The catalytic efficiency of Fe-porphyrins supported on multi-walled carbon nanotubes in the oxidation of olefins and sulfides with molecular oxygen

Article abstract of DOI:10.1039/c7nj01530e

A Fe-porphyrin catalyst was anchored covalently onto functionalized multi-walled carbon nanotubes. The heterogeneous catalyst was characterized by powder X-ray diffraction, transmission electron microscopy, scanning electron microscopy, FTIR, UV-Vis spectroscopy and thermogravimetric analysis (TGA). The catalyst loading on the nanotube support was determined by atomic absorption spectroscopy (AAS). The aerobic oxidation of olefins and sulfides under ambient conditions was efficiently enhanced under the influence of the separable nanocatalyst. This oxidation system was found to oxidize various sulfides to sulfones and different olefins to their respective epoxides as major products. Aerobic oxidation generally requires high temperatures and/or long reaction times to occur. However, in this research, the aerobic oxidation of olefins and sulfides was studied in the presence of Fe-porphyrin which was immobilized onto multi-wall carbon nanotubes at room temperature and with a short reaction time of 60 min. The separation and recycling of the catalyst and the byproduct of the oxidant were simple, effective, and economic in this clean oxidation method. FTIR and leaching experiments after seven successive cycles showed that the catalyst was very strongly anchored to the carbon nanotubes.

Full text of DOI:10.1039/c7nj01530e

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The catalytic efficiency of Fe–porphyrins supported
on multi-walled carbon nanotubes in the oxidation
of olefins and sulfides with molecular oxygen†
Cite this:DOI: 10.1039/c7nj01530e
Saeed Rayati * and Fatemeh Nejabat
A Fe–porphyrin catalyst was anchored covalently onto functionalized multi-walled carbon nanotubes. The
heterogeneous catalyst was characterized by powder X-ray diffraction, transmission electron microscopy,
scanning electron microscopy, FTIR, UV-Vis spectroscopy and thermogravimetric analysis (TGA). The
catalyst loading on the nanotube support was determined by atomic absorption spectroscopy (AAS). The
aerobic oxidation of olefins and sulfides under ambient conditions was efficiently enhanced under
the influence of the separable nanocatalyst. This oxidation system was found to oxidize various sulfides to
sulfones and different olefins to their respective epoxides as major products. Aerobic oxidation generally
requires high temperatures and/or long reaction times to occur. However, in this research, the aerobic
oxidation of olefins and sulfides was studied in the presence of Fe–porphyrin which was immobilized onto
multi-wall carbon nanotubes at room temperature and with a short reaction time of 60 min. The
separation and recycling of the catalyst and the byproduct of the oxidant were simple, effective, and
economic in this clean oxidation method. FTIR and leaching experiments after seven successive cycles
showed that the catalyst was very strongly anchored to the carbon nanotubes.
Received 6th May 2017,
Accepted 27th June 2017
DOI: 10.1039/c7nj01530e
rsc.li/njc
can be greatly decreased by immobilization of metalloporphyrins
Introduction
16–19
on different solid supports.
Since biological aerobic oxidation
The selective oxidation of organic substrates particularly by reactions include highly developed electron and proton transfer
2
0
metal salts and complexes is of high significance from an steps to activate molecular oxygen, the catalytic oxygenation
1
–4
academic, as well as industrial, point of view. The substantial of organic compounds with molecular oxygen at physiological
2
1,22
use of toxic and harmful oxidants in these catalytic systems has temperatures should involve particular reducing agents.
minimized considerably the applied significance of bio-inspired Besides, the low product selectivity obtained in aerobic oxidations
oxidation catalysis during the past few decades. Using nontoxic, is an important drawback for employing molecular oxygen as an
2
3
environmentally friendly and green oxidants such as hydrogen oxidant in the catalytic oxidations especially in epoxidation.
peroxide or molecular oxygen has attracted growing interest in Although there are several reports in the literature on the catalytic
5
–7
catalytic oxidation in recent years. Great interest is dedicated epoxidation with molecular oxygen as an oxidant, almost in all of the
to molecular oxygen because of its important advantages such as cases high temperatures and/or extremely long reaction times have
24–26
high active oxygen content, low cost, fast kinetics and relatively been selected as reaction conditions.
Herein, we wish to report
the aerobic oxidation of olefins and sulfides with excellent selectivity
8
–10
good reproducibility with no byproduct during the reaction.
Cytochrome P-450 as a significant monooxygenase enzyme in the presence of meso-tetrakis(p-hydroxyphenyl)porphyrinato-
catalyzes the aerobic oxidation of different organic substrates. iron(III) chloride anchored to functionalized multi-walled carbon
Consequently, metalloporphyrins as biomimetic catalysts have nanotubes [Fe(THPP)Cl@MWCNTs] under ambient conditions.
1
1–14
been used for this critical transformation for many years.
The heterogeneous catalyst was easily separated by simple
However, a problem generally encountered in metalloporphyrin filtration and reused at least five times without any significant
catalyzed oxidation processes is the deactivation of the catalyst deactivation of the heterogeneous metalloporphyrin.
1
1,15
by rapid oxidative degradation of metalloporphyrins,
which
Experimental section
Materials and characterization techniques
Department of Chemistry, K.N. Toosi University of Technology, P.O. Box 16315-1618,
Tehran 15418, Iran. E-mail: rayati@kntu.ac.ir, srayati@yahoo.com;
Fax: +98 21 22853650; Tel: +98 21 22850266
The chemicals, reagents and solvents were purchased from
†
Electronic supplementary information (ESI) available. See DOI: 10.1039/c7nj01530e Aldrich, Merck or Fluka chemical companies and were employed
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without further purification. Modified MWCNTs (multi-walled
carbon nanotubes containing –COOH groups, purity 96%) were
purchased from Shenzhen NTP Factory (China). Specifications of
MWCNT-COOH used in this study were as follows: outer diameter
(
10–20 nm), inner diameter (5–10 nm), length (30 mm), –COOH
2
À1
content (2%) and specific surface area (200 m g ). The electronic
absorption spectra were recorded on a PerkinElmer Lambda 25
UV-Vis spectrophotometer. Analyses for liquid samples were
accomplished using a quartz cell with 1 cm path length. The
morphology of the solid catalysts was investigated using trans-
mission electron microscopy (TEM) (Philips EM208). Thermo-
gravimetric analysis (TGA) was carried out using a Mettler-Toledo
TGA 851e. The modified MWCNTs and heterogeneous catalysts
were identified by X-ray powder diffraction (XRD) using a STOE
Scheme 1 Schematic preparation of Fe(THPP)Cl@MWCNTs.
a
diffractometer with Cu-K radiation (l = 0.15418 nm) at 40 kV and
4
0 mA. Infrared spectra were recorded (KBr pellets) on an ABB
À1
Bomem: FTLA 2000–100 in the 400 to 4000 cm range. The iron
content of the catalyst was determined by atomic absorption
spectrophotometry (AAS) with flame atomization (Varian AA240).
Quantification was performed by a standard addition method.
Gas chromatography experiments (GC) were performed with a Fig. 1 TEM image of the Fe(THPP)Cl@MWCNT.
Shimadzu GC-14B equipped with a flame ionization detector
(
3
FID) with a SAB-5 capillary column (phenyl methyl siloxane
0 m  320 mm  0.25 mm).
X-ray diffraction (XRD) patterns of MWCNTs and Fe(THPP)Cl@
MWCNTs in the 2y range between 101 and 801 are shown in Fig. 2.
Comparison of the XRD patterns of the original MWCNTs and
Fe(THPP)Cl@MWCNTs indicates that after immobilization of
General procedure for the oxidation reaction
Catalytic experiments were carried out in a 5 mL test tube. In a metalloporphyrin the characteristic diffraction peaks of the carbon
typical procedure, 0.95 mmol isobutyraldehyde (IBA) was added nanotubes at around 261 (sharp peak) and 431 (Fig. 2a and b) due
30
to a mixture of sulfide (0.063 mmol) or olefin (0.095 mmol) and to the graphitic planes remain intact. But the characteristic peaks
catalyst (0.0021 mmol) in CH CN (1 mL). Next, a balloon was of the attached Fe–porphyrin were not observed clearly, which
3
purged with oxygen and fixed on the top of the test tube. The suggests that the amount of Fe is probably small inside the walls of
mixture was stirred for 60 min, and eventually, the products the MWCNTs and the catalyst is also well dispersed onto the
31
were characterized by GC.
surface of the MWCNTs. The absence of iron peaks indicates
good dispersion of metalloporphyrin onto the surface of the solid
support.
Results and discussion
Preparation and characterization of Fe(THPP)Cl@MWCNTs
Fig. 3 shows the FT-IR spectra of MWCNTs and Fe(THPP)Cl@
MWCNTs to ensure the esterification step. The appearance of a
À1
À1
strong band at 1741 cm and 1387 cm due to the CQO and
the C–O stretch of the ester group, respectively, can be attributed
to the esteric bond constructed between the modified carbon
The free base porphyrin and metalloporphyrin were prepared
2
7
and purified by the methods reported previously (see S-1 in
the ESI†). Fe(THPP)Cl is anchored onto modified MWCNTs
through covalent bonding between the hydroxyl group of the
Fe–porphyrin and the carboxylic acid group of the MWCNTs
À1
nanotubes and metalloporphyrin. The signals at 1634 cm and
À1
1
510 cm were related to the stretching of the phenyl rings at
the meso positions and the CQN stretch at the center of the
2
8,29
according to previous reports
(see S-2 in the ESI†). The
32
porphyrin respectively.
schematic immobilization of Fe(THPP)Cl onto modified
MWCNTs is illustrated in Scheme 1.
The heterogeneous catalyst was characterized by different
techniques. The amount of catalyst loading on the modified
MWCNTs was estimated by atomic absorption spectroscopy
(AAS), and based on this value, the Fe content of the catalyst
was determined to be about 381 mmol per gram of the catalyst.
Transmission electron microscopy (TEM) of Fe(THPP)Cl
supported on the modified MWCNTs (Fig. 1) showed the
stability of the MWCNTs after iron porphyrin immobilization.
Fe(THPP)Cl nanoparticles are not visible on the surfaces of the
MWCNTs.
Fig. 2 XRD patterns of MWCNTs (a) and Fe(THPP)Cl@MWCNTs (b).
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Table 1 The effect of solvents on the aerobic oxidation of styrene
catalyzed by Fe(THPP)Cl@MWCNTs at room temperature
a
Entry
Solvent
Conversion (%)
1
2
3
4
5
6
7
Methanol
Ethanol
Water
1,2-Dichloroethane
Chloroform
Dichloromethane
Acetonitrile
0
0
0
38
85
91
100
a
Reaction conditions: the catalyst : styrene : isobutyraldehyde molar ratio
(balloon), solvent: 1 mL, reaction time: 60 min.
is 1 : 45 : 450 and O
2
highest conversion (100%) was achieved in acetonitrile after
0 min at room temperature. So, acetonitrile was used for the
6
Fig. 3 FT-IR spectra of MWCNTs (a) and Fe(THPP)Cl@MWCNTs (b).
rest of the optimizations.
The epoxidation reaction was tested in air (without any
special oxygen source) and the results were compared with the
results of the catalytic activity of the supported Fe–porphyrin for
epoxidation with molecular oxygen (preserved with a filled balloon).
Based on the results, the lower conversion in air with respect to
molecular oxygen is related to the oxygen content of air (Fig. 5).
Therefore, it should also affect the kinetics of the reaction.
The effectiveness of isobutyraldehyde as a reducing agent in the
epoxidation of styrene with molecular oxygen was investigated, and
the results show that the oxidation reaction did not proceed in the
absence or even in the presence of a small amount of isobutyr-
aldehyde (Table 2), which implies that the catalyst could not
activate molecular oxygen without aldehyde. It should be noted
that catalytic aerobic oxidation reactions in the absence of any
reducing agent need high temperatures and/or extremely long
Fig. 4 UV-Vis absorption spectra of (a) Fe(THPP)Cl and (b)
Fe(THPP)Cl@MWCNTs in DMF.
33,34
reaction times.
The optimum amount of IBA as a reducing
UV-Vis absorption spectra of dispersed Fe(THPP)Cl@MWCNTs
in DMF exhibit characteristic absorption peaks almost at the same
wavelengths of Fe(THPP)Cl, with a Soret band at 418 nm, which
confirms the presence of metalloporphyrins on the surface of the
modified carbon nanotubes (Fig. 4). Also this spectrum demon-
strates that the structure of Fe–porphyrin remains the same after
supporting on the modified MWCNTs (Fig. 4).
Also other characterization methods were used and all the
characterization studies completely confirm the presence of Fe–
porphyrin on the surface of the MWCNTs (see S-3 in the ESI†).
agent was also investigated and the results are presented in Table 2.
According to the results presented in Table 2, the aerobic oxidation
of styrene needs an IBA/substrate ratio of 5 for completion.
Different amounts of the catalyst were used in the oxidation
of styrene in order to find the optimum concentration of the
catalyst, and based on the results, the highest conversion was
achieved with 5.5 mg (2.2 mol%) of the catalyst. Furthermore, a
blank experiment revealed that the presence of the catalyst is
essential for the oxidation reaction (Fig. 6).
In order to establish the general applicability of the method,
2
oxidation of various olefins with O and isobutyraldehyde was
Heterogeneous catalytic oxidation of olefins
In order to evaluate the catalytic activity of the prepared
catalyst, oxidation of styrene with molecular oxygen or air in
the presence of Fe(THPP)Cl@MWCNTs and isobutyraldehyde
was studied at room temperature. Blank experiments showed
that the reaction did not proceed in the absence of the catalyst. In
a search for suitable reaction conditions to achieve the highest
conversion, the effects of different parameters that may affect the
conversion and selectivity of the reaction including the solvent,
the reaction time and the amount of isobutyraldehyde (IBA) as a
reducing agent were studied in detail.
The solvent can play an important role in the oxidation
reactions. The influence of different solvents on the oxidation
Fig. 5 The effect of the oxygen source on the catalytic activity of
Fe(THPP)Cl@MWCNTs for the epoxidation of styrene in CH CN (reaction
3
of styrene with O
2
was studied. Based on the data of Table 1, the conditions as mentioned in Table 1).
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Table 2 The effect of the isobutyraldehyde/substrate molar ratio on the
aerobic oxidation of styrene catalyzed by Fe(THPP)Cl@MWCNTs at room
temperature
Table 3 Oxidation of various olefins with O
oxygen donor in the presence of Fe(THPP)Cl@MWCNTs in CH
temperature
2
/isobutyraldehyde as the
3
CN at room
a
a
Entry
IBA/substrate
Conversion (%) Entry Alkene
Conversion (%) Selectivity to epoxide (%)
1
2
3
4
0
2.5
5
0
0
100
100
1
2
3
95
100
100
100
94
10
a
Reaction conditions: Fe(THPP)Cl@MWCNTs : styrene : isobutyraldehyde
(balloon), solvent: 1 mL, reaction time: 60 min;
is 1 : 45 : X and O
conversions and selectivities were determined by GC; epoxy styrene as
isolated yield.
2
100
4
5
73
96
85
78
6
7
100
93
b
61
24
100
8
100
a
The cat : alkene : IBA molar ratio is 1 : 45 : 225 and O
CN, reaction time: 60 min. Conversions and
catalyzed by Fe(THPP)Cl@MWCNTs (reaction conditions: the Cat : styrene : selectivities were determined by GC based on the starting alkene.
2
(balloon),
3
Fig. 6 The catalytic system for the epoxidation of styrene in CH CN
solvent: 1 mL of CH
3
b
IBA molar ratio is X : 45 : 450 and O
2
(balloon), reaction time: 90 min).
Epoxy trans stilbene was the sole product.
carried out in the presence of a catalytic amount of Fe(THPP)Cl@
MWCNTs in CH CN under the optimized reaction conditions
Table 3). As the results show, oxidation of alkenes leads to the
3
(
formation of the epoxide as the major product (78–100%). Also,
based on the results, the existence of electron withdrawing sub-
stituents decreased the catalytic activity of the heterogeneous
catalyst (Table 3, entries 4 and 5).
Catalytic epoxidation with molecular oxygen and aldehyde
in the presence of metalloporphyrins usually was considered a
free radical reaction based on the literature review as shown in
5
,35,36
Scheme 2.
by H abstraction from aldehyde in the presence of iron(III) Scheme 2 The proposed mechanism for the oxidation of olefins with O2/
porphyrin. In the second step, the acyl radical reacts with O
to IBA in the presence of Fe–porphyrin.
In the first step, an acyl radical was generated
2
give an acyl peroxy radical. Then, the acyl peroxy radical reacts
with another aldehyde to give the peroxy acid and another acyl
radical. Next, the high-valent Fe porphyrin intermediate could sulfides were subjected to the oxidation protocol using O /IBA
2
be generated by the reaction of another iron porphyrin with under a catalytic amount of Fe(THPP)Cl@MWCNTs (see S-4 in
peroxy acid. Finally, epoxidation of olefins can easily occur in the ESI†). Oxidation of methyl phenyl sulfide, diphenyl sulfide,
the presence of the active high-valent metal–oxo intermediate. benzyl phenyl sulfide, phenyl vinyl sulfide and 4-chlorophenyl
methyl sulfide led to excellent conversion (100%) after 90 min at
Heterogeneous catalytic oxidation of sulfides
room temperature under the optimized conditions. It is worth
Encouraged by the significant results achieved in the aerobic mentioning that sulfone was the isolated product.
epoxidation of olefins under the mild conditions, the activity of
Recycling procedure of Fe(THPP)Cl@MWCNTs
this catalytic system towards organic sulfides was also studied.
To confirm the efficiency of the catalyst, a blank experiment One of the most attractive features of the heterogeneous
was performed by using methyl phenyl sulfide and diphenyl catalyst is its capability of recovery from the reaction medium
sulfide as model substrates, which did not afford any product. In and reusability. The recovery of the catalyst from the liquid
order to establish the general applicability of the method, various medium was easy. For each of the repeated processes, after the
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In summary, Fe–porphyrin was coordinatively anchored onto
MWCNTs. Significant activity was observed for Fe(THPP)Cl@
MWCNTs toward the oxidation of olefins and sulfides under
mild conditions. For achieving the best yield of the product,
different parameters such as the solvent, reaction time, amount
of catalyst and reducing agent were investigated. The research
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oxygen as a standard ‘‘green’’ oxidant and also the simple reusability
of the catalyst is cost-effective and environment-friendly, and thus a
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