MWCNTs-ZrO2 as a reusable heterogeneous catalyst for the synthesis of N-heterocyclic scaffolds under green reaction medium

Article abstract of DOI:10.1002/aoc.5906

A simple, efficient, and facile heterogeneous multi-walled carbon nanotubes-zirconia nanocomposite (MWCNTs-ZrO₂) has been synthesized using natural feedstock coconut juice (água-de-coco do Ceará). The synthesized catalyst was characterized by Fourier transform infrared spectroscopy, X-ray diffraction, field emission scanning electron microscopy, and X-ray photoelectron spectroscopy analysis. The heterogeneous nanocomposite has been used for one-pot synthesis of various N-heterocyclic compounds like pyrazoles, 1,2-disubstituted benzimidazoles, 2-arylbenzazoles, and 2,3-dihydroquinazolin-4(1H)-ones under green reaction medium at room temperature. This novel method has several advantages, such as short reaction time, simple work-up, excellent yield, and green reaction conditions. The catalyst was recycled up to four times without significant loss in catalytic activity.

Full text of DOI:10.1002/aoc.5906

Received: 15 January 2020
DOI: 10.1002/aoc.5906
Revised: 21 May 2020
Accepted: 8 June 2020
F U L L P A P E R
MWCNTs-ZrO2 as a reusable heterogeneous catalyst for
the synthesis of N-heterocyclic scaffolds under green
reaction medium
Bipasa Halder
| Flora Banerjee
| Ahindra Nag
Department of Chemistry, Indian Institute
of Technology Kharagpur, Kharagpur,
721302, India
A simple, efficient, and facile heterogeneous multi-walled carbon nanotubes-
zirconia nanocomposite (MWCNTs-ZrO2) has been synthesized using natural
feedstock coconut juice (água-de-coco do Ceará). The synthesized catalyst was
characterized by Fourier transform infrared spectroscopy, X-ray diffraction,
field emission scanning electron microscopy, and X-ray photoelectron spectros-
copy analysis. The heterogeneous nanocomposite has been used for one-pot
synthesis of various N-heterocyclic compounds like pyrazoles, 1,2-disubstituted
benzimidazoles, 2-arylbenzazoles, and 2,3-dihydroquinazolin-4(1H)-ones under
green reaction medium at room temperature. This novel method has several
advantages, such as short reaction time, simple work-up, excellent yield, and
green reaction conditions. The catalyst was recycled up to four times without
significant loss in catalytic activity.
Correspondence
Ahindra Nag, Department of Chemistry,
Indian Institute of Technology Kharagpur,
Kharagpur 721302, India.
Email: ahinnag@chem.iitkgp.ernet.in
Funding information
UGC, NEW DELHI, INDIA, Grant/Award
Number: Ref No.- 21/06/2015(i)EU-V
K E Y W O R D S
coconut juice, heterogeneous catalyst, MWCNTs-ZrO2, N-heterocyclic compounds, one-pot synthesis
1 | INTRODUCTION
CuO/ZrO2,[14] {[HMIM]C(NO2)3},[15] Fe3O4@Si@MoO2,[16]
and CPS-CDMNPs.[17]
Nitrogen-containing
heterocyclic
(azaheterocyclic)
Benzimidazole derivatives are fused N-heterocyclic
skeletons that possess different biological properties such as
anti-inflammatory,[18] antiprotozoal,[19] antihelminthic,[20]
antimicrobial,[21] and anticancer activities,[22] 5-lipoxy
genase inhibitory activities,[23] and antihypertensive activi-
ties.[24] Recently, several methods have emerged for the
synthesis of 1,2-disubstituted benzimidazole derivatives in
the presence of various catalysts such as ionic liquid-
coated ZnO nanoparticles,[25] chitosan-supported Fe3O4,[26]
Zn-proline,[27] and trifluoroethanol,[28] as well as benzimid-
azole derivatives synthesized from sodium dodecyl sulfate
micelles,[29] Er(OTf)3,[30] and graphene sulfonic acid[31]
catalysts. In addition, benzothiazole motifs are of out-
standing significance due to their anticancer, antitumor,
and antibacterial activities.[29] Several methodologies have
privileged scaffolds are widely dispersed in diverse fields
such as synthetic organic chemistry, pivotal intermediates
in drug industry, and biological and pharmaceutical
areas.[1] Many pyrazole derivatives in synthetic chemistry,
and amino pyrazoles are fascinating due their numerous
properties, for example antifungal,[2] antimicrobial,[3]
antiviral,[4] antihyperglycemic,[5] antitumor,[6] anti-
inflammatory,[7] analgesic,[8] antidepressant,[9] and anti-
convulsant.[10] Of the many reported methods, most of
the conventional approaches for the synthesis of pyrazoles
are multicomponent reactions (MCRs) between
a
variety of aldehydes, malononitrile, and phenylhydrazine,
which are synthesized using different types of catalyst,
such as I2,[11] GO-TiO2,[12] polyethylene glycol:H2O,[13]
Appl Organomet Chem. 2020;e5906.
wileyonlinelibrary.com/journal/aoc
© 2020 John Wiley & Sons, Ltd.
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https://doi.org/10.1002/aoc.5906

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HALDER ET AL.
been disclosed for the synthesis of benzothiazoles, such
as SDS micelles,[29] nano-Fe2O3,[32] ButONa,[33] and
oxalic/malonic acid.[34]
2,3-dihydroquinazolin-4(1H)-ones (DHQs) are N-hetero
bicyclic compounds that have wide-ranging application as
drug intermediates in the pharmaceutical and biological
fields.[35] A number of methods have been established
for the synthesis of DHQs over silica sulfuric acid,[36] beta
cyclodextrin,[37] PEG-400,[35] Co-CNTs,[38] montmorillonite
K-10,[39] and [Ce(L-Pro)2]2(Oxa).[40]
Heterogeneous catalysis is widely used and has
become important for organic transformations in recent
years.[41] Heterogeneous catalysts have many advantages
over homogeneous catalysts, such as simple work-up,
easy handling, reusability, and recoverability.[42] Notably,
carbon nanotubes (CNTs) have attracted much attention
in the field of heterogeneous catalysis[43] due to their
high surface area, unique morphology, network, physical
properties, and high electrical conductivity.[44] Recently,
carbon nanotubes and their composites have been used
as catalysts in many organic reactions as well as in the
pharmaceutical industry.[38,45]
Among the various ceramic oxide nanoparticles,
ZrO2 has been used as a heterogeneous catalyst because
of its high dielectric properties and thermal stability,
acid–base properties and good mechanical strength.[46]
Likewise, it has many applications in the field of
science and technology.[47] Therefore multi-walled
CNTs-ZrO2 (MWCNTs-ZrO2) composites have been
used in the field of Electromagnetic interference
shielding.[48] Hybridization of MWCNTs-ZrO2 compos-
ites has not been used in organic synthesis as a hetero-
geneous catalyst till now.
From the perspective of green chemistry, water has
attracted attention as a reaction medium due to its cheap,
environmentally friendly, nontoxic, and nonflammable
properties.[49] Recently, plant parts have been used as a
biocatalyst for organic transformations and generating
nanoparticles also due to its mild reaction condition and
eco-friendly aqueous medium.[50] In this context, coconut
juice was introduced to the field of green nanotechnol-
ogy. Coconut juice, known as água-de-coco do Ceará
(ACC), is a sweet refreshing juice. Each 100 g of coconut
juice contains water (94.99 g), proteins (0.72 g), fat
(0.2 g), carbohydrates (3.71 g), fiber (1.1 g), sugar (2.61 g),
electrolytes such as sodium (105 mg) and potassium
(250 mg), minerals such as calcium (24 mg), magnesium
(25 mg), and phosphorous (20 mg), ascorbic acid
(2.4 mg), and pantothenic acid (0.043 mg).[51] It is
already known that many fruit extracts, which are
acidic in nature, have been used to synthesize ZrO2
nanoparticles.[52] We report here the first synthesis of
ZrO2 and MWCNTs-ZrO2 nanoparticles using ACC. Due
to the acidic nature of ACC (pH = 5.06), it accelerates the
formation of MWCNTs-ZrO2 nanocomposite.
We describe the synthesis of MWCNTs-ZrO2
nanocomposite from ACC and demonstrate its applica-
tion as an efficient heterogeneous catalyst for the
synthesis of pyrazoles by a multicomponent reaction of
phenylhydrazine, malononitrile, aldehydes, synthesis of
1,2-disubstituted benzimidazoles and benzimidazoles
from o-phenylenediamine and benzaldehydes, synthesis
of benzothiazoles from o-aminothiophenol and benzalde-
hydes, and synthesis of 2,3-dihydroquinazolin-4(1H)-ones
from 2-aminobenzamide and benzaldehydes at room
temperature (Scheme 1).
2 | RESULTS AND DISCUSSION
2.1 | Characterizations of the synthesized
nanocomposite
ZrO2 and MWCNTs-ZrO2 were synthesized using a
simple refluxing method from the natural feedstock
ACC. The bonding, crystallinity, morphology, and atomic
composition of the product were examined by Fourier
transform infrared (FT-IR) spectroscopy, X-ray diffraction
(XRD), field emission scanning electron microscopy
(FE-SEM), and X-ray photoelectron spectroscopy (XPS),
respectively.
The FT-IR spectra of oxidized MWCNT, ZrO2, and
MWCNTs-ZrO2 composite were recorded in the range of
4,000–400 cm−1 and the findings are displayed in
Figure 1. The prominent peaks in spectra of oxidized
MWCNT and ZrO2 at ꢀ1,222 and 769 cm−1 correspond
to C–O bond stretching in different chemical environ-
ments and C–H bending vibration, respectively.[53] The
two peaks in the spectra of ZrO2 can be attributed to
some metabolite and protein, such as the flavonoids and
terpernoids present in coconut juice.[50f] Broad peaks
centered at ꢀ3,360, 1,611, and 1,349 cm−1 appeared in
the ZrO2 spectrum due to the stretching and bending
modes of the –OH group originating from adsorbed
moisture.[53c,54] Notably, the weak and broad peak cen-
tered at ꢀ968 cm−1 is related to the Zr–O stretching
mode. The FT-IR spectrum of the MWCNTs-ZrO2 com-
posite exhibited superimposition of the spectral features
of both oxidized MWCNTs and ZrO2, accompanied by a
change in peak intensity. Weak peaks centered at ꢀ1,120
and 1,017 cm−1 appeared in the spectrum for the
MWCNTs-ZrO2 composite due to the stretching mode of
the aromatic C=C and C–O groups, respectively,
presenting different chemical environment.[48] These
findings clearly signify successful incorporation of ZrO2
into MWCNTs.

HALDER ET AL.
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SCHEME 1 Synthesis of N-heterocyclic
scaffolds in the presence of MWCNTs-ZrO2
FIGURE 2 XRD pattern of oxidized MWCNTs, ZrO2, and
FIGURE 1 FT-IR spectra of oxidized MWCNTs, ZrO2, and
MWCNTs-ZrO2 composite
MWCNTs-ZrO2 composite
XRD patterns of oxidized MWCNTs, ZrO2, and
MWCNTs-ZrO2 composites are shown in Figure 2. It can
be seen that diffraction peaks for oxidized MWCNTs
occur at 2θ = ꢀ26ꢁ and ꢀ43ꢁ, corresponding to the (002)
and (100) planes, respectively, of MWCNTs.[55] Further-
more, the appearance of two broad peaks centered at
around ꢀ30ꢁ (002) and 55ꢁ (100) confirmed the forma-
tion of amorphous from ZrO2 (JCPDS no. 37–1,484).[56]
XRD patterns for the MWCNTs-ZrO2 composite had
diffraction peaks for both ZrO2 and oxidized MWCNTs.
Notably, the breadth of the oxidized MWCNT peak
centered at ꢀ 26ꢁ increased and the peak intensity of the
same peak decreased with incorporation of amorphous
ZrO2 in MWCNTs. These findings clearly indicate incor-
poration of ZrO2 into the MWCNT matrix.
FE-SEM images of ZrO2, oxidized MWCNTs, and
MWCNTs-ZrO2 composite are shown in Figure 3. ZrO2
appears to exist as agglomerated spherical nanoparticles.[48]

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HALDER ET AL.
FIGURE 3 FE-SEM images of
(a) ZrO2, (b) oxidized MWCNT, and
(c) low-magnification and (d) high-
magnification images of MWCNTs-ZrO2
composite
Oxidized MWCNTs exhibit nanotube-like morphology sta-
cked together. FE-SEM images of MWCNTs-ZrO2 compos-
ite show that ZrO2 nanoparticles are clearly incorporated
in the MWCNT matrix. Notably, the regular shape of the
Zr 3d spectrum at 183.7 and 186.2 eV, corresponding to Zr
3d3/2 and Zr 3d5/2, respectively.[57a] Deconvoluated peaks
at 284.9, 286.3, and 288.2 eV in the high-resolution C 1 s
spectrum are associated with the C–C, C–O, and C=O
functionalities, respectively.[57a] The fine spectrum of O 1 s
in the as-prepared composite reveals peaks at 531.3, 532.4,
and 533.5 eV, corresponding to Zr–O, C=O, and O–C=O,
respectively.[57] All these findings point to the oxidized
nature of MWCNTs and the presence of ZrO2 in the
MWCNTs-ZrO2 composite.
[48]
MWCNTs remains intact even after synthesis with ZrO2.
The valence state and elemental analysis of MWCNTs-
ZrO2 composite were analyzed by XPS (Figure 4).
Figure 4a shows the full survey spectrum of the MWCNTs-
ZrO2 composite, which confirms the presence of C, O, and
Zr elements. Figure 4b shows deconvoluated peaks for the
FIGURE 4 XPS spectra of
MWCNTs-ZrO2 composite: (a) full
survey spectra, (b) Zr 3d, (c) C 1 s, and
(d) O 1 s

HALDER ET AL.
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2.2 | One-pot synthesis of pyrazole
derivatives
was 92% in water at room temperature (Table 1, entry 5).
Solvents with different polarities, such as EtOH, MeOH,
dichloromethane (DCM), MeCN, and H2O, were used to
determine the best solvent and H2O was found to be the
best reaction medium for the synthesis of pyrazole
(Table 1, entries 1–11). It is noteworthy that there was no
significant improvement in the yield of product on
increasing the amount of catalyst from 10 mg to 20 mg
(Table 1, entries 5 and 6). Further increasing the temper-
ature had no effect on the yield of the desired product
(Table 1, entry 11). After optimization of the reaction
conditions, the scope of the reaction was extended by
using different aromatic aldehydes, phenylhydrazine, and
malononitrile to obtain the products 4a–p, which are
summarized in Figure 5. In all cases, aromatic aldehydes
containing electron-withdrawing as well as electron-
donating compounds showed good to excellent yield. The
structures of all products were characterized by FT-IR,
1H NMR, and 13C NMR spectroscopy analysis.
The catalytic activity of MWCNTs-ZrO2 was examined for
the multicomponent reaction of substituted benzaldehydes,
phenyl hydrazine, and malononitrile (Scheme 2). To opti-
mize the suitable reaction conditions for the synthesis of
pyrazole derivatives, p-nitro benzaldehyde (1 mmol), phe-
nyl hydrazine (1 mmol), and malononitrile (1 mmol) were
chosen as model reactants in the presence of MWCNTs-
ZrO2 nanocomposite and the results are summarized in
Table 1. When the reaction was carried out in the absence
of a catalyst, no product was formed after 10 hr at room
temperature or at high temperature (Table 1, entries 1 and
2). The same reaction was performed in the presence of
MWCNT (10 mg) and ZrO2 (10 mg) in water (5 ml) as
solvent at room temperature, giving the products in yields
of 35% and 40%, respectively (Table 1, entries 3 and 4).
When the reaction was carried out in the presence of
MWCNTs-ZrO2 catalyst (10 mg), the yield of the product
2.3 | One-pot synthesis of 1,2-disubstituted
benzimidazole derivatives
As a result of the success of the synthesis of pyrazole
reactions, the utility of the nanocomposite MWCNTs-
ZrO2 was extended to the synthesis of 1,2-disubstituted
SCHEME 2 Synthesis of pyrazole derivatives 4
TABLE 1 Optimization of the reaction conditions for the synthesis of pyrazole 4a[a]
Entry
Catalyst (mg)
–
Solvent
H2O
Temperature (ꢁC)
Time (hr)
Yield (%)[b]
1
r.t.
60
10
10
5
–
–
2
–
H2O
3
MWCNT (10)
H2O
r.t.
r.t.
r.t.
r.t.
r.t.
r.t.
r.t.
r.t.
60
35
40
92
92
88
78
82
85
92
4
ZrO2 (10)
H2O
5
5
MWCNTs-ZrO2 (10)
MWCNTs-ZrO2 (20)
MWCNTs-ZrO2 (10)
MWCNTs-ZrO2 (10)
MWCNTs-ZrO2 (10)
MWCNTs-ZrO2 (10)
MWCNTs-ZrO2 (10)
H2O
1.5
1.5
1.5
1.5
1.5
1.5
1.5
6
H2O
7
EtOH
MeCN
DCM
MeOH
H2O
8
9
10
11
aReaction conditions: p-nitro benzaldehyde (1 mmol), phenylhydrazine (1 mmol), malononitrile (1 mmol), and MWCNTs-ZrO2
nanocomposite in solvent (5 ml).
bIsolated yield.

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HALDER ET AL.
FIGURE 5 Synthesis of pyrazole derivatives in the presence of MWCNTs-ZrO2 nanocomposite
benzimidazole by cyclocondensation of
1
mol of
2.4 | One-pot synthesis of 2-arylbenzazole
o-phenylenediamine and 2 mol of various benzaldehydes
(Scheme 3).
derivatives
At the outset of the present study and to optimize the
reaction conditions for the synthesis of disubstituted
benzimidazole derivatives, o-phenylenediamine (1 mmol)
and p-hydroxybenzaldehyde (2 mmol) in the presence of
MWCNTs-ZrO2 nanocomposite with different solvents
were chosen as a model reaction and the results are
summarized in Table 2.
The outlook of o-substituted aniline system has been
continued for the synthesis of 2-aryl benzimidazole and
2-aryl benzothiazole derivatives (Scheme 4) under the
same reaction conditions as shown in Table 2 (entry 8).
For the synthesis of 2-aryl benzimidazole derivatives,
o-phenylenediamine (1 mmol) and various aromatic
benzaldehydes (1 mmol) with MWCNTs-ZrO2 nano-
composite were stirred in EtOH. For the synthesis of
2-aryl benzothiazole derivatives, 2-aminothiophenol
(1 mmol) was used instead of o-phenylenediamine with
various aromatic aldehydes (1 mmol) and MWCNTs-ZrO2
nanocomposite (10 mg) in EtOH. All compounds 9a–j are
summarized in Figure 7. From Figure 7, it is concluded
that aromatic aldehydes containing electron-donating
as well as electron-withdrawing compounds showed
good to excellent yield. The structures of all products were
The cyclocondensations of o-phenylenediamine
(1 mmol) and various aromatic benzaldehydes (2 mmol)
with MWCNTs-ZrO2 nanocomposite in EtOH under opti-
mized reaction conditions are summarized in Figure 6.
In all cases, aromatic aldehydes containing electron-
withdrawing as well as electron-donating compounds
showed good to excellent yield. The structures of the
1
products 7a–o were characterized by FT-IR, H NMR,
and 13C NMR spectroscopy analysis.
1
characterized by FT-IR, H NMR, and 13C NMR spectros-
copy analysis.
2.5 | One-pot synthesis of
dihydroquinazolinone derivatives
The catalytic activity of MWCNTs-ZrO2 was investigated
for the synthesis of dihydroquinazolinone derivatives,
SCHEME 3 Synthesis of 1,2-disubstituted benzimidazole
derivatives 7

HALDER ET AL.
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TABLE 2 Optimization of the reaction conditions for the synthesis of disubstituted benzimidazole 7b[a]
Entry
Solvent/catalyst
H2O/10 mg
Temperature (ꢁC)
Time (hr)
Yield (%)[b]
1
2
3
4
5
6
7
8
9
r.t.
r.t.
r.t.
r.t.
r.t.
r.t.
r.t.
r.t.
60
2
40
45
50
60
62
75
85
92
92
H2O/20 mg
2
MeCN/10 mg
DCM/10 mg
CHCl3/10 mg
MeOH/10 mg
EtOH/10 mg
EtOH/10 mg
EtOH/20 mg
2
2.5
2.5
2
2
3
3
Bold emphasis highlight the optimal reaction conditions giving excellent yield.
aReaction conditions: o-phenylenediamine (1 mmol), p-hydroxybenzaldehyde (2 mmol), and MWCNTs-ZrO2 nanocomposite in solvent
(5 ml).
bIsolated yield.
FIGURE 6 Synthesis of 1,2-disubstituted benzimidazole derivatives in the presence of MWCNTs-ZrO2 nanocomposite

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HALDER ET AL.
SCHEME 5 Synthesis of dihydroquinazolinone derivatives 12
SCHEME 4 Synthesis of 2-arylbenzazole derivatives 9
keeping water as a solvent at room temperature
(Scheme 5).
acetate and water, and the catalyst was recovered by
above stated procedure. In this way, the catalyst
was recovered four times and a slight loss of catalytic
activity was observed for each reaction. The results are
shown in Figure 9.
To optimize the reaction conditions, the reaction
between 2-aminobenzamide (1 mmol) and 4-hydroxy-
3,5-dimethoxy benzaldehyde (1 mmol) was used as a
model reaction in the presence of MWCNTs-ZrO2
nanocomposite and the results are summarized in
Table 3.
3 | EXPERIMENTAL
The products obtained from the condensation of
2-aminobenzamide (1 mmol) and various aromatic benz-
aldehydes (1 mmol) with MWCNTs-ZrO2 nanocomposite
in H2O under optimal conditions are summarized in
Figure 8 and have moderately good yield. The structures
3.1 | Materials and characterization
All reagents were purchased from Merck, Sigma-Aldrich,
and Fluka, Bangalore. The reactions were monitored
using precoated aluminum TLC plates (Mark silica
60, f254). XRD measurements were recorded at room tem-
perature to characterize MWCNTs, oxidized MWCNTs,
and MWCNTs-ZrO2 using a Phillip, Holland, Bangalore
instrument with CuKα radiation at λ = 0.1541 nm. The
morphology of the samples was studied by FE-SEM on a
Carl Zeiss, Bangalore at an accelerating voltage of 20 kV.
XPS of the nanocomposite was examined using a PHI
1
of compounds 12a–e were characterized by FT-IR, H
NMR, and 13C NMR spectroscopy analysis.
To demonstrate the efficiency of the present protocol,
the results obtained for the synthesis of compounds 4f,
7b, 9h, and 12b were compared with some previous
reported methods (Table 4). From Table 4, it is clear that
the present system is comparable with reported proce-
dures with respect to solvent and yields.
Catalyst reusability was screened for the synthesis
of products 4f, 7b, 9h, and 12b. For compounds 4f and
12b the catalyst was recovered from the aqueous part
of the reaction mixture after work-up with ethyl acetate.
The catalyst was washed with EtOH and DCM several
times, then dried in an oven at 60ꢁC. For compounds
7b and 9h, the reaction mixtures were evaporated
under reduced pressure to remove EtOH. The residues
were extracted after a conventional work-up with ethyl
5000 Versa Probe II system, Bangalore using a
microfocused (100 μm, 25 W, 15 kV) monochromatic Al
Kα source (hν = 1486.6 eV). NMR spectra were recorded
on a Bruker Avance, Bangalore 400 MHz, 500 MHz or
600 MHz spectrometer using DMSO-d6 or CDCl3 as the
solvent. The multiplicity was marked as s (singlet), d
(doublet), t (triplet), m (multiplet), and or (broad singlet).
Melting points were determined in open capillary
using an Electrothermal 9,100, Bangalore and were
FIGURE 7 Synthesis of
2-arylbenzazole derivatives in the presence
of MWCNTs-ZrO2 nanocomposite

HALDER ET AL.
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TABLE 3 Optimization of the reaction conditions for the synthesis of dihydroquinazolinone 12e[a]
Entry
Solvent/catalyst
H2O/10 mg
Temperature (ꢁC)
Time (hr)
Yield (%)[b]
1
2
3
4
5
6
r.t.
r.t.
r.t.
r.t.
r.t.
r.t.
4
84
84
80
78
62
56
H2O/20 mg
5
EtOH/10 mg
MeOH/10 mg
DCM/10 mg
CHCl3/10 mg
4
4
4
4.5
Bold emphasis highlight the optimal reaction conditions giving excellent yield.
aReaction conditions: 2-aminobenzamide (1 mmol), 4-hydroxy-3,5-dimethoxy benzaldehyde (1 mmol), and MWCNTs-ZrO2 nanocomposite
in solvent (5 ml).
bIsolated yield.
3.3 | Preparation of MWCNTs-ZrO2
composite
MWCNTs were chemically oxidized by refluxing using
concentrated HNO3 and to form products referred to as
MWCNT Ox.[58] Next, 2 wt% of oxidized MWCNTs was
added to 30 ml of 0.3 M ZrO(NO3)2.H2O solution using
coconut juice as solvent with vigorous stirring followed
by its ultrasonication for 30 min. Subsequently, the whole
dispersion was refluxed at 100ꢁC for 24 hr. The black
product obtained was washed with distilled water until
neutral pH was reached followed by drying in a vacuum
FIGURE 8 Synthesis of dihydroquinazolinone derivatives in
the presence of MWCNTs-ZrO2 nanocomposite
oven at 60ꢁC. This product is MWCNTs-ZrO2.
uncorrected. FT-IR spectra were recorded in the
wavenumber range 4,000–400 cm−1 on a Perkin-Elmer,
Bangalore FT-IR spectrometer RXI.
3.4 | General procedure for the synthesis
of pyrazoles
A mixture of phenylhydrazine (1, 1 mmol), benzaldehyde
(2, 1 mmol), manolonitrile (3, 1 mmol), and MWCNTs-
ZrO2 nanocomposite (10 mg) in 5 ml of water was stirred
in a round-bottomed flask at room temperature for
1.5 hr. After completion of the reaction, the reaction
mixture was extracted with ethyl acetate. The organic
phase was dried over anhydrous Na2SO4 and removed
under reduced pressure. Finally, the residue was washed
with water and recrystallized from ethanol to afford the
pure products 4a–p. The products were characterized by
FT-IR, 1H and 13C NMR spectroscopic analysis.
3.2 | Preparation of ZrO2
A solution (30 ml) of 0.3 M ZrO(NO3)2.H2O using coco-
nut juice as solvent was vigorously stirred followed by
ultrasonication for 30 min. Subsequently, the solution
was subjected to refluxing at 100ꢁC for 24 hr. Finally, the
white product was washed with distilled water until
neutral pH was obtained, then it was dried in a vacuum
oven at 60ꢁC.

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HALDER ET AL.
TABLE 4 Comparison of synthesis of products 4f (entries 1–5), 7b (entries 6–10), 9h (entries 11–13), and 12b (entries 14–17) with
reported protocols
Sl no.
Compound
Catalyst
Condition
Yield (%)
Reference
1
I2
Water/r.t./6 min
Ultrasonic irradiation/45 min
Water/40ꢁC/1.5 hr
Solvent-free/r.t./20 min
Water/r.t./1.5 hr
Ball mill/r.t./22 min
EtOH/r.t./4.5 hr
r.t./30 min
92
90
89
90
92
95
91
93
94
92
95
76
90
84
84
81
85
[11]
2
PEG:H2O
[13]
3
4f
CuO/ZrO2
[14]
4
Fe3O4@Si@MoO2
MWCNTs-ZrO2
Ionic liquid-coated ZnO nanoparticles
Chitosan-supported Fe3O4
TFE
[16]
5
This work
[25]
6
7
[26]
8
7b
[28]
9
SDS micelles
MWCNTs-ZrO2
SDS micelles
ButONa
Water/25ꢁC/10 min
[29]
10
11
12
13
14
15
16
17
Water/r.t./3 hr
This work
[29]
Water/25ꢁC/10 min
Toluene/100ꢁC/24 hr
Water/r.t./1.5 hr
100–110ꢁC
Water/55–60ꢁC
EtOH/50–55ꢁC
9h
[33]
MWCNTs-ZrO2
PEG-400
This work
[35]
β-CD
[37]
12b
[Ce(L-Pro)2]2 (Oxa)
MWCNTs-ZrO2
[40]
Water/r.t./3 hr
This work
ButONa, Sodium tert-butoxide; CuO, Copper (II) oxide; EtOH, ethanol; Fe3O4, Iron oxide; H2O = Water; I2, Iodine; L-Pro, L-Proline; MoO2, Molybdenum
dioxide; MWCNTs-ZrO2, multi-walled carbonnanotubes-zirconia; Oxa, Oxalate; PEG, Polyethylene glycol; PEG-400, Polyethylene glycol-400; SDS, Sodium
dodecyl sulfate; TFE, Trifluoroethanol; ZnO, Zinc oxide; ZrO2, Zirconia; β-CD, beta-cyclodextrin.
FIGURE 9 Reusability of MWCNTs-ZrO2
for synthesis of 4f, 7b, 9h, and 12b products
3.5 | General procedure for the synthesis
of 1,2-disubstituted benzimidazoles
was washed with water and methanol to afford the pure
products 7a–o. The products were characterized by
FT-IR, 1H and 13C NMR spectroscopic analysis.
A mixture of o-phenylenediamine (5, 1 mmol), benzalde-
hyde (6, 2 mmol), and MWCNTs-ZrO2 (10 mg) in 5 ml of
ethanol was stirred in a round-bottomed flask at room
temperature for 3 hr. After completion of the reaction,
the solvent was removed under reduced pressure and the
residue was extracted with ethyl acetate and water. The
ethyl acetate extract was dried over anhydrous Na2SO4
and removed under reduced pressure. Finally, the residue
3.6 | General procedure for the synthesis
of 2-aryl benzimidazoles and 2-aryl
benzothiazoles
A mixture of o-phenylenediamine (5a, 1 mmol), benzal-
dehyde (8, 1 mmol), and MWCNTs-ZrO2 (10 mg) in 5 ml

HALDER ET AL.
11 of 13
of ethanol was stirred in a round-bottomed flask at room
temperature for 2 hr. After completion of the reaction, an
identical procedure (serial no. 3.5) was adopted to get
pure products 9a–e. A similar procedure was used for
reaction of o-aminothiophenol (5b, 1 mmol) and benzal-
dehyde (8, 1 mmol) in the presence of MWCNTs-ZrO2
(10 mg) for the synthesis of 2-aryl benzothiazoles 9f–j.
3.8.3 | 1-(4-methylbenzyl)-2-(p-tolyl)-1H-
benzimidazole (7a)
White solid, m.p. 128–130ꢁC.[28] IR (KBr) ʋmax: 2941,
1,609, 1,516, 1,451, 748 cm−1. 1H NMR (400 MHz, CDCl3)
δ: 7.89 (d, 1H, J = 8.0 Hz), 7.61 (d, 2H, J = 7.9 Hz),
7.32–7.28 (m, 1H), 7.25 (d, 2H,
J = 7.7 Hz),
1
The products were characterized by FT-IR, H and 13C
7.20 (d, 2H, J = 5.7 Hz), 7.13 (d, 2H, J = 7.7 Hz), 6.99
(d, 2H, J = 7.7 Hz), 5.39 (s, 2H), 2.40 (s, 3H),
2.33 (s, 3H) ppm. 13C NMR (100 MHz, CDCl3)
δ: 154.3, 143.2, 140.1, 137.4, 136.1, 133.5, 129.7, 129.5,
129.2, 127.2, 125.9, 122.9, 122.6, 119.8, 110.6, 48.2, 21.5,
21.1 ppm.
NMR spectroscopic analysis.
3.7 | General procedure for the synthesis
of dihydroquinazolinones
A mixture of anthranilamide (10, 1 mmol), benzaldehyde
(11, 1 mmol), and MWCNTs-ZrO2 (10 mg) in 5 ml of
water was stirred in a round-bottomed flask at room
temperature for 4 hr. After completion of the reaction,
the reaction mixture was extracted with ethyl acetate.
The organic phase was dried over anhydrous Na2SO4
and evaporated under reduced pressure. Finally,
the residue was washed with water and recrystallized
from ethanol to afford the pure products 12a–e. The
3.8.4 | 1-(4-hydroxy-3-chloro-benzyl)-
2-(4-hydroxy-3-chloro-phenyl)-1H-
benzimidazole (7j)
White solid, m.p. 232ꢁC. IR (KBr) ʋmax: 3407, 2,532,
1
1,600, 1,407, 745 cm−1. H NMR (600 MHz, DMSO-d6)
δ: 10.82 (s, 1H), 10.23 (s, 1H), 7.69 (s, 2H), 7.51
(dd, 2H, J = 12.8, 5.9 Hz), 7.25–7.24 (m, 2H), 7.11
(d, 1H, J = 8.4 Hz), 7.03 (s, 1H), 6.87 (d, 1H, J = 8.4 Hz),
6.74 (d, 1H, J = 8.4 Hz), 5.45 (s, 2H) ppm. 13C NMR
(150 MHz, DMSO-d6) δ: 155.0, 152.9, 152.6, 143.0, 136.3,
131.0, 129.4, 129.1, 128.3, 126.4, 123.1, 122.7, 122.3, 120.5,
120.3, 119.6, 117.4, 117.2, 111.5, 47.0 ppm.
1
products were characterized by FT-IR, H and 13C NMR
spectroscopic analysis.
3.8 | Spectral data for selected compounds
3.8.1 | 5-amino-3-(4-chlorophenyl)-
1-phenyl-1H-pyrazole-4-carbonitrile (4b)
3.8.5 | 2-(4-hydroxy-3-ethoxyphenyl)-1H-
benzimidazole (9d)
White solid, m.p. 130–132ꢁC.[11] IR (KBr) ʋmax: 3312,
3,035, 2,234, 1,679, 1,585, 1,490, 1,257, 1,089, 746 cm−1
.
Chrome yellow solid, m.p. 108–110ꢁC.[59] IR (KBr) ʋmax
3273, 2,914, 2,391, 1,587, 1,451, 1,265 cm−1 1H NMR
:
1H NMR (600 MHz, CDCl3) δ: 7.64 (s, 2H), 7.60
(d, 2H, J = 8.3 Hz), 7.36 (d, 2H, J = 8.3 Hz), 7.32–7.28
(m, 2H), 7.13 (d, 2H, J = 7.9 Hz), 6.92 (t, 1H, J = 7.2 Hz)
ppm. 13C NMR (150 MHz, CDCl3) δ: 158.3, 144.4, 135.9,
134.0, 133.9, 131.8, 130.1, 129.4, 128.8, 127.3, 120.4,
112.8 ppm.
.
(600 MHz, DMSO-d6) δ: 12.65 (bs, 1H), 9.48 (s, 1H),
7.74 (s, 1H), 7.62 (dd, 1H, J = 8.1, 1.1 Hz), 7.55 (s,
2H), 7.16 (dd, 2H, J = 5.5, 2.9 Hz), 6.94 (d, 1H,
J = 8.2 Hz), 4.15 (q, 2H, J = 6.9 Hz), 1.40 (t, 3H,
J = 6.9 Hz) ppm. 13C NMR (150 MHz, DMSO-d6) δ:
152.3, 149.2, 147.4, 122.1, 121.9, 120.1, 116.2, 112.0,
64.4, 15.2 ppm.
3.8.2 | 5-amino-3-(4-hydroxy-
3,5-dimethoxyphenyl)-1-phenyl-1H-
pyrazole-4-carbonitrile (4o)
3.8.6 | 2-(4-nitrophenyl)-benzothiazole (9g)
Orange oil. IR (KBr) ʋmax: 3453, 2,932, 2,214, 1,592, 1,497,
Yellow solid, m.p. 230–232ꢁC.[29] IR (KBr) ʋmax: 3051,
1
1,453, 1,112 cm−1
.
1H NMR (400 MHz, DMSO-d6)
1,630, 1,602, 1,502, 1,336, 763 cm−1. H NMR (500 MHz,
δ: 10.10 (s, 1H), 8.55 (s, 1H), 7.73 (s, 1H), 7.18 (t, 2H,
J = 7.4 Hz), 7.03 (d, 2H, J = 7.9 Hz), 6.89 (s, 2H), 6.69
(t, 1H, J = 7.2 Hz), 3.79 (s, 6H) ppm. 13C NMR (100 MHz,
DMSO-d6) δ: 148.6, 146.0, 137.7, 136.6, 129.5, 126.7,
118.7, 112.3, 103.7, 56.4 ppm.
CDCl3) δ: 8.09 (d, 1H, J = 8.1 Hz), 8.06 (d, 2H, J = 8.4 Hz),
7.93 (d, 1H, J = 7.9 Hz), 7.54–7.49 (m, 3H), 7.43
(t, 1H, J = 7.6 Hz) ppm. 13C NMR (125 MHz, CDCl3)
δ: 166.6, 154.1, 137.1, 135.1, 132.2, 129.3, 128.7, 126.5,
125.4, 123.3, 121.7 ppm.

12 of 13
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3.8.7 | 2-(3,4-dihydroxyphenyl)-
2,3-dihydroquinazolin-4(1H)-one (12b)
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White solid, m.p. 291–293ꢁC.[35] IR (KBr) ʋmax: 3402,
3,244, 3,158, 1,645, 1,616, 1,508, 1,265 cm−1. H NMR
1
(600 MHz, DMSO-d6) δ: 9.02 (s, 1H), 8.98 (s, 1H), 8.07
(s, 1H), 7.61 (d, 1H, J = 6.6 Hz), 7.23 (d, 1H, J = 6.6 Hz),
6.92 (s, 2H), 6.724 (s,), 6.66 (d, 3H, J = 6 Hz), 5.58 (s, 1H)
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146.1, 145.5, 133.7, 132.8, 127.8, 118.5, 117.4, 115.5, 115.3,
114.8, 114.7, 67.1 ppm.
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MWCNTs-ZrO2 nanocomposite was successfully synthe-
sized from the natural feedstock ACC, which acts as a
green, environmentally friendly, and nontoxic solvent.
Because of the acidic nature of ACC, it is the main source
of protons and stimulates the formation of MWCNTs-
ZrO2. The nanocomposite was characterized by FT-IR,
XRD, FE-SEM, and XPS analysis. It is noteworthy that
the synthesized nanocomposite was used as a heteroge-
neous and reusable nanocatalyst in the field of organic
synthesis for the first time. Various N-heterocyclic com-
pounds such as pyrazoles, disubstituted benzimidazoles,
2-arylbenzazoles, and dihydroquinazolinones were syn-
thesized using the MWCNTs-ZrO2 nanocomposite under
ecofriendly reaction conditions. The major advantages of
the protocol are short reaction time, good to excellent
yield, green reaction conditions, and recyclability without
significant loss in catalytic activity, and these factors
open up a new horizon in organic synthesis.
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ACKNOWLEDGMENTS
We thank University Grants Commission, New Delhi for
financial support of this work [Ref No.- 21/06/2015(i)EU-V].
ꢀ
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ORCID
Bipasa Halder https://orcid.org/0000-0002-0138-9084
Flora Banerjee https://orcid.org/0000-0003-3257-8942
Ahindra Nag https://orcid.org/0000-0001-5924-8981
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Note. r.t., room temperature.
Note. r.t., room temperature.
Note. r.t., room temperature.
SUPPORTING INFORMATION
Additional supporting information may be found online
in the Supporting Information section at the end of this
article.
How to cite this article: Halder B, Banerjee F,
Nag A. MWCNTs-ZrO2 as a reusable
heterogeneous catalyst for the synthesis of
N-heterocyclic scaffolds under green reaction
medium. Appl Organomet Chem. 2020;e5906.
https://doi.org/10.1002/aoc.5906
[48] K. Ghosh, S. K. Srivastava, S. Puravankara, J. Nanosci.
Nanotechnol. 2019, 19, 3367.