Immobilized Lignin Peroxidase-Like Metalloporphyrins as Reusable Catalysts in Oxidative Bleaching of Industrial Dyes

Abstract

Synthetic and bioinspired metalloporphyrins are a class of redox-active catalysts able to emulate several enzymes such as cytochromes P450, ligninolytic peroxidases, and peroxygenases. Their ability to perform oxidation and degradation of recalcitrant compounds, including aliphatic hydrocarbons, phenolic and non-phenolic aromatic compounds, sulfides, and nitroso-compounds, has been deeply investigated. Such a broad substrate specificity has suggested their use also in the bleaching of textile plant wastewaters. In fact, industrial dyes belong to very different chemical classes, being their effective and inexpensive oxidation an important challenge from both economic and environmental perspective. Accordingly, we review here the most widespread synthetic metalloporphyrins, and the most promising formulations for large-scale applications. In particular, we focus on the most convenient approaches for immobilization to conceive economical affordable processes. Then, the molecular routes of catalysis and the reported substrate specificity on the treatment of the most diffused textile dyes are encompassed, including the use of redox mediators and the comparison with the most common biological and enzymatic alternative, in order to depict an updated picture of a very promising field for large-scale applications.

Keywords: biomimetic; immobilization; lignin peroxidase; metalloporphyrins; textile dyes; wastewaters.

Scheme 1

Biological oxidations involving heme proteins. Adapted from references [43,44].

Scheme 2

Cytochrome P450 reaction.

Scheme 3

Main oxidation reaction pathways catalyzed by CYP450 enzymes. Adapted from references [44,47,48,49,50].

Scheme 4

The catalytic cycle of CYP450 including the shorter “peroxide shunt”, and the structure of the Fe(III) complex of protoporphyrin IX. Adapted from [38,47,51,52,53,54].

Figure 1

Structures illustrating the so-called first, second and third generation porphyrins [25].

Scheme 5

The synthetic methods usually employed to obtain 5,10,15,20-tetrasubstituted porphyrins from the condensation of different aldehydes with pyrrole.

Scheme 6

Some of the most common methods for the covalent immobilization of metalloporphyrins.

Figure 2

Immobilization of metalloporphyrins through coordination bond. The most common ligands are imidazole (a) to emulate the active site of ligninolytic peroxidases [28,143]; pyridine (b) [26,31]; and mercapto function (c) to emulate the active site of cytochrome P450 and peroxygenase [30].

Figure 3

Core structure of some of the most used textile dyes. (a) azo; (b) anthraquinone; (c) indigo; (d) cationic; (e) triphenylmethane; (f) other cationic dyes with condensed tricyclic systems derived from acridine/phenazine/phenoxazine or similar structures, where one heteroatom bears a net positive charge.

Figure 4

Substituted phenylazo derivatives of substituted 1- or 2-naphthols studied as substrates of the iron(III) complex of 5,10,15,20-tetrakis(2,6-dichloro-3-sulfonatophenyl)porphyrin (TDCSP).

Scheme 7

The reductions of metalloporphyrin Cpd II analog can follow three main pathways: oxygen transfer (OT), hydrogen atom transfer (HAT), and electron transfer (ET) (S = substrate). OT path under these conditions was not observed.

Scheme 8

At alkaline pH (almost 12) Cpd I analog converts 1-phenylazo-2-naphthol dyes in 1-carbinols. These in turn can be cleaved unsymmetrically by Cpd II (X = –NO₂ or –OCH₃).

Scheme 9

At pH 9.30 a nucleophilic attack of a hydroxide ion at position 4 of the naphthol ring takes place after the reaction with Cpd I analog. No cleavage of azo moiety was detected, whereas Wallach rearrangement occurs.

Scheme 10

At neutral pH, the oxidizing species was Cpd II analog through HAT transfer. The arising radical from the azo dye can in turn react with molecular oxygen, yielding the same product observed at alkaline pH.

Figure 5

Several substituted azobenzenes have been proved to be substrates of Fe(TDCPP)Cl/MCPBA system.

Scheme 11

Sudan II was bleached in the presence of the Fe(TDCPP)Cl/MCPBA system with oxidative unsymmetrical cleavage of the azo linkage.

Scheme 12

Fe(TDCPP)Cl/MCPBA system in the case of azo dyes with N,N-dimethylamino substituent gives rise to gradual demethylation, followed by oxidation, leading to a complex mixture of products.

Figure 6

Three azo dyes (Acid Orange 7, Acid Orange 52, and Basic Orange 33) were used as the substrates in the first study about bleaching by Mn(III) porphyrins.

Scheme 13

Acid Red 27 (amaranth) is cleaved by Mn porphyrins in various products, such as the salts of naphthalene-1-sulfonic acid, 1-naphthol-4-sulfonic acid, and 2-hydroxy-1,4-naphthoquinone (lawsone), concomitantly to the formation of a dimer.

Figure 7

Methyl Orange (please, refer to Figure 6) and Sudan IV have been the subject of a comparative bleaching study by dichloro- or dibromophenyl meso-substituted Mn porphyrins.

Scheme 14

Alizarin Red S is bleached by a Mn porphyrin complex through a HAT mechanism leading to incomplete mineralization.

Figure 8

Reactive Red 195 structure.

Scheme 15

Phenosafranine structure.

Figure 9

Brilliant Green (a triphenylmethane) and Rhodamine B (a tricyclic immonium/oxonium cation) structures.

Figure 10

Iron(II) tetrakis(5,6-dichloro-1,4-dithiin)porphyrazine is a catalyst strictly resembling the structure of “normal” metalloporphyrins.