Sulfonated reduced graphene oxide as a highly efficient catalyst for direct amidation of carboxylic acids with amines using ultrasonic irradiation

Article abstract of DOI:10.1016/j.ultsonch.2015.10.009

Sulfonated reduced graphene oxide nanosheets (rGO-SO₃H) were prepared by grafting sulfonic acid-containing aryl radicals onto chemically reduced graphene oxide (rGO) under sonochemical conditions. rGO-SO₃H catalyst was characterized by Fourier-transform infrared (FT-IR) spectroscopy, Raman spectroscopy, scanning electron microscopy (SEM), X-ray diffraction (XRD), thermogravimetric analysis (TGA), differential scanning calorimetry (DSC) and X-ray photoelectron spectroscopy (XPS). rGO-SO₃H catalyst was successfully applied as a reusable solid acid catalyst for the direct amidation of carboxylic acids with amines into the corresponding amides under ultrasonic irradiation. The direct sonochemical amidation of carboxylic acid takes place under mild conditions affording in good to high yields (56-95%) the corresponding amides in short reaction times.

Full text of DOI:10.1016/j.ultsonch.2015.10.009

Ultrasonics Sonochemistry 29 (2016) 371–379
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Short communication
Sulfonated reduced graphene oxide as a highly efficient catalyst for
direct amidation of carboxylic acids with amines using ultrasonic
irradiation
Maryam Mirza-Aghayan a, , Mahdieh Molaee Tavana a, Rabah Boukherroub b
⇑
a Chemistry and Chemical Engineering Research Center of Iran (CCERCI), P.O. BOX 14335-186, Tehran, Iran
b Institut d’Electronique, de Microélectronique et de Nanotechnologie (IEMN, UMR 8520), Avenue Poincaré – CS 60069, 59652 Villeneuve d’Ascq, France
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 6 August 2015
Received in revised form 12 October 2015
Accepted 12 October 2015
Available online 22 October 2015
Sulfonated reduced graphene oxide nanosheets (rGO-SO3H) were prepared by grafting sulfonic
acid-containing aryl radicals onto chemically reduced graphene oxide (rGO) under sonochemical
conditions. rGO-SO3H catalyst was characterized by Fourier-transform infrared (FT-IR) spectroscopy,
Raman spectroscopy, scanning electron microscopy (SEM), X-ray diffraction (XRD), thermogravimetric
analysis (TGA), differential scanning calorimetry (DSC) and X-ray photoelectron spectroscopy (XPS).
rGO-SO3H catalyst was successfully applied as a reusable solid acid catalyst for the direct amidation of
carboxylic acids with amines into the corresponding amides under ultrasonic irradiation. The direct
sonochemical amidation of carboxylic acid takes place under mild conditions affording in good to high
yields (56–95%) the corresponding amides in short reaction times.
Keywords:
Sulfonated reduced graphene oxide
Carboxylic acids
Amines
Ó 2015 Elsevier B.V. All rights reserved.
Direct amidation
Solid acid catalyst
Sonochemical reaction
1. Introduction
oxidant has been proposed for the synthesis of amides [17].
Bantreil et al. established a domino reaction for the formation of
Amide is one of the most important functional groups in organic
chemistry; amides are present in many natural products, peptides,
synthetic polymers, pharmaceuticals, and biological systems [1].
Several amide derivatives have biological properties such as anti-
tumor, antifungal, antihistamine, anthelmintic, and antibacterial
activity [2,3]. They are also useful and valuable intermediates for
the synthesis of various important compounds [4]. Usually amides
can be synthesized by the reaction of carboxylic acids [5] or their
derivatives such as halides [6], esters [7], and carboxylic salts [8]
with amines. A number of methods have been developed for the
synthesis of amide compounds, including rearrangement of aldox-
imes [9], transamidation of amides with amines [10,11], cross cou-
pling of amides with aryl halides [12], aminocarbonylation of aryl
halides [13] or terminal alkynes [14], hydration of nitriles [15] and
Ugi reaction [16]. The catalytic procedures developed for oxidative
amidation of aldehydes [17], alcohols [18–21] and alkylarenes [22]
with amines or amine salts are quite attractive from atom econ-
omy and green chemistry view points. Recently, N-heterocyclic
carbene (NHC)-catalyzed oxidative amidation of aromatic aldehy-
des with amines in the presence of N-bromosuccinimide as an
benzamides in one step from various benzyl alcohols in the pres-
ence of copper salt [18]. Recently, Wang et al. investigated direct
oxidative amidation between methylarenes and free amines in
water by employing tert-butyl hydroperoxide as the environmen-
tal benign oxidant with the co-catalysis of tetrabutyl-ammonium
iodide and FeCl3 in the presence of 4 Å molecular sieves [22].
Nevertheless, some of the reported methods require stoichiometric
amounts of coupling reagents and suffer from poor atom efficiency
or the use of highly hazardous reagents. Despite the low reactivity
of acids, direct amidation is still the most preferred industrial
process from both atom economy and environmental points of
view. Thus, it is desirable to use a cheap and environmentally benign
catalyst for direct amidation in order to overcome these problems.
Solid acids have the great potential to replace liquid acids as
environmentally benign acid catalysts [23,24]. Acidic carbons,
based on the concept of green chemistry, were investigated as
stable and highly active protonic acid catalysts for several acid-
catalyzed reactions [25,26]. Graphene oxide and graphite oxide
have been effectively applied as useful heterogeneous catalysts
for certain organic transformations [27–32]. Recently, we have
demonstrated the efficiency of graphite oxide as a solid acid
catalyst for esterification of organic acids with alcohols [33] and
ring-opening of epoxides with various alcohols [34].
⇑
Corresponding author.
E-mail address: m.mirzaaghayan@ccerci.ac.ir (M. Mirza-Aghayan).
http://dx.doi.org/10.1016/j.ultsonch.2015.10.009
1350-4177/Ó 2015 Elsevier B.V. All rights reserved.

372
M. Mirza-Aghayan et al. / Ultrasonics Sonochemistry 29 (2016) 371–379
HO3S
SO3H
(a)
(b)
O
O
O
HO
OH
O
O
O
O
O
1. )))
OH
OH
OH
OH
HO
2. NaBH4
O
HO
3. )))
O
O
O
4. Diazonium salt
OH
HO3S
SO3H
Graphite oxide
Sulfonated reduced graphene oxide (rGO-SO3H)
O
H
N
O
cat. rGO-SO3H, )))
R''
N
+
R
R
OH
R'
R''
R'
Scheme 1. (a) Synthesis of rGO-SO3H; (b) direct sonochemical amidation of carboxylic acids with amine derivatives using rGO-SO3H catalyst.
Table 1
Different conditions for the direct amidation of acetic acid with anilinea.
NH2
O
O
+
H3C
NH
OH
H3C
Entry
Catalyst
Conditions
Time
Yield (%)
1
2
3
4
5
6
7
8
9
–
–
Solvent-free, room temperature
Acetonitrile, reflux
Solvent-free, room temperature
Solvent-free, room temperature
Solvent-free, 70 °C
72 h
48 h
48 h
8 h
20
32
29
45
86
92
94
–
Graphite oxide (5 mg)
rGO-SO3H (5 mg)
rGO-SO3H (5 mg)
rGO-SO3H (5 mg)
rGO-SO3H (5 mg)
–
2 h
Bath ultrasonic, solvent-free, 70 °C
20 min
20 min
100 min
20 min
20 min
Bath ultrasonic, solvent-free, room temperature
Bath ultrasonic, solvent-free, room temperature
Bath ultrasonic, solvent-free, room temperature
Bath ultrasonic, solvent-free, room temperature
Graphite oxide (5 mg)
rGO-SO3H (2.5 mg)
8
27
10
a
Reaction conditions: acetic acid (1 mmol) and aniline (1 mmol).
Sonochemical waves were employed to exfoliate graphite oxide
KBr disks with a Bruker Vector 22 FT-IR spectrometer. Raman spec-
tra were recorded using a dispersive Raman spectrophotometer
Bruker model SENTRRA. Scanning electron microscopy (SEM) and
energy dispersive X-ray (EDX) data were obtained using a VEGA3
LMU TESCAN SEM. X-ray diffraction (XRD) data were performed
using a Bruker D8 Advance Theta-2theta diffractometer. Thermo-
gravimetric analysis (TGA) was performed on a NETZSCH TG 209
F1. Differential scanning calorimetry (DSC) was carried out on a
NETZSCH DSC 204 F1 Phoenix. X-ray photoelectron spectroscopy
(XPS) were collected on a VG ESCALAB MK-II spectrometer with
and functionalized graphene oxide. An original approach was
recently proposed by Maktedar et al. for the direct functionaliza-
tion of graphene oxide with 6-aminoindazole through sonochemi-
cal nucleophilic substitution reaction [35]. Wang et al. applied
sulfonated graphene as a solid catalyst for the ester-exchange
reaction [36]. Sulfonated graphene catalyst prepared by grafting
sulfonic acid-containing aryl radicals onto the two-dimensional
surface of graphene was successfully used for the etherification
of glycerol with isobutene [37].
In continuation of our efforts on the use of graphite oxide, we
investigate in this work a simple procedure for the preparation of
sulfonated reduced graphene oxide (rGO-SO3H) (Scheme 1a). The
catalytic activity of rGO-SO3H as a reusable solid acid catalyst
was further investigated for the direct amidation of carboxylic
acids with amines into the corresponding amides under ultrasonic
irradiation (Scheme 1b). Although the effect of ultrasound in
chemical reactions is known [38], to the best of our knowledge,
there are only a few examples on the amidation reaction [39–41].
Al Ka as the excitation source (hm = 1486.6 eV) operated at
10.5 kV and 20 mA at a pressure better than 10ꢀ8 Pa. GC–MS
analysis was performed on a FISON GC 8000 series TRIO 1000 gas
chromatograph equipped with a capillary column CP Sil.5 CB,
60 m ꢁ 0.25 mm i.d. 1H and 13C NMR spectra were recorded on a
Bruker 300 and 75 MHz spectrometer using tetramethylsilane as
internal standard. Elemental analyses were performed on
ThermoFinigan Flash EA 1112 series elemental analyzer.
a
2.2. Synthesis of sulfonated reduced graphene oxide nanosheets
(rGO-SO3H)
2. Experimental section
2.1. Analysis and characterization of materials used in this study
Typically, 150 mg of graphite oxide [30] powder in 150 mL
water were mixed in a reaction container using bath ultrasonic
with a frequency of 37 kHz for 30 min. A solution of 1.2 g NaBH4
in 30 mL water was added dropwise into the graphite oxide
Ultrasonic irradiation was accomplished with an Elmasonic P
ultrasonic cleaning unit (bath ultrasonic) with a frequency of
37 kHz and 100% output power. IR spectra were recorded from

M. Mirza-Aghayan et al. / Ultrasonics Sonochemistry 29 (2016) 371–379
373
Table 2
Ultrasound-assisted direct amidation of various acids with amines using rGO-SO3H catalyst.
O
H
N
O
cat. rGO-SO3H, )))
R''
+
R
N
R
OH
R'
R''
R'
Entry
1
Acid
Amine
Amide
Time
20
Yielda (%)
94 [9]
O
O
O
NH2
H3C
OH
OH
H3C
N
H
2
3
30
20
82 [9]
Cl
O
Cl
NH2
H3C
H3C
H3C
O
N
H
O
O
NH2
87 [47]
OH
OH
H3C
N
H
4
5
6
NH2
20
20
20
95 [48]
76 [12]
80 [7]
O
H3C
H3C
N
H
O
Cl
O
Cl
NH2
OH
N
H
O
O
NH2
O
O
OH
OH
N
H
7
20
73 [7]
NH
O
N
O
8
9
20
20
81 [49]
75 [50]
O
Cl
O
Cl
O
NH2
Cl
Cl
Cl
OH
N
H
O
O
NH
Cl
N
OH
OH
O
10
11
20
20
92 [51]
85 [52]
O
O
NH2
N
H
O
O
O
O
N
N
O
NH2
OH
OH
N
H
12
13
20
20
80 [53]
94 [54]
NH
N
O
O
NH2
O
OH
OH
N
H
O2
N
O2N
14
15
40
40
56 [55]
73 [55]
O
O
NH
O
N
O
O
O
NH2
OH
N
H
N
N
(continued on next page)

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M. Mirza-Aghayan et al. / Ultrasonics Sonochemistry 29 (2016) 371–379
Table 2 (continued)
Entry
16
Acid
N
Amine
Amide
Time
40
Yielda (%)
81
O
O
O
O
NH2
OH
OH
N
H
N
N
17
NH2
40
84
N
H
N
a
Isolated product.
Fig. 1. FT-IR spectra of (a) graphite oxide and (b) rGO-SO3H nanosheets.
dispersion, with the pH adjusted to 9–10 by addition of 5 wt.%
Na2CO3 solution. The mixture was heated at 80 °C for 1 h after
which the brown graphene oxide solution turned black. Next, the
mixture was washed with water and centrifuged three times at
3700 rpm for 20 min. The partially reduced graphene oxide (rGO)
was sonicated in 150 mL water using bath ultrasonic with a
frequency of 37 kHz for 30 min and cooled in an ice bath. Then, a
diazonium salt was prepared from the reaction of 92 mg sulfanilic
acid and 1 mL of 1 N HCl in 10 mL water in an ice bath at 0 °C,
followed by the addition of 36 mg NaNO2 in 10 mL water until
all reactants had dissolved. The diazonium solution was poured
into the rGO solution at 0 °C and stirred overnight at room temper-
ature. The black solution was centrifuged at 3700 rpm for 20 min
and washed with deionized water three times. The resultant
black residue was filtered followed by dehydration on a rotary
evaporator under vacuum and dried in oven at 80 °C for 2 h.
achieved by column chromatography using n-hexane/EtOAc:
100/3 as eluent. The spectroscopic data of the obtained esters were
compared with authentic samples. Spectroscopic data:
N-(2,4-dimethylphenyl)isonicotinamide (entry 16, Table 2): Color-
less oil, IR (KBr)
m = 3298, 3082, 2931, 1655, 1549, 903, 755,
693 cmꢀ1 1H NMR (300 MHz, CDCl3) d = 2.69 (s, 3H, CH3), 3.36
;
(s, 3H, CH3), 6.79–7.02 (m, 3H, CH Arom), 7.31–7.47 (m, 4H, CH
Arom), 10.30 (s, 1H, NH); 13C NMR (75 MHz, CDCl3) d = 18.9, 24.8,
112.3, 121.9, 130.4, 132.2, 135.2, 142.1, 144.4, 152.7, 166.2; MS
(EI) (70 eV), m/z (%): 226 (7) [M]+, 148 (32), 121 (100), 107 (53),
105 (5), 78 (5).
N-propylisonicotinamide (entry 17, Table 2): Yellow oil; IR (KBr)
m
= 3261, 3057, 2931, 1665, 1500, 961, 752, 665 cmꢀ1 1H NMR
;
(300 MHz, CDCl3) d = 0.85 (t, J = 6.6 Hz, 3H, CH3), 1.23–1.66 (m,
2H, CH2CH3), 2.91 (t, J = 7.4 Hz, 2H, CH2CH2CH3), 7.31–8.32 (m,
5H, CH Arom and NH); 13C NMR (75 MHz, CDCl3) d = 13.2, 23.7,
42.6, 123.9, 144.8, 149.1, 166.7; MS (EI) (70 eV), m/z (%): 164 (5)
[M]+, 121 (34), 106 (53), 86 (25), 78 (5), 43 (100).
2.3. Typical procedure for the direct amidation of carboxylic acids with
amines under ultrasonication
3. Results and discussion
To a mixture of carboxylic acid (1 mmol) and amine (1 mmol)
was added rGO-SO3H (5 mg). The resulting mixture was sonicated
in an Elmasonic P ultrasonic cleaning unit (ultrasonic bath) with a
frequency of 37 kHz and 100% output power at room temperature
for the time indicated in Table 2. Then ethyl acetate was added and
the mixture was filtered through a sintered funnel and extracted
with ethyl acetate. The organic layer was dried over Na2SO4,
filtered and evaporated under reduced pressure. Purification was
3.1. Characterization of rGO-SO3H nanosheets catalyst
The overall process of sulfonation was monitored by FT-IR spec-
troscopy. FT-IR spectrum of graphite oxide shows peaks at about
3369, 1729, 1584, 1219 and 1049 cmꢀ1 assigned to OAH, C@O,
C@C, (CAO) epoxy and (CAO) alkoxy groups, respectively (Fig. 1)
[42,43]. The comparison of FT-IR spectra of graphite oxide and

M. Mirza-Aghayan et al. / Ultrasonics Sonochemistry 29 (2016) 371–379
375
Fig. 2 shows the Raman spectra of graphite oxide and rGO-SO3H
nanosheets. Two characteristic peaks at 1311 and 1594 cmꢀ1 for
graphite oxide and at 1313 and 1585 cmꢀ1 for rGO-SO3H occur in
each spectrum, corresponding to the D and G bands, respectively.
The ID/IG ratio in rGO-SO3H (2.07) is greater than that of graphite
oxide (1.56). The result suggests that some of the oxygenated
groups are removed by NaBH4 under sonochemical conditions.
The comparison of SEM images of graphite oxide and rGO-SO3H
nanosheets confirms its exfoliation after grafting of sulfonic acid-
containing aryl radicals onto chemically rGO under ultrasonication.
It should be noted that the presence of sulfur with 6.25 at.% in EDX
spectra suggests the presence of the sulfonated group in rGO-SO3H
nanosheets (Fig. 3). CHNS analysis indicates the presence of sulfur
in 4.74% in rGO-SO3H nanosheets.
The XRD pattern of graphite oxide displays a single diffraction
peak at 2h = 11.8°, suggesting the formation of graphite oxide upon
graphite oxidation by the Hummer’s method (Fig. 4a) [43]. After
chemical reduction of exfoliated graphite oxide with NaBH4 and
after grafting sulfonic acid containing aryl radicals, the peak of
graphite oxide at 2h = 11.8° decreased and a new broad peak at
2h = 26.3° appeared, confirming the formation of rGO-SO3H
(Fig. 4b) [45]. This result suggests graphite oxide reduction to
rGO-SO3H by partial removal of oxygenated functional groups.
The interlayer spacing (d-spacing) value for graphite oxide was
0.21 nm, while the interlayer spacing of rGO-SO3H was calculated
to be 0.78 nm, which revealed the increase of distance between
rGO-SO3H nanosheets.
Fig. 5 displays the TGA curves of graphite oxide and rGO-SO3H
samples. The overall weight loss of 51% for graphite oxide occurs
in three successive steps. The first one is a steady weight loss of
7% attributed to the vaporization of adsorbed water molecules
and occurs at around 100 °C. Then a rapid loss of 25% due to the
decomposition of the oxygen-containing functional groups such
as hydroxyl, epoxy, carbonyl, and carboxyl groups in the tempera-
ture range of 100–208 °C. Finally, a weight loss of 19% that can be
Fig. 2. Raman spectra of (a) graphite oxide and (b) rGO-SO3H nanosheets.
rGO-SO3H nanosheets evidently suggests that chemical changes
occurred during its preparation. Indeed, in the rGO-SO3H FT-IR
spectrum, the peak at 1729 cmꢀ1 is highly decreased, which
indicates a partial reduction of graphite oxide in the sulfonation
process [44]. The presence of a broad band at about 1629 cmꢀ1
due to C@C stretching modes clearly suggests partial restoration
of the aromatic network during the chemical process [42,43]. The
appearance of peaks at about 1381 and 1202 cmꢀ1 due to the
S@O symmetric and asymmetric stretching modes, respectively
confirms the successful decoration of sulfonic groups on the
rGO-SO3H surface. The peaks at about 652 and 603 cmꢀ1 are
associated with SAO and SAC stretching modes, respectively,
signifying the existence of covalent sulfonic acid groups on the
surface of rGO-SO3H nanosheets (Fig. 1) [44].
Fig. 3. SEM images (a) graphite oxide and (b) rGO-SO3H; (c) and (d) their respective EDX spectra.

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M. Mirza-Aghayan et al. / Ultrasonics Sonochemistry 29 (2016) 371–379
Fig. 6. DSC curves of (a) graphite oxide and (b) rGO-SO3H nanosheets.
The XPS wide-scan spectrum of rGO-SO3H nanosheets showed
two peaks at 531.0 and 282.8 eV which correspond to O1s and
C1s, respectively (Fig. 7a). The presence of sulfur in rGO-SO3H
nanosheets was confirmed by two characteristic peaks of S2s and
S2p at 214.0 and 166.1 eV, respectively [46]. High resolution XPS
spectrum of S2p displays a single peak at 168.4 eV which corre-
spond to SAO bond (Fig. 7b). This result indicated the successful
incorporation of sulfur group in rGO-SO3H nanosheets. Moreover,
the high resolution XPS spectrum of C1s exhibits two peaks at
284.6 and 288.8 eV corresponding to sp2 C@C bonding and the
carboxylic acid group, respectively (Fig. 7c). It should be noted that
the characteristic peak at 286.9 eV which corresponding to CAOH
and CAOAC moieties not clearly observed. This result suggests that
some of the oxygenated groups in rGO-SO3H nanosheets are
reduced by NaBH4 under sonochemical conditions.
Fig. 4. XRD patterns of (a) graphite oxide and (b) rGO-SO3H nanosheets.
3.2. Catalytic performance of rGO-SO3H nanosheets
Initially, we screened the direct amidation of acetic acid
(1 mmol) with aniline (1 mmol) under different conditions. The
results are summarized in Table 1. First, the direct amidation of
acetic acid (1 mmol) with aniline in absence of catalyst at room
temperature in solvent-free condition or in reflux of acetonitrile
afforded the corresponding amide in 20% and 32% yield after 72
and 48 h, respectively (entries 1 and 2, Table 1). When the amida-
tion of acetic acid was performed in presence of 5 mg of graphite
oxide in absence of solvent, the corresponding amide was obtained
in 29% yield after 48 h (entry 3, Table 1). The amidation of acetic
acid in the presence of 5 mg of rGO-SO3H at room temperature
gave N-phenylacetamide in 45% yield after 8 h (entry 4, Table 1).
Next, this reaction was carried out at 70 °C and the obtained
N-phenylacetamide was isolated in 86% yield after 2 h (entry 5,
Table 1). The comparison of entries 2 and 5 clearly indicates that
rGO-SO3H catalyst has an important role in the direct amidation
by decreasing the reaction time and increasing the yield. In the
next step, we examined the effect of ultrasonic irradiation on this
chemical transformation. The reaction was carried out in the pres-
ence of 5 mg of rGO-SO3H in an Elmasonic P ultrasonic cleaning
unit (ultrasonic bath) with a frequency of 37 kHz and 100% output
power at 70 °C. A decrease in the reaction time was observed;
N-phenylacetamide was isolated in 92% yield only after 20 min
(entry 6, Table 2). It should be noted that a similar result was
obtained when the reaction was performed at room temperature,
suggesting that room temperature was enough for this reaction
(entry 7, Table 1). In the absence of rGO-SO3H, the direct amidation
of acetic acid with aniline gave the starting material after 100 min
Fig. 5. TGA analysis of (a) graphite oxide and (b) rGO-SO3H nanosheets.
attributed to the combustion of the carbon skeleton is observed in
the temperature range of 208–900 °C (Fig. 5a) [42]. rGO-SO3H
nanosheets show overall less than 30% weight loss in the same
temperature range (Fig. 5b). The weight loss of rGO-SO3H
nanosheets takes place in four successive steps. The first one is a
weight loss of 17% at around 300 °C and three successive steps
with a slow decrease of weight loss of 12% in the temperature
range of 300–900 °C. The weight loss in the temperature range
400–580 °C can be attributed to decomposition of sulfonated
groups [42].
Fig. 6 displays the DSC curves of graphite oxide and rGO-SO3H
samples. The DSC curves of graphite oxide and rGO-SO3H
confirmed the results obtained by TGA analysis. The DSC curve of
graphite oxide displays a broad peak at 170 °C, indicating the
decomposition of thermally labile oxygen containing groups over
the surface of graphite oxide. This peak is much decreased in
DSC curve of rGO-SO3H (Fig. 6b), indicating that rGO-SO3H
nanosheets are more stable than graphite oxide during heating.

M. Mirza-Aghayan et al. / Ultrasonics Sonochemistry 29 (2016) 371–379
377
Fig. 7. (a) XPS wide-scan and high resolution spectra of (b) S2p and (c) C1s of rGO-SO3H nanosheets.
Table 3
under otherwise similar experimental conditions (entry 8, Table 1).
Reusability of rGO-SO3H catalyst.
This result clearly supported the positive effect of rGO-SO3H cata-
lyst on the amidation process. Similarly using graphite oxide as
catalyst afforded N-phenylacetamide in 8% yield along with the
starting material under ultrasonic irradiation (entry 9, Table 1).
Finally, we examined the effect of a lower rGO-SO3H amount
(2.5 mg) on the direct amidation of acetic acid with aniline (entry
10, Table 1). The results indicated that 5 mg of rGO-SO3H was
optimum for this reaction.
Run
1st
94
2nd
90
3rd
87
4th
90
5th
6th
7th
(89)
Yielda (%)
52 (90)
(88)
a
The number in parentheses indicates the yield of amide by using the recycled
acidified rGO-SO3H catalyst.
10–13, Table 2). Similarly, cinnamic acid reacted with morpholine
to give the corresponding amide in 56% yield after 40 min
sonication (entry 14, Table 2). Finally, we examined the direct
sonochemical amidation of an hetero aromatic carboxylic acid,
isonicotinic acid, with aniline, 2,4-dimethylaniline and n-propyl
amine using rGO-SO3H catalyst under ultrasonic bath irradiation.
The corresponding amides were obtained in 73%, 81% and 84%
yields after 40 min. (entries 15–17, Table 2). The results indicated
that through a sonochemical reaction, direct amidation of aliphatic,
aromatic and hetero aromatic carboxylic acids with aromatic, ben-
zylic, hetro aromatic, linear and cyclic aliphatic amines produced
the corresponding amides in good to high yield in presence of
rGO-SO3H catalyst in short reaction times at room temperature.
The results, obtained using this procedure, are comparable to
those reported in the literature [5,9,12]. For example, Allen et al.
[5] prepared N-benzylpropionamide in 81% yield by the reaction
of propionic acid with benzyl amine in presence of 5 mol% ZrCl4
in toluene at 110 °C for 5 h. N-(4-chlorophenyl)propionamide
was synthesized by CAN coupling of propionamide to aryl halides
in 94% yield using 5 mol% CuI catalyst in presence of 10 mol% N,N0-
dimethylethylene diamine and 2–2.5 eq. cesium fluoride in
tetrahydrofuran as solvent at room temperature after 18–24 h [12].
To evaluate the reusability of the rGO-SO3H catalyst after com-
pletion of the amidation reaction, ethyl acetate was added and the
mixture was filtered through a sintered funnel to recover the
catalyst. The reaction of acetic acid and aniline in the presence of
Under the improved reaction conditions, direct amidation of
various carboxylic acids with a variety of amines in the presence
of rGO-SO3H (5 mg) under ultrasonic bath irradiation at room
temperature gave the corresponding amides. The results are
summarized in Table 2. Under these experimental conditions, the
reaction of aniline, 4-chloroaniline, benzyl amine and n-propyl
amine with acetic acid afforded the corresponding amides in
82–95% yields after 20–30 min. of sonication (entries 1–4, Table 2).
Similarly, direct amidation of 4-chloroaniline, benzyl amine and
morpholine with propionic acid using this method produced the
corresponding amides in 76%, 80% and 73% yield, respectively
under ultrasonic bath irradiation after 20 min. (entries 5–7,
Table 2). Direct amidation of a carboxylic acid containing an
electron-donating group such as 2-chloroacetic acid, with
4-chloroaniline and morpholine in presence of rGO-SO3H (5 mg)
resulted in the formation of the corresponding amide derivatives
in 81% and 75% yields, respectively after 20 min. (entries 8–9,
Table 2). In the next step, we investigated the direct amidation
of aromatic carboxylic acids such as benzoic acid and
4-nitrobenzoic acid with aromatic, hetro aromatic, cyclic and
linear aliphatic amines under ultrasonic bath irradiation.
N-(2,4-Dimethylphenyl)benzamide,
N-(pyridin-4-yl)benzamide,
morpholino(phenyl)methanone and 4-nitro-N-propylbenzamide
were obtained in 92%, 85%, 80% and 94% yields, respectively in
presence of rGO-SO3H after 20 min at room temperature (entries

378
M. Mirza-Aghayan et al. / Ultrasonics Sonochemistry 29 (2016) 371–379
(a)
R
O
O
O
R'
rGO-SO3H, )))
- H2O
rGO-SO3H, )))
O- H3+NR'
+
R'NH2
R
R
N
H
OH
O
(b)
OH
HO
R
R
rGO-SO3H
OH
+
+
I
R'NH3
R'NH2
NH2
R'
-
- rGO-SO3
rGO-SO3H, ))) H+ transfer
+
O
OH2
HO
R
)))
R'
H2O
+
R
N
H
NH
R'
-
rGO-SO3H
rGO-SO3
Scheme 2. Proposed mechanism for direct amidation of carboxylic acids with amines catalyzed by rGO-SO3H.
5 mg of the recovered rGO-SO3H was performed for five consecu-
tive cycles. The recycled rGO-SO3H was efficient for the direct
amidation of acetic acid into the corresponding amide even after
four consecutive times (Table 3). However we observed a loss of
activity of catalyst after the 5th run; the corresponding amide
was obtained in 52% yield. Thus, the obtained recycled rGO-SO3H
after 4th run was washed with a solution of hydrogen chloride
(10%) and water, and centrifuged in water (3 times) for 20 min at
3700 rpm and then filtered and dried in an oven at 80 °C for 2 h.
The recycled acidified rGO-SO3H was efficient for the direct
amidation of acetic acid with aniline into the corresponding amide
after seven consecutive cycles (Table 3).
5. Conclusion
In conclusion, a simple technique was used to synthesize
rGO-SO3H nanosheets by grafting sulfonic acid-containing aryl
radicals onto rGO under sonochemical conditions. Through a
sonochemical reaction we successfully achieved direct amidation
of various carboxylic acids with
a variety of amines using
rGO-SO3H as a reusable solid acid catalyst. This new and simple
ultrasonic procedure is advantageous as the amidation takes place
at room temperature, in short reaction times and in good to high
yields, reusable catalyst for seven consecutive cycles with a very
simple work-up procedure.
Acknowledgment
4. Mechanism
The authors would like to thank the Iran National Science
Foundation (INSF) for financial support.
In addition to providing mechanical effects, cavitation induced
by ultrasound can support many homogeneous and heterogeneous
reactions. The basis of many applications of ultrasound is acoustic
cavitation. The formation, growth and collapse of microbubbles
generate high local pressure and release of heat energy. The result
is an important increase of temperature and pressure of up to
several thousand degrees Kelvin and several hundred bars. It is
assumed that this cavitation bubble collapse provides the essential
energy for a chemical reaction. The mechanism of direct amidation
of carboxylic acids with amines using rGO-SO3H catalyst under
ultrasonic irradiation is not very clear. However, a reasonable
and appropriate mechanism is suggested for this reaction, as out-
lined in Scheme 2. The reaction may proceed through the initial
formation of a stable ammonium carboxylate salt (Scheme 2a).
This salt can undergo a subsequent dehydration leading to the
formation of amide product [56]. Effect of ultrasound and high acid
capacity of rGO-SO3H catalyst enhanced the formation of the
ammonium carboxylate salt and the dehydration step. However,
the reaction can proceed via pathway (b). In this pathway, high
acid capacity of rGO-SO3H catalyst leads to the conversion of
amine into ammonium salt. The intermediate I, obtained from
the reaction between ammonium salt and carboxylic acid, under-
goes a subsequent proton transfer leading to the formation of
amide bond and water elimination [57]. Effect of ultrasound and
high acid capacity of rGO-SO3H catalyst enhanced the proton
transfer and the dehydration step.
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