDCPP or PEG-MnMPMME. The decoloration rate increased with increasing concentration of imidazole and then showed a maximum value when [imidazole]/[PEG-MnDCPP] ) 100 or [imidazole]/[PEGMnMPMME] ) 1000.14 Imidazole enhanced the rate constant in comparison to that in the absence of imidazole,

especially for PEG-MnDCPP, as shown in Table 2. Thus, interactions between the porphyrin and imidazole play a crucial role for the decoloration rate, consistent with the peroxide oxidation of toluene, as described later. The role of imidazole is likely to be that imidazole coordinates to Mn as a ligand and also works as a base catalyst, as described in Mansuy’s paper and shown in Scheme 2,7 in which manganese porphyrins decompose H2O2 and transfer more or less than one oxygen atom to the azo dye presumably via a high-valent MndO intermediate such

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Nango et al.

Table 3. Hydroxylation of Toluene by H2O2 Catalyzed by Halogenated Manganese Porphyrins in a Heterogeneous Systema Products (µ mol dm-3) Mn porphyrins

o-cresol

p-cresol

totalb

PEG-MnPFPPBr8 PEG-MnPFPP PEG-MnDCPPBr8 PEG-MnDCPP

1.3 2.4 1.5 0.6

1.4 2.3 1.5 0.5

2.7 4.7 3.0 1.1

PhCH2OH

PhCHO

PhCOOH

totalc

4.2 4.0 11.1 15.0

1.2 4.2 3.2 6.0

0.2 0.3 1.6 1.2

5.6 8.5 15.9 22.2

a Conditions: [Mn porphyrin], 2.0 µmol dm-3; [imidazole], 140 µmol dm-3; [toluene], 6.0 mmol dm-3; [CH Cl ], 2.0 mL; [31% H O aq], 2 2 2 2 0.80 mL in CH2Cl2/H2O at 40 °C, 2 h. Analysis of products was made by GC/MS and HPLC by comparison with authentic samples. Yield b determination was made by using calibration curves with acetophenone as an internal standard. Total products of aromatic ring hydroxylation. c Total products of methyl group hydroxylation.

as Mn(IV)dO in the bleaching solution when imidazole is present. The importance of imidazole as an axial Mn ligand is to favor the heterolytic cleavage of H2O2, leading to the MndO intermediate and H2O, over a possible homolytic cleavage leading to a hydroxyl radical and to favor monooxygenation of azo dye over H2O2 dismutation at the level of an imidazolesMndO intermediate. Its role as a base catalyst could be important for the formation of a MndO intermediate from Mn(III) and H2O2. However, excess imidazole may quench radical species, causing the inhibition of the decoloration as described below (see Figure 2). As is apparent from Table 2, the decoloration rates increase in the order PEG-PFPPBr8 < PEGMnDCPPBr8 < PEG-MnPFPP < PEG-MnDCPP, coinsitent with the side-chain oxidation of toluene as described later, implying that the main active species in the reaction are radical species for MnDCPP.1g,7 These results imply that the hydroxyl radicals react with the organic azo coloring agent, destroying the chromophore, as described in the following peroxiation of toluene.13,16 Peroxide Oxidation of Toluene. Polyethylene glycollinked manganese halogenated porphyrin-catalyzed peroxidation of simple aromatic hydrocarbons such as toluene was examined to elucidate the catalytic effect of the halogenated porphyrins on the peroxidation of azo dyes. Total phenol products or total side oxidation products of toluene for the peroxide oxidation catalyzed by manganese halogenated porphyrins are summarized in Table 3. As is apparent from Table 3, the total side-chain oxidation products (benzyl alcohol, benzaldehyde, and benzoic acid) on toluene increase in the order PEG-MnPFPPBr8 < PEG-MnPFPP < PEG-MnDCPPBr8 < PEG-MnDCPP, and in contrast, the total phenol products increase in the order PEG-MnDCPP < PEG-PFPPBr8 ∼ PEG-MnDCPPBr8 < PEG-MnPFPP, consistent with the order for the peroxide oxidation catalyzed by manganese porphyrin monomer as described previously.1g The chemoselectivity of the peroxide oxidation depends on the porphyrin structure. That is, a more electron-sufficient porphyrin such as PEG-MnDCPP catalyzes efficiently the side-chain oxidation of an aromatic hydrocarbon like toluene, while a more electron-deficient porphyrin such as PEG-MnPFPP catalyzes efficiently the aromatic ring hydroxylation although the data do not seem to entirely support this conclusion (see Tables 1 and 3). Interestingly, selective side chain oxidations on toluene are observed especially when PEG-MnDCPP is used, implying that the main active species for the reaction are radical species for MnDCPP but electrophilic species for MnPFPP.1g,7 More experiments are necessary to draw conclusions about the relative importance of these two factors on the chemoselectivity variations. Furthermore, for example as shown in Figure 3, the phenol product increased with increasing concentration of imidazole and then showed a maximum value when [imidazole]/[PEG-MnDCPP] ) 80, while the product was not observed when imidazole was not present.

Figure 3. Influence of imidazole/PEG-MnDCPP ratio on the hydroxylation of toluene by H2O2 catalyzed PEG-MnDCPP in CH2Cl2/H2O at 40 °C for 2 h: [PEG-MnDCPP], 2.0 × 10-6 mol dm-3; [toluene], 6.0 × 10-3 mol dm-3; [CH2Cl2], 2.0 mL; [31% H2O2 aq], 0.80 mL; O, total side-chain oxidation products; b, total aromatic hydroxylation products. Table 4. Rate Constants for Decoloration Reaction of Azo Dyes Catalyzed by Manganese Porphyrin Derivatives in the Presence of Imidazole with H2O2 at 25 °C and pH 8.0a kobs (102 min-1) catalyst

C.I. Acid Orange 7

C.I. Acid Orange 52

C.I. Basic Orange 33

none PEG-MnDCPPb PEG-MnMPMMEc

0.16 14.8 18.7

0.08 16.4 31.4

0.08 13.0 2.23

a H O , 3 × 10-2 mol dm-3; C.I. Acid Orange 7, 1 × 10-4 mol 2 2 dm-3; C.I. Acid Orange 52, C.I. Basic Orange 33, 5 × 10-5 mol dm-3. b PEG-MnDCPP, 1 × 10-5 mol dm-3; imidazole, 1 × 10-3 mol dm-3. c PEG-MnMPMME, 1 × 10-5 mol dm-3; imidazole, 1 × 10-2 mol dm-3. Cited from ref 14.

This result indicates that the imidazole plays a crucial role in the hydroxylation. The role of imidazole is likely to be as a ligand and also as a base catalyst, as described above,7 in which manganese porphyrins decompose H2O2 and transfer more or less than one oxygen atom to alkanes presumably via a high-valent MndO intermediate such as Mn(IV)dO when imidazole is present, as shown in Sceme 2. The importance of imidazole as an axial Mn ligand is to favor the heterolytic cleavage of H2O2, leading to the MndO intermediate, and to favor hydrocarbon monooxygenation over H2O2 dismutation at the level of an imidazolesMndO intermediate while its role as a base catalyst could be important for the formation of an MndO intermediate from Mn(III) and H2O2. However, further investigations will be necessary to determine the various roles indicated by imidazole in oxidations by H2O2 catalyzed by Mn porphyrins. Effect of Structure of Azo Dyes. The decoloration rate of various azo dyes, C.I. Acid Orange 7, C.I. Acid

Peroxide Decoloration of Azo Dyes

Langmuir, Vol. 14, No. 12, 1998 3277 Table 5. Kinetic Constantsa

manganese porphyrin derivative

imidazole (102 mol dm-3)

PEG-MnPFPPBr8 PEG-MnPFPP PEG-MnDCPPBr8 PEG-MnDCPP PEG-MnDCPP PEG-MnDCPP PEG(550)-MnDCPP PEG-MnMPMMEb

1 1 1 1 0.1 0.01 0.1 1

a

k2 (min-1)

KM (104 mol dm-3)

k2/KM (10-4 mol dm-3 min-1)

0.33

0.40

0.83

0.87 2.22 1.06 0.50 1.78

0.83 1.22 0.95 1.00 0.89

1.05 1.81 1.11 0.50 2.0

PEG-Mn porphyrin ) 1 × 10-5 mol dm-3. b Cited from ref 14.

Figure 4. Variation of initial rate constant V0/P0 for PEGMnDCPP and PEG(550)-MnDCPP in the presence of imidazole as a function of substrate concentration under excess substrate: O, PEG-MnDCPP; b, PEG(550)-MnDCPP.

Orange 52, and C.I. Basic Orange 33, with hydrogen peroxide catalyzed by manganese porphyrins was measured in the presence of imidazole. Table 4 summarized the rate constants for the decoloration reaction of the azo dyes. As is apparent from Table 4, an enhanced decoloration rate of C.I. Acid Orange 52 or C.I. Basic Orange 33 was observed due to the presence of manganese porphyrins, consistent with the result of C.I. Acid Orange 7 as described above. Interestingly, large differences of the rate constants between these azo dyes were not observed for PEG-MnDCPP. It is likely that the complex forming between the azo dye and the manganese porphyrin complex with hydrogen peroxide in the polymer domain plays an important role in the difference of the decoloration rate between azo dyes, as described later (Table 5). A more detailed effect of the structure of azo dyes on the decoloration will be reported elsewhere.14 Kinetics of the Decoloration. The decoloration of azo dyes by hydrogen peroxide catalyzed by PEG-linked manganese porphyrins was enhanced, depending upon the structure of porphyrins and azo dyes, especially when imidazole is present. To further examine the effect of structures of porphyrin and azo dyes on the decoloration rate, kinetic measurements in the presence of imidazole were made. Figure 4 showed the relation between the decoloration rate of C.I. Acid Orange 7 and the concentration of hydrogen peroxide in the presence of polyethylene glycol-linked manganese porphyrins. The decoloration rate of C.I. Acid Orange 7 increased with increasing concentration of hydrogen peroxide and then saturated. The rate-limiting step for this decoloration is likely to be oxidation of the dye by hydrogen peroxide, but the reaction may be governed by incorporation of the dye into the metallo complex, as observed in enzyme-like catalytic reactions.17 The kinetics of cleavage of C.I. Acid Orange 7 by hydrogen peroxide in the presence of manganese por-

Figure 5. Linear transform, following eq 4, of data illustrated in Figure 4.

phyrins were analyzed in a format similar to that used in enzymatic catalyses.19 If C represents one catalytic site of the high-valent intermediate on the manganese porphyrin in the presence of hydrogen peroxide and S represents the substrate, then the following scheme may be formulated: κ1

κ2

S + C y\ z SC 98 product + C κ -1

(2)

For certain sets of the experiments, the initial concentration of substrate S0 was kept in great excess over that of catalytic sites C0. Under these circumstances, S0 . C0, initial velocities V0 are measured, and the steady-state expression for the scheme in eq 1 is

κ0 ) κ2C0 /(KM + S0)

(3)

where κ0 is the initial rate constant, V0/S0, and KM ) (κ-1 + κ2)/κ1. Then

C0 /k0 ) (KM/k2) + (1/k2)S0

(4)

For example, Figure 4 illustrates the experiments under conditions of S0 . C0, for which, for the polyethylene glycollinked manganese porphyrin derivative PEG-MnDCPP or PEG(550)-MnDCPP, a plateau is reached with increasing concentration of the substrate. Since S and C appear symmetrically in the rate steps of eq 2, each species must exhibit saturation behavior, and each does in this decoloration reaction. The data for S0 . C0 have been fitted to eq 4, as is illustrated in Figure 5. From the parameters of the lines shown in Figure 5, the kinetic constants given in Table 5 were determined. These results indicated that the kinetics of cleavage of C.I. Acid Orange 7 by hydrogen peroxide catalyzed by polyethylene glycollinked manganese porphyrins were able to deal with a (19) Nango, M.; Kimura, Y.; Ihara, Y.; Kuroki, K. Macromolecules 1988, 21, 2330-2335.

3278 Langmuir, Vol. 14, No. 12, 1998

format similar to that used in enzymatic catalyses, because of linear lines. As is shown in eq 3, we can assume that KM is responsible for the complex forming between the substrate and the manganese porphyrin with hydrogen peroxide in the polymer domain and that k2 is responsible for the oxidation by the metalloporphyrin complex with hydrogen peroxide in the polymer domain. Thus, if there are some differences between k2 and KM, we can see the effect of porphyrin structure and the effect of imidazole on the decoloration reaction. Comparing the kinetic constants between PEGlinked manganese porphyrins, we note that the difference of the second-order rate parameter k2/KM between PEGMnDCPP and PEG-MnPFPP is affected by the contribution of k2 rather than KM (see Table 5). This result implies that the increased decoloration in the presence of PEGMnDCPP in comparison to PEG-MnPFPP is contributed at the k2 step. Thus, the oxidation process by metalloporphyrin with hydrogen peroxide plays an important role in the decoloration of the dye under these conditions, in which main active species in the reaction are radical species, as described above. Furthermore, comparing the kinetic constants between PEG-linked manganese chlorinated porphyrins with molecular weights of PEG, we note that the difference of the second-order rate parameter k2/KM between PEG-MnDCPP and PEG(550)-MnDCPP is affected by the contribution of k2 rather than KM. This result also implies that the difference of the molecular weight of PEG-linked manganese porphyrins affects the oxidation process of the decoloration. Alternatively, we note that the second-order rate parameter k2/KM catalyzed by PEG-MnDCPP is affected by the concentration of imidazole, contributed by k2 rather than KM. Thus, imidazole enhanced the oxidation of the metalloporphyrin with hydrogen peroxide in the polymer domain. These results indicated that polyethylene glycol-linked manganese halogenated porphyrins decomposed hydrogen peroxide and transferred one oxygen atom to the azo dye presumably via high-valent intermediate of manganese porphyrin in the bleaching solution, depending on the oxidation potential of the manganese halogenated porphyrins, the molecular weight of PEG, and the concentration of imidazole. Interestingly, large differences of the rate constants between these azo dyes were not observed for PEG-MnDCPP as shown in Table 4. Comparing the kinetic constants for decoloration of azo dyes catalyzed by PEG-MnMPMME,14 we note the difference of the secondorder rate parameter k2/KM between 2.00 (104 mol dm-3/ min) for C.I. Acid Orange 7 and 0.19 (104 mol dm-3/min) for C.I. Basic Orange 33. The difference is affected by the contribution of KM rather than k2, in which k2 ) 1.78 (min-1) and KM ) 0.89 (104 mol dm-3) for C.I. Acid Orange

Nango et al.

7 and k2 ) 1.75 (min-1) and KM ) 9.28 (104 mol dm-3) for C.I. Basic Orange 33 (see PEG-MnMPMME in Table 5 and ref 14).14 This result implies that the difference in the decoloration rate between these azo dyes is due to the KM step. Thus, the complex forming between the substrate and the manganese porphyrin complex with hydrogen peroxide in the polymer domain plays an important role in the difference on the decoloration rate between these azo dyes. The role of the polymer domain could be important for favoring the complex formation between manganese porphyrin and azo dyes, getting oxidized. Conclusion The polyethylene glycol-linked manganese porphyrins PEG-MnDCCP and PEG-MnDCCPBr8 (Chart 1) were synthesized. Polyethylene glycol-linked manganese porphyrins with H2O2 in the presence of imidazole catalyze efficiently the peroxide oxidation of azo dyes (Chart 2) under mild conditions such as pH 8.0 and 25 °C. This result is the first reported that manganese porphyrins catalyze the decoloration of the azo dye by hydrogen peroxide in aqueous solution under mild conditions. The oxidation depended upon the structure of the porphyrins, the molecular weight of PEG, and the presence of imidazole. The effect of the porphyrin structures on the peroxide oxidations is discussed. The maximum decoloration rate of C.I. Acid Orange 7 was observed for PEGMnDCPP, and no or less decoloration was observed for PEG-MnPFPPBr8, implying that the hydroxyl radicals react with the organic coloring agent, destroying the chromophore, consistent with the peroxide oxidation of toluene, in which the main active species of the reaction are electrophilic species for MnPFPPBr8 but radical species for MnDCPP. Interestingly, large differences of the rate constants between these azo dyes were not observed for PEG-MnDCPP. The kinetics of the decoloration of the azo dye by hydrogen peroxide catalyzed by PEG-linked manganese porphyrins in the presence of imidazole were evaluated. The increased decolorations due to the presence of manganese halogenated porphyrins and also the presence of imidazole are attributed to the oxidation process by manganese porphyrins with hydrogen peroxide in the polymer domain rather than the complex formation between manganese porphyrins and the azo dye. Acknowledgment. M.N. thanks Mmes. Kenichi Okada, Mitsutaka Matsuura, Naoto Matsushima, and Kenichi Ichikawa for the preparation of manganese porphyrins. The present work was partially supported by a Grantin-Aid from from the Ministry of Education, Science and Culture, Japan. LA970644T