Sulfonic-acid-functionalized activated carbon made from tea leaves as green catalyst for synthesis of 2-substituted benzimidazole and benzothiazole

Article abstract of DOI:10.1007/s11164-017-3187-x

Abstract: A simple and efficient procedure for synthesis of 2-substituted benzimidazole and benzothiazole has been developed by using sulfonic-acid-functionalized activated carbon as heterogeneous catalyst. The activated material was prepared from matured tea leaf in presence of phosphoric acid as activating agent. The final catalyst was prepared by anchoring –SO₃H group on the surface of the activated carbon. The catalyst could be easily recovered and reused for more than three catalytic cycles without significant loss in catalytic activity. The catalytic performance of the catalyst was found to be superior to that of a similar catalyst prepared from montmorillonite K10. Graphical Abstract: [Figure not available: see fulltext.].

Full text of DOI:10.1007/s11164-017-3187-x

Res Chem Intermed
DOI 10.1007/s11164-017-3187-x
Sulfonic-acid-functionalized activated carbon made
from tea leaves as green catalyst for synthesis of
2-substituted benzimidazole and benzothiazole
Mridusmita Goswami1 Mintu Maan Dutta1 Prodeep Phukan1
•
•
Received: 11 July 2017 / Accepted: 30 October 2017
Ó Springer Science+Business Media B.V., part of Springer Nature 2017
Abstract Asimple andefficientprocedurefor synthesisof2-substitutedbenzimidazole and
benzothiazole has been developed by using sulfonic-acid-functionalized activated carbon as
heterogeneous catalyst. The activated material was prepared from matured tea leaf in pres-
ence of phosphoric acid as activating agent. The final catalyst was prepared by anchoring –
SO3Hgroup onthe surfaceof theactivatedcarbon. The catalystcouldbeeasilyrecoveredand
reused for more than three catalytic cycles without significant loss in catalytic activity. The
catalytic performance of the catalyst was found to be superior to that of a similar catalyst
prepared from montmorillonite K10.
Graphical Abstract
&
Prodeep Phukan
pphukan@yahoo.com
1
Department of Chemistry, Gauhati University, Guwahati, Assam 781 014, India
123

M. Goswami et al.
Keywords Activated carbon Á Sulfonic acid Á Heterogeneous catalysis Á
Benzimidazole Á Benzothiazole
Introduction
Benzimidazoles and benzothiazoles are considered to be attractive building blocks
by synthetic chemists due to their presence as active substructures in biologically
significant compounds. These structural motifs are associated with a wide variety of
pharmacological activities such as antiviral, antifungal, anticancer, antiulcer,
antihypertensive, and antihistaminic effects [1–8]. Benzimidazole derivatives
exhibit considerable activity against several viruses such as human immunodefi-
ciency virus (HIV) and herpes simplex virus type 1 (HSV-1), and also act as
antitumor and antimicrobial agents [9–12]. Benzothiazole derivatives have appli-
cations as enzyme inhibitors, antitumor agents, fluorescent materials, and in vivo
imaging agents [13–16].
Due to the extensive utility of these heterocycles, a variety of methods for their
synthesis have been widely reported. The most common synthetic method involves
condensation of 1,2-phenylenediamine or 2-aminothiophenol with aldehydes
[17–19]. In case of some other synthetic approaches, orthoesters [20, 21], carboxylic
acid [22–24], alcohols [25], acid chlorides [26] or diketones [27, 28] have also been
used instead of aldehydes.
Recently, Martins et al. [29] reported a procedure for synthesis of benzimidazole
using Ce(NO3)3Á6H2O as promoter in dimethylformamide (DMF). Shirini et al. [30]
and coworkers synthesized benzimidazole derivatives using 1,10-disulfo-[2,20-
bipyridine]-1,10-diium chloride ionic liquid as homogeneous catalyst. Hanoon et al.
[31] described synthesis of benzimidazole derivatives in presence of ionic-liquid-
functionalized reduced graphene oxide as catalyst. Another method using graphene
oxide as catalyst for synthesis of benzimidazoles and benzothiazoles was reported
by Dhopte [32]. Use of CuI nanoparticles for synthesis of 2-substituted benzim-
idazoles requires use of molecular oxygen as oxidant [33]. Although there are
several reports on synthesis of these two heterocycles, most of the traditional
synthetic methods suffer from one or more drawbacks, such as high temperature,
long reaction time, low yield of product, use of toxic organic solvents, harsh
reaction condition, use of expensive and hazardous catalyst, etc. Taking into
account the various drawbacks of existing synthetic methods, there is a need to
develop environmentally benign methodologies for synthesis of benzimidazoles and
benzothiazoles.
We report herein an efficient environmentally friendly method for synthesis of
benzimidazoles and benzothiazoles in presence of sulfonic-acid-functionalized
activated carbon made from matured tea leaf (MTLAC–SA). Sulfonic-acid-
functionalized montmorillonite K10 (K10–SA) was also used as catalyst for
comparison. Although both catalysts were found to be efficient for synthesis of these
heterocycles, MTLAC–SA showed better catalytic activity than K10–SA.
123

Sulfonic-acid-functionalized activated carbon made from…
Experimental
Materials and apparatus
The Brunauer–Emmett–Teller (BET) surface area of all the synthesized materials
was determined from the N2 adsorption–desorption isotherm at -196 °C using a
surface area analyzer (Beckmann Coulter SA-3100). Scanning electron microscopy
(SEM) and energy-dispersive X-ray analysis (EDX) of the synthesized materials
were carried out using a JEOL JSM-6360 scanning electron microscope. Fourier-
transform infrared (FT-IR) spectra were recorded on a Shimadzu IRAffinity-1
spectrophotometer. 1H nuclear magnetic resonance (NMR) spectra were obtained in
CDCl3 and dimethylsulfoxide (DMSO) using a Bruker 300 MHz instrument.
Synthesis of catalysts
Synthesis of activated carbon (MTLAC)
Activated carbon was synthesized by following a previously reported procedure
[34]. The raw material, i.e., matured tea leaf (MTL), for preparation of activated
carbon, was collected from a tea garden of Upper Assam. MTL was washed
thoroughly with distilled water to remove adhering dust and impurities, dried,
ground, and sieved to obtain 60-mesh-size particles. MTL powder was soaked in
85% H3PO4 at ratio of 1:3 (MTL:H3PO4, w/v) and kept in an oven for 3 h at 60 °C.
Acid-impregnated MTL was carbonized in a tube furnace under nitrogen
atmosphere. It was heated from room temperature to 500 °C (at heating rate of
5 °C/min) with 1 h holding time at 100 °C intervals. After cooling to room
temperature, the activated carbon was washed thoroughly with distilled water until
the washing water became neutral, then dried in oven at 100 °C for about 8 h. The
resulting activated carbon (MTLAC) was ground, sieved, and kept in a desiccator
for further use.
Synthesis of sulfonic-acid-functionalized MTLAC and K10
Sulfonic-acid-functionalized activated carbon was prepared as follows [35]:
Initially, MTLAC powder (200 mg) was dispersed in 5 mL dry CH2Cl2 by
ultrasonic bath for 30 min. Chlorosulfonic acid (0.2 mL) was added dropwise to an
ice-cooled solution of MTLAC over a period of 30 min at room temperature. After
completion of the addition, the mixture was stirred for another 3 h for complete
dissipation of HCl from the reaction vessel. The resulting product was filtered and
washed with ethanol and water and finally dried in an oven at 80 °C to obtain
sulfonic-acid-functionalized activated carbon (MTLAC–SA). Sulfonic-acid-func-
tionalized montmorillonite K10 was prepared by the same procedure as above and is
denoted as K10–SA.
123

M. Goswami et al.
General procedure for synthesis of benzimidazole and benzothiazole derivatives
Aldehyde (1 mmol), ortho-phenylenediamine or 2-aminothiophenol (1.1 mmol),
synthesized catalyst (10 wt%), and ethanol (3 mL) were mixed and stirred at room
temperature for appropriate time. After reaction completion as confirmed by thin-
layer chromatography (TLC), the usual work-up procedure was carried out and the
crude product was purified using column chromatography to obtain the pure
product. Both the catalysts were recovered after reaction, washed with ethanol,
dried in an oven for about 1 h, and reused for seven cycles successfully.
Experimental data for synthesized benzimidazole and benzothiazole
derivatives
2-Phenyl-1H-benzimidazole (1a)
1
M.p. 287–290 °C (lit. 290–291 °C) [36]; H NMR (300 MHz, CDCl3 ? DMSO-
d6): d (ppm): 7.18–7.20 (m, 2H), 7.47–7.64 (m, 5H), 8.16 (d, J = 7.8 Hz, 2H).
2-p-Tolyl-1H-benzimidazole (1b)
1
M.p. 261–264 °C (lit. 262–265 °C) [36]; H NMR (300 MHz, CDCl3 ? DMSO-
d6): d (ppm): 2.33 (s, 3H), 7.18–7.23 (m, 4H), 7.59–7.62 (m, 2H), 8.05 (d,
J = 8.1 Hz, 2H).
2-(4-Methoxyphenyl)-1H-benzimidazole (1c)
1
M.p. 224–226 °C (lit. 225–226 °C) [36]; H NMR (300 MHz, CDCl3 ? DMSO-
d6): d (ppm): 3.76 (s, 3H), 6.90 (d, J = 8.7 Hz, 2H), 7.13–7.16 (m, 2H), 7.53–7.56
(m, 2H), 8.09 (d, J = 8.7 Hz, 2H).
2-(2-Methoxyphenyl)-1H-benzimidazole (1d)
1
M.p. 163–165 °C (lit. 157–158 °C) [37]; H NMR (300 MHz, CDCl3 ? DMSO-
d6): d (ppm): 3.99 (s, 3H), 6.97–7.06 (m, 2H), 7.16–7.19 (m, 2H), 7.36 (t,
J = 8.7 Hz, 1H), 7.60–7.64 (m, 2H), 8.45–8.47 (d, J = 6 Hz, 1H).
2-(3,4-Dimethoxyphenyl)-1H-benzimidazole (1e)
1
M.p. 186–188 °C (lit. 180–181 °C) [38]; H NMR (300 MHz, CDCl3 ? DMSO-
d6): d (ppm): 3.86 (s, 6H), 6.87 (d, J = 8.4 Hz, 1H), 7.16–7.20 (m, 2H), 7.56–7.59
(m, 2H), 7.71 (d, J = 8.1 Hz, 1H), 7.79 (s, 1H).
2-(4-Chlorophenyl)-1H-benzimidazole (1f)
1
M.p. 286–288 °C (lit. 288–290 °C) [36]; H NMR (300 MHz, CDCl3 ? DMSO-
d6): d (ppm): 7.22 (s, 2H), 7.62 (d, J = 7.8 Hz, 4H), 8.17 (d, J = 7.5 Hz, 2H).
123

Sulfonic-acid-functionalized activated carbon made from…
2-(4-Fluorophenyl)-1H-benzimidazole (1g)
1
M.p. 248–252 °C (lit. 251 °C) [17]; H NMR (300 MHz, CDCl3 ? DMSO-d6): d
(ppm): 7.08 (t, J = 8.7 Hz, 2H), 7.18–7.21 (m, 2H), 7.57–7.60 (m, 2H), 8.14–8.19
(m, 2H).
2-(2-Fluorophenyl)-1H-benzimidazole (1h)
1
M.p. 223–225 °C (lit. 232–235 °C) [39]; H NMR (300 MHz, CDCl3 ? DMSO-
d6): d (ppm): 7.20–7.25 (m, 1H), 7.30–7.36 (m, 3H), 7.45 (t, J = 7.5 Hz, 1H), 7.52
(s, 1H), 7.86 (s, 1H), 8.52 (t, J = 7.5 Hz, 1H), 10.02 (s, 1H).
2-(4-Bromophenyl)-1H-benzimidazole (1i)
1
M.p. 295–298 °C (lit. 294–296 °C) [17]; H NMR (300 MHz, CDCl3 ? DMSO-
d6): d (ppm): 7.12–7.15 (m, 2H), 7.51 (d, J = 8.7 Hz, 4H), 7.99 (d, J = 8.7 Hz,
2H).
2-(4-Nitrophenyl)-1H-benzimidazole (1j)
1
M.p. 317–320 °C (lit. 319–321 °C) [36]; H NMR (300 MHz, CDCl3 ? DMSO-
d6): d (ppm): 7.19–7.22 (m, 2H), 7.50–7.57 (m, 2H), 8.26 (t, J = 7.8 Hz, 2H), 8.37
(d, J = 8.7 Hz, 2H).
2-(3-Nitrophenyl)-1H-benzimidazole (1k)
1
M.p. 196–199 °C (lit. 203–205 °C) [40]; H NMR (300 MHz, CDCl3 ? DMSO-
d6): d (ppm): 7.38–7.24 (m, 2H), 7.65 (t, J = 7.8 Hz, 3H), 8.24 (d, J = 7.8 Hz, 1H),
8.63 (d, J = 7.5 Hz, 1H), 9.09 (s, 1H).
2-Heptyl-1H-benzimidazole (1l)
1
M.p. 134–138 °C (lit. 138–140 °C) [38]; H NMR (300 MHz, CDCl3 ? DMSO-
d6): d (ppm): 0.80–0.87 (m, 3H), 1.21–1.35 (m, 8H), 1.82–1.89 (m, 2H), 2.99 (t,
J = 8.1 Hz, 2H), 6.16 (s, 1H), 7.23–7.27 (m, 2H), 7.60 (s, 2H).
2-(1H-pyrrol-2-yl)-1H-benzimidazole (1m)
1
M.p. 275–277 °C (lit. 273–275 °C) [37]; H NMR (300 MHz, CDCl3 ? DMSO-
d6): d (ppm): 6.11 (t, J = 5.4 Hz, 1H), 6.87 (s, 1H), 6.93 (d, J = 3 Hz, 1H),
7.04–7.08 (m, 2H), 7.36–7.40 (m, 2H), 11.59 (s, 1H).
123

M. Goswami et al.
2-(Anthracen-9-yl)-1H-benzimidazole (1n)
1
M.p. 255 °C (lit. 261–263 °C) [41]; H NMR (300 MHz, CDCl3 ? DMSO-d6): d
(ppm): 7.33–7.44 (m, 6H), 7.61 (d, J = 7.8 Hz, 2H), 7.72–7.75 (m, 2H), 7.98 (d,
J = 8.4 Hz, 2H), 8.52 (s, 1H).
2-(Naphthalen-1-yl)-1H-benzimidazole (1o)
1
M.p. 205–207 °C (lit. 212–215 °C) [38]; H NMR (300 MHz, CDCl3 ? DMSO-
d6): d (ppm): 7.08–7.10 (m, 2H), 7.35 (s, 2H), 7.52 (s, 2H), 7.68 (d, J = 4.2 Hz,
1H), 7.76 (d, J = 8.1 Hz, 2H), 8.15 (d, J = 8.4 Hz, 1H), 8.58 (s, 1H).
2-(Furan-2-yl)-1H-benzimidazole (1p)
1
M.p. 284–285 °C (lit. 278–283 °C) [38]; H NMR (300 MHz, CDCl3 ? DMSO-
d6): d (ppm): 6.44–6.46 (m, 1H), 7.10–7.13 (m, 2H), 7.26 (t, J = 3.3 Hz, 1H),
7.46–7.50 (m, 3H).
2-Styryl-1H-benzimidazole (1q)
1
M.p. 200–204 °C (lit. 200–202 °C) [17]; H NMR (300 MHz, CDCl3 ? DMSO-
d6): d (ppm): 7.04–6.96 (m, 3H), 7.11–7.18 (m, 3H), 7.33–7.40 (m, 4H), 7.57 (d,
J = 16.8 Hz, 1H).
4-(1H-Benzimidazol-2-yl) benzene-1,2-diol (1r)
1
M.p. 255–257 °C (lit. 258 °C) [42]; H NMR (300 MHz, CDCl3 ? DMSO-d6): d
(ppm): 6.69 (d, J = 8.4 Hz, 1H), 6.90–6.93 (m, 2H), 7.30–7.33 (m, 3H), 7.46 (s,
1H).
5,6-Dichloro-2-p-tolyl-1H-benzimidazole (1s)
M.p. 220–221 °C (lit. 223–225 °C) [43]; 1H NMR (300 MHz, DMSO-d6): d (ppm):
2.37 (s, 3H), 7.36 (d, J = 7.8 Hz, 2H), 7.81 (s, 2H), 8.03 (d, J = 6 Hz, 2H).
6-Methyl-2-p-tolyl-1H-benzimidazole (1t)
1
M.p. 104–105 °C (lit. 101–103 °C) [44]; H NMR (300 MHz, CDCl3): d (ppm):
2.27 (s, 3H), 2.39 (s, 3H), 7.07–7.00 (m, 3H), 7.33 (s, 1H), 7.46 (d, J = 8.1 Hz, 2H),
8.07 (d, J = 7.8 Hz, 2H).
2-Phenylbenzothiazole (2a)
1
M.p. 112–115 °C (lit. 111–112 °C) [36]; H NMR (300 MHz, CDCl3): d (ppm):
6.69 (d, J = 8.4 Hz, 1H), 6.90–6.93 (m, 2H), 7.30–7.33 (m, 3H), 7.46 (s, 1H).
123

Sulfonic-acid-functionalized activated carbon made from…
2-p-Tolylbenzothiazole (2b)
M.p. 82–86 °C (lit. 81–83 °C) [36]; 1H NMR (300 MHz, CDCl3): d (ppm): 2.43 (s,
3H), 7.30–7.41 (m, 3H), 7.50 (t, J = 7.8 Hz, 1H), 7.90 (d, J = 7.5 Hz, 1H), 7.99 (d,
J = 7.8 Hz, 2H), 8.07 (d, J = 8.4 Hz, 1H).
2-(4-Methoxyphenyl)benzothiazole (2c)
M.p. 125 °C (lit. 120–121 °C) [36]; 1H NMR (300 MHz, CDCl3): d (ppm): 3.90 (s,
3H), 7.02 (d, J = 9 Hz, 2H), 7.38 (t, J = 7.8 Hz, 1H), 7.50 (t, J = 7.2 Hz, 1H),
7.89 (d, J = 8.1 Hz, 1H), 8.07 (d, J = 8.7 Hz, 3H).
2-(2-Methoxyphenyl)benzothiazole (2d)
M.p. 108 °C (lit. 110–112 °C) [45]; 1H NMR (300 MHz, CDCl3): d (ppm): 4.08 (s,
3H), 7.07–7.18 (m, 2H), 7.37–7.42 (m, 1H), 7.45–7.53 (m, 2H), 7.94 (d,
J = 8.1 Hz, 1H), 8.13 (d, J = 7.8 Hz, 1H), 8.56 (d, J = 9.6 Hz, 1H).
2-(4-Fluorophenyl)benzothiazole (2e)
M.p. 100 °C (lit. 100–102 °C) [46]; 1H NMR (300 MHz, CDCl3): d (ppm): 7.20 (t,
J = 8.7 Hz, 2H), 7.41 (t, J = 7.8 Hz, 1H), 7.52 (t, J = 7.8 Hz, 1H), 7.91 (d,
J = 7.8 Hz, 1H), 8.07–8.13 (m, 3H).
2-(2-Fluorophenyl)benzothiazole (2f)
M.p. 91–93 °C (lit. 95–98 °C) [39]; 1H NMR (300 MHz, CDCl3): d (ppm):
7.23–7.37 (m, 2H), 7.42–7.58 (m, 3H), 7.96 (d, J = 8.1 Hz, 1H), 8.19 (d,
J = 8.4 Hz, 1H), 8.50 (t, J = 8.1 Hz, 1H).
2-(2,4-Dichlorophenyl)benzothiazole (2g)
M.p. 148 °C (lit. 151–152 °C) [45]; 1H NMR (300 MHz, CDCl3): d (ppm):
7.40–7.48 (m, 2H), 7.55 (t, J = 7.5 Hz, 2H), 7.97 (d, J = 7.8 Hz, 1H), 8.13 (d,
J = 8.1 Hz, 1H), 8.24 (d, J = 8.4 Hz, 1H).
2-(3-Nitrophenyl)benzothiazole (2h)
M.p. 190 °C (lit. 194–197 °C) [45]; 1H NMR (300 MHz, CDCl3): d (ppm): 7.46 (t,
J = 7.8 Hz, 1H), 7.56 (t, J = 8.1 Hz, 1H), 7.70 (t, J = 8.1 Hz, 1H), 7.96 (d,
J = 7.5 Hz, 1H), 8.13 (d, J = 8.4 Hz, 1H), 8.35 (d, J = 9.3 Hz, 1H), 8.43 (d,
J = 7.5 Hz, 1H), 8.95 (s, 1H).
123

M. Goswami et al.
2-(4-Bromophenyl)benzothiazole (2i)
M.p. 100 °C (lit. 101–102 °C) [36]; 1H NMR (300 MHz, CDCl3): d (ppm): 7.42 (t,
J = 7.5 Hz, 1H), 7.52 (t, J = 7.5 Hz, 1H), 7.65 (d, J = 8.4 Hz, 2H), 7.90–7.98 (m,
3H), 8.08 (d, J = 8.1 Hz, 1H).
2-Heptyl-1H-benzothiazole (2j)
1H NMR (300 MHz, CDCl3): d (ppm): 0.88 (t, J = 6.6 Hz, 3H), 1.29–1.48 (m, 8H),
1.82–1.92 (m, 2H), 3.11 (t, J = 9 Hz, 2H), 7.34 (t, J = 7.5 Hz, 1H), 7.45 (t,
J = 7.2 Hz, 1H), 7.83 (d, J = 7.8 Hz, 1H), 7.98 (d, J = 8.1 Hz, 1H).
2-(Naphthalen-1-yl)benzothiazole (2k)
1
M.p. 126–128 °C (lit. 128–130 °C) [46]; H NMR (300 MHz, CDCl3): d (ppm):
7.42 (t, J = 7.5 Hz, 1H), 7.50–7.58 (m, 3H), 7.88–8.00 (m, 4H), 8.13 (d,
J = 8.1 Hz, 1H), 8.23 (d, J = 8.1 Hz, 1H), 8.58 (s, 1H).
Results and discussion
Synthesis of catalysts
Initially, activated carbon was synthesized by following a procedure reported by us
[34]. After ensuring formation of activated carbon from matured tea leaves, the
surface of activated carbon was further modified by incorporating sulfonic acid
functionality on the surface by using chlorosulfonic acid in DCM (Fig. 1).
For surface modification, carbon sample was taken in DCM and kept under
sonication for 30 min to produce homogeneous dispersion. The dispersion was
further kept on an ice bath, and appropriate amount of chlorosulfonic acid was
added slowly dropwise. To incorporate maximum amount of sulfonic acid
functionality on the carbon surface, a series of experiments was conducted using
different amounts of chlorosulfonic acid. It was found that a maximum of 1 mL
chlorosulfonic acid per gram of sample could be used for surface modification.
Beyond this amount, the carbon sample was converted to a sticky mass after
Fig. 1 Schematic representation of the process of synthesis of catalysts
123

Sulfonic-acid-functionalized activated carbon made from…
Table 1 Surface area of
Sample
SBET (m2/g)
materials
MTLAC
MTLAC–SA
K10
1313.4
1169.3
218.2
204
K10–SA
900
MTLAC
MTLAC-SA
K10
K10-SA
750
600
450
300
150
0
0.0
0.2
0.4
0.6
0.8
1.0
Relative Pressure (Ps/Po)
Fig. 2 N2 adsorption–desorption isotherms of synthesized materials
chlorosulfonic acid treatment. For comparison of the catalytic activity, commer-
cially available montmorillonite K10 sample was also modified by incorporating
sulfonic acid functionality. It is noteworthy that montmorillonite K10 has already
been exploited as catalyst for this particular transformation.
The BET surface area of all the synthesized materials was determined from N2
adsorption–desorption isotherms at - 196 °C. The surface texture and elemental
content of the synthesized materials were characterized by scanning electron
microscopy (SEM) and energy-dispersive X-ray (EDX) spectroscopic analysis. To
confirm the functional groups present in the samples, Fourier-transform infrared
(FT-IR) spectra were recorded in the scanning range from 4000 to 400 cm-1
.
Boehm titration was carried out to determine the total surface acidity of the
synthesized catalysts.
123

M. Goswami et al.
Fig. 3 Scanning electron micrographs of a MTLAC, b MTLAC–SA, c K10, and d K10–SA
Table 2 EDX analysis of
MTLAC and MTLAC–SA
Element (wt%)
Sample
MTLAC
MTLAC–SA
C K
O K
S K
96.22
3.78
–
73.13
25.63
1.24
Table 3 EDX analysis of K10
and K10–SA
Element (wt%)
Sample
K10
K10–SA
O K
Al K
Si K
S K
50.87
15.81
33.33
–
60.83
11.58
25.41
1.19
BET analysis
The surface area of the synthesized materials was determined by nitrogen
adsorption–desorption method using the BET equation. Prior to analysis, the
samples were dried at 100 °C then degassed under vacuum at 120 °C for 2 h. The
123

Sulfonic-acid-functionalized activated carbon made from…
surface area of the materials is presented in Table 1, and the nitrogen adsorption–
desorption isotherms are shown in Fig. 2. All the isotherms were found to be of
type II as per International Union of Pure and Applied Chemistry (IUPAC)
classification. From Table 1, it is seen that MTLAC had higher surface area than
K10. After surface functionalization, there was a decrease in surface area for all the
materials. Such decrease in surface area may be due to introduction of functional
groups onto surface-active sites of the parent material.
SEM and EDX analysis
The surface morphology of MTLAC–SA and K10–SA was studied by SEM (Fig. 3).
The samples were coated with a thin layer of gold using a sputter coater prior to
analysis. As shown in the figure, MTLAC–SA had porous surface, in good
agreement with its higher BET surface area among the two catalysts.
Energy-dispersive X-ray (EDX) spectroscopic analysis was carried out for in situ
chemical analysis of the synthesized materials; the results are presented in Tables 2
and 3. Presence of one new element (S) in MTLAC–SA and K10–SA confirmed
successful functionalization of sulfonic acid group on the surface of the synthesized
materials.
MTLAC
MTLAC-SA
S-O
S=O
K10
S-OH
K10-SA
S-O
S-OH
S=O
4000
3500
3000
2500
2000
1500
1000
500
Wavenumber (cm-1)
Fig. 4 FT-IR spectra of synthesized materials
123

M. Goswami et al.
Table 4 Results of Boehm
titration
Sample
Amount of acidic surface groups (mmol/g)
MTLAC–SA
K10–SA
0.51
0.45
FT-IR analysis
FT-IR spectra of all the synthesized materials are presented in Fig. 3. The signal
observed at around 620 cm-1 can be attributed to presence of S–O stretching
vibration. The signal observed at around 1120 cm-1 is due to presence of S=O
symmetric stretching vibration. The broad band at around 3400 cm-1 can be
assigned to S–OH stretching vibration (Fig. 4).
Boehm titration
Boehm titration was carried out to determine the amount of acidic surface groups on
the synthesized materials [47]. Synthesized materials (0.15 g) were mixed with
30 mL 0.05 N NaOH solution. The samples were sealed and shaken for 24 h, then
10 mL filtrate was titrated with 0.05 N HCl. The number of acidic surface groups
was calculated based on the assumption that NaOH neutralizes acidic groups present
on the material surface. The results of the experiment are presented in Table 4,
revealing that MTLAC–SA had slightly more acidic surface groups than K10–SA.
Optimization of reaction conditions
The reaction of 4-methylbenzaldehyde (1 mmol) and ortho-phenylenediamine
(1.1 mmol) was selected as model reaction to establish the best reaction conditions.
The reaction was first performed without any catalyst for about 5 h at room
temperature, obtaining trace (8%) yield. In presence of the synthesized catalyst
(MTLAC–SA), 92% yield was obtained in 45 min.
The effect of different solvent systems for synthesis of imidazole derivatives was
studied; the results are summarized in Table 5. The reaction was carried out using
solvents EtOH, MeOH, water, and acetonitrile. EtOH and acetonitrile showed
almost equal results regarding yield of product. Considering the environmental
effect, EtOH was chosen as a suitable solvent for the reaction. The model reaction
was also optimized in terms of catalyst concentration. The results are summarized in
Table 5. In absence of catalyst, the reaction did not produce good yield. However,
addition of 5 wt% (with respect to the weight of aldehyde) catalyst resulted in a
sharp increase of yield within a short period of time. Addition of 10 wt% catalyst
was found to be sufficient for the reaction. Further increase in the amount of catalyst
up to 15 wt% did not show any effective increase in product yield.
These optimum conditions were also used for synthesis of benzothiazole
derivatives.
123

Sulfonic-acid-functionalized activated carbon made from…
123

M. Goswami et al.
MTLAC-SA
K10-SA
92
90
88
86
84
82
80
78
76
74
Benzimidazole
Benzothiazole
Fig. 5 Comparison of catalytic activity of MTLAC–SA and K10–SA
To compare the catalytic activity of MTLAC–SA with that of K10–SA, the
reaction between 4-methylbenzaldehyde and ortho-phenylenediamine or ortho-
aminothiophenol was carried out using both catalysts. The results obtained are
depicted in Fig. 5.
From this figure, it is observed that MTLAC–SA showed better catalytic activity
in comparison with K10–SA. This can be attributed to the acid capacity of the
studied catalyst. According to the Boehm titration results (Table 4), the acid
capacity of MTLAC–SA was higher than that of K10–SA. Also, the higher surface
area of the catalyst (Table 1) provides a large number of active sites. These are the
two major reasons for the superior catalytic performance of MTLAC–SA to that of
K10–SA.
After optimizing the reaction conditions, the versatility of this method was
examined by performing the reaction using different substituted aldehydes.
Aldehydes with both electron-withdrawing and electron-donating substituent were
used successfully for synthesis of corresponding imidazole and thiazole derivatives
in excellent yield. The results are presented in Table 6. Using the same optimum
condition, benzothiazole derivatives were also synthesized; the results obtained are
summarized in Table 6. From this table one can say that the catalyst is very
effective for benzothiazole synthesis as well.
The effectiveness and applicability of the synthesized catalysts were compared
with some reported catalysts in terms of reaction time, percentage yield, and
reaction temperature. For this study, synthesis of 2-(4-methoxyphenyl)-1H-
benzimidazole and 2-(4-methoxyphenyl)benzothiazole were taken as model reac-
tions (Table 7).
123

Sulfonic-acid-functionalized activated carbon made from…
Table 6 Synthesis of benzimidazole and benzothiazole derivatives using MTLAC-SA catalyst
NH2
XH
N
X
Catalyst (10 wt%)
Ethanol (3 mL), rt
R1
R1 CHO
R
R
X=NH, 1a- t
X=S, 2a-k
R1= Aryl, alkyl
X= NH, S
MTLAC–SA
Entry
1
R
H
X
Substrate
Product
Time Yielda
(min)
(%)
NH
1a
45
85
CHO
2
3
4
H
H
H
NH
NH
1b
1c
1d
45
45
45
92
94
92
OHC
CH3
OHC
OCH3
H3CO
OHC
NH
OCH3
5
6
H
H
NH
NH
1e
1f
45
60
92
82
OHC
OCH3
OHC
Cl
7
8
H
H
NH
NH
1g
1h
50
60
88
85
OHC
F
F
OHC
9
H
H
H
NH
NH
NH
1i
1j
60
45
60
82
84
80
OHC
OHC
Br
10
11
NO2
NO2
1k
OHC
12
13
H
H
NH
NH
1l
65
50
75
92
OHC
H
N
1m
OHC
CHO
14
15
H
H
NH
NH
1n
1o
60
60
86
90
CHO
123

M. Goswami et al.
Table 6 continued
MTLAC–SA
Time Yielda
Entry
R
X
Substrate
O
Product
(min)
(%)
16
17
H
H
NH
NH
1p
1q
60
85
OHC
OHC
60
60
84
87
OH
18
19
20
H
NH
1r
1s
1t
OHC
OH
4,5-dichloro NH
60
60
83
90
OHC
CH3
4-CH3
NH
OHC
CH3
21
22
23
24
H
H
H
H
S
S
S
S
2a
2b
2c
2d
60
45
45
45
85
92
94
90
CHO
OHC
CH3
OHC
OCH3
H3CO
OHC
25
26
H
H
S
S
2e
2f
45
60
87
84
OHC
F
F
OHC
Cl
27
28
H
H
S
S
2g
2h
60
60
81
82
OHC
Cl
NO2
OHC
OHC
29
30
31
H
H
H
S
S
S
2i
2j
60
65
60
85
75
85
Br
OHC
CHO
2k
Reaction condition: aldehyde (1 mmol), o-phenylenediamine/o-aminothiophenol (1.1 mmol), catalyst
10 wt%
aIsolated yield, column chromatography
123

Sulfonic-acid-functionalized activated carbon made from…
Table 7 Comparative study of present versus reported methods
Catalyst
Reaction time (min) Temperature Yield (%) References
Montmorillonite K10a
Nano-ZnOa,b
Nanoporous aluminosilicateb
2400
80
R.t.
47.1
70
[48]
Reflux
Reflux
R.t.
[49]
270
300
270
45
88
[50]
b
MnFe2O4
Copper triflateb
82
[51]
Reflux
R.t.
88
[52]
SA-functionalized activated carbona,b
94
This study
This study
SA-functionalized montmorillonite
K10a
90
R.t.
87
aReaction performed with 4-methoxybenzaldehyde and o-aminothiophenol
bReaction performed with 4-methoxybenzaldehyde and o-phenylenediamine
100
90
80
70
60
50
40
30
20
10
0
MTLAC-SA
K10-SA
1
2
3
4
5
6
7
No. of Cycles
Fig. 6 Recyclability of both catalysts
Recyclability of catalysts
To check the recyclability of the catalysts, the reaction between 4-methylbenzalde-
hyde and ortho-phenylenediamine was carried out using both catalysts.
After the reaction, the catalysts were separated from the reaction mixture, washed
with ethanol, then dried. The recovered catalysts were further used for the same
reaction to check the catalytic activity. All the catalysts were successfully used for
seven catalytic cycles without much decrease in product yield, as shown in Fig. 6.
123

M. Goswami et al.
Conclusions
An efficient, simple, and green method was developed for synthesis of benzimi-
dazole and benzothiazole derivatives using sulfonic-acid-modified activated carbon
(MTLAC–SA) as very effective and recyclable catalyst. The catalytic activity of
MTLAC–SA was found to be superior to that of a catalyst prepared by modification
of montmorillonite K10 (K10–SA). The synthesis procedure for both catalysts was
very simple and cost effective. Short reaction time, good to excellent yield of
product, room-temperature condition, and use of inexpensive and reusable catalysts
make this method both economically and environmentally suitable.
Acknowledgements Financial support from DST [grant no. SR/NM/NS-18/2011(G)] is gratefully
acknowledged. M.M.D. thanks UGC for a research fellowship. We also acknowledge support from the
Department of Chemistry, Gauhati University, SAIF-GU, SAIF-NEHU, and CIF-IITG for all the
analytical facilities used during the course of this investigation.
References
1. M. Gaba, S. Singh, C. Mohan, Eur. J. Med. Chem. 76, 494 (2014)
2. M.J. Tebbe, W.A. Spitzer, F. Victor, S.C. Miller, C.C. Lee, T.R. Sattelberg Sr., E. Mckinney, J.C.
Tang, J. Med. Chem. 40, 3937 (1997)
3. V.K. Vyas, M. Ghate, Mini-Rev. Med. Chem. 10, 1366 (2010)
4. A. Mavrova, K.K. Anichina, D.I. Vuchev, J.A. Tsenov, P.S. Denkova, M.S. Kondeva, M.K. Micheva,
Eur. J. Med. Chem. 41, 1412 (2006)
5. N.C. Desai, N.R. Shihory, G.M. Kotadiya, P. Desai, Eur. J. Med. Chem. 82, 480 (2014)
6. V. Zaharia, A. Ignat, N. Palibroda, B. Ngameni, V. Kuete, C.N. Fokunang, M.L. Moungang, B.T.
Ngadjui, Eur. J. Med. Chem. 45, 5080 (2010)
7. A. Kamal, M.N.A. Khan, K.S. Reddy, K. Rohini, Bioorg. Med. Chem. 15, 1004 (2007)
8. F. Azam, B.A. El-Gnidi, I.A. Alkskas, M.A. Ahmed, J. Enzyme Inhib. Med. Chem. 25, 818 (2010)
9. A.R. Porcari, R.V. Devivar, L.S. Kucera, J.C. Drach, L.B. Townsend, J. Med. Chem. 41, 1252 (1998)
10. M.T. Migawa, J.L. Giradet, J.A. Walker, G.W. Koszalka, S.D. Chamber-Lain, J.C. Drach, L.B.
Townsend, J. Med. Chem. 41, 1242 (1998)
11. W.A. Denny, G.W. Rewcastle, B.C. Bagley, J. Med. Chem. 33, 814 (1990)
12. T. Forseca, B. Gigante, T.L. Gilchrist, Tetrahedron 57, 1793 (2001)
13. S. Aiello, G. Wells, E.L. Stone, H. Kadri, R. Bazzi, D.R. Bell, M.F.G. Stevens, C.S. Matthews, T.D.
Bradshaw, A.D. Westwell, J. Med. Chem. 51, 5135 (2008)
14. S.J. Choi, H.J. Park, S.K. Lee, S.W. Kim, G. Han, H.Y.P. Choo, Bioorg. Med. Chem. 14, 1229 (2006)
15. D. Alagille, R.M. Baldwin, G.D. Tamagnan, Tetrahedron Lett. 46, 1349 (2005)
16. S.J. Ji, H.B. Shi, Dyes Pigm. 70, 246 (2006)
17. J. Azizian, P. Torabi, J. Noei, Tetrahedron Lett. 57, 185 (2016)
18. A. Dandia, V. Parewa, K.S. Rathore, Catal. Comm. 28, 90 (2012)
19. R. Shelkar, S. Sarode, J. Nagarkar, Tetrahedron Lett. 54, 6986 (2013)
20. Z.H. Zhang, J.J. Li, Y.Z. Gao, Y.H. Liu, J. Heterocycl. Chem. 44, 1509 (2007)
21. D. Villemin, M. Hammadi, B. Martin, Synth. Commun. 26, 2895 (1996)
22. Y. Wang, K. Sarris, D.R. Sauer, S.W. Djuric, Tetrahedron Lett. 47, 4823 (2006)
23. A.K. Chakraborti, C. Selvam, G. Kaur, S. Bhagat, Synlett 5, 0851 (2004)
24. X. Wen, J.E. Bakali, R. Deprez-Poulain, B. Deprez, Tetrahedron Lett. 53, 2440 (2012)
25. G.M. Raghavendra, A.B. Ramesha, C.N. Revanna, K.N. Nandeesh, K. Mantelingu, K.S. Rangappa,
Tetrahedron Lett. 52, 5571 (2011)
26. R.N. Nadaf, S.A. Siddiqui, T. Daniel, R.J. Lahoti, K.V. Srinivasan, J. Mol. Catal. A: Chem. 214, 155
(2004)
27. M.S. Mayo, X. Yu, X. Zhou, X. Feng, Y. Yamamoto, M. Bao, Org. Lett. 16, 764 (2014)
28. M.S. Mayo, X. Yu, X. Zhou, X. Feng, Y. Yamamoto, M. Bao, J. Org. Chem. 79, 6310 (2014)
123

Sulfonic-acid-functionalized activated carbon made from…
29. G.M. Martins, T. Puccinelli, R.A. Gariani, F.R. Xavier, C.C. Silveira, S.R. Mendes, Tetrahedron Lett.
58, 1969 (2017)
30. F. Shirini, M. Abedini, M. Seddighi, F.S. Arbosara, Res. Chem. Intermed. 41, 7683 (2015)
31. H.D. Hanoon, E. Kowsari, M. Abdouss, H. Zandi, M.H. Ghasemi, Res. Chem. Intermed. 43, 1751
(2017)
32. K.B. Dhopte, R.S. Zambare, A.V. Patwardhan, P.R. Parag, RSC Adv. 6, 8164 (2016)
33. P.L. Reddy, R. Arundhathi, M. Tripathi, D.S. Rawat, RSC Adv. 6, 53596 (2016)
34. M. Goswami, L. Borah, D. Mahanta, P. Phukan, J. Porous Mater. 21, 1025 (2014)
35. H. Naeimi, S. Mohamadabadi, Dalton Trans. 43, 12967 (2014)
36. E. Dezfoolinezhad, K. Ghodrati, R. Badri, New J. Chem. 40, 4575 (2016)
37. L.-H. Du, X.-P. Luo, Synth. Commun. 40, 2880 (2010)
38. M.A. Chari, D. Shobha, T. Sasaki, Tetrahedron Lett. 52, 5575 (2011)
39. Y.-C. Lin, N.-C. Li, Y.-J. Cherng, J. Heterocycl. Chem. 51, 808 (2014)
40. R. Srinivasulu, K.R. Kumar, P.V.V. Satyanarayana, Green Sustain. Chem. 4, 33 (2014)
41. D. Saha, A. Saha, B.C. Ranu, Green Chem. 11, 733 (2009)
42. S.S. Panda, S.C. Jain, Synth. Commun. 41, 729 (2011)
43. O. Ravi, A. Shaikh, A. Upare, K.K. Singarapu, S.R. Bathula, J. Org. Chem. 82, 4422 (2017)
44. K. Bahrami, M.M. Khodaei, F. Naali, J. Org. Chem. 73, 6835 (2008)
45. G.F. Chen, L.Y. Zhang, H.M. Jia, B.H. Chen, J.T. Li, S.X. Wang, G.Y. Bai, Res. Chem. Intermed. 39,
2077 (2013)
46. M. Chhabra, S. Sinha, S. Banerjee, P. Paira, Bioorg. Med. Chem. Lett. 26, 213 (2016)
47. H.P. Boehm, Adv. Catal. 16, 179 (1966)
48. G.F. Chen, N. Xiao, J.S. Yang, H.Y. Li, B.H. Chen, L.F. Han, Res. Chem. Intermed. 41, 5159 (2015)
49. A. Teimouria, A.N. Chermahinib, H. Salavatia, L. Ghorbanianc, J. Mol. Catal. A: Chem. 373, 38
(2013)
50. M.A. Chari, D. Shobha, E.L.-R. Kenawy, S.S. Al-Deyab, B.V. Subba Reddy, A. Vinu, Tetrahedron
Lett. 51, 5195 (2010)
51. G. Brahmachari, S. Laskar, P. Barik, RSC Adv. 3, 14245 (2013)
52. M.A. Chari, P. Sadanandam, D. Shobha, K. Mukkanti, J. Heterocycl. Chem. 47, 153 (2010)
123