Manganese(III) porphyrin supported onto multi-walled carbon nanotubes for heterogeneous oxidation of synthetic textile dyes and 2,6-dimethylphenol by tert -butyl hydroperoxide

Article abstract of DOI:10.1016/j.crci.2015.12.001

Manganese porphyrin has been supported onto multi-walled carbon nanotubes (Mn(TCPP)OAc@MWCNT) and characterized by powder X-ray diffraction, FT-IR, atomic absorption and UV-vis spectroscopy, field emission scanning electron microscopy (FE-SEM) and also thermogravimetric analysis (TGA). The TGA curve shows that the nanocatalyst was thermally stable up to almost 350 °C. This catalyst was found to be able to oxidize different synthetic textile dyes in aqueous media over a wide pH range at ambient temperature with tert-butyl hydroperoxide (TBHP) as the oxygen source. The influence of some important parameters such as the initial pH of the dye solution, temperature and concentration of the catalyst, the oxidant and the co-catalyst were investigated. Also, the ability of this heterogeneous catalyst to oxidize 2,6-dimethylphenol (with excellent selectivity for quinone (86%)) with TBHP in acetonitrile was evaluated. The separation and recycling of the catalyst is simple and the catalyst can be used several successive cycles without significant decrease in catalytic activity.

Full text of DOI:10.1016/j.crci.2015.12.001

C. R. Chimie xxx (2016) 1e10
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Full paper/Memoire
Manganese(III) porphyrin supported onto multi-walled
carbon nanotubes for heterogeneous oxidation of synthetic
textile dyes and 2,6-dimethylphenol by tert-butyl
hydroperoxide
Saeed Rayati*, Zahra Sheybanifard
Department of Chemistry, K.N. Toosi University of Technology, P.O. Box 16315-1618, 15418 Tehran, Iran
a r t i c l e i n f o
a b s t r a c t
Article history:
Manganese porphyrin has been supported onto multi-walled carbon nanotubes (Mn(TCPP)
OAc@MWCNT) and characterized by powder X-ray diffraction, FT-IR, atomic absorption
and UVevis spectroscopy, field emission scanning electron microscopy (FE-SEM) and also
thermogravimetric analysis (TGA). The TGA curve shows that the nanocatalyst was ther-
mally stable up to almost 350 ꢀC. This catalyst was found to be able to oxidize different
synthetic textile dyes in aqueous media over a wide pH range at ambient temperature with
tert-butyl hydroperoxide (TBHP) as the oxygen source. The influence of some important
parameters such as the initial pH of the dye solution, temperature and concentration of the
catalyst, the oxidant and the co-catalyst were investigated. Also, the ability of this het-
erogeneous catalyst to oxidize 2,6-dimethylphenol (with excellent selectivity for quinone
(86%)) with TBHP in acetonitrile was evaluated. The separation and recycling of the catalyst
is simple and the catalyst can be used several successive cycles without significant
decrease in catalytic activity.
Received 1 July 2015
Accepted 3 December 2015
Available online xxxx
Keywords:
Heterogeneous oxidation
Mn-porphyrin
Carbon nanotubes
Textile dyes
2,6-Dimethylphenol
ꢀ
© 2015 Academie des sciences. Published by Elsevier Masson SAS. All rights reserved.
1. Introduction
efforts have been made and numerous methods have been
utilized, the problem remains unsolved [3e4]. Recently,
In recent decades, increasing productions in the chem-
ical industry has led to the making of millions of tons of
unwanted compounds which are often harmful from an
environmental point of view. Thus, the degradation and
elimination of these compounds has been subject of
extensive academic research. Synthetic textile dyes and
industrial pigments are essential chemicals which are used
extensively in industries such as textiles, papers, leathers,
cosmetics, food and additives [1,2]. Textile and dyeing in-
dustries produce a large quantity of coloured waste com-
pounds and thus, the removal of coloured waste from
industrial wastewater is imperative. Although substantial
catalytic oxidation processes have been widely used for the
transformation of the pollutants into more functional and
non-toxic compounds [5e13]. Among different catalytic
systems, synthetic metalloporphyrin complexes have been
extensively examined as models of cytochrome P-450 over
the last few decades [14e18].
Homogeneous metalloporphyrin catalytic systems are
very effective in catalytic oxidation reactions but problems
such as oxidative degradation of porphyrin and the
impossibility of separating the products and recovering the
catalyst have led to the scarce use of these complexes.
Studies have indicated that the use of solids as supports for
the immobilization of metalloporphyrin complexes can be
a suitable way of improving their efficiency and stability
[19e22]. Among the various possible solid supports, carbon
* Corresponding author.
E-mail addresses: rayati@kntu.ac.ir, srayati@yahoo.com (S. Rayati).
http://dx.doi.org/10.1016/j.crci.2015.12.001
ꢀ
1631-0748/© 2015 Academie des sciences. Published by Elsevier Masson SAS. All rights reserved.
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S. Rayati, Z. Sheybanifard / C. R. Chimie xxx (2016) 1e10
nanotubes can be a good choice due to their unique prop-
erties such as structural characteristics, high surface area
and the lack of solubility in most solvents [23,24]. We have
reported the ability of supported Mn- and Fe-porphyrins in
the oxidation of alkenes with tert-butyl hydroperoxide
(TBHP) and H2O2 [25e28]. In the present research, a het-
erogeneous catalyst was prepared by covalent bonding
between Mn(TCPP)OAc and functionalized carbon nano-
tubes and its catalytic efficiency was examined for oxida-
tive degradation of different synthetic textile dyes using
TBHP in aqueous media.
Other hazardous materials are phenolic compounds
which are inexpensive and readily available and have been
used in petrochemical and pharmaceutical industries
[29,30]. Oxidation reactions can be a suitable way for
removing these compounds and turning them into more
beneficial compounds such as quinones which are promi-
nent intermediates in organic syntheses [31e33]. In general,
in comparison with other compounds such as hydrocarbons
and sulfides, metalloporphyrins are less frequently consid-
ered in the oxidation of phenolic compounds. Hence, we
checked the ability of this catalytic system for the oxidation
of 2,6-dimethylphenol in acetonitrile.
was equipped with
a
Agilent Zorbox C8 column
(4.6 mm ꢂ 150 mm, 5
mm) and whose mobile phase was
composed of 0.01 mol/L of acetonitrile-ammonium acetate
(30/70, v/v) solution (pH ¼ 6.8) and also, 20
mL of sample
was injected by an autosampling device. The eluent from the
chromatographic column sequentially entered the UVevis
diode array detector. The mass spectrometer was equipped
with an electrospray ionization (ESI) source and operated in
negative ion mode. Oxidation of 2,6-dimethylphenol was
monitored by gas chromatography (Shimadzu GC-14B)
equipped with a flame ionization detector (FID) with a
SAB-5 capillary column (phenyl methyl siloxane
30 m ꢂ 320 mm ꢂ 0.25 mm). In the GC experiments, n-
octane was used as an internal standard.
2.3. Catalyst preparation ((Mn(TCPP)OAc) and its supported
form (Mn(TCPP)OAc@MWCNT))
Meso-tetrakis(4-carboxyphenyl)porphyrin (H2TCPP) and
meso-tetrakis-(4-carboxyphenyl)porphyrinatomanganese-
(III) acetate (Mn(TCPP)OAc) were prepared according to
methods described in the literature studies [34,35]. The
Mn(III) porphyrin complex was anchored covalently onto
the functionalized multi-walled carbon nanotubes by the
following procedure: to a mixture of 500 mg of MWCNT-OH
in 50 mL DMF, 360 mg of Mn(TCPP)OAc, 400 mg of 2-(1H-
benzotriazole-1-yl)-1,1,3,3-tetramethyluronium tetrafluoro-
borate (TBTU) and 300 mg of N,N0-diisopropylamine (DIPEA)
were added and the mixture was stirred for 48 h at room
temperature. The dark solid product was filtered and
washed several times with DMF for removing the weakly
adsorbed metalloporphyrins and was eventually dried at
room temperature [26,36].
2. Experimental section
2.1. Materials
Functionalized multi-walled carbon nanotubes con-
taining 3.06 wt% hydroxyl groups with an outside diameter
of 10e20 nm, inside diameter of 5e10 nm, length of ~30 mm
and specific surface area of >200 m2/g were used as the
solid support. Other materials and solvents were purchased
from Merck or Fluka chemical companies and used as
received. Distilled water was utilized throughout the dye
oxidative degradation processes.
2.4. Catalytic oxidative degradation of the synthetic textile
dyes
2.2. Physico-chemical characterization techniques
The catalytic reactions were run in 5 mL test tubes or
10 mL glass flasks in a water bath. To a 500 mg/dm3
aqueous solution of dye (3.3 cm3 of methyl orange (MO),
3.2 cm3 of methylene blue (MB), 4 cm3 of crystal violet (CV)
and 6.9 cm3 of congo red (CR) that are equal to
5 ꢂ 10ꢁ6 mol), 1 mg of the catalyst and 9 ꢂ 10ꢁ3 cm3 of
TBHP as the oxidant were added. Also, various amounts of
imidazole as the co-catalyst have been used. The solution
pH was adjusted with ammonium acetate buffer solutions
(0.01 M). To monitor the oxidative degradation process,
solution samples were taken out at given time intervals and
measured by UVevis spectroscopy and the calibration
curve using the characteristic band of each dye (MO:
464 nm, CR: 498, MB: 660 nm, and CV: 590 nm).
Field emission scanning electron microscopy (FE-SEM)
images were obtained on a Hitachi S-4160 with an acceler-
ating voltage of 20 kV. Thermogravimetric analysis (TGA)
was carried out with a Mettler-ToledoTGA 851e apparatus at
a heating rate of 10 K minꢁ1 in a nitrogen atmosphere. FT-IR
spectra were recorded on an ABB Bomem: FTLA 2000-100 in
the range of 400e4000 cmꢁ1 using KBr pellets. A Varian
AA240 atomic absorption spectrometer was utilized to
determine the amount of Mn-porphyrin complex absorbed
onto the solid support. The electronic spectra were taken on
a single beam spectrophotometer (Cam Spec-M330 model)
in a 2 mm path length quartz cell in the range of
200e800 nm. For adjusting the pH values of the dye solu-
tion a PHS-3C pH meter was utilized. A Bruker FT-NMR 500
(500 MHz) spectrometer was utilized to record 1H NMR
spectra in CDCl3 solution. The powder XRD patterns were
recorded using an STOE STADIP diffractometer equipped
with a scintillation detector, secondary monochromator and
2.5. Oxidation of 2,6-dimethylphenol
0.06 mmol of 2,6-dimethylphenol in 0.5-mL acetonitrile,
1.7 mg imidazole as the co-catalyst and 2.5 mg of the
supported catalyst have been added in a 5 mL test tube. The
oxidation reaction was run at ambient temperature after
adding 23 mL of TBHP to the reaction mixture. The progress
of the reaction was monitored by GC. After the completion
of the reaction, the reaction mixture was centrifuged to
Cu-K
a
1 radiation (
l
¼ 1.5406 Å). In order to reveal the
reaction products arising from the oxidative degradation of
dyes, high-performance liquid chromatography mass
spectrometry (HPLC-MS) (Agilent 6410) was used which
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3
separate the catalyst. Oxidation products were separated
on pre-coated F254 silica gel plates (Merck), 2 mm thick-
ness, and 20 ꢂ 20 cm, using n-hexane/ethyl acetate (0.1:
0.9) as the eluent and were identified using 1H NMR by
comparison with authentic samples.
OAc@MWCNT confirmed that the bands detected in Fig. 3b
are due to the Mn-porphyrin, because the UVevis spectrum
of MWCNT-OH does not exhibit any bands in the target
region.
After anchoring the metalloporphyrin, the Soret band
(474 nm) shows a red-shift (4 nm) in comparison with the
metalloporphyrin in solution. The steric constraints of the
3. Results and discussion
support which caused
a modification of the metal-
loporphyrin can be an explanation for this behaviour
[12,38e40].
3.1. Characterization of the supported catalyst (Mn(TCPP)
OAc@MWCNT)
Atomic absorption spectroscopy exhibited that the
metal content of the supported catalyst was 497.4 mmol per
Mn-porphyrin has been covalently anchored onto the
functionalized multi-walled carbon nanotubes using TBTU
as an impressive coupling reagent for esterification in the
presence of DIPEA as a base. The proposed structure of the
supported catalyst is presented in Fig. 1.
gram catalyst. This amount is higher than other immobili-
zation approaches in the literature studies [37,41] which
may be due to the use of a very effective uronium salt as a
coupling reagent.
The morphology of the prepared catalyst indicates that
carbon nanotubes are aggregated and their nanotube na-
ture has been kept after anchoring the metalloporphyrin
complexes to them (Fig. 4). These results are in agreement
with those obtained by other studies [25,41].
The covalent bonding of the supported catalyst was
investigated by comparison of the FT-IR spectra of
Mn(TCPP)OAc and Mn(TCPP)OAc@MWCNT (Fig. 2). The
peaks at 1649.62 cmꢁ1 and 1652.11 cmꢁ1 in the FT-IR
spectra of the Mn-porphyrin and supported catalyst were
attributed to the stretching vibrations of the phenyl rings in
the meso positions of the porphyrin molecule [37]. The
sharp peak at 1724.51 cmꢁ1 was ascribed to the esteric
linkage between Mn-porphyrin and MWCNT-OH (Fig. 2b).
The most informative spectroscopic data, regarding the
covalent bonding of Mn(TCPP)OAc onto MWCNT-OH, were
obtained by comparison of the UVevis spectra of Mn(TCPP)
OAc, MWCNT-OH and Mn(TCPP)OAc@MWCNT (Fig. 3). The
absorption spectrum of Mn(TCPP)OAc in DMF exhibits a
sharp peak at 470 nm (Soret bond) and two weak absorp-
tions at 566 nm and 601 nm (Q bonds) as shown in Fig. 3a.
The UVevis spectrum of MWCNT-OH and Mn(TCPP)
Fig. 5 shows the X-ray diffraction patterns of MWCNT
and Mn(TCPP)OAc@MWCNT in the range of 2
The diffraction peaks on the 2
q
¼ 10ꢀe90ꢀ.
q
values of 25.5ꢀ, 42ꢀ and 44ꢀ
which are related to the (002), (100) and (101) planes,
respectively, are characteristic diffraction peaks of carbon
nanotubes (Fig. 5b). The similarity of the XRD pattern
before and after supporting onto nanotubes confirms that
the Mn-porphyrin complex was dispersed onto MWCNTs
and also the nature of the MWCNTs has been preserved
[37,42].
Thermogravimetric analysis of Mn(TCPP)OAc@MWCNT
is illustrated in Fig. 6a. A comparison with the Mn(TCPP)
OAc (Fig. 6b) shows that the heterogeneous catalyst ꢀis
thermally stable up to 350 ꢀC. The mass loss below 300 C
corresponds to the loss of humidity [28]. The mass decline
around 350e700 ꢀC may be due to the decomposition of
Mn(TCPP)OAc followed by the oxidation of MWCNTs [37].
C
O
O
H
3.2. Catalytic oxidative degradation of the synthetic textile
dyes
H
O
c N
O
C
A
O
N
N
Mn
The synthesized heterogeneous catalyst was utilized for
the catalytic oxidative degradation of methyl orange (MO)
as a model which is often used in analytical chemistry as an
acid-base indicator. The UVevis spectrum of this com-
pound in water displays two peaks at 270 nm and 464 nm,
the latter of which attributed to the n/л* transition of the
N]N group [43]. The influence of different parameters
including pH, presence of imidazole as the axial base, re-
action time, temperature, and amount of catalyst and
oxidant were evaluated by changing one parameter while
the others were kept constant.
N
C
O
O
H
O=C
O
H
OH
O
There is minor oxidation in the absence of either the
catalyst or the TBHP (Table 1, entries 1 and 2). In addition,
the adsorption of methyl orange onto the MWCNT-OH was
very low (Table 1, entry 3). Catalytic activity of Mn-
porphyrins has been usually affected by the presence of
nitrogenous bases [44,45]. Thereby, the effect of imidazole
as a co-catalyst on the oxidation of MO has been tested
(Table 1, entries 4e10). According to the results, the use of
O
H
O
H
O
H
Fig. 1. Schematic representation of the 5, 10, 15, 20-tetrakis(4-carboxyphenyl)
porphyrinatomanganese(III) acetate supported onto the surface of function-
alized multi-walled carbon nanotubes (Mn(TCPP)OAc@MWCNT).
Please cite this article in press as: S. Rayati, Z. Sheybanifard, Manganese(III) porphyrin supported onto multi-walled carbon
nanotubes for heterogeneous oxidation of synthetic textile dyes and 2,6-dimethylphenol by tert-butyl hydroperoxide, Comptes
Rendus Chimie (2016), http://dx.doi.org/10.1016/j.crci.2015.12.001

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S. Rayati, Z. Sheybanifard / C. R. Chimie xxx (2016) 1e10
Fig. 2. FT-IR spectra of (a) Mn(TCPP)OAc and (b) Mn(TCPP)OAc@MWCNT.
13.6 mg imidazole enhanced the catalytic efficiency to 79%,
and a decrease in catalytic efficiency was observed beyond
13.6 mg. Our results also show that the oxidation efficiency
was affected by the amount of catalyst and oxidant (Table 1,
entries 8 and 11e15). When the catalyst amount increased
from 0.5 to 1 mg the catalytic efficiency increased by about
25% (from 54% to 79%), but with an increase in the amount
of catalyst from 1 mg to 2 mg the catalytic efficiency
improved by only 9% (from 79% to 88%). Another investi-
gation was done on the oxidant concentration and the re-
sults show that when the oxidant amount was raised from
3 ꢂ 10ꢁ3 to 9 ꢂ 10ꢁ3 cm3, the catalytic efficiency enhanced
from 51% to 79% and beyond this concentration the cata-
lytic efficiency improved by only 8% (from 79 to 87%). The
oxidative degradation of the catalyst in higher amounts of
catalyst and oxidant can be account for these observations.
Thus, for checking other parameters such as reaction time,
pH and temperature, 1 mg of catalyst and 9 ꢂ 10ꢁ3 cm3 of
the oxidant were chosen.
In order to determine the optimum reaction time, the
catalytic reaction was also studied at various reaction
times. The results show that the oxidative degradation
Fig. 3. UVevis spectra of (a) Mn(TCPP)OAc, (b) Mn(TCPP)OAc@MWCNT and
(c) MWCNT-OH.
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nanotubes for heterogeneous oxidation of synthetic textile dyes and 2,6-dimethylphenol by tert-butyl hydroperoxide, Comptes
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5
Fig. 4. FE-SEM images of the supported catalyst (Mn(TCPP)OAc@MWCNT) in two views.
wide range of pH is very important. So, an attempt was done
to examine the effect of initial pH of the dye solution on the
oxidative degradation of MO. The pH was adjusted at 5, 7, 9
and 11 using ammonium acetate buffer and experiments
were run for 1 h. According to the results presented in the
Figs. 9 and 10, catalytic efficiency of the supported catalyst
was accelerated by enhancing the pH values. Presumably,
the lower catalytic degradation of the MO under acidic
conditions may be due to the dissociation of TBHP from the
surface of the catalyst [49]. According to the literature
studies [50,51] in the absence of the catalyst, MO is stable
even in the presence of strong oxidants such as TBHP at low
pH values and it can be oxidized by peroxides at high pH
values but the reaction is very slow. In the presence of the
Mn(TCPP)OAc@MWCNT as the catalyst, oxidation of MO can
be performed over a wide range of pH from acidic to alkaline
conditions in the shorter reaction time.
Fig. 5. XRD patterns of (a) Mn(TCPP)OAc@MWCNT and (b) MWCNT-OH.
The UVevis spectra of methyl orange before and after
oxidation reaction are displayed in Fig. 11. Oxidative
degradation of the dye causes a decrease in the peak at
464 nm which is attributed to the n/л* transition.
The oxidative degradation products of the MO were
monitored by HPLC-MS. Two pathways are proposed for
oxidative cleavage of azo dyes: (i) asymmetrical and (ii)
symmetrical. It is believed that both of these pathways
occur during oxidative degradation reaction of azo dyes
[52,53]. In the present study, the negative ion mode of MS
analysis was employed and six main products were
detected. N,N-dimethyl-p-phenylenediamine (m/z 136) and
4-aminobenzenesulfonate (m/z 172) were proposed for
symmetrical azo bond cleavage and also benzenesulfonate
(m/z 157), 4-(dimethylamino)-phenol (m/z 137), 4-
carboxybenzenesulfonate (m/z 201) and 4-diazenyl-N,N-
dimethylaniline (m/z 149) were proposed for asymmetrical
azo bond cleavage. Two types of cleavage are presented in
Fig. 12.
proceeded sluggishly after 4 h. When the reaction time was
increased from 4 h to 12 h, only a little increase in the
catalytic efficiency was observed (about 10%) (Fig. 7).
Temperature is one of the most important factors in the
catalytic reactions, especially in the catalytic processes
utilized in environmental and industrial applications.
Therefore, the impact of temperature on the performance
of the supported catalyst was probed and the results are
shown in Fig. 8. The oxidative degradation process was
performed at 10, 40, 60 and 90 ꢀC. In order to carry out
more precise investigation, the reactions were run in the
shorter reaction times (1 h and 2 h). When the temperature
increased from 40 to 60 ꢀC in 1 h, the degradation of MO
increased from 66% to 85%. While at 90 ꢀC, the degradation
of MO reached 77%. Thus the best temperature for catalytic
activity is 60 ꢀC. At this temperature, the rate of oxidative
degradation reached to 91% after 4 h.
Industrial wastes usually have various pH values and dye
oxidation is strongly affected by varying the dye solution pH
[46e48]. Thus, performance of a catalytic system over a
Performance of this catalytic system was also studied in
the oxidation of the three other dyes: congo red (CR),
Please cite this article in press as: S. Rayati, Z. Sheybanifard, Manganese(III) porphyrin supported onto multi-walled carbon
nanotubes for heterogeneous oxidation of synthetic textile dyes and 2,6-dimethylphenol by tert-butyl hydroperoxide, Comptes
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S. Rayati, Z. Sheybanifard / C. R. Chimie xxx (2016) 1e10
Fig. 6. TGA curves of (a) Mn(TCPP)OAc@MWCNT and (b) Mn(TCPP)OAc.
Table 1
The screening amount of the catalyst, oxidant and co-catalyst in the
oxidation of MO using TBHP catalysed by using the supported catalyst
(Mn(TCPP)OAc@MWCNT) at room temperature for 4 h.a
Entry
Catalyst
amount
(mmol)
Oxidant
amount
(mmol)
Co-catalyst
amount
(mg)
Oxidative
degradation
(%)
1
2
3
4
5
6
7
8
None
0.075
None
None
0.075
0.075
0.075
0.075
0.075
0.075
0.075
0.075
0.075
0.025
0.050
0.125
None
None
None
None
3.4
10
5
0.497
MWCNT-OH
0.497
0.497
0.497
0.497
0.497
0.497
0.497
0.248
0.994
0.497
0.497
0.497
13
11
43
58
75
79
78
71
54
88
51
67
87
6.8
10.2
13.6
20.4
54.4
13.6
13.6
13.6
13.6
13.6
Fig. 7. Percentage of MO oxidative degradation as
a function of time.
9
Experimental conditions: 1 mg of catalyst, 9 ꢂ 10ꢁ3 cm3 of TBHP, 3.3 cm3 of
500 mg/dm3 aqueous solution of MO and reactions were run at room
temperature.
10
11
12
13
14
15
The results show that 10% or 25% of the MB or CV have
been adsorbed onto the surface of the supported catalyst.
The results for oxidation of MB and CV with TBHP are
presented in Table 2 (entries 3e6). The obtained results
a
Reaction conditions: Molar ratio of the catalyst:TBHP is 1:150 and
3.3 cm3 of 500 mg/dm3 aqueous solution of MO. The initial reaction pH is
6.
methylene blue (MB) and crystal violet (CV) (Table 2). CR is
a diazo dye and its UVevis spectrum shows a maximum
absorbance around 498 nm in aqueous solution. Control
experiment was performed in the presence of the oxidant
without the supported catalyst and only 4% of the absor-
bance peak at 498 nm was decreased after 4 h. In addition,
the adsorption process of this dye was investigated in the
presence of the supported catalyst without an oxidant and
only 7% of the CR was adsorbed onto the surface of the
supported catalyst after 4 h. In the presence of both the
oxidant and supported catalyst, 74% oxidation of the CR
occurred after 4 h (Table 2, entry 1) while in the pH ¼ 11,
82% oxidation of the CR will be obtained (Table 2, entry 2).
MB and CV show maximum absorbance around 660 and
590 nm, respectively.
Fig. 8. Percentage of MO oxidative degradation as a function of temperature.
Experimental conditions: 1 mg of catalyst, 9 ꢂ 10ꢁ3 cm3 of TBHP, 3.3 cm3 of
500 mg/dm3 aqueous solution of MO.
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3.3. Oxidation of 2,6-dimethylphenol
In addition to the oxidation of different synthetic textile
dyes, the ability of the prepared heterogeneous catalyst to
oxidize 2,6-dimethylphenol has been investigated (Fig. 13).
According to the results (Fig. 14), this catalytic system
was also active in the oxidation of 2,6-dimethylphenol with
high selectivity toward quinones. Control experiments
show that the existence of the catalyst is essential (only 20%
oxidation of 2,6-dimethylphenol will be achieved after 24 h
in the absence of the catalyst). In the presence of the
catalyst and oxidant, the efficiency of the catalyst increased
with increasing the reaction time and the highest conver-
sion (96%) was obtained after 5 h.
Fig. 9. The effect of pH values on the oxidative degradation of MO. Exper-
imental conditions: 1 mg of catalyst, 9 ꢂ 10ꢁ3 cm3 of TBHP, 3.3 cm3 of
500 mg/dm3 buffer solution of MO and reactions were run for 1 h.
3.4. Proposed catalytic mechanism
In the proposed catalytic cycle, it seems that the oxidant
can be coordinated to the metal center and compound I or/
and II (Scheme 1) will be formed which can be used for
oxidation of phenol or oxidative degradation of dyes
[10,55]. The formation of quinone in the oxidation of phe-
nols may be explained by two routes: (a) a radical mech-
anism initiated in the presence of the high valent metal oxo
species [56e59] or (b) direct oxygenation from compound I
analogue [60].
3.5. Catalyst stability
Fig. 10. Comparison of the catalytic activity of Mn(TCPP)OAc@MWCNT with
In order to study the stability and reusability of the
supported catalyst, 2,6-dimethylphenol was selected as a
model substrate. A series of sequential experiments were
done in which the used catalyst was centrifuged, washed
thoroughly with acetonitrile and methanol, dried at room
temperature and then employed without any treatment in
the next run (Fig. 15). Interestingly, the catalyst maintains a
considerable part of its catalytic efficiency after reusing
several times and reaches to about 56% after the sixth re-
action. Hence, this catalyst can be considered as an excel-
lent option for industrial applications.
or without buffer solution. Experimental conditions:
1 mg of catalyst,
9 ꢂ 10ꢁ3 cm3 of TBHP, 6.5 cm3 of 500 mg/dm3 buffer solution of dye and
reactions were run at room temperature.
4. Conclusions
In this research, a simple heterogeneous catalyst was
prepared through covalent bonding between Mn-
porphyrin and multi-walled carbon nanotubes. This het-
erogeneous catalyst exhibits efficient catalytic activity in
the oxidative degradation of synthetic textile dyes, in
aqueous solution over a wide range of pH at ambient
temperature using TBHP. By using HPLC-MS analysis, six
main products were detected and two main pathways were
proposed for oxidative degradation of MO. On the other
hand, this simple supported catalyst was successfully able
to perform the selective and efficient oxidation of 2,6-
dimethylphenol as a phenolic substrate toward quinones
under mild conditions. Furthermore, this catalyst can be
recovered and reused several times and according to the
results, this catalytic system can be suitable for a potential
industrial application.
Fig. 11. UVevis absorption spectra of the reaction mixture (a) before the
oxidation reaction and (b) after the oxidation reaction in the presence of
1 mg of catalyst, 9 ꢂ 10ꢁ3 cm3 of TBHP, and 3.3 cm3 of 500 mg/dm3 buffer
solution of MO. The reactions were run for
temperature.
1
h
at pH
¼
11 at room
confirmed that Mn(TCPP)OAc@MWCNT is a suitable het-
erogeneous catalyst for oxidation of various dyes.
Comparison of this catalytic system with previously
reported systems shows that higher dye degradation or
lower reaction time was achieved [3,12,54].
Please cite this article in press as: S. Rayati, Z. Sheybanifard, Manganese(III) porphyrin supported onto multi-walled carbon
nanotubes for heterogeneous oxidation of synthetic textile dyes and 2,6-dimethylphenol by tert-butyl hydroperoxide, Comptes
Rendus Chimie (2016), http://dx.doi.org/10.1016/j.crci.2015.12.001

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S. Rayati, Z. Sheybanifard / C. R. Chimie xxx (2016) 1e10
Fig. 12. Possible pathways for oxidative cleavage of MO using TBHP catalysed by Mn(TCPP)OAc@MWCNT.
Table 2
The monitoring of the oxidation of the three other dyes using TBHP cat-
alysed by Mn(TCPP)OAc@MWCNT at room temperature.
Entry Dye
pH
Time (h) Oxidative
degradation (%)
1
2
3
4
5
6
Congo red
Congo red
Methylene blue Without buffer
Methylene blue 11
Without buffer
11
4
1
4
1
4
e
74
82
69
90
73
e
Crystal violet
Without buffer
11
Crystal violeta
a
Crystal violet is insoluble at pH ¼ 11.
Fig. 14. Oxidation of 2,6-dimethylphenol using TBHP catalysed by Mn(TCPP)
OAc@MWCNT at room temperature. The molar ratio of catalyst: co-catalyst:
substrate: oxidant is 1:20:50:150 in acetonitrile (0.5 mL).
Fig. 13. Oxidation of 2,6-dimethylphenol.
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nanotubes for heterogeneous oxidation of synthetic textile dyes and 2,6-dimethylphenol by tert-butyl hydroperoxide, Comptes
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S. Rayati, Z. Sheybanifard / C. R. Chimie xxx (2016) 1e10
9
O
O- Na+
S
O
N
N
Oxidative Products
N
tBu
H
O
TBHP
ImH
MnIV
ImH
MnV
OAc
tBu
OH
O
or
MnIII
CNT
CNT
OH
CNT
Mn
CNT
ImH
O
O
ImH
OAc
I
II
ImH
MnV
O
O
ImH
MnIV
CNT
CNT
O
OH
III
III
OH
OH
O
O
MnIII
CNT
ImH
[ox.]
OH
Scheme 1. Proposed mechanism for oxidation of dye and 2,6-dimethylphenol.
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Fig. 15. Reusability of the supported catalyst, Mn(TCPP)OAc@MWCNT, in the
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Acknowledgements
Financial support of this work by K.N. Toosi University of
Technology is gratefully acknowledged.
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nanotubes for heterogeneous oxidation of synthetic textile dyes and 2,6-dimethylphenol by tert-butyl hydroperoxide, Comptes
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