Functionalized multi-walled carbon nanotubes as an efficient reusable heterogeneous catalyst for green synthesis of N-substituted pyrroles in water

Article abstract of DOI:10.1039/c5ra12185j

In this protocol, the functionalization of multi-walled carbon nanotubes (MWCNTs) was carried out and the resulting sulfonated MWCNTs were characterized by FT-IR, XRD, SEM, BET, EDX, XPS and Raman spectroscopy that are each discussed separately in the text. Then, the MWCNT-SO₃H composite was applied as an efficient, recyclable heterogeneous catalyst for the synthesis of N-substituted pyrroles via the reaction of 2,5-dimethoxy tetrahydrofuran with primary amines under clean and mild conditions. In this reaction, the N-substituted pyrroles were obtained as beneficial and significant products in short reaction times (30-65 min) and good to excellent yields (40-92%) with high purity. The products were obtained through a simple work up procedure and characterized by FT-IR, ¹H NMR and ¹³C NMR. After the end of the reaction, the nanocatalyst was recovered and reused several times without efficient loss of its activity for the preparation of N-substituted pyrroles.

Full text of DOI:10.1039/c5ra12185j

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Functionalized multi-walled carbon nanotubes as
an efficient reusable heterogeneous catalyst for
green synthesis of N-substituted pyrroles in water†
Cite this: RSC Adv., 2015, 5, 76221
*
Hossein Naeimi and Mahla Dadaei
In this protocol, the functionalization of multi-walled carbon nanotubes (MWCNTs) was carried out and
the resulting sulfonated MWCNTs were characterized by FT-IR, XRD, SEM, BET, EDX, XPS and Raman
spectroscopy that are each discussed separately in the text. Then, the MWCNT–SO3H composite
was applied as an efficient, recyclable heterogeneous catalyst for the synthesis of N-substituted
pyrroles via the reaction of 2,5-dimethoxy tetrahydrofuran with primary amines under clean and
mild conditions. In this reaction, the N-substituted pyrroles were obtained as beneficial and
significant products in short reaction times (30–65 min) and good to excellent yields (40–92%) with
high purity. The products were obtained through a simple work up procedure and characterized
by FT-IR, 1H NMR and 13C NMR. After the end of the reaction, the nanocatalyst was recovered
and reused several times without efficient loss of its activity for the preparation of N-substituted
pyrroles.
Received 24th June 2015
Accepted 1st September 2015
DOI: 10.1039/c5ra12185j
www.rsc.org/advances
used for the oxidation of CNTs is the use of aqueous solutions
of oxidizing agents. The oxidation of MWCNTs using acid-
Introduction
sonication has been studied by Xing et al., who demon-
strated that the dispersion stability and solubility of the
treated MWCNTs in water improved signicantly.20 Ozone in
the presence of an oxidative ozonide cleavage reagent (H2O2)
has been described as a good oxidizing agent.21 Some
methods for sulfonating nanotubes have been reported to be
hydrothermally and microwave assisted under various
conditions.22–26
The substituted pyrroles are a major class of heteroaromatic
molecules that are components in a diverse range of biologically
active natural products and industrially useful compounds.
Pyrroles are present in different bioactive drug molecules
such as immunosuppressant, anti-inammatory and antitumor
agents.27 In view of their high importance, many methodologies
have been developed for the synthesis of pyrrole structures.28
Among them, the Paal–Knorr29 synthesis reminds the most
useful preliminary method for the produced pyrroles. Some of
the catalysts that have been utilized in these methods are;
glacial acetic acid,30 P2O5,31 FeCl3$7H2O,32 TfOH33 and b-
cyclodextrin.34
Since carbon nanotubes were detected by Iijima in 1991, they
have received remarkable attention. These elongated tubular
macromolecules, consisting of regular carbon hexagons in a
concentric procedure with both ends normally capped by
fullerene-like structures, can be seen as a graphite sheet rolled
into a nanoscale tubular form as single-walled carbon nano-
tubes (SWCNTs), or with additional graphene tubes around the
core of a SWCNT as multi-walled carbon nanotubes
(MWCNTs).1
Functionalized carbon nanotubes possess vast potential as
components of sensors and nanoscale electronics. The
panorama of applications has led to the successful function-
alization of single-walled carbon nanotubes (SWCNTs) and
multi-waledl carbon nanotubes (MWCNTs).2–10 The chemical
modication of CNTs is a method employed to increase their
number of applications. Different methods such as changing
the chemical behaviour using highly concentrated acids,11,12
wrapping, sheathing or graing the CNTs with polymer
chains13–16 and functionalizing in plasma conditions17–19 have
been used to better the functionalities of the CNTs surfaces.
The chemical oxidation of nanotubes is generally achieved
using gaseous or liquid oxidants. A more general procedure
In continuation of our previous research on catalytic reac-
tions for the synthesis of heterocyclic compounds,35,36 herein,
we hope to report the efficient sulfonation of MWCNTs and the
application of the resulting MWCNTs–SO3H composites as a
heterogeneous catalyst for the synthesis of N-aryl pyrroles.
These heterocycles were produced through the treatment of 2,5-
dimethoxy tetrahydrofuran with various primary aromatic
amines in water as a green media.
Department of Organic Chemistry, Faculty of Chemistry, University of Kashan, Kashan,
87317, Islamic Republic of Iran. E-mail: naeimi@kashanu.ac.ir; Fax: +98
3615912397; Tel: +98 3615912388
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c5ra12185j
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Experimental
General information
All commercially available reagents were used without further
purication and purchased from the Merck Chemical Company
in high purity. The used solvents were puried by standard
procedures. IR spectra were obtained as KBr pellets on a Perkin-
Elmer 781 spectrophotometer and on an Impact 400 Nicolet
FTIR spectrophotometer. 1H NMR and 13C NMR were recorded
in CDCl3 and d6-DMSO solvents on a Bruker DRX-400 spec-
trometer with tetramethylsilane as an internal reference. UV-vis
spectra were recorded with a Perkin-Elmer 550 spectropho-
tometer in a CH2Cl2 solvent. The nanostructures were charac-
terized using a Holland Philips Xpert X-ray powder diffraction
(XRD) diffractometer (CuK, radiation, k ¼ 0.154056 nm), at a
scanning speed of 2ꢀ minꢁ1 from 10ꢀ to 100ꢀ (2Ø). The
morphological study of the nanocomposites was investigated by
scanning electron microscopy (SEM, Leo 1455VP). The Electron
Dispersive X-ray (EDX) analysis of the catalyst was performed on
a Philips Holland model XL30. The surface area analysis (BET)
was performed using an automated gas adsorption analyzer
(NANO SORD). The Raman spectra were recorded with an
Almega Thermo Nicolet Dispersive Raman spectrometer excited
at 532 nm. X-ray photoelectron spectroscopy (XPS) spectra were
measured on an ESCA-3000 electron spectrometer with non-
monochromatized Mg Ka X-rays as the excitation source.
Melting points were obtained with a Yanagimoto micro melting
point apparatus and are uncorrected. The purity determination
of the substrates and reaction monitoring were accomplished
by TLC on silica-gel polygram SILG/UV 254 plates (from Merck
Company).
Scheme 1 Preparation of the MWCNTs–SO3H catalyst.
different times in water. The progress of the reactions was
monitored by TLC (ethyl acetate : n-hexane 6 : 4). Aer the
completion of the reaction, the catalyst along with the solid
product was collected by ltration from the reaction mixture.
The ltrated solid was dissolved in chloroform, the catalyst was
separated from the reaction mixture and washed with chloro-
form. The solvent was removed in vacuum to give the crude
product. Then, the crude product was recrystallized from
ethanol, the N-aryl pyrroles were obtained as pure products. The
recovered catalyst (0.019 g) was reused in the reaction for 4 runs.
All of the pure products were characterized by comparison of
their physical and spectral data with authentic samples.37–47
Results and discussion
General procedure for the synthesis of MWCNTs–SO3H.
Preparation of the MWCNTs–SO3H catalyst was performed in
two steps, as described in Scheme 1. MWCNTs (1 g) and
deionized water (100 ml) were added to a beaker and sonicated
for 30 min. Aer sonication, the solvent was removed under
vacuum and the obtained MWCNTs were transferred to another
ask containing 25 ml HNO3 (63%) and 25 ml HCl (37%), and
stirred at 80 ꢀC for 4 h under a nitrogen atmosphere. Then, the
solution was ltered under vacuum, washed throughout with
Preparation and characterization of the catalyst
In the rst step, the MWCNTs were converted to MWCNTs–
COOH with HCl (37%) and HNO3 (63%). This step was carried
out in order to maintain the purity of the MWCNTs and to
remove fullerene, amorphous carbon and catalyst particles such
as Fe, Co, Ni and increase the reactivity. This step also intro-
duced oxygen comprising groups, mainly the carboxyl groups
on the MWCNTs.48,49 Aer this step, sulfuric acid (98%) was
added to MWCNTs–COOH to obtain MWCNTs–SO3H.
ꢀ
deionized water and dried at 100 C for 10 h. In this step, the
MWCNTs–COOH material was obtained. The MWCNTs–COOH
(1 g) and deionized water (50 ml) were sonicated for 15 min.
Then, the water was ltered and the MWCNTs–COOH material
was dried. Then, 40 ml H2SO4 (98%) was added to the set-up at
250–270 ꢀC for 20 h under a nitrogen atmosphere. Aer cooling
the solution, the liquid was ltered and washed throughout
with deionized water several times. The obtained solid was
dried at 100 ꢀC for 12 h. Aer this step, the MWCNTs–SO3H
composite was obtained and used as a catalyst in the synthesis
of N-aryl pyrroles.
General procedure for the synthesis of N-aryl pyrroles.
MWCNTs–SO3H (0.04 g) was added as a catalyst to a mixture of
2,5-dimethoxy tetrahydrofuran (1.2 mmol) and different
primary aromatic amines (1 mmol) in water (5 ml) as solvent.
Then, the reaction mixture was heated (80 ꢀC) and stirred at
Fig. 1 (a) FT-IR spectrum of the MWCNTs, (b) FT-IR spectrum of
MWCNTs–SO3H.
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Fig. 2 (a) XRD spectrum of the pristine MWCNTs, (b) XRD spectrum of
MWCNTs–SO3H.
Fig. 6 The XPS spectra of MWCNTs–SO3H in the regions of C1s, O1s,
and S2p.
Fig. 3 (a) SEM image of the MWCNTs, (b) SEM image of MWCNTs–
SO3H.
Table 1 Surface area of the MWCNTs and MWCNTs–SO3H
S.A (total)
Name
S.A (BET) (m2 gꢁ1
)
(m2)
MWCNTs
MWCNTs–SO3H
62.775
30.925
0.816
0.619
Table 2 Investigation of the different catalysts at 80 ꢀC in water
Entry
Catalyst
Time (h)
Yield (%)
1
2
3
4
5
6
7
8
9
CuFe2O4
Fe3O4
TiO2
ZnS
MWCNTs–SO3H
MWCNTs
MWCNTs–COOH
MWCNTs/H2SO4 (a drop)
Ph-SO3H
12
5
12
8
1
7
5
5
3
40
30
—
—
30
—
10
—
17
Fig. 4 The EDX pattern of the MWCNTs–SO3H catalyst.
The prepared catalyst was characterized by SEM, EDX, XRD,
FT-IR, BET, XPS and Raman spectroscopy. The spectroscopic
data of FT-IR conrmed that the functionalization of the
MWCNTs with sulfonic acid groups had occurred (Fig. 1a
and b).
This spectrum shows peaks at about 1400 and 1100 cmꢁ1
which correspond to the SO2 asymmetric and symmetric
Fig. 5 Raman spectra of (a) the MWCNTs, and (b) MWCNTs–SO3H.
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MWCNTs–SO3H is shown in Fig. 4. For MWCNTs–SO3H, the
results obtained by EDX were as follows (%): C, 95.80; O, 1.73; S,
2.47. This image conrmed that oxygen and sulphur were
contained in the functionalized MWCNTs. Furthermore, the
EDX image infers that functional groups such as sulfonic acid
were attached to the MWCNTs.
Scheme 2 Reaction of 2,5-dimethoxy tetrahydrofuran and primary
Another technique used to characterize MWCNTs is Raman
spectroscopy. In the 1300–1600 cmꢁ1 region of the spectrum,
two bands are observed showing the characteristics of
MWCNTs. These bands point to the graphite band (G-band) and
the disorder and defects of the structure, named the D-band. A
higher ratio between the intensities of the D-band and the G-
band, noted as the ID/IG value, corresponds to a higher
proportion of sp3 carbon atoms. Fig. 5 shows the Raman spectra
of the MWCNTs and MWCNTs–SO3H. As shown in Fig. 5a, the
spectrum exhibits two peaks at about 1345 and 1583 cmꢁ1. The
feature at 1583 cmꢁ1 is the G-band and the origin of the line at
1345 cmꢁ1 is the D-band with regards to the MWCNTs. Also, in
Fig. 5b, the two peaks at 1349 and 1583 cmꢁ1 are the D-band
and the G-band of MWCNTs–SO3H, respectively. It was calcu-
lated that the ID/IG values of the MWCNTs and MWCNTs–SO3H
are 0.85 and 1.70, respectively. This increase in the ID/IG value
indicates that some of the sp2 carbon atoms were converted to
sp3 carbon atoms on the side walls of MWCNTs. This matter can
be related to the functionalization of the MWCNTs by sulfonic
acid groups.
XPS spectra were measured to investigate the chemical
valences of the functional groups attached to the surface of the
MWCNTs. As shown in Fig. 6, the spectra of the C1s region, O1s
region and S2p region were assigned. The main peak appears at
284.9 eV with a non-symmetrical edge. This peak generally
refers to the sp2 carbon–carbon double bond or the sp3 carbon–
carbon single bond.50 Also, this peak is related to the sp2-
hybridized graphite carbon. The peak at 288.7 eV can be
determined to be due to the carbon atoms bound to oxygen or
sulphur in functionalized groups e.g. carboxylic acid or sulfonic
acid groups.50 The O1s region shows that all the peaks occur in
the range of 530.9 to 534.2 eV, with a peak maxima at ꢂ533.1
eV.22 In the S2p region, the main peak of sulphur in the sulfonic
acid groups was observed at 169.2 eV. The peak at 169.2 eV is
related to a higher oxidation state of sulphur in the SO3H
groups.22
aromatic amines.
Table 3 Optimization of the catalyst amount at 80 ꢀC in water
Amount of catalyst
Entry
(g)
Time (min)
Yield (%)
1
2
3
4
5
0.01
0.02
0.03
0.04
0.05
120
120
90
45
45
30
45
60
92
92
Table
4 Optimization of different solvents in the presence of
MWCNTs–SO3H (0.04 g) at 80 ꢀ
C
Entry
Solvent
Time (min)
Yield (%)
1
2
3
4
EtOH
CH3CN
DMF
30
30
30
30
5
40
80
90
H2O
Table 5 Optimization of temperature in the presence of MWCNTs–
SO3H (0.04 g) in water
Entry
Temp. (ꢀC)
Time (min)
Yield (%)
1
2
3
4
5
60
70
80
45
45
45
45
45
70
83
92
90
87
90
100
stretching modes, respectively. In addition, in the low frequency
part of the spectrum, the line at 670 cmꢁ1 is assigned to the S–O
stretching mode and the peak shown at 582.96 cmꢁ1 is assigned
to the C–S stretching mode, suggesting the existence of covalent
sulfonic acid groups. The broad band centred at about 3400
cmꢁ1 is the contribution of the acidic OH group. The XRD
images of the MWCNTs before and aer the H2SO4 treatment
are shown in Fig. 2a and b. As can be seen in Fig. 2, the XRD
pattern for the MWCNTs (Fig. 2a) has a slight difference in peak
width compared to the peak related to MWCNTs–SO3H (Fig. 2b)
suggesting that the structure of the MWCNTs was not changed
aer the functionalization to MWCNTs–SO3H.
The surface area of the initial MWCNTs and MWCNTs–SO3H
as a target solid catalyst was determined by a BET technique.
The corresponding results were indicated in Table 1. The
decrease in the total and unit surface of the MWCNTs aer
sulfonation proved the presence of –SO3H as acidic groups on
the surface of the solid catalyst (Table 1). It can be concluded
that the sulfonation of carbon nanotubes had taken place,
because the physical adsorption of nitrogen gas aer func-
tionalization using sulphuric acid was decreased to nearly 50%
lower than that for the raw MWCNTs.
The SEM images of the MWCNTs before and aer the H2SO4
treatment are shown in Fig. 3. Compared with the primitive
MWCNTs, the functionalized MWCNTs were covered by a layer
and they are severely entangled and form a big agglomeration
that results in thickened MWCNTs. Also, the EDX image of
In order to investigate the catalytic activity of MWCNTs–
SO3H, it was compared compared with other catalysts. The
reaction of 2,5-dimethoxy tetrahydrofuran with 4-methoxy
aniline in a 1.2 : 1 molar ratio was done in the presence of
various catalysts under thermal conditions in water as a solvent.
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Table 6 Synthesis of various N-aryl pyrroles using MWCNTs–SO3H (0.04 g) as catalyst under thermal conditions (80 ꢀC) in water
Entry
Amine
Product
Time (min) : yielda (%)
1
2
3
45 : 88
50 : 89
45 : 92
4
5
50 : 87
30 : 92
6
30 : 90
7
8
55 : 88
50 : 88
9
55 : 78
10
11
12
65 : 40
60 : 72
60 : 70
13b
14b
15c
720 : 62
360 : 66
10 : 70
16c
10 : 77
a
c
Isolated yields. b Reaction conditions: 10% MgI2(OEt2)n, CH3CN, 80 ꢀC, Lit.51 Reaction conditions: AcOH, MW, 170 ꢀC, Lit.37
It was found that for the reactions in the presence of CuFe2O4,
Then, the optimization of the catalyst amount in the reaction
Fe3O4, TiO2, ZnS nano particles, MWCNTs, MWCNTs–COOH, was investigated. The reaction of 2,5-dimethoxy tetrahydrofuran
MWCNTs/H2SO4 and Ph-SO3H, the products were not obtained with 4-methoxy aniline in a 1.2 : 1 molar ratio was carried out in
in suitable yields (Table 2), while the reaction in the presence of different concentrations of catalyst under thermal conditions in
MWCNTs–SO3H as a heterogeneous catalyst in the same water. The corresponding results are presented in Table 3. By
conditions achieved appropriate results on the basis of yield consideration of the results in Table 3, the optimum amount of
and reaction time (Table 2, entry 5). Then, the reaction was MWCNTs–SO3H used as a catalyst in this reaction was obtained
carried out in the presence of MWCNTs–SO3H (Scheme 2).
as 0.04 g per one mol of aniline derivative (Table 3, entry 4).
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Fig. 7 Reusability of the catalyst in the reaction.
Scheme 3 Plausible reaction mechanism for the preparation of N-aryl
pyrroles.
summarized in Table 6. In this study, N-aryl pyrroles as prod-
ucts in an efficient method were prepared through the reaction
of 2,5-dimethoxy tetrahydrofuran and various primary aromatic
amines in the presence of a heterogeneous catalytic amount of
MWCNTs–SO3H (0.04 g) under thermal conditions in water. A
comparison of the present method with previous works using
other catalysts was done and the related results were indicated
in Table 6, entries 13–16.37,51
Fig. 8 1H NMR spectrum of 1-(3-nitro phenyl)-1H-pyrrole.
Recycling of the catalyst
For the practical application of heterogeneous catalysts, the life-
time of the MWCNTs–SO3H and its level of reusability are very
important features. At the end of each reaction, the catalyst was
isolated by ltration, washed exhaustively with chloroform and
ethanol, and dried at 100 ꢀC for 24 h before being used with
fresh materials. The catalyst was reused for 4 runs; the yields
ranged from 95% to 90% (Fig. 7).
The structures of N-aryl pyrroles as products were conrmed
by physical and spectroscopic data such as their melting point,
IR, 1H NMR and 13C NMR spectroscopy. The 1H NMR spectrum
of 1-(3-nitrophenyl)-1H-pyrrole shows signals around d ¼ 6.33–
7.34 ppm that are assigned to the protons of the aromatic rings
(Fig. 8). In the 13C NMR spectra of the above mentioned
compound, signals are shown at around d ¼ 110.0–150.0 ppm
that are assigned to the carbon atoms of the aromatic rings
(Fig. 9).
A plausible mechanism52 for the formation of N-aryl pyrroles
is presented in Scheme 3. As can be seen, in the rst step, the
acetal group of 2,5-dimethoxy tetrahydrofuran was protonated
to form compound a. Then, the nucleophilic attack of hydroxyl
group of the substrate to the carbon atom of the acetal occurred
to produce the intermediate b. Next, b was protonated by
MWCNTs–SO3H and dehydrated to convert to the dicarbonyl
compound (d). Then, d was protonated by MWCNTs–SO3H,
then nucleophilic attack of the amino group to the carbonyl
Fig. 9 13C NMR spectrum of 1-(3-nitro phenyl)-1H-pyrrole.
Aer the optimization of catalyst amount, the reaction was
carried out in various solvents (Table 4). The best results were
obtained using water as solvent (Table 4, entry 4). Then, the
reaction of 2,5-dimethoxy tetrahydrofuran with 4-methoxy
aniline (1.2 : 1 molar ratio) in the presence of MWCNTs–SO3H
catalyst (0.04 g) in aqueous solution was carried out at various
temperatures (Table 5). The best results were obtained at 80 ꢀC
(Table 5, entry 3).
To ascertain the limitation of this protocol, the reactions of
2,5-dimethoxy tetrahydrofuran with several primary aromatic
amines were carried out according to the general experimental
procedure. The corresponding products and their results are
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group of e took place to produce f. Finally, f was protonated, 16 Y. Lin, A. M. Rao, B. Sadanadan, E. A. Kenik and Y. P. Sun, J.
cyclized and dehydrated to give N-aryl pyrrole as a desired
Phys. Chem., 2002, 106, 1294.
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Conclusion
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22 G. S. Duesberg, R. Graupner, P. Downes, A. Minett, L. Ley,
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24 H. Yu, Y. Jin, Z. Li, F. Peng and H. Wang, J. Solid State Chem.,
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In this research, the synthesis of N-aryl pyrroles using 2,5-
dimethoxy tetrahydrofuran with different primary aromatic
amines was described. This reaction was performed in the
presence of a catalytic amount of MWCNTs–SO3H (0.04 g) under
thermal conditions. The corresponding products were obtained
in excellent yields, high purity and short reaction times. The
products were conrmed by physical and spectroscopic data
such as their melting point, FTIR, 1H NMR, 13C NMR
spectroscopy.
Acknowledgements
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The authors are grateful to the University of Kashan for sup-
porting this work by grant number 159148/52.
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