Citrus Extract Modified Graphene Oxide as a Green and Heterogeneous Organocatalyst for the Synthesis of Imidazole Derivatives

Article abstract of DOI:10.14233/ajchem.2020.22578

A naturally benign convention was created with a surface change of graphene oxide by citrus extract as catalyst was prepared by a straight-forward chemical modification method. The prepared catalyst′s catalytic activity was examined by the synthesis of imidazole derivatives at room temperature. It shows a strong acidic catalytic and sustainable organocatalyst. The prepared catalyst was characterized using different analytical procedures like elemental analysis, Fourier transforms infrared spectroscopy (FT-IR), powder X-ray diffraction (PXRD), energy-dispersive X-ray analysis (EDS), scanning electron microscopy images (SEM) and transmission electron microscopy images (TEM) analysis. The catalytic activity shows high activity and can be reused without significant loss of catalytic activity after five times. A present catalyst works easily under room temperature.

Full text of DOI:10.14233/ajchem.2020.22578

Asian Journal of Chemistry; Vol. 32, No. 9 (2020), 2375-2380
A
SIAN
J
OURNAL OF HEMISTRY
C
https://doi.org/10.14233/ajchem.2020.22578
Citrus Extract Modified Graphene Oxide as a Green and Heterogeneous
Organocatalyst for the Synthesis of Imidazole Derivatives
1,*
1,*
2
1
S. ARUNKUMAR , S. CHIDAMBARA VINAYAGAM , S. LAKSHMANAN and S. ARUL ANTONY
1
2
Department of Chemistry, Presidency College, Chennai-600005, India
Department of Chemistry, BIHER, Bharath University, Chennai-600073, India
*
Corresponding author: E-mail: arunchemphd666@gmail.com
Received: 8 December 2019; Accepted: 23 July 2020;
Published online: 20 August 2020;
AJC-20041
A naturally benign convention was created with a surface change of graphene oxide by citrus extract as catalyst was prepared by a
straight-forward chemical modification method. The prepared catalyst′s catalytic activity was examined by the synthesis of imidazole
derivatives at room temperature. It shows a strong acidic catalytic and sustainable organocatalyst. The prepared catalyst was characterized
using different analytical procedures like elemental analysis, Fourier transforms infrared spectroscopy (FT-IR), powder X-ray diffraction
(
PXRD), energy-dispersive X-ray analysis (EDS), scanning electron microscopy images (SEM) and transmission electron microscopy
images (TEM) analysis. The catalytic activity shows high activity and can be reused without significant loss of catalytic activity after five
times. A present catalyst works easily under room temperature.
Keywords: Graphene oxide, Organocatalyst, Citric acid, Imidazoles.
catalyst for many of the organic synthesis. For example, synthsis
of amines (different types of nitrogens) on the graphene surface
is one of the most widely recognized strategies for covalent
functionalization. For instance of the utility of these function-
alized materials, the expansion of long, aliphatic amine function-
alities were exhibited to increase the activity on chemical
reactions [4].
Imidazoles are classified in a fused heterocyclic and poss-
essed a diverse range of pharmaceutical activities. Because of
their potential and occurrence in nature, the ongoing explor-
ation gave a few strategies need to made their subsidiaries and
created many short approach for their amalgamation. These
incorporate buildup of anthranilamide with aldehydes in
nearness of acid, base catalysts and metal salts [5]. N-Bromo
INTRODUCTION
Among different nanomaterials graphene is a novel and
carbon-based nanomaterials and also, it has pulled in as a result
of its exceptional physical and chemical properties [1]. The unique
properties of graphene are its high substance of carbon and
oxygen proportion, its practical gathering change and surface
properties were involving an appealing organocatalyst for bio-
medical applications for instance biosensor demonstrating,
tranquilize conveyance and bacterial inhibition [2]. Recently,
carbon materials are probably the best alternative for the gene-
ration of various organic compound synthesis. Because, they
have extraordinary properties, for example, huge explicit surface
area, high permeable structure and solid connections among
carbon and hydrogen atoms. Thus, graphene oxide is a promi-
sing contender for a wide assortment of synergist applications
succinamide [6], CuCl
2
[7], silica chloride [8], K
3
PO [9], etc.
4
catalysts demonstrated the limitations, for example, longer
reaction time, encompassing conditions, homogeneous nature
of the catalysts which makes the procedure tedious and exor-
bitant. However, aldehydes and ketone require more reaction
times and delivering lowest yields. However, aldehydes and
ketone require more reaction times and producing minimum
yields. In such cases, poor yield were observed like aromatic
[
3]. Graphene oxide sheet have chemically reactive oxygen
functional group, such as, carboxylic acid functionalities at their
edges (as indicated by the generally known Lerf-Klinowski
model) and hydroxyl and epoxy functionalities are available
on the basal planes. Our definitive mean to do the chemical
modification on graphene oxide sheets would use as an organo-
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2
376 Arunkumar et al.
Asian J. Chem.
ketone is the major drawback of the imidazole synthesis.Acidic
carbons, in light of the idea of greener blend, were accounted
for as increasingly steady and high dynamic proton rich catalysts
for a few acid based reactions. Graphene oxide and its alter-
ations were successfully applied as heterogeneous catalyst for
some organic reaction conversion [10]. Graphene is a slender
type of carbon with various trademark, has been exceptional
consideration on both the theoretical and practical applications
in ongoing years [11].
In this work, on a fundamental level, graphene oxide was
prepared from graphite as indicated by the modified Hummer′s
method and then the prepared graphene oxide was scattered
into the fluid citrus extract at room temperature and stirred over-
night to get a heterogeneous organocatalyst, which was utilized
in the synthesis of imidazole derivatives.
were added dropwise with constant stirring. The resultant
solution was stirred at room temperature for 3 h and the obtained
black precipitate was washed thoroughly and labeled as CA/
GO [12].
Synthesis of imidazole derivatives:A mixture of organo-
catalyst (5 mg), 1 mmol of 2-aminobenzamide was stirred at
30 ºC for 10 min. Subsequently, corresponding aldehydes (1.2
mmol) was added. The resulting mixture was stirred at the
same temperature until the reaction was completed. Then the
catalyst was separated by simple filtration. The upper organic
phase with the product was concentrated under reduced pressure.
The crude products were purified by column chromatography
(Scheme-I).
Spectral data
2
-Phenyl-2, 3-dihydroquinazolin-4(1H)-one (3a):Yield:
1
9
0%; white solid, m.p. 178-180 ºC. H NMR (300 MHz,
EXPERIMENTAL
DMSO) δ: 12.62 (s, 1H), 8.26 (dd, J = 8.8, 5.5 Hz, 2H), 8.17 (d,
Analytical reagent grade graphite powder, sodium nitrate,
potassium permangnate, citric acid and hydrogen peroxide
J = 8.0 Hz, 1H), 7.91-7.82 (m, 1H), 7.75 (d, J = 8.0 Hz, 1H),
13
7
.60-7.50 (m, 1H), 7.41 (t, J = 8.8 Hz, 2H). C NMR (75
(
30%) were purchased from Sigma-Aldrich and used without
MHz, DMSO) δ: 152.30 (s), 135.47 (s), 131.21 (d, J = 8.9 Hz),
28.13 (s), 127.45 (s), 126.69 (s), 116.61 (s), 116.32 (s), 41.19
s), 40.92 (s), 40.64 (s), 40.36 (s), 40.08 (s), 39.80 (s), 39.52 (s).
ES-MS (M+1) Calculated (m/z): 224.09. Anal. calcd. (found) %
O: C, 74.98 (74.95); H, 5.39 (5.36); N, 12.49 (12.41).
-(p-Tolyl)-2,3-dihydroquinazolin-4(1H)-one (3b):
further purifications.
1
(
Synthesis of heterogeneous organocatalyst: Graphene
oxide (GO) was synthesized from graphite powder using modified
Hummer′s method [8], then citric acid modified graphene oxide
was synthesized by the following procedure, 200 mg of grap-
hene oxide was dispersed in 50 mL of water via sonication.
To a dispersed solution, 50 mL of 10 M citric acid solution
for C14H N
12 2
2
1
Yield: 81%; white solid, m.p. 184-186 ºC. H NMR (300 MHz,
R
O
O
CA/GO
Water
H
NH2
+
H
R
NH NH
NH2
O
CH3
OH
Br
CN
H
H
H
H
H
NH NH
NH NH
NH NH
NH NH
NH NH
O
O
O
O
O
3c
3
a
3b
3d
3e
I
Cl
NO2
NO2
O N
H
2
H
H
H
H
NH NH
NH NH
NH NH
NH NH
NH NH
O
O
O
O
O
3f
3g
3h
3i
3j
Scheme-I: CA/GO catalyzed imidazole derivative synthesis with different substitutions

Vol. 32, No. 9 (2020)
Citrus Extract Modified Graphene Oxide as Organocatalyst for the Synthesis of Imidazole Derivatives 2377
1
3
DMSO) δ: 12.44 (s, 1H), 8.15 (d, J = 7.9 Hz, 1H), 7.83 (dd, J
7.9 Hz, 2H), 7.60 (dd, J = 16.2, 8.2 Hz, 1H). C NMR (75
MHz, DMSO) δ: 133.37 (s), 129.47 (s), 41.19 (s), 40.77 (d, J
= 20.9 Hz), 40.63-40.61 (m), 40.36 (s), 40.08 (s), 39.80 (s),
39.52 (s). ES-MS (M+1) calculated (m/z): 269.08.Anal. calcd.
=
11.1, 4.2 Hz, 1H), 7.67 (d, J = 8.0 Hz, 1H), 7.51 (dd, J =
1
5.7, 7.8 Hz, 2H), 7.45-7.37 (m, 1H), 7.37-7.28 (m, 2H), 2.55-
13
2
.46 (m, 3H). C NMR (75 MHz, DMSO) δ: 135.23 (s), 131.35
(
s), 130.68 (s), 129.90 (s), 128.19 (s), 127.42 (s), 126.55 (d, J =
.9 Hz), 41.37 (s), 41.09 (s), 40.81 (s), 40.63-39.82 (m), 39.70
s), 20.33 (s). ES-MS (M+1) calculated (m/z): 238.11. Anal.
Calcd. (found) % for C15 O: C, 75.61 (75.63); H, 5.92
5.90); N, 11.76 (11.76).
-(4-Hydroxyphenyl)-2,3-dihydroquinazolin-4(1H)-
(found) % for C14
N, 15.61 (15.63).
H
11
N
3
O : C, 62.45 (62.42); H, 4.12 (4.15);
3
8
(
2-(4-Nitrophenyl)-2,3-dihydroquinazolin-4(1H)-one
(3h):Yield: 81%. H NMR (300 MHz, DMSO) δ: 12.47 (s, 1H),
8.10 (dd, J = 15.4, 8.0 Hz, 3H), 7.82 (t, J = 7.6 Hz, 1H), 7.71
(d, J = 8.1 Hz, 1H), 7.49 (t, J = 7.4 Hz, 1H), 7.34 (d, J = 8.1 Hz,
2H). C NMR (75 MHz, DMSO) δ: 142.27 (s), 135.33 (s),
130.82 (s), 130.00 (s), 128.52 (s), 127.17 (s), 126.67 (s), 120.94
(s), 41.36 (s), 41.08 (s), 40.81 (s), 40.53 (s), 40.25 (s), 39.97
(s), 39.69 (s), 21.80 (s). ES-MS (M+1) calculated (m/z): 269.08.
1
H
14
N
2
(
2
1
13
one (3c):Yield: 81%; white solid, m.p. 180-182 ºC. H NMR
300 MHz, DMSO) δ: 12.41 (s, 1H), 8.14 (dd, J = 17.2, 8.4
Hz, 3H), 7.80 (t, J = 6.9 Hz, 1H), 7.69 (d, J = 7.8 Hz, 1H),
(
13
7
.47 (t, J = 7.4 Hz, 1H), 7.07 (d, J = 8.9 Hz, 2H). C NMR (75
MHz, DMSO) δ: 162.63 (s), 152.61 (s), 135.09 (s), 130.20
s), 128.04 (s), 126.63 (d, J = 9.1 Hz), 125.66 (s), 114.66 (s),
Anal. calcd. (found) % for C14
4.11 (4.11); N, 15.63 (15.64).
H
11
N
3
O : C, 62.46 (62.45); H,
3
(
8
4
3
0.13 (s), 79.69 (s), 79.27 (s), 56.16 (s), 41.26 (s), 40.98 (s),
0.70 (s), 40.28 (d, J = 20.7 Hz), 40.10-39.96 (m), 39.86 (s),
9.59 (s). ES-MS (M+1) calculated (m/z): 240.09.Anal. Calcd.
2-(2-Nitrophenyl)-2,3-dihydroquinazolin-4(1H)-one
(3i):Yield: 81%; yellow solid, m.p. 190-192 ºC. H NMR (300
1
MHz, DMSO) δ: 12.97 (d, J = 122.0 Hz, 1H), 8.77 (d, J = 5.9
(
found) % for C14
H
12
N
2
2
O : C, 69.99 (69.91); H, 5.03 (5.02);
Hz, 3H), 8.26-8.06 (m, 3H), 7.92-7.72 (m, 3H), 7.57 (dd, J =
13
N, 11.66 (11.68).
10.8, 3.9 Hz, 1H). C NMR (75 MHz, DMSO) δ: 151.56 (s),
151.10 (s), 135.55 (s), 128.56 (s), 128.17 (s), 126.78 (s), 122.45
(s), 42.65-41.29 (m), 41.19 (s), 40.77 (d, J = 21.1 Hz), 40.21
(d, J = 21.0 Hz), 39.80 (s), 39.51 (s). ES-MS (M+1) calculated
3
-(4-Oxo-1,2,3,4-tetrahydroquinazolin-2-yl)benzonitrile
1
(
3d):Yield: 81%; white solid, m.p. 182-184 ºC. H NMR (300
MHz, DMSO) δ: 12.64 (s, 1H), 8.29 (dd, J = 8.8, 5.5 Hz, 2H),
.19 (d, J = 8.0 Hz, 1H), 7.94- 7.85 (m, 1H), 7.78 (d, J = 8.0
8
(m/z): 269.08.Anal. calcd. (found) % for C14
H
11
N
3
3
O : C, 62.46
13
Hz, 1H), 7.64-7.53 (m, 1H), 7.44 (t, J = 8.8 Hz, 2H). C NMR
(62.46); H, 4.11 (4.11); N, 15.63 (15.63).
(
(
(
75 MHz, DMSO) δ: 135.47 (s), 131.21 (d, J = 8.9 Hz), 127.45
s), 126.69 (s), 116.61 (s), 116.32 (s), 41.19 (s), 40.92 (s), 40.64
s), 40.36 (s), 40.08 (s), 39.80 (s), 39.52 (s). ES-MS (M+1)
2-(4-Iodophenyl)-2,3-dihydroquinazolin-4(1H)-one
(3j):Yield: 81%; white solid, m.p. 172-174 ºC. H NMR (300
1
MHz, DMSO) δ: 12.97 (d, J = 122.0 Hz, 1H), 8.77 (d, J = 5.9
calculated (m/z): 249.27.Anal. calcd. (found) % for C15
C, 72.28 (72.30); H, 4.45 (4.46); N, 16.86 (16.88).
H
11
3
N O:
Hz, 3H), 8.26-8.06 (m, 3H), 7.92-7.72 (m, 3H), 7.57 (dd, J =
13
10.8, 3.9 Hz, 1H). C NMR (75 MHz, DMSO) δ: 151.56 (s),
151.10 (s), 135.55 (s), 128.56 (s), 128.17 (s), 126.78 (s), 122.45
(s), 42.65-41.29 (m), 41.19 (s), 40.77 (d, J = 21.1 Hz), 40.21
(d, J = 21.0 Hz), 39.80 (s), 39.51 (s). ES-MS (M+1) calculated
2
-(4-Methoxyphenyl)-2,3-dihydroquinazolin-4(1H)-
1
one (3e):Yield: 81%; white solid, m.p. 172-174 ºC. H NMR
300 MHz, DMSO) δ: 12.41 (s, 1H), 8.12 (d, J = 6.7 Hz, 1H),
(
7
7
.80 (d, J = 6.3 Hz, 1H), 7.70 (d, J = 6.1 Hz, 1H), 7.56 (s, 1H),
(m/z): 349.99.Anal. calcd. (found) % for C14
H
11
2
N O: C, 48.02
13
.52-7.41 (m, 1H), 6.39 (s, 1H), 2.42 (s, 2H). C NMR (75
(48.03); H, 3.17 (3.18); N, 8.00 (8.04).
MHz, DMSO) δ: 145.71 (s), 135.34 (s), 127.55 (s), 126.89
s), 119.85 (s), 116.71 (s), 108.73 (s), 41.18 (s), 40.91 (s),
0.63 (s), 40.35 (s), 40.07 (s), 39.79 (s), 39.52 (s), 14.42 (s).
ES-MS (M+1) calculated (m/z): 254.11. Anal. calcd. (found) %
: C, 70.85 (70.86); H, 5.55 (5.54); N, 11.02 (11.03).
-(4-Chlorophenyl)-2,3-dihydroquinazolin-4(1H)-one
RESULTS AND DISCUSSION
(
4
In present work, citrus extracted modified graphene oxide
as a heterogeneous nanocatalyst was synthesized and utilized
for the synthesis of imidazole derivatives under mild conditions
in a short times. The graphene oxide was prepared via modified
Hummer′s method further the citric acid was covalently bonded
to graphene oxide nanosheets and then characterized by using
several analytical techniques.
FTIR studies: FT-IR spectrum [Fig. 1A(i)] of modified
graphene oxide revealed characteristic peaks at 1064 (C-O),
1273 (C-O-C), 1381 (C-OH) and 1726 (C=O), while the band
for C15
H N O
14 2 2
2
1
(
3f):Yield: 81%; white solid, m.p. 190-192 ºC. H NMR (300
MHz, DMSO) δ: 12.60 (s, 1H), 8.15 (dd, J = 12.1, 8.4 Hz,
4
8
H), 7.89-7.77 (m, 2H), 7.72 (d, J = 8.1 Hz, 1H), 7.60 (d, J =
.6 Hz, 2H), 7.51 (t, J = 7.4 Hz, 1H). C NMR (75 MHz,
13
DMSO) δ: 162.63 (s), 152.61 (s), 135.09 (s), 130.20 (s), 128.04
s), 126.63 (d, J = 9.1 Hz), 125.66 (s), 114.66 (s), 80.13 (s),
(
-1
7
4
9.69 (s), 79.27 (s), 56.16 (s), 41.26 (s), 40.98 (s), 40.70 (s),
0.28 (d, J = 20.7 Hz), 40.10-39.96 (m), 39.86 (s), 39.59 (s).
at 1622 cm can be attributed to the C=C vibration of oxidized
graphene sheets [11,13]. The new peaks [Fig. 1A(ii)] appeared
-1
ES-MS (M+1) calculated (m/z): 258.06. Anal. calcd. (found)
OCl: C, 65.00 (65.02); H, 4.29 (4.28); N, 10.83
at 3350, 1627, 1315 and 779 cm which are corresponds to O-H
streching, C-H bending, C-H streching and O-H bending,
%
for C14H N
11 2
-1
(
10.84).
-(4-Nitrophenyl)-2,3-dihydroquinazolin-4(1H)-one
3g):Yield: 81%; yellow solid, m.p. 196-198 ºC. H NMR (300
respectively. The high intensity of 1627 cm is due to the overlap
of amide C=O stretching along with O-H bending.
2
1
(
EDX analysis: The EDX and elemental analysis confirmed
that citric acid was coupled on the surface of graphene sheets
and the results were also compared with oxidized graphene
MHz, DMSO) δ: 12.78 (s, 1H), 8.34 (d, J = 8.4 Hz, 2H), 8.18
(
d, J = 7.3 Hz, 1H), 8.06 (t, J = 7.7 Hz, 2H), 7.84 (dt, J = 13.2,

2
378 Arunkumar et al.
Asian J. Chem.
(B)
(i)
(A)
(ii)
(i)
(ii)
0
10
20
30
40
(°)
50
60
70
80
4000
3500
3000
2500
2000
1500
1000
500
2
θ
–1
Wavenumber (cm )
(D)
(C)
62.53%
i) 76.89%
(
(ii)
(ii)
22.5%
14.97%
(i)
18.42%
4.69%
1000
1500
2000
–1
Wavenumber (cm )
C K
N K
O K
C K
N K
O K
Element content
Element content
Fig. 1. FT-IR (A); PXRD (B); EDX (C) and Raman (D) spectrum of GO (i) and CA/GO (ii)
sheets [14]. The analyzed results show the carbon 76.89% and
oxygen 18.42% in graphene oxide [Fig. 1C(i)], whereas Fig.
Raman analysis:The D and G bands are two intense Raman
features in the spectra of materials. They were represented by
-1
1
C(ii) shows the composition of heterogeneous nanocatalyst,
peaks at around 1350-1320 and 1585-1570 cm , respectively.
which consisted carbon 62.53% and oxygen 22.5%. EDX
analysis clearly shows the presence of 22.5% of oxygen which
indicated different types of oxygen were doped on the graphene
sheets [15].
In some cases, the peak called D′ also appeared at 1625-1602
-1
cm . The G-band is generally a doubly degenerated phonon mode
2
of sp carbon network and the 2D-band is the second-order
Raman scattering process. However, due to the defects, a weak
-1
Graphene oxide and heterogeneous nanocatalyst were
characterized by X-ray diffraction. It can be observed from
Fig. 1B(i) that graphene oxide spectrum was appeared the sharp
peak at 2θ = 12.3º, which is very close to the reported XRD
pattern of graphene oxide [16]. On the other hand, after the
citric acid treatment of graphene oxide, different types of
modifications were observed on surface of graphene sheets.
The XRD peak position shifted to 2θ = 26.1º similar to the
XRD peak position of amine-graphene composite. This result
illustrates the d-spacing of graphene oxide (d = 0.83 nm),
which decreases after covalent treatment graphene sheets. This
clearly indicates a significant number of oxygen groups was
occurred on the surface of graphene oxide (d = 0.335nm) [17].
In addition, a peak of heterogeneous nanocatalyst at 2θ = 26.1º
with broader width and weaker intensity than pristine graphene
oxide (2θ = 12.3º) is due to the insertion of oxygen atom in to
the graphene sheets. So the sheet was loss its crystal structure
that means the degree of disorder increased with the oxidised
graphene layers.
D-band determined at 1348 cm is observed [18,19]. (Fig. 1D)
shows the degree of structural deformations of the graphene oxide
and heterogeneous nanocatalyst. The intensity ratio of D band to
G band provides the different for the amount of structural defects
between graphene oxide and heterogeneous nanocatalyst. The
nanocatalyst was found to have an ID/IG ratio of 1.35, obviously
larger than 0.91 observed for graphene oxide. The downshift of
the G peak from graphene oxide in hetero-geneous nanocatalyst
can be related to the electron rich oxygen group.
Morphological studies: The SEM images of the synthe-
sized heterogeneous nanocatalyst and its precursor are presented
in Fig. 2a-b. It is observed that a single sheet of graphene oxide
is composed of a few layers which are loosely stacked on each
other [20]. Whereas SEM image of heterogeneous nanocatalyst
revealed that as-prepared catalyst consisted of randomly
aggregated thin sheets which are closely associated, forming
a porous and disordered network [21].
Fig. 3a shows the TEM image of graphene oxide with a
crumpled silk like morphology, which is the characteristic

Vol. 32, No. 9 (2020)
Citrus Extract Modified Graphene Oxide as Organocatalyst for the Synthesis of Imidazole Derivatives 2379
TABLE-1
SOLVENT OPTIMIZATION OF CA/GO CATALYZED
a
IMIDAZOLE DERIVATIVE SYNTHESIS
O
H
O
CA/GO
Water
NH NH
NH2
+
H
O
NH2
i
ii
b
Entry
Solvent
Ethanol
Water
Catalyst load (mg)
Yield (%)
1
2
3
4
5
6
7
8
9
10
10
10
10
10
10
10
10
10
10
10
77
98
73
57
53
52
72
68
–
Fig. 2. SEM images of GO (a) and CA/GO (b)
Ethanol:water
Methanol:water
Methanol
Tetrahydrofuran
iso-propyl alcohol
Toluene
1,4-Dioxane
Ethyl acetate
Acetonitrile
1
1
0
1
70
68
a
Reaction conditions: 2-Aminobenzamide (1 mmol), aldehydes (1.2
i
ii
mmol), CA/GO catalyst (5 mg), solvent (5 mL), 0.5 h at room temp.;
All are isolated yield.
b
Fig. 3. TEM images of GO (a) and CA/GO (b)
100
feature of single layer of graphene sheets. As seen in Fig. 3b,
TEM image of heterogeneous nanocatalyst in the graphene
nanosheet has a typical crumpled surface with random stacking,
which might be attributed to the defective structure formed
upon exfoliation and the presence of foreign oxygen atoms [22].
Optimization of solvent: In order to identify a reliable
solvent for the synthesized heterogeneous nanocatalyst catal-
yzed in the imidazole derivatives synthesis, a series of solvent
such as ethanol, methanol, isopropanol, acetonitrile, tetra-
hydrofuran, 1,4-dioxane, toluene, ethanol:water, methanol:
water and water were employed. The reaction was performed
using organic solvents and the desired imidazoles with a good
yield upto 77% (Table-1). To improve more benign nature of
the carbocatalyst and practice greener protocols, the experi-
ment was tried water as medium, the results were encouraging
and excellent yield were observed up to 98% without further
purification (entry 2, Table-1). Finally, water medium at room
temperature was chosen as an optimum medium for the hetero-
geneous nanocatalyst for the catalyzed synthesis of compound
8
0
0
6
40
20
0
0
1
2
3
4
5
6
7
8
Catalyst load (mg)
Fig. 4. Catalyst load optimization of CA/GO imidazole derivative synthesis
3
a.
Reusability: The recyclability of as-prepared hetero-
geneous orangocatalyst (5 mg) was optimized in the reaction
between 2-aminobenzamide (1 mmol) and aldehydes (1.2
mmol). After performing the first run, the reaction mixture
was taken in water (5 mL) and after the reaction was completed
as monitored by TLC. The catalyst was separated by using
simple filtration and the filtered liquid containing product
was isolated by vacuum distillation. The catalyst was then
washed with methanol and acetone followed by drying to get
the free floating heterogeneous orangocatalyst for further use.
Recovered catalyst was little bit lower compared to the quantity
used in the first run, which may be due to the loss of some
acid functional groups involved in the reaction. However, the
Dosage of catalyst: Furthermore, catalyst load has a pivotal
role in organic synthesis. Even though, the solvent and temp-
erature optimization involve 10 mg of catalyst and further
catalyst load optimization was carried out from 1 mg onwards
(
Fig. 4). It was observed that 5 mg of heterogeneous nanocatalyst
is adequate for the complete conversion of the corresponding
a, with > 98% yield without any byproducts. Similarly, time
3
optimization results also revealed that within 30 min, all the
reactants were consumed and converted into the desired 3a.
Furthermore, the present catalytic system is highly specific
towards the formation of 3a, rather than other intermediate
even in the presence of excess equivalence of reactants.

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2
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4
5
Number of cycle
1
1
Fig. 5. Reusability and recovery of CA/GO in the model reaction
https://doi.org/10.1039/C6NR07448K
Conclusion
In summary, citric acid on graphene nanosheet was synth-
14. M.M. Islam, S.N. Faisal,A.K. Roy, S.Ansari, K. Konstantinov, D. Cardillo
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esized via simple chemical modification method. Efficient,
imidazole derivatives were preceded over metal-free, citric acid
graphene nanosheets under mild conditions. The surface analysis
for a prepared heterogeneous orangocatalyst demonstrates that
the graphene oxide was well functionalized with citric acid.
The citric acid-grafted graphene oxide displayed a superior
acidic behaviour and exhibited remarkable catalytic activity.
Sustainable nature of the catalyst is very high and more stable
even after few cycles.
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CONFLICT OF INTEREST
20. M. Du, J. Sun, J. Chang, F. Yang, L. Shi and L. Gao, RSC Adv., 4, 42412
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(
The authors declare that there is no conflict of interests
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