A novel ternary GO@SiO2-HPW nanocomposite as an efficient heterogeneous catalyst for the synthesis of benzazoles in aqueous media

Article abstract of DOI:10.1002/jccs.201800323

A new solid acid catalyst, consisting of 12-phosphotungstic heteropoly acid (HPW) supported on graphene oxide/silica nanocomposite (GO@SiO₂), has been developed via immobilizing HPW onto an amine-functionalized GO/SiO₂ surface through coordination interaction (GO@SiO₂-HPW). The GO@SiO₂-HPW nanocomposite was characterized by Fourier transform infrared (FT-IR) spectroscopy, thermogravimetric analysis (TGA), scanning electron microscopy (SEM), and powder X-ray diffraction (XRD). The prepared nanocomposite could be dispersed homogeneously in water and further used as a heterogeneous, reusable, and efficient catalyst for the synthesis of benzimidazoles and benzothiazoles by the reaction of 1,2-phenelynediamine or 2-aminothiophenol with different aldehydes.

Full text of DOI:10.1002/jccs.201800323

Received: 18 August 2018
Accepted: 17 January 2019
DOI: 10.1002/jccs.201800323
A R T I C L E
A novel ternary GO@SiO2-HPW nanocomposite as an efficient
heterogeneous catalyst for the synthesis of benzazoles in aqueous
media
Setareh Habibzadeh1 | Ghasem Firouzzadeh Pasha2 | Mahmood Tajbakhsh2 |
Nasim Amiri Andi1 | Ehsan Alaee3
1Department of Chemistry, Babol Noshirvani
A new solid acid catalyst, consisting of 12-phosphotungstic heteropoly acid
University of Technology, Babol, Iran
2Faculty of Chemistry, University of Mazandaran,
Babolsar, Iran
3Neonatal and Children's Health Research Center,
Golestan University of Medical Sciences,
Gorgan, Iran
(HPW) supported on graphene oxide/silica nanocomposite (GO@SiO2), has been
developed via immobilizing HPW onto an amine-functionalized GO/SiO2 surface
through coordination interaction (GO@SiO2-HPW). The GO@SiO2-HPW nano-
composite was characterized by Fourier transform infrared (FT-IR) spectroscopy,
thermogravimetric analysis (TGA), scanning electron microscopy (SEM), and pow-
der X-ray diffraction (XRD). The prepared nanocomposite could be dispersed
homogeneously in water and further used as a heterogeneous, reusable, and effi-
cient catalyst for the synthesis of benzimidazoles and benzothiazoles by the reac-
tion of 1,2-phenelynediamine or 2-aminothiophenol with different aldehydes.
Correspondence
Setareh Habibzadeh, Department of Chemistry,
Babol Noshirvani University of Technology,
Babol 47148-71167, Iran.
Email: habibzadeh@nit.ac.ir
Funding information
Babol Noshirvani University of Technology,
Babol, Iran
KEYWORDS
benzimidazole, benzothiozole, graphene oxide, heterogeneous catalyst,
heteropoly acid
The recently reported methods mostly employed homoge-
neous catalysts that require special work-up and separation
1
| INTRODUCTION
Benzazoles such as benzimidazoles and benzothiazoles are
considered an important class of heterocyclic compounds pos-
sessing interesting biological and pharmacological properties
such as antifungal,[1] antioxidant,[2] anti-inflammatory,[3]
antibacterial,[4] antiviral[5] and anticancer activity.[6] Up to
now, several methods have been employed for the synthesis
of these heterocyclic moieties. The conventional synthetic
method for the preparation of benzimidazoles or benzothia-
zoles is based on the condensation of o-substituted aminoaro-
matics with carboxylic acids or their derivatives (nitriles,
amidates, orthoesters) in the presence of a strong acid as cata-
lyst. However, the most attractive method for the synthesis of
benzimidazoles is the reaction of 1,2-phenelynediamine with
aldehydes.[7] Similarly, benzothiazoles are also synthesized
by reacting 2-aminothiophenol with aldehydes.
techniques for the recovery and recycling from the reaction
environment. Some other catalysts such as mesoporous titania-
iron(III) oxide,[8] CuO/silica,[9] β-cyclodextrin,[10] Pt-TiO2,[11]
α-MoO3 nanobelts,[12] Fe3O4/SiO2/(CH2)3N+Me3Br− core–
3
shell nanoparticles,[13] and Fe3O4-SiO2-(NH4)6Mo7O24 mag-
netic core–shell nanocomposite[14] were also used in alleviating
some of the limitations. Most of these methods suffer from
some drawbacks, such as the use of highly toxic or expensive
reagent, long reaction times, tedious work-up procedures, low
yields of products, and, in some cases, harsh reaction condi-
tions. In view of these drawbacks, the development of an effi-
cient and convenient method for the synthesis of benzazoles
would be valuable.
Heteropoly acids (HPAs), especially 12-phosphotungstic
heteropoly acid (HPW), have attracted extensive interest in
the last decade as a solid acid catalyst. The main
© 2019 The Chemical Society Located in Taipei & Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
J Chin Chem Soc. 2019;1–11.
http://www.jccs.wiley-vch.de
1

2
HABIBZADEH ET AL.
disadvantage of HPAs as catalysts is their low specific sur-
face area (1–10 m2/g), which hinders accessibility of the
reactants to the acidic sites. Consequently, it becomes neces-
sary to disperse HPAs on supports that possess large surface
area. HPAs have been reported to catalyze the reaction of
aldehydes with o-substituted aminoaromatics[15–17] Rafiee
and co-workers compared the catalytic activity of a sup-
ported HPA (γ-Fe2O3@SiO2-PW) catalyst and several other
catalysts for this reaction and found that supported HPA is
more active than others, substantially due to its stronger
acidity.[18]
In recent years, graphene oxide (GO), also termed as
“carbocatalyst,” has been widely used as a support for het-
erogeneous nanocatalysts due to its intrinsic properties such
as the large surface area, abundant functional sites, high
chemical stability, low toxicity, and facile hybridization with
other functional nanomaterials.[19] Nanocomposites of GO
and nonmetallic oxide particles (such as GO/SiO2) have
superior properties compared to GO and nonmetallic oxide
particles, which is attributed to the synergistic effect of the
pristine materials.
on an Electrothermal model 9100 apparatus using open glass
1
capillary: the reported values are uncorrected. H NMR and
13C NMR spectra were obtained on a Bruker Avance spec-
trometer at 400.13 and 100.61 MHz, respectively, with TMS
as internal standard.
2.2 | Catalyst preparation
2.2.1
| Preparation of GO
GO was prepared from natural graphite by using a modified
Hummers’ method.[20] Typically, 1.0 g of graphite was
added to a mixture of concentrated H2SO4 (125 mL) and
H3PO4 (15 mL) and stirred for 12 hr at room temperature.
Subsequently, 3.0 g of KMnO4 was added, and the obtained
mixture was oxidized for 3 hr. Then, 200 mL of DI water
and 6 mL H2O2 (30%) were added slowly to the reaction
mixture and stirred for 30 min at 5ꢀC. Finally, the GO was
collected by centrifugation at 8,000 rpm for 10 min and
washed twice with 25 mL of 10% HCl and twice with
25 mL DI water.
As far as we know, in spite of the development of
GO/SiO2-supported catalysts, GO/SiO2-supported HPW
nanocomposite (GO@SiO2-HPW) has still not been
reported. In this paper, we report a simple, green, and effi-
cient method for the synthesis of benzimidazoles and ben-
zothiazoles using GO@SiO2-HPW in water (Scheme 1).
2
| EXPERIMENTAL
2.1 | Materials and methods
All reagents and solvents were purchased from Merck,
Fluka, and Aldrich chemical companies and used without
further purification. Double-distilled, deionized (DI) water
was used throughout the studies.
Fourier transform infrared (FT-IR) spectra were recorded
on a Broker Vector spectrophotometer. Thermogravimetric
analysis (TGA) was carried out on a TGA (model
951, Dupont) apparatus at the heating rate of 10ꢀC/min
under a nitrogen atmosphere. X-ray powder diffraction
(XRD) pattern was recorded using Cu Kα radiation source
on a Philips PW 1830 diffractometer. Scanning electron
microscopy (SEM) studies were performed using a Vega
Tescan-XMU microscope. The progress of reactions were
checked by thin-layer chromatography (TLC) on precoated
plates (Merck silica gel 60, F254) using UV light as a visual-
izing agent. Melting points of the products were determined
FIGURE 1 Schematic diagram of the preparation of GO@SiO2-HPW
O
NH2
XH
N
X
GO/SiO2-HPW
Water
R2
R2
H
X: NH, S
SCHEME 1 One-pot synthesis of benzazoles catalyzed by GO/SiO2-HPW
FIGURE 2 FT-IR of spectra of GO, GO@SiO2, GO@SiO2-APTES, and
in water
fresh GO@SiO2-HPW catalyst

HABIBZADEH ET AL.
3
2.2.2
| Preparation of GO@SiO2
A mixture of GO (0.5 g), tetraethylorthosilicate (TEOS,
7 mL), and ammonium hydroxide (1 mL) in ethanol
(40 mL) was stirred at room temperature. After 15 hr, the
obtained solid mixture was filtered and washed with DI
water (2 × 5 mL) and ethanol (2 × 5 mL) alternately and
dried at 60ꢀC for 12 hr to give GO@SiO2.
2.2.3
| Preparation of GO@SiO2-HPW
GO@SiO2 (1.5 g) and 3-aminopropyl triethoxy silane
(APTES; 6 mL) were stirred in 10 mL of an aqueous ethanol
solution (80% ethanol +20% DI water, by volume) at 80 C
FIGURE 3 TGA curves of GO@SiO2 and GO@SiO2-HPW
ꢀ
for 24 hr. The resultant mixture was filtered and washed
with ethanol (4 × 5 mL) and dried at 60ꢀC for 12 hr to
obtain GO@SiO2-APTES. In the next step, 0.5 g of
12-tungstophosphoric acid (H3PW12O40) was dissolved in
25 mL of DI water, and then 1.0 g of GO@SiO2-APTES
was added to the solution with vigorous stirring under ambi-
ent temperature for 12 hr. The final GO@SiO2-HPW prod-
uct was obtained by similar steps of filtering, washing, and
drying. A schematic diagram of the synthesis approach for
the preparation of GO@SiO2-HPW is shown in Figure 1.
2.3 | General procedure for the synthesis benzazoles
FIGURE 4 XRD pattern of GO@SiO2-HPW
A mixture of aldehyde (1 mmol), amine (1 mmol), and
GO@SiO2-HPW (0.01 g) as a catalyst in a 25-mL round-
bottom flask equipped with a condenser was stirred at
δC 165.9, 153.8, 135.4, 135.1, 134.5, 132.1, 129.6, 127.3,
126.9, 126.4, 123.6, 123.0, 122.9.
ꢀ
100 C for an appropriate time. After completion of the reac-
tion, as indicated by TLC, the reaction mixture was diluted
with ethanol and subsequently centrifuged to get the catalyst.
Evaporation of the solvent of the filtrate under reduced pres-
sure gave the crude product. The pure product was obtained
by recrystallization in ethanol. All products were character-
ized by spectroscopy (1H NMR and 13C NMR spectra) and
melting points.
3
| RESULTS AND DISCUSSION
3.1 | Catalyst characterization
FT-IR spectra of GO, GO@SiO2, GO@SiO2-APTES, and
fresh GO@SiO2-HPW catalyst are compared in Figure 2.
The bare GO gave five intense peaks at 3,422, 1,721, 1,627,
1,227, and 1,057 cm−1, corresponding to the stretching
vibrations of the O–H, C=O, C=C, C–OH, and C–O groups
Spectral data for the selected compounds:
2-(4-Chlorophenyl)-1H-benzimidazole (Table 2, entry 1)
1
White solid, m.p. = 290–292ꢀC; H NMR (400 MHz,
DMSO-d6): δH 13.1 (br, 1H, NH), 8.19 (d, J = 8.8 Hz, 2H),
7.61–7.64 (m, 4H, Ar), 7.19–7.24 (m, 2H, Ar); 13C NMR
(100 MHz, DMSO-d6): δC 150.6, 135.0, 129.5, 129.5,
128.6, 122.8.
4-(1H-Benzimidazol-2-yl)benzonitrile (Table 2, entry 4)
1
White solid, m.p. = 260–262ꢀC; H NMR (400 MHz,
3
DMSO-d6): δH 13.2 (s, 1H, NH), 8.34 (dd, J = 6.8 Hz,
3
4
4J = 1.6 Hz, 2H), 8.03 (dd, J = 6.8 Hz, J = 1.6 Hz, 2H),
7.72 (d, J = 8 Hz, 1H), 7.58 (d, J = 8 Hz, 1H), 7.22–7.30
(m, 2H, Ar); 13C NMR (100 MHz, DMSO-d6): δC 133.5,
127.4, 123.9, 122.7, 119.8, 119.1, 112.3, 112.2, 112.1.
2-Phenyl-1,3-benzothiazole (Table 4, entry 1)
1
White solid, m.p. = 112–114ꢀC; H NMR (400 MHz,
DMSO-d6): δH 8.24–8.25 (m, 1H, Ar), 8.16–8.18 (m, 1H,
Ar), 8.05–8.10 (m, 2H, Ar), 7.77–7.79 (m, 1H, Ar),
7.47–7.59 (m, 4H, Ar); 13C NMR (100 MHz, DMSO-d6):
FIGURE 5 SEM images of (a) GO, (b) GO@SiO2, (c) fresh GO@SiO2-
HPW, and (d) used GO@SiO2-HPW catalysts

4
HABIBZADEH ET AL.
TABLE 1 Optimization of the reaction conditions
Entry
Amount of catalyst (Mol %)
Solvent
Time (min)
Temp. (ꢀC)
Reflux
Reflux
Reflux
Reflux
Reflux
Reflux
Reflux
Reflux
Reflux
Reflux
Yield (%)a
1
2
3
4
5
6
7
8
9
10
1
Toluene
90
90
90
90
90
90
90
90
85
90
75
65
92
80
88
60
96
80
96
90
1
Acetonitrile
DMF
1
1
Ethanol
1
Ethanol/water (1:1 v/v)
No solvent
Water
1
1
0.5
2
Water
Water
3
Water
a
Isolated yields.
TABLE 2 Synthesis of benzimidazoles using GO@SiO2-HPW in watera
Entry
Amine
Aldehyde
Product
Time (min)
Yield (%)b [ref.]
1
90
96 [24]
2
3
90
90 [25]
85 [26]
120
4
5
70
96 [26]
87 [27]
120
6
7
110
70
94 [24]
95 [25]

HABIBZADEH ET AL.
5
TABLE 2 (Continued)
Entry
Amine
Aldehyde
Product
Time (min)
Yield (%)b [ref.]
8
95
85 [25]
9
130
100
80 [28]
10
85 [25]
11
12
13
14
130
160
150
150
80 [29]
70 [30]
85 [31]
82 [31]
15
80
95 [25]
16
17
90
80
92 [32]
95 [33]
18
120
87 [32]

6
HABIBZADEH ET AL.
TABLE 2 (Continued)
Entry
Amine
Aldehyde
Product
Time (min)
Yield (%)b [ref.]
19
130
85 [34]
20
21
150
150
88 [31]
84 [31]
22
23
110
110
90 [32]
88 [32]
24
25
160
180
70 [34]
44 [30]
26
27
160
160
65 [34]
50 [34]
28c
100
85 [35]
a
b
c
Reaction conditions: 1,2-phenylenediamine (1 mmol), aldehyde (1 mmol), GO@SiO2-HPW (1 mol%).
Isolated yields.
Reaction conditions: 1,2-phenylenediamine (1 mmol), aldehyde (2 mmol), GO@SiO2-HPW (1 mol%).

HABIBZADEH ET AL.
7
of GO, respectively.[21] For the GO@SiO2 sample, the new
peak at 1,081 cm−1 was assigned to the Si−O−C stretching
vibration.[22] GO@SiO2-APTES showed the characteristic
stretching vibration of the CH2 groups of the aminopropyl
at 2θ = 5–10ꢀ. The peaks at 2θ = 35.8ꢀ, 58.2ꢀ, and 62.8ꢀ
can be attributed to HPW.
The surface morphologies and particle sizes of GO,
GO@SiO2, and fresh and used GO@SiO2-HPW catalysts
were investigated using SEM. As shown in Figure 5a, GO
sheets showed smooth surfaces and layered structures. The
SEM image of GO@SiO2 in Figure 5b indicates that the
SiO2 particles are nearly spherical with particle sizes in the
range 30–40 nm. The SEM image of fresh GO@SiO2-HPW
catalyst (Figure 5c) also showed morphology similar to that
of GO@SiO2. The used GO@SiO2-HPW catalyst indicated
agglomeration of particles with size up to 100 nm
(Figure 5d).
moieties at 2,936 cm−1
.
The formation of the GO@SiO2-HPW catalyst was con-
firmed by the new peaks at 805 and 1,080 cm−1 attributed to
the stretching vibrations of W–O and P–O bonds of the
HPW Keggin anions, respectively.[23]
TGA was used to evaluate the thermal stability of
GO@SiO2 and GO@SiO2-HPW. The TGA curves in
ꢀ
Figure 3 demonstrate a weight loss around 100 C, which
was related to the loss of absorbed water molecules. The
ꢀ
weight loss of GO@SiO2-HPW below 100 C (1.0%) was
3.2 | Catalyst activity
lower compared to that of GO@SiO2 (3.0%), which indi-
cates that the hydrophilic feature of GO@SiO2 was reduced
after modification with the hydrophobic APTES functional
group.[24] Moreover, the pronounced weight loss in the tem-
perature range 250–600ꢀC (10.0%) can be attributed to the
degradation of APTES (Figure 3). The content of APTES on
the GO@SiO2-HPW surface was calculated based on TGA
as 1.7 mmol/g. These results proved that GO@SiO2-HPW
could endure about 250ꢀC, which was much higher than the
reaction temperature (100ꢀC) in the following catalytic
experiments.
The XRD pattern of GO@SiO2-HPW showed character-
istic diffraction peak at 2θ = 11.8ꢀ, confirming the forma-
tion of graphite oxide upon graphite oxidation by the
Hummer's method (Figure 4). In addition, the amorphous
SiO2 in GO@SiO2-HPW was identified from the broad peak
To optimize the reaction conditions, we selected the reaction
of 1,2-diaminobenzene with 4-chlorobenzaldehyde as a
model reaction.
As shown in Table 1, water and various organic solvents
such as toluene, acetonitrile, N,N-dimethyl formamide
(DMF), ethanol, and ethanol/water mixture (1:1 v/v) were
investigated using 1 mol% of catalyst, and it was found that
water was the best solvent for this transformation.
Next, the effect of catalyst loading was optimized by
employing the reaction with 0.5, 1, 2, and 3 mol% of
GO@SiO2-HPW in water at reflux temperature (Table 1,
entries 7–10). The best loading was found to be 1 mol%. A
further increase in the amount of catalyst (up to 3 mol%) did
not have any significant effect on the product yield or
reaction time.
TABLE 3 Synthesis of imidazolines using GO@SiO2-HPW in water
Yield (%)a
[ref.]
Entry
Amine
Aldehyde
Product
Time (min)
1
120
85 [36]
2
3
100
140
120
88 [36]
82 [36]
80 [37]
4
a
Isolated yields.
b
Reaction conditions: 1,2-ethylenediamine (1 mmol), aldehyde (1 mmol), GO@SiO2-HPW (1 mol%).

8
HABIBZADEH ET AL.
TABLE 4 Synthesis of benzothiazoles using GO@SiO2-HPW in water
Yield (%)a
[ref.]
Entry
Amine
Aldehyde
Product
Time (min)
1
55
96 [38]
2
3
4
42
45
50
92 [38]
90 [39]
96 [38]
5
6
50
65
85 [40]
90 [41]
7
90
78 [42]
8
9
70
40
85 [38]
94 [43]
10
50
95 [42]

HABIBZADEH ET AL.
9
TABLE 4 (Continued)
Yield (%)a
[ref.]
Entry
Amine
Aldehyde
Product
Time (min)
11
80
78 [44]
80 [42]
80 [42]
12
13
75
80
14
80
85 [44]
15
16
45
85
80 [45]
75 [45]
a
Isolated yields.
b
Reaction conditions: 2-aminothiophenol (1 mmol), aldehyde (1 mmol), GO@SiO2–HPW (1 mol%).
After finding the optimized reaction conditions, to evalu-
be seen that electron-donating and electron-withdrawing
substituents do not show any difference in the reaction
yields.
ate the scope and limitations of this reaction, we studied the
reactions of 1,2-phenylenediamines with various aldehydes
(Table 2). Aliphatic and aromatic aldehydes containing
either electron-donating or electron-withdrawing substituents
reacted and afforded good to high yields of the products. It is
worth noting that the reactions of aromatic aldehydes posses-
sing electron-donating groups required longer reaction times
compared to aromatic aldehydes with electron-withdrawing
groups. The catalyst was also found to be very active for the
preparation of benzimidazoles from a sterically hindered
aldehyde such as naphthaldehyde (Table 2, entry 6).
In the case of synthesis of benzimidazoles, the reaction
of 1,2-phenylenediamine and aldehyde in 1:1 ratio gave only
2-substituted benzimidazoles, whereas when this ratio was
changed to 1:3, then 1,2-disubstituted benzimidazole was
obtained in good yield under same reaction conditions
(Table 5). It is worth noting that most of the reported proce-
dures generate mono-substituted benzimidazoles as a target
product.
Employing the same procedure given above but using
1,2-ethylenediamine instead of 1,2-phenylenediamine, the
corresponding 2-substituted imidazolines were obtained in
excellent yields with various substituents (Table 3).
Encouraged by these results, the applicability of this pro-
tocol was further extended for the synthesis of benzothia-
zoles (Table 4). Aromatic aldehydes with electron-donating
or electron-withdrawing substituents were used, and, as
expected, gave good to excellent yield of the products. It can
3.3 | Catalyst reusability
The recovery and reuse of a catalyst is highly desirable dur-
ing its practical application. In this regard, the recyclability
of GO@SiO2-HPW was investigated in a model reaction of
1,2-diaminobenzene with 4-chlorobenzaldehyde under opti-
mized reaction conditions. After the reaction was completed,
the catalyst was separated by centrifugation. Then the cata-
lyst was washed with methanol (3 × 5 mL) and

10
HABIBZADEH ET AL.
TABLE 5 Synthesis of 1,2-disubstituted benzimidazole using GO@SiO2-HPW in water
Entry
Amine
Aldehyde
Product
Time (min)
Yield (%)a [ref.]
1
100
80 [45]
2
85
88 [45]
a
Isolated yields.
b
Reaction conditions: 1,2-phenylenediamine (1 mmol), aldehyde (3 mmol), GO@SiO2–HPW (1 mol%).
TABLE 6 Reusability of GO@SiO2-HPW catalyst
ORCID
Entry
Reaction cycle
First
Time (min)
Yield (%)a
Setareh Habibzadeh https://orcid.org/0000-0002-5534-
8210
1
2
3
4
5
90
90
90
90
90
96
95
90
87
85
Second
Third
Fourth
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This work was supported by the Babol Noshirvani Univer-
sity of Technology, Babol, Iran.

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How to cite this article: Habibzadeh S, Firouzzadeh
Pasha G, Tajbakhsh M, Amiri Andi N, Alaee E. A
novel ternary GO@SiO2-HPW nanocomposite as an
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benzazoles in aqueous media. J Chin Chem Soc.
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