Amidosulfonic acid supported on graphitic carbon nitride: novel and straightforward catalyst for Paal–Knorr pyrrole reaction under mild conditions

Article abstract of DOI:10.1007/s00706-021-02771-1

A novel heterogeneous acidic catalyst was prepared by in situ immobilization of amidosulfonic acid (NH₂SO₃H) on graphitic carbon nitride (g-C₃N₄) under hydrothermal conditions. The textural morphology of NH₂SO₃H/g-C₃N₄ nanocomposite was characterized via powder X-ray diffraction, FT-IR, TGA, EDX, and scanning electron microscopy. The spatial arrangement of the amidosulfonic acid on the surface of g-C₃N₄ leads to the construction of a unique solid acid structure, resulting in a substantial enhancement of catalytic activity toward a high efficient preparation of pyrroles by Paal–Knorr reaction. The reactions undergo completion readily with good to excellent yields, with simple purification in an environmentally friendly manner. The NH₂SO₃H/g-C₃N₄ nanocomposite can be readily recycled, and no noteworthy reduction in the catalytic activity detected after four runs. Graphic abstract: [Figure not available: see fulltext.]

Full text of DOI:10.1007/s00706-021-02771-1

Monatshefte für Chemie - Chemical Monthly
https://doi.org/10.1007/s00706-021-02771-1
ORIGINAL PAPER
Amidosulfonic acid supported on graphitic carbon nitride: novel
and straightforward catalyst for Paal–Knorr pyrrole reaction
under mild conditions
Asieh Azhdari1 · Najmedin Azizi1,2 · Haleh Sanaeishoar1 · Elham Tahanpesar1
Received: 5 February 2021 / Accepted: 20 April 2021
© Springer-Verlag GmbH Austria, part of Springer Nature 2021
Abstract
A novel heterogeneous acidic catalyst was prepared by in situ immobilization of amidosulfonic acid (NH2SO3H) on graphitic
carbon nitride (g-C3N4) under hydrothermal conditions. The textural morphology of NH2SO3H/g-C3N4 nanocomposite was
characterized via powder X-ray difraction, FT-IR, TGA, EDX, and scanning electron microscopy. The spatial arrangement
of the amidosulfonic acid on the surface of g-C3N4 leads to the construction of a unique solid acid structure, resulting in a
substantial enhancement of catalytic activity toward a high efcient preparation of pyrroles by Paal–Knorr reaction. The
reactions undergo completion readily with good to excellent yields, with simple purifcation in an environmentally friendly
manner. The NH2SO3H/g-C3N4 nanocomposite can be readily recycled, and no noteworthy reduction in the catalytic activity
detected after four runs.
Graphic abstract
Keywords Amidosulfonic acid · Nanocomposite · Pyrrole · Paal–Knorr reaction · g-C3N4
Introduction
systems for green organic synthesis has continued to attract
considerable synthetic attention. The traditional industrial
catalysts are rare precious metals and metal oxides. The
increasingly thoughtful environmental contamination prob-
lems limited their real-world uses to a certain extent that the
most important disadvantages. Carbon-based catalysts have
progressively been applied to interchange the modern metal-
based catalysts for improving environmental safety [2–5].
Carbon nitrides as metal-free polymeric porous materi-
als have become a research hot spot for unique electrical,
optical, and structural properties. The suitable photocata-
lytic activity, inexpensive starting materials, straightforward
construction, excellent thermal and chemical stability, and
environmentally friendly nature of graphitic carbon nitride
(g-C3N4) have made massive applications in the felds of
Sustainable chemistry is a transformative approach and strat-
egy in the industrial processes that cause negligible environ-
mental infuence, and it explores decreasing and restricting
toxic waste at its source. Greenization of routine industrial
procedures and the expansion of innovative, ecologically
friendly processes depend signifcantly on improving cata-
lyst designs [1]. The interest in developing efcient catalytic
* Najmedin Azizi
azizi@ccerci.ac.ir
1
Department of Chemistry, Ahvaz Branch, Islamic Azad
University, Ahvaz, Iran
2
Chemistry and Chemical Engineering Research Center
of Iran, P.O. Box 14335-186, Tehran, Iran
Vol.:(0123456789)
1 3

A. Azhdari et al.
photocatalytic reaction, energy generation, catalysts, and
ecological remediation [6]. Carbon nitrides were obtained
via simple polymerization of inexpensive nitrogen-based-
monomers like cyanamide, urea, thiourea, and melamine
by thermal polycondensation reaction [7–18]. Nevertheless,
pure g-C3N4 has been restricted by the low photocatalysis
yields and specifc surface area, so various modifcations
in the preparation of g-C3N4 have been examined in recent
years [19–23].
separable and inexpensive heterogeneous catalyst under eco-
friendly conditions for the synthesis of pyrrole derivatives.
By variation of the two components, we have developed
efcient and operationally simple protocols in which both
support and catalyst are employed in catalytic quantities.
The combination of the NH2SO3H/g-C3N4 nanocomposite
involves two steps: preparation of the g-C3N4 nanosheet sup-
port and grafting of amidosulfonic acid layered graphitic
carbon nitride by simple mortar grinding. The structural,
morphological, and elemental compositions of new nano-
composite were studied using XRD, SEM, EDS, TGA, and
FT-IR spectroscopy.
Heterocyclic structures are involved in all biological pro-
cesses, such as energy provision, the transmission of nerve
impulses, and the transfer of genetic information in nature.
Nitrogen-containing aromatic and non-aromatic heterocy-
cles hold important roles among other heterocycles due to
their signifcant biological activity [24]. Furthermore, the
pyrrole derivatives are important azaheterocycles with vari-
ous natural, pharmaceuticals, and agrochemicals activities
[25]. Therefore, bearing in mind that the pyrrole nucleus
has recently gained increasing importance, there is a con-
tinuous pursuit of designing new synthetic methods and the
techniques for the deployment of known ways to produce
these heterocyclic compounds from low-cost and read-
ily available starting materials [26–28]. Due to this high
level of importance and utility, considerable interest has
been paid to the synthesis of pyrrole derivatives in recent
years. Among the various approaches for the synthesis of
pyrroles, the most frequently accepted methods are Bar-
ton–Zard, Paal–Knorr, Clauson-Kaas, and Hantzsch reac-
tions [29, 30]. The Paal–Knorr reaction is one of the oldest
and simplest protocols for synthesizing biologically and
pharmacologically active pyrrole scafolds. Generally, the
Paal–Knorr reaction is carried out with Lewis acid or Brøn-
sted acid as catalysts (i.e., H2SO4, p-toluenesulfonic acid,
and BiCl3) [31] with appropriately substituted 1,4-dicar-
bonyl compounds in organic solvents [32]. However, these
approaches have disadvantages, for example, moderate
yields, toxic, moisture-sensitive reagents and catalysts, and
tedious work-up. Despite the growing number of pyrrole
core publications containing heterocyclic scafolds, there
are a few practically simple reports under environmentally
benign reactions [33–35].
The SEM image of the NH2SO3H/g-C3N4 nanocompos-
ite showed an agglomerate of small sheet-like nanoparti-
cles with cracks and pores on the surface of g-C3N4. Fur-
thermore, it can observe that NH2SO3H spherical particles
attached to the sheet-like structure of g-C3N4 (Fig. 1).
The EDS spectra of nanocomposite are shown in Fig. 2.
It demonstrates that the nanocomposite consists of S, O,
C, and N elements, which also supports the existence of an
interfacial connection between the most active and essential
tri-s-triazine terminal-linked g-C3N4 and NH2SO3H.
FT-IR spectroscopy also confrmed the structure and
functional groups of the prepared nanocomposite. The broad
peak at around 3000–3500 cm−1 is mainly the stretching
vibrational modes of N–H and the O–H bonds of the -NH2
groups in the C3N4 and NH2SO3H and adsorbed H2O from
the air. The peaks in the range of 11,200–1695 cm−1 are
observed in the nanocomposite, which is the typical hepa-
tize-based molecular units’ characteristic vibrations (Fig. 3).
The XRD analysis provides valuable evidence for the
formation of NH2SO3H/mpg-C3N4 nanocomposite (Fig. 4).
The wide-angle XRD pattern of NH2SO3H/g-C3N4 con-
frms the phase structure of the graphitic carbon nitride and
NH2SO3H.
In the thermal gravimetric analysis (TGA), the synthe-
sized NH2SO3H/g-C3N4 was heated from room temperature
to 700 °C. The frst weight loss below 200 °C can be attrib-
uted to the vaporization of water (2.96%) and the second
one from 200 to 700 °C corresponded to the slow structure
decomposition of NH2SO3H (4.43%). The TGA result has
also verifed the attachment of NH2SO3H on the surface of
g-C3N4 nanoparticles (Fig. 5).
Results and discussion
Catalyst surface acidity was characterized by applying
the acid–base back titration using HCl and NaOH aqueous
solutions. 100 mg of composite was dispersed in 50 cm3
of 0.1 M NaOH solution and slowly shaken at room tem-
perature for 24 h and then was centrifuged. Next the alkali
solution was titrated against a 0.1 M HCl standard solution.
Acid density is determined by acid–base back titration was
2.86 mmol/g.
As part of our ongoing interest in developing novel hetero-
geneous carbon-based support as a new catalyst for organic
transformations [36–39], we recently developed a novel class
of functional graphitic carbon nitride catalysts. We applied
it to the environmentally benign synthesis of biologically
active heterocyclic compounds. Encouraged by this success
and aiming to indicate this nanocomposite catalysis’s ef-
ciency, we decided to introduce (NH2SO3H/g-C3N4) a new
After the nanocomposite’s preparation and characteri-
zation, their activity was evaluated by pyrrole synthesis
1 3

Amidosulfonic acid supported on graphitic carbon nitride: novel and straightforward catalyst…
Fig. 1 SEM images of
NH2SO3H/mpg-C4N4 nanocom-
posite
Fig. 2 EDX analysis of
NH2SO3H/mpg-C3N4 nanocom-
posite
1 3

A. Azhdari et al.
Fig. 3 a IR spectra of g-C3N4;
b IR spectra of amidosul-
fonic acid; c IR spectra of
NH2SO3H/g-C3N4 nanocatalyst
in the model reaction to optimize the reaction conditions
(Table 1). A 20-mg nanocomposite with aniline (1.0 mmol)
and 2,5-hexanedione (1.0 mmol) was added to 1.0 cm3 of
ethanol, and the solution was stirred for 100 min at room
temperature. The model reaction was carried out without
nanocomposite, and it was found that no reaction takes
place, and the starting materials were recovered. The nano-
composite elements g-C3N4 and NH2SO3H alone are also
used as catalysts for the model reaction and low conversion
found. In the next step, a wide variety of supported acidic
C3N4 were prepared to investigate their catalytic role in the
pyrrole synthesis. It was found that all heterogeneous solid
acid-catalyzed reactions defantly. However, amidosulfonic
acid (Table 1, entry 11) and sulfonic acid (Table 1, entry 10)
gives an excellent conversion under the same conditions.
It was found that 20 mg catalyst (Table 1, entry 10) was
efcient to aford incredible conversion, and the additional
amount of catalyst (50 mg) did not increase the yields nota-
bly (Table 1, entry 12). The lower amount of catalyst (5 mg)
needed a longer reaction time for maximum efciency, and a
lower yield was obtained (Table 1, entry 13). Solvent plays
a vital role in these transformations, so various kinds of
common organic solvents are investigated (Table 1, entries
12–17). The optimum conversion in ethanol as solvent was
achieved. After a survey of diferent reaction conditions, the
optimal conditions were obtained using one equivalent of
aniline and one equivalent of 2,5-hexandione in the presence
of 20 mg of catalyst in ethanol for 100 min at r.t. (Table 1).
Under the optimal reaction conditions mentioned above,
we carried out a series of reactions using various aromatic
amines with 2,5-hexanedione or 2,5-dimethoxytetrahy-
drofuran in the presence of 20 mg of NH2SO3H/g-C3N4 to
1 3

Amidosulfonic acid supported on graphitic carbon nitride: novel and straightforward catalyst…
Fig. 4 XRD spectra of
NH2SO3H/g-C3N4 nanocom-
posite
Fig. 5 TGA curve of NH2SO3H/g-C3N4 nanocomposite
explore the scope of this present protocol (Table 2). For the
aromatic amines, the nature of substituents present in the
aromatic ring afects the yields of reactions. The reaction
proceeded well using aromatic amine with electron-donating
groups such as alkyl, alkoxy, and amino in all positions as
substrates (Table 2). In the cases of strong electron-with-
drawing groups such as nitro, cyanide, and halogen on
aniline, a moderate to good yield could be achieved. With
the satisfactory results of aromatic amines, we also investi-
gated the diferent primary and secondary aliphatic amines
in this system to establish the full scope and the applicability
of this reaction. The condensation reaction of acyclic and
cyclic aliphatic amines smoothly gave corresponding pyrrole
derivatives in good to excellent yields (Table 2).
1 3

A. Azhdari et al.
Table 1 The efect of various parameters on the synthesis of pyrrole
Entry
Catalysts
Catalyst/mg
Time/min
Solvent
Yield/%
1
2
3
4
5
6
7
8
–
–
300
240
240
240
240
240
240
240
240
120
100
100
240
240
240
240
240
Ethanol
Ethanol
Ethanol
Ethanol
Ethanol
Ethanol
Ethanol
Ethanol
Ethanol
Ethanol
Ethanol
Ethanol
Ethanol
DMF
23
18
78
65
73
64
35
67
64
90
93
93
71
48
35
42
43
g-C3N4
Sulfamic acid
20
20
20
20
20
20
20
20
20
20
50
5
g-C3N4/acidic Al2O3
g-C3N4/HCl
g-C3N4/CF3COOH
g-C3N4/basic Al2O3
g-C3N4/sulfanilic acid
g-C3N4/H2SO4
9
10
11
12
13
14
15
16
17
g-C3N4 @ SO3H
g-C3N4/amidosulfonic acid
g-C3N4/amidosulfonic acid
g-C3N4/amidosulfonic acid
g-C3N4/amidosulfonic acid
g-C3N4/amidosulfonic acid
g-C3N4/amidosulfonic acid
g-C3N4/amidosulfonic acid
20
20
20
20
Ethyl acetate
THF
H2O
Reaction conditions: aniline (1.0 mmol), 2,5-hexanedione (1.0 mmol), 1.0 cm3 solvent, and catalyst
In addition to high catalytic activity and low-cost of the
nanocomposite, the facile separation and recycling is another
desirable feature for practical application in industrial pro-
cesses. So the reusability of NH2SO3H/g-C3N4 was tested
in the model reaction for 10 mmol scale and the yields cor-
responding to four consecutive runs are shown in Fig. 6.
After each run, the nanocomposite was recovered by simple
centrifugation and reused after treatment with ethyl acetate
for the next runs the slight decrease in the yield after fve
cycles, indicating excellent stability under air without appar-
ent deactivation.
provides easy and cost-efective access to the N-substituted
pyrrole derivatives in good to attractive yields under envi-
ronmentally benign conditions. This fnding demonstrates
that the NH2SO3H/g-C3N4 nanocomposite is a promis-
ing heterogeneous catalyst with a simple, convenient, and
straightforward work-up process.
Experimental
All starting materials and nanocomposite resources were
commercially available and were purchased from Aldrich
or Merck Company and were used as received. Water and
other solvents were twice distilled. 1H NMR spectra were
recorded on a 500-MHz NMR spectrometer using CDCl3
or DMSO-d6 as a solvent. Melting points were recorded on
Buchi 535 melting point apparatus.
To show the efectiveness of the NH2SO3H/g-C3N4 com-
posite with some reported procedures for the preparation of
N-substituted pyrrole derivatives, the results of comparison
with reported catalysts are represented in Table 3.
Conclusion
General procedure for the synthesis of g‑C3N4
In summary, NH2SO3H/g-C3N4 nanocomposite synthesized
by immobilizing NH2SO3H on the surface of g-C3N4 under
grinding conditions as a green technique with the simple,
fast, efcient, and less-toxic procedure. The obtained catalyst
shows the excellent catalyst activity in the synthesis of vari-
ous pyrrole derivatives under mild conditions. The method
Bulk g-C3N4 simply prepared by hydrothermal calcining of
melamine in air. Usually, 10 g of melamine was placed in
a covered 50 cm3 alumina crucible and heated 550 °C for
4 h in a tube furnace at a heating rate of 5 °C/ min and
kept for 3 h at 550 °C with a loose cover. After cooling to
1 3

Amidosulfonic acid supported on graphitic carbon nitride: novel and straightforward catalyst…
Table 2 Synthesis of various N-substituted pyrroles in green catalytic system
Entry
1
Amine
Yield/%a,b
Time/min
M.p./°C found
References
1
2
1a
1b
93
82
100
140
52–54
59–60
53–55 [53]
58–60 [41]
3
4
1a
1b
98
90
130
150
Liq.
Liq. [52]
49–50
50–51 [39]
5
1b
79
190
182–184
44–46
184–186 [41]
46–47 [49]
6
1b
69
180
7
8
1a
1b
74
76
190
190
74–75
92–93
74–76 [53]
93–94 [49]
9
1b
81
160
51
50–51 [41]
10
11
1a
1b
90
79
160
190
57–58
88–89
57–58 [53]
87–88 [49]
12
13
14
1a
1a
1a
83
75
91
180
180
180
62–62
57–59
95–97
64–66 [50]
58–60 [52]
95–98 [28]
15
1a
97
180
Liq.
Liq. [51]
16
17
1a
1a
76
79
190
190
Liq.
Liq.
Liq. [29]
Liq. [34]
aIsolated yields
bReaction conditions: amine (1.0 mmol), 2,5-hexanedione or 2,5-dimethoxytetrahydrofuran (1.0 mmol), 1.0 cm3 solvent, and 20 mg catalyst, r.t.,
100–190 min
1 3

A. Azhdari et al.
Fabrication of NH2SO3H/g‑C3N4 nanocomposite
In a typical synthesis, 400 mg of amidosulfonic acid was
mixed with 1 g of exfoliated g-C3N4 nanosheet ground in
the crucible for three min until it gained steady powder
obtained (Fig. 7).
General procedure for the synthesis of pyrrole
To a solution of amine (1.0 mmol) and 2,5-hexanedione or
2,5-dimethoxytetrahydrofuran (1.0 mmol) in 1 cm3 ethanol
at room temperature 20 mg NH2SO3H/g-C3N4 added. The
reaction mixture was stirred at r.t. for 100–190 min and dur-
ing this time, the reaction was monitored by GC or TLC.
After completion of the reactions, 5 cm3 warm ethyl acetate
was added, and the catalyst was removed using a centri-
fuge and washed with ethyl acetate, and reused for the next
reactions. The solvent was removed under reduced pressure.
Then, the crude product was recrystallized from ethanol
or by column chromatography (n-hexane/ethyl acetate) to
aford the desired pure products [40–53].
Fig. 6 Recyclability test of the NH2SO3H/g-C3N4 in synthesis of
pyrrole; reaction conditions: aniline (1.0 mmol), 2,5-hexanedione
(1.0 mmol), 1.0 cm3 ethanol, and 20 mg catalyst NH2SO3H/g-C3N4
r.t., 180 min
room temperature, a light yellow powder was obtained and
stored for further use. The bulk g-C3N4 is used as the precur-
sor for the thermally exfoliated g-C3N4 porous nanosheets
in the air. In detail, 2 g of bulk carbon nitride in 10 cm3
uncovered crucible is heated to 550 °C with a heating rate
of 2 °C min−1 and maintained for 3 h until white g-C3N4
nanosheet obtained.
Table 3 Comparison of the activity of NH2SO3H/g-C3N4 nanocomposite with the reported methods
Entry
Catalyst
Reaction conditions
Time/min
Yield/%
References
1
2
3
4
5
6
7
8
Montmorillonite K-10
KOH
Squaric acid
CuI
FeCl3
Dowex 50w
FePO4
Dichloromethane, 90 ℃
Dimethyl sulfoxide, 100 ℃
Water, 60 ℃
Dimethyl sulfoxide, 20 ℃
Water, 60 ℃
Water, 130 ℃
Neat, r.t
Neat, r.t
60
97
63
90
88
95
85
99
95
96
96
90
98
92
93
[44]
[45]
[46]
[47]
[48]
[43]
[41]
[40]
[40]
[40]
[42]
[33]
[35]
This work
180
240
120
120
300
480
600
600
600
75
KSF
9
Bi(NO3)3
Polystyrene
PS/AlCl3
ZrCl4
DES
NH2SO3H/g-C3N4
Neat, r.t
10
11
12
13
14
Water/ethanol, r.t
Acetonitrile, 80 ℃
US, r.t
DES
Ethanol, r.t
7
10
100
1 3

Amidosulfonic acid supported on graphitic carbon nitride: novel and straightforward catalyst…
Fig. 7 Preparation of nanocomposite
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